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  • richardmitnick 3:16 pm on September 18, 2016 Permalink | Reply
    Tags: , , Researchers see individual atoms keep away from each other or bunch up as pairs, Superconductivity   

    From MIT: “For first time, researchers see individual atoms keep away from each other or bunch up as pairs” 

    MIT News
    MIT News
    MIT Widget

    September 15, 2016
    Jennifer Chu

    1
    “Learning from this model, we can understand what’s really going on in these superconductors, and what one should do to make higher-temperature superconductors, approaching hopefully room temperature,” says Martin Zwierlein, professor of physics and principal investigator in MIT’s Research Laboratory of Electronics. Illustration: Sampson Wilcox

    2
    Illustration of atoms on a lattice. Credit: Christine Daniloff/MIT. Science Alert

    Observations of atomic interactions could help pave way to room-temperature superconductors.

    If you bottle up a gas and try to image its atoms using today’s most powerful microscopes, you will see little more than a shadowy blur. Atoms zip around at lightning speeds and are difficult to pin down at ambient temperatures.

    If, however, these atoms are plunged to ultracold temperatures, they slow to a crawl, and scientists can start to study how they can form exotic states of matter, such as superfluids, superconductors, and quantum magnets.

    Physicists at MIT have now cooled a gas of potassium atoms to several nanokelvins — just a hair above absolute zero — and trapped the atoms within a two-dimensional sheet of an optical lattice created by crisscrossing lasers. Using a high-resolution microscope, the researchers took images of the cooled atoms residing in the lattice.

    By looking at correlations between the atoms’ positions in hundreds of such images, the team observed individual atoms interacting in some rather peculiar ways, based on their position in the lattice. Some atoms exhibited “antisocial” behavior and kept away from each other, while some bunched together with alternating magnetic orientations. Others appeared to piggyback on each other, creating pairs of atoms next to empty spaces, or holes.

    The team believes that these spatial correlations may shed light on the origins of superconducting behavior. Superconductors are remarkable materials in which electrons pair up and travel without friction, meaning that no energy is lost in the journey. If superconductors can be designed to exist at room temperature, they could initiate an entirely new, incredibly efficient era for anything that relies on electrical power.

    Martin Zwierlein, professor of physics and principal investigator at MIT’s NSF Center for Ultracold Atoms and at its Research Laboratory of Electronics, says his team’s results and experimental setup can help scientists identify ideal conditions for inducing superconductivity.

    “Learning from this atomic model, we can understand what’s really going on in these superconductors, and what one should do to make higher-temperature superconductors, approaching hopefully room temperature,” Zwierlein says.

    Zwierlein and his colleagues’ results appear in the Sept. 16 issue of the journal Science. Co-authors include experimentalists from the MIT-Harvard Center for Ultracold Atoms, MIT’s Research Laboratory of Electronics, and two theory groups from San Jose State University, Ohio State University, the University of Rio de Janeiro, and Penn State University.

    “Atoms as stand-ins for electrons”

    Today, it is impossible to model the behavior of high‐temperature superconductors, even using the most powerful computers in the world, as the interactions between electrons are very strong. Zwierlein and his team sought instead to design a “quantum simulator,” using atoms in a gas as stand-ins for electrons in a superconducting solid.

    The group based its rationale on several historical lines of reasoning: First, in 1925 Austrian physicist Wolfgang Pauli formulated what is now called the Pauli exclusion principle, which states that no two electrons may occupy the same quantum state — such as spin, or position — at the same time. Pauli also postulated that electrons maintain a certain sphere of personal space, known as the “Pauli hole.”

    His theory turned out to explain the periodic table of elements: Different configurations of electrons give rise to specific elements, making carbon atoms, for instance, distinct from hydrogen atoms.

    The Italian physicist Enrico Fermi soon realized that this same principle could be applied not just to electrons, but also to atoms in a gas: The extent to which atoms like to keep to themselves can define the properties, such as compressibility, of a gas.

    “He also realized these gases at low temperatures would behave in peculiar ways,” Zwierlein says.

    British physicist John Hubbard then incorporated Pauli’s principle in a theory that is now known as the Fermi-Hubbard model, which is the simplest model of interacting atoms, hopping across a lattice. Today, the model is thought to explain the basis for superconductivity. And while theorists have been able to use the model to calculate the behavior of superconducting electrons, they have only been able to do so in situations where the electrons interact weakly with each other.

    “That’s a big reason why we don’t understand high-temperature superconductors, where the electrons are very strongly interacting,” Zwierlein says. “There’s no classical computer in the world that can calculate what will happen at very low temperatures to interacting [electrons]. Their spatial correlations have also never been observed in situ, because no one has a microscope to look at every single electron.”

    Carving out personal space

    Zwierlein’s team sought to design an experiment to realize the Fermi-Hubbard model with atoms, in hopes of seeing behavior of ultracold atoms analogous to that of electrons in high-temperature superconductors.

    The group had previously designed an experimental protocol to first cool a gas of atoms to near absolute zero, then trap them in a two-dimensional plane of a laser-generated lattice. At such ultracold temperatures, the atoms slowed down enough for researchers to capture them in images for the first time, as they interacted across the lattice.

    At the edges of the lattice, where the gas was more dilute, the researchers observed atoms forming Pauli holes, maintaining a certain amount of personal space within the lattice.

    “They carve out a little space for themselves where it’s very unlikely to find a second guy inside that space,” Zwierlein says.

    Where the gas was more compressed, the team observed something unexpected: Atoms were more amenable to having close neighbors, and were in fact very tightly bunched. These atoms exhibited alternating magnetic orientations.

    “These are beautiful, antiferromagnetic correlations, with a checkerboard pattern — up, down, up, down,” Zwierlein describes.

    At the same time, these atoms were found to often hop on top of one another, creating a pair of atoms next to an empty lattice square. This, Zwierlein says, is reminiscent of a mechanism proposed for high-temperature superconductivity, in which electron pairs resonating between adjacent lattice sites can zip through the material without friction if there is just the right amount of empty space to let them through.

    Ultimately, he says the team’s experiments in gases can help scientists identify ideal conditions for superconductivity to arise in solids.

    Zwierlein explains: “For us, these effects occur at nanokelvin because we are working with dilute atomic gases. If you have a dense piece of matter, these same effects may well happen at room temperature.”

    Currently, the team has been able to achieve ultracold temperatures in gases that are equivalent to hundreds of kelvins in solids. To induce superconductivity, Zwierlein says the group will have to cool their gases by another factor of five or so.

    “We haven’t played all of our tricks yet, so we think we can get colder,” he says.

    This research was supported in part by the National Science Foundation, the Air Force Office of Scientific Research, the Army Research Office, and the David and Lucile Packard Foundation.

    The study has been published in Science.

    See the full article here .

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  • richardmitnick 11:44 am on August 18, 2016 Permalink | Reply
    Tags: , , Superconductivity   

    From BNL: “Scientists Uncover the Origin of High-Temperature Superconductivity in Copper-Oxide Compound” 

    Brookhaven Lab

    August 17, 2016
    Ariana Tantillo
    atantillo@bnl.gov
    (631) 344-2347

    Peter Genzer,
    genzer@bnl.gov
    (631) 344-3174

    Analysis of thousands of samples reveals that the compound becomes superconducting at an unusually high temperature because local electron pairs form a “superfluid” that flows without resistance.

    1
    (Clockwise from left) Brookhaven Lab physicists Ivan Bozovic, Anthony Bollinger, and Jie Wu, and postdoctoral researcher Xi He with the atomic layer-by-layer molecular beam epitaxy system used to synthesize more than 2,500 thin films of a copper-oxide compound called LSCO. The team studied LSCO to understand why it can become superconducting at a much higher temperature than the ultra-chilled temperatures required by conventional superconductors.

    Since the 1986 discovery of high-temperature superconductivity in copper-oxide compounds called cuprates, scientists have been trying to understand how these materials can conduct electricity without resistance at temperatures hundreds of degrees above the ultra-chilled temperatures required by conventional superconductors. Finding the mechanism behind this exotic behavior may pave the way for engineering materials that become superconducting at room temperature. Such a capability could enable lossless power grids, more affordable magnetically levitated transit systems, and powerful supercomputers, and change the way energy is produced, transmitted, and used globally.

    Now, physicists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory have an explanation for why the temperature at which cuprates become superconducting is so high. After growing and analyzing thousands of samples of a cuprate known as LSCO for the four elements it contains (lanthanum, strontium, copper, and oxygen), they determined that this “critical” temperature is controlled by the density of electron pairs—the number of electron pairs per unit area. This finding, described in a Nature paper published August 17, challenges the standard theory of superconductivity, which proposes that the critical temperature depends instead on the strength of the electron pairing interaction.

    “Solving the enigma of high-temperature superconductivity has been the focus of condensed matter physics for more than 30 years,” said Ivan Bozovic, a senior physicist in Brookhaven Lab’s Condensed Matter Physics and Materials Science Department who led the study. “Our experimental finding provides a basis for explaining the origin of high-temperature superconductivity in the cuprates—a basis that calls for an entirely new theoretical framework.”

    According to Bozovic, one of the reasons cuprates have been so difficult to study is because of the precise engineering required to generate perfect crystallographic samples that contain only the high-temperature superconducting phase.

    “It is a materials science problem. Cuprates can have up to 50 atoms per unit cell and the elements can form hundreds of different compounds, likely resulting in a mixture of different phases,” said Bozovic.

    That’s why Bozovic and his research team grew their more than 2,500 LSCO samples by using a custom-designed molecular beam epitaxy system that places single atoms onto a substrate, layer by layer. This system is equipped with advanced surface-science tools, such as those for absorption spectroscopy and electron diffraction, that provide real-time information about the surface morphology, thickness, chemical composition, and crystal structure of the resulting thin films.

    “Monitoring these characteristics ensures there aren’t any irregular geometries, defects, or precipitates from secondary phases in our samples,” Bozovic explained.

    In engineering the LSCO films, Bozovic chemically added strontium atoms, which produce mobile electrons that pair up in the copper-oxide layers where superconductivity occurs. This “doping” process allows LSCO and other cuprates—normally insulating materials—to become superconducting.

    For this study, Bozovic added strontium in amounts beyond the doping level required to induce superconductivity. Earlier studies on this “overdoping” had indicated that the density of electron pairs decreases as the doping concentration is increased. Scientists had tried to explain this surprising experimental finding by attributing it to different electronic orders competing with superconductivity, or electron pair breaking caused by impurities or disorder in the lattice. For example, they had thought that geometrical defects, such as displaced or missing atoms, could be at play.


    Brookhaven Lab physicist Ivan Bozovic explains why a copper-oxide compound can conduct electricity without resistance at temperatures well above those required by conventional superconductors.

    To test these explanations, Bozovic and his team measured the magnetic and electronic properties of their engineered LSCO films. They used a technique called mutual inductance to determine the magnetic penetration depth (the distance a magnetic field transmits through a superconductor), which indicates the density of electron pairs.

    Their measurements established a precise linear relationship between the critical temperature and electron pair density: both continue to decrease as more dopant is added, until no electrons pair up at all, while the critical temperature drops to near-zero Kelvin (minus 459 degrees Fahrenheit). According to the standard understanding of metals and conventional superconductors, this result is unexpected because LSCO becomes more metallic the more it is overdoped.

    “Disorder, phase separation, or electron pair breaking would have the reverse effect by introducing scattering that impedes the flow of electrons, thus making the material more resistive, i.e., less metallic,” said Bozovic.

    If Bozovic’s team is correct that critical temperature is controlled by electron pair density, then it seems that small, local pairs of electrons are behind the high temperature at which cuprates become superconducting. Previous experiments have established that the size of electron pairs is much smaller in cuprates than in conventional superconductors, whose pairs are so large that they overlap. Understanding what interaction makes the electron pairs so small in cuprates is the next step in the quest to solve the mystery of high-temperature superconductivity.

    Bozovic’s team included Brookhaven physicists Anthony Bollinger and Jie Wu, supported by funding provided by DOE’s Office of Science, and postdoctoral researcher Xi He, supported by the Gordon and Betty Moore Foundation.

    See the full article here .

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  • richardmitnick 11:23 am on July 28, 2016 Permalink | Reply
    Tags: , , , Superconductivity   

    From Carnegie: “New Material Could Advance Superconductivity” 

    Carnegie Institution for Science
    Carnegie Institution for Science

    July 28, 2016

    No writer credit found

    1
    At center, in green, is the new three-atom hydrogen “chain.” It is surrounded by several “normal” two-atom molecules of hydrogen, also in green. The new chain configuration appears in the new material NaH7, which was produced under high pressure and high temperature conditions. The new material could change the superconductivity landscape and be useful for hydrogen storage in hydrogen fuel cells. Image courtesy Duck Young Kim

    Scientists have looked for different ways to force hydrogen into a metallic state for decades. A metallic state of hydrogen is a holy grail for materials science because it could be used for superconductors, materials that have no resistance to the flow of electrons, which increases electricity transfer efficiency many times over. For the first time researchers, led by Carnegie’s Viktor Struzhkin, have experimentally produced a new class of materials blending hydrogen with sodium that could alter the superconductivity landscape and could be used for hydrogen-fuel cell storage. The research is published in Nature Communications.

    It had been predicted that certain hydrogen-rich compounds consisting of multiple atoms of hydrogen with so-called alkali metals like lithium, potassium or sodium, could provide a new chemical means to alter the compound’s electronic structure. This, in turn, may lead the way to metallic high-temperature superconductors.

    “The challenge is temperature,” explained Struzhkin. “The only superconductors that have been produced can only exist at impractically cold temperatures. In recent years, there have been predictions of compounds with several atoms of hydrogen coupled with alkali metals that could exist at more practical temperatures. They are theorized to have unique properties useful to superconductivity.”

    Now, the predictions have been confirmed. The Struzhkin team included Carnegie researchers Duck Young Kim, Elissaios Stavrou, Takaki Muramatsu, Ho-Kwang Mao, and Alexander Goncharov, with researchers from other institutions.*

    The team used theory to guide their experiments and measured the samples using both a method that reveals the atomic structure (X-ray diffraction) and a method that identifies molecules by characteristics such as their minute vibrations and rotations (Raman spectroscopy). Theoretically, the sodium/hydrogen material would be stable under pressure, have metallic characteristics and unique structures, and show superconducting properties.

    The team conducted high-pressure/high-temperature experiments. Matter under these extreme conditions can morph into new structures with new properties. They squeezed lithium and sodium samples in a diamond anvil cell to enormous pressures while heating the samples using a laser. At pressures between 300,000 and 400,000 atmospheres (30-40 gigapascals, or GPa) and temperatures of about 3100°F (2000 kelvin), they observed, for the first time, structures of “polyhydrides,” sodium with 3 hydrogen atoms (NaH3) and NaH7—sodium with seven atoms of hydrogen—in very unusual configurations. Three negative charged hydrogen atoms in the NaH7 material lined up and looked like one-dimensional hydrogen chains, which is a new phase that is very different from pure hydrogen.

    “This configuration was originally predicted to exist in 1972, more than 40 years ago,” remarked Duck Young Kim. “It turns out that our experiments are in complete agreement with the theory, which predicted the existence of NaH3. The bonus is that we also observed the compound with seven hydrogen atoms.”

    Struzhkin reflected, “Further work needs to be done to see if materials in this class can be produced at lower temperatures and pressures. But this new class of matter opens up a whole new world of possibilities.”

    *Other researcher include Chris Pickard with the University College, London; Richard Needs of the Cavendish Laboratory in the UK; and Vitali Prakapenda of the University of Chicago. This work was supported by the DOE/BES; the Energy Frontier Research in Extreme Environments Center (EFree); the Engineering and Physical Sciences Research Council (EPSRC) of the UK; DARPA; and NSFC.

    See the full article here .

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  • richardmitnick 12:43 pm on April 13, 2016 Permalink | Reply
    Tags: , , Superconductivity   

    From BNL: “Elusive State of Superconducting Matter Discovered after 50 Years” 

    Brookhaven Lab

    Karen McNulty Walsh
    (631) 344-8350
    kmcnulty@bnl.gov

    Peter Genzer
    (631) 344-3174
    genzer@bnl.gov

    1
    A schematic image representing a periodic variation in the density of Cooper pairs (pairs of blue arrows pointing in opposite directions) within a cuprate superconductor. Densely packed rows of Cooper pairs alternate with regions having lower pair density and no pairs at all. Such a “Cooper pair density wave” was predicted 50 years ago but was just discovered using a unique “scanning Josephson tunneling microscope. No image credit

    The prediction was that “Cooper pairs” of electrons in a superconductor could exist in two possible states. They could form a “superfluid” where all the particles are in the same quantum state and all move as a single entity, carrying current with zero resistance — what we usually call a superconductor. Or the Cooper pairs could periodically vary in density across space, a so-called “Cooper pair density wave.” For decades, this novel state has been elusive, possibly because no instrument capable of observing it existed.

    Now a research team led by J.C. Séamus Davis, a physicist at Brookhaven Lab and the James Gilbert White Distinguished Professor in the Physical Sciences at Cornell, and Andrew P. Mackenzie, Director of the Max-Planck Institute CPMS in Dresden, Germany, has developed a new way to use a scanning tunneling microscope (STM) to image Cooper pairs directly.

    The studies were carried out by research associate Mohammed Hamidian (now at Harvard) and graduate student Stephen Edkins (St. Andrews University in Scotland), working as members of Davis’ research group at Cornell and with Kazuhiro Fujita, a physicist in Brookhaven Lab’s Condensed Matter Physics and Materials Science Department.

    Superconductivity was first discovered in metals cooled almost to absolute zero (-273.15 degrees Celsius or -459.67 Fahrenheit). Recently developed materials called cuprates – copper oxides laced with other atoms – superconduct at temperatures as “high” as 148 degrees above absolute zero (-125 Celsius). In superconductors, electrons join in pairs that are magnetically neutral so they do not interact with atoms and can move without resistance.

    Hamidian and Edkins studied a cuprate incorporating bismuth, strontium, and calcium (Bi2Sr2CaCu2O8) using an incredibly sensitive STM that scans a surface with sub-nanometer resolution, on a sample that is refrigerated to within a few thousandths of a degree above absolute zero.

    At these temperatures, Cooper pairs can hop across short distances from one superconductor to another, a phenomenon known as Josephson tunneling. To observe Cooper pairs, the researchers briefly lowered the tip of the probe to touch the surface and pick up a flake of the cuprate material. Cooper pairs could then tunnel between the superconductor surface and the superconducting tip. The instrument became, Davis said, “the world’s first scanning Josephson tunneling microscope.”

    Flow of current made of Cooper pairs between the sample and the tip reveals the density of Cooper pairs at any point, and it showed periodic variations across the sample, with a wavelength of four crystal unit cells. The team had found a Cooper pair density wave state in a high-temperature superconductor, confirming the 50-year-old prediction.

    A collateral finding was that Cooper pairs were not seen in the vicinity of a few zinc atoms that had been introduced as impurities, making the overall map of Cooper pairs into “Swiss cheese.”

    The researchers noted that their technique could be used to search for Cooper-pair density waves in other cuprates as well as more recently discovered iron-based superconductors.

    This work was supported by a grant to Davis from the EPiQS Program of the Gordon and Betty Moore Foundation and by the U.S. Department of Energy’s Office of Science. The collaboration also included scientists in Scotland, Germany, Japan and Korea.

    Science paper: Detection of a Cooper-pair density wave in Bi2Sr2CaCu2O8+x

    These authors contributed equally to this work.
    M. H. Hamidian, S. D. Edkins & Sang Hyun Joo

    Authors and Affiliations

    Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA
    M. H. Hamidian
    Laboratory of Atomic and Solid State Physics, Department of Physics, Cornell University, Ithaca, New York 14853, USA
    S. D. Edkins, A. Kostin, M. J. Lawler, E.-A. Kim & J. C. Séamus Davis
    School of Physics and Astronomy, University of St Andrews, Fife KY16 9SS, UK
    S. D. Edkins, A. P. Mackenzie & J. C. Séamus Davis
    Institute of Applied Physics, Department of Physics and Astronomy, Seoul National University, Seoul 151-747, South Korea
    Sang Hyun Joo & Jinho Lee
    Center for Correlated Electron Systems, Institute of Basic Science, Seoul 151-742, South Korea
    Sang Hyun Joo & Jinho Lee
    Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8568, Japan
    H. Eisaki & S. Uchida
    Department of Physics, University of Tokyo, Bunkyo, Tokyo 113-0011, Japan
    S. Uchida
    Department of Physics, Binghamton University, Binghamton, New York 13902-6000, USA
    M. J. Lawler
    Max Planck Institute for Chemical Physics of Solids, D-01187 Dresden, Germany
    A. P. Mackenzie
    Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA
    K. Fujita & J. C. Séamus Davis
    Kavli Institute at Cornell for Nanoscale Science, Cornell University, Ithaca, New York 14853, USA
    J. C. Séamus Davis

    Contributions

    M.H.H., S.D.E., A.K., and J.L. developed the SJTM techniques and carried out the experiments. K.F., H.E. and S.U. synthesized and characterized the samples. M.H.H., S.D.E., A.K., S.H.J. and K.F. developed and carried out analyses. E.-A.K. and M.J.L. provided theoretical guidance. A.P.M., J.L. and J.C.S.D. supervised the project and wrote the paper with key contributions from M.H.H., S.D.E. and K.F. The manuscript reflects the contributions and ideas of all authors.

    See the full article here .

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  • richardmitnick 5:57 am on March 5, 2016 Permalink | Reply
    Tags: , Superconductivity,   

    From UC Riverside: “A Proposed Superconductivity Theory Receives Exclusive Experimental Confirmation” 

    UC Riverside bloc

    UC Riverside

    March 4, 2016
    Iqbal Pittalwala

    UC Riverside physicist Chandra Varma proposed the theory

    Superconductivity – a quantum phenomenon in which metals below a certain temperature develop flow of current with no loss or resistance – is one of the most exciting problems in physics, which has resulted in investments worldwide of enormous brain power and resources since its discovery a little over a century ago. Many prominent theorists, Nobel laureates among them, have proposed theories for new classes of superconducting materials discovered several decades later, followed by teams of experimentalists working furiously to provide solid evidence for these theories. More than 100,000 research papers have been published on the new materials.

    One such theory began with a proposal in 1989 by Chandra Varma while at Bell Laboratories, NJ, and now a distinguished professor of physics and astronomy at the University of California, Riverside. At UC Riverside, he further developed the theory and proposed experiments to confirm or refute it. That theory has now been experimentally proven to be a consistent theory by physicists in China and Korea.

    The experimental results, published in Science Advances today (March 4), now allow for a clear discrimination of theories of high-temperature superconductivity, favoring one and ruling others out. The research paper is titled Quantitative determination of pairing interactions for high-temperature superconductivity in cuprates.

    “At the core of most models for the high-temperature superconductivity in cuprates lies the idea of the electron-electron pairing,” said Lev P. Gor’kov, a theoretical physicist at Florida State University who is renowned for making the most important formal advance in the superconductivity field in 1958, while at the Soviet Academy of Sciences. “The paper by Prof. Chandra Varma and his colleagues from China and Korea is the daring and successful attempt to extract the relevant electron-electron interactions directly from experiment. Their elegant approach opens new prospects also for studying the superconductivity mechanisms in other systems with strongly correlated electrons.”

    A boon to technology

    Superconductors are used in magnetic-imaging devices in hospitals. They are used, too, for special electrical switches. The electromagnets used in the Large Hadron Collider at CERN use superconducting wire. Large-scale use of superconductivity, however, is not feasible presently because of cost. If superconductors could be made cheaply and at ordinary temperatures, they would find wide use in power transmission, energy storage and magnetic levitation.

    First discovered in the element mercury in 1911, superconductivity is said to occur when electrical resistance in a solid vanishes when that solid is cooled below a characteristic temperature, called the transition temperature, which varies from material to material. Transition temperatures tend to be close to 0 K or -273 C. At even slightly higher temperatures, the materials tend to lose their superconducting properties; indeed, at room temperature most superconductors are very poor conductors. In 1987, some high-temperature superconductors, called cuprates, were discovered by physicists Georg Bednorz and Alexander Müller, so named because they all contain copper and oxygen. These new materials have properties which have raised profound new questions. Why these high-temperature superconductors perform as they do has remained unknown.

    A brief history lesson

    The superconductivity problem was considered solved by a theory proposed in 1957: the BCS theory of superconductivity. This comprehensive theory, developed by physicists John Bardeen, Leon Cooper and John Schrieffer (the first letter of their last names gave the theory its name), explained the behavior of superconducting materials as resulting from electrons forming pairs, with each pair being strongly correlated with other pairs, allowing them all to function coherently as a single entity. Concepts in the BCS theory and its elaborations have influenced all branches of physics, ranging from elementary particle physics to cosmology.

    “But in the cuprates, some of the founding concepts of the physics of interacting particles, such as the quasi-particle concept, were found to be invalid,” Varma said. “The physical properties of superconductors above the superconducting transition temperature were more remarkable than the superconductivity itself. Subsequently, almost all the leading theoretical physicists in the world proposed different directions of ideas and calculations to explain these properties as well as superconductivity. But very few predictions stemming from these ideas were verified, and specific experiments were not in accord with them.”

    A quasi-particle is a packet of energy and momentum that can, in some respects, be regarded as a particle. It is a physical concept, which allows detailed calculation of properties of matter.

    In 1989, while at Bell Laboratories, Varma and some collaborators proposed that the breakdown of the quasi-particle concept occurs due to a simple form of quantum-critical fluctuations – fluctuations which are quantum in nature and occur when symmetry of matter breaks down, such as at the phase transition critical point near absolute zero of temperature.

    In physics, symmetry is said to occur when some change in orientation or movement by any amount leaves the physical situation unchanged (empty space, for example, has symmetry because it is everywhere the same). Relativity, quantum theory, crystallography and spectroscopy involve notions of symmetry.

    “It was at this time that we introduced the concept of marginal Fermi-liquids or marginal quasi-particles through which various properties of superconductivity were explained,” Varma said. “We also provided some definitive predictions, which could only be tested in 2000 by a new technique called Angle Resolved Photoemissions or ARPES.”

    Varma explained that in 1989 there was also no evidence that the same quantum-critical fluctuations promoted the superconductivity transition.

    “There was no theory for the cause of such quantum-critical fluctuations or for the symmetry which must change near absolute zero to realize them,” he said.

    In 1997, Varma proposed transitions to a new class of symmetries, in which the direction of time was picked by the direction of currents. These currents, he suggested, begin to spontaneously flow in each microscopic cell of the cuprates. Since 2004, a group of French scientists at Saclay has been reporting evidence of such symmetries in every high-temperature superconducting compound it could investigate with neutron scattering. Several other kinds of experiments by other research groups are in accord also.

    Varma cautioned that some unresolved issues persist. His group is proposing experiments to address them.

    In 2003, the year Varma moved to UC Riverside, he formulated a theory for how quantum fluctuations coupled to electrons give rise to the observed symmetry in superconductivity.

    “This was a completely new kind of coupling,” he said. “It had very remarkable and unusual predictions for experiments designed to decipher such a coupling.”

    ARPES to the rescue

    In 2010, Varma became aware of high-quality laser-based ARPES in a laboratory at the Institute of Physics in the Chinese Academy of Sciences, Beijing, China. A collaboration with physicist Xingjiang Zhou at the institute ensued, with numerical analysis of the data being done by Han-Yong Choi, a physicist at SungKyunKwan University, Korea, who, in the past, worked with Varma at UCR.

    Zhou’s team made several improvements in the ARPES technique, which ensured that the quality of data was high and reproducible enough to have full confidence.

    “The data obtained and the analysis we describe in our paper are conclusive on the most important issues relevant to superconductivity,” Varma said. “Our conclusions – namely, that the quantum fluctuations promoting superconductivity are the same as those that lead to the marginal Fermi-liquid and they are consistently of the form predicted, being stretched exponentially in time in a scale-invariant way relative to stretching in space – also have no theoretical approximations. They are as precise as the quality of the data allows. They also unambiguously address the question of symmetry of superconductivity. Further, they rule out many of the alternative ideas that have been proposed on this problem in the last thirty years since the original discovery. Our observations of the breakdown of time-reversal symmetry and of the fluctuations that follow complete major aspects of our understanding of these problems.”

    Varma, Zhou and Choi were joined in the research by Jin Mo Bok (first author of the paper) and Jong Ju Bae at SungKyunKwan University, Korea; and Wentai Zhang, Junfeng He, Yuxiao Zhang and Li Yu at the Institute of Physics, Chinese Academy of Sciences, Beijing, China. Varma was partially supported by a grant from the National Science Foundation.

    About Varma

    After he received his doctoral degree in physics from the University of Minnesota, Varma joined Bell Labs in 1969, one of the most coveted positions at the time for young physicists anywhere in the world. The following year, he became a permanent member of the laboratory. He was the head of the theoretical physics department at Bell Labs from 1983 to 1987, and was awarded the Distinguished Member of Research in 1988. He has served as a visiting professor at the University of Chicago, Stanford University, MIT, the College de France in Paris, France, and at CNRS, France; and a senior visiting fellow at Cavendish Lab at Cambridge University. In 2000, he was selected to the Lorentz Visiting Chair at Leiden University, the Netherlands. In 2009, he held a Miller Professorship at UC Berkeley.

    He is a fellow of the American Physical Society and of the American Association for the Advancement of Science. A member of the World Academy of Sciences, he is the recipient of the Alexander Humboldt Prize and the Bardeen Prize for theoretical advances in superconductivity.

    Varma has published nearly 200 scientific papers, which have in all about 18,000 citations. He has made seminal contributions to the theory of glasses, to Kondo and mixed valence and heavy-fermion phenomena, novel forms of superconductivity, charge density waves, co-existing magnetic and superconducting states, the Higgs boson in superconductors, quantum criticality, singular Fermi-liquids and associated superconductivity.

    See the full article here .

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    The University of California, Riverside is one of 10 universities within the prestigious University of California system, and the only UC located in Inland Southern California.

    Widely recognized as one of the most ethnically diverse research universities in the nation, UCR’s current enrollment is more than 21,000 students, with a goal of 25,000 students by 2020. The campus is in the midst of a tremendous growth spurt with new and remodeled facilities coming on-line on a regular basis.

    We are located approximately 50 miles east of downtown Los Angeles. UCR is also within easy driving distance of dozens of major cultural and recreational sites, as well as desert, mountain and coastal destinations.

     
  • richardmitnick 5:09 pm on February 5, 2016 Permalink | Reply
    Tags: , , , , Superconductivity   

    From SA: “Taming Superconductors with String Theory” 

    Scientific American

    Scientific American

    February 4, 2016
    Kevin Hartnett, Quanta Magazine

    The physicist Subir Sachdev borrows tools from string theory to understand the puzzling behavior of superconductors.

    String theory was devised as a way to unite the laws of quantum mechanics with those of gravity [General Relativity], with the goal of creating the vaunted theory of everything.

    Subir Sachdev is taking the “everything” literally. He’s applying the mathematics of string theory to a major problem at the other end of physics — the behavior of a potentially revolutionary class of materials known as high-temperature superconductors.

    Superconductivity
    Superconductivity

    These materials are among the most promising and the most perplexing. Unlike regular superconductors, which need to be cooled almost to absolute zero (–273.15 degrees Celsius) to pass a frictionless current of electricity, high-temperature superconductors yield the same remarkable performance under more accommodating conditions. Since the first high-temperature superconductor was discovered in 1986, physicists have found other materials that exhibit superconductivity at successively higher temperatures, with the current record standing at –70 degrees Celsius.

    This progress has occurred despite the fact that physicists don’t understand how these superconductors work. Broadly speaking, many condensed-matter physicists study how electrons — the carriers of electrical current — move through a given material. In an ordinary conductor like copper or gold, the electrons flow through a lattice formed by the copper or gold atoms. In an insulator like diamond, electrons tend to stay put. In superconductors, electrons move through the underlying atomic lattice with no energy loss at all. For three decades, physicists have been unable to develop a comprehensive theory that explains how electrons in high-temperature superconductors behave.

    A particularly interesting question is how the behavior of the material changes with temperature — in particular, how conductors transition from ordinary to super as the temperature drops. Scientists call this a “quantum phase change,” with the two phases being the property of the material on either side of the transition temperature.

    Sachdev, a condensed-matter physicist at Harvard University, explains that the challenge is one of scale. A typical chunk of material has trillions upon trillions of electrons. When those electrons interact with one another — as they do in superconductors — they become impossible to keep track of. In some phases of matter, physicists have been able to overcome this scale issue by modeling swarms of electrons as quasiparticles, quantum excitations that behave a lot like individual particles. But the quasiparticle strategy doesn’t work in high-temperature superconductors, forcing physicists to look for another way to impose collective order on the behavior of electrons in these materials.

    In 2007 Sachdev had a startling insight: He realized that certain features of string theory correspond to the electron soup found in high-temperature superconductors. In the years since, Sachdev has developed models in string theory that offer ways to think about the electron behavior in high-temperature superconductors. He’s used these ideas to design real-world experiments with materials like graphene — a flat sheet of carbon atoms — which have properties in common with the materials that interest him.

    In a forthcoming paper in Science, he and his collaborators use methods borrowed from string theory to correctly predict experimental results related to the flow of heat and electrical charge in graphene. Now he hopes to apply his insights to high-temperature superconductors themselves.

    Quanta Magazine spoke with Sachdev about how the electrons in high-temperature superconductors are related to black holes, his recent success with graphene, and why the biggest name in condensed-matter physics is skeptical that the string-theory approach works at all. An edited and condensed version of the interview follows.

    QUANTA MAGAZINE: What’s going on inside a high-temperature superconductor?
    SUBIR SACHDEV: The difference between old materials and the new materials is that in older materials, electrons conduct electricity independent of one another. They obey the exclusion principle, which says electrons can’t occupy the same quantum state at the same time and that they move independently of one another. In the new materials that I, and many others, have been studying, it’s clear that this independent-electron model fails. The general picture is that they move cooperatively and, in particular, they’re entangled — their quantum properties are linked.

    This entanglement makes high-temperature superconductors much more complicated to model than regular superconductors. How have you been looking at the problem?
    Generally I approach this through the classification of the quantum phases of matter. Examples of simple quantum phases are simple metals like silver and gold, or simple insulators like diamonds. Many of these phases are well-understood and appear everywhere in our daily lives. Since we discovered high-temperature superconductors, and many other new materials, we’ve been trying to understand the other physical properties that can emerge when you have trillions of electrons obeying quantum principles and also interacting with each other. At the back of my mind is the hope that this broad attack on classifying quantum phases of matter will lead to a deeper understanding of high-temperature superconductors.

    How far have you gotten?
    There has been great progress in understanding the theory of quantum phase transitions, which involves taking two phases of quantum matter that are very different from each other and adjusting some parameter — say, pressure on a crystal — and asking what happens when the material goes from one phase to the other. There has been a huge amount of progress for a wide class of quantum phase transitions. We now understand many different kinds of phases we didn’t know existed before.

    But a full theory of how electrons behave in high-temperature superconductors has been difficult to develop. Why?
    If you have a single electron moving through a lattice, then you really only need to worry about the different positions that electron can occupy. Even though the number of positions is large, that pretty much is something you can handle on a computer.

    But once you start talking about many electrons, you have to think about it very differently. One way to think about it is to imagine that each site on the lattice can be either empty or full. With N sites it’s 2N, so the possibilities are unimaginably vast. In this vast set of possibilities, you have to classify what are reasonable things an electron would tend to do. That in a nutshell is why it’s a difficult problem.

    Returning to phase transitions, you’ve spent a lot of time studying what happens to a high-temperature superconductor when it grows too warm. At this point, it becomes a so-called “strange metal.” Why would understanding strange metals help you to understand high-temperature superconductors?
    If you start with a superconductor and raise the temperature, there’s a critical temperature at which the superconductivity disappears. Right above this temperature you get a type of metal that we call a strange metal because many of its properties are very different from ordinary metals. Now imagine reversing the path, so that the phase of a system is changing from a strange-metal state to a superconducting state as it goes below the critical temperature. If we’re going to determine the temperature at which this happens, we need to compare the energies of the quantum states on either side of the critical temperature. But strange metals look strange in every respect, and we have only the simplest models for their physical properties.

    What makes strange metals so different from other unique quantum phases?
    In certain phases, [quantum] excitations generally behave like new emergent particles. They are quasiparticles. Their inner structure is very complicated, but from the outside they look like ordinary particles. The quasiparticle theory of many-body states pretty much applies to all states we’ve discovered in the older materials.

    Strange metals are one of the most prominent cases we know where quasiparticle theory fails. That’s why it’s so much harder to study them, because this basic tool of many-body theory doesn’t apply.

    You had the idea that string theory might be useful for understanding quantum phases that lack quasiparticles, like strange metals. How is string theory useful in this setting?
    From my point of view, string theory was another powerful mathematical tool for understanding large numbers of quantum-entangled particles. In particular, there are certain phases of string theory in which you can imagine that the ends of strings are sticking to a surface. If you are an ant moving on the surface, you only see the ends of the string. To you, these ends look like particles, but really the particles are connected by a string that goes to an extra dimension. To you, these particles sitting on the surface will appear entangled, and it is the string in the extra dimension which is entangling the particles. It’s a different way of describing entanglement.

    Now you could imagine continuing that process, not just with two electrons, but with four, six, infinitely many electrons, looking at the different entangled states the electrons can form. This is closely connected to the classification of phases of matter. It’s a hierarchical description of entanglement, where each electron finds a partner, and then the pairs entangle with other pairs, and so on. You can build this hierarchical structure using the stringy description. So it is one approach to talking about the entanglement of trillions of electrons.

    This application of string theory to strange metals has some interesting implications. For instance, it’s led you to draw connections between strange metals and the properties of black holes. How do you get from one to the other?
    In the string-theory picture, [changing the density of electrons] corresponds to putting a charge on a black hole. Many people have been studying this in the last five years or so — trying to understand things about strange metals from the properties of charged black holes. I have a recent paper in which I actually found a certain artificial model of electrons moving on a lattice where many properties precisely match the properties of charged black holes.

    I’ve read that Philip Anderson, considered by many people to be the most-influential living condensed-matter physicist, is skeptical that string theory is really useful for understanding strange metals. Do you know if that’s true?
    I think that’s correct. He’s told me himself that he doesn’t believe any of this, but, you know, what can I say, he’s a brilliant man with his own point of view. I would say that when we first proposed the idea in 2007, it certainly sounded crazy. A lot of progress has been made since then. I have a new paper with Philip Kim and others where it turns out that with graphene, which is a slightly less-strange metal, many of the methods inspired by string theory have led to quantitative predictions that have been verified by experiments.

    I think that’s been one of the best successes of the string-theory methods so far. It literally works; you can get the numbers right. But graphene is a simple system, and whether these methods are going to work for high-temperature superconductors hasn’t yet been proven.

    Could you say more about why Anderson might be skeptical of the approach you’ve taken?
    If you go back and actually look at string-theory models, on the surface they look very different from the kinds of models you need for high-temperature superconductors. You look at the stringy models and their constituents, and it appears absurd that these are connected to the constituents of the high-temperature superconductors. But if you take the point of view that, OK, I’m not literally saying this model is going to be found in [high-temperature superconductors], this is just a model that helps me make progress on difficult issues, like how do materials without quasiparticles behave, string theory gives you examples of one of these materials that’s reliably solvable.

    How literally are you using string theory? Is it a direct application, or are you drawing inspiration from it?
    It’s closer to the inspiration side of things. Once you’ve solved the model, it gives you a lot of insight into other models that you may not be able to solve. After six or seven years of work closer to the string-theory side, we think we’ve learned a lot. For us the next step appears to be working in more realistic systems using inspiration we got from more solvable models.

    How might the string-theory models, plus the work on graphene, put you in a position to understand the properties of high-temperature superconductors?
    As you change the density of electrons in high-temperature superconductors, there’s a much more dramatic change in which the electrons go from a regime where it seems only a few electrons are mobile to one where all electrons are mobile. We’re understanding that there’s a special point called the optimal density where there seems to be a dramatic change in the quantum state of electrons. And right near this point is where the strange metal is also observed. We’re trying to work out microscopic theories of this special point where the quantum state changes, and stringy models can teach us a lot about such quantum-critical points. Once we have the full framework, we’re hopeful and optimistic that we can take many of the insights from graphene and apply them to this more complicated model. That’s where we are.

    See the full article here .

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  • richardmitnick 4:31 pm on February 5, 2016 Permalink | Reply
    Tags: , Superconductivity,   

    From Waterloo: “Waterloo physicists discover new properties of superconductivity” 

    U Waterloo bloc

    University of Waterloo

    February 4, 2016
    Victoria Van Cappellen
    Rose Simone

    Superconductivity could have implications for creating technologies like ultra-efficient power grids and magnetically levitating vehicles. New superconductivity findings published in journal Science.

    Physicists at the University of Waterloo have led an international team that has come closer to understanding the mystery of how superconductivity, an exotic state that allows electricity to be conducted with practically zero resistance, occurs in certain materials.

    Superconductivity

    Physicists all over the world are on a quest to understand the secrets of superconductivity because of the exciting technological possibilities that could be realized if they could make it happen at closer to room temperatures. In conventional superconductivity, materials that are cooled to nearly absolute zero ( −273.15 Celsius) exhibit the fantastic property of electrons pairing up and being able to conduct electricity with practically zero resistance. If superconductivity worked at higher temperatures, it could have implications for creating technologies such as ultra-efficient power grids, supercomputers and magnetically levitating vehicles.

    The new findings from an international collaboration, led by Waterloo physicists David Hawthorn, Canada Research Chair Michel Gingras, doctoral student Andrew Achkar and post-doctoral student Zhihao Hao, present direct experimental evidence of what is known as electronic nematicity – when electron clouds snap into an aligned and directional order – in a particular type of high-temperature superconductor. The results, published in the prestigious journal Science, may eventually lead to a theory explaining why superconductivity occurs at higher temperatures in certain materials.

    The findings show evidence of electronic nematicity as a universal feature in cuprate high-temperature superconductors. Cuprates are copper-oxide ceramics composed of two-dimensional layers or planes of copper and oxygen atoms separated by other atoms. They are known as the best of the high-temperature superconductors. In the 1980s, materials that exhibit superconductivity under somewhat warmer conditions (but still -135 Celsius, so far from room temperature) were discovered. But how superconductivity initiates in these high-temperature superconductors has been challenging to predict, let alone explain.

    “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,” says Hawthorn. “These patterns and symmetries have important consequences for superconductivity – they can compete, coexist or possibly even enhance superconductivity.”

    Scientists use soft x-ray scattering in superconductivity research

    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.

    Canadian Light Source
    Canadian Lightsource synchrotron

    Specifically, they looked at 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. The term nematicity commonly refers to when liquid crystals spontaneously align under an electric field in liquid crystal displays. In this case, the electron orbitals enter the nematic state as the temperature drops below a critical point.

    Cuprates can made to be superconducting by adding elements that will remove electrons from the material, a process known as “doping.”

    A material can be optimally doped to achieve superconductivity at the highest and most accessible temperature, but in studying how superconductivity happens, physicists often work with material that is “underdoped,” which means the level of doping is less than the level that maximizes the superconducting temperature.

    Results from this study show electronic nematicity likely occur in all underdoped cuprates.

    Physicists also want to understand the relation of nematicity to a phenomenon known as charge density wave fluctuations. Normally, the electrons are in a nice, uniform distribution, but charge-ordering can cause the electrons to bunch up, like ripples on a pond. This sets up a competition, whereby the material is fluctuating between the superconducting and non-superconducting states until the temperature cools enough for the superconductivity to win.

    Future work will tackle how electrons can be tuned for superconductivity

    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.

    Hawthorn and Gingras are both Fellows of the Canadian Institute For Advanced Research. Gingras holds the Canada Research Chair in Condensed Matter Theory and Statistical Mechanics and spent time at the Perimeter Institute for Theoretical Physics as a visiting researcher while this work was being carried out.

    Other Canadian collaborators include the Canadian Light Source and H. Zhang and Y.-J. Kim from the University of Toronto.

    See the full article here .

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    In just half a century, the University of Waterloo, located at the heart of Canada’s technology hub, has become a leading comprehensive university with nearly 36,000 full- and part-time students in undergraduate and graduate programs.

    Consistently ranked Canada’s most innovative university, Waterloo is home to advanced research and teaching in science and engineering, mathematics and computer science, health, environment, arts and social sciences. From quantum computing and nanotechnology to clinical psychology and health sciences research, Waterloo brings ideas and brilliant minds together, inspiring innovations with real impact today and in the future.

    As home to the world’s largest post-secondary co-operative education program, Waterloo embraces its connections to the world and encourages enterprising partnerships in learning, research, and commercialization. With campuses and education centres on four continents, and academic partnerships spanning the globe, Waterloo is shaping the future of the planet.

     
  • richardmitnick 11:14 am on February 1, 2016 Permalink | Reply
    Tags: , Superconductivity,   

    From TUM: “Superconductivity in the land of the “heavy fermions” 

    Techniche Universitat Munchen

    Techniche Universitat Munchen

    01.02.2016
    Prof. Dr. Erwin Schuberth
    Technical University Munich
    Chair for Technical Physics (E23)
    Walther-Meißner-Str. 8, 85748 Garching, Germany

    Superconductivity at low temps ytterbium rhodium and silicon.
    Chips of the intermetallic Ytterbium-Rhodium-Silicide (YbRh2Si2) shimmer golden under the microscope. One unit of the superimposed scale corresponds to 0,1 mm. – Photo: Marc Tippmann / TUM

    An international research team has discovered nonclassical superconductivity at extremely low temperatures in a compound of ytterbium, rhodium, and silicon. The project was a collaboration among physicists of the Technical University of Munich (TUM), the Walther Meissner Institute of the Bavarian Academy of Sciences in Garching, the Max Planck Institute for Chemical Physics of Solids in Dresden, Rice University (Houston, USA), and Renmin University (Beijing, China).

    Superconductors transport electrical current completely without resistance and are therefore of high interest for technology. While there is a physical explanation for classical superconductivity, it is not yet clear how the phenomenon comes about in high-temperature superconductors. Researchers worldwide are searching for models and examples that could explain it and bring them closer to the long-term goal of achieving room-temperature superconductivity.

    Now a mechanism by which superconductivity arises at low temperatures in a compound of ytterbium, rhodium, and silicon (YbRh2Si2) has been discovered by an international team: Prof. Erwin Schuberth of TUM and the Walther Meissner Institute of the Bavarian Academy of Sciences, Prof. Frank Steglich, director of the Max Planck Institute for Chemical Physics of Solids in Dresden, Prof. Qimiao Si of Rice University, and Prof. Rong Yu of Renmin University.

    In the land of the “heavy fermions”

    In contrast to neutrons and protons, the building blocks of the atomic nucleus, electrons are extremely light. They belong to the class of particles known as fermions. In special materials and under particular conditions, though, they behave as if they were a thousand times heavier. The intermetallic compound of ytterbium, rhodium, and silicon that the researchers investigated is a typical representative of such a material, a so-called heavy-fermion system.

    “There is already compelling evidence that unconventional superconductivity is linked, in both copper-based and iron-based high-temperature superconductors, to quantum fluctuations that alter the magnetic order of the materials at ‘quantum critical points,’ watershed thresholds that mark the transition from one quantum phase to another,” Qimao Si says. “This work provides the first evidence that similar processes bring about superconductivity in heavy-fermion systems.”

    Ultralow temperatures

    In Frank Steglich’s research group, heavy-fermion systems such as the ytterbium-rhodium-silicide have been intensively studied for more than 15 years. In earlier investigations, an external magnetic field enabled the researchers to obtain quantum fluctuations but inhibited the transition to superconductivity.

    For their new experiments, the scientists used a nuclear demagnetization cryostat at the Bavarian Academy of Science’s Walther Meissner Institute in Garching. This device can cool samples down to a temperature of 400 millionths of a degree Kelvin. At a transition temperature of two millikelvin, the samples suddenly became superconducting.

    In specific-heat measurements, the study’s authors were surprised to see that the effective mass of the charge carriers in their compound appeared to increase by a further factor of 1000 when the material was cooled below the superconducting transition temperature. “That shows clearly that in the domain of ultralow temperatures, interactions with the nuclear spin of the surrounding atoms are at work,” says Erwin Schuberth. “They form a magnetic order that makes superconductivity possible.”

    Nuclear spins rearrange themselves

    When theorists Qimiao Si and Rong Yu analyzed the measurement results, they found that the prerequisite for the superconductivity is a special arrangement of the nuclear spins of the ytterbium. According to their theory, at extremely low temperatures the nuclear spins link up and arrange themselves in a manner that competes with and decisively weakens the antimagnetic electronic order. In this way, the electronic “quantum critical” fluctuations come to bear – the driving force for the superconductivity.

    “This work shows that the emergence of nonclassical superconductivity in the vicinity of antiferromagnetic instabilities is a general phenomenon,” says Frank Steglich. “It is not confined to the cuprates and organic superconductors, but also occurs in the heavy-fermion materials, model substances for quantum materials with extremely strong electronic correlations.”

    This research was supported by the German Research Foundation (DFG), the Robert A. Welch Foundation, and the National Science Foundation (NSF).

    Publication:

    Emergence of superconductivity in the canonical heavy-electron metal YbRh2Si2
    Erwin Schuberth, Marc Tippmann, Lucia Steinke, Stefan Lausberg, Alexander Steppke, Manuel Brando, Cornelius Krellner, Christoph Geibel, Rong Yu, Qimiao Si, Frank Steglich; Science, 29.01.2016 – DOI: 10.1126/science.aaa9733

    See the full article here .

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    Techniche Universitat Munchin Campus

    Technische Universität München (TUM) is one of Europe’s top universities. It is committed to excellence in research and teaching, interdisciplinary education and the active promotion of promising young scientists. The university also forges strong links with companies and scientific institutions across the world. TUM was one of the first universities in Germany to be named a University of Excellence. Moreover, TUM regularly ranks among the best European universities in international rankings.

     
  • richardmitnick 5:43 pm on January 29, 2016 Permalink | Reply
    Tags: , , Superconductivity   

    From Cornell: “First self-assembled superconductor structure created” 

    Cornell Bloc

    Cornell University

    Jan. 29, 2016
    Tom Fleischman

    Cornell first self assembled semiconductor
    The Wiesner Group at Cornell University has synthesized the first block copolymer self-assembly-derived nanostructured superconductor. Shown is an example of a bismuth-based superconductor levitating a magnet, with simulated and electron microscope images of the nanostructured material. Credit: Cornell University via phys.org

    Building on nearly two decades’ worth of research, a multidisciplinary team at Cornell has blazed a new trail by creating a self-assembled, three-dimensional gyroidal superconductor.

    Ulrich Wiesner, the Spencer T. Olin Professor of Engineering, led the group, which included researchers in engineering, chemistry and physics.

    The group’s findings are detailed in a paper published in Science Advances, Jan. 29.

    Wiesner said it’s the first time a superconductor, in this case niobium nitride (NbN), has self-assembled into a porous, 3-D gyroidal structure. The gyroid is a complex cubic structure based on a surface that divides space into two separate volumes that are interpenetrating and contain various spirals. Pores and the superconducting material have structural dimensions of only around 10 nanometers, which could lead to entirely novel property profiles of superconductors.

    Superconductivity for practical uses, such as in magnetic resonance imaging (MRI) scanners and fusion reactors, is only possible at near absolute zero (-459.67 degrees below zero), although recent experimentation has yielded superconducting at a comparatively balmy 94 degrees below zero.

    “There’s this effort in research to get superconducting at higher temperatures, so that you don’t have to cool anymore,” Wiesner said. “That would revolutionize everything. There’s a huge impetus to get that.”

    Superconductivity, in which electrons flow without resistance and the resultant energy-sapping heat, is still an expensive proposition. MRIs use superconducting magnets, but the magnets constantly have to be cooled, usually with a combination of liquid helium and nitrogen.

    Wiesner and frequent co-author Sol Gruner had been dreaming for over two decades about making a gyroidal superconductor in order to explore how this would affect the superconducting properties. The difficulty was in figuring out a way to synthesize the material.

    The breakthrough was the decision to use NbN as the superconductor. This was born from a conversation between Wiesner and Cornell physicist James Sethna, a co-author on the paper. Wiesner recalled asking Sethna what he thought of the possibility of a gyroidal superconductor, and what material should be used.

    Sethna, who was writing a paper on superconductors at the time, felt that NbN would be the best option.

    Wiesner’s group started by using organic block copolymers to structure direct sol-gel niobium oxide (Nb2O5) into three-dimensional alternating gyroid networks by solvent evaporation-induced self-assembly. Simply put, the group built two intertwined gyroidal network structures, then removed one of them by heating in air at 450 degrees.

    The team’s discovery featured a bit of “serendipity,” Wiesner said. In the first attempt to achieve superconductivity, the niobium oxide (under flowing ammonia for conversion to the nitride) was heated to a temperature of 700 degrees. After cooling the material to room temperature, it was determined that superconductivity had not been achieved. The same material was then heated to 850 degrees, cooled and tested, and superconductivity had been achieved.

    “We tried going directly to 850, and that didn’t work,” Wiesner said. “So we had to heat it to 700, cool it and then heat it to 850 and then it worked. Only then.”

    Wiesner said the group is unable to explain why the heating, cooling and reheating works, but “it’s something we’re continuing to research,” he added.

    Limited previous study on mesostructured superconductors was due, in part, to a lack of suitable material for testing. The work by Wiesner’s team is a first step toward more research in this area.

    “Now that we have these periodically nanostructured and porous materials, we can start to ask questions about structure property relationships,” he said. “Or we can fill the pores with a second material, that may be magnetic or a semiconductor, and then study the properties of these new superconducting composites with very large interfacial areas.”

    This latest effort is groundbreaking in terms of bringing together the organic and inorganic science communities, Wiesner said.

    “We are saying to the superconducting community, ‘Hey, look guys, these organic block copolymer materials can help you generate completely new superconducting structures and composite materials, which may have completely novel properties and transition temperatures. This is worth looking into,’” Wiesner said.

    Wiesner, whose paper on laser heating-induced structures from block copolymer directed self-assembly was published in Science on July 3, noted that his team’s work points to the collaborative nature of much of the research going on at Cornell. Students, grad students and professors are more identified by their fields of study and not their departments, he said.

    “There is a lot of interaction among these different departments, facilitated by the field structure at Cornell,” he said. “At most places, they are siloed, where at Cornell, even the administrative setup is already encouraging and facilitating interdisciplinary research.”

    Co-lead authors on the paper, titled “Block copolymer self-assembly directed synthesis of mesoporous gyroidal superconductors,” were Spencer Robbins and Peter Beaucage, graduate students in the fields of chemistry and chemical biology, and materials science and engineering, respectively. Robbins, who graduated in January 2015, is now a materials scientist at San Francisco-based TeraPore Technologies, a startup company out of the Wiesner group.

    Other team members included Francis DiSalvo, the John A. Newman Professor of Chemistry and Chemical Biology; Gruner, the John L. Wetherill Professor of Physics; and Bruce van Dover, chair of the Department of Materials Science and Engineering.

    The work was supported by grants from the National Science Foundation and the U.S. Department of Energy, and it made use of the Cornell Center for Materials Research Shared Facility, the Cornell High Energy Synchrotron Source, the Cornell NanoScale Science and Technology Facility, and the Kavli Institute at Cornell for Nanoscale Science.

    See the full article here .

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    Once called “the first American university” by educational historian Frederick Rudolph, Cornell University represents a distinctive mix of eminent scholarship and democratic ideals. Adding practical subjects to the classics and admitting qualified students regardless of nationality, race, social circumstance, gender, or religion was quite a departure when Cornell was founded in 1865.

    Today’s Cornell reflects this heritage of egalitarian excellence. It is home to the nation’s first colleges devoted to hotel administration, industrial and labor relations, and veterinary medicine. Both a private university and the land-grant institution of New York State, Cornell University is the most educationally diverse member of the Ivy League.

    On the Ithaca campus alone nearly 20,000 students representing every state and 120 countries choose from among 4,000 courses in 11 undergraduate, graduate, and professional schools. Many undergraduates participate in a wide range of interdisciplinary programs, play meaningful roles in original research, and study in Cornell programs in Washington, New York City, and the world over.

     
  • richardmitnick 9:31 am on September 18, 2015 Permalink | Reply
    Tags: , , Superconductivity,   

    From DESY: “X-rays reveal electron puddles in ceramic superconductors” 

    DESY
    DESY

    2015/09/17
    No Writer Credit

    1
    The superconducting current (red tubes) running in the interstitial space between puddles of electronic crystals. Credit: Alessandro Ricci/DESY

    Using high-energy X-rays, an international team of scientists has discovered a surprising inner structure of a special class of superconductors: Within these so-called high-temperature superconductors, the electrons form puddles of varying sizes throughout the material. This finding helps to understand the microscopic origin of high-temperature superconductivity that is still not fully known. The team reports its observations in the journal Nature.

    Superconductors are materials that can transport electric currents completely without loss. This feature makes them attractive for a wide spectrum of technical applications. Unfortunately, classic superconductors have to be cooled down to temperatures near absolute zero (minus 273,15 degrees Celsius) to work. This limits their application to a few special purposes. However, a couple of decades ago it was discovered that certain ceramics can become superconducting at much higher temperatures. Despite their name, these high-temperature superconductors still have to be cooled down, but not as much as classic superconductors. Some copper oxides (cuprates) can become superconducting at minus 170 degrees Celsius, for instance.

    High-temperature superconductors work different from classic superconductors, and with a better understanding of their function, the design of a room temperature superconductor might become possible one day. To investigate the microstructure of a high-temperature cuprate superconductor (HgBa2CuO4+y), the team led by Alessandro Ricci of DESY, Antonio Bianconi of the Rome International Centre for Materials Science Superstripes (RICMASS) and Gaetano Campi of the Italian Council of National Research (CNR) looked at it with high-energy X-rays at DESYs synchrotron light source DORIS (beamline BW5), the Italian synchrotron Elettra and the European Synchrotron Radiation Source ESRF.

    DESY DORIS
    DORIS

    Elettra Synchrotron Italy
    ElettraESRF
    ESRF

    Here they used a special space resolved diffraction technique (called scanning micro X-ray diffraction) that allows to investigate the microscopic aggregation of electrons in small crystalline domains.

    In conventional materials like metals and semiconductors, the electrons, carriers of the electric charge, move homogenous, like a liquid spreading out evenly in a canal. For many decades scientists believed that superconductivity also had to appear as a homogenous order in the material. By contrast, in the high-temperature cuprate superconductor investigated, the electrons start to aggregate and form puddles at minus 20 degrees Celsius already. „We discovered that the sizes of these puddles vary widely, like the chunks of a molten iceberg or the steam bubbles in a boiling pot“, explains Ricci. While the average puddle measures about 4 nanometres (millionths of a millimetre) across, puddles as large as 40 nanometres could be seen. The distribution of the puddle sizes can be described by a power-law which is typical for self-organisation.

    The scientists could show that the puddles fill the whole material, leaving free interstitial space. Not all electrons become aggregated in these puddles. The electric current, which is carried by pairs of electrons that have remained free, has to flow around the puddles. As the authors found, the interstitial space between the puddles can be described by a special form of geometry: While the world around us usually follows the rules of Euclidean geometry, in the interstitial space of the high-temperature superconductor a hyperbolic geometry applies, as Ricci point out. „These results open new avenues for the design of superconducting materials, and thus could advance the search for a room temperature superconductor.“

    The team consisted of scientists from DESY, RICMASS, CNR, ESRF, Elettra, the University of Twente in The Netherlands, the Queen Mary University of London, the Swiss Federal Institute of Technology, the Moscow State University and Ghent University in Belgium.

    Reference:
    „Inhomogeneity of charge-density-wave order and quenched disorder in a high-Tc superconductor“; G. Campi, A. Bianconi, A. Ricci et al.; Nature, 2015; DOI: 10.1038/nature14987

    See the full article here .

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    DESY is one of the world’s leading accelerator centres. Researchers use the large-scale facilities at DESY to explore the microcosm in all its variety – from the interactions of tiny elementary particles and the behaviour of new types of nanomaterials to biomolecular processes that are essential to life. The accelerators and detectors that DESY develops and builds are unique research tools. The facilities generate the world’s most intense X-ray light, accelerate particles to record energies and open completely new windows onto the universe. 
That makes DESY not only a magnet for more than 3000 guest researchers from over 40 countries every year, but also a coveted partner for national and international cooperations. Committed young researchers find an exciting interdisciplinary setting at DESY. The research centre offers specialized training for a large number of professions. DESY cooperates with industry and business to promote new technologies that will benefit society and encourage innovations. This also benefits the metropolitan regions of the two DESY locations, Hamburg and Zeuthen near Berlin.

     
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