Mathis on Graphene? Any hints?

Graphene nanoribbons

Graphene nanoribbons (GNRs, also called nano-graphene ribbons or nano-graphite ribbons) are strips of graphene with width less than 50 nm. Graphene ribbons were introduced as a theoretical model by Mitsutaka Fujita and coauthors to examine the edge and nanoscale size effect in graphene.[2][3][4]



Nanotomy

Large quantities of width-controlled GNRs can be produced via graphite nanotomy,[5] where applying a sharp diamond knife on graphite produces graphite nanoblocks, which can then be exfoliated to produce GNRs. GNRs can also be produced by "unzipping" or axially cutting nanotubes.[6] In one such method multi-walled carbon nanotubes were unzipped in solution by action of potassium permanganate and sulfuric acid.[7] In another method GNRs were produced by plasma etching of nanotubes partly embedded in a polymer film.[8] More recently, graphene nanoribbons were grown onto silicon carbide (SiC) substrates using ion implantation followed by vacuum or laser annealing.[9][10][11] The latter technique allows any pattern to be written on SiC substrates with 5 nm precision.[12]

Epitaxy

GNRs were grown on the edges of three-dimensional structures etched into silicon carbide wafers. When the wafers are heated to approximately 1,000 °C (1,270 K; 1,830 °F), silicon is preferentially driven off along the edges, forming nanoribbons whose structure is determined by the pattern of the three-dimensional surface. The ribbons had perfectly smooth edges, annealed by the fabrication process. Electron mobility measurements surpassing one million correspond to a sheet resistance of one ohm per square— two orders of magnitude lower than in two-dimensional graphene.[13]

https://en.wikipedia.org/wiki/Graphene_nanoribbon

Silicene Transistors

The U.S. Army Research Laboratory has been supporting research on silicene since 2014. The stated goals for research efforts were to analyze atomic scale materials, such as silicene, for properties and functionalities beyond existing materials, like graphene.[40] In 2015, Deji Akinwande, led researchers at the University of Texas, Austin in conjunction with Alessandro Molle's group at CNR, Italy, and collaboration with U.S. Army Research Laboratory and developed a method to stabilize silicene in the air and reported a functional silicene field effect transistor device. An operational transistor’s material must have bandgaps, and functions more effectively if it possesses a high mobility of electrons. A bandgap is an area between the valence and conduction bands in a material where no electrons exist. Although graphene has a high mobility of electrons, the process of forming a bandgap in the material reduces many of its other electric potentials.[41]

https://en.wikipedia.org/wiki/Silicene
https://en.wikipedia.org/wiki/2D_silica
https://en.wikipedia.org/wiki/Borophene
https://en.wikipedia.org/wiki/Germanene
https://en.wikipedia.org/wiki/Stanene
https://en.wikipedia.org/wiki/Plumbene

"P-wave states"...hmm could that just be the alignment of Mathis' charge field?

----------

Graphene's Superconductive Power Has Finally Been Unlocked, And It's Crazier Than We Expected
FIONA MACDONALD
20 JAN 2017

It's official: graphene has been made into a superconductor in its natural state - which means electrical current can flow through it with zero resistance.

Last year, physicists managed to do this by doping graphene with calcium atoms, but this is the first time researchers have achieved superconductivity in the material without having to alter it. And the results so far show that the material achieves an incredibly rare type of superconductivity that's even crazier and more powerful than scientists expected.

The new research is a big deal even for a material as innately impressive as graphene, seeing as superconductivity is the key to more efficient electronics, better power grids, and new medical technology.

"It has long been postulated that, under the right conditions, graphene should undergo a superconducting transition, but can't," said one of the researchers, Jason Robinson from the University of Cambridge in the UK.

Now, he says his team has managed to awaken that ability. And it appears graphene isn't just a normal superconductor - it could be conducting current with no resistance as a result of an unconfirmed and elusive type of superconductivity called p-wave state. Further research is needed to confirm this result, but it's a pretty intriguing possibility.

Already an over-achiever, graphene is a two-dimensional sheet of carbon atoms that's super flexible, harder than diamond, and stronger than steel.

But since its discovery in 2004, researchers have suspected that graphene might also have the ability to be a superconductor - which means it could shuttle electrons through it with no resistance at all.

Even materials that are good conductors are still inefficient in comparison to superconductors - for example, energy companies lose about 7 percent of their energy as heat as a result of resistance in the grid.

We already use superconducting materials to create the strong magnetic fields needed in MRI machines and maglev trains, but right now, these materials only become superconductive at temperatures of around –269 degrees Celsius (–452.2 degrees Fahrenheit), which is incredibly expensive, and not exactly practical.

If we could find a way to achieve superconductivity sustainably and at high temperatures, it would open up a the possibility of supercomputers that run without resistance and more efficient medical technology - and graphene is often thought of as a prime candidate for achieving this, given all of its other weird and wonderful properties.

Researchers succeeded in making graphene a superconductor last year by inserting calcium atoms into its lattice. And other teams have achieved a similar result by placing it on a superconducting material.

But in the new study, researchers from the University of Cambridge were able to activate the dormant potential of graphene without it being influenced by another material.

"Placing graphene on a metal can dramatically alter the properties so it is technically no longer behaving as we would expect," said one of the team, Angelo di Bernardo.

"What you see is not graphene's intrinsic superconductivity, but simply that of the underlying superconductor being passed on."

Instead, the team achieved superconductivity by coupling it with a material called praseodymium cerium copper oxide (PCCO).

That might sound similar to what happened in previous experiments - after all, they're still putting graphene on top of another material - but the difference here is that PCCO is a type of superconducting material called a cuprate, which has well-understood electronic properties.

So the team was able to clearly distinguish the superconductivity in PCCO from the superconductivity in graphene.

And what they saw was crazier than they'd expected.

Superconductivity occurs when electrons pair up and travel more efficiently through a material.

The spin alignment, or symmetry, of these electron pairs changes depending on the type of superconductivity involved. For example, in PCCO, the electrons pair up with a spin state that's antiparallel, which is called d-wave state.

But what the team saw happening in graphene was very different - they found evidence of a rare and still unverified type of superconductivity called p-wave state occurring.

"What we saw in the graphene was, in other words, a very different type of superconductivity than in PCCO," said Robinson.

"This was a really important step because it meant that we knew the superconductivity was not coming from outside it and that the PCCO was therefore only required to unleash the intrinsic superconductivity of graphene."

It remains unclear exactly what kind of superconductivity the team unleashed in graphene - but if it's confirmed that it really is the elusive p-wave form, it could prove once and for all that this type of superconductivity exists, and allow researchers to study it properly for the first time.

P-wave superconductivity was first proposed in 1994, when Japanese researchers found evidence of it occurring in a crystal material called strontium ruthenate. But the crystal is too bulky to study it well enough to achieve the type of proof scientists need to confirm that the state exists.

If it's happening in graphene, it would be a lot easier to investigate.
....
(more at link)

https://www.sciencealert.com/graphene-s-superconductive-power-has-finally-been-unlocked-and-it-s-crazier-than-we-expected

Keep in mind these papers:
1/13/18, What Rutherford really Proved about the Electron. Also more information about the positron.
http://milesmathis.com/electron.pdf

1/5/18, Graphene. Where we see all the mainstream explanations are wrong. Why? Because they are based on electron bonding theory, which is a fudge from the first word.
http://milesmathis.com/graphene.pdf


-------------

Unconventional superconductivity in magic-angle graphene superlattices

Yuan Cao, Valla Fatemi, Shiang Fang, Kenji Watanabe, Takashi Taniguchi, Efthimios Kaxiras & Pablo Jarillo-Herrero

Nature volume 556, pages 43–50 (05 April 2018) | Download Citation

Abstract

The behaviour of strongly correlated materials, and in particular unconventional superconductors, has been studied extensively for decades, but is still not well understood. This lack of theoretical understanding has motivated the development of experimental techniques for studying such behaviour, such as using ultracold atom lattices to simulate quantum materials. Here we report the realization of intrinsic unconventional superconductivity—which cannot be explained by weak electron–phonon interactions—in a two-dimensional superlattice created by stacking two sheets of graphene that are twisted relative to each other by a small angle. For twist angles of about 1.1°—the first ‘magic’ angle—the electronic band structure of this ‘twisted bilayer graphene’ exhibits flat bands near zero Fermi energy, resulting in correlated insulating states at half-filling. Upon electrostatic doping of the material away from these correlated insulating states, we observe tunable zero-resistance states with a critical temperature of up to 1.7 kelvin. The temperature–carrier-density phase diagram of twisted bilayer graphene is similar to that of copper oxides (or cuprates), and includes dome-shaped regions that correspond to superconductivity. Moreover, quantum oscillations in the longitudinal resistance of the material indicate the presence of small Fermi surfaces near the correlated insulating states, in analogy with underdoped cuprates. The relatively high superconducting critical temperature of twisted bilayer graphene, given such a small Fermi surface (which corresponds to a carrier density of about 1011 per square centimetre), puts it among the superconductors with the strongest pairing strength between electrons. Twisted bilayer graphene is a precisely tunable, purely carbon-based, two-dimensional superconductor. It is therefore an ideal material for investigations of strongly correlated phenomena, which could lead to insights into the physics of high-critical-temperature superconductors and quantum spin liquids.

https://www.nature.com/articles/nature26160

-----------------


Insulator or superconductor? Physicists find graphene is both

When rotated at a "magic angle," graphene sheets can form an insulator or a superconductor.

Jennifer Chu | MIT News Office
March 5, 2018
Press Inquiries


It’s hard to believe that a single material can be described by as many superlatives as graphene can. Since its discovery in 2004, scientists have found that the lacy, honeycomb-like sheet of carbon atoms — essentially the most microscopic shaving of pencil lead you can imagine — is not just the thinnest material known in the world, but also incredibly light and flexible, hundreds of times stronger than steel, and more electrically conductive than copper.

Now physicists at MIT and Harvard University have found the wonder material can exhibit even more curious electronic properties. In two papers published today in Nature, the team reports it can tune graphene to behave at two electrical extremes: as an insulator, in which electrons are completely blocked from flowing; and as a superconductor, in which electrical current can stream through without resistance.

Researchers in the past, including this team, have been able to synthesize graphene superconductors by placing the material in contact with other superconducting metals — an arrangement that allows graphene to inherit some superconducting behaviors. This time around, the team found a way to make graphene superconduct on its own, demonstrating that superconductivity can be an intrinsic quality in the purely carbon-based material.

The physicists accomplished this by creating a “superlattice” of two graphene sheets stacked together — not precisely on top of each other, but rotated ever so slightly, at a “magic angle” of 1.1 degrees. As a result, the overlaying, hexagonal honeycomb pattern is offset slightly, creating a precise moiré configuration that is predicted to induce strange, “strongly correlated interactions” between the electrons in the graphene sheets. In any other stacked configuration, graphene prefers to remain distinct, interacting very little, electronically or otherwise, with its neighboring layers.

The team, led by Pablo Jarillo-Herrero, an associate professor of physics at MIT, found that when rotated at the magic angle, the two sheets of graphene exhibit nonconducting behavior, similar to an exotic class of materials known as Mott insulators. When the researchers then applied voltage, adding small amounts of electrons to the graphene superlattice, they found that, at a certain level, the electrons broke out of the initial insulating state and flowed without resistance, as if through a superconductor.

“We can now use graphene as a new platform for investigating unconventional superconductivity,” Jarillo-Herrero says. “One can also imagine making a superconducting transistor out of graphene, which you can switch on and off, from superconducting to insulating. That opens many possibilities for quantum devices.”

A large-scale interpretation of the moiré patterns formed when one graphene lattice is slightly rotated at a “magic angle,” with respect to a second graphene lattice.

A 30-year gap

A material’s ability to conduct electricity is normally represented in terms of energy bands. A single band represents a range of energies that a material’s electrons can have. There is an energy gap between bands, and when one band is filled, an electron must embody extra energy to overcome this gap, in order to occupy the next empty band.

A material is considered an insulator if the last occupied energy band is completely filled with electrons. Electrical conductors such as metals, on the other hand, exhibit partially filled energy bands, with empty energy states which the electrons can fill to freely move.

Mott insulators, however, are a class of materials that appear from their band structure to conduct electricity, but when measured, they behave as insulators. Specifically, their energy bands are half-filled, but because of strong electrostatic interactions between electrons (such as charges of equal sign repelling each other), the material does not conduct electricity. The half-filled band essentially splits into two miniature, almost-flat bands, with electrons completely occupying one band and leaving the other empty, and hence behaving as an insulator.

“This means all the electrons are blocked, so it’s an insulator because of this strong repulsion between the electrons, so nothing can flow,” Jarillo-Herrero explains. “Why are Mott insulators important? It turns out the parent compound of most high-temperature superconductors is a Mott insulator.”

(more at link)
https://news.mit.edu/2018/graphene-insulator-superconductor-0305
----------------------------

p-wave triggered superconductivity in single-layer graphene on an electron-doped oxide superconductor

A. Di Bernardo, O. Millo, M. Barbone, H. Alpern, Y. Kalcheim, U. Sassi, A. K. Ott, D. De Fazio, D. Yoon, M. Amado, A. C. Ferrari, J. Linder & J. W. A. Robinson
Nature Communications volume 8, Article number: 14024 (2017) | Download Citation


Abstract

Electron pairing in the vast majority of superconductors follows the Bardeen–Cooper–Schrieffer theory of superconductivity, which describes the condensation of electrons into pairs with antiparallel spins in a singlet state with an s-wave symmetry. Unconventional superconductivity was predicted in single-layer graphene (SLG), with the electrons pairing with a p-wave or chiral d-wave symmetry, depending on the position of the Fermi energy with respect to the Dirac point. By placing SLG on an electron-doped (non-chiral) d-wave superconductor and performing local scanning tunnelling microscopy and spectroscopy, here we show evidence for a p-wave triggered superconducting density of states in SLG. The realization of unconventional superconductivity in SLG offers an exciting new route for the development of p-wave superconductivity using two-dimensional materials with transition temperatures above 4.2 K.


https://www.nature.com/articles/ncomms14024

In Miles' words:

-------------
1/5/18, Graphene. Where we see all the mainstream explanations are wrong. Why? Because they are based on electron bonding theory, which is a fudge from the first word.
http://milesmathis.com/graphene.pdf

We are told that electrons propagating through Graphene lose their mass, becoming quasi-particles, but
that is absurd. What really happens is that the electrons are spin-stripped by the material, becoming
photons. The reason they have to be described currently by the 2-D Dirac equation rather than the
Schrodinger equation is that the Schrodinger equation is faulty, especially regarding spin ½ particles.
Since the whole theory behind spin ½ is also faulty, we see where the mess came from. But why
would Graphene spin-strip an electron down to a real photon? Again because the applied current is
moving opposite to the internal current of Graphene. Charge can move both directions here, but
electrons can't. The electrons they are talking about are electrons that came in with the applied current,
so they are moving with what we are calling the anticharge here. We will say they are spinning right.
But the internal charge profile of the Graphene, as created by the metal, is charge, or left spinning.
Therefore, when the introduced electron hits the field of the Graphene, it will be spun down. If its
outer spin is spun down completely, this is the same as a spin-strip. That spin is gone. An electron that
loses its outer spin becomes a photon. Therefore, under the right circumstances, we should see
Graphene “producing” X-rays. And guess what, it does. At that link to phys.org, we are told that
electrons moving through Graphene plasmons cause them to release X-rays. But the electrons aren't
releasing X-rays, they are becoming X-rays. That would be pretty easy to prove, since they now have
electron counters. All they have to do is count electrons. They will find that the X-ray production
leads to an electron reduction.
--------

Also:
https://milesmathis.forumotion.com/t259-graphene-s-crazier-than-we-thought-can-handle-1000-times-more-current-than-regular-material
https://milesmathis.forumotion.com/t292-newly-discovered-phenomenon-shows-electrons-can-move-much-faster-than-expected

An interesting French company with a new Graphene creation process:

-----------

Our unique graphene
Why our Graphene is (so) wanted?

Per definition, graphene is a perfect, defect-free single layer of carbon, exhibiting exceptional properties. In reality, there is no large-scale production of such high-quality material.

There are different types of graphene available on the market, exhibiting different properties and therefore different advantages and disadvantages. e.g. purity, quality, and the scalability to industrial production.

Carbon Waters masters a graphene production method which sets us apart from other suppliers and brings the possibility of using “real” graphene.

Our product, “Eau de graphène”, exhibits exceptional properties. He is singled out in particular by the absence of organic solvents and surfactants. Additionally, our high-quality graphene dispersion offers versatility without the security limitations of graphene in powder form or VOCs contents.
What is graphene identity card of grap'up the graphene by carbon waters

Why our graphene is more flexible?

Carbon Waters can produce graphene directly in a liquid dispersion which allows a very broad flexibility in applications.

As our graphene can be produced in aqueous and organic solvents, it is compatible with a great variety of formulation matrices.

Moreover, this graphene form is also adapted for a lot of industrial deposition process: electrodeposition, spray, inkjet and others.

------
https://www.carbon-waters.com/surface-treatments-unit/

Our expertise on surface treatments
Graphene-enhanced surface treatments

When a protection against external environment is required, a surface treatment can be an efficient way to optimize resistance and improve the material properties.

At Carbon Waters, we have developed an expertise in surface treatment using graphene.

Taking advantage of graphene impermeability (barrier to gas and solvents) and chemical inertness, we provide solutions tailored to client’s requirements.

How to prevent corrosion with graphene surface treatment

Our deposition techniques

We can provide graphene-enhanced surface treatment adjusted to a wide range of materials: metals, plastics, polymers and composites.

Our capabilities include various deposition techniques such as electrodeposition, spray, dip-coating and inkjet.

Each technique has its own specificity and we work closely with our clients to define the most effective protocol according to their needs in order to deliver the best added-value.

https://www.carbon-waters.com/wp-content/uploads/2019/10/what-is-graphene-carbon-waters.png.webp

---

Graphene Barrier effects

Thanks to its high quality, our graphene shows very good barrier-enhancing properties.

This barrier effect provides an excellent protection against solvents (water-based and organic), extreme pH, temperature, radiations.

https://www.extremetech.com/electronics/198563-researchers-make-graphene-magnetic-clearing-the-way-for-faster-everything

More at link..

Researchers make graphene magnetic, clearing the way for faster everything

By Ryan Whitwam on
January 29, 2015 at 4:14 pm
14 Comments

Graphene has many fantastic properties that could change the course of human civilization. It’s chemically stable, highly conductive, and incredibly strong. One thing it is not, however, is magnetic. This is one of the issues cited by the likes of IBM, which has tried to dampen expectations for a future of super-efficient microprocessors built on graphene. That might not be a problem much longer, though. Scientists from the University of California, Riverside have successfully created graphene that has magnetic properties.

To make this happen, the team started with a sheet of regular (but still awesome) non-magnetic graphene. The graphene was placed on a layer of magnetic yttrium iron garnet, which actually transferred its magnetic properties to the graphene without disrupting its structure or other properties. Most magnetic substances interfere with graphene’s ability to conduct electricity, but yttrium iron garnet is also an electric insulator. That meant it was unlikely to negatively affect the graphene’s electron transport properties.

When removed and exposed to a magnetic field, the team found their treated graphene’s Hall voltage depended on the magnetic linearity of the yttrium iron garnet. This told the scientists that their graphene was magnetic all on its own and that magnetism had come from exposure to the yttrium iron garnet layer. This property should last indefinitely as it is not the result of depositing material on the graphene, but comes from the graphene itself.

yttrium iron garnet

It has been possible in the past to create magnetized graphene, but this always relied on adding additional magnetic compounds or coatings to the raw graphene — often lead or iron. This rather defeats the purpose of graphene by adding additional complications. It’s already hard enough to produce large quantities of sufficiently pure graphene without adding these additional complications. The addition of extra atoms to graphene’s single atom structure also screws up its electrical properties in the same way exposure to non-insulating magnetic materials can. What’s different this time is that the graphene is still just pure graphene.

More at link...

https://m.phys.org/news/2017-02-layers-graphene-reveals-kind-magnet.html

Three layers of graphene reveals a new kind of magnet

February 23, 2017 , Tata Institute of Fundamental Research


A spectrum of the three layer graphene as a function of magnetic field and density of electrons. Credit: Biswajit Datta, Mandar Deshmukh

Metals have a large density of electrons and to be able to see the wave nature of electrons one has to make metallic wires that are only a few atoms wide. However, in graphene - one atom thick graphite—the density of electrons is much smaller and can be changed by making a transistor. As a result of the low density of electrons the wave nature of electrons, as described by quantum mechanics, is easier to observe in graphene.

Often in metals like copper the electron is scattered every 100 nanometers, a distance roughly 1000 times smaller than the diameter of human hair, due to impurities and imperfections. Electrons can travel much longer in graphene, upto distances of 10 micrometer, a distance roughly 10 times smaller than the diameter of human hair. This is realized by sandwiching graphene between layers of boron nitride. The layers of boron nitride have few imperfections to impede the flow of electrons in graphene.

Once electrons travel long distances, implying there are few imperfections, one notices the faint whispers of electrons "talking to each other".

Link: https://arxiv.org/ftp/arxiv/papers/1303/1303.2391.pdf


Electrostatic Graphene Loudspeaker

Qin Zhouand A. Zettla
Center of Integrated Nanomechanical Systems, University of California at Berkeley, Berkeley, California 94720, USA

Department of Physics, University of California at Berkeley, Berkeley, California 94720, USAMaterials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

Graphene has extremely low mass density and high mechanical strength, key qualities for efficient wide-frequency-response electrostaticaudio speaker design. Low mass ensures good high frequency response, while high strength allows for relatively large free-standing diaphragms necessary for effective low frequency response. Here we report on construction and testing of a miniaturized graphene-based electrostatic audio transducer. The speaker/earphone is straightforward in design and operation and has excellent frequency response across the entire audio frequency range (20HZ –20kHz), with performance matching or surpassing commercially available audio earphones.

How Graphene makes a faster, cooler and safer battery

Updated: Dec 4, 2019

We know many of you are curious about how graphene works and how we utilize it to bring you a faster, cooler and safer charging experience.  

https://www.realgrapheneusa.com/graphene

Why are current lithium batteries so limited?​

Heat is the number one major cause. As the device begins charging, heat is generated based on resistivity of conductor. Generated heat increases the resistivity of conductor. Since the conductor is hotter, the resistivity is higher which means the device charges even more heat. This creates a positive feedback loop that can spiral out of control and cause the battery to catch on fire.

To prevent this, batteries will regulate the speed of charging but this results in battery charging speeds slowing to a crawl.

What are the benefits of using Graphene composite?

Graphene is a near perfect conductor of electricity. This allows electricity to flow without hindrance. This dramatically slows the heating process lithium batteries face while allowing charging speeds up to 5 times as fast. This also increases the battery life by 5 times the charging cycles.

Graphene also evenly disperses heat acting as a cooling system. Graphene already generates less heat due to extremely low resistivity. But graphene also conducts heat evenly across battery to help cool the battery.

​
Learn more at: https://www.realgrapheneusa.com/graphene
-----------------

Lunch with Microsoft President, Brad Smith

CEO of Real Graphene USA, Samuel Gong, had lunch with Microsoft's President, Brad Smith, at the Nixon Library in Yorba Linda growing awareness for the Real Graphene brand.

The closed-doored lunch, hosted at the Nixon Library, was held to discuss several topics in upcoming technology among many industry leaders. During the lunch, Samuel Gong spoke with Brad Smith about the prospects of advancing technologies and how he viewed their integration into society. They also spoke about creating environments where innovation and new ideas can flourish during tense relations between the USA and China.

After the lunch, Brad and Samuel discussed several things such as the potential of graphene composite batteries in future devices. Real Graphene power banks can be used to charge all of Microsoft's Surface devices. However, if Real Graphene batteries could be integrated into the Surface devices themselves, it would allow for charging speeds well beyond the current capabilities.

https://www.realgrapheneusa.com/post/design-a-stunning-blog

Ballistic miniband conduction in a graphene superlattice

   Menyoung Lee1, John R. Wallbank2, Patrick Gallagher1, Kenji Watanabe3, Takashi Taniguchi3, Vladimir I. Fal’ko2,4, David Goldhaber-Gordon1,*

See all authors and affiliations
Science  30 Sep 2016:
Vol. 353, Issue 6307, pp. 1526-1529
DOI: 10.1126/science.aaf1095

Abstract

Rational design of long-period artificial lattices yields effects unavailable in simple solids. The moiré pattern in highly aligned graphene/hexagonal boron nitride (h-BN) heterostructures is a lateral superlattice with high electron mobility and an unusual electronic dispersion whose miniband edges and saddle points can be reached by electrostatic gating. We investigated the dynamics of electrons in moiré minibands by measuring ballistic transport between adjacent local contacts in a magnetic field, known as the transverse electron focusing effect. At low temperatures, we observed caustics of skipping orbits extending over hundreds of superlattice periods, reversals of the cyclotron revolution for successive minibands, and breakdown of cyclotron motion near van Hove singularities. At high temperatures, electron-electron collisions suppress focusing. Probing such miniband conduction properties is a necessity for engineering novel transport behaviors in superlattice devices.

In solids, the quantum nature of electrons generates band structure, which controls conduction and optical properties. Similarly, longer-period superlattices generate minibands that disperse at a finer energy scale over a reduced Brillouin zone, enabling phenomena such as negative differential conductance and Bloch oscillations (1–3). However, fabricating long-range periodic patterns that strongly modulate the potential to form well-separated minibands without undermining the material quality and electron coherence remains challenging. Most experiments on laterally patterned semiconductor heterostructures have revealed classical commensurability effects (4–6), which do not require well-formed and separated minibands. Despite evidence for Fermi surface reconstruction in a patterned superlattice, details of Fermi surfaces were obscured by poor separation between minibands and consequent magnetic breakdown across weakly avoided crossings (7).

The arrival of high-quality graphene/h-BN van der Waals heterostructures with misalignment angle below 1° (8, 9) has drastically changed the situation. In such systems, the periodic potential for electrons in graphene is imposed by the hexagonal moiré pattern generated by the incommensurability and misalignment between the two crystals (10–12). Formation of minibands for Dirac electrons has been demonstrated by scanning tunneling (13), capacitance (14), and optical (15) spectroscopies, as well as magnetotransport (16–19). These studies have elucidated the electronic structure known as the Hofstadter butterfly, which emerges in a quantizing magnetic field (20). By contrast, a small magnetic field may be treated semiclassically. Then the connection between the miniband dispersion ε(k) and transport properties is established by the equations of motion for an electron in an out-of-plane magnetic field Embedded Image,Embedded Image (1)where e is the charge on the electron, ħ is the Planck constant divided by 2π, and the relation between carrier velocity v and momentum ħk is approximately v = vk/k (v ≈ 106 m/s) close to the Dirac point of graphene’s spectrum (10, 11, 13, 14).

The shape of the cyclotron orbit in a two-dimensional (2D) metal is a 90° rotation of the shape of the Fermi surface, and the carrier revolves along it clockwise or counterclockwise. Electron trajectories near the boundary of a metal open into skipping orbits (21), which drift in the direction determined by the effective charge of the carrier. These skipping orbits bunch along caustics (22–27), leading to the transverse electron focusing (TEF) effect (22, 23). Experimentally, TEF takes place when the magnetic field is tuned such that caustics of skipping orbits, emanating from an emitter E, end up at a collector C, located at position x = L along the boundary. Then a voltage VC is induced at C, proportional to the current IE injected into E. Figure 1B illustrates skipping orbits and caustics in a material with an isotropic Fermi surface, such as unperturbed graphene near the Dirac point, where TEF occurs for B = Bj ≡ 2jħkF/±eL (for j = 1, 2, …). An equidistant series of peaks (oscillations) appears in the focusing “spectrum”—the nonlocal magnetoresistance VC/IE(B) (Fig. 1C, lower trace), from which the Fermi momentum ħkF and the sign of effective charge ±e can be inferred. TEF was initially used to study the Fermi surfaces of bulk metals (22, 28) and was later extended to 2D systems (23), including graphene (29).
.....

The direct observation and manipulation of ballistic transport is a powerful probe of the low-energy physics of an electron system. Here, the quasiparticles propagate freely from the emitter to the collector through ballistic trajectories as long as πL/2 = 10 μm, which is 700 in dimensionless units of the superlattice period. Ballistic motion of ultracold atoms has been seen in homogeneous optical lattices as large as 100 unit cells (37), but in the solid state, the mean free path of electrons in semiconductor 1D superlattices has been limited to 10 unit cells (38). Our experiment elucidates the key features of miniband electron dynamics in a moiré superlattice and points toward further explorations of novel transport effects. For instance, the saddle-point VHS could host exotic effects caused by enhanced electron-electron interactions (19, 39), and valley-contrasting physics could be accessed by taking advantage of the severe trigonal warping of minibands (40). For technology, such a clear validation of the miniband conduction properties suggests that graphene/h-BN (and perhaps other moiré superlattices) may be a practical platform for devices based on miniband physics. Efficient photocurrent generation at the edge of a graphene superlattice in a magnetic field (41) may be caused by the skipping orbits we have observed; furthermore, THz devices such as the Bloch oscillator can benefit from the much longer scattering times in this system.

link: https://science.sciencemag.org/content/353/6307/1526.full
..................

Exceptional ballistic transport in epitaxial graphene nanoribbons

Jens Baringhaus, Ming Ruan, Frederik Edler, Antonio Tejeda, Muriel Sicot, Amina Taleb-Ibrahimi, An-Ping Li, Zhigang Jiang, Edward H. Conrad, Claire Berger, Christoph Tegenkamp & Walt A. de Heer

Nature volume 506, pages349–354(2014)Cite this article

Abstract

Graphene nanoribbons will be essential components in future graphene nanoelectronics1. However, in typical nanoribbons produced from lithographically patterned exfoliated graphene, the charge carriers travel only about ten nanometres between scattering events, resulting in minimum sheet resistances of about one kilohm per square2,3,4,5. Here we show that 40-nanometre-wide graphene nanoribbons epitaxially grown on silicon carbide6,7 are single-channel room-temperature ballistic conductors on a length scale greater than ten micrometres, which is similar to the performance of metallic carbon nanotubes. This is equivalent to sheet resistances below 1 ohm per square, surpassing theoretical predictions for perfect graphene8 by at least an order of magnitude. In neutral graphene ribbons, we show that transport is dominated by two modes. One is ballistic and temperature independent; the other is thermally activated. Transport is protected from back-scattering, possibly reflecting ground-state properties of neutral graphene. At room temperature, the resistance of both modes is found to increase abruptly at a particular length—the ballistic mode at 16 micrometres and the other at 160 nanometres. Our epitaxial graphene nanoribbons will be important not only in fundamental science, but also—because they can be readily produced in thousands—in advanced nanoelectronics, which can make use of their room-temperature ballistic transport properties.

https://www.nature.com/articles/nature12952

Link: https://iopscience.iop.org/article/10.1088/1367-2630/aa8ec5


The focusing effect of electron flow and negative refraction in three-dimensional topological insulators

Kai-Tong Wang1, Yanxia Xing2,5, King Tai Cheung3

, Jian Wang3, Hui Pan4 and Hong-Kang Zhao1,5

Published 24 October 2017 • 2017 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft
New Journal of Physics, Volume 19, October 2017
Download Article PDF DownloadArticle ePub

Abstract

We numerically study the focusing effect induced by a single p–n junction in three-dimensional topological insulators (3D TIs). It is found that, for either surface states or bulk states of 3D TIs, the corresponding electrons injected from the n/p region can be perfectly focused at the symmetric position in the p/n region. These results suggest that the focusing effect is a general phenomenon in materials which can be described by massless or massive Dirac equations. We also find that the focusing effect is robust against moderate random disorders. In the presence of external magnetic fields, the focusing effect remains good, but the position of the focus point oscillates periodically due to the finite size effect.

1. Introduction

In the 1960s, Veselago theoretically predicted the existence of negative refractive index materials, i.e., left-handed materials [1, 2]. After about 30 years, the first artificial left-handed material was experimentally verified [3]. In general, electromagnetic negative refraction can only be realized in artificial metamaterials with negative epsilon and μ, [4] where epsilon and μ are the electric permittivity and the magnetic permeability, respectively. When concerning the wave vector and the group velocity, the optical rays and the electron flow (the electron's de Broglie wave) are similar. Hence the negative refraction would be achieved in massless Dirac fermion materials, such as graphene. In such a material, the negative refraction is directly related to the perfect Veselago lens [5] and the Klein paradox [6].

The existence of negative refraction in massless Dirac material is natural. The electrons and holes in massless Dirac material are conjugately linked, and the chiralities (or dispersions) in the conduction band and valence band are opposite. Therefore, the potential barrier induced by a p–n junction (PNJ) in massless Dirac materials is highly transparent for the charge carriers [7]. As a result, the electron flow would be negatively refracted and symmetrically focused by the straight interface of the PNJ in the linear dispersion region [8]. Beyond the linear region, the statement on Dirac fermion fall through, but the focusing effect still exists [9]. It means the negative refraction is not limited to two-dimensional (2D) massless Dirac materials. In fact, as shown in figure 1, when electrons with momentum $({k}_{x},{k}_{\parallel })$ and velocity $({v}_{x},{v}_{\parallel })$ penetrate through the PNJ and become holes with momentum $(-{k}_{x},{k}_{\parallel })$, due to the opposite dispersion for electrons and holes, the velocity of holes becomes $({v}_{x},-{v}_{\parallel })$. Then the negative refraction is formed. As a result, the electron flow is focused by the straight interface induced by the PNJ. Here, '∥' denotes the direction along y for a 2D system or y–z plane for three-dimensional systems. So, there are two essential conditions to the focusing effect of the electron flow. One is the opposite dispersions in the conduction band and valence band, the other is the nearly transparent PNJ. In principle, besides massless Dirac fermions, [10, 11] all gapless semi-metals and topological materials [12, 13] described by the quadratic massive Dirac equation in 2D or beyond 2D, such as the 3D topological insulator (TI), are expected to have the same effect. Considering the helical resolved characteristics of the TI materials, the focusing effect in TIs can have great potential in the applications of helicity-based electronic optics [12].
....
4. Conclusion

In summary, based on the tight-binding Hamiltonian and NEGF technique, the focusing effect of electron flows in 3D TIs with a single PNJ is systematically studied. It is found that, for either surface states or bulk states of 3D TIs, the electron flow injected from the n/p region can be perfectly focused at the symmetric position in the p/n region of the PNJ. These facts suggest that the focusing effect is a general phenomenon in TIs described by massless or massive Dirac equations. However, focusing patterns are destroyed when the Fermi energy of electron flows is at the band edges, where surface and bulk states are mixed. This exception is attributed to the incompatible dispersion relations of the surface and bulk states of TIs. We also found the focusing pattern is robust against moderate random disorders, but the focusing intensity are severely reduced at strong disorders. In the presence of a weak magnetic field, the focusing effect remains well, but the position of the focus point oscillates periodically due to the finite size effect. These numerical findings are beneficial to the application of topological materials.

https://iopscience.iop.org/article/10.1088/1367-2630/aa8ec5/pdf

History and Physics of the Klein Paradox
A Calogeracos, N Dombey
(Submitted on 24 May 1999)

The early papers by Klein, Sauter and Hund which investigate scattering off a high step potential in the context of the Dirac equation are discussed to derive the 'paradox' first obtained by Klein. The explanation of this effect in terms of electron-positron production is reassessed. It is shown that a potential well or barrier in the Dirac equation can become supercritical and emit positrons or electrons spontaneously if the potential is strong enough. If the well or barrier is wide enough, a seemingly constant current is emitted. This phenomenon is transient whereas the tunnelling first calculated by Klein is time-independent. It is shown that tunnelling without exponential suppression occurs when an electron is incident on a high barrier, even when the barrier is not high enough to radiate. Klein tunnelling is therefore a property of relativistic wave equations and is not necessarily connected to particle emission. The Coulomb potential is investigated and it is shown that a heavy nucleus of sufficiently large Z will bind positrons. Correspondingly, as Z increases the Coulomb barrier should become increasingly transparent to positrons. This is an example of Klein tunnelling. Phenomena akin to supercritical positron emission may be studied experimentally in superfluid.

https://arxiv.org/pdf/quant-ph/9905076.pdf
.....
We begin with a summary of the Dirac equation in one dimension in the presence of a potential V(x) and show how Klein’s original result for RS and TS is obtained. We go on to the papers of Sauter in 1931, who replaced Klein’s potential step with a barrier with a finite slope, and then to Hund in 1940 who realised that the Klein potential step gives rise to the production of pairs of charged particles when the potential strength is sufficiently strong. This result although not well known is a precursor of the famous results of modern quantum field theory of Schwinger [6] and Hawking [7] which show that particles are spontaneously produced in the presence of strong electric and gravitational fields. In Part II we turn to the underlying physics of the Klein paradox and show that particle production and Klein tunnelling arise naturally in the Dirac equation: when a potential well is deep enough it becomes super critical (defined as the potential strength for which the bound state energy E=−m) and positrons will be spontaneously produced. Supercriticality is well-understood [8], [9] and can occur in the Coulomb potential with finite nuclear size when thenuclear chargeZ >137. Positron production via this mechanism has been the subject of experimental investigations in heavy ion collisions for many years. We then show that if a potential well is wide enough, a steady but transient current will flow when the potential becomes supercritical. In order to analyse these processes it is necessary to introduce the concept of vacuum charge. We consider the implications of these concepts for the Coulomb potential and for other physical phenomena and we end by pointing out that Klein was unfortunate in that the example he chose to calculate was pathological.

(Miles' solves this with the charge field.)

Mar 16, 2012

Engineered piezoelectric graphene could yield dramatic degree of control in nanotechnology


(Nanowerk News) In what became known as the 'Scotch tape technique," researchers first extracted graphene with a piece of adhesive in 2004. Graphene is a single layer of carbon atoms arranged in a honeycomb, hexagonal pattern. It looks like chicken wire.

Graphene is a wonder material. It is one-hundred-times better at conducting electricity than silicon. It is stronger than diamond. And, at just one atom thick, it is so thin as to be essentially a two-dimensional material. Such promising physics have made graphene the most studied substance of the last decade, particularly in nanotechnology. In 2010, the researchers who first isolated it shared the Nobel Prize.

Yet, while graphene is many things, it is not piezoelectric. Piezoelectricity is the property of some materials to produce electric charge when bent, squeezed or twisted. Perhaps more importantly, piezoelectricity is reversible. When an electric field is applied, piezoelectric materials change shape, yielding a remarkable level of engineering control.

lithium atoms (red) adhered to a graphene lattice that will produce electricity when bent, squeezed or twisted

This illustration shows lithium atoms (red) adhered to a graphene lattice that will produce electricity when bent, squeezed or twisted.

Conversely, the graphene will deform when an electric field is applied, opening new possibilities in nanotechnology.

Illustration: Mitchell Ong, Stanford School of Engineering

Piezoelectrics have found application in countless devices from watches, radios and ultrasound to the push-button starters on propane grills, but these uses all require relatively large, three-dimensional quantities of piezoelectric materials.

Now, in a paper published in the journal ACS Nano ("Engineered Piezoelectricity in Graphene"), two materials engineers at Stanford have described how they have engineered piezoelectrics into graphene, extending for the first time such fine physical control to the nanoscale.

https://dx.doi.org/doi:10.1021/nn204198g
Straintronics

"The physical deformations we can create are directly proportional to the electrical field applied and this represents a fundamentally new way to control electronics at the nanoscale," said Evan Reed, head of the Materials Computation and Theory Group at Stanford and senior author of the study. "This phenomenon brings new dimension to the concept of 'straintronics' for the way the electrical field strains—or deforms—the lattice of carbon, causing it to change shape in predictable ways."
"Piezoelectric graphene could provide an unparalleled degree of electrical, optical or mechanical control for applications ranging from touchscreens to nanoscale transistors," said Mitchell Ong, a post-doctoral scholar in Reed's lab and first author of the paper.

Using a sophisticated modeling application running on high-performance supercomputers, the engineers simulated the deposition of atoms on one side of a graphene lattice — a process known as doping — and measured the piezoelectric effect.
They modeled graphene doped with lithium, hydrogen, potassium and fluorine, as well as combinations of hydrogen and fluorine and lithium and fluorine on either side of the lattice. Doping just one side of the graphene, or doping both sides with different atoms, is key to the process as it breaks graphene's perfect physical symmetry, which otherwise cancels the piezoelectric effect.
The results surprised both engineers.
"We thought the piezoelectric effect would be present, but relatively small. Yet, we were able to achieve piezoelectric levels comparable to traditional three-dimensional materials," said Reed. "It was pretty significant.

More at link: https://www.nanowerk.com/news/newsid=24620.php?utm_source=feedburner&utm_medium=email&utm_campaign=Feed%3A+nanowerk%2FagWB+%28Nanowerk+Nanotechnology+News%29&utm_content=Yahoo%21+Mail

Graphene Repairs Holes By Knitting Itself Back Together, Say Physicists

Enter Konstantin Novoselov at the University of Manchester and a few pals who have spent more than a few hours staring at graphene sheets through an electron microscope to see how it behaves.

Today, these guys say they've discovered why graphene appears so unpredictable. It turns out that if you make a hole in graphene, the material automatically knits itself back together again.

Novoselov and co made their discovery by etching tiny holes into a graphene sheet using an electron beam and watching what happens next using an electron microscope. They also added a few atoms of palladium or nickel, which catalyse the dissociation of carbon bonds and bind to the edges of the holes making them stable.

They found that the size of the holes depended on the number of metal atoms they added--more metal atoms can stabilise bigger holes.

But here's the curious thing. If they also added extra carbon atoms to the mix, these displaced the the metal atoms and reknitted the holes back together again.

Novoselov and co say the structure of the repaired area depends on the form in which the carbon is available. So when available as a hydrocarbon, the repairs tend to contain non-hexagonal defects where foreign atoms have entered the structure.

http://www.technologyreview.com/view/42 ... 2012-07-11


http://arxiv.org/abs/1207.1487

More unexpected properties:

---------------

New study reveals unexpected softness of bilayer graphene

May 1, 2020 , Queen Mary, University of London
graphene

This visualisation shows layers of graphene used for membranes. Credit: University of Manchester
In the study, published in the journal Physical Review B, the researchers showed that bilayer graphene, consisting of two layers of graphene, was noticeably softer than both two-dimensional (2-D) graphene and three-dimensional (3-D) graphite along the stacking direction.

This surprising result differs from previous research which showed that 2-D graphene, a flat single layer of carbon atoms arranged in a honeycomb structure had many of the same mechanical properties as 3-D graphite, which is a naturally occurring form of carbon made up from a very weak stack of many layers of graphene.

Measuring stiffness

Graphene is a 2-D material, but has 3-D properties such as its stiffness in the 'out-of-plane' direction, perpendicular to the plane of the graphene sheets.

The behaviour of π electrons within multilayer graphene determine its out-of-plane stiffness. In this study, the researchers found that when bilayer graphene is compressed out-of-plane, some π electrons are 'squeezed' through the graphene planes, which are impenetrable to small molecules such as water. This response makes the material softer and much easier to compress.

Dr. Yiwei Sun, lead author of the study from Queen Mary University of London, said: "Our previous study showed that 2-D graphene and 3-D graphite have many of the same mechanical properties, so we were surprised to see that bilayer graphene is much softer than both of these materials. We think that the softness of bilayer graphene results from the 'squeezing' of pi-electronic orbitals through the graphene layers. For example, if the bread on a burger is replaced by a bagel it is even easier to compress because the contents can be squeezed out of the bagel hole."

Realising potential

Often hailed as a 'wonder material', graphene has the highest known thermal and electrical conductivity and is stronger than steel, as well as being light, flexible and transparent.

It was discovered in 2004 by peeling off graphene flakes from bulk graphite (used in pencil leads and lubricants) using sticky tape.

Stacking the graphene flakes one on top of the other provides more possibilities as the material's extraordinary properties are determined by interactions between its stacked layers. Its unique characteristics can also be fine-tuned for various applications by stacking other 2-D materials, such as boron nitride and molybdenum disulphide, to graphene.

More at link:  https://phys.org/news/2020-05-news-story-reveals-unexpected-softness.html

.......

30 Mar 2020 in Research & Technology

Graphene displays unexpected permeability
In a two-step process, diatomic hydrogen flips through the monolayer’s intrinsic pores.

More at link: https://physicstoday.scitation.org/do/10.1063/PT.6.1.20200330a/full/

Christine Middleton

The angstrom-scale hexagonal voids in a sheet of graphene resemble the holes in chicken wire. But despite the material’s appearance, studies have shown that defect-free graphene is nearly impermeable. It blocks all liquids and gases; only protons have been found to pass through (see Physics Today, February 2015, page 11). Quantum calculations indicate that any atom or molecule attempting to cross faces an all but insurmountable energy barrier of a few electron volts or more.

Now Pengzhan Sun, Andre Geim (both at the University of Manchester in the UK), and their collaborators have found another exception to graphene’s impermeability: hydrogen gas.

Diagram of graphene membrane setup
Credit: Adapted from P. Z. Sun et al., Nature 579, 229, 2020

The researchers’ experimental setup, depicted in the first image, resembled that used in previous experiments to test graphene’s permeability. Micron-scale wells of trapped air were sealed with graphene membranes and surrounded by the gas being investigated. If any gas molecules got into a well, its membrane would balloon outward because of the increased pressure inside the well, and that deflection could be measured using atomic force microscopy. The new setup’s main advantage was an improved seal between the graphene and the well. Previous experiments with oxidized silicon wells had leaky seals, so Sun, Geim, and coworkers instead used graphite and hexagonal boron nitride wells. The graphene membranes formed leak-proof seals with the atomically flat ring-shaped surfaces surrounding each well.

The researchers tested a few dozen wells with helium—the smallest and potentially leakiest gas—and saw no membrane deflection over 30 days. Neon, nitrogen, oxygen, argon, krypton, and xenon also showed no discernable penetration. The precise measurements set an upper bound on graphene’s permeability, which is around that of a 1-km-thick piece of glass. However, when they tested hydrogen, the researchers were surprised to see a slow but steady upward membrane deflection of a few nanometers.

Diagram of graphene with chemisorbed hydrogen
Credit: Adapted from P. Z. Sun et al., Nature 579, 229, 2020

Although the researchers couldn’t directly observe the hydrogen transport, theoretical predictions for one route agreed with the observed leak rates. Hydrogen molecules first chemisorb on the membrane. That process is facilitated by the graphene’s local curvature; numerous ripples in the sheet make it more reactive

Princeton team detects a cascade of electronic transitions in "magic-angle" twisted bilayer graphene

A team of researchers at Princeton has looked for the origins of the unusual behavior known as magic-angle twisted bilayer graphene, and detected signatures of a cascade of energy transitions that could help explain how superconductivity arises in this material.

"This study shows that the electrons in magic-angle graphene are in a highly correlated state even before the material becomes superconducting, "said Ali Yazdani, Professor of Physics and the leader of the team that made the discovery. "The sudden shift of energies when we add or remove an electron in this experiment provides a direct measurement of the strength of the interaction between the electrons."

This is significant because these energy jumps provide a window into the collective behaviors of electrons, such as superconductivity, that emerge in magic-angle twisted bilayer graphene, a material composed of two layers of graphene in which the top sheet is rotated by a slight angle relative to the other.

In everyday metals, electrons can move freely through the material, but collisions among electrons and from the vibration of atoms give rise to resistance and the loss of some electrical energy as heat - which is why electronic devices get warm during use.

In superconducting materials, electrons cooperate. "The electrons are kind of dancing with each other," said Biao Lian, a postdoctoral research associate in the Princeton Center for Theoretical Science and one of the co-first authors of the study. "They have to collaborate to go into such a remarkable state."

By some measures, magic-angle graphene, discovered two years ago by Pablo Jarillo-Herrero and his team at the Massachusetts Institute of Technology (MIT), is one of the strongest superconductors ever discovered. Superconductivity is relatively robust in this system even though it occurs when there are very few freely moving electrons.



The researchers set out to explore how the unique crystal structure of magic-angle graphene enables collective behaviors. Electrons not only have a negative charge, but also two other characteristics: angular momentum or "spin," and possible movements in the crystal structure known as "valley" states. Combinations of spin and valley make up the various "flavors" of electrons.

The team particularly wanted to know how these flavors affect collective behaviors, so they conducted their experiments at temperatures just slightly above the point at which the electrons become strongly interacting, which the researchers likened to the parent phase of the behaviors.

"We measured the force between the electrons in the material at higher temperatures in the hopes that understanding this force will help us understand the superconductor that it becomes at lower temperatures," said Dillon Wong, a postdoctoral research fellow in the Princeton Center for Complex Materials and a co-first author.

They used a scanning tunneling microscope, in which a conductive metal tip can add or remove an electron from magic-angle graphene and detect the resulting energy state of that electron.

Because strongly interacting electrons resist the addition of a new electron, it costs some energy to add the additional electron. The researchers can measure this energy and from it determine the strength of the interaction force.

"I'm literally putting an electron in and seeing how much energy it costs to shove this electron into the cooperative bath," said Kevin Nuckolls, a graduate student in the Department of Physics, also a co-first author.

The team found that the addition of each electron caused a jump in the amount of energy needed to add another one - which would not have been the case if the electrons were able to go into the crystal and then move freely among the atoms. The resulting cascade of energy transitions resulted from an energy jump for each of the electrons' flavors -- since electrons need to assume the lowest energy state possible while also not being of the same energy and same flavor as other electrons at the same location in the crystal.

More at link: https://www.graphene-info.com/princeton-team-detects-cascade-electronic-transitions-magic-angle-twisted

June 22, 2020
A new symmetry-broken parent state discovered in twisted bilayer graphene

by Weizmann Institute of Science

The symmetry-breaking phase transition in magic-angle graphene. The four ‘flavours’ of Dirac electrons filling their energy levels are represented by four ‘liquids’ filling conical glasses.

Credit: Weizmann Institute of Science

In 2018 it was discovered that two layers of graphene twisted one with respect to the other by a "magic" angle show a variety of interesting quantum phases, including superconductivity, magnetism and insulating behaviors. Now, a team of researchers from the Weizmann Institute of Science led by Prof. Shahal Ilani of the Condensed Matter Physics Department, in collaboration with Prof. Pablo Jarillo-Herrero's group at MIT, have discovered that these quantum phases descend from a previously unknown high-energy "parent state" with an unusual breaking of symmetry.

Graphene is a flat crystal of carbon, just one atom thick. When two sheets of this material are placed on top of each other, misaligned at small angle, a periodic "moiré" pattern appears. This pattern provides an artificial lattice for the electrons in the material. In this twisted bilayer system the electrons come in four "flavors": spins "up" or "down," combined with two "valleys" that originate in the graphene's hexagonal lattice. As a result, each moiré site can hold up to four electrons, one of each flavor.

While researchers already knew that the system behaves as a simple insulator when all the moiré sites are completely full (four electrons per site), Jarillo-Herrero and his colleagues discovered to their surprise, in 2018, that at a specific "magic" angle, the twisted system also becomes insulating at other integer fillings (two or three electrons per moiré site). This behavior, exhibited by magic-angle twisted bilayer graphene (MATBG), cannot be explained by single particle physics, and is often described as a "correlated Mott insulator." Even more surprising was the discovery of exotic superconductivity close to these fillings. These findings led to a flurry of research activity aiming to answer the big question: what is the nature of the new exotic states discovered in MATBG and similar twisted systems?

Imaging magic-angle graphene electrons with a carbon nanotube detector

The Weizmann team set out to understand how interacting electrons behave in MATBG using a unique type of microscope that utilizes a carbon nanotube single-electron transistor, positioned at the edge of a scanning probe cantilever. This instrument can image, in real space, the electric potential produced by electrons in a material with extreme sensitivity.

"Using this tool, we could image for the first time the 'compressibility' of the electrons in this system—that is, how hard it is to squeeze additional electrons into a given point in space," explains Ilani. "Roughly speaking, the compressibility of electrons reflects the phase they are in: In an insulator, electrons are incompressible, whereas in a metal they are highly compressible."

Compressibility also reveals the "effective mass" of electrons. For example, in regular graphene the electrons are extremely "light," and thus behave like independent particles that practically ignore the presence of their fellow electrons. In magic-angle graphene, on the other hand, electrons are believed to be extremely "heavy" and their behavior is thus dominated by interactions with other electrons ‒ a fact that many researchers attribute to the exotic phases found in this material. The Weizmann team therefore expected the compressibility to show a very simple pattern as a function of electron filling: interchanging between a highly-compressible metal with heavy electrons and incompressible Mott insulators that appear at each integer moiré lattice filling.

To their surprise, they observed a vastly different pattern. Instead of a symmetric transition from metal to insulator and back to metal, they observed a sharp, asymmetric jump in the electronic compressibility near the integer fillings.

"This means that the nature of the carriers before and after this transition is markedly different," says study lead author Uri Zondiner. "Before the transition the carriers are extremely heavy, and after it they seem to be extremely light, reminiscent of the 'Dirac electrons' that are present in graphene." (See Mathis on stripping to "photons")

The same behavior was seen to repeat near every integer filling, where heavy carriers abruptly gave way and light Dirac-like electrons re-emerged.

But how can such an abrupt change in the nature of the carriers be understood? To address this question, the team worked together with Weizmann theorists Profs. Erez Berg, Yuval Oreg and Ady Stern, and Dr. Raquel Quiroez; as well as Prof. Felix von-Oppen of Freie Universität Berlin. They constructed a simple model, revealing that electrons fill the energy bands in MATBG in a highly unusual "Sisyphean" manner: when electrons start filling from the "Dirac point" (the point at which the valence and conduction bands just touch each other), they behave normally, being distributed equally among the four possible flavors. "However, when the filling nears that of an integer number of electrons per moiré superlattice site, a dramatic phase transition occurs," explains study lead author Asaf Rozen. "In this transition, one flavor 'grabs' all the carriers from its peers, 'resetting' them back to the charge-neutral Dirac point."

"Left with no electrons, the three remaining flavors need to start refilling again from scratch. They do so until another phase transition occurs, where this time one of the remaining three flavors grabs all the carriers from its peers, pushing them back to square one. Electrons thus need to climb a mountain like Sisyphus, being constantly pushed back to the starting point in which they revert to the behavior of light Dirac electrons," says Rozen. While this system is in a highly symmetric state at low carrier fillings, in which all the electronic flavors are equally populated, with further filling it experiences a cascade of symmetry-breaking phase transitions that repeatedly reduce its symmetry.

A 'parent state'

"What is most surprising is that the phase transitions and Dirac revivals that we discovered appear at temperatures well above the onset of the superconducting and correlated insulating states observed so far," says Ilani. "This indicates that the broken symmetry state we have seen is, in fact, the 'parent state' out of which the more fragile superconducting and correlated insulating ground states emerge."

The peculiar way in which the symmetry is broken has important implications for the nature of the insulating and superconducting states in this twisted system.

"For example, it is well known that stronger superconductivity arises when electrons are heavier. Our experiment, however, demonstrates the exact opposite: superconductivity appears in this magic-angle graphene system after a phase transition has revived the light Dirac electrons. How this happens, and what it tells us about the nature of superconductivity in this system compared to other more conventional forms of superconductivity remain interesting open questions," says Zondiner.

A similar cascade of phase transitions was reported in another paper published in the same Nature issue by Prof. Ali Yazdani and colleagues at Princeton University. "The Princeton team studied MATBG using a completely different experimental technique, based on a highly-sensitive scanning tunneling microscope, so it is very reassuring to see that complementary techniques lead to analogous observations," says Ilani.

The Weizmann and MIT researchers say they will now use their scanning nanotube single-electron-transistor platform to answer these and other basic questions about electrons in various twisted-layer systems: What is the relationship between the compressibility of electrons and their apparent transport properties? What is the nature of the correlated states that form in these systems at low temperatures? And what are the fundamental quasiparticles that make up these states?

More at link:
https://phys.org/news/2020-06-symmetry-broken-parent-state-bilayer-graphene.html

News Release 25-Jun-2020
Science study: Chemists achieve breakthrough in the synthesis of graphene nanoribbons

Martin-Luther-Universität Halle-Wittenberg

Graphene Nanoribbons might soon be much easier to produce. An international research team led by Martin Luther University Halle-Wittenberg (MLU), the University of Tennessee and Oak Ridge National Laboratory in the U.S. has succeeded in producing this versatile material for the first time directly on the surface of semiconductors. Until now, this was only possible on metal surfaces. The new approach also enables scientists to customise the properties of the nanoribbons. Storage technology is one of the potential applications of the material. The research team reports on its results in the upcoming issue of Science.

For years, graphene has been regarded as the material of the future. In simple terms, it is a two-dimensional carbon surface that resembles a honeycomb. This special structure gives the material distinctive properties: for example, it is extremely stable and ultra-light. There is a particular interest in graphene nanoribbons as they are a semiconductor material that could be used, for instance, in the electrical and computer industry. "This is why many research groups around the world are focusing their efforts on graphene nanoribbons," explains chemist Professor Konstantin Amsharov at MLU. These ribbons, which are only nanometres in size, are made up of just a few carbon atoms wide. Their properties are determined by their shape and width. When graphene research was just beginning, the bands were produced by cutting up larger sections. "This process was very complicated and imprecise," says Amsharov.

He and colleagues from Germany, the U.S. and Poland, have now succeeded in simplifying the production of the coveted nanoribbons. The team produces the material by joining together individual atoms, which enables the properties to be customised. The researchers have succeeded for the first time in producing the ribbons on the surface of titanium oxide, a non-metallic material. "Until now, the ribbons were mainly synthesised on gold surfaces. This is not only comparatively expensive, but also impractical," explains Amsharov. The problem with this approach is that gold conducts electricity. This would directly negate the properties of the graphene nanoribbons, which is why this method has only been used in basic research. However, the gold was needed as a catalyst to produce the nanoribbons in the first place. In addition, the nanoribbons had to be transferred from the gold surface to another surface - a very tricky undertaking. The new approach discovered by Amsharov and his colleagues solves this set of problems.

More at link: https://www.eurekalert.org/pub_releases/2020-06/mh-sc062320.php

............


Graphene Nanomesh: New Nanotechnology ‘Brick’ for Modern Micromachines
TOPICS:2D MaterialsGrapheneNanotechnology

By Japan Advanced Institute of Science and Technology May 7, 2020

Researchers at Japan advanced institute of science and technology (JAIST) have successfully fabrication the suspended graphene nanomesh in a large area by the helium ion beam microscopy. 6nm diameter nanopores were pattern on the 1.2 um long and 500 nm wide suspended graphene uniformly. By systematically controlling the pitch (nanopore’s center to nanopore’s center) from 15 nm to 50 nm, a series of stable graphene nanomesh devices were achieved. This provides a practical way to investigate the intrinsic properties of graphene nanomesh towards the application for gas sensing, phonon engineering, and quantum technology.

Graphene, with its excellent electrical, thermal and optical properties, is promising for many applications in the next decade. It is also a potential candidate instead of silicon to build the next generation of electrical circuits. However, without a bandgap, it is not straightforward to use graphene as field-effect transistors (FETs). Researchers tried to cut the graphene sheet into a small piece of graphene nanoribbon and observed the bandgap opening successfully. However, the current of graphene nanoribbons is too low to drive the integrated circuit. In this case, the graphene nanomesh is pointed out by introducing periodical nanopores on the graphene, which is also considered as very small graphene nanoribbon array.

A research team led by Dr Fayong Liu and Professor Hiroshi MIZUTA has demonstrated in collaboration with researchers at the National Institute of Advanced Industrial Science and Technology (AIST) that large area suspended graphene nanomesh is quickly achievable by the helium ion beam microscopy with sub-10 nm nanopore diameter and well-controlled pitches. Comparing to slow speed TEM patterning, the helium ion beam milling technique overcomes the speed limitation, and meanwhile, provides a high imaging resolution. With the initial electrical measurements, it has found that the thermal activation energy of the graphene nanomesh increased exponentially by increasing the porosity of the graphene nanomesh. This immediately provides a new method for bandgap engineering beyond the conventional nanoribbon method. The team plans to continue exploring graphene nanomesh towards the application of phonon engineering.

“Graphene nanomesh is a kind of new ‘brick’ for modern micromachine systems. Theoretically, we can generate many kinds of periodical patterns on the original suspended graphene, which tunes the property of the device to the direction for a special application, in particular nanoscale thermal management” says Prof. Hiroshi Mizuta

More at link: https://scitechdaily.com/graphene-nanomesh-new-nanotechnology-brick-for-modern-micromachines/

Researchers Create ‘Water’ That Can Corrode Diamonds

Mar 15, 2013 by Natali Anderson

Scientists from Singapore and Belgium have successfully altered the properties of water, making it corrosive enough to etch diamonds.

Diamond particles (Jason Nadler / Uic.edu / Sci-News.com)

Diamond particles (Jason Nadler / Uic.edu / Sci-News.com)

The team achieved this by attaching a layer of graphene on diamond and heating to high temperatures – water molecules trapped between them become highly corrosive.

While diamond is known to be a material with superlative physical qualities, little is known about how it interfaces with graphene, an one-atom thick substance composed of pure carbon.

The researchers sought to explore what happens when a layer of graphene, behaving like a soft membrane, is attached on diamond, which is also composed of carbon. To encourage bonding between the two rather dissimilar carbon forms, the researchers heated them to high temperatures.

At elevated temperatures, the team noted a restructuring of the interface and chemical bonding between graphene and diamond. As graphene is an impermeable material, water trapped between the diamond and graphene can’t escape. At a temperature that is above 400 degree Celsius, the trapped water transforms into a distinct supercritical phase.

“We show for the first time that graphene can trap water on diamond, and the system behaves like a ‘pressure cooker’ when heated. Even more surprising, we found that such superheated water can corrode diamond. This has never been reported,” said Prof Loh Kian Ping from the National University of Singapore, senior author of a paper published in the journal Nature Communications.

Due to its transparent nature, the graphene bubble-on-diamond platform provides a novel way of studying the behaviors of liquids at high pressures and high temperature conditions, which is traditionally difficult.

“The applications from our experiment are immense. In the industry, supercritical water can be used for the degradation of organic waste in an environmentally friendly manner. Our work can be also applicable to the laser-assisted etching of semiconductor or dielectric films, where the graphene membrane can be used to trap liquids,” Prof Loh said.

More at link: http://www.sci-news.com/othersciences/nanotechnologies/article00939.html

New graphene super batteries charge up in seconds and last virtually forever

ENVIRONMENT

Environment  Conservation  Energy
07/25/2016 under Art, Energy, Environment, News, Science

VIEW SLIDESHOW

With the aid of one of the strongest materials on Earth, a researcher at Australia’s Swinburne University has created a battery that charges up super fast and can be used over and over and over again, without losing efficiency. Researcher Han Lin developed the battery using a form of carbon called graphene, which is commonly heralded as one of the strongest materials on the planet. The new supercapacitor addresses many of the shortcomings of traditional lithium ion batteries, beating them in charging time, lifespan, and also environmental impact.

Continue reading below

Our Featured Videos

Swinburne University, australia, melbourne, han lin, battery, graphene, supercapacitor, carbon, new battery technology
Researchers around the globe have worked on expanding the capabilities of supercapacitors for many years, but they are typically limited in storage capacity. Han overcame this problem by adding sheets of graphene, which have a large surface area for energy storage due to the material’s honeycomb structure. The material is also strong and flexible at the same time. The researcher used a 3D printer to create the graphene sheets, resulting in a cost-effective energy storage method that could someday replace the batteries in our cell phones and electric cars.

Related: Melbourne’s Advanced Technologies Centre by H2O Architects looks like a gigantic LEGO brick

The new supercapacitor’s ultra-quick charging time—just seconds compared to the minutes or hours needed by a lithium-based battery—is its primary selling point, as it eliminates the inconvenience of long charging times. The graphene-enhanced battery also costs less than a traditional lithium ion battery over the course of its lifetime, due to its unique ability to withstand more recharges without losing strength.

Han presented his new supercapacitor at Fresh Science Victoria 2016 earlier this year.

+ Swinburne University

Via Phys.org

https://inhabitat.com/new-graphene-super-batteries-charge-up-in-seconds-and-last-virtually-forever/

Also:

A New Battery Built by Samsung Uses Graphene to Charge Five Times Faster

https://futurism.com/a-new-battery-built-by-samsung-uses-graphene-to-charge-five-times-faster

Korean Researchers Develop Device That Can Control Speed of Light
By Yoon Young-sil May 16, 2018, 10:21

Korean researchers have developed a device that can control the speed of light. The new device is expected to accelerate the development of next-generation optical communication equipment.

Professor Kim Teun-teun of the Center for Integrated Nanostructure Physics (CINAP) of the Institute for Basic Science (IBS) announced that his team has developed a graphene-based metamaterial device that can control the speed of light, in cooperation with a research team led by Professor Min Beom-ki of KAIST.

Korean researchers have developed a device that can control the speed of light.
Korean researchers have developed a device that can control the speed of light.

Traveling faster than anything in the world, light is the most useful means for data transmission. However, light should be converted into electrical signals to transmit data. In this process, the speed of light is slowed down due to the limitations of electronic signal processors.

By combining graphene with a metamaterial, the researchers have developed a device that can slow down the speed of light and then accelerate it. The new device is expected to help develop next-generation optical communication devices.

A metamaterial is a material engineered to have a property that is not found in nature. Graphene is a single, tightly packed layer of carbon atoms that are bonded together in a hexagonal honeycomb lattice.

http://www.businesskorea.co.kr/news/articleView.html?idxno=22292

......

New graphene-based metasurface capable of independent amplitude and phase control of light

A KAIST research team in collaboration with the University of Wisconsin-Madison theoretically developed a graphene-based active metasurface capable of independent amplitude and phase control of mid-infrared light. This research gives a new insight into modulating the mid-infrared wavefront with high resolution by solving the problem of the independent control of light amplitude and phase, which has remained a long-standing challenge.

Light modulation technology is essential for developing future optical devices such as holography, high-resolution imaging, and optical communication systems. Liquid crystals and a microelectromechanical system (MEMS) have previously been utilized to modulate light. However, both methods suffer from significantly limited driving speeds and unit pixel sizes larger than the diffraction limit, which consequently prevent their integration into photonic systems.

The metasurface platform is considered a strong candidate for the next generation of light modulation technology. Metasurfaces have optical properties that natural materials cannot have, and can overcome the limitations of conventional optical systems, such as forming a high-resolution image beyond the diffraction limit. In particular, the active metasurface is regarded as a technology with a wide range of applications due to its tunable optical characteristics with an electrical signal.

However, the previous active metasurfaces suffered from a correlation between light amplitude control and phase control. This problem is caused by the modulation mechanism of conventional metasurfaces. Conventional metasurfaces have been designed such that a metaatom only has one resonance condition, but a single resonant design inherently lacks the degrees of freedom to independently control the amplitude and phase of light.

The research team made a metaunit by combining two independently controllable metaatoms, dramatically improving the modulation range of active metasurfaces. The proposed metasurface can control the amplitude and phase of the mid-infrared light independently with a resolution beyond the diffraction limit, thus allowing complete control of the optical wavefront.

The research team theoretically confirmed the performance of the proposed active metasurface and the possibility of wavefront shaping using this design method. Furthermore, they developed an analytical method that can approximate the optical properties of metasurfaces without complex electromagnetic simulations. This analytical platform proposes a more intuitive and comprehensively applicable metasurface design guideline.

https://www.graphene-info.com/new-graphene-based-metasurface-capable-independent-amplitude-and-phase-control

NEWS AND VIEWS  05 MARCH 201
Novel electronic states seen in graphene



A simple system made from two sheets of graphene has been converted from an insulator to a superconductor. The finding holds promise for opening up studies of an unconventional form of superconductivity.


In two papers in Nature, Cao et al.1,2 report the discovery of new electronic ground states in twisted bilayer graphene — a pair of single-atom-thick sheets of carbon atoms, stacked with their honeycomb lattices rotated out of alignment. The authors interpret one of these states2 as a correlated Mott insulator, a non-conducting state produced by strong repulsive interactions between electrons. The other1 is a superconductor, a state of zero electrical resistance produced by effective attractive interactions between electrons. The insulator turns into the superconductor when a small number of charge carriers are added to the graphene. This connection between the states is unlikely to be a coincidence — as Sherlock Holmes might have commented, “the universe is rarely so lazy”.


Read the paper: Unconventional superconductivity in magic-angle graphene superlattices
Cao et al. show that the stacking of graphene sheets allows access to a new family of materials with electronic behaviours that are exquisitely sensitive to the atomic alignment between the layers, which affects interlayer electron motion. This finding might surprise physicists, because electronic behaviour is usually dominated by whichever of the associated processes has the largest energy scale. But, in this case, there’s a conundrum: the energy associated with electron motion between atoms within a layer is of the order of electronvolts, whereas the energy for electron motion between layers3 is, at most, hundreds of millielectronvolts.
https://www.nature.com/articles/d41586-018-02660-4

Stanford team finds novel form of magnetism in twisted bi-layer graphene

July 19 2019

(The Hall Effect mentioned again....Cr6)

Stanford physicists recently observed a novel form of magnetism, predicted but never seen before, that is generated when two graphene sheets are carefully stacked and rotated to a special angle. The researchers suggest the magnetism, called orbital ferromagnetism, could prove useful for certain applications, such as quantum computing.

bi-layer graphene between hBN gives off orbital ferromagnetism image

Optical micrograph of the assembled stacked structure, which consists of two graphene sheets sandwiched between two protective layers made of hexagonal boron nitride. (Image: Aaron Sharpe)

“We were not aiming for magnetism. We found what may be the most exciting thing in my career to date through partially targeted and partially accidental exploration,” said study leader David Goldhaber-Gordon, a professor of physics at Stanford’s School of Humanities and Sciences. “Our discovery shows that the most interesting things turn out to be surprises sometimes.”

The Stanford researchers inadvertently made their discovery while trying to reproduce a previous finding - in early 2018, Pablo Jarillo-Herrero’s group at MIT announced that they had coaxed a stack of two subtly misaligned sheets of graphene – twisted bilayer graphene – to conduct electricity without resistance, a property known as superconductivity.

The discovery was a confirmation of a nearly decade-old prediction that graphene sheets rotated to a very particular angle should exhibit interesting phenomena.

When stacked and twisted, graphene forms a superlattice with a repeating interference, or moiré, pattern. “It’s like when you play two musical tones that are slightly different frequencies,” Goldhaber-Gordon said. “You’ll get a beat between the two that’s related to the difference between their frequencies. That’s similar to what you get if you stack two lattices atop each other and twist them so they’re not perfectly aligned.”



Physicists theorized that the particular superlattice formed when graphene rotated to 1.1 degrees causes the normally varied energy states of electrons in the material to collapse, creating what they call a flat band where the speed at which electrons move drops to nearly zero. Thus slowed, the motions of any one electron becomes highly dependent on those of others in its vicinity. These interactions lie at the heart of many exotic quantum states of matter.

“I thought the discovery of superconductivity in this system was amazing. It was more than anyone had a right to expect,” Goldhaber-Gordon said. “But I also felt that there was a lot more to explore and many more questions to answer, so we set out to try to reproduce the work and then see how we could build upon it.”

While attempting to duplicate the MIT team’s results, Goldhaber-Gordon and his group introduced two seemingly unimportant changes.

First, while encapsulating the graphene sheets in thin layers of hexagonal boron nitride, the researchers inadvertently rotated one of the protective layers into near alignment with the twisted bilayer graphene.

“It turns out that if you nearly align the boron nitride lattice with the lattice of the graphene, you dramatically change the electrical properties of the twisted bilayer graphene,” said study co-first author Aaron Sharpe, a graduate student in Goldhaber-Gordon’s lab.

Secondly, the group intentionally overshot the angle of rotation between the two graphene sheets. Instead of 1.1 degrees, they aimed for 1.17 degrees because others had recently shown that twisted graphene sheets tend to settle into smaller angles during the manufacturing process.

“We figured if we aim for 1.17 degrees, then it will go back toward 1.1 degrees, and we’ll be happy,” Goldhaber-Gordon said. “Instead, we got 1.2 degrees.”

The consequences of these small changes didn’t become apparent until the Stanford researchers began testing the properties of their twisted graphene sample. In particular, they wanted to study how its magnetic properties changed as its flat band – that collection of states where electrons slow to nearly zero – was filled or emptied of electrons.

While pumping electrons into a sample that had been cooled close to absolute zero, Sharpe detected a large electrical voltage perpendicular to the flow of the current when the flat band was three-quarters full. Known as a Hall voltage, such a voltage typically only appears in the presence of an external magnetic field – but in this case, the voltage persisted even after the external magnetic field had been switched off.

This anomalous Hall effect could only be explained if the graphene sample was generating its own internal magnetic field. Furthermore, this magnetic field couldn’t be the result of aligning the up or down spin state of electrons, as is typically the case for magnetic materials, but instead must have arisen from their coordinated orbital motions.

“To our knowledge, this is the first known example of orbital ferromagnetism in a material,” Goldhaber-Gordon said. “If the magnetism were due to spin polarization, you wouldn’t expect to see a Hall effect. We not only see a Hall effect, but a huge Hall effect.”

The researchers estimate that the magnetic field near the surface of their twisted graphene sample is about a million times weaker than that of a conventional refrigerator magnet, but this weakness could be a strength in certain scenarios, such as building memory for quantum computers.

“Our magnetic bilayer graphene can be switched on with very low power and can be read electronically very easily,” Goldhaber-Gordon said. “The fact that there’s not a large magnetic field extending outward from the material means you can pack magnetic bits very close together without worrying about interference.”

https://www.graphene-info.com/stanford-team-finds-novel-form-magnetism-twisted-bi-layer-graphene

Twisted bilayer graphene responds strongly to infrared light

02 Sep 2020 Isabelle Dumé

Graphene. Credit: University of Texas at Dallas

The list of surprising behaviours in “twisted bilayer” graphene (TBG) just keeps getting longer. The material – which is made by stacking two sheets of graphene on top of one another, and then rotating one of them so that the sheets are slightly misaligned – was already known to support a wide array of insulating and superconducting states, depending on the strength of an applied electric field. Now researchers in the US have uncovered yet another oddity: when TBG is exposed to infrared light, its ability to conduct electricity changes. According to Fengnian Xia of Yale University, Fan Zhang of the University of Texas at Dallas, and colleagues, this finding could make it possible to develop a new class of infrared detectors using these stacked carbon sheets.

A single layer of graphene consists of a simple repetition of carbon atoms arranged in a two-dimensional hexagonal lattice. In this pristine state, the material does not have an electronic bandgap – that is, it is a gapless semiconductor. However, when two graphene sheets are placed on top of each other and slightly misaligned, they form a moiré pattern, or superlattice. In this new arrangement, the unit cell of the 2D crystal expands to a huge extent, as if it were artificially “stretched” in the two in-plane directions. This stretching dramatically changes the material’s electronic interactions.

From magic angle to twistronics
The misalignment angle in TBG is critically important. For example, at a so-called “magic” misalignment angle of 1.1°, the material switches from an insulator to a superconductor (that is, able to carry electrical current with no resistance below 1.7 K), as a team at the Massachusetts Institute of Technology (MIT) discovered in 2018.

The existence of such strongly correlated effects – which were first theoretically predicted in 2011 by Allan MacDonald and Rafi Bistritzer of the University of Texas at Austin – kick-started the field of “twistronics”. In this fundamentally new approach to device engineering, the weak coupling between different layers of 2D materials, like graphene, can be used to manipulate the electronic properties of these materials in ways that are not possible with more conventional structures, simply by varying the angle between the two layers.

Infrared light affects TBG’s conductance
Xia and Zhang’s teams have now studied how TBG interacts with infrared light – something that has never been investigated before. In their experiments, they shone light in the mid-infrared region of the spectrum, with a wavelength of between 5 and 12 microns, onto samples of TBG and measured how the electrical conductance varied at different twist angles. They found that the conductance reached a peak at 1.81° and that the photoresponse of the material was much stronger compared to untwisted bilayer graphene. This is because the twist significantly enhances the interactions between light and the material and induces a narrow bandgap (as well as superlattice-enhanced density of states). They also found that this strong photoresponse fades at a twist angle of less than 0.5°, as the bandgap closes.

Further investigations by the team revealed that the TBG absorbs the incident energy of the photons from the infrared light. This increases its temperature, which, in turn, produces an enhanced photocurrent.

Ferromagnetism appears in twisted bilayer graphene

The results suggest that the conducting mechanism in TBG is fundamentally connected to the period of the moiré pattern, and the superlattice produced, which is itself connected to the twist angle between the two graphene layers, Zhang explains. The twist angle is thus clearly very important in determining the material’s electronic properties, Xia adds, with smaller twist angles producing a larger moiré periodicity.

Towards a new class of infrared detectors
The researchers, who report their work in Nature Photonics, now hope to find out whether they can combine photoresponsivity and superconductivity in TBG. “Can shining a light induce or somehow modulate superconductivity? That will be very interesting to study,” Zhang says.

More at link: https://physicsworld.com/a/twisted-bilayer-graphene-responds-strongly-to-infrared-light/

Polycyclic Aromatic Hydrocarbons as Potential Building Blocks for Organic Solar Cells

Cyril Aumaitre et al. Chem Rec. 2019 Jun.

Abstract

Since the discovery of graphene in the early 2000's, polycyclic aromatic hydrocarbons (PAHs) have been resurrected and new synthetic tools have been developed to prepare unprecedented structures with unique properties. One application that has been overlooked for this class of molecules is organic solar cells (OSCs). In this account, we present the recent development in the preparation of moderate to low band gap PAHs that could potentially be used as semiconducting materials in OSCs. Our focus is directed toward all-carbon PAHs as well as their polymeric analogs.

Keywords: Anthanthrene; Carbon-rich molecules; Organic semiconductors; Polycyclic aromatic hydrocarbons; Solar cells.

2019 The Chemical Society of Japan & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

https://pubmed.ncbi.nlm.nih.gov/31106986/

.........

Polycyclic Aromatic Hydrocarbons Adsorption onto Graphene: A DFT and AIMD Study

Bing Li et al. Materials (Basel). 2018.
Free PMC article Show details

Abstract

Density functional theory (DFT) calculations and ab-initio molecular dynamics (AIMD) simulations were performed to understand graphene and its interaction with polycyclic aromatic hydrocarbons (PAHs) molecules. The adsorption energy was predicted to increase with the number of aromatic rings in the adsorbates, and linearly correlate with the hydrophobicity of PAHs. Additionally, the analysis of the electronic properties showed that PAHs behave as mild n-dopants and introduce electrons into graphene; but do not remarkably modify the band gap of graphene, indicating that the interaction between PAHs and graphene is physisorption. We have also discovered highly sensitive strain dependence on the adsorption strength of PAHs onto graphene surface. The AIMD simulation indicated that a sensitive and fast adsorption process of PAHs can be achieved by choosing graphene as the adsorbent. These findings are anticipated to shed light on the future development of graphene-based materials with potential applications in the capture and removal of persistent aromatic pollutants.

https://pubmed.ncbi.nlm.nih.gov/29751556/

.........

Adsorption of Polycyclic Aromatic Hydrocarbons by Graphene and Graphene Oxide Nanosheets

Jun Wang†‡, Zaiming Chen†‡, and Baoliang Chen†‡*
View Author Information
Cite this: Environ. Sci. Technol. 2014, 48, 9, 4817–4825
Publication Date:March 31, 2014
https://doi.org/10.1021/es405227u
Copyright 2014 American Chemical Society

Abstract

Abstract Image

The adsorption of naphthalene, phenanthrene, and pyrene onto graphene (GNS) and graphene oxide (GO) nanosheets was investigated to probe the potential adsorptive sites and molecular mechanisms. The microstructure and morphology of GNS and GO were characterized by elemental analysis, XPS, FTIR, Raman, SEM, and TEM. Graphene displayed high affinity to the polycyclic aromatic hydrocarbons (PAHs), whereas GO adsorption was significantly reduced after oxygen-containing groups were attached to GNS surfaces. An unexpected peak was found in the curve of adsorption coefficients (Kd) with the PAH equilibrium concentrations. The hydrophobic properties and molecular sizes of the PAHs affected the adsorption of G and GO. The high affinities of the PAHs to GNS are dominated by π–π interactions to the flat surface and the sieving effect of the powerful groove regions formed by wrinkles on GNS surfaces. In contrast, the adsorptive sites of GO changed to the carboxyl groups attaching to the edges of GO because the groove regions disappeared and the polar nanosheet surfaces limited the π–π interactions. The TEM and SEM images initially revealed that after loading with PAH, the conformation and aggregation of GNS and GO nanosheets dramatically changed, which explained the observations that the potential adsorption sites of GNS and GO were unusually altered during the adsorption process.

https://pubs.acs.org/doi/suppl/10.1021/es405227u/suppl_file/es405227u_si_001.pdf

International Journal of Hydrogen Energy
Volume 44, Issue 57, 15 November 2019, Pages 30486-30498
Supercritical water gasification of naphthalene over iron oxide catalyst: A ReaxFF molecular dynamics
study

Author links open overlay panelYouHanabJinliZhangac
https://doi.org/10.1016/j.ijhydene.2019.09.215
Get rights and content

Highlights
•
SCW served not only as H source but also as O source.

•
Iron oxide provided lattice oxygen, catalyze SCW and C–C breaking.

•
Carbon deposition, lattice oxygen and iron loss caused catalyst deactivation.

•
Catalyst regeneration mechanism in O2 environment was also revealed.


Abstract

ReaxFF molecular dynamics simulation has been employed to investigate the iron oxide-catalyzed supercritical water gasification (SCWG) of naphthalene (NAP), a common component of refractory polycyclic aromatic hydrocarbons. Simulation results showed that synergistic effects between SCW and iron oxide catalyst enormously promoted the degradation of NAP and the production of H2 and CO. During the gasification process, SCW served not only as H source for H2 generation but also as O source for CO generation and lattice oxygen recompense, while the major roles of iron oxide catalyst were to provide lattice oxygen with high hydrogen-abstraction ability, catalyze SCW to produce more active species, and weaken the C–C bonds. The effects of different parameters were subsequently revealed: increasing the use of H2O molecules raised H2 and CO yields along with the lattice oxygen supplement but slowed the rate of CO generation, high hydrogen recovery was achieved at high NAP concentration accompanied by a low carbon gasification efficiency. Our simulated results further demonstrated that the deactivation of iron oxide catalyst was caused by carbon deposition, lattice oxygen exhaustion and iron loss. SCW media effectively inhibited the iron loss, while calcination in O2 environment could successfully regenerate the iron oxide catalyst by cleaning up the carbon deposition and replenishing the lattice oxygen.

https://www.sciencedirect.com/science/article/abs/pii/S0360319919336638

Facile synthesis of iron oxides/reduced graphene oxide composites: application for electromagnetic wave absorption at high temperature

Lili Zhang, Xinxin Yu, […]Changle Chen
Scientific Reports volume 5, Article number: 9298 (2015) Cite this article

1694 Accesses

61 Citations

1 Altmetric

Metricsdetails

Abstract

Iron oxides/reduced graphene oxide composites were synthesized by facile thermochemical reactions of graphite oxide and FeSO4·7H2O. By adjusting reaction temperature, α-Fe2O3/reduced graphene oxide and Fe3O4/reduced graphene oxide composites can be obtained conveniently. Graphene oxide and reduced graphene oxide sheets were demonstrated to regulate the phase transition from α-Fe2O3 to Fe3O4 via γ-Fe2O3, which was reported for the first time. The hydroxyl groups attached on the graphene oxide sheets and H2 gas generated during the annealing of graphene oxide are believed to play an important role during these phase transformations. These samples showed good electromagnetic wave absorption performance due to their electromagnetic complementary effect. These samples possess much better electromagnetic wave absorption properties than the mixture of separately prepared Fe3O4 with rGO, suggesting the crucial role of synthetic method in determining the product properties. Also, these samples perform much better than commercial absorbers. Most importantly, the great stability of these composites is highly advantageous for applications as electromagnetic wave absorption materials at high temperatures.

Introduction

Nowadays, severe electromagnetic (EM) radiation is being generated everywhere due to the increasing use of wireless communication tools, local area network, personal digital devices and so on. EM radiation has become a serious pollution issue, not only influencing the operation of electronic devices, but also affecting human health and raising problems concerning their military applications1,2. In this regard, high performance EM wave absorption materials have attracted more and more attention as an effective strategy to solve these problems. The desired properties for ideal EM wave absorption materials include strong absorption capability, wide absorption range, lightweight, good thermal and oxidation stability, etc. Most conventional EM wave absorption materials are magnetic or metallic particles with electromagnetic parameters not functioning well in the GHz range. Combining with the high density and phase instability, their practical applications have been greatly limited. In contrast, carbon-based materials (carbon black, graphite flakes, carbon fiber, carbon nanotubes, reduced graphene oxide, etc) could potentially solve these issues due to their unique properties such as low density, high complex permittivity and superior thermal stability. Unfortunately, their EM wave absorption property mainly originates from dielectric loss because of their non-magnetic feature. The preparation of carbon-based composite materials with magnetic particles could efficiently solve this problem via controllable modifications of their dielectric and magnetic properties3,4.

Recently, rGO was reported to demonstrate enhanced EM wave absorption, comparing with graphite, carbon nanotubes and high quality graphene5. This was attributed to their defects and functional groups. However, the value of EM wave absorption is only −7 dB5, −3 dB away from the minimum requirement for practical applications (−10 dB). Various iron oxides/rGO composites have been explored to address this issue. Recently, Zhang et al. reported a maximum absorption of −33.5 dB from a rGO/α-Fe2O3 composite hydrogel prepared via a two-step process3. He et al. reported a facile solvothermal route to prepare laminated rGO/Fe3O4 composites, with reflection loss (RL) below −10 dB at 2 GHz and a maximum absorption of −26.4 dB6. Yin et al. fabricated rGO/γ-Fe2O3 composite with RL of −59.65 dB at 10.09 GHz7. Among these fabrication methods including hydrothermal, solvothermal, sol-gel process and chemical route3,6,7,8,9, most of them suffer from complicated and time-consuming procedures, which greatly limit their potential large scale application. The development of a facile, cost-effective and scalable method to synthesize iron oxides/rGO composite with high EM wave absorbing performance is highly desired.

A huge disadvantage of the conventional magnetic absorbing materials is the loss magnetic properties and consequently EM wave absorbing properties under high temperatures10. In fact, the temperature increment due to the conversion of electromagnetic energy into heat may cause serious damage to magnetic absorbing materials and related11. This is especially true for military stealth materials for radar cross section (RCS) reduction. The heat generated on the surface of hyper-velocity missiles, bombers, rockets, aircrafts and spacecrafts due to friction can result in high temperatures (600–800°C), leading to composition changes and even destruction the EM wave absorbing materials.

Herein, we report a simple, efficient and scalable procedure for the synthesis of iron oxides/rGO composite from thermochemical oxidation of FeSO4·7H2O and reduction of graphite oxide. Interestingly, the initially formed α-Fe2O3/rGO composite was converted to Fe3O4/rGO composite via γ-Fe2O3/rGO intermediate when the temperature was increased from 500°C to 800°C. In contrast, treating FeSO4·7H2O powders without the aid of rGO under the same conditions generated α-Fe2O3 at every temperature point. This is the first example of phase transition between α-Fe2O3, γ-Fe2O3 and Fe3O4 regulated by rGO. All three composites showed great EM wave absorption abilities. More importantly, the thermal stability of the Fe3O4/rGO composite at up to 800°C may open up a whole new field for high temperature application of carbon-based composite materials as EM wave absorption materials.

https://www.nature.com/articles/srep09298.pdf

.
Physicists build circuit that generates clean, limitless power from graphene
https://phys.org/news/2020-10-physicists-circuit-limitless-power-graphene.html
OCTOBER 2, 2020
by University of Arkansas

A team of University of Arkansas physicists has successfully developed a circuit capable of capturing graphene's thermal motion and converting it into an electrical current.

"An energy-harvesting circuit based on graphene could be incorporated into a chip to provide clean, limitless, low-voltage power for small devices or sensors," said Paul Thibado, professor of physics and lead researcher in the discovery.

The findings, published in the journal Physical Review E, are proof of a theory the physicists developed at the U of A three years ago that freestanding graphene—a single layer of carbon atoms—ripples and buckles in a way that holds promise for energy harvesting.

Airman. Hey Cr6, Looks like progress, couldn’t wait to share. At room temperature, a single layer of Graphene has been observed to produce ‘an alternating current’, associated with the graphene’s slight upward and downward buckling motions which they attribute to the presence of positive and negative charge. Attaching an integrated circuit with battery, diode, capacitor, switches and a load, shows the circuit can run continuously, powered only by the graphene’s slow Brownian motion.

Of course we know that Brownian motion at some room temperature is caused by the charge field.    
.

Very cool LTAM! Thanks for sharing this. Interesting that the discovery didn't follow expectations or orthodoxy when the effects were noticed. Good to see this.

This too. Graphene batteries might provide a bright future for C.F. powered devices.
https://www.youtube.com/watch?v=dnE1nO6o-do

Stanene is also something to use.

New wonder material 'stanene' could replace graphene with 100% electrical conductivity

Even copper's high electrical conductivity is beginning to hold back computers as scientists push the material to its limits

James Vincent
@jjvincent
Wednesday 27 November 2013 17:21
A new material made from a single layer of tin atoms could make history by becoming the world’s first electrical conductor to work at 100 per cent efficiency. This would make it even more conductive than 'wonder material' graphene.

The new material has not yet been fabricated but has been christened “stanene”, a combination of the Latin word for tin (stannum) and the suffix found in the word graphene.

Click here to find out more about the possibilities of graphene

Stanene was discovered by researchers from the US Department of Energy’s (DOE) SLAC National Accelerator Laboratory and Stanford University and could revolutionize computing by replacing the copper wires still used in modern computer chips.

"Stanene could increase the speed and lower the power needs of future generations of computer chips, if our prediction is confirmed by experiments that are underway in several laboratories around the world," Shoucheng Zhang told Phys.org, a physics professor at Stanford and a team leader on the project.

How could stanene help make faster computers?
Up until now we've relied on copper to relay electricity in various forms, and for good reason. As well as being cheap and ductile (this means it can be easily drawn into strips) copper is also very conductive.

However, modern computer chips deploy the metal on a scale that would be unimaginable to past generations. Technology site Extremetech has noted that in a modern chip the size of your thumbnail there can be up to sixty miles of copper wiring, with some of the strands just atoms thick.

At this point scientists are pushing the limits of the material, channelling so much electricity through it that the material's electrical resistance causes the wires to heats, potentially setting it on fire. If stanene fulfils on scientists’ promises then chips could get smaller and faster without running this risk of overheating.

As we continue to scale down computer chips we begin to strain the limits of what copper can handle.
As we continue to scale down computer chips we begin to strain the limits of what copper can handle.
How does stanene work?
Stanene is what is known as a ‘topological insulator’, meaning its interior is an insulator but it conducts electrons along its surface. By making the material only a single atom thick, the stanene is essentially all surface, allowing it to conduct electricity with 100 per cent efficiency.

"The magic of topological insulators is that by their very nature, they force electrons to move in defined lanes without any speed limit, like the German autobahn," said Zhang. "As long as they're on the freeway – the edges or surfaces – the electrons will travel without resistance."

By adding fluorine atoms to the mix, the scientist claim they can retain this level of efficiency at temperatures of up to 100 degrees Celsius, allowing the material to be used in computers, where processors typically run at temperatures of between 40 and 90 degrees Celsius.

However, there are many obstacles standing between stanene and mainstream use (not limited to the difficulties of manufacturing one-atom thick wires on an industrial scale) and without working samples of the material available it is perhaps a little early to get excited.

https://www.independent.co.uk/news/science/new-wonder-material-stanene-could-replace-graphene-100-electrical-conductivity-8967573.html

Graphene used to make graphene-copper composite that’s 500 times stronger

By Sebastian Anthony on August 27, 2013 at 8:59 am

This site may earn affiliate commissions from the links on this page.

Graphene, as metal hexagon

Researchers at the Korean Advanced Institute of Science and Technology (KAIST) have created composite materials using graphene that are up to 500 times stronger than the raw, non-composite material. This is the first time that graphene has been successfully used to create strong composite materials — and due to the tiny amounts of graphene used (just 0.00004% by weight) this breakthrough could lead to much faster commercial adoption than pure graphene, which is still incredibly hard to produce in large quantities.

The reason these composites are so strong is that the graphene stops the metal atoms from slipping and dislocating under stress. In a solid metal, if a slip plane forms (due to stress), the atoms will readily slip apart, causing a fracture. The layers of graphene stop the metal atoms from sliding — the metal atoms cannot physically pass through the super-strong graphene — so no fractures can form (pictured above). It’s essentially the metallic equivalent of steel-reinforced concrete. In case you were wondering, this is also one of the primary reasons why metals are nearly always used in alloy form — because there’s a mix of different metal atoms with different atom sizes, it’s much harder for slip planes to form.

More at link: https://www.extremetech.com/extreme/164961-graphene-used-to-make-graphene-copper-composite-thats-500-times-stronger

India's Log 9 Materials develops graphene-based metal-air battery

India-based Log 9 Materials is working on graphene-based metal-air batteries, that in theory may even lead to electric vehicles that run on water.

India's Log 9 Materials develops graphene-based metal-air battery image

LOG 9 BATTERY COMPOSITION (LOG 9 MATERIALS)

The metal air batteries use a metal as anode, air (oxygen) as cathode and water as an electrolyte. A graphene rod is used in the air cathode of the batteries. Since Oxygen has to be used as the cathode, the cathode material has to be porous to let the air pass, a property in which graphene excels. According to Akshay Singhal, co-founder of Log 9 Materials, the graphene used in the electrode is able to increase the battery efficiency by five times at one-third the cost.

The Company explains that the most immediate advantage of such a battery would be the eliminated need to recharge. Instead of the lithium-ion batteries that power most of the EVs (and gadgets), the metal-air batteries by Log 9 do not require any charging and instead, run on a refueling mechanism that only required water as the fuel. So every hundred kilometers.

On a global level, this would again reduce the dependence upon the limited elements such as Cobalt which are used majorly in the making of lithium ion batteries. It would also eliminate the need for huge investments in charging stations and other related infrastructure. This also means less electricity requirement, which is again mostly generated through fossil fuels at the moment.

However, the idea of using metal-air batteries for electric vehicles and other applications is not new or unique to Log 9. Several companies have been working towards it and many others have shown interest in the concept, including Tesla, that currently rules the electric vehicle market share.



There are, however, several challenges that restrict the adoption of such batteries in vehicles. A major one of them is the irreversible loss in battery performance through corrosion. As per Kartik Hajela (Co-Founder), Log 9 is relying upon the material competency to prevent corrosion in the prone parts. The firm also has an IP generated around the process.

Hajela also mentioned how the team faced difficulties in driving power out of the batteries. Despite this, however, the team has managed to increase the power per cell by 4 to 5 times.

Kartik mentions that the metal-air batteries and their product development cycle are currently being optimised to make them feasible for the market. The POC has been done for the same. The company aims to launch the product by 2020 for stationary purposes and begin the commercial prototyping around the same time for mobility applications. He says the cost of the said batteries would be “on par or cheaper” with the current batteries. This is a prediction though, based on the raw materials and process of making it. The cost will be cut down (if so) through the use of graphene.

To date, Log 9 Materials has already come up with a range of graphene-based products like Log 9 Oil sorbent, a filtration media and a Post-Purification Filter (PPuF) for smokers. Through these innovations, it has managed to garner seed investment from GEMS Micro-VC Fund and a pre-series A investment by various firms.

More at link: https://www.graphene-info.com/indias-log-9-materials-develops-graphene-based-metal-air-battery

Recent paper from Princeton on TBG. Still finding the unexpected:

https://www.princeton.edu/news/2020/12/14/magic-angle-graphene-and-creation-unexpected-topological-quantum-states

More at link:

‘Magic’ angle graphene and the creation of unexpected topological quantum states
Tom Garlinghouse for the Department of Physics
Dec. 14, 2020 2:04 p.m.


Electrons inhabit a strange and topsy-turvy world. These infinitesimally small particles have never ceased to amaze and mystify despite the more than a century that scientists have studied them. Now, in an even more amazing twist, physicists have discovered that, under certain conditions, interacting electrons can create what are called “topological quantum states.” This finding, which was recently published in the journal Nature, holds great potential for revolutionizing electrical engineering, materials science and especially computer science.

Topological states of matter are particularly intriguing classes of quantum phenomena. Their study combines quantum physics with topology, which is the branch of theoretical mathematics that studies geometric properties that can be deformed but not intrinsically changed. Topological quantum states first came to the public’s attention in 2016 when three scientists — Princeton’s Duncan Haldane, who is Princeton’s Thomas D. Jones Professor of Mathematical Physics and Sherman Fairchild University Professor of Physics, together with David Thouless and Michael Kosterlitz — were awarded the Nobel Prize for their work in uncovering the role of topology in electronic materials. 



A Princeton-led team of physicists have discovered that, under certain conditions, interacting electrons can create what are called “topological quantum states,” which, has implications for many technological fields of study, especially information technology. To get the desired quantum effect, the researchers placed two sheets of graphene on top of each other with the top layer twisted at the “magic” angle of 1.1 degrees, which creates a moiré pattern. This diagram shows a scanning tunneling microscope imaging the magic-angle twisted bilayer graphene.

“The last decade has seen quite a lot of excitement about new topological quantum states of electrons,” said Ali Yazdani, the Class of 1909 Professor of Physics at Princeton and the senior author of the study. “Most of what we have uncovered in the last decade has been focused on how electrons get these topological properties, without thinking about them interacting with one another.”

But by using a material known as magic-angle twisted bilayer graphene, Yazdani and his team were able to explore how interacting electrons can give rise to surprising phases of matter.

The remarkable properties of graphene were discovered two years ago when Pablo Jarillo-Herrero and his team at the Massachusetts Institute of Technology (MIT) used it to induce superconductivity — a state in which electrons flow freely without any resistance. The discovery was immediately recognized as a new material platform for exploring unusual quantum phenomena.

Yazdani and his fellow researchers were intrigued by this discovery and set out to further explore the intricacies of superconductivity.
But what they discovered led them down a different and untrodden path.

“This was a wonderful detour that came out of nowhere,” said Kevin Nuckolls, the lead author of the paper and a graduate student in physics. “It was totally unexpected, and something we noticed that was going to be important.”

Following the example of Jarillo-Herrero and his team, Yazdani, Nuckolls and the other researchers focused their investigation on twisted bilayer graphene.
“It’s really a miracle material,” Nuckolls said. “It’s a two-dimensional lattice of carbon atoms that’s a great electrical conductor and is one of the strongest crystals known.”
Graphene is produced in a deceptively simple but painstaking manner: a bulk crystal of graphite, the same pure graphite in pencils, is exfoliated using sticky tape to remove the top layers until finally reaching a single-atom-thin layer of carbon, with atoms arranged in a flat honeycomb lattice pattern.
To get the desired quantum effect, the Princeton researchers, following the work of Jarillo-Herrero, placed two sheets of graphene on top of each other with the top layer angled slightly. This twisting creates a moiré pattern, which resembles and is named after a common French textile design. The important point, however, is the angle at which the top layer of graphene is positioned: precisely 1.1 degrees, the “magic” angle that produces the quantum effect.
“It’s such a weird glitch in nature,” Nuckolls said, “that it is exactly this one angle that needs to be achieved.” Angling the top layer of graphene at 1.2 degrees, for example, produces no effect.

The researchers generated extremely low temperatures and created a slight magnetic field. They then used a machine called a scanning tunneling microscope, which relies on a technique called “quantum tunneling” rather than light to view the atomic and subatomic world. They directed the microscope’s conductive metal tip on the surface of the magic-angle twisted graphene and were able to detect the energy levels of the electrons.

They found that the magic-angle graphene changed how electrons moved on the graphene sheet. “It creates a condition which forces the electrons to be at the same energy,” said Yazdani. “We call this a ‘flat band.’”

When electrons have the same energy — are in a flat band material — they interact with each other very strongly. “This interplay can make electrons do many exotic things,” Yazdani said.

One of these “exotic” things, the researchers discovered, was the creation of unexpected and spontaneous topological states.

“This twisting of the graphene creates the right conditions to create a very strong interaction between electrons,” Yazdani explained. “And this interaction unexpectedly favors electrons to organize themselves into a series of topological quantum states.”

The researchers discovered that the interaction between electrons creates topological insulators: unique devices that whose interiors do not conduct electricity but whose edges allow the continuous and unimpeded movement of electrons. This diagram depicts the different insulating states of the magic-angle graphene, each characterized by an integer called its “Chern number,” which distinguishes between different topological phases.

Specifically, they discovered that the interaction between electrons creates what are called topological insulators. These are unique devices that act as insulators in their interiors, which means that the electrons inside are not free to move around and therefore do not conduct electricity. However, the electrons on the edges are free to move around, meaning they are conductive. Moreover, because of the special properties of topology, the electrons flowing along the edges are not hampered by any defects or deformations. They flow continuously and effectively circumvent the constraints — such as minute imperfections in a material’s surface — that typically impede the movement of electrons.

.
Pardon me Cr6, I’ve rarely if ever redirect posts here after several years, yet this must be my third repost/redirect today. Coincidence?

The following links to our discussion and possible charge field model of Twisted Bilayer (TBG) Graphene from the Flying Saucers? thread back to this main graphene thread, Mathis on Graphene? Any hints?

https://milesmathis.forumotion.com/t453p100-flying-saucers#6410

1 degree TBG, the rotation center is near the top left.

Back to Mathis on Graphene? Any hints?

Cr6 wrote. Recent paper from Princeton on TBG. Still finding the unexpected:

https://www.princeton.edu/news/2020/12/14/magic-angle-graphene-and-creation-unexpected-topological-quantum-states

More at link:

‘Magic’ angle graphene and the creation of unexpected topological quantum states
Tom Garlinghouse for the Department of Physics
Dec. 14, 2020 2:04 p.m.

Electrons inhabit a strange and topsy-turvy world. These infinitesimally small particles have never ceased to amaze and mystify despite the more than a century that scientists have studied them. Now, in an even more amazing twist, physicists have discovered that, under certain conditions, interacting electrons can create what are called “topological quantum states.” This finding …

Airman. Do I detect a note of sarcasm Cr6? Having gone through our recent TBG discussion, I found this Department of Physics paper surprisingly easy to read. They found superconductivity at a 1.1 degree TBG twist. The main point of the paper however seems to be introducing the reader to the exciting new world of “infinitesimally small” “topsy-turvy" "interacting electrons” “creating new phases of matter” and “exotic topological quantum states”.

Exploding quantum theory indeed.
.

Future batteries, coming soon: Charge in seconds, last months and power over the air

Chris Hall, Editor · 7 October 2020 ·

Vertically aligned carbon nanotube electrode
NAWA Technologies has designed and patented an Ultra Fast Carbon Electrode, which is says is a game-changer in the battery market. It uses a vertically-aligned carbon nanotube (VACNT) design and NAWA says it can boost battery power ten fold, increase energy storage by a factor of three and increase the lifecycle of a battery five times. The company sees electric vehicles as being the primary beneficiary, reducing the carbon footprint and cost of battery production, while boosting performance. NAWA says that 1000km range could become the norm, with charging times cut to 5 minutes to get to 80 per cent. The technology could be in production as soon as 2023.

A cobalt-free lithium-ion battery
Researchers at the University of Texas have developed a lithium-ion battery that doesn't use cobalt for its cathode. Instead it switched to a high percentage of nickel (89 per cent) using manganese and aluminium for the other ingredients. "Cobalt is the least abundant and most expensive component in battery cathodes," said Professor Arumugam Manthiram, Walker Department of Mechanical Engineering and director of the Texas Materials Institute. "And we are completely eliminating it." The team says they have overcome common problems with this solution, ensuring good battery life and an even distribution of ions.

SVOLT unveils cobalt free batteries for EVs
While the emission-reducing properties of electric vehicles are widely accepted, there's still controversy around the batteries, particularly the use of metals like cobalt. SVOLT, based in Changzhou, China, has announced that it has manufactured cobalt-free batteries designed for the EV market. Aside from reducing the rare earth metals, the company is claiming that they have a higher energy density, which could result in ranges of up to 800km (500 miles) for electric cars, while also lengthening the life of the battery and increasing the safety. Exactly where we'll see these batteries we don't know, but the company has confirmed that it's working with a large European manufacturer.

More at link: https://www.pocket-lint.com/gadgets/news/130380-future-batteries-coming-soon-charge-in-seconds-last-months-and-power-over-the-air

Hi Airman....Here's a new paper from a few months ago on Twistronics.

.....

Tunable strongly coupled superconductivity in magic-angle twisted trilayer graphene

Jeong Min Park, Yuan Cao, […]Pablo Jarillo-Herrero
Nature volume 590, pages249–255 (2021)Cite this article

16k Accesses

8 Citations

Abstract

Moiré superlattices1,2 have recently emerged as a platform upon which correlated physics and superconductivity can be studied with unprecedented tunability3,4,5,6. Although correlated effects have been observed in several other moiré systems7,8,9,10,11,12,13,14,15,16,17, magic-angle twisted bilayer graphene remains the only one in which robust superconductivity has been reproducibly measured4,5,6. Here we realize a moiré superconductor in magic-angle twisted trilayer graphene (MATTG)18, which has better tunability of its electronic structure and superconducting properties than magic-angle twisted bilayer graphene. Measurements of the Hall effect and quantum oscillations as a function of density and electric field enable us to determine the tunable phase boundaries of the system in the normal metallic state. Zero-magnetic-field resistivity measurements reveal that the existence of superconductivity is intimately connected to the broken-symmetry phase that emerges from two carriers per moiré unit cell. We find that the superconducting phase is suppressed and bounded at the Van Hove singularities that partially surround the broken-symmetry phase, which is difficult to reconcile with weak-coupling Bardeen–Cooper–Schrieffer theory. Moreover, the extensive in situ tunability of our system allows us to reach the ultrastrong-coupling regime, characterized by a Ginzburg–Landau coherence length that reaches the average inter-particle distance, and very large TBKT/TF values, in excess of 0.1 (where TBKT and TF are the Berezinskii–Kosterlitz–Thouless transition and Fermi temperatures, respectively). These observations suggest that MATTG can be electrically tuned close to the crossover to a two-dimensional Bose–Einstein condensate. Our results establish a family of tunable moiré superconductors that have the potential to revolutionize our fundamental understanding of and the applications for strongly coupled superconductivity.

https://www.nature.com /articles/s41586-021-03192-0

.

Cr6 wrote. Hi Airman....Here's a new paper from a few months ago on Twistronics.

Hi Cr6, thanks for the suggestion, I haven't considered graphene in months. By the way, the graphene of a whole nuther
dimension - tubular, vertically-aligned carbon nanotube design boosting battery performance you posted here last time
looks interesting.  

Unfortunately,    - not, the paper, Tunable strongly coupled superconductivity in magic-angle twisted trilayer
graphene is behind the paywall, costing $8.99 for that single paper or $199 for journal access for one year. As far as
I can tell, there's no good reason to pay for any graphene quantum superconducting explanation. They appear to be based
on electron interactions(??). Of course we know that's wrong, the electrons are just 'along for the ride', the driving force is
the charge field - which the mainstream ignores.   

I see Twistronics attempts to duplicate the same superconductivity results using the same graphene magic-angle approach,
with other elements, including doping bilayer graphene. I saw somewhere that trilayer graphene has displayed superconductivity
without any magic twist. Anyway, Nature.com has many free, open access papers covering some of these developments.
For example, here's a News Feature greatly 'clarifying' the whole graphene magic angle superconductivity breakthrough
from 2 Jan 2019, How 'magic angle' graphene is stirring up physics, https://www.nature.com/articles/d41586-018-07848-2

I doubt I could charge field interpret most of the mainstream 'facts', but they deserve some discussion. Unfortunately,    
- for real, there doesn't appear to be much interest.
.

Rayleigh Scattering mentioned.
http://milesmathis.com/bright3.pdf


Flexural phonons in supported graphene: from pinning to localization

Wei L Z Zhao et al. Sci Rep. 2018.

Abstract

We identify graphene layer on a disordered substrate as a system where localization of phonons can be observed. Generally, observation of localization for scattering waves is not simple, because the Rayleigh scattering is inversely proportional to a high power of wavelength. The situation is radically different for the out of plane vibrations, so-called flexural phonons, scattered by pinning centers induced by a substrate. In this case, the scattering time for vanishing wave vector tends to a finite limit. One may, therefore, expect that physics of the flexural phonons exhibits features characteristic for electron localization in two dimensions, albeit without complications caused by the electron-electron interactions. We confirm this idea by calculating statistical properties of the Anderson localization of flexural phonons for a model of elastic sheet in the presence of the pinning centers. Finally, we discuss possible manifestations of the flexural phonons, including the localized ones, in the electronic thermal conductance.

Conflict of interest statement
The authors declare no competing interests.

https://pubmed.ncbi.nlm.nih.gov/30389980/#&gid=article-figures&pid=figure-1-uid-0

Keep in mind Miles' paper: http://milesmathis.com/hall.pdf


https://news.utdallas.edu/science-technology/graphene-quantum-hall-effect-2021/


Physicists Discover Novel Quantum Effect in Bilayer Graphene
By Amanda Siegfried | Nov. 2, 2021

UT Dallas physicists are studying the exotic behavior of bilayer graphene, which is a naturally occurring, two-atom thin layer of carbon atoms arranged in two honeycomb lattices stacked together.

Theorists at The University of Texas at Dallas, along with colleagues in Germany, have for the first time observed a rare phenomenon called the quantum anomalous Hall effect in a very simple material. Previous experiments have detected it only in complex or delicate materials.

Dr. Fan Zhang, associate professor of physics in the School of Natural Sciences and Mathematics, is an author of a study published on Oct. 6 in the journal Nature that demonstrates the exotic behavior in bilayer graphene, which is a naturally occurring, two-atom thin layer of carbon atoms arranged in two honeycomb lattices stacked together.

The quantum Hall effect is a macroscopic phenomenon in which the transverse resistance in a material changes by quantized values in a stepwise fashion. It occurs in two-dimensional electron systems at low temperatures and under strong magnetic fields. In the absence of an external magnetic field, however, a 2D system may spontaneously generate its own magnetic field, for example, through an orbital ferromagnetism that is produced by interactions among electrons. This behavior is called the quantum anomalous Hall effect.

“When the rare quantum anomalous Hall effect was investigated previously, the materials studied were complex,” Zhang said. “By contrast, our material is comparably simple, since it just consists of two layers of graphene and occurs naturally.”

Dr. Thomas Weitz, an author of the study and a professor at the University of Göttingen, said: “Additionally, we found quite counterintuitively that even though carbon is not supposed to be magnetic or ferroelectric, we observed experimental signatures consistent with both.”

In research published in 2011, Zhang, a theoretical physicist, predicted that bilayer graphene would have five competing ground states, the most stable states of the material that occur at a temperature near absolute zero (minus 273.15 degrees Celsius or minus 459.67 degrees Fahrenheit). Such states are driven by the mutual interaction of electrons whose behavior is governed by quantum mechanics and quantum statistics.

“We predicted that there would be five families of states in bilayer graphene that compete with each other to be the ground state. Four have been observed in the past. This is the last one and the most challenging to observe,” Zhang said.

In experiments described in the Nature article, the researchers found eight different ground states in this fifth family that exhibit the quantum anomalous Hall effect, ferromagnetism and ferroelectricity simultaneously.

“We also showed that we could choose among this octet of ground states by applying small external electric and magnetic fields as well as controlling the sign of charge carriers,” Weitz said.

   “We predicted, observed, elucidated and controlled a quantum anomalous Hall octet, where three striking quantum phenomena — ferromagnetism, ferroelectricity and zero-field quantum Hall effect — can coexist and even cooperate in bilayer graphene.”

   Dr. Fan Zhang, associate professor of physics in the School of Natural Sciences and Mathematics

The ability to control the electronic properties of bilayer graphene to such a high degree might make it a potential candidate for future low-dissipation quantum information applications, although Zhang and Weitz said they are primarily interested in revealing the “beauty of fundamental physics.”

“We predicted, observed, elucidated and controlled a quantum anomalous Hall octet, where three striking quantum phenomena — ferromagnetism, ferroelectricity and zero-field quantum Hall effect — can coexist and even cooperate in bilayer graphene,” Zhang said. “Now we know we can unify ferromagnetism, ferroelectricity and the quantum anomalous Hall effect in this simple material, which is amazing and unprecedented.”

Other authors of the Nature article include UT Dallas physics doctoral student Tianyi Xu and researchers from the University of Göttingen and the Ludwig Maximilian University of Munich.

Zhang’s research is funded by the U.S. Army Combat Capabilities Development Command’s Army Research Laboratory and the National Science Foundation.

Media Contact: Amanda Siegfried, UT Dallas, 972-883-4335, amanda.siegfried@utdallas.edu, or the Office of Media Relations, UT Dallas, (972) 883-2155, newscenter@utdallas.edu.
Tagged: Dr. Fan Zhang NSM Physics research

https://news.mit.edu/2021/physicists-discover-important-new-property-graphene-0208

Physicists discover important new property for graphene
Unconventional form of ferroelectricity could impact next-generation computing.
Elizabeth Thomson | Materials Research Laboratory
Publication Date:
February 8, 2021



MIT researchers and colleagues recently discovered an important — and unexpected — electronic property of graphene, a material discovered only about 17 years ago that continues to surprise scientists with its interesting physics. The work, which involves structures composed of atomically thin layers of materials that are also biocompatible, could usher in new, faster information-processing paradigms. One potential application is in neuromorphic computing, which aims to replicate the neuronal cells in the body responsible for everything from behavior to memories.

The work also introduces new physics that the researchers are excited to explore.

“Graphene-based heterostructures continue to produce fascinating surprises. Our observation of unconventional ferroelectricity in this simple and ultra-thin system challenges many of the prevailing assumptions about ferroelectric systems, and it may pave the way for an entire generation of new ferroelectrics materials,” says Pablo Jarillo-Herrero, the Cecil and Ida Green Professor of Physics at MIT and leader of the work, which involved a collaboration with five other MIT faculty from three departments.

A new property

Graphene is composed of a single layer of carbon atoms arranged in hexagons resembling a honeycomb structure. Since the material’s discovery, scientists have shown that different configurations of graphene layers can give rise to a variety of important properties. Graphene-based structures can be either superconductors, which conduct electricity without resistance, or insulators, which prevent the movement of electricity. They have even been found to display magnetism.

In this work, which was reported in Nature, the MIT researchers and colleagues show that bilayer graphene can also be ferroelectric. This means that positive and negative charges in the material can spontaneously separate into different layers.

In most materials, opposite charges are attracted to each other; they want to combine. Only the application of an electric field will force them to opposite sides, or poles. In a ferroelectric material, no external electric field is necessary to keep the charges apart, giving rise to a spontaneous polarization. However, the application of an external electric field does have an effect: an electric field of opposite direction will cause the charges to switch sides and reverse the polarization.

For all of these reasons, ferroelectric materials are used in a variety of electronic systems, from medical ultrasounds to radio frequency identification cards.

Conventional ferroelectrics, however, are insulators. The MIT-led team’s ferroelectric based on graphene operates through a completely different mechanism — different physics — that allows it to conduct electricity. And that opens up myriad additional applications. “What we’ve found here is a new type of ferroelectric material,” says Zhiren "Isaac" Zheng, an MIT graduate student in physics and first author of the Nature paper.

Qiong Ma PhD '16, a co-author of the paper and an assistant professor at Boston College, puts the work in perspective. “There are challenges associated with conventional ferroelectrics that people have been working to overcome. For example, the ferroelectric phase becomes unstable as the device continues to be miniaturized. With our material, some of those challenges may be automatically solved.” Ma conducted the current work as a postdoc through MIT’s Materials Research Laboratory (MRL).

In addition to Jarillo-Herrero, Zheng, and Ma, additional authors of the paper are Zhen Bi of Pennsylvania State University; Sergio de la Barrera, a postdoc in the MRL; Ming-Hao Liu of National Cheng Kung University; Nannan Mao, a postdoc in MIT’s Research Laboratory of Electronics; Yang Zhang, a postdoc in the MRL; Natasha Kiper of ETH Zürich; Professor Jing Kong of MIT’s Department of Electrical Engineering and Computer Science; William Tisdale, the ARCO Career Development Professor in MIT’s Department of Chemical Engineering; Professor Ray Ashoori of the MIT Department of Physics; Professor Nuh Gedik of the Department of Physics; Liang Fu, MIT’s Lawrence C. (1944) and Sarah W. Biedenharn Career Development Associate Professor of Physics, and Su-Yang Xu of Harvard University.

Important patterns

The structure the team created is composed of two layers of graphene — a bilayer — sandwiched between atomically thin layers of boron nitride (BN) above and below. Each BN layer is at a slightly different angle from the other. Looking from above, the result is a unique pattern called a moiré superlattice. A moiré pattern, in turn, “can dramatically change the properties of a material,” Zheng says.

Jarillo-Herrero’s group demonstrated an important example of this in 2018. In that work, also reported in Nature, the researchers stacked two layers of graphene. Those layers, however, weren’t exactly on top of each other; rather, one was slightly rotated at a “magic angle” of 1.1 degrees. The resulting structure created a moiré pattern that in turn allowed the graphene to be either a superconductor or an insulator depending on the number of electrons in the system as provided by an electric field. Essentially the team was able to “tune graphene to behave at two electrical extremes.”

“So by creating this moiré structure, graphene is not graphene anymore. It almost magically turns into something very, very different,” Ma says.

In the current work, the researchers created a moiré pattern with sheets of graphene and boron nitride that has resulted in a new form of ferroelectricity. The physics involved in the movement of electrons through the structure is different from that of conventional ferroelectrics.

“The ferroelectricity demonstrated by the MIT group is fascinating,” says Philip Kim, a professor of physics and applied physics at Harvard University, who was not involved in the research. “This work is the first demonstration that reports pure electronic ferroelectricity, which exhibits charge polarization without ionic motion in the underlying lattice. This surprising discovery will surely invite further studies that can reveal more exciting emergent phenomena and provide an opportunity to utilize them for ultrafast memory applications.”

The researchers aim to continue the work by not only demonstrating the new material’s potential for a variety of applications, but also developing a better understanding of its physics. “There are still many mysteries that we don’t fully understand and that are fundamentally very intriguing,” Ma says.

This work was supported by the U.S. Department of Energy, the Gordon and Betty Moore Foundation, the U.S. Air Force Office of Scientific Research, the U.S. National Science Foundation, the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, and the Taiwan Ministry of Science and Technology.

Looks like more tip-toe'ing around Mathis' C.F.:
......
New tricks for finding better superconductive materials

by Vienna University of Technology

https://phys.org/news/2021-10-superconductive-materials.html


Hard-to-access parameters

Theoretical models describing the behavior of such superconductors already exist. The problem, however, is that in order to use these models, one must know certain material parameters that are difficult to determine. "The charge transfer energy plays a key role," explains Jan Kuneš. "This value tells us how much energy you have to add to the system to transfer an electron from a nickel atom to an oxygen atom."

Unfortunately, this value cannot be measured directly, and theoretical calculations are extremely complicated and imprecise. Therefore, Atsushi Hariki, a member of Jan Kuneš' research group, developed a method to determine this parameter indirectly: When the material is examined with X-rays, the results also depend on the charge transfer energy. "We calculated details of the X-ray spectrum that are particularly sensitive to this parameter and compared our results with measurements of different X-ray spectroscopy methods," explains Jan Kuneš. "In this way, we can determine the appropriate value—and this value can now be inserted into the computational models used to describe the superconductivity of the material."

Important prerequisite for the search for better nickelates

Thus, for the first time, it has now been possible to explain the electronic structure of the material precisely and to set up a parameterised theoretical model for describing superconductivity in nickelates. "With this, we can now get to the bottom of the question of how the mechanics of the effect can be explained at the electronic level," says Jan Kuneš. "Which orbitals play a decisive role? Which parameters matter in detail? That's what you need to know if you want to find out how to improve this material further, so that one day you might be able to produce new nickelates whose superconductivity persists up to even significantly higher temperatures."

+ Explore further
Superconductivity: It's hydrogen's fault
More information: Keisuke Higashi et al, Core-Level X-Ray Spectroscopy of Infinite-Layer Nickelate: LDA+DMFT Study, Physical Review X (2021). DOI: 10.1103/PhysRevX.11.041009

A new paper on Bi-layer Graphene:
...

Magic angles in twisted bilayer graphene near commensuration: towards a hyper-magic manifold


The Bistritzer-MacDonald continuum model (BM model) describes the low-energy moiré bands for twisted bilayer graphene (TBG) at small twist angles. We derive a generalized continuum model for TBG near any commensurate twist angle, which is characterized by a complex inter-layer hopping at commensurate AA stackings (rather than the real hopping in the BM model), a real inter-layer hopping at commensurate AB/BA stackings, and a global energy shift. The complex phase of the AA stacking hopping and the twist angle together define a single angle parameter ϕ0. We compute the model parameters for the first six distinct commensurate TBG configurations, among which the 38.2∘ configuration may be within experimentally observable energy scales. We identify the first magic angle for any ϕ0 at a condition similar to that of the BM model. At this angle, the lowest two moiré bands at charge neutrality become flat (except in the vicinity of the ΓM point) and retain fragile topology, but lose particle-hole symmetry. We further identify a hyper-magic manifold in the parameter space at ϕ0=±π/2, where seven or more moiré bands around charge neutrality become flat simultaneously. The lowest two moiré flat bands in the hyper-magic manifold have fragile (trivial) topology when close to (far from) the chiral limit with zero AA hopping.

Michael G. Scheer, Kaiyuan Gu, Biao Lian

https://arxiv.org/abs/2203.06163v1

Moore’s Law: Scientists Just Made a Graphene Transistor Gate the Width of an Atom

By

 Jason Dorrier

 -

Mar 13, 2022

5,176

There’s been no greater act of magic in technology than the sleight of hand performed by Moore’s Law. Electronic components that once fit in your palm have long gone atomic, vanishing from our world to take up residence in the quantum realm.

But we’re now brushing the bitter limits of this trend. In a paper published in Nature this week, scientists at Tsinghua University in Shanghai wrote that they’ve built a graphene transistor gate with a length of 0.34 nanometers (nm)—or roughly the size of a single carbon atom.

The gate, a chip component that switches transistors on and off, is a critical measure of transistor size. Previous research had already pushed gate lengths to one nanometer and below. By scaling gate lengths down to the size of single atoms, the latest work sets a new mark that’ll be hard to beat. “In the future, it will be almost impossible for people to make a gate length smaller than 0.34 nm,” the paper’s senior author Tian-Ling Ren told IEEE Spectrum. “This could be the last node for Moore’s Law.”

Etching a 2D Sandwich

Transistors have a few core components: the source, the drain, the channel, and the gate. Electrical current flows from the source, through the channel, past the gate, and into the drain. The gate switches this current on or off depending on the voltage applied to it.

Recent advances in extreme transistor gate miniaturization rely on some fascinating materials. In 2016, for example, researchers used carbon nanotubes—which are single-atom-thick sheets of carbon rolled into cylinders—and a 2D material called molybdenum disulfide to achieve a gate length of one nanometer. Silicon is a better semiconductor, as electrical currents encounter more resistance in molybdenum disulfide, but when gate lengths dip below five nanometers, electrons leak across the gates in silicon transistors. Molybdenum disulfide’s natural resistance prevents this leakage at the tiniest scales.

https://singularityhub.com/2022/03/13/moores-law-scientists-just-made-a-graphene-transistor-gate-the-width-of-an-atom/

A paper on Salt-Graphene Batteries using novel stacking with monographene layers.

https://www.science.org/doi/epdf/10.1126/sciadv.abf0812

https://phys.org/news/2021-08-janus-graphene-doors-sustainable-sodium-ion.html

http://dx.doi.org/10.1126/sciadv.abf0812

More at link:
.........

Janus graphene opens doors to sustainable sodium-ion batteries
by Chalmers University of Technology

Janus graphene opens doors to sustainable sodium-ion batteries

Sodium is one of the most abundant and affordable metals in the world. Now researchers at Chalmers University of Technology, Sweden, present a concept that allows sodium-ion batteries to match the capacity of todays lithium-ion batteries. Using a novel type of graphene, they stacked specially designed graphene sheets with molecules in between. The new material allows the sodium ions (in green) to efficiently store energy. Credit: Marcus Folino and Yen Strandqvist/Chalmers University of Technology

In the search for sustainable energy storage, researchers at Chalmers University of Technology, Sweden, present a new concept to fabricate high-performance electrode materials for sodium batteries. It is based on a novel type of graphene to store one of the world's most common and cheap metal ions—sodium. The results show that the capacity can match today's lithium-ion batteries.

Even though lithium ions work well for energy storage, lithium is an expensive metal with concerns regarding its long-term supply and environmental issues.

Sodium, on the other hand, is an abundant low-cost metal, and a main ingredient in seawater (and in kitchen salt). This makes sodium-ion batteries an interesting and sustainable alternative for reducing our need for critical raw materials. However, one major challenge is to increase the capacity.

At the current level of performance, sodium-ion batteries cannot compete with lithium-ion cells. One limiting factor is the graphite, which is composed of stacked layers of graphene, and used as the anode in today's lithium-ion batteries.

The ions intercalate in the graphite, which means that they can move in and out of the graphene layers and be stored for energy usage. Sodium ions are larger than lithium ions and interact differently. Therefore, they cannot be efficiently stored in the graphite structure. But the Chalmers researchers have come up with a novel way to solve this.

Janus graphene opens doors to sustainable sodium-ion batteries
The material used in the study has a unique artificial nanostructure. The upper face of each graphene sheet has a molecule that acts as both spacer and active interaction site for the sodium ions. Each molecule in between two stacked graphene sheets is connected by a covalent bond to the lower graphene sheet and interacts through electrostatic interactions with the upper graphene sheet. The graphene layers also have uniform pore size, controllable functionalisation density, and few edges. Credit: Yen Strandqvist/Chalmers University of Technology

"We have added a molecule spacer on one side of the graphene layer. When the layers are stacked together, the molecule creates larger space between graphene sheets and provides an interaction point, which leads to a significantly higher capacity," says researcher Jinhua Sun at the Department of Industrial and Materials Science at Chalmers and first author of the scientific paper, published in Science Advances.

Ten times the energy capacity of standard graphite

Typically, the capacity of sodium intercalation in standard graphite is about 35 milliampere hours per gram (mA h g-1). This is less than one tenth of the capacity for lithium-ion intercalation in graphite. With the novel graphene the specific capacity for sodium ions is 332 milliampere hours per gram—approaching the value for lithium in graphite. The results also showed full reversibility and high cycling stability.

"It was really exciting when we observed the sodium-ion intercalation with such high capacity. The research is still at an early stage, but the results are very promising. This shows that it's possible to design graphene layers in an ordered structure that suits sodium ions, making it comparable to graphite," says Professor Aleksandar Matic at the Department of Physics at Chalmers.

"Divine" Janus graphene opens doors to sustainable batteries

The study was initiated by Vincenzo Palermo in his previous role as Vice-Director of the Graphene Flagship, a European Commission-funded project coordinated by Chalmers University of Technology.

Janus graphene opens doors to sustainable sodium-ion batteries
Researchers at Chalmers University of Technology, Sweden, present a new concept to fabricate high-performance electrode materials for sodium-ion batteries. It is based on a novel type of graphene to store one of the world's most common and cheap metal ions – sodium. The results show that the capacity can match today’s lithium-ion batteries. Credit: Marcus Folino/Chalmers University of Technology

The novel graphene has asymmetric chemical functionalisation on opposite faces and is therefore often called Janus graphene, after the two-faced ancient Roman God Janus—the God of new beginnings, associated with doors and gates, and the first steps of a journey. In this case the Janus graphene correlates well with the roman mythology, potentially opening doors to high-capacity sodium-ion batteries.

"Our Janus material is still far from industrial applications, but the new results show that we can engineer the ultrathin graphene sheets—and the tiny space in between them—for high-capacity energy storage. We are very happy to present a concept with cost-efficient, abundant and sustainable metals," says Vincenzo Palermo, Affiliated Professor at the Department of Industrial and Materials Science at Chalmers.

More on the material: Janus graphene with a unique structure


......

Sodium-based batteries could make your smartphone cheaper and cleaner
More information: Jinhua Sun et al, Real-time imaging of Na+ reversible intercalation in "Janus" graphene stacks for battery applications, Science Advances (2021). DOI: 10.1126/sciadv.abf0812

Journal information: Science Advances

Provided by Chalmers University of Technology



Sodium-based batteries could make your smartphone cheaper and cleaner
Jun 06, 2018

Proliferation of electric vehicles based on high-performance, low-cost sodium-ion battery
Jun 18, 2021

Making sodium-ion batteries that last
Feb 15, 2017

Replacing lithium with sodium in batteries
Jul 17, 2020

Research aims to improve rechargeable batteries by focusing on graphene oxide paper
Dec 18, 2014

Sodium-ion batteries are potential power technology of future
Sep 23, 2015
Load comments (0)

Keeping the energy in the room
6 hours ago

Study: How placentas evolved in mammals
6 hours ago

New method boosts the study of regulation of gene activity
8 hours ago

Hidden in genetics: The evolutionary relationships of two groups of ancient invertebrates revealed
9 hours ago

'Soft' CRISPR may offer a new fix for genetic defects
9 hours ago

Dinosaurs took over amid ice, not warmth, says a new study of ancient mass extinction
9 hours ago

Mining's effect on fish warrants better science-based policies
9 hours ago

Advocating a new paradigm for electron simulations
10 hours ago

New study reveals impact of plastic on small mammals, as four out of seven species identified as 'plastic positive'
11 hours ago

Triply eclipsing stellar systems
11 hours ago

It takes three: The genetic mutations that made rice cultivation possible
11 hours ago




Science X Daily and the Weekly Email Newsletter are free features that allow you to receive your favorite sci-tech news updates in your email inbox


Archive
Phys.org 2003 - 2022 powered by Science X Network

Miles mentioned the Muon in a recent paper:
http://milesmathis.com/muonmag.pdf

Interesting word "decay" associated with the Muon. In charge field terms, it sounds like a spin-up or spin-down where the spin state changes enough the structure "disappears" or "decays".  
-------

AUGUST 1, 2022

Study finds nickelate superconductors are intrinsically magnetic
by Glennda Chui, SLAC National Accelerator Laboratory

A muon, center, spins like a top within the atomic lattice of a thin film of superconducting nickelate. These elementary particles can sense the magnetic field created by the spins of electrons up to a billionth of a meter away. By embedding muons in four nickelate compounds at the Paul Scherrer Institute in Switzerland, researchers at SLAC and Stanford discovered that the nickelates they tested host magnetic excitations whether they're in their superconducting states or not—another clue in the long quest to understand how unconventional superconductors can conduct electric current with no loss. Credit: Jennifer Fowlie/SLAC National Accelerator Laboratory

Electrons find each other repulsive. Nothing personal—it's just that their negative charges repel each other. So getting them to pair up and travel together, like they do in superconducting materials, requires a little nudge.

In old-school superconductors, which were discovered in 1911 and conduct electric current with no resistance, but only at extremely cold temperatures, the nudge comes from vibrations in the material's atomic lattice.

But in newer, "unconventional" superconductors—which are especially exciting because of their potential to operate at close to room temperature for things like zero-loss power transmission—no one knows for sure what the nudge is, although researchers think it might involve stripes of electric charge, waves of flip-flopping electron spins that create magnetic excitations, or some combination of things.

In the hope of learning more by looking at the problem from a slightly different angle, researchers at Stanford University and the Department of Energy's SLAC National Accelerator Laboratory synthesized another unconventional superconductor family—the nickel oxides, or nickelates. Since then, they've spent three years investigating the nickelates' properties and comparing them to one of the most famous unconventional superconductors, the copper oxides or cuprates.

And in a paper published in Nature Physics today, the team reported a significant difference: Unlike in the cuprates, the magnetic fields in nickelates are always on.

Magnetism: Friend or foe?

Nickelates, the scientists said, are intrinsically magnetic, as if each nickel atom were clutching a tiny magnet. This is true whether the nickelate is in its non-superconducting, or normal, state or in a superconducting state where electrons have paired up and formed a sort of quantum soup that can host intertwining phases of quantum matter. Cuprates, on the other hand, are not magnetic in their superconducting state.

"This study looked at fundamental properties of the nickelates compared to the cuprates, and what that can tell us about unconventional superconductors in general," said Jennifer Fowlie, a postdoctoral researcher at SLAC's Stanford Institute for Materials and Energy Sciences (SIMES) who led the experiments.

Some researchers think magnetism and superconductivity compete with each other in this type of system, she said; others think you can't have superconductivity unless magnetism is close by.

"While our results don't settle that question, they do highlight where more work should probably be done," Fowlie said. "And they mark the first time that magnetism has been examined in both the superconducting and the normal state of nickelates."

Harold Hwang, a professor at SLAC and Stanford and director of SIMES, said, "This is another important piece of the puzzle that the research community is putting together as we work to frame the properties and phenomena at the heart of these exciting materials."

Enter the muon

Few things come easy in this field of research, and studying the nickelates has been harder than most.

While theorists predicted more than 20 years ago that their chemical similarity to the cuprates made it likely that they could host superconductivity, nickelates are so difficult to make that it took years of trying before the SLAC and Stanford team succeeded.

Even then, they could only make thin films of the material—not the thicker chunks needed to explore its properties with common techniques. A number of research groups around the world have been working on easier ways to synthesize nickelates in any form, Hwang said.

So the research team turned to a more exotic method, called low-energy muon spin rotation/relaxation, that can measure the magnetic properties of thin films and is available only at the Paul Scherrer Institute (PSI) in Switzerland.

Muons are fundamental charged particles that are similar to electrons, but 207 times more massive. They stick around for just 2.2 millionths of a second before decaying. Positively charged muons, which are often preferred for experiments like these, decay into a positron, a neutrino and an antineutrino. Like their electron cousins, they spin like tops and change the direction of their spin in response to magnetic fields. But they can "feel" those fields only in their immediate surroundings—up to about one nanometer, or a billionth of a meter, away.

At PSI, scientists use a beam of muons to embed the little particles in the material they want to study. When the muons decay, the positrons they produce fly off in the direction the muon is spinning. By tracing the positrons back to their origins, researchers can see which way the muons were pointing when they winked out of existence and thus determine the material's overall magnetic properties.

Finding a workaround

The SLAC team applied to do experiments with the PSI system in 2020, but then the pandemic made it impossible to travel in or out of Switzerland. Fortunately, Fowlie was a postdoc at the University of Geneva at the time and already planning to come to SLAC to work in Hwang's group. So she started the first round of experiments in Switzerland with a team led by Andreas Suter, a senior scientist at PSI and an expert in extracting information about superconductivity and magnetism from muon decay data.

After arriving at SLAC May 2021, Fowlie immediately started making various types of nickelate compounds the team wanted to test in their second round of experiments. When travel restrictions ended, the team was finally able to go back to Switzerland to finish the study.

The unique experimental setup at PSI allows scientists to embed muons at precise depths in the nickelate materials. From this, they were able to determine what was going on in each super-thin layer of various nickelate compounds with slightly different chemical compositions. They discovered that only the layers that contained nickel atoms were magnetic.

More at link:
https://phys.org/news/2022-08-nickelate-superconductors-intrinsically-magnetic.html

Looks like these researchers on graphene are starting to look at Mathis' papers:
----------
Physicists discover a “family” of robust, superconducting graphene structures
The findings could inform the design of practical superconducting devices.

Jennifer Chu | MIT News Office
Publication Date:July 8, 2022

PRESS INQUIRIES
Cooper pairs in magic-angle multilayer graphene
Caption:An illustration showing superconducting Cooper pairs in magic-angle multilayer graphene family. The adjacent layers are twisted in an alternating fashion.
Credits:Credit: Ella Maru Studio
graphic showing 2D layers of graphene
Caption:MIT physicists have established twisted graphene as a new “family” of robust superconductors, each member consisting of alternating graphene layers, stacked at precise angles.
Credits:Courtesy of the researchers
Previous image Next image
When it comes to graphene, it appears that superconductivity runs in the family.

Graphene is a single-atom-thin material that can be exfoliated from the same graphite that is found in pencil lead. The ultrathin material is made entirely from carbon atoms that are arranged in a simple hexagonal pattern, similar to that of chicken wire. Since its isolation in 2004, graphene has been found to embody numerous remarkable properties in its single-layer form.

In 2018, MIT researchers found that if two graphene layers are stacked at a very specific “magic” angle, the twisted bilayer structure could exhibit robust superconductivity, a widely sought material state in which an electrical current can flow through with zero energy loss. Recently, the same group found a similar superconductive state exists in twisted trilayer graphene — a structure made from three graphene layers stacked at a precise, new magic angle.

Now the team reports that — you guessed it — four and five graphene layers can be twisted and stacked at new magic angles to elicit robust superconductivity at low temperatures. This latest discovery, published this week in Nature Materials,  establishes the various twisted and stacked configurations of graphene as the first known “family” of multilayer magic-angle superconductors. The team also identified similarities and differences between graphene family members.

The findings could serve as a blueprint for designing practical, room-temperature superconductors. If the properties among family members could be replicated in other, naturally conductive materials, they could be harnessed, for instance, to deliver electricity without dissipation or build magnetically levitating trains that run without friction.

“The magic-angle graphene system is now a legitimate ‘family,’ beyond a couple of systems,” says lead author Jeong Min (Jane) Park, a graduate student in MIT’s Department of Physics. “Having this family is particularly meaningful because it provides a way to design robust superconductors.”

Park’s MIT co-authors include Yuan Cao, Li-Qiao Xia, Shuwen Sun, and Pablo Jarillo-Herrero, the Cecil and Ida Green Professor of Physics, along with Kenji Watanabe and Takashi Taniguchi of the National Institute for Materials Science in Tsukuba, Japan.

“No limit”

Jarillo-Herrero’s group was the first to discover magic-angle graphene, in the form of a bilayer structure of two graphene sheets placed one atop the other and slightly offset at a precise angle of 1.1 degrees. This twisted configuration, known as a moiré superlattice, transformed the material into a strong and persistent superconductor at ultralow temperatures.

The researchers also found that the material exhibited a type of electronic structure known as a “flat band,” in which the material’s electrons have the same energy, regardless of their momentum. In this flat band state, and at ultracold temperatures, the normally frenetic electrons collectively slow down enough to pair up in what are known as Cooper pairs — essential ingredients of superconductivity that can flow through the material without resistance.

While the researchers observed that twisted bilayer graphene exhibited both superconductivity and a flat band structure, it wasn’t clear whether the former arose from the latter.

“There was no proof a flat band structure led to superconductivity,” Park says. “Other groups since then have produced other twisted structures from other materials that have some flattish band, but they didn’t really have robust superconductivity. So we wondered: Could we produce another flat band superconducting device?”

As they considered this question, a group from Harvard University derived calculations that confirmed mathematically that three graphene layers, twisted at 1.6 degrees, would exhibit also flat bands, and suggested they may superconduct. They went on to show there should be no limit to the number of graphene layers that exhibit superconductivity, if stacked and twisted in just the right way, at angles they also predicted. Finally, they proved they could mathematically relate every multilayer structure to a common flat band structure — strong proof that a flat band may lead to robust superconductivity.

“They worked out there may be this entire hierarchy of graphene structures, to infinite layers, that might correspond to a similar mathematical expression for a flat band structure,” Park says.

Shortly after that work, Jarillo-Herrero’s group found that, indeed, superconductivity and a flat band emerged in twisted trilayer graphene — three graphene sheets, stacked like a cheese sandwich, the middle cheese layer shifted by 1.6 degrees with respect to the sandwiched outer layers. But the trilayer structure also showed subtle differences compared to its bilayer counterpart.

“That made us ask, where do these two structures fit in terms of the whole class of materials, and are they from the same family?” Park says.

An unconventional family

In the current study, the team looked to level up the number of graphene layers. They fabricated two new structures, made from four and five graphene layers, respectively. Each structure is stacked alternately, similar to the shifted cheese sandwich of twisted trilayer graphene.

The team kept the structures in a refrigerator below 1 kelvin (about -273 degrees Celsius), ran electrical current through each structure, and measured the output under various conditions, similar to tests for their bilayer and trilayer systems.

Overall, they found that both four- and five-layer twisted graphene also exhibit robust superconductivity and a flat band. The structures also shared other similarities with their three-layer counterpart, such as their response under a magnetic field of varying strength, angle, and orientation.

These experiments showed that twisted graphene structures could be considered a new family, or class of common superconducting materials. The experiments also suggested there may be a black sheep in the family: The original twisted bilayer structure, while sharing key properties, also showed subtle differences from its siblings. For instance, the group’s previous experiments showed the structure’s superconductivity broke down under lower magnetic fields and was more uneven as the field rotated, compared to its multilayer siblings.

The team carried out simulations of each structure type, seeking an explanation for the differences between family members. They concluded that the fact that twisted bilayer graphene’s superconductivity dies out under certain magnetic conditions is simply because all of its physical layers exist in a “nonmirrored” form within the structure. In other words, there are no two layers in the structure that are mirror opposites of each other, whereas graphene’s multilayer siblings exhibit some sort of mirror symmetry. These findings suggest that the mechanism driving electrons to flow in a robust superconductive state is the same across the twisted graphene family.

“That’s quite important,” Park notes. “Without knowing this, people might think bilayer graphene is more conventional compared to multilayer structures. But we show that this entire family may be unconventional, robust superconductors.”

....
more at link: https://news.mit.edu/2022/superconducting-graphene-family-0708

Also:
http://jarilloherrero.mit.edu/research/gated-bilayer-graphene/
https://news.mit.edu/2021/magic-trilayer-graphene-superconductor-magnet-0721

Still calling out effects from the "Hall Effect" and "Van Der Waal Forces" than Mathis has debunked (from 2018):



---------------

An interview with Pablo Jarillo-Herrero and Allan MacDonald
---------
A new twist on graphene: an interview with Pablo Jarillo-Herrero and Allan MacDonald
Philip Ball

National Science Review, Volume 9, Issue 4, April 2022, nwac005, https://doi.org/10.1093/nsr/nwac005
Published: 13 January 2022

Abstract
Graphene is the building block of graphite, made of carbon atoms bonded into sheets of hexagonal rings just a single atom thick. Although such isolated sheets had been predicted for many decades to exist, and had been grown on other surfaces, interest in this material exploded after the discovery in 2004 that single sheets could be made easily and cheaply by separating them mechanically from graphite flakes (a process called exfoliation). Although graphene is often advertised as a ‘wonder material’—electronically conducting, transparent and extremely strong and flexible—much of the interest in it is more fundamental. As a 2D conductor, graphene shows unusual electronic and magnetic properties that enable the study of quantum-mechanical effects of confinement and of correlations between electron motions—some of which might find applications in electronic devices. The excitement of this discovery was reflected in the award of the 2010 Nobel Prize in Physics to two pioneers in the field: Andre Geim and Konstantin Novoselov of the University of Manchester in the UK.

This rich behavior is broadened still further when two graphene sheets are brought close enough to interact with one another. In particular, the electronic properties may then depend on the relative orientation of the sheets: how aligned the two ‘honeycomb’ lattices are. Two grids superimposed on one another may create ‘superlattices’: regularities at larger scales than the grid spacing, due to registry (commensurability) between the two at certain angles. This so-called moiré effect is sometimes evident for two closely spaced grid-like fences seen from afar. Experimentally exploring the electronic properties of such ‘twisted bilayer graphene’ requires an ability to precisely control the position and orientation of the two sheets. These phenomena are now recognized as generic to other 2D materials, such as hexagonal sheets of boron nitride. They have revealed a fertile playground for condensed-matter physics. In particular, striking electronic properties appear at certain ‘magic-angle’ twists of the layers.

NSR spoke to two of the leading experts in the study of magic-angle twisted bilayer graphene (MATBG): experimentalist Pablo Jarillo-Herrero of the Massachusetts Institute of Technology and theorist Allan MacDonald of the University of Texas at Austin.

Issue Section: Interview
THE DISCOVERY OF MAGIC ANGLES
NSR: How did the discovery of unusual electronic behavior in twisted bilayer graphene came about? Were such effects predicted before they were observed?

PJ-H: Many theory groups started to work on twisted bilayer graphene from around 2007. In late 2009, the group of Eva Andrei reported an investigation of twisted bilayer graphene using scanning tunneling microscopy (STM) [G. Li et al., Nat Phys 2010; 6: 109]. They observed peaks in their data that they interpreted as arising from features in the electronic structure known as Van Hove singularities [where the density of electronic states diverges]—which seemed to change with the twist angle. In particular, the separation between the peaks seemed to extrapolate to zero for a twist angle of ∼1.16°. Around the same time, two theory groups worked on the theory of twisted bilayer graphene at very small angles: Eric Suárez Morell and collaborators in Chile [E. S. Morell et al., Phys Rev B 2010; 82: 121407] and Rafi Bistritzer and Allan MacDonald in Texas [R. Bistritzer and A. MacDonald, Proc Natl Acad Sci USA 2011; 108: 12233]. Both groups predicted the existence of ‘flat’ electronic bands in twisted bilayer graphene at an angle of 1.1–1.5°. Bistritzer and MacDonald coined the term ‘magic angle’: the angle for which the velocity of electrons at the Fermi level [the energy level electrons will occupy at zero temperature] goes to zero.

Experimentalist Pablo Jarillo-Herrero's group was the first to fabricate magic-angle graphene materials (courtesy of Prof. Pablo Jarillo-Herrero).
Open in new tabDownload slide
Experimentalist Pablo Jarillo-Herrero's group was the first to fabricate magic-angle graphene materials (courtesy of Prof. Pablo Jarillo-Herrero).

Allan MacDonald was one of the first theorists who predicted the special properties of magic-angle graphene (courtesy of Prof. Allan MacDonald).
Open in new tabDownload slide
Allan MacDonald was one of the first theorists who predicted the special properties of magic-angle graphene (courtesy of Prof. Allan MacDonald).

My own group started to work on twisted bilayer graphene around 2010. In 2016, we showed that twisted bilayer graphene with a 1.8° twist angle already exhibited an interesting modification of the electronic structure: the velocity of the electrons at the Fermi level was substantially reduced, in agreement with theoretical calculations [Y. Cao et al., Phys Rev Lett 2016; 117: 116804]. This is what motivated us to continue working on reducing the twist angle and to investigate devices with twist angles very close to 1.1°. In 2017, we managed to fabricate several devices with twist angles in the range of 1–1.2°, and in these devices we made two unexpected discoveries. First, MATBG can become an insulator due to correlation effects when the charge density is tuned to a certain integer number of electrons or holes per moiré unit cell [Y. Cao et al., Nature 2018; 556: 80]. Second, if you dope the material to add a bit of extra charge density, then MATBG becomes an electrically tunable superconductor with one of the highest ratios of critical temperature to Fermi temperature [Y. Cao et al., Nature 2018; 556: 43]. These characteristics are very reminiscent of other strongly correlated quantum materials and superconductors. The fact that we could obtain these results in such a seemingly simple system, just two layers of graphene rotated to a precise angle, and with such a degree of tunability, is one of the key reasons why our discovery attracted a lot of attention. These discoveries were totally unexpected and not predicted theoretically.

AM: My understanding of the history—beyond what is in the publication record—comes from Eva Andrei [https://arxiv.org/pdf/2008.08129.pdf]. Eva was the first person to measure interesting changes in electronic structure, seeing features in STM density-of-states measurements on bilayer flakes that accidentally had moiré effects. Eva told me that the observation came first, and motivated the theory by Antonio Castro-Neto and João Lopes dos Santos.

My interest in graphene moiré electronic structure started from a conversation I had with Ed Conrad at Georgia Tech, who showed me some data from angle-resolved photoemission spectroscopy that I did not understand. As my postdoc Rafi Bistritzer and I built up our picture, we discovered that our calculations predicted that the velocity of graphene electrons would fall to zero at a discrete set of twist angles—we called them magic angles. The largest magic angle was ∼1°. This was a complete surprise to us, and we immediately recognized that it implied a rich platform for strong correlated-electron physics. Sometime later, we noticed that a group from Chile had independently obtained at least a glimmer of the magic-angle physics. At this time, we had no idea whether or not experimentalists would be able to fabricate samples with controlled twist angles to see this physics. My colleague Emanuel Tutuc did a lot of work in this direction, some of which helped to inform Pablo's parallel efforts.

NSR: What induced you to study this system? It seems now that it offers a playground for looking at correlated-electron phenomena in a controlled way—but was that the expectation, or did the discovery come as a surprise?

PJ-H: Originally, my motivation to study twisted bilayer graphene was the intuition that this ‘new knob’ in condensed-matter physics, namely the possibility to change the twist angle, had to yield interesting physics. In condensed-matter physics, the systems are typically quite complex, and when one explores unchartered territory, surprises often arise. In the specific case of magic-angle graphene, my motivation was to find interesting correlated insulator states that I though may appear when one would shift the Fermi energy in graphene to the Van Hove singularities. [NSR: New electronic phases such as superconductivity have been previously observed when the Fermi energy is close to such singularities.] We did find insulators—but to our surprise they were of a different type, where the insulating behavior occurs for an integer number of electrons per moiré unit cell, rather than because of the Van Hove singularities. This was a very nice surprise. An even bigger surprise was the discovery of superconductivity, which no one had predicted.

The magic-angle effect is a sort of ‘resonant’ condition … [where] it is as ‘easy’ for the electrons to go through one graphene layer as it is for them to ‘tunnel’ to the other graphene layer.

—Pablo Jarillo-Herrero

AM: Our original theoretical finding of magic-angle effects was not expected on the basis of earlier experiments, and we had a lot of problems getting our work published because the referees thought that the result must be incorrect. Around this time, I was elected to the US National Academy of Sciences, which allows new members to publish a lightly reviewed inaugural article in the journal PNAS. I decided to give up on a long fight with referees and just publish our findings that way.

Following that paper, I tried to find other instances where interesting moiré superlattice physics would be observable. I proposed the possibility of realizing topological exciton bands [F. Wu et al., Phys Rev Lett 2017; 118: 147401] and a number of other proposals related to optical properties. I also proposed that layered transition-metal dichalcogenide (TMD) moiré systems would yield quite different physics compared to graphene multilayers. This part of the moiré field has now really taken off experimentally.

A PLAYGROUND FOR NEW PHYSICS
NSR: There seems to be a wide range of electronic states produced from these graphene systems, from insulators to superconductors and magnetic systems. What is the underlying physics that produces such a menagerie of states, and what are the key factors that determine the properties?

PJ-H: Well, we are still trying to fully understand these systems. But your basic observation is true: MATBG, and now several other moiré systems, exhibit an incredibly rich set of correlated behaviors. The origin seems to be in the fact that these systems have narrow electronic bands (meaning the electrons have very little kinetic energy) and hence the interaction energy [due to electrostatic repulsion] between the electrons plays a dominant role. Once you have strong interactions between the electrons, then many possible many-body ground states (for example, superconductivity, correlated insulators, magnetism, etc.) become possible. We are seeing all of these because moiré systems are highly tunable.

AM: There are many analogies between strong correlations in graphene multilayers and strong correlations in quantum Hall physics. This connection was clarified by work of Eslam Khalaf, Ashvin Vishwanath and collaborators at Harvard, Mike Zaletel and others. Ultimately it related to topological properties of the electronic bands. At the same time, these systems have an aspect of the quasi-2D Hubbard model [one of the first and simplest lattice models of strongly correlated electrons]. MATBG seems to be a marriage of the quantum Hall effect [which arises in 2D systems] and high-temperature superconductivity—it's an amazing system.

NSR: Can you explain the magic-angle effect? What makes certain relative orientations of the graphene sheets ‘special’?

It's a whole new paradigm for making artificial tunable crystals, and we’re still just scratching the surface. We’ll see what happens—that's what makes science fun.

—Allan MacDonald

PJ-H: The magic-angle effect is a sort of ‘resonant’ condition. One can think of the magic angle as the angle for which the electronic structure is such that it is as ‘easy’ for the electrons to go through one graphene layer as it is for them to ‘tunnel’ to the other graphene layer. In much simpler terms, an analogy for why electrons in MATBG display such varied behavior is this: when electrons have lots of kinetic energy (that is, they move very fast), they barely have time to interact. But in MATBG, the electrons are very slow, and hence they have lots of opportunity to interact with each other as they pass each other by.

NSR: The interplay of insulating and superconducting behavior in this system seems to mirror that seen in the copper-oxide high-temperature superconductors. Is similar physics at work? Does this behavior in fact help us to understand the origin of superconductivity in such materials?

PJ-H: There are indeed many similarities between the phase diagrams of MATBG and cuprate superconductors, but there are also many differences too. For example, the symmetries of the lattices and the topological properties of the electronic structures are very different. Also, the electrons in the cuprates have degenerate spin [all the same], whereas the spin states for MATBG are somewhat richer. So we do not know yet whether understanding MATBG will help us to understand the origin of superconductivity in cuprates. My intuition is that it will, but it's too early to tell.

AM: We do not yet have completely confident answers to these questions, but we are making progress. There are many similarities between high-temperature superconductors and MATBG systems—the proximity to magnetic order and to Fermi-surface reconstructions is most intriguing. In my opinion, there is every reason to expect that we will continue to make progress in understanding MATBG superconductivity by performing new experiments and testing theoretically proposed scenarios, and that this progress will have implications for understanding high-temperature superconductivity. The possibility of tuning the charge-carrier density in situ, and the possibility of adjusting the system properties in other ways (for example, by varying the distance to gates, the dielectric environment and the in-plane magnetic field) is an important advantage for MATBG.

NSR: What is the role of dimensionality here? Are these behaviors dependent on the fact that this is a (quasi-)2D system? And in that regard, does this behavior connect with the work on low-dimensional quantum many-body problems such as the quantum Hall effect (QHE)?

PJ-H: Dimensionality is quite important for various reasons. Among them: MATBG is highly tunable electrically because of the 2D geometry; the electronic structure (for example, the density of states) depends on dimensionality; and interaction effects can be also strongly dependent on dimensionality—for example, screening by electrons is very different in 1D, 2D and 3D. Regarding quantum Hall physics, there is a deep connection in that both the QHE and the electronic bands in MATBG (and several other related moiré systems) are topological in nature. That's why the latter can show interesting quantum Hall effects, even at zero magnetic field [unlike the standard QHE].

AM: Electronic correlations tend to be stronger in lower-dimensional systems and have more scope to yield surprising many-electron states, including fractional quantum Hall effect (FQHE) systems, MATBG, bilayer and trilayer graphene. The topological picture of the QHE provides a link between MATBG and FQHE physics. One experimental proof of this connection is the common appearance of anomalous quantum Hall states (that is, QHE without a magnetic field) in MATBG.

CHALLENGES, APPLICATIONS AND OPPORTUNITIES
NSR: How does one study these systems experimentally? Is it now routine to make good-quality single-layer graphene? How do you control the relative orientation of the sheets?

PJ-H: It is very standard to obtain very-high-quality single-layer graphene, by mechanical exfoliation of graphite for example. Thousands of groups around the world can do this. What is a lot more tricky is to be able to stack on top of each other two graphene sheets with a very well controlled angle of rotation, and that's even harder for small angles such as the magic angle of 1.1°. There are only ∼15 groups around the world that can currently fabricate MATBG, but more are joining all the time. It's something that can be easily learned if someone shows you. Before the pandemic, we had plenty of groups come to MIT and learn about it, and those groups now have reproduced and extended many of our results.

AM: It is amazing what has been accomplished already. If it were possible to develop techniques to control twist angles even more finely, and make the twist angles even more uniform, it would speed up progress in the field.

NSR: What are the key questions that still remain to be explored in these systems? What are you personally most eager to study now?

PJ-H: There are many key questions remaining to be explored. Perhaps one of the most important is the exact mechanism for superconductivity and the symmetry of the order parameter. Right now, experiments and theory seem to point towards a very unconventional origin for the superconductivity (though not

There are only ∼15 groups around the world that can currently fabricate MATBG, but more are joining all the time.

—Pablo Jarillo-Herrero

everybody agrees, and some people still think MATBG could be a [standard] electron–phonon-mediated superconductor in a very unusual parameter regime). But we still need to study this in more detail. I’m personally very eager to discover and study new moiré systems, new superconductors and their correlated and topological behavior. I think we have barely scratched the surface of the many hundreds of possible moiré systems we can build, with very different constituents, geometries and complexity.

AM: I think that it is important to nail down the origin of superconductivity in MATBG. I am actively working on this question.

One important hope is that we will be able to realize fractional anomalous quantum Hall systems (also known as fractional Chern insulators) in MATBG or in TMD moirés that show the quantum anomalous Hall effect. Given the flexibility of the moiré superlattices, there is a good chance that favorable conditions could be discovered or engineered. FQH states are one of the possible hosts for topological quantum computing too.

NSR: There seem to be many potential degrees of freedom to explore in such systems. For example, there is now interest in going beyond bilayer systems to trilayers—what is predicted and/or observed there? What about using hetero-bilayers, such as with boron nitride or other 2D materials?

AM: I am very interested in identifying new layered materials that can host new classes of moiré superlattices—each case will bring its own universe of new physics. Already just with TMD and twisted graphene moirés, we have examples of itinerant [mobile] electron ferromagnetism—but with quite low ordering temperatures. It's interesting to think about how ordering temperatures can be increased and what the ultimate limits are. Because the moiré superlattice systems can be tuned in so many ways, there is some scope for optimism. It's a whole new paradigm for making artificial tunable crystals, and we’re still just scratching the surface. We’ll see what happens—that's what makes science fun.

PJ-H: Indeed, the possibilities are pretty much endless. Just earlier this year, two independent groups (Philip Kim's group and my group) discovered superconductivity in magic-angle twisted trilayer graphene (MATTG). The magic angle turns out to be a bit different (∼1.6°), which was theoretically predicted a couple of years ago, so we knew where to look. The superconductivity in MATTG turns out to be even more interesting than in MATBG, because it is stronger and much more tunable. Using hetero-bilayers can indeed add many things, and the discovery of the quantum anomalous Hall effect (QAHE) in MATBG aligned with boron nitride was one of the earliest examples.

NSR: More generally, MATBG systems exemplify an explosion of interest in the past two decades in strongly correlated electrons, which has yielded the discovery of a vast zoo of quantum materials, such as topological insulators, Majorana zero modes, Weyl semimetals and so on. What has triggered the explosion of interest? Is there any emerging sense of a unified view of the quantum electronic phases of matter, or are we still very much in the phase of discovery and surprise?

PJ-H: Condensed-matter physics saw two ‘revolutions’ in the 1980s, with the discovery of the integer/FQH effects (which brought topology into the field) and with the discovery of high-temperature superconductivity (which brought strong correlations to the forefront). Since then, these two communities (working on topological and strongly correlated systems) did not interact very closely, because the systems were quite different. Then in the 2000s, three major disruptions occurred: the discovery of graphene and 2D crystalline materials; the theoretical prediction and, soon after, the experimental discovery of topological insulators; and the discovery of a second family of high-temperature superconductors, the iron pnictides. Yet still these communities remained largely separate. MATBG has merged these three communities, because it has characteristics of all of them. This topic of ‘moiré quantum matter’ has stimulated very interesting discussions among all of these people.

AM: In my view, we are still very much in a phase of discovery and surprise, but I am very optimistic that these new classes of strongly correlated systems will lead to a broader and deeper view of strong-electron-correlation physics.

NSR: Are there any likely practical applications of such systems? In particular, does there seem any likelihood that MATBG might find its way into device technology?

PJ-H: This is always very hard to predict. For now, my group and the overall community are motivated by the beautiful fundamental physics that we can explore in these systems. Having said that, MATBG is an electrically tunable superconductor or, in engineering terms, a superconducting field-effect transistor, so one could easily imagine many applications for it, if the community can manage to fabricate MATBG on a large scale. Applications easy to imagine include tunable superconducting quantum bits, tunable quantum photodetectors and classical cryogenic computing.

AM: Personally, I am very interested in identifying potential applications—for optical properties perhaps, or for spintronics [information processing using spins]. Interfaces with TMDs may prove useful in adjusting spin–orbit interaction strengths—something that will be key for spintronics.

NSR: What is your impression of the research in this area in China?

PJ-H: There has been lots of interest in China from the point of view of theoretical physics. In terms of experimental work, China currently has only a few groups with the nanofabrication experience to produce high-quality moiré quantum systems. Those groups (the most renowned of which is that of Yuanbo Zhang (张远波) at Fudan University) are producing very good work. Given the recent rapid evolution of scientific research in China, I expect many more experimental groups will start working on this subject in the coming years.

My former student Yuan Cao (曹原) is a very remarkable scientist in many respects. He is very smart, hard-working, creative and effective. He was not only the first author in the two discovery papers I mentioned above, and hence a young leader in the field, but has continued to make outstanding contributions to the field since then. He has received several awards, even at his young age, including the McMillan Award (the most prestigious award for young condensed-matter physicists) and very recently the International Sackler Prize in Physics. I consider myself extremely fortunate to have worked with him. I think I have learned, at the very least, as much from him as he may have learned from me. I believe he will be one of the leading scientists of his generation.

AM: Fengcheng Wu (吴冯成), a former student in my group who did important early work on TMD moirés, including optical and electronic properties, and on MATBG superconductivity, is now a professor at Wuhan University and is a leading researcher in the field. Wang Yao (姚望) at Hong Kong University is a leading theoretical researcher on optical properties of TMD moirés. The QAHE was first observed at Tsinghua University in magnetic topological insulators. MATBG has provided a second example, with interesting similarities and differences.

Most of the time, the new understanding is just a detail that is already understood by someone—but every now and again it will be something truly new.

—Allan MacDonald

NSR: What or who are your key sources of inspiration for this work? And what advice would you give to young researchers entering the field?

PJ-H: There are many colleagues whom I find very creative and who have inspired my group's approach to experimental condensed-matter physics. These include Paul McEuen (Cornell), Andre Geim (Manchester) and Amir Yacoby (Harvard). And, of course, my PhD advisor Leo Kouwenhoven at Delft and my postdoc advisor Philip Kim at Harvard had a big influence in my formation. To a young researcher, I’d say: be adventurous and take risks. Try to follow your interests and don’t let others put a ceiling on your ambitions.

AM: I’ve been doing this a long time now. I have developed a great appreciation for the ability of experiment to surprise. My approach to doing fundamental theory in materials science is to try to identify exciting new phenomena that are likely to be observable experimentally if someone would just look. My intuition is based very much on known experimental results and by thinking about why different types of theoretical approximations are successful—or unsuccessful—in describing nature. Trying to deepen theoretical understanding of phenomena that have been observed but remain mysterious is nearly as much fun.

I would advise young researchers to develop their own independent way of thinking about their scientific area. Whenever you encounter something that you do not understand, dig deeper until you do. Most of the time, the new understanding is just a detail that is already understood by someone—but every now and again it will be something truly new.

Notes
Philip Ball writes for NSR from London.

The Author(s) 2022. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd.

https://academic.oup.com/nsr/article/9/4/nwac005/6506475?login=false

Here we go "novel light interacting"...like Mathis' C.F.? Article is from a few years ago in 2018:
-----------
Magic-angle graphene Superlattice

Apart from stacking two-dimensional building blocks on top of each other, the properties of Van der Waals heterostructures can be also tuned by introducing a twist angle between different layers. In particular, we create and study two-dimensional superlattices made of two graphene sheets rotated slightly, at a magic angle of 1.1 degrees.

Interestingly, this system exhibits strongly correlated electronic properties. For a carrier density that corresponds to half-filling of each set of degenerate supperlatice bands, the material behaves as a Mott insulator, showing correlated insulating or non-conducting states as a result of their bands being flat near zero Fermi energy. Upon electrostatic doping of the material away from these states, in other words when adding small amounts of electrons to the graphene supperlattice, zero-resistance states are found, turning our system into a superconductor with a critical temperature of approximately 1.7K.

Twisted bilayer graphene (TBG) is a precisely tunable, purely carbon-based, two dimensional superconductor which can be crucial for understanding strongly correlated phenomena such as the physics of high-critical-temperature superconductors or quantum spin liquids.

More recently, we have discovered magic-angle twisted trilayer graphene (MATTG), which can be tuned with electric displacement field, in addition to carrier density. It was revealed that this system could reach the ultra-strong coupling regime of superconductivity.

Our recent articles about the topic:
Superlattice-Induced Insulating States of TBG Rev. Mod. Phys. 117, 116804 (2016)
Correlated insulator behaviour at half-filling in TBG Nature 556, 80-84 (2018)
Unconventional superconductivity in TBG Nature 556, 43-50 (2018)
Tunable ultra-storngly coupled superconductivity in MATTG Nature 590, 249-255 (2021)

Two Dimensional Magnetism

http://jarilloherrero.mit.edu/wp-content/uploads/2018/10/Fig1A-page-001.jpg

The magnetic properties of a layered crystal can dramatically change when it is cleaved to the few-layer limit. One example is chromium triiodide (CrI3), a van der Waals magnetic insulator that displays out-of-plane ferromagnetic behavior in the monolayer. Surprisingly, it adopts an alternating ferromagnetic alignment in few-layer crystals, giving a layered antiferromagnetic ground state in the bilayer.

We use both transport and optical methods to probe and manipulate these ultrathin magnets with potential applications in spintronics. By electrostatic doping, we can electrically control the magnetic ground state of these materials. Moreover, we can electrically probe the magnetic state using van der Waals magnetic tunnel junctions fabricated from few-layer CrI3 crystals. This arises from the sensitive dependence of quantum electron tunneling on the barrier’s magnetic state. Using these tools, we can explore diverse magnetic ground states including multiferroics, quantum spin liquids, and frustrated magnetism in layered metal halides.

Our recent articles about the topic:
Discovery of ferromagnetism in monolayer CrI3 Nature 546,270-273 (2017)
Electrical control of magnetism in bilayer CrI3 Nature Nanotechnology 13, 544-548 (2018)
Tunneling as a probe of magnetism in few-layer CrI3 Science 360, 1218-1222 (2018)
Connecting magnetism and stacking order in CrCl3 Nature Physics , in press (2019)

Optics and optoelectronics of low dimensional materials

Low dimensional materials (LDMs), such as quantum dots, graphene, nanotubes, in which one or more spatial dimensions are strongly suppressed to restrict the quantum mechanical wave function of electrons inside, exhibit diverse and intriguing physical phenomena that are very different from their bulk counterparts. The quantum size effect alters the electronic band structure vastly, leading to novel light-matter interactions and allowing for dramatic electrical control and efficient detection of light. Moreover, recent progress in convenient assembly of LDMs with ultraclean interfaces, boost the emerging field in condensed matter physics focusing on versatile heterostructures with distinct electronic and optoelectronic properties from their original constituents. This has important implications both for the fundamental physics of electrons in low dimensional systems as well as for potential applications in photodetection and valleytronics.

General Reading of Review Articles:
Graphene photonics and optoelectronics Nature Photonics 4, 611 – 622 (2010) http://www.nature.com/nphoton/journal/v4/n9/full/nphoton.2010.186.html
Photodetectors based on graphene, other two-dimensional materials and hybrid systems Nature Nanotechnology 9, 780–793 (2014)
Two-dimensional material nanophotonics Nature Photonics 8, 899–907 (2014)

http://jarilloherrero.mit.edu/research/

Since Jarillo Herro is positing so much on Fermi. Here are a few Mathis' papers on Fermi...there is a lot of "Chaos Theory" in the background with unexpected properties related to Magic Angle graphene:

http://milesmathis.com/fermi.pdf

Miles Mathis wrote:
The Specific Heat Problem of Electrons another major mainstream fudge
by Miles Mathis
First published July 22, 2015
Updated July 28, 2015

This problem—also listed as Electron Heat Capacity—is another one the mainstream has been hiding for decades, one I wish to pull out of the closet and exhibit on the front lawn. The gatekeepers have hired hoards of people to try to waste my time in various ways to prevent me from writing any more of these papers, but their gambits are failing. Here I am again.

As we have seen in previous papers, almost as soon as the electron was discovered it was used as the field particle for nearly everything, including of course heat transfer and heat capacity. The Drude- Lorentz Model of 1900 tapped the electron as the basic field particle, and that has not changed in the 115 years since then. Although the mainstream now has ubiquitous evidence the electron is not the field particle, it keeps fudging the old equations over and over to convince students it is. The Drude- Sommerfeld model is now such a huge pile of finesses it should be an eternal embarrassment to any real scientist and to the fields of physics and chemistry, but rather than admit that they just keep finessing it. New finesses are added each year to answer new experiments, as we have seen.* We have looked at many later indications that the electron was not the field particle in those previous papers, but here we will look at the first indication—one that should have been (and almost was) fatal to electron theory. This was the fact that electrons did not add to specific heat. Using the theory of the time, it was expected that electrons should add appreciably to specific heat; but experiments could not show even 1% of that expectation. This was an early “catastrophe,” on a par with the later vacuum catastrophes and dark matter meltdowns. They tell you this problem was solved, but it wasn't. To prove that, we will look at some of the solutions posted by the mainstream on the internet. Hyperphysics.edu is the top listed answer to a search on this subject. Here is that answer: The electrons in the metal which contribute to conduction are very close to the Fermi level, " ripples on the Fermi sea". But to contribute to bulk specific heat, all the valence electrons would have to receive energy from the nominal thermal energy kT. But The Fermi energy is much greater than kT and the ovewhelming majority of the electrons cannnot receive such energy since there are no available energy levels within kT of their energy. Apparently Hyperphysics operates without Spell-check. It also apparently operates without Logic- check. That entire paragraph is just back-engineered speculation and we have no evidence to support it. In fact, we have about a century of evidence to refute it. To start with, the Fermi level is simply a level of energy in a given substance, also called chemical potential. That potential is then assigned by the mainstream to electrons arbitrarily, but if that assignment had been true, the theory shouldn't have needed a continuous pile of magical pushes over the decades. The magical pushes are precisely what should have indicated to honest physicists and chemists that the initial assignment was mistaken. As it turns out, the Fermi energy (and Fermi level) can just as easily be assigned to charge photons, and if that assignment is made we no longer need all the magical pushes like quantum tunneling, band structures, electron holes, ideal crystals, and so on. Assigning the Fermi energy to photons instead of electrons immediately simplifies all solid state theory, conduction theory, and heat theory by many orders of magnitude.

It also solves the electron problem of specific heat. If the electron isn't the field particle of either conduction or heat, then the original expectations vanish. This also ties into the problem of heat capacity, which I have already solved in a previous paper. See below where I gloss it again for good measure. Also notice how the explanation at Hyperphysics elides from conducted electrons to valence electrons.

But conducted electrons must be free: how else are they conducted from place to place? Valence electrons aren't free. The definition of a valence electron is “one that is associated with an atom.” Heat can't be transferred by electrons associated with atoms, unless they are proposing the atoms are dragged along in conduction as well. So whether or not valence electrons are responding to kT is beside the point. The original problem concerned the fact that conducted electrons were not adding to the heat, and that is even admitted at Hyperphysics. Where? In their statement of the original problem:

One of the great mysteries in physics in the early part of the 20th century was why electrons didn't appear to contribute to specific heat. How could they contribute to electrical conduction and heat conduction and not to specific heat?
If they are contributing to electrical conduction or heat conduction, they aren't valence electrons. So the entire Hyperphysics explanation is just misdirection. As more proof of that, we can compare the Hyperphysics explanation to the explanation at the number two site that comes up on a search. This is the site at Drexel University. There, it says this in the first box: When a metal specimen is heated from absolute zero, not every conduction electron gains an energy ~k T as expected classically.

See, they say “conduction electron.” That would be “free electron,” not “valence electron.” These major sites can't even fudge you in the same way on the same longstanding question. Beyond that, you can't have valence electrons in a Fermi gas, since a valence is a type of charge interaction. When talking of Fermi models, the fermions are non-interacting, which precludes charge interaction.
Here is the entire first box at Drexel:

First, they tell you the cause of the discrepancy is the Pauli exclusion principle. But then if you look at all the other stuff in the box, you find nothing to do with the Pauli exclusion principle. Like Hyperphysics, they try to trap 99% of the electrons “down here,” but that trapping isn't a function of the PEP. To start with, the PEP applies to electrons in orbitals or shells, not to free electrons. There is no reason free electrons can't have the same or very similar energies, since they aren't trying to occupy the same place. If they are the field particle of heat or conduction, then they must be conducted, which means that aren't in those orbitals. So the electrons “down there” are just a fiction created to fudge an answer here. Nothing in the old theory would indicate conducted electrons should be trapped in lower energy levels, and in fact they should occupy higher energy levels, simply because they are both “free” and electrons. Being the smallest fermions, the electrons should have the highest kinetic energies.

To see the continuation of this fudge, let us look at the second box at Drexel:
We are supposed to be getting a “qualitative argument,” but as you see we are just getting pushed math. Pushed math isn't a qualitative argument. Math is quantitative, last time I looked. The authors are just assuming what they are trying to prove, then writing equations for it. Not only is that not qualitative, it isn't an argument. Look closely: they say: “only a fraction can be excited thermally.” But we have no experimental evidence for that. They are just assuming that is the cause. They are assuming what they are expected to argue for. There is no argument or theory here, just a push to a conclusion. No mechanics is offered, just unsupported diagrams.

This reminds us that all this talk about the Fermi energy and Fermi level doesn't even apply to real substances. Both refer to the energy difference between the highest and lowest occupied single-particle states in a quantum system of non-interacting fermions at absolute zero temperature. In a Fermi gas the lowest occupied state is taken to have zero kinetic energy, whereas in a metal the lowest occupied state is typically taken to mean the bottom of the conduction band.

In real life, there are no such things as non-interacting fermions, and this is due to a little thing called charge. Fermions are charged, and the charge defines the interactions. For the same reason, there is no such thing as a “single-particle state.” Real particles don't create Fermi gases, or anything like them, not even near absolute zero. Just as they later fudged solid-state physics with ideal crystals, here they are fudging you with manufactured quantum systems which do not and cannot exist. Ask yourself why they would define these problems in terms of fake systems of fake particles? Why would you theorize about non-interacting fermions, when part of the definition of fermion was that they interacted strongly via charge? Isn't such theory simply perverse

http://milesmathis.com/pasta.pdf

Miles Mathis wrote:But after getting up for a sandwich and sitting on that idea for about 15 minutes, I saw my mistake.

The answer was staring me in the face, as usual. The charge field IS in the Fermi Pasta models, since they are using math from Chaos theory. Since Chaos theory comes from tweaking Newton's equations—which contain charge without anyone knowing it (see last link)—that is the problem there as well. In short, Newton's equations are unified field equations and they include the charge field. This is why the Lagrangian is a differential: the second term includes a charge field variation Newton missed, fine tuning the equation and making it match data much better. Although we are taught this was achieved with advanced mathematical analysis by Laplace and Lagrange, using differential equations, it actually wasn't. It was achieved by splitting Newton's field equations into two terms that, as a differential, were able to better express the field. In this form, the charge field was able to work in opposition to the gravity field, as it must. But since Laplace and Lagrange never understood their equations were unified, because Newton's equation had already been unified from the start, the whole field was thrown into chaos. The mathematicians of Perturbation theory and then Chaos theory thought they were dealing with various remaining inequalities, ie unknowns, when what they were dealing with was the charge field. The charge field was unknown in a way, but it wasn't an unknown in the sense Chaos theorists think, since it isn't unknowable. It isn't random and isn't therefore “chaotic”.

Now that we know what is causing the “inequalities”, we can dissolve these unknowns out of the field equations. Which is what I have done. So the problem with Fermi Pasta is that, like the Chaos theorists before them, they didn't understand that their equations included the charge field. They thought they had a fairly simple computer model, but they forgot to tell the computer that the equations included the charge field. The charge field then allowed feedback loops to work in the model, creating mysterious periodicities. But once you understand that the charge field is included in any equations that come from Newton's field, and understand how the charge field works at the ground level, all those mysteries dissolve.

New explanation emerges for robust superconductivity in three-layer graphene
15 Mar 2022 Isabelle Dumé

trilayer graphene superconductivity

Circular Fermi surfaces create a ring-like shape in trilayer graphene. (Courtesy: IST Austria)
Last year, experimentalists at Harvard University in the US made an unexpected discovery: three layers of graphene are better than two at conducting electricity without resistance. At the time, the reasons for this unusual superconducting behaviour were unclear. Now, however, theorists in Austria and Israel have come up with an explanation that sheds more light on the origins of superconductivity in trilayer graphene, while also helping to explain other anomalies in recent experiments on this 2D carbon material.

Graphene is an atomically-thin sheet of carbon atoms arranged in a 2D hexagonal lattice. When two sheets of graphene are placed on top of each other and slightly misaligned, the positions of the atoms form a moiré pattern or “stretched” superlattice that dramatically changes the interactions between their electrons. The degree of misalignment is very important: in 2018, researchers at the Massachusetts Institute of Technology (MIT) discovered that at a “magic” angle of 1.1°, the material switches from being an insulator to a superconductor. The explanation for this is behaviour is that, as is the case for conventional superconductors, electrons with opposite spins pair up to form “Cooper pairs” that then move though the material without any resistance below a certain critical transition temperature Tc (in this case, 1.7 K).

Three years later, the Harvard experimentalists observed something similar happening in (rhombohedral) trilayer graphene, which they made by stacking three sheets of the material at small twist angles with opposite signs. In their work, the twist angle between the top and middle layer was 1.5° while that between the middle and bottom layer was -1.5°. By measuring the current that circulated through the layers when a voltage was applied to them, the researchers found that trilayer graphene remains superconducting at higher temperatures than is the case in two-layer graphene. Indeed, the measured Tc was about 40% higher, at 2.3 K. They also found evidence of something even more surprising: superconductivity in the trilayer system seemed to involve strong interactions between electrons, rather than weak ones as is the case in most conventional superconductors.

Natural allotrope of carbon
The bilayer and trilayer graphene materials in these studies must be constructed layer by layer, and their mechanical instability introduces certain complications for scientists seeking to explain their properties. For this reason, theorists led by Maksym Serbyn and Areg Ghazaryan from the Institute of Science and Technology (IST) Austria and Erez Berg and Tobias Holder from Israel’s Weizmann Institute of Science chose instead to study superconductivity in crystalline rhombohedral trilayer graphene. Although rare, this allotrope of carbon does occur naturally, and is mechanically stable.

In this material, the researchers identified two further phenomena that they say are difficult to reconcile with conventional superconductivity. The first, Ghazaryan explains, is that above a threshold temperature of about 13 K, the electrical resistance of the material should increase linearly with increasing temperature. However, previous experimental work found that the rate of increase remained constant up to 23 K.

READ MORESpin-orbit coupling in graphene
Magic-angle graphene switches from superconductor to ferromagnet

The second phenomenon is that pairing between electrons of opposite spin implies a coupling that contradicts another experimentally observed feature: the presence of a nearby configuration in which the spins are fully aligned – that is, magnetism. “In our work, we show that both these observations can be explained if one assumes that an interaction between electrons provides the ‘glue’ that holds electrons together,” Serbyn adds. “This leads to unconventional superconductivity.”

The theorists also found that the material has two so-called Fermi surfaces (the boundary between occupied and unoccupied electron energy states that defines many of properties of metals and semiconductors) with a circular shape. According to a theory from the 1960s, such circular surfaces favour a mechanism for superconductivity based only on electron interactions.

The findings are detailed in Physical Review Letters.

https://physicsworld.com/a/new-explanation-emerges-for-robust-superconductivity-in-three-layer-graphene/

Looks like 2023 may be the year of customized heterostructures for overcoming "challenges":
--------

Graphene and 2D materials

The isolation of graphene at The University of Manchester led to the discovery of a whole family of 2D materials, including hexagonal boron nitride and molybdenum disulphide.

These can be combined with graphene to create new 'designer materials' to produce applications originally limited to science fiction.

   It is probably fair to say that research on 'simple graphene' has already passed its zenith.

   — Professor Sir Andre Geim, Nature 2013

Designer materials

Combinations of these 2D materials are called heterostructures - tiny towers with different layers of different materials. Any combination is possible, which means new materials can be built from the ground up on an atomic level to create materials tailored to exact functions.

-----

The challenges

   Globally, corrosion costs more than $2 trillion per year (World Corrosion Organization).
   Approximately 3.5 million people die each year due to inadequate water supply, sanitation and hygiene (United Nations).
   Businesses that produce and process materials make up 15% of the UK's GDP (Policy Exchange).
   Transport accounts for a fifth of the UK's carbon emissions (Department of Energy and Climate Change).
   New nuclear build is valued at £60 billion in the UK (Department for Business, Innovation and Skills and Department of Energy and Climate Change).
   Just 5.2% of UK energy consumption in 2013 was provided by renewable sources (Department of Energy and Climate Change).
   Deep-sea platforms drill for oil 10km below the seabed, at temperatures of more than 200°F and under pressures of 20,000psi (BP).
   Salt canopies above drilling sites can be taller than Mount Kilimanjaro (BP).

How are we tackling them?

   Single-layer graphene is a million times thinner than a human hair and will revolutionise healthcare, water and consumer electronics.
   Dalton Nuclear Institute's paper on welding for nuclear new build received more than 230 citations over a decade.
   BP has four senior staff permanently on site at the University, giving them an immediate pipeline to our expertise.
   We're home to the global knowledge base in 2D materials, with over 300 people working on graphene, two Nobel laureates and more than £170 million of current investment.
   Our 3D characterisation capability is enabling us to study the properties of new protective coatings for materials such as aluminium used in planes.
   An aero engine, developed by Rolls-Royce with the University, is 25% more fuel efficient than its closest competitor.

https://www.graphene.manchester.ac.uk/learn/advanced-materials/

------------------------

Graphene, 2D Materials and Carbon Nanotubes Market Size in 2023 SWOT Analysis by Top Players till 2028

Published: Jan. 11, 2023 at 1:53 p.m. ET

Jan 11, 2023 (The Expresswire) -- ""Graphene, 2D Materials and Carbon Nanotubes Market"" Research report Insights 2023 | Number of Tables and Figures [ 186 ] Graphene, 2D Materials and Carbon Nanotubes Market cover market size for segment by Applications [3D Printing, Aerospace, Automotive, Electronics, Oil And Gas, Others], By Types [Carbon Nanotubes, Graphene, Other 2-D Materials ] offers wide-ranging forecasts from 2023-2028. According to Our Research the [ Chemical and Material ] Industry is dominated and accounted for the major income share In 2022. It describes the gap betweensupply and consumption,tables and figures,SWOT analysisof theleading Key Playersin the Graphene, 2D Materials and Carbon Nanotubes Report.

New Report Spread Across [ 135 Pages ].

Who is the key manufacturer in the Global?

Some of the PROMINENT PLAYERS in the global Graphene, 2D Materials and Carbon Nanotubes market include:

● 2D Carbon (Changhzou)
● Abalonyx
● Advanced Graphene Products
● AIST
● Alpha Assembly
● AMO
● anderlab Technologies
● Angstron
● Applied Graphene Materials
● Arkema
● AzTrong
● Bayer
● biDimensional
● Birla Carbon
● Bluestone Global Tech
● Bosch
● Brewer Science
● BTU International
● Cabot
● Cambridge Graphene Centre
● Cambridge Nanosystems


Get a Sample Copy of the Graphene, 2D Materials and Carbon Nanotubes Market Report 2023

What are Industry Insights?

MWCNTs are mainly produced using the C-CVD process (catalytic chemical vapor deposition). The evolution of accumulated global production for MWCNTs is shown below. Note here that the commercialization efforts start around 2005/2006. The super hype then sets in, leading to a rush to install capacity. This pushes the industry into a state of overcapacity, and still worse, pushes many to produce a CNT that is not good enough to meaningfully displace carbon black or similar.
Report Overview
Due to the COVID-19 pandemic and Russia-Ukraine War Influence, the global market for Graphene, 2D Materials and Carbon Nanotubes estimated at USD million in the year 2023, is projected to reach a revised size of USD million by 2028, growing at a CAGR of % during the forecast period 2023-2028.
CNTs are almost thirty years old already. In this time, they have gone through almost the entire hype curve, rising from their academic origins toward their peak of hype before nearly disappearing into the valley of disillusionment. CNTs have however been making a quiet comeback and have now indeed entered a phase of volume growth.

The report includes six parts, dealing with:

1.) Basic Information

2.) Asia Graphene, 2D Materials and Carbon Nanotubes Market

3.) North American Graphene, 2D Materials and Carbon Nanotubes Market

4.) European Graphene, 2D Materials and Carbon Nanotubes Market

5.) Market Entry and Investment Feasibility

6.) Report Conclusion

Get a Sample PDF of report -https://www.marketgrowthreports.com/enquiry/request-sample/21991567

The Global Graphene, 2D Materials and Carbon Nanotubes market is likely to growth at a substantial rate during the forecast period, between 2023 and 2028. In 2023, the market is growing at a steady rate and with the increasing adoption of tactics by key players, the market is predicted to rise over the projected horizon.

The report is divided into three parts:

Part I- Market Overview

Part II- Market Data

Part III- Strategic Recommendations

Graphene, 2D Materials and Carbon Nanotubes Market - Competitive and Segmentation Analysis:

This Graphene, 2D Materials and Carbon Nanotubes Market report offers detailed analysis supported by reliable statistics on sale and revenue by players for the period 2015-2023. The report also includes company description, major business, Graphene, 2D Materials and Carbon Nanotubes product introduction, recent developments and Graphene, 2D Materials and Carbon Nanotubes sales by region, type, application and by sales channel.

Report further studies the market development status and future Graphene, 2D Materials and Carbon Nanotubes Market trend across the world. Also, it splits Graphene, 2D Materials and Carbon Nanotubes market Segmentation by Type and by Applications to fully and deeply research and reveal market profile and prospects. (more at link below)

https://www.marketwatch.com/press-release/graphene-2d-materials-and-carbon-nanotubes-market-size-in-2023-swot-analysis-by-top-players-till-2028-latest-report-2023-01-11