Mathis on Graphene? Any hints?

Page 2 of 4 Previous  1, 2, 3, 4  Next

Go down

Re: Mathis on Graphene? Any hints?

Post by Cr6 on Sat Mar 10, 2018 2:18 am

This came out recently...might need to review Miles' paper one more time to see if the charge field aligns differently with a "twist":
-------
(more at link....)

Give double-layer graphene a twist and it superconducts
A ‘magic angle’ lets electrons flow freely

By Emily Conover
4:10pm, March 8, 2018
graphene

DOUBLE UP  A device made of two layers of graphene (illustrated) can conduct electricity without resistance when one layer is rotated relative to the other.

Theasis/istockphoto

LOS ANGELES — Give a graphene layer cake a twist and it superconducts — electrons flow freely through it without resistance. Made up of two layers of graphene, a form of carbon arranged in single-atom-thick sheets, the structure’s weird behavior suggests it may provide a fruitful playground for testing how certain unusual types of superconductors work, physicist Pablo Jarillo-Herrero of MIT reported March 7 at a meeting of the American Physical Society.

The discovery, also detailed in two papers published online in Nature on March 5, could aid the search for a superconductor that functions at room temperature, instead of the chilly conditions required by all known superconductors. If found, such a substance could replace standard conductors in various electronics, promising massive energy savings.

Layered graphene’s superconductivity occurs when the second layer of graphene is twisted relative to the first, at a “magic angle” of about 1.1 degrees, and when cooled below 1.7 kelvins (about –271° Celsius). Surprisingly, Jarillo-Herrero and colleagues report, the same material can also be nudged into becoming an insulator — in which electrons are stuck in place — by using an electric field to remove electrons from the material. That close relationship with an insulator is a characteristic shared by certain types of high-temperature superconductors, which function at significantly warmer temperatures than other superconductors, although they still require cooling.

https://www.sciencenews.org/article/give-double-layer-graphene-twist-and-it-superconducts
....


APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session K35: 2D Materials - Superconductivity and Charge Density Waves I
8:00 AM–11:00 AM, Wednesday, March 7, 2018
LACC Room: 409B
Sponsoring Unit: DMP
Chair: Daniel Rhodes, Columbia Univ
Abstract: K35.00007 : Topology, correlations, and superconductivity in 2D
9:12 AM–9:48 AM
Abstract Presenter: Pablo Jarillo-Herrero (Physics, MIT)

In this talk I will review our recent quantum electronic transport experiments in a variety of 2D materials and van der Waals heterostructures, where we show interplay between topology, strong electron-electron correlations and electrically tunable superconductivity. In some of these materials, different phases can be achieved simply by tuning the electric field applied to the material.

http://meetings.aps.org/Meeting/MAR18/Session/K35.7

....

(more at link....)

Some high-temperature superconductors might not be so odd after all
Finding hidden swirls of electric current shows that the material’s behavior matches standard theory

By Emily Conover
7:00am, December 8, 2017
illustration of a superconductor

VORTEX FOUND Newly observed swirls of electric current in a high-temperature superconductor (shown in an artist’s conception) may indicate that the unusual material fits within the standard theoretical picture.

©️ XAVIER RAVINET/UNIGE
Magazine issue: Vol. 193, No. 1, January 20, 2018, p. 11

A misfit gang of superconducting materials may be losing their outsider status.

Certain copper-based compounds superconduct, or transmit electricity without resistance, at unusually high temperatures. It was thought that the standard theory of superconductivity, known as Bardeen-Cooper-Schrieffer theory, couldn’t explain these oddballs. But new evidence suggests that the standard theory applies despite the materials’ quirks, researchers report in the Dec. 8 Physical Review Letters.

All known superconductors must be chilled to work. Most must be cooled to temperatures that hover above absolute zero (–273.15° Celsius). But some copper-based superconductors work at temperatures above the boiling point of liquid nitrogen (around –196° C). Finding a superconductor that functions at even higher temperatures — above room temperature — could provide massive energy savings and new technologies (SN: 12/26/15, p. 25). So scientists are intent upon understanding the physics behind known high-temperature superconductors.

When placed in a magnetic field, many superconductors display swirling vortices of electric current — a hallmark of the standard superconductivity theory. But for the copper-based superconductors, known as cuprates, scientists couldn’t find whirls that matched the theory’s predictions, suggesting that a different theory was needed to explain how the materials superconduct. “This was one of the remaining mysteries,” says physicist Christoph Renner of the University of Geneva. Now, Renner and colleagues have found vortices that agree with the theory in a high-temperature copper-based superconductor, studying a compound of yttrium, barium, copper and oxygen.

Vortices in superconductors can be probed with a scanning tunneling microscope. As the microscope tip moves over a vortex, the instrument records a change in the electrical current. Renner and colleagues realized that, in their copper compound, there were two contributions to the current that the probe was measuring, one from superconducting electrons and one from nonsuperconducting ones. The nonsuperconducting contribution was present across the entire surface of the material and masked the signature of the vortices.

Subtracting the nonsuperconducting portion revealed the vortices, which behaved in agreement with the standard superconductivity theory. “That, I think, is quite astonishing; it's quite a feat,” says Mikael Fogelström of Chalmers University of Technology in Gothenburg, Sweden, who was not involved with the research.

https://www.sciencenews.org/article/some-high-temperature-superconductors-might-not-be-so-odd-after-all

Cr6
Admin

Posts : 1099
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

Post by Cr6 on Sat Mar 10, 2018 3:11 am

It is curious that the bigger "lattice" type molecules often display "superconducting" properties at very cold temps. I wonder if at cold temps/high pressure they just become charge-field tamped down "blocks" and certain charge field streams then just penetrate across these non-cycling or reduced cycling atom-blocks (charge field reduced cold atoms). Basically, "recycling" per Mathis stops and then flow occurs around the "semi-frozen" atoms? In many of these cases, the charge cycling balance is off.


(Miles)
Europium
Atomic Number: 63

240c. Period 6 Why Isn't Hafnium a Noble Gas?
http://milesmathis.com/haf.pdf
We have more evidence of my diagrams from Europium, whose density goes way down compared to Samarium. That is because Samarium has gone all blue in the inner levels, with two protons on both sides of each inner hole. That only gives us four parcels of charge through a hole that can take five, but remember, this 5-stack contains a single proton, and that proton is not part of an alpha. This means that there is charge leakage around that inner proton (in the sandwich), so the 5-stack can't really channel 5 proton's worth of charge. The inner proton channels, but it doesn't spin up the charge a fifth amount. So the charge strength of the 5-stack stays at 4. Therefore, Europium is actually at its inner limit. It can't put any more protons in the inner levels. So it switches to a different plan, one more like we saw with Dysprosium:

We can see why that is considerably less dense, since it has more mass out in the 4th level and less on the axis. We can also see that Europium now has enough protons to work with that it can bump up all the numbers in the 4th level by one. In this way, it avoids having the same number at each pole. Instead of 1 North and two South, it has 2 north and 3 South. That solves the problem of equal charge.

This means that once again Europium isn't doing what it is doing to find a +3 oxidation number. That is just a side-effect of a deeper mechanics.

....
Notes on Dysprosium as SC:


Journal of Inorganic and Organometallic Polymers and Materials

September 2012, Volume 22, Issue 5, pp 1081–1086 | Cite as
Growth of the Dysprosium–Barium–Copper Oxide Superconductor Nanoclusters in Biopolymer Gels

Clusters of DyBa2Cu3O7−y high TC type II nanosuperconductor were prepared by sol–gel method in the presence of biopolymer chitosan. At the first step, the precursor and biopolymer were aggregated into amorphous matrix and then hydrogels were formed by thermogelling. Nucleation and growth of discrete nanoparticles is controlled by the biopolymer gel owing to retention of the fibrous nature of the chitosan at high temperatures up to 500 °C. After heating to 900 °C and complete decomposition of BaCO3, nanoparticles of DyBa2Cu3O7−y superconductor with diameter of 10–20 nm in the form of nanoclusters are prepared. Critical temperature (Tc) of the nanoparticles was found to be above 83 K. Characterizations of specimens were performed using scanning electron microscopy and transmission electron microscopy, supported by other techniques including XRD diffraction, energy dispersive X-ray, FT-IR spectrum and magnetic susceptibility measurements.

https://rd.springer.com/article/10.1007%2Fs10904-012-9687-7

July 2016, Volume 29, Issue 7, pp 1787–1791 | Cite as
The Effect of Dy Doping on the Magnetic Behavior of YBCO Superconductors

Abstract

The effect of dysprosium (Dy) doping on yttrium barium copper oxide (YBCO) prepared by conventional solid-state reaction method has been investigated by means of XRD, AC susceptibility, and DC magnetization measurements. AC susceptibility measurements for sintered YBCO pellets have been performed as a function of temperatures at constant frequency and AC field amplitude in the absence of a DC bias field. DC magnetization measurements were done at 5, 20, and 77 K upon zero field cooling (ZFC) process. The magnetization measurements showed a paramagnetic behavior existing at high magnetic fields. The magnetic field dependence of critical current density of the samples has been estimated from DC magnetization data. The partial Dy substitution for Y on YBCO superconductors improves the bulk critical current density at high magnetic fields and at high-temperature regions (higher than 20 K).

https://link.springer.com/article/10.1007/s10948-016-3493-3

....
Posted: May 16, 2009
Europium found to be a superconductor
(Nanowerk News) Of the 92 naturally occurring elements, add another to the list of those that are superconductors.

James S. Schilling, Ph.D., professor of physics in Arts & Sciences at Washington University in St. Louis, and Mathew Debessai, Ph.D., — his doctoral student at the time — discovered that europium becomes superconducting at 1.8 K (-456 °F) and 80 GPa (790,000 atmospheres) of pressure, making it the 53rd known elemental superconductor and the 23rd at high pressure.

Debessai, who received his doctorate in physics at Washington University's Commencement May 15, 2009, is now a postdoctoral research associate at Washington State University.

"It has been seven years since someone discovered a new elemental superconductor," Schilling said. "It gets harder and harder because there are fewer elements left in the periodic table."


This discovery adds data to help improve scientists' theoretical understanding of superconductivity, which could lead to the design of room-temperature superconductors that could be used for efficient energy transport and storage.

The results are published in the May 15, 2009, issue of Physical Review Letters in an article titled "Pressure-induced Superconducting State of Europium Metal at Low Temperatures."

Schilling's research is supported by a four-year $500,000 grant from the National Science Foundation,Division of Materials Research.

Europium belongs to a group of elements called the rare earth elements. These elements are magnetic; therefore, they are not superconductors.

"Superconductivity and magnetism hate each other. To get superconductivity, you have to kill the magnetism," Schilling explained.

Of the rare earths, europium is most likely to lose its magnetism under high pressures due to its electronic structure. In an elemental solid almost all rare earths are trivalent, which means that each atom releases three electrons to conduct electricity.

"However, when europium atoms condense to form a solid, only two electrons per atom are released and europium remains magnetic. Applying sufficient pressure squeezes a third electron out and europium metal becomes trivalent. Trivalent europium is nonmagnetic, thus opening the possibility for it to become superconducting under the right conditions," Schilling said.

Schilling uses a diamond anvil cell to generate such high pressures on a sample. A circular metal gasket separates two opposing 0.17-carat diamond anvils with faces (culets) 0.18 mm in diameter. The sample is placed in a small hole in the gasket, flanked by the faces of the diamond anvils.
Pressure is applied to the sample space by inflating a doughnut-like bellow with helium gas. Much like a woman in stilettos exerts more pressure on the ground than an elephant does because the woman's force is spread over a smaller area, a small amount of helium gas pressure (60 atmospheres) creates a large force (1.5 tons) on the tiny sample space, thus generating extremely high pressures on the sample.

Unique electrical, magnetic properties

Superconducting materials have unique electrical and magnetic properties. They have no electrical resistance, so current will flow through them forever, and they are diamagnetic, meaning that a magnet held above them will levitate.

These properties can be exploited to create powerful magnets for medical imaging, make power lines that transport electricity efficiently or make efficient power generators.

However, there are no known materials that are superconductors at room temperature and pressure. All known superconducting materials have to be cooled to extreme temperatures and/or compressed at high pressure.

"At ambient pressure, the highest temperature at which a material becomes superconducting is 134 K (-218 °F). This material is complex because it is a mixture of five different elements. We do not understand why it is such a good superconductor," Schilling said.

Scientists do not have enough theoretical understanding to be able to design a combination of elements that will be superconductors at room temperature and pressure. Schilling's result provides more data to help refine current theoretical models of superconductivity.

"Theoretically, the elemental solids are relatively easy to understand because they only contain one kind of atom," Schilling said. "By applying pressure, however, we can bring the elemental solids into new regimes, where theory has difficulty understanding things.

"When we understand the element's behavior in these new regimes, we might be able to duplicate it by combining the elements into different compounds that superconduct at higher temperatures."
Schilling will present his findings at the 22nd biennial International Conference on High Pressure Science and Technology in July 2009 in Tokyo, Japan.

https://www.nanowerk.com/news/newsid=10666.php

Cr6
Admin

Posts : 1099
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

Post by Cr6 on Sat Mar 10, 2018 3:37 am

28 August 1993
Technology: US Navy superconductor sails past Britain

By ELISABETH GEAKE

The US Navy is pioneering the use of high-temperature superconductors
in working machinery with a newly delivered sonar system. Now British scientists
are urging the government to fund more research into superconductivity.

Meanwhile the American Superconductor Corporation of Westboro, Massachusetts,
and the US Naval Undersea Warfare Center, based in Connecticut, have finished
a prototype sonar system incorporating bismuth strontium copper oxide, which
is superconducting at 73 K (-200 °C). It was delivered two years ahead
of schedule.

Nick Kerley of Oxford Instruments in Eynsham says, ‘It is a very good
demonstrator for high-temperature superconductors. The message (is) that
real practical devices are beginning to appear on the horizon – it is no
longer a dream.’ He was among the group who visited the US, and says Britain
must now decide on a device which is within its capabilities, and start
making a prototype by the end of the year. ‘I’d like to think the DTI would
be influenced and call for demonstration proposals,’ he says. Tim Button
of ICI Superconductors in Billingham, Cleveland, thinks the DTI may act
more quickly now, but he says: ‘I always felt it was urgent anyway. The
science isn’t any better in the US but the transfer into usable technology
is strides ahead of what we could do.’

The American instrument is an acoustic transducer, which converts electrical
energy into sound waves. It is used in sensitive sonar systems probing shallow
seas, such as the Persian Gulf. The company says this is the first time
high-temperature superconductors, which lose all their electrical resistance
at temperatures below about 113 K, have been combined with the ordinary
room-temperature electronics that control them. The transducer, which cost
$800 000, is similar to a hi-fi speaker: it has a coil made of the superconductor
and generates a magnetic field when an alternating electric current passes
through it. Inside the coil is a rod of terbium dysprosium, which is a magneto-strictive
material – its length changes with the magnetic field. The rod is connected
to pistons immersed in the sea, whose movements generate sound waves. The
US Navy has tested it successfully in a 30-metre-deep test lake.

The superconductor is refrigerated at between 50 and 70 K, which is
also the optimum operating temperature for the terbium rod. ‘The marriage
of the high-temperature superconductor and the terbium dysprosium gives
rise to low frequencies and high powers not available before
,’ says Greg
Yurek, the company president. This means the system can detect quiet submarines.
A coil made from an ordinary conductor such as copper would overheat from
the electric current, while one made of a low-temperature superconductor,
which is only superconducting up to about 10 K, would be too expensive to
cool.

https://www.newscientist.com/article/mg13918883-000-technology-us-navy-superconductor-sails-past-britain/

Cr6
Admin

Posts : 1099
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

Post by Cr6 on Sat Mar 10, 2018 3:41 am

Terbium–neodymium co-doping in Bi sites on the BPSCCO bismuth cuprate superconductor

Morsy M A Sekkina, Hosny A El-Daly and Khaled M Elsabawy1

Published 19 November 2003 • 2004 IOP Publishing Ltd
Superconductor Science and Technology, Volume 17, Number 1

Abstract

The parent BiPbSr2Ca2Cu3O10 (BPSCCO) and samples of the general formula Bi1-(x+y)NdxTbyPbSr2Ca2Cu3O10, where x = y = 0.05, 0.1 and 0.2, were prepared using the conventional high-temperature solid-state reaction technique. The superconducting measurements proved that the best value of Tc, 108 K, is for the sample with x = y = 0.1 mol% while the lowest value of Tc, 101 K, is for the sample with maximum dopant concentration x = y = 0.2 mol%. The evaluated crystalline lattice structure of the prepared samples mainly belongs to the superconductive tetragonal phase (2223) besides the secondary (2212) phase. Also, thermogravimetric and differential thermal analyses were studied on the green mixture showing an endothermic peak at 790 °C corresponding to the superconductive phase formation. The microstructures of the prepared samples were investigated by scanning electron microscopy and energy dispersive x-ray analysis.

http://iopscience.iop.org/article/10.1088/0953-2048/17/1/016

Cr6
Admin

Posts : 1099
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

Post by Cr6 on Sat Mar 10, 2018 4:00 am

The Ytterby Elements

About a half hours drive out of Stockholm, Sweden, there’s a tiny nondescript town. During the 18th – 20th centuries there was slightly more life here, with a mine operating out of the town.


Ytterby mine, roughly 1910. Courtesy of Tekniska museet on Flickr

One day in 1787, Carl Arrhenius, an army lieutenant, discovered a strange, unusually heavy black ore in the mine. Over the next 100 years, many different elements where discovered from this one ore, later named gadolinite.

These elements are all “rare earth” elements, which, despite the name, are not particularly rare. Their rarity comes more from the difficulty in separating them from each other. It took many scientists many years of research and arguing to discover them all.

Now the little town of Ytterby is not the first place you’d pick to hold a scientific record. Yet it does, as it’s the origin of the names of not one, but four elements. Of the elements in the original gadolinite ore, yttrium, terbium, erbium, and ytterbium all ended up named after the town.

Yttrium

The first element in Ytterby’s story is yttrium. A Finnish chemist, Johan Gadolin, received a sample of the mysterious black ore from Arrhenius. He isolated an unknown element – which later become known as yttrium.

Yttrium’s claim to fame is a discovery made two centuries later in 1987. Yttrium barium copper oxide, a material containing yttrium, is a superconductor at “high” temperatures. Superconductors are strong magnets at low temperatures, and are used in places like in MRIs in hospitals. It is worth noting that to scientists researching superconductors, -180ºC is a high temperature, although to anyone else on the planet it seems freezing. This is because to reach these temperatures, it needs liquid helium cooling. As liquid helium is both expensive and in short supply, hopefully a material like this will soon replace conventional superconductors in MRIs and scientific instruments.

A superconductor becomes magnetic at low temperatures and floats above another magnet. Courtesy of Trevor Prentice on Flickr

Terbium

Further along the table lives terbium. Another Swedish man called Carl, this time Carl Gustaf Mosander, found two more elements in the black gadolinite. A thoroughly unimaginative guy, he named all three after the mine where they were first found – naming Gadolin’s element yttrium, and the other two terbium and erbium.

Terbium is another rare-earth, and is often used in TV screens to make yellow and green phosphors. It’s also mixed with blue and red phosphors to make white light in LEDs.
SONY DSC

Terbium sulfate glowing green under UV light. Terbium green is often used in TV phosphors. Credit Chemical Elements, A Virtual Museum

Terbium is versatile, and can also be used in single molecules that act as tiny bar magnets. Usually magnets need hundreds and thousands of individual atoms or molecules. One of these terbium molecules can instead act as a magnet on its own.

The reason why this is important is due to the device you’re reading this on. The storage in computers and phones is made up of tiny little magnetic areas coded with data. These magnetic bits have gotten smaller and smaller as well as faster with time, as computers have gone from filling an entire room to fitting in your pocket. We are now reaching the limit of how small we can make these magnets, but imagine if single molecules could be used instead! With terbium, the dream of minuscule computers may come true.

Erbium

Now erbium is just confusing! The name, as you may have noticed, is very close to that of terbium. This led to some mishaps when the two where first discovered.

Out of the black gadolinite came different coloured element oxides. The original “erbium” was the yellowish coloured stuff, while “terbium” the bright pink stuff. These two were simple enough to separate from the white yttrium. However, when they were discovered people weren’t convinced that they were two separate elements, and by the time they sorted it out, the original names were switched!

These days, erbium refers to the element whose salts are a beautiful rose-pink. If you buy a pair of rose-coloured glasses, chances are there’s erbium!

Small amounts of erbium are also used with yttrium in Er:YAG lasers. They’re useful because the light from these lasers doesn’t travel through the human body. They are therefore useful for dermatology and dentistry, where only the skin or surface of a tooth needs to be treated.

A rare earth (neodymium and yttrium Nd:YAG) laser. Similar lasers are made with erbium or ytterbium and yttrium. Courtesy of Ben Williams on Flickr

Ytterbium


Finally, we have ytterbium. Ytterbium was discovered a lot later than the other three elements, from a sample of erbium by a Swiss chemist Jean Charles Galissard de Marignac. Just like Mosander, he had little imagination, and decided again to name the element after the town.

Ytterbium is also used like erbium in yttrium-based Yb:YAG lasers. Its most interesting use though is in atomic clocks.

Atomic clocks use a vibrating atom to keep time. The ytterbium clock is even more accurate than the caesium atomic clock currently used to define the second. This improved accuracy means that super super fast things in earth sciences and astronomy can be measured.

The Ytterby elements in the lab. Author’s own

Countless other applications of these Ytterby elements exist, and they’re the focus of a lot of research. Any dedicated rare-earth chemist will eventually make the trip to Ytterby to visit the birthplace of these versatile elements.

However, the story doesn’t end with these four. From gadolinite and the mine in Ytterby, 6 other rare-earth elements were discovered:

Scandium: named after Scandinavia

Gadolinium: named after Johan Gadolin

Dysprosium: from the Greek “hard to find”

Holmium: named after Stockholm county, where Ytterby is found

Thulium: after “Thulia”, a Greek name for Scandinavia

Lutetium: from the Latin name for Paris (where the mineral sample was analysed)

https://blogs.unimelb.edu.au/sciencecommunication/2016/10/15/the-ytterby-elements/

Cr6
Admin

Posts : 1099
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

Post by Cr6 on Sat Mar 10, 2018 4:11 am

Polonium's Superconductivity
-------

Quasi-two-dimensional superconductivity from dimerization of atomically ordered AuTe2Se4/3 cubes
J. G. Guo ORCID: orcid.org/0000-0003-3880-30121, X. Chen ORCID: orcid.org/0000-0003-3477-22041,2, X. Y. Jia3, Q. H. Zhang1, N. Liu1,2, H. C. Lei4, S. Y. Li3,5, L. Gu1,6,7, S. F. Jin1,6 & X. L. Chen1,6,7

Nature Communicationsvolume 8, Article number: 871 (2017)
doi:10.1038/s41467-017-00947-0
Download Citation

Abstract

The emergent phenomena such as superconductivity and topological phase transitions can be observed in strict two-dimensional (2D) crystalline matters. Artificial interfaces and one atomic thickness layers are typical 2D materials of this kind. Although having 2D characters, most bulky layered compounds, however, do not possess these striking properties. Here, we report quasi-2D superconductivity in bulky AuTe2Se4/3, where the reduction in dimensionality is achieved through inducing the elongated covalent Te–Te bonds. The atomic-resolution images reveal that the Au, Te, and Se are atomically ordered in a cube, among which are Te–Te bonds of 3.18 and 3.28 Å. The superconductivity at 2.85 K is discovered, which is unraveled to be the quasi-2D nature owing to the Berezinsky–Kosterlitz–Thouless topological transition. The nesting of nearly parallel Fermi sheets could give rise to strong electron–phonon coupling. It is proposed that further depleting the thickness could result in more topologically-related phenomena.

Introduction

The dimensional reduction or degeneracy usually induces the significant change of electronic structure and unexpected properties. The monolayer, interface and a few layers of bulky compounds are typical resultant forms of low dimensionality. The two dimensional (2D) material, for instance, graphene, is found to have a linear energy dispersion near Fermi energy (EF) and possess a number of novel properties1,2,3. Monolayer MoS2 exhibits a direct energy gap of 1.8 eV4 and pronounced photoluminescence5, in contrast to trivial photo-response in bulky MoS2 with an indirect band-gap.

2D superconductivity (SC), a property closely related to dimensional reduction, has been observed in a variety of crystalline materials like ZrNCl6, NbSe27, and MoS28 recently through the electric-double layer transistor (EDLT)9 technique. Many emerged properties, i.e., the well-defined superconducting dome, metallic ground state and high upper critical field6, 8, significantly differ from those of intercalated counterparts. Besides, the lack of in-plane inversion symmetry in the outmost layer of MoS2/NbSe2 with strong Ising spin-orbital coupling induces a valley polarization7, 8. In the scenario of low-dimensional interface, the unexpected 2D SC10, 11, the remarkable domed-shaped superconducting critical temperature (Tc)12, pseudo-gap state13, and quantum criticality14 have been demonstrated in La(Al,Ti)O3/SrTiO3(001) film. Very recently, the interface between Bi2Te3 and FeTe thin films displayed 2D SC evidenced by Berezinsky–Kosterlitz–Thouless (BKT) transition at 10.1 K15. The tentative explanations are related to the strong Rashba-type spin–orbit interactions in the 2D limit.

At the moment, the way to fabricating low-dimensional materials generally involves molecule beam epitaxy and exfoliation from the layered compounds. The top-down reduction processes usually are sophisticated and time consuming for realizing scalable and controllable crystalline samples. There are other chemical routes to tuning dimensionality by means of either changing the size of intercalated cations or incorporating additional anions. It is reported that increasing the size of alkaline-earth metals Ae (Ae=Mg, Ca, and Ba) between [NiGe] ribbons can reduce three dimensional (3D) structure to quasi-1 dimensional one16. In addition, the ternary CaNiGe can be converted into ZrCuSiAs-type CaNiGeH by forming additional Ca–H bonds, which exhibits different properties owing to the emergence of 2D electronic states17. The metastable Au1−x Te x (0.6 < x < 0.85) show an α-type polonium structure18, 19, in which the Au and Te disorderly locate at the eight corners of a simple cubic unit cell. The Tc fluctuates in the range of 1.5–3.0 K, but the mechanism of SC has been barely understood20. Besides, the equilibrium phase AuTe2, known as calaverite, is a non-superconducting compound, in which distorted AuTe6 octahedra are connected by Te–Te dimers21.

Through incorporating more electronegative Se anions, we fabricate a new layered compound AuTe2Se4/3 by conventional high temperature solid-state reaction. In a basic cube subunit, the Se anions attract electrons from Te and lead to the ordered arrangement of Au, Te and Se atoms. The cubes stack into strip through Te–Te dimers at 3.18 Å and 3.28 Å along the a- and b-axis, respectively, which composes 2D layers due to the existence of weak Te–Te interaction (~4 Å) along the c-axis. Electrical and magnetic measurements demonstrate that the SC occurs at 2.85 K. Furthermore, this SC exhibits 2D nature evidenced by the BKT transition in the thin crystals. The observed results are interpreted according to the crystallographic and electronic structure in reduced-dimensionality.

https://www.nature.com/articles/s41467-017-00947-0


Cr6
Admin

Posts : 1099
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

Post by Cr6 on Sat Mar 10, 2018 4:12 am

Hydrogen‐rich superconductors at high pressures
Advanced Review
Hui Wang, Xue Li, Guoying Gao, Yinwei Li, Yanming Ma
Published Online: Sep 05 2017
DOI: 10.1002/wcms.1330

Abstract

The hydrogen‐rich superconductors stabilized at high‐pressure conditions have been the subject of topic interests. There is an essential hope that hydrogen‐rich superconductors are promising candidates of room‐temperature superconductors. Recent advances in first‐principles crystal structure prediction techniques have opened up the possibility of reliable prediction of superconductive structures, and subsequent superconductivity calculations based on phonon‐mediated superconducting mechanism revealed a general appearance of high temperature superconductivity in pressurized hydrides. Theory‐orientated experiments at high pressure discovered a number of hydrogen‐rich superconductors, among which sulfur hydrides exhibit a remarkably high superconducting critical temperature reaching 203 K. In this review, we discuss the emerging research activities towards hydrogen‐rich superconductors at high pressures and outlook the future direction in the field. WIREs Comput Mol Sci 2018, 8:e1330. doi: 10.1002/wcms.1330

This article is categorized under:

   Structure and Mechanism > Computational Materials Science

Images
The crystal structure of (a) the Pm‐3n phase of YH3, (b) the Im‐3m phase of H3S, (c) the R‐3m phase of H3S, (d) the P63/mmm phase of TeH4, (e) the Cmmm phase of RbH3, (f) the Cc phase of H5Cl, (g) the Im‐3m phase of CaH6, and (h) the I4/mmm phase of CaH4. Hydrogen and the heavier element are shown by the small and large balls, respectively.
[ Normal View | Magnified View ]
(a) Calculated PHDOS (top panel) and the Eliashberg phonon spectral function α2F(ω)/ω, electron–phonon integral λ(ω) (lower panel) of SiH4(H2)2 at 250 GPa. (b) Top panel is the collected estimated Tc values of SiH4(H2)2, GeH4(H2)2, and AlH3H2 at 250 GPa, and the lower panel describes the contributions of the low‐frequency vibrations from heavy atoms (green bars), the intermediate‐frequency intermolecular vibrations (blue bars), and the high‐frequency phonons from the H2 vibrons (yellow bars) to the total λ.
[ Normal View | Magnified View ]
(a) Calculated PHDOS (top panel) and the Eliashberg phonon spectral function α2F(ω)/ω, electron–phonon integral λ(ω) (lower panel) of AsH8 at 350 GPa. (b) Top panel is the collected estimated Tc values of H4I, PoH4, LiH6, PbH8, AsH8, and MgH12 at selected pressures, and the lower panel describes the contributions of the low‐frequency vibrations from metal atoms (green bars), the intermediate‐frequency intermolecular vibrations (blue bars), and the high‐frequency phonons from the H2 vibrons (yellow bars) to the total λ.
[ Normal View | Magnified View ]
(a) The crystal structure and electron localization function of the Im‐3m phase of CaH6. (b) Phonon dispersion curves of Im‐3m‐CaH6 (left panel). Olive circles indicate the phonon line width with a radius proportional to the strength. Calculated Eliashberg phonon spectral function α2F(ω) and electron–phonon integral λ(ω) (right panel). Band structures of (c) CaH6 and (d) Ca0H6 (Im‐3m‐CaH6 with Ca removed), respectively.
[ Normal View | Magnified View ]
(a) The crystal structure and electron localization function (ELF) of the Im‐3m phase of H3S. (b) Calculated Eliashberg phonon spectral function α2F(ω) and electron–phonon integral λ(ω) (top panel) and the PHDOS (lower panel) of Im‐3m‐H3S at 200 GPa. (c) The crystal structure of the Pm‐3n phase of GaH3. (d) Calculated Eliashberg phonon spectral function α2F(ω)/ω and electron–phonon integral λ(ω) (top panel) and the PHDOS (lower panel) of Pm‐3n‐GaH3 at 160 GPa.
[ Normal View | Magnified View ]

References

http://wires.wiley.com/WileyCDA/WiresArticle/wisId-WCMS1330.html
---------

Ionic hydrides

Ionic or saline hydrides are composed of hydride bound to an electropositive metal, generally an alkali metal or alkaline earth metal. The divalent lanthanides such as europium and ytterbium form compounds similar to those of heavier alkali metal. In these materials the hydride is viewed as a pseudohalide. Saline hydrides are insoluble in conventional solvents, reflecting their non-molecular structures. Ionic hydrides are used as bases and, occasionally, as reducing reagents in organic synthesis.[6]

C6H5C(O)CH3 + KH → C6H5C(O)CH2K + H2

Typical solvents for such reactions are ethers. Water and other protic solvents cannot serve as a medium for ionic hydrides because the hydride ion is a stronger base than hydroxide and most hydroxyl anions. Hydrogen gas is liberated in a typical acid-base reaction.

NaH + H2O → H2 (g) + NaOH ΔH = −83.6 kJ/mol, ΔG = −109.0 kJ/mol

Often alkali metal hydrides react with metal halides. Lithium aluminium hydride (often abbreviated as LAH) arises from reactions of lithium hydride with aluminium chloride.

4 LiH + AlCl3 → LiAlH4 + 3 LiCl
............

Pseudohalogen
From Wikipedia, the free encyclopedia

The pseudohalogens are polyatomic analogues of halogens, whose chemistry, resembling that of the true halogens, allows them to substitute for halogens in several classes of chemical compounds[1]. Pseudohalogens occur in pseudohalogen molecules, inorganic molecules of the general forms Ps–Ps or Ps–X (where Ps is a pseudohalogen group), such as cyanogen; pseudohalide anions, such as cyanide ion; inorganic acids, such as hydrogen cyanide; as ligands in coordination complexes, such as ferricyanide; and as functional groups in organic molecules, such as the nitrile group. Well-known pseudohalogen functional groups include cyanide, cyanate, thiocyanate, and azide.
https://en.wikipedia.org/wiki/Pseudohalide

Cr6
Admin

Posts : 1099
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

Post by Cr6 on Sat Mar 10, 2018 4:34 am

WHAT'S HOT IN... PHYSICS , May/Jun 2009
The Good Samarium Hots Up Superconductivity
by Simon Mitton

The Physics Hot Papers in this period show that superconductivity research is now hotting up, thanks to the unexpected discovery of a new class of iron-based superconductors. Papers #2, #3, #7 and #9 capture the tremendous interest stimulated by the recent discovery of superconductivity at Tc = 26 K in Hideo Hosonothe iron-based oxypnictide La(O1-xFx)FeAs (Y. Kamihari, et al., J. Am. Chem. Soc., 130 [11]: 3296-7, 2008; currently #1 in the Chemistry Top Ten). That research, by Hideo Hosono and colleagues of the Tokyo Institute of Technology, put high-temperature superconductivity back on the agenda with a bang.

Paper #2 describes an experiment designed by Xian Hui Chen, and conducted together with colleagues at the University of Science and Technology, Hefei, China. They followed up on the Japanese discovery paper by looking at superconductivity in a related compound, SmFeAsO1-x Fx, in which samarium is substituted for lanthanum. They aimed to see how high they could push Tc in Jonathan Baggera layered rare-earth superconductor. In doing so they broke the record for a non-copper-oxide superconductor, by reaching Tc = 43 K, comfortably above the previous record of 39 K for magnesium diboride.

The Sm-doped material is intriguing: according to Chen, it has Tc above that suggested by standard BCS theory, which argues for the oxypnictides being unconventional superconductors. Furthermore, the jump in Tc from 26 K to 43 K just by substituting Sm for La immediately suggested that further research would produce higher Tc in layered oxypnictides doped with F.

That’s where #3 takes us: in it Zhi-An Ren and colleagues from Beijing, China, report Tc = 55 K in the same F-doped compound. In fact, related experiments by this group, in which they also substituted Ce, Pr, and Nd, have shown that FeAs superconductors constitute a new family with Tc > 50 K. The high-citation rate of #3 is partly driven by the comprehensive information it gives on fabrication. The materials are grown using a high-pressure technique similar to that used for turning graphite to diamond.

Zhi-An Ren’s collaboration is also responsible for #7, in which they point out that the compounds have a simple structure of alternating FeAs and ReO layers (where Re is a rare earth). Instead of doping with F to achieve superconductivity, they created vacancies of oxygen atoms in the lattice. That move creates more electron carriers, which should be a more efficient approach to the realization of superconductivity. And indeed, tuning the O content leads to the occurrence of superconductivity in a way that resembles the situation in cuprates. That’s encouraging because the parallels between the two compounds suggest that the arsenides with O vacancies rather than F doping could be the more competitive choice for higher Tc.

Newcomer #9 is a paper that neatly illustrates how research on F-doped arsenides may contribute to fundamental physics. The experiments described in this paper show how F doping suppresses spin-density-wave (SDW) instabilities and leads to superconductivity. SDW is a low-energy ordered state that occurs at low temperatures. SDW inhibits the onset of superconductivity.

Superconductivity is one of the most dramatic phenomena in condensed matter physics. Part of the motivation for the groups in China and Japan is the ultimate goal: the realization of the phenomenon at room temperature. There are plenty of physicists who will state informally that room temperature operation is about as likely as cold fusion, or hot fusion. But fast progress has energized research. In 2008 there were at least seven international symposia devoted to Fe-based superconductors, and those events have no doubt propelled the citation rates. For researchers it’s a matter of striking while the iron is hot!

Dr. Simon Mitton is a Fellow of St. Edmund’s College, Cambridge, U.K.

http://archive.sciencewatch.com/ana/hot/phy/09mayjun-phy/

Cr6
Admin

Posts : 1099
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

Post by Cr6 on Sat Mar 10, 2018 4:42 am

Princeton’s Robert Cava: From Superconductivity to Topological Insulators
Scientist Interview: March 2011

SW: What do you consider the critical questions in high-Tc that still have to be answered?

Well, why do these things superconduct at all, and what’s going on in these materials at a very, very local scale? The most interesting physics is probably occurring at a scale of tens of angstroms. You can see wonderfully complicated things going on at the nanometer-length scale with different kinds of experimental probes. It’s not just a uniform sea of electrons like we learned a piece of copper is. It’s a very inhomogeneous distribution of electrons doing all kinds of crazy stuff. The more people look, the more complicated it gets.

SW: Are any researchers seriously looking for new high-Tc superconductors anymore, or is that part also done?

A small number of people are, and every once in a while a big surprise appears—somebody finds a new superconductor that nobody expected. In 2008, for example, a Japanese group found a new superconductor, a combination of iron, arsenic, lanthanum, oxygen, and fluorine that was superconducting at 26 Kelvin. What made it so interesting is that the superconductor seems to arise from the iron and arsenic, and the iron should typically give you a magnet.

Until the high-temperature superconductors came along, people thought magnetism and superconductivity were incompatible, whereas in many cases they’re probably just two sides of the same coin. You can change a magnet into a superconductor and a superconductor into a magnet by changing some chemical parameter.

So in 2008, a group of Japanese researchers discovered that iron and arsenic are the basis of a new class of superconductors whose superconductivity and magnetism seem to be related (Y. Kamihara, et al., J. Am. Chem. Soc., 130[11]: 3296-7, 2008; see also ).

After that big discovery, another group discovered that the temperature of the superconductor could go up to 50 or 60 Kelvin with the right combination of elements. That makes these the second-highest-temperature superconductors known, and with a whole new element involved—not copper anymore, but iron.

Of course, thousands of people also jumped onto this new one really fast. The interesting difference between now and 1986 is that back then you had to hear about the discovery through word of mouth. Somebody talked to somebody who talked to somebody on the other side of the world. Occasionally, a fax of a preprint appeared. Information didn’t travel very fast.

SW: Let’s begin at the beginning. How far have high-temperature superconductors come in the quarter-century since they were discovered, and what are the key research areas still being studied?


I’d say there are a couple of things that are still going on. First, there’s no universally accepted theory yet about why they work. We know a lot about them, and we have much of the phenomenology worked out, but we still have no theory about what makes them superconducting that the community as a whole accepts. That’s a remarkable situation, if you ask me. It goes to show how complicated physics can be sometimes.

There are so many interesting phenomena that occur in conjunction with the superconductivity that the whole package has not really been put together yet in a way that satisfies everybody, at least not like the BCS theory explains basic superconductivity. There’s nothing yet established that will go into the textbooks as explaining it. There’s a lot of action on the theoretical side, and it’s very sophisticated, but nobody has explained it all.

http://archive.sciencewatch.com/inter/aut/2011/11-mar/11marCava/

Cr6
Admin

Posts : 1099
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

Post by Cr6 on Sun Apr 01, 2018 1:12 am

Physicists find clues to the origins of high-temperature superconductivity
January 18, 2018 by Lisa Zyga, Phys.org feature
cuprate superconductors

Figure showing the conversion between incoherent and coherent electron correlations in the cuprates’ non-superconducting and superconducting states, respectively. Credit: Li et al. Published in Nature Communications.

Ever since cuprate (copper-containing) superconductors were first discovered in 1986, they have greatly puzzled researchers. Cuprate superconductors have critical superconducting temperatures—the point at which their electrical resistance drops to zero—of up to 138 K at ambient pressure, which far exceeds the critical temperatures of other superconductors and is even higher than what is thought possible based on theory.

Now in a new study, researchers have discovered the existence of a positive feedback loop that greatly enhances the superconductivity of cuprates and may shed light on the origins of high-temperature cuprate superconductivity—considered one of the most important open questions in physics.

The researchers, Haoxiang Li et al., at the University of Colorado at Boulder and the École Polytechnique Fédérale de Lausanne, have published a paper on their experimental ARPES (Angle Resolved Photoemission Spectroscopy) results on high-temperature cuprate superconductors in a recent issue of Nature Communications.

As the researchers explain, the positive feedback mechanism arises from the fact that the electrons in the non-superconducting cuprate state are correlated differently than in most other systems, including in conventional superconductors, which have strongly coherent electron correlations. In contrast, cuprates in their non-superconducting state have strongly incoherent "strange-metal" correlations, which are at least partly removed or weakened when the cuprates become superconducting.

Due to these incoherent electron correlations, it has been widely believed that the framework that describes conventional superconductivity—which is based on the notion of quasiparticles—cannot accurately describe cuprate superconductivity. In fact, some research has suggested that cuprate superconductors have such unusual electronic properties that even attempting to describe them with the notion of particles of any kind becomes useless.

This leads to the question of, what role, if any, do the strange-metal correlations play in high-temperature cuprate superconductivity?

The main result of the new paper is that these correlations don't simply disappear in the cuprate superconducting state, but instead get converted into coherent correlations that lead to an enhancement of the superconductive electron pairing. This process results in a positive feedback loop, in which the conversion of the incoherent strange-metal correlations into a coherent state increases the number of superconductive electron pairs, which in turn leads to more conversion, and so on.

The researchers found that, due to this positive feedback mechanism, the strength of the coherent electron correlations in the superconducting state is unprecedented, greatly exceeding what is possible for conventional superconductors. Such a strong electron interaction also opens up the possibility that cuprate superconductivity might occur due to a completely unconventional pairing mechanism—a purely electronic pairing mechanism that could arise solely due to quantum fluctuations.

"We experimentally discover that the incoherent electron correlations in the strange metal 'normal state' are converted to coherent correlations in the superconducting state that help strengthen the superconductivity, with an ensuing positive feedback loop," coauthor Dan Dessau at the University of Colorado at Boulder told Phys.org. "Such a strong positive feedback loop should strengthen most conventional pairing mechanisms but could also allow for a truly unconventional (purely electronic) pairing mechanism."

(more at link...)

https://phys.org/news/2018-01-physicists-clues-high-temperature-superconductivity.html

.....

Scientists find new magnetic state in iron-based superconductor
27 February 2018

The Ames scientists created a variant of the iron arsenide CaKFe4As4 by substituting in cobalt and nickel at precise locations. This slightly distorted the atomic arrangements, inducing the new magnetic order while retaining the material’s superconducting properties. Image: Ames Laboratory, US Department of Energy.

Scientists at the US Department of Energy's Ames Laboratory have discovered a state of magnetism that may be the missing link in understanding the relationship between magnetism and unconventional superconductivity. The research, recently reported in a paper in Nature Quantum Materials, provides tantalizing new possibilities for attaining superconducting states in iron-based materials.

"In the research of quantum materials, it's long been theorized that there are three types of magnetism associated with superconductivity," explained Paul Canfield, a senior scientist at Ames Laboratory and a distinguished professor of physics and astronomy at Iowa State University. "One type is very commonly found, another type is very limited and only found in rare situations, and this third type was unknown, until our discovery."

The scientists suspected that the material they studied, the iron arsenide CaKFe4As4, was such a strong superconductor because there was an associated magnetic ordering hiding nearby. Creating a variant of the material by substituting in cobalt and nickel at precise locations, known as ‘doping’, slightly distorted the atomic arrangements, which induced the new magnetic order while retaining the material’s superconducting properties.

(more at link...)
https://www.materialstoday.com/metals-alloys/news/new-magnetic-state-ironbased-superconductor/

Cr6
Admin

Posts : 1099
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

Post by Cr6 on Sun Apr 01, 2018 1:16 am

Physicists "learn the rules" of magnetic states in newly published research

September 18, 2017 by Laura Millsaps, Ames Laboratory

Credit: Ames Laboratory

Ames Laboratory scientists have found new insight to the "rules" of how magnetic states emerge and are suppressed, creating a guide for discovery of other materials with superconducting capabilities. The discovery was made through the study of the transition metal compound LaCrGe3 under temperature, pressure, and magnetic field changes.

These findings are in a newly published paper in Nature Communications Materials and are part of Ames Laboratory's broader scope of condensed matter research dedicated to discovering new and novel states of magnetism.

"If we find transition metal-based systems with some sort of magnetism and suppress it to lower and lower temperatures, sometimes new states like superconductivity pop up, and that's what we hope for," said Paul Canfield, a senior scientist at Ames Laboratory and a Distinguished Professor and the Robert Allen Wright Professor of Physics and Astronomy at Iowa State University.In the case of LaCrGe3 the ferromagnetism is suppressed and, for certain pressures and fields, new antiferromagnetic states emerge. "By studying how magnetism is suppressed," said Canfield "we are beginning to learn some of the rules for when superconductivity, or other novel states, appear and when they don't."

Explore further: Making ferromagnets stronger by adding non-magnetic elements

More information: Udhara S. Kaluarachchi et al. Tricritical wings and modulated magnetic phases in LaCrGe3 under pressure, Nature Communications (2017). DOI: 10.1038/s41467-017-00699-x

https://phys.org/news/2017-09-physicists-magnetic-states-newly-published.html


Last edited by Cr6 on Sun Apr 01, 2018 1:22 am; edited 1 time in total

Cr6
Admin

Posts : 1099
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

Post by Cr6 on Sun Apr 01, 2018 1:18 am

Contact: Ariana Tantillo, (631) 344-2347, or Peter Genzer, (631) 344-3174share:
Bringing a Hidden Superconducting State to Light

High-power light reveals the existence of superconductivity associated with charge "stripes" in the copper-oxygen planes of a layered material above the temperature at which it begins to transmit electricity without resistance

February 16, 2018

Physicist Genda Gu holds a single-crystal rod of LBCO—a compound made of lanthanum, barium, copper, and oxygen—in Brookhaven's state-of-the-art crystal growth lab. The infrared image furnace he used to synthesize these high-quality crystals is pictured in the background.

UPTON, NY—A team of scientists has detected a hidden state of electronic order in a layered material containing lanthanum, barium, copper, and oxygen (LBCO). When cooled to a certain temperature and with certain concentrations of barium, LBCO is known to conduct electricity without resistance, but now there is evidence that a superconducting state actually occurs above this temperature too. It was just a matter of using the right tool—in this case, high-intensity pulses of infrared light—to be able to see it.

Reported in a paper published in the Feb. 2 issue of Science, the team’s finding provides further insight into the decades-long mystery of superconductivity in LBCO and similar compounds containing copper and oxygen layers sandwiched between other elements. These “cuprates” become superconducting at relatively higher temperatures than traditional superconductors, which must be frozen to near absolute zero (minus 459 degrees Fahrenheit) before their electrons can flow through them at 100-percent efficiency. Understanding why cuprates behave the way they do could help scientists design better high-temperature superconductors, eliminating the cost of expensive cooling systems and improving the efficiency of power generation, transmission, and distribution. Imagine computers that never heat up and power grids that never lose energy.

“The ultimate goal is to achieve superconductivity at room temperature,” said John Tranquada, a physicist and leader of the Neutron Scatter Group in the Condensed Matter Physics and Materials Science Department at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory, where he has been studying cuprates since the 1980s. “If we want to do that by design, we have to figure out which features are essential for superconductivity. Teasing out those features in such complicated materials as the cuprates is no easy task.”

The copper-oxygen planes of LBCO contain “stripes” of electrical charge separated by a type of magnetism in which the electron spins alternate in opposite directions. In order for LBCO to become superconducting, the individual electrons in these stripes need to be able to pair up and move in unison throughout the material.

Previous experiments showed that, above the temperature at which LBCO becomes superconducting, resistance occurs when the electrical transport is perpendicular to the planes but is zero when the transport is parallel. Theorists proposed that this phenomenon might be the consequence of an unusual spatial modulation of the superconductivity, with the amplitude of the superconducting state oscillating from positive to negative on moving from one charge stripe to the next. The stripe pattern rotates by 90 degrees from layer to layer, and they thought that this relative orientation was blocking the superconducting electron pairs from moving coherently between the layers.

“This idea is similar to passing light through a pair of optical polarizers, such as the lenses of certain sunglasses,” said Tranquada. “When the polarizers have the same orientation, they pass light, but when their relative orientation is rotated to 90 degrees, they block all light.”

However, a direct experimental test of this picture had been lacking—until now.

One of the challenges is synthesizing the large, high-quality single crystals of LBCO needed to conduct experiments. “It takes two months to grow one crystal, and the process requires precise control over temperature, atmosphere, chemical composition, and other conditions,” said co-author Genda Gu, a physicist in Tranquada’s group. Gu used an infrared image furnace—a machine with two bright lamps that focus infrared light onto a cylindrical rod containing the starting material, heating it to nearly 2500 degrees Fahrenheit and causing it to melt—in his crystal growth lab to grow the LBCO crystals.

Collaborators at the Max Planck Institute for the Structure and Dynamics of Matter and the University of Oxford then directed infrared light, generated from high-intensity laser pulses, at the crystals (with the light polarization in a direction perpendicular to the planes) and measured the intensity of light reflected back from the sample. Besides the usual response—the crystals reflected the same frequency of light that was sent in—the scientists detected a signal three times higher than the frequency of that incident light.

“For samples with three-dimensional superconductivity, the superconducting signature can be seen at both the fundamental frequency and at the third harmonic,” said Tranquada. “For a sample in which charge stripes block the superconducting current between layers, there is no optical signature at the fundamental frequency. However, by driving the system out of equilibrium with the intense infrared light, the scientists induced a net coupling between the layers, and the superconducting signature shows up in the third harmonic. We had suspected that the electron pairing was present—it just required a stronger tool to bring this superconductivity to light.”

(more at link...)
https://www.bnl.gov/newsroom/news.php?a=112737




Cr6
Admin

Posts : 1099
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

Post by Cr6 on Sun Apr 01, 2018 1:27 am

Cuprate Superconductors

Schematic phase diagram of a hole doped cuprate superconductor and crystal structure of CuO2 planes

High-transition-temperature (Tc) superconductivity in copper oxides (cuprates) is one of the most intriguing emergent phenomena in strongly correlated electron systems. It has attracted great attention since its discovery because Tc can exceed the boiling temperature of liquid nitrogen, which is much higher than the putative limit of Tc ~ 40 K derived from the BCS theory for conventional superconductivity.

The cuprate superconductors have a layered crystal structure consisting of CuO2 planes separated by charge reservoir layers, which may dope electrons or holes into the CuO2 planes. On doping holes, the antiferromagnetic Mott insulating phase of the parent compounds disappears and superconductivity emerges. Tc follows a dome-like shape as a function of doping, with a maximum Tc around 16% doped per CuO2 plaquette. A similar phase diagram is seen on doping electrons, albeit with a more robust antiferromagnetic phase and a lower Tc. On the hole-doped side, there exists an enigmatic state above Tc called the pseudogap, where the electron density of states within certain momentum region is suppressed.

https://arpes.stanford.edu/research/quantum-materials/cuprate-superconductors

....

Viewpoint: Cuprate Superconductors May Be Conventional After All


   Can-Li Song, Department of Physics, Tsinghua University, Beijing 100084, China
   Qi-Kun Xue, Department of Physics, Tsinghua University, Beijing 100084, China

December 4, 2017• Physics 10, 129
Experiments on a copper-based high-temperature superconductor uncover the existence of vortex states—a hallmark of conventional superconductivity.

APS/Alan Stonebraker
Figure 1: Bardeen-Cooper-Schrieffer theory predicts that vortex states are formed in conventional type-II superconductors in a magnetic field. Renner and co-workers have observed such states in the high-Tc cuprate YBa2Cu3O7−

Renner and colleagues’ experiment shows that applying a magnetic field to Y123 produces vortex states similar to those seen in conventional type-II BCS superconductors. While this result doesn’t yet tell us why copper oxides exhibit superconductivity at such high temperatures, it strongly suggests that superconductivity in copper oxides may be of the conventional BCS type. This conclusion is consistent with recent studies, which showed that several types of high- Tc superconductors—including monolayer FeSe films ( Tc>65K) [8] and pressurized H2S ( Tc=203K) [9]—can also be described by BCS theory. Within this context, the work of Renner’s group may signal that we are getting close to the discovery of a unifying mechanism for high- Tc superconductivity.

Several caveats about this work, however, are worth mentioning. First, the simple hypothesis of an additive tunneling conductance from SC and NSC channels might be questionable: The electrons in the SC channel may interact with those in other channels, complicating the interpretation of the measurements. Further experiments in other materials where the NSC channel has been well characterized will show whether the assumption is justified. Second, the samples may be in a “dirty” regime (in which the mean-free path of electrons is shorter than the coherence length of superconducting Cooper pairs). In such a regime, the vortices do not host bound states, but the subtraction process could lead to signals that could be misinterpreted as the ZBCPs associated with vortex states. Ruling out this possibility would require a quantification of the ZBCP amplitudes. Finally, the vortices measured by the authors exhibit an unexpected variability in their ZBCP spectra: for some vortices the ZBCP splits into a double peak when measured away from the vortex centers, whereas for others it does not. The authors attribute such variability to effects of disorder in the lattice of vortices. Experiments using cleaner samples will be necessary to confirm this interpretation.

...

Iridates and cuprate superconductors: the similarities are more than skin deep
July 11, 2014

https://physics.aps.org/articles/pdf/10.1103/Physics.10.129

https://dash.harvard.edu/handle/1/17465318


So far, the superconductivity community has documented that cuprates have certain characteristics that seem to set them apart and provide a ‘recipe’ for high-temperature superconductivity: their copper atoms sit in a two-dimensional square lattice, have a spin-½ ion, and “talk” to each other magnetically through a phenomenon known as Heisenberg exchange.

The thought goes that if other materials share these characteristics, they could be undiscovered superconductors—or if they are missing just one or two ingredients, they could provide clues to how cuprate superconductivity works. (Studying systems that are very similar, but with one ingredient missing, is a classic scientific technique).

An international Argonne-led team decided to try their luck with iridium oxides. These, it turns out, have all three characteristics—they are built up of spin-½ ions on a two-dimensional grid structure, and the ions follow the expectations of Heisenberg exchange behavior—but no one has observed superconductivity in them. Yet. The reason is that most cuprates need to have mobile charges in the form of extra electrons or holes added in a process called doping before they become superconducting, and iridates have been very resistant to this process.

B.J. Kim, one of the Argonne authors, thought to add potassium ions to the surface as a way of providing these charges. As he measured the electronic structure, he found that a very thin top layer of the iridate started to behave as though it were doped into a metallic state, the same kind of doping needed for cuprate superconductors. More importantly, Kim found the doped iridate shared a key electronic structure found in the cuprates—a Fermi arc that evolved into a closed Fermi surface as he added more potassium ions or warmed the sample.

Although they did not find evidence for superconductivity, the team plans to continue exploring this material and related iridium oxides.

“The work is an existence proof that these very special Fermi arcs can be found in materials other than cuprates—which could itself suggest that the family of superconductors could be much larger than currently known,” said Argonne scientist John Mitchell, one of the coauthors of the study. “It will also make us look very carefully at how these arcs may or may not result from the same underlying physics as the cuprates.”

...

Cr6
Admin

Posts : 1099
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

Post by Cr6 on Sun Apr 01, 2018 8:45 pm

Here are the base elements of CaKFe4As4. How to get the N-S going in one direction only with the molecule :


Cr6
Admin

Posts : 1099
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

Post by LongtimeAirman on Mon Apr 02, 2018 1:56 pm

.
Cr6, I believe you’re asking for a suggested configuration for a superconducting molecule, CaKFe4As4.

All those double alphas - which I think makes Fe such a good two-way conductor - make all four elements very self-similar. What is the superconducting definition? It could be as simple as aligning the molecule properly on a tabletop.

The first configuration that springs into mind is the single file filament, but that makes no sense to me at all.
Four As in a square configuration, (the south layer), each topped with an Fe (the north layer). The north pole position might be K and the south pole Ca.


Nevyn, I see you’re at Atomic Viewer 0.9. I know you’ve built plenty of molecules. Can users build molecules?
.

LongtimeAirman
Admin

Posts : 1080
Join date : 2014-08-10

View user profile

Back to top Go down

Re: Mathis on Graphene? Any hints?

Post by Nevyn on Mon Apr 02, 2018 9:18 pm

Here's my proposed alignment. Iron and Arsenic are the work horses. They have all of those protons in their centers that can take a lot of charge and they are very balanced in the carousel levels. Potassium and Calcium are going to be the top and bottom atoms because they don't have carousel protons.

Iron will sit on top of Arsenic with the single hook proton of Arsenic bonding with the 2 hook protons on the bottom of Iron. That gives us a very balanced molecule with 3 protons bonding them together. This creates an increased potential for charge flow through both of them.

Potassium and Calcium are very similar to each other with the only difference being an extra proton on the bottom of Calcium. We want a balanced molecule so we put Potassium on top of Iron and Calcium underneath Arsenic. This gives us a 3 proton bond between Calcium and Arsenic to match the 3 proton bond between Arsenic and Iron, keeping a smooth flow through the internals. The bond between Potassium and Calcium is not quite as strong as it only contains 2 protons. This may mean that there is a slight difference between photon flow and anti-photon flow, but not much.

We actually have 4 Iron and 4 Arsenic atoms to play with and we just stack them in FeAr pairs, always leaving K and Ca as the intake/exhaust ports. This does create some bonds between Fe and Ar that contains 4 protons but this just boosts the charge potential even further. These bonds will never operate at their full potential because the rest of the molecule can not pull in that amount of charge.

How does it work?

Iron will usually create magnetic fields because it is quite balanced and has a large carousel level. Not quite as much for Arsenic because it has a differential from top to bottom which indicates an increase in through-charge rather than magnetic charge. This is a good thing because we want through-charge and minimal magnetic charge. Arsenic will essentially convert Iron into a better conductor. Potassium and Calcium do not have carousel levels so they are already good for through-charge. This gives us a nice straight path through the molecule with minimal carousel interaction. Those carousel levels are not useless though. They do still emit some charge which is used to protect the internal through-charge stream from outside influence.

That analysis assumed no funky proton re-arrangements like Miles describes for Neodymium. I admit that I get a bit lost in the possibilities when I think about that. I don't think it is necessary here but it may play some part.

I tried to create an image of this molecule but I don't have the usual software that I use and GIMP is being a little bitch (translation: I don't understand how to use it and don't have the time to figure it out right now). I could use my old desktop Atomic Viewer to put them together but it takes too much time to get all of the bonds aligned.

No, web-based AV can not build molecules yet. That might be my big project for this year. No promises because I haven't felt the desire to dive back in to any of this lately but I'm sure that will change soon enough. You guys have piqued my interests a few times with some good discussions but I haven't taken that leap quite yet.
avatar
Nevyn
Admin

Posts : 1399
Join date : 2014-09-11

View user profile http://www.nevyns-lab.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

Post by Cr6 on Tue Apr 10, 2018 12:19 am

Thanks for showing the connections.

Found a few more recent articles. A few newer SC molecules are coming out:

---------


Graphene's sleeping superconductivity awakens
January 19, 2017, University of Cambridge
graphene
Credit: AlexanderAlUS/Wikipedia/CC BY-SA 3.0

(more at link... https://phys.org/news/2017-01-graphene-superconductivity-awakens.html )

Researchers have found a way to trigger the innate, but previously hidden, ability of graphene to act as a superconductor - meaning that it can be made to carry an electrical current with zero resistance.

The finding, reported in Nature Communications, further enhances the potential of graphene, which is already widely seen as a material that could revolutionise industries such as healthcare and electronics. Graphene is a two-dimensional sheet of carbon atoms and combines several remarkable properties; for example, it is very strong, but also light and flexible, and highly conductive.

Since its discovery in 2004, scientists have speculated that graphene may also have the capacity to be a superconductor. Until now, superconductivity in graphene has only been achieved by doping it with, or by placing it on, a superconducting material - a process which can compromise some of its other properties.

But in the new study, researchers at the University of Cambridge managed to activate the dormant potential for graphene to superconduct in its own right. This was achieved by coupling it with a material called praseodymium cerium copper oxide (PCCO).

Superconductors are already used in numerous applications. Because they generate large magnetic fields they are an essential component in MRI scanners and levitating trains. They could also be used to make energy-efficient power lines and devices capable of storing energy for millions of years.

Superconducting graphene opens up yet more possibilities. The researchers suggest, for example, that graphene could now be used to create new types of superconducting quantum devices for high-speed computing. Intriguingly, it might also be used to prove the existence of a mysterious form of superconductivity known as "p-wave" superconductivity, which academics have been struggling to verify for more than 20 years.

The research was led by Dr Angelo Di Bernardo and Dr Jason Robinson, Fellows at St John's College, University of Cambridge, alongside collaborators Professor Andrea Ferrari, from the Cambridge Graphene Centre; Professor Oded Millo, from the Hebrew University of Jerusalem, and Professor Jacob Linder, at the Norwegian University of Science and Technology in Trondheim.

"It has long been postulated that, under the right conditions, graphene should undergo a superconducting transition, but can't," Robinson said. "The idea of this experiment was, if we couple graphene to a superconductor, can we switch that intrinsic superconductivity on? The question then becomes how do you know that the superconductivity you are seeing is coming from within the graphene itself, and not the underlying superconductor?"

Similar approaches have been taken in previous studies using metallic-based superconductors, but with limited success. "Placing graphene on a metal can dramatically alter the properties so it is technically no longer behaving as we would expect," Di Bernardo said. "What you see is not graphene's intrinsic superconductivity, but simply that of the underlying superconductor being passed on."

PCCO is an oxide from a wider class of superconducting materials called "cuprates". It also has well-understood electronic properties, and using a technique called scanning and tunnelling microscopy, the researchers were able to distinguish the superconductivity in PCCO from the superconductivity observed in graphene.

Superconductivity is characterised by the way the electrons interact: within a superconductor electrons form pairs, and the spin alignment between the electrons of a pair may be different depending on the type - or "symmetry" - of superconductivity involved. In PCCO, for example, the pairs' spin state is misaligned (antiparallel), in what is known as a "d-wave state".

By contrast, when graphene was coupled to superconducting PCCO in the Cambridge-led experiment, the results suggested that the electron pairs within graphene were in a p-wave state. "What we saw in the graphene was, in other words, a very different type of superconductivity than in PCCO," Robinson said. "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 what type of superconductivity the team activated, but their results strongly indicate that it is the elusive "p-wave" form. If so, the study could transform the ongoing debate about whether this mysterious type of superconductivity exists, and - if so - what exactly it is.

In 1994, researchers in Japan fabricated a triplet superconductor that may have a p-wave symmetry using a material called strontium ruthenate (SRO). The p-wave symmetry of SRO has never been fully verified, partly hindered by the fact that SRO is a bulky crystal, which makes it challenging to fabricate into the type of devices necessary to test theoretical predictions.

https://phys.org/news/2017-01-graphene-superconductivity-awakens.html

....

A different spin on superconductivity—Unusual particle interactions open up new possibilities in exotic materials
April 7, 2018, University of Maryland
(more at link...
https://phys.org/news/2018-04-superconductivityunusual-particle-interactions-possibilities-exotic.html
)

When you plug in an appliance or flip on a light switch, electricity seems to flow instantly through wires in the wall. But in fact, the electricity is carried by tiny particles called electrons that slowly drift through the wires. On their journey, electrons occasionally bump into the material's atoms, giving up some energy with every collision.

The degree to which electrons travel unhindered determines how well a material can conduct electricity. Environmental changes can enhance conductivity, in some cases drastically. For example, when certain materials are cooled to frigid temperatures, electrons team up so they can flow uninhibited, without losing any energy at all—a phenomenon called superconductivity.

Now a team of researchers from the University of Maryland (UMD) Department of Physics together with collaborators has seen exotic superconductivity that relies on highly unusual electron interactions. While predicted to occur in other non-material systems, this type of behavior has remained elusive. The team's research, published in the April 6 issue of Science Advances, reveals effects that are profoundly different from anything that has been seen before with superconductivity.

Electron interactions in superconductors are dictated by a quantum property called spin. In an ordinary superconductor, electrons, which carry a spin of ½, pair up and flow uninhibited with the help of vibrations in the atomic structure. This theory is well-tested and can describe the behavior of most superconductors. In this new research, the team uncovers evidence for a new type of superconductivity in the material YPtBi, one that seems to arise from spin-3/2 particles.

"No one had really thought that this was possible in solid materials," explains Johnpierre Paglione, a UMD physics professor and senior author on the study. "High-spin states in individual atoms are possible but once you put the atoms together in a solid, these states usually break apart and you end up with spin one-half. "

Finding that YPtBi was a superconductor surprised the researchers in the first place. Most superconductors start out as reasonably good conductors, with a lot of mobile electrons—an ingredient that YPtBi is lacking. According to the conventional theory, YPtBi would need about a thousand times more mobile electrons in order to become superconducting at temperatures below 0.8 Kelvin. And yet, upon cooling the material to this temperature, the team saw superconductivity happen anyway. This was a first sign that something exotic was going on inside this material.

After discovering the anomalous superconducting transition, researchers made measurements that gave them insight into the underlying electron pairing. They studied a telling feature of superconductors—their interaction with magnetic fields. As the material undergoes the transition to a superconductor, it will try to expel any added magnetic field from its interior. But the expulsion is not completely perfect. Near the surface, the magnetic field can still enter the material but then quickly decays away. How far it goes in depends on the nature of the electron pairing, and changes as the material is cooled down further and further.

To probe this effect, the researchers varied the temperature in a small sample of the material while exposing it to a magnetic field more than ten times weaker than the Earth's. A copper coil surrounding the sample detected changes to the superconductor's magnetic properties and allowed the team to sensitively measure tiny variations in how deep the magnetic field reached inside the superconductor.

The measurement revealed an unusual magnetic intrusion. As the material warmed from absolute zero, the field penetration depth for YPtBi increased linearly instead of exponentially as it would for a conventional superconductor. This effect, combined with other measurements and theory calculations, constrained the possible ways that electrons could pair up. The researchers concluded that the best explanation for the superconductivity was electrons disguised as particles with a higher spin—a possibility that hadn't even been considered before in the framework of conventional superconductivity.

The discovery of this high-spin superconductor has given a new direction for this research field. "We used to be confined to pairing with spin one-half particles," says Hyunsoo Kim, lead author and a UMD assistant research scientist. "But if we start considering higher spin, then the landscape of this superconducting research expands and just gets more interesting."


Cr6
Admin

Posts : 1099
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

Post by Cr6 on Tue Apr 10, 2018 1:35 am

Looks like YPtBi is at a higher stack level overall but the same mix of Charge is at play. "Y" couldn't resolve in the browser with Nevyn's Periodic table so I used Nb. Again the key question is how these molecules bond with each other to create a N-S or other strong (non-magnetic per Nevyn) directional flow? Always remember that Nb makes the strongest natural magnets:

https://www.nevyns-lab.com/mathis/app/AtomicViewer/AtomicViewer.php




Cr6
Admin

Posts : 1099
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

Post by Cr6 on Tue Apr 10, 2018 2:13 am

We are going to have to use Nevyn's Periodic Table to diagram each of these Superconducting molecules/materials. I bet a definitive "form" will come out showing the binding for better determination (and perhaps even prediction?) to indicate what actually creates a superconductor since official theory at a complete loss apparently.


Last edited by Cr6 on Fri Apr 13, 2018 2:19 am; edited 1 time in total

Cr6
Admin

Posts : 1099
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

Post by Nevyn on Tue Apr 10, 2018 8:20 pm

The higher elements have not been added to AV because they are very large and the possible changes from one element in the table to the next (i.e. how to add the next proton) are difficult to choose between. If anyone finds evidence for these elements bonding to other elements, hopefully known ones, then I might be able to create a few more.
avatar
Nevyn
Admin

Posts : 1399
Join date : 2014-09-11

View user profile http://www.nevyns-lab.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

Post by Nevyn on Tue Apr 10, 2018 8:22 pm

I've been thinking about how to start a Molecular Builder. I'm trying a few different ideas, just in my head at the moment, on how to build a molecule. There always seems to be something that makes each approach unusable for some scenario. The main problem is complex structures like rings. They require special handling as they sometimes need to bend the atoms themselves. I'm thinking about ignoring them for now and just getting something working. I could make something to handle the types of structures that are being discussed here. Most cases seem easy enough but I want to create a framework that can work for most, if not all, cases.

I wouldn't hold my breath waiting for it though. It will still take me some time to get something that is operational.

The best approach I have come up with so far requires the user to select the bond type before the atoms. The bond will be rendered in some way so that the user can see where to attach atoms to it. The atoms will then be aligned and rendered. While in building mode, you will be able to see all bonds which will, hopefully, look like mechanical linkages. Jared, if you're interested, maybe you could help me with some 3D models for the bonds. You are certainly more graphically artistic than I am. When not in build mode, the bonds will not be visible and the atoms will move closer together, more like an actual bond between elements.

I initially wanted a more free-flowing system where the user could drop atoms in and drag them around. The system would look for potential bond points that were near the atom being moved and if dropped near one of them, it would create the bond. The more I thought about it though, the more difficult it looked.

I think the Bond-First approach will be easier than the Drag'n'Drop system. In the very least, it will be easier to start with and I might see other ways as I develop that. I sometimes have to remind myself that I don't need to solve all problems in the first go.
avatar
Nevyn
Admin

Posts : 1399
Join date : 2014-09-11

View user profile http://www.nevyns-lab.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

Post by Cr6 on Fri Apr 13, 2018 2:31 am

Totally agree Nevyn... it can get very complex quickly with charge flow replications of classical "bonding".

The electron.pdf is really good for looking at this. He puts a whole slew of new requirements for the flows with (positrons versus the electron):
...
(Mathis)
In fact, we now have strong experimental evidence I am correct. Ultra-intense lasers have been used to irradiate a gold target, producing very large numbers of positrons. There is no indication of beta decay or the weak force in this phenomenon. Rather, it appears to me that the energy of the laser is able to knock positrons directly out of the south poles of the gold nuclei. Properly aimed, the laser may even be knocking positrons out of other positions as well, including positions in the carousel level. Because hundreds of billions of positrons are produced this way—not just a few—it strongly indicates the positrons were in the gold nuclei to start with.

I remind my readers of several things at this juncture. It is known that positrons are created in
radioactive decay, but currently they are said to be created by invoking the weak force (mainly in beta decay). I have shown you this is completely ad hoc as well as unnecessary. If they are already in the nucleus, their appearance in decay doesn't have to be explained as a creation by some new-fangled process. They aren't created, they are simply freed from the south pole and other normal locations in the nucleus. And, as we will see below, in most cases, the leptons “produced” in decay aren't ionized at all. They aren't freed from the nucleus at all. They are free both before and after the decay.
...
I will be told that in most cases, Argon is produced here by electron capture. Only rarely is Argon
produced by positron emission. Yes, but notice that electron capture and positron emission are almost the same thing. The electron and positron are opposite, and the words capture and emission are opposite. So, what we are seeing is not that the positron emission happens less often, but that it is harder for us to detect. It is hard for us to detect the positron leaving the site of this impact, so we simply assume it usually isn't happening. But it is always happening. Only a fraction of the time will we be able to detect the positron leaving. Why? Because in leaving, it overwrites the track of the incoming electron. Since we detect these particles by the tracks they leave, if it overwrites the electron track perfectly, we can't detect it. This means there is no such thing as either electron capture or positron emission (during beta decay). All these events are collisions, not captures or emissions.

Cr6
Admin

Posts : 1099
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

Post by Cr6 on Mon Apr 16, 2018 11:37 pm

Some superconductors can also carry currents of 'spin'
April 16, 2018, University of Cambridge

Conceptual image of spin current flow in a superconductor. Credit: Jason Robinson

Researchers have shown that certain superconductors—materials that carry electrical current with zero resistance at very low temperatures—can also carry currents of 'spin'. The successful combination of superconductivity and spin could lead to a revolution in high-performance computing, by dramatically reducing energy consumption.

Spin is a particle's intrinsic angular momentum, and is normally carried in non-superconducting, non-magnetic materials by individual electrons. Spin can be 'up' or 'down', and for any given material, there is a maximum length that spin can be carried. In a conventional superconductor electrons with opposite spins are paired together so that a flow of electrons carries zero spin.

A few years ago, researchers from the University of Cambridge showed that it was possible to create electron pairs in which the spins are aligned: up-up or down-down. The spin current can be carried by up-up and down-down pairs moving in opposite directions with a net charge current of zero. The ability to create such a pure spin supercurrent is an important step towards the team's vision of creating a superconducting computing technology which could use massively less energy than the present silicon-based electronics.

Now, the same researchers have found a set of materials which encourage the pairing of spin-aligned electrons, so that a spin current flows more effectively in the superconducting state than in the non-superconducting (normal) state. Their results are reported in the journal Nature Materials.

"Although some aspects of normal state spin electronics, or spintronics, are more efficient than standard semiconductor electronics, the large-scale application has been prevented because the large charge currents required to generate spin currents waste too much energy," said Professor Mark Blamire of Cambridge's Department of Materials Science and Metallurgy, who led the research. "A fully-superconducting method of generating and controlling spin currents offers a way to improve on this."

In the current work, Blamire and his collaborators used a multi-layered stack of metal films in which each layer was only a few nanometres thick. They observed that when a microwave field was applied to the films, it caused the central magnetic layer to emit a spin current into the superconductor next to it.

(more at link... https://phys.org/news/2018-04-superconductors-currents.html#nRlv )
....

https://phys.org/news/2015-06-efficient-conversion-currents-superconductor.html


Graphene single photon detectors

September 6, 2017, ICFO
https://phys.org/news/2017-09-graphene-photon-detectors.html

Current detectors are efficient at detecting incoming photons that have relatively high energies, but their sensitivity drastically decreases for low frequency, low energy photons. In recent years, graphene has shown to be an exceptionally efficient photo-detector for a wide range of the electromagnetic spectrum, enabling new types of applications for this field.

Thus, in a recent paper published in the journal Physical Review Applied, and highlighted in APS Physics, ICFO researcher and group leader Prof. Dmitri Efetov, in collaboration with researchers from Harvard University, MIT, Raytheon BBN Technologies and Pohang University of Science and Technology, have proposed the use of graphene-based Josephson junctions (GJJs) to detect single photons in a wide electromagnetic spectrum, ranging from the visible down to the low end of radio frequencies, in the gigahertz range.

Cr6
Admin

Posts : 1099
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

Post by Cr6 on Mon Apr 16, 2018 11:41 pm


Light may unlock a new quantum dance for electrons in graphene
January 15, 2018 by Nina Beier, Joint Quantum Institute


Electrons carry out this choreography—known as the fractional quantum Hall effect—in graphene. Interestingly, tuning the interactions between electrons can coax them into different quantum Hall dance patterns, but it requires a stronger magnet or an entirely different sample—sometimes with two layers of graphene stacked together.

The new work, which is a collaboration between researchers at JQI and the City College of New York, proposes using laser light to circumvent some of these experimental challenges and even create novel quantum dances. The light can prod electrons into jumping between orbits of different energies. As a result, the interactions between the electrons change and lead to a different dance pattern, including some that have never been seen before in experiments. The intensity and frequency of the light alter the number of electrons in specific orbits, providing an easy way to control the electrons' performance. "Such a light-matter interaction results in some models that have previously been studied theoretically," says Mohammad Hafezi, a JQI Fellow and an author of the paper. "But no experimental scheme was proposed to implement them."

Unlocking those theoretical dances may reveal novel quantum behavior. Some may even spawn exotic quantum particles that could collaborate to remain protected from noise—a tantalizing idea that could be useful in the quest to build robust quantum computers.

https://phys.org/news/2018-01-quantum-electrons-graphene.html

Cr6
Admin

Posts : 1099
Join date : 2014-08-09

View user profile http://milesmathis.forumotion.com

Back to top Go down

Re: Mathis on Graphene? Any hints?

Post by Sponsored content


Sponsored content


Back to top Go down

Page 2 of 4 Previous  1, 2, 3, 4  Next

Back to top


 
Permissions in this forum:
You cannot reply to topics in this forum