Magnets levitate above a superconductor: New properties of superconductors discovered

Magnets levitate above a superconductor: New properties of superconductors discovered

Date:
   February 4, 2016
Source:
   University of Waterloo
Summary:
   New findings may eventually lead to a theory of how superconductivity initiates at the atomic level, a key step in understanding how to harness the potential of materials that could provide lossless energy storage, levitating trains and ultra-fast supercomputers.

A magnet levitating above a cuprate high temperature superconductor. New findings from an international collaboration led by Canadian scientists may eventually lead to a theory of how superconductivity initiates at the atomic level, a key step in understanding how to harness the potential of materials that could provide lossless energy storage, levitating trains and ultra-fast supercomputers.

Credit: Robert Hill/University of Waterloo

New findings from an international collaboration led by Canadian scientists may eventually lead to a theory of how superconductivity initiates at the atomic level, a key step in understanding how to harness the potential of materials that could provide lossless energy storage, levitating trains and ultra-fast supercomputers.

Professor David Hawthorn, Professor Michel Gingras, doctoral student Andrew Achkar, and post-doctoral fellow Dr. Zhihao Hao from University of Waterloo's Department of Physics and Astronomy have experimentally shown that electron clouds in superconducting materials can snap into an aligned and directional order called nematicity.

"It has become apparent in the past few years that the electrons involved in superconductivity can form patterns, stripes or checkerboards, and exhibit different symmetries -- aligning preferentially along one direction," said Professor Hawthorn. "These patterns and symmetries have important consequences for superconductivity -- they can compete, coexist or possibly even enhance superconductivity. "

Their results, published today in the journal Science, present the most direct experimental evidence to date of electronic nematicity as a universal feature in cuprate high-temperature superconductors.

"In this study, we identify some unexpected alignment of the electrons -- a finding that is likely generic to the high temperature superconductors and in time may turn out be a key ingredient of the problem," said Professor Hawthorn.

Superconductivity, the ability of a material to conduct an electric current with zero resistance, is best described as an exotic state in high temperature superconductors -- challenging to predict, let alone explain.

The scientists used a novel technique called soft x-ray scattering at the Canadian Light Source synchrotron in Saskatoon to probe electron scattering in specific layers in the cuprate crystalline structure. Specifically, the individual cuprate (CuO2) planes, where electronic nematicity takes place, versus the crystalline distortions in between the CuO2 planes.

Electronic nematicity happens when the electron orbitals align themselves like a series of rods -- breaking their unidirectional symmetry apart from the symmetry of the crystalline structure.

The term "nematicity" commonly refers to when liquid crystals spontaneously align under an electric field in liquid crystal displays. In this case, it is the electronic orbitals that enter the nematic state as the temperature drops below a critical point.

Recent breakthroughs in high-temperature superconductivity have revealed a complex competition between the superconductive state and charge density wave order fluctuations. These periodic fluctuations in the distribution of the electrical charges create areas where electrons bunch up in high- versus low-density clouds, a phenomenon that is now recognized to be generic to the underdoped cuprates.

Results from this study show electronic nematicity also likely occurs in underdoped cuprates. Understanding the relation of nematicity to charge density wave order, superconductivity and an individual material's crystalline structure could prove important to identifying the origins of the superconducting and so-called pseudogap phases.

The authors also found the choice of doping material impacts the transition to the nematic state. Dopants, such as strontium, lanthanum, and even europium added to the cuprate lattice, create distortions in the lattice structure which can either strengthen or weaken nematicity and charge density wave order in the CuO2 layer.

Although there is not yet an agreed upon explanation for why electronic nematicity occurs, it may ultimately present another knob to tune in the quest to achieve the ultimate goal of a room temperature superconductor.

"Future work will tackle how electronic nematicity can be tuned, possibly to advantage, by modifying the crystalline structure," says Hawthorn.
....
Journal Reference:

   A. J. Achkar, M. Zwiebler, C. McMahon, F. He, R. Sutarto, I. Djianto, Z. Hao, M. J. P. Gingras, M. Hucker, G. D. Gu, A. Revcolevschi, H. Zhang, Y.- J. Kim, J. Geck, D. G. Hawthorn. Nematicity in stripe-ordered cuprates probed via resonant x-ray scattering. Science, 2016; 351 (6273): 576 DOI: 10.1126/science.aad1824

Mathis'-Neyn's Boron:

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http://www.nanowerk.com/nanotechnology-news/newsid=43015.php
(more at link...)

Posted: Mar 31, 2016
Flat boron is a superconductor
(Nanowerk News) Rice University scientists have determined that two-dimensional boron is a natural low-temperature superconductor. In fact, it may be the only 2-D material with such potential.

Rice theoretical physicist Boris Yakobson and his co-workers published their calculations that show atomically flat boron is metallic and will transmit electrons with no resistance. The work appears this month in the American Chemical Society journal Nano Letters ("Can Two-Dimensional Boron Superconduct?").

Electrons with opposite momenta and spins pair up via lattice vibrations

Electrons with opposite momenta and spins pair up via lattice vibrations at low temperatures in two-dimensional boron and give it superconducting properties, according to new research by theoretical physicists at Rice University. (Image: Evgeni Penev/Rice University)
The hitch, as with most superconducting materials, is that it loses its resistivity only when very cold, in this case between 10 and 20 kelvins (roughly, minus-430 degrees Fahrenheit). But for making very small superconducting circuits, it might be the only game in town.
The basic phenomenon of superconductivity has been known for more than 100 years, said Evgeni Penev, a research scientist in the Yakobson group, but had not been tested for its presence in atomically flat boron.

"It's well-known that the material is pretty light because the atomic mass is small," Penev said. "If it's metallic too, these are two major prerequisites for superconductivity. That means at low temperatures, electrons can pair up in a kind of dance in the crystal."

"Lower dimensionality is also helpful," Yakobson said. "It may be the only, or one of very few, two-dimensional metals. So there are three factors that gave the initial motivation for us to pursue the research. Then we just got more and more excited as we got into it."

Electrons with opposite momenta and spins effectively become Cooper pairs; they attract each other at low temperatures with the help of lattice vibrations, the so-called "phonons," and give the material its superconducting properties, Penev said. "Superconductivity becomes a manifestation of the macroscopic wave function that describes the whole sample. It's an amazing phenomenon," he said.

It wasn't entirely by chance that the first theoretical paper establishing conductivity in a 2-D material appeared at roughly the same time the first samples of the material were made by laboratories in the United States and China. In fact, an earlier paper by the Yakobson group had offered a road map for doing so.

That 2-D boron has now been produced is a good thing, according to Yakobson and lead authors Penev and Alex Kutana, a postdoctoral researcher at Rice. "We've been working to characterize boron for years, from cage clusters to nanotubes to planer sheets, but the fact that these papers appeared so close together means these labs can now test our theories," Yakobson said.

"In principle, this work could have been done three years ago as well," he said. "So why didn't we? Because the material remained hypothetical; okay, theoretically possible, but we didn't have a good reason to carry it too far.

"But then last fall it became clear from professional meetings and interactions that it can be made. Now those papers are published. When you think it's coming for real, the next level of exploration becomes more justifiable," Yakobson said.

Boron atoms can make more than one pattern when coming together as a 2-D material, another characteristic predicted by Yakobson and his team that has now come to fruition. These patterns, known as polymorphs, may allow researchers to tune the material's conductivity "just by picking a selective arrangement of the hexagonal holes," Penev said.
He also noted boron's qualities were hinted at when researchers discovered more than a decade ago that magnesium diborite is a high-temperature electron-phonon superconductor. "People realized a long time ago the superconductivity is due to the boron layer," Penev said. "The magnesium acts to dope the material by spilling some electrons into the boron layer. In this case, we don't need them because the 2-D boron is already metallic."

Penev suggested that isolating 2-D boron between layers of inert hexagonal boron nitride (aka "white graphene") might help stabilize its superconducting nature.

Physicists discover flaws in superconductor theory
April 8, 2016

(more at link... http://phys.org/news/2016-04-physicists-flaws-superconductor-theory.html#nRlv )

Physicists discover flaws in superconductor theory

This image of a magnet levitated over a high-temperature superconductor array shows rectangular TFMs (black) levitating a heavy ferromagnet (silver) above a container of liquid nitrogen. Credit: Weinstein/University of Houston

University of Houston physicists report finding major theoretical flaws in the generally accepted understanding of how a superconductor traps and holds a magnetic field. More than 50 years ago, C.P. Bean, a scientist at General Electric, developed a theoretical explanation known as the "Bean Model" or "Critical State Model."

The basic property of superconductors is that they represent zero "resistance" to electrical circuits. In a way, they are the opposite of toasters, which resist electrical currents and thereby convert energy into heat. Superconductors consume zero energy and can store it for a long period of time. Those that store magnetic energy —known as "trapped field magnets" or TFMs—can behave like a magnet.

In the Journal of Applied Physics, the researchers describe experiments whose results exhibited "significant deviations" from those of the Critical State Model. They revealed unexpected new behavior favorable to practical applications, including the possibility of using TFMs in myriad new ways.

Much of modern technology is already based on magnets. "Without magnets, we'd lack generators [electric lights and toasters], motors [municipal water supplies, ship engines], magnetrons [microwave ovens], and much more," said Roy Weinstein, lead author of the study, and professor of physics emeritus and research professor at the University of Houston.

Generally, the performance of a device based on magnets improves as the strength of the magnet increases, up to the square of the increase. In other words, if a magnet is 25 times stronger, the device's performance can range from 25 to 625 times better.

TFMs are clearly intriguing, but their use has been largely held back by the challenge of getting the magnetic field into the superconductor. "A more tractable problem is the need to cool the superconductor to the low temperature at which it superconducts," Weinstein explained.

"Bean assumed the superconductor had zero resistance and that the basic laws of electromagnetism, developed circa 1850, were correct," Weinstein said. "And he was able to predict how and where an external magnetic field would enter a superconductor."

The method widely used today is to apply a magnetic field to a superconductor via a pulse field magnet after the superconductor is cooled. Bean's model predicted, and until now experiments confirmed, that to push as much magnetic field as possible into a superconductor, the pulsed field must be at least twice as strong, and more typically over 3.2 times as strong, as the resulting field of the TFM.

But, this severely limits the applicability of TFMs. "It's difficult and expensive to produce fields of more than 12 tesla," said Weinstein. "If Bean's theory held true, this cost and practicality barrier would limit TFMs used within products to a maximum of typically 3.75 tesla."

Minor problems with Bean's Critical State Model emerged shortly after it was published, according to Weinstein. Any chink in theoretical armor is worthy of an exploratory experiment, and this is what motivated Weinstein and his colleagues.

They discovered that for certain constraints on a magnetic pulse, Bean's model is far off base, and a significantly different spatial distribution of field occurs. "Great increases in field occur suddenly, in a single leap, whereas Bean's model predicts a steady, slow increase," Weinstein said.

All of this new, unexpected behavior is repeatable and controllable. "The most encouraging is that we can now produce full-strength TFMs with a pulse strength 1.0 times that of the TFM," he added.

...

The researchers are still within the "early days" of this work and have already disproven their first thoughts concerning what is causing their results. "We're now essentially spelunking in a dark cave without lights—it's frustrating, but exciting," Weinstein said.

In terms of applications for their discovery, the researchers suggest the ability to replace a $100,000 low-temperature superconducting magnet in a research X-ray machine with a $300 TFM, or possibly replace a motor with one that is a quarter of the size of an existing one. There are many other potential applications, such as an energy-efficient ore separator, noncontact magnetic gears that will not wear or require repair, a red blood separator with 50 percent improved yield, and even an automated docking system for spacecraft.

Weinstein and colleagues are now searching for fast, short-term support that will allow them to continue their research to explain this new phenomenon. "While we now know enough to apply our new discovery to significantly improve a large number of devices, we don't yet fully know what's going on in terms of the basic laws of physics," he noted.

More information: "Anomalous results observed in magnetization of bulk high temperature superconductors - a windfall for applications," by Roy Weinstein, Drew Parks, Ravi-Persad Sawh, Keith Carpenter and Kent Davey, Journal of Applied Physics April 7, 2016, DOI: 10.1063/1.4945018


Read more at: http://phys.org/news/2016-04-physicists-flaws-superconductor-theory.html#jCp