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  • richardmitnick 2:44 pm on March 4, 2017 Permalink | Reply
    Tags: , , , Superconductivity   

    From COSMOS: “Resistance is futile: the super science of superconductivity” 

    Cosmos Magazine bloc


    30 May 2016 [Re-issued?]
    Cathal O’Connell

    From maglev trains to prototype hoverboards and the Large Hadron Collider – superconductors are finding more and more uses for modern technology. What superconductors are and how they work.

    A superconducting ceramic operates at the relatively high temperature of 123 Kelvin in a Japanese lab.

    What are superconductors?

    All the electronic devices around you – your phone, your computer, even your bedside lamp – are based on moving electrons through materials. In most materials, there is an opposition to this movement (kind of like friction, but for electrons) called electrical resistance, which wastes some of the energy as heat.

    This is why your laptop heats up during use, and the same effect is used to boil water in a kettle.

    Superconductors are materials that carry electrical current with exactly zero electrical resistance. This means you can move electrons through it without losing any energy to heat.

    Sounds amazing. What’s the catch?

    The snag is you have to cool a superconductor below a critical temperature for it to work. That critical temperature depends on the material, but it’s usually below -100 °C.

    A room temperature superconductor, if one could be found, could revolutionise modern technology, letting us transmit power across continents without any loss.

    How was superconductivity discovered?

    When you cool a metal, its electrical resistance tends to decrease. This is because the atoms in the metal jiggle around less, and so are less likely to get in an electrons way.

    Around the turn of the 19th century, physicists were debating what would happen at absolute zero, when the jiggling stops altogether.

    Some wondered whether the resistance would continue to decrease until it reached zero.

    Others, such as Lord Kelvin (after whom the temperature scale is named), argued that the resistance would become infinite as electrons themselves would stop moving.

    In April 1911, Dutch physicist Heike Kamerlingh Onnes cooled a solid mercury wire to 4.2 Kelvin and found the electrical resistance suddenly vanished – the mercury became a perfect conductor. It was a shocking discovery, both because of the abruptness of the change, and the fact it happened still a good four degrees above absolute zero.

    Kamerlingh Onnes had discovered superconductivity, although it took another 40 years for his results to be fully explained.

    What’s the explanation for superconductivity?

    It turns out there are at least two kinds of superconductivity, and physicists can only explain one of them.

    In the simplest case, when you cool a single element down below its critical temperature (as with the mercury example above) physicists can explain superconductivity pretty well: it arises from a weird quantum effect which causes the electrons to pair up within the material. When paired, the electrons gain the ability to flow through the material without getting knocked about by atoms.

    But more complex materials, such as some ceramics which are superconducting at higher temperatures, can’t be explained using this theory.

    Physicists don’t have a good explanation for what causes superconductivity in these “non-traditional superconductor” materials, although the answer must be another quantum effect which links up the electrons in some way.

    What are high-temperature superconductors?

    Physicists have a loose definition of what a “high temperature” is. In this case, it usually means anything above 70 Kelvin (or -203 °C). They choose this temperature because it means the superconductor can be cooled using liquid nitrogen, making it relatively cheap to run (liquid nitrogen only costs about 10-15 cents a litre.)

    The threshold temperature for superconductivity has been increasing for decades. The current record (-70 °C) is held by hydrogen sulfide (yes, the same molecule that gives rotten eggs their distinctive smell).

    The hope is that one day scientists will produce a material that superconducts at room temperature with no cooling required.

    What are superconductors used for now?

    Superconductors are used to make incredibly strong magnets for magnetic levitation (maglev) trains, for the magnetic resonance imaging (MRI) machines in hospitals, and to keep particles on track as they race around the Large Hadron Collider.

    CERN LHC particles
    CERN LHC particles

    The reason superconductors can make strong magnets comes down to Faraday’s law (a moving electric field creates a magnetic field). With no resistance, you can create a huge current, which makes for a correspondingly large magnetic field.

    For example, maglev trains have a series of superconducting coils along each wagon. Each superconductor contains a permanent electric current of about 700,000 amperes.

    The Japanese SCMaglev’s EDS suspension is powered by the magnetic fields induced either side of the vehicle by the passage of the vehicle’s superconducting magnets.


    The current runs round and round the coil without ever winding down, and so the magnetic field it generates is constant and incredibly strong. As the train passes over other electromagnets in the track, it levitates.

    With no friction to slow them down, maglev trains can reach over 600 kilometres per hour, making them the fastest in the world.

    A prototype hoverboard designed by Lexus also uses superconducting magnets for levitation

    Lexus via Wired

    What uses might superconductors have in the future?

    About 6% of all the electricity generated by power plants is lost in transmitting and distributing it around the country along copper wires.

    By replacing copper wires with superconducting wires, we could potentially transmit electrical power across entire continents without any loss. The problem, at the moment, is this would be ludicrously expensive.

    In 2014, the German city of Essen installed a kilometre-long superconducting cable for transmitting electrical power. It can transmit five times more power than a conventional cable, and with hardly any loss, although it’s a complicated bit of kit.

    To keep the superconductor below its critical temperature, liquid nitrogen must be pumped through the core and the whole thing is encased in several layers of insulation, a bit like a thermos flask.

    For a more practical solution, we’ll need to wait for cheap superconductors that can operate closer to room temperature, an advance that can be expected to take decades.

    Closer to reality, perhaps, are superconducting computers. Scientists have already developed computer chips based on superconductors, such as the Hypres Superconducting Microchip. Using such processors could lead to supercomputers requiring 1/50Oth the power of a regular supercomputer.

    Hypres Superconducting Microchip, Incorporating 6000 Josephson Junctions. Noimage credit. http://www.superconductors.org/uses.htm

    See the full article here .

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  • richardmitnick 3:28 pm on December 23, 2016 Permalink | Reply
    Tags: , , Laser Pulses Help Scientists Tease Apart Complex Electron Interactions, Superconductivity   

    From BNL: “Laser Pulses Help Scientists Tease Apart Complex Electron Interactions” 

    Brookhaven Lab

    December 20, 2016
    Karen McNulty Walsh
    (631) 344-8350

    Peter Genzer
    (631) 344-3174

    A microscopic image of one of the bismuth strontium calcium copper oxide samples the scientists studied using a new high-speed imaging technique. Color changes show changes in sample height and curvature to dramatically reveal the layered structure and flatness of the material. No image credit.

    Scientists studying high temperature superconductors—materials that carry electric current with no energy loss when cooled below a certain temperature—have been searching for ways to study in detail the electron interactions thought to drive this promising property. One big challenge is disentangling the many different types of interactions—for example, separating the effects of electrons interacting with one another from those caused by their interactions with the atoms of the material.

    Now a group of scientists including physicists at the U.S. Department of Energy’s Brookhaven National Laboratory has demonstrated a new laser-driven “stop-action” technique for studying complex electron interactions under dynamic conditions. As described in a paper just published in Nature Communications, they use one very fast, intense “pump” laser to give electrons a blast of energy, and a second “probe” laser to measure the electrons’ energy level and direction of movement as they relax back to their normal state.

    “By varying the time between the ‘pump’ and ‘probe’ laser pulses we can build up a stroboscopic record of what happens—a movie of what this material looks like from rest through the violent interaction to how it settles back down,” said Brookhaven physicist Jonathan Rameau, one of the lead authors on the paper. “It’s like dropping a bowling ball in a bucket of water to cause a big disruption, and then taking pictures at various times afterward,” he explained.

    Brookhaven Lab physicists Peter Johnson (rear) and Jonathan Rameau. No image credit.

    The technique, known as time-resolved, angle-resolved photoelectron spectroscopy (tr-ARPES), combined with complex theoretical simulations and analysis, allowed the team to tease out the sequence and energy “signatures” of different types of electron interactions. They were able to pick out distinct signals of interactions among excited electrons (which happen quickly but don’t dissipate much energy), as well as later-stage random interactions between electrons and the atoms that make up the crystal lattice (which generate friction and lead to gradual energy loss in the form of heat).

    But they also discovered another, unexpected signal—which they say represents a distinct form of extremely efficient energy loss at a particular energy level and timescale between the other two.

    “We see a very strong and peculiar interaction between the excited electrons and the lattice where the electrons are losing most of their energy very rapidly in a coherent, non-random way,” Rameau said. At this special energy level, he explained, the electrons appear to be interacting with lattice atoms all vibrating at a particular frequency—like a tuning fork emitting a single note. When all of the electrons that have the energy required for this unique interaction have given up most of their energy, they start to cool down more slowly by hitting atoms more randomly without striking the “resonant” frequency, he said.

    The frequency of the special lattice interaction “note” is particularly noteworthy, the scientists say, because its energy level corresponds with a “kink” in the energy signature of the same material in its superconducting state, which was first identified by Brookhaven scientists using a static form of ARPES. Following that discovery, many scientists suggested that the kink might have something to do with the material’s ability to become a superconductor, because it is not readily observed above the superconducting temperature.

    But the new time-resolved experiments, which were done on the material well above its superconducting temperature, were able to tease out the subtle signal. These new findings indicate that this special condition exists even when the material is not a superconductor.

    “We know now that this interaction doesn’t just switch on when the material becomes a superconductor; it’s actually always there,” Rameau said.

    The scientists still believe there is something special about the energy level of the unique tuning-fork-like interaction. Other intriguing phenomena have been observed at this same energy level, which Rameau says has been studied in excruciating detail.

    It’s possible, he says, that the one-note lattice interaction plays a role in superconductivity, but requires some still-to-be-determined additional factor to turn the superconductivity on.

    “There is clearly something special about this one note,” Rameau said.

    Members of the research team: Peter Johnson and Jonathan Rameau of Brookhaven Lab with Laurenz Rettig, Manuel Ligges, and Isabella Avigo and their time-resolved ARPES experimental setup at the University Duisburg-Essen, Germany.

    Work at Brookhaven National Laboratory was supported by the Center for Emergent Superconductivity, an Energy Frontier Research Center headquartered at Brookhaven National Laboratory and funded by the DOE Office of Science. Additional funding was provided by the National Science Foundation, the Aspen Center for Physics, the Laboratory Directed Research and Development Program of Lawrence Berkeley National Laboratory, and by the McDevitt bequest at

    Georgetown University. Computational resources were provided by the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility headquartered at Lawrence Berkeley National Laboratory. Additional support came from Deutsche Forschungsgemeinschaft, the Mercator Research Center Ruhr, and from the European Union within the seventh Framework Program.

    See the full article here .

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    BNL Campus

    One of ten national laboratories overseen and primarily funded by the Office of Science of the U.S. Department of Energy (DOE), Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. The Laboratory’s almost 3,000 scientists, engineers, and support staff are joined each year by more than 5,000 visiting researchers from around the world.Brookhaven is operated and managed for DOE’s Office of Science by Brookhaven Science Associates, a limited-liability company founded by Stony Brook University, the largest academic user of Laboratory facilities, and Battelle, a nonprofit, applied science and technology organization.

  • richardmitnick 6:53 am on November 26, 2016 Permalink | Reply
    Tags: A distinct state of matter, , , , New Clues Emerge in 30-Year-Old Superconductor Mystery, Nonlinear optical rotational anisotropy, Pseudogap, Superconductivity   

    From Caltech: “New Clues Emerge in 30-Year-Old Superconductor Mystery” 

    Caltech Logo



    Whitney Clavin
    (626) 395-1856

    An artistic representation of the data showing the breaking of spatial inversion and rotational symmetries in the pseudogap region of superconducting materials—evidence that the pseudogap is a distinct phase of matter. Rings of light reflected from a superconductor reveal the broken symmetries. Credit: Hsieh Lab/Caltech

    One of the greatest mysteries of experimental physics is how so-called high-temperature superconducting materials work. Despite their name, high-temperature superconductors—materials that carry electrical current with no resistance—operate at chilly temperatures less than minus 135 degrees Celsius. They can be used to make superefficient power cables, medical MRIs, particle accelerators, and other devices. Cracking the mystery of how these materials work could lead to superconducting devices that operate at room temperatures—and could revolutionize electrical devices, including laptops and phones.

    In a new paper in the journal Nature Physics, researchers with the Institute for Quantum Information and Matter at Caltech have at last solved one piece of this enduring puzzle. They have confirmed that a transitional phase of matter called the pseudogap—one that occurs before these materials are cooled down to become superconducting—represents a distinct state of matter, with properties very different from those of the superconducting state itself.

    When matter transitions from one state, or phase, to another—say, water freezing into ice—there is a change in the ordering pattern of the materials’ particles. Physicists previously had detected hints of some type of ordering of electrons inside the pseudogap state. But exactly how they were ordering—and whether that ordering constituted a new state of matter—was unclear until now.

    “A peculiar property of all these high-temperature superconductors is that just before they enter the superconducting state, they invariably first enter the pseudogap state, whose origins are equally if not more mysterious than the superconducting state itself,” says David Hsieh, professor of physics at Caltech and principal investigator of the new research. “We have discovered that in the pseudogap state, electrons form a highly unusual pattern that breaks nearly all of the symmetries of space. This provides a very compelling clue to the actual origin of the pseudogap state and could lead to a new understanding of how high-temperature superconductors work.”

    The phenomenon of superconductivity was first discovered in 1911. When certain materials are chilled to super-cold temperatures, as low as a few degrees above absolute zero (a few degrees Kelvin), they carry electrical current with no resistance, so that no heat or energy is lost. In contrast, our laptops are not made of superconducting materials and therefore experience electrical resistance and heat up.

    Chilling materials to such extremely low temperatures requires liquid helium. However, because liquid helium is rare and expensive, physicists have been searching for materials that can function as superconductors at ever-higher temperatures. The so-called high-temperature superconductors, discovered in 1986, are now known to operate at temperatures up to 138 Kelvin (minus 135 degrees Celsius) and thus can be cooled with liquid nitrogen, which is more affordable than liquid helium. The question that has eluded physicists, however—despite three Nobel Prizes to date awarded in the field of superconductivity—is exactly how high-temperatures superconductors work.

    The dance of superconducting electrons

    Materials become superconducting when electrons overcome their natural repulsion and form pairs. This pairing can occur under extremely cold temperatures, allowing the electrons, and the electrical currents they carry, to move unencumbered. In conventional superconductors, electron pairing is caused by natural vibrations in the crystal lattice of the superconducting material, which act like glue to hold the pairs together.

    But in high-temperature superconductors, this form of “glue” is not strong enough to bind the electron pairs. Researchers think that the pseudogap, and how electrons order themselves in this phase, holds clues about what this glue may constitute for high-temperature superconductors. To study electron ordering in the pseudogap, Hsieh and his team have invented a new laser-based method called nonlinear optical rotational anisotropy. In the method, a laser is pointed at the superconducting material; in this case, crystals of ytttrium barium copper oxide (YBa2Cu3Oy). An analysis of the light reflected back at half the wavelength compared to that going in reveals any symmetry in the arrangement of the electrons in the crystals.

    Broken symmetries point to new phase

    Different phases of matter have distinct symmetries. For example, when water turns into ice, physicists say the symmetry has been “broken.”

    “In water,” Hsieh explains, “the H2O molecules are pretty randomly oriented. If you were swimming in an infinite pool of water, your surroundings look the same no matter where you are. In ice, on the other hand, the H2O molecules form a regular periodic network, so if you imagine yourself submerged in an infinite block of ice, your surroundings appear different depending on whether you are sitting on an H or O atom. Therefore, we say that the translational symmetry of space is broken in going from water to ice.”

    With the new tool, Hsieh’s team was able to show that the electrons cooled to the pseudogap phase broke a specific set of spatial symmetries called inversion and rotational symmetry. “As soon as the system entered the pseudogap region, either as a function of temperature or the amount of oxygen in the compound, there was a loss of inversion and rotational symmetries, clearly indicating a transition into a new phase of matter,” says Liuyan Zhao, a postdoctoral scholar in the Hsieh lab and lead author of the new study. “It is exciting that we are using a new technology to solve an old problem.”

    “The discovery of broken inversion and rotational symmetries in the pseudogap drastically narrows down the set of possibilities for how the electrons are self-organizing in this phase,” says Hsieh. “In some ways, this unusual phase may turn out to be the most interesting aspect of these superconducting materials.”

    The Nature Physics study, entitled A global-inversion-symmetry-broken phase inside the pseudogap region of YBa2Cu3Oy, was funded by the Army Research Office, the National Science Foundation, the Gordon and Betty Moore Foundation, the Canadian Institute for Advanced Research, and the Natural Sciences and Engineering Research Council. Other authors are C. A. Belvin of Wellesley College, Massachusetts; R. Liang, D.A. Bonn, and W.N. Hardy of the University of British Columbia, Vancouver; and N.P. Armitage of The Johns Hopkins University, Baltimore.

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    The California Institute of Technology (commonly referred to as Caltech) is a private research university located in Pasadena, California, United States. Caltech has six academic divisions with strong emphases on science and engineering. Its 124-acre (50 ha) primary campus is located approximately 11 mi (18 km) northeast of downtown Los Angeles. “The mission of the California Institute of Technology is to expand human knowledge and benefit society through research integrated with education. We investigate the most challenging, fundamental problems in science and technology in a singularly collegial, interdisciplinary atmosphere, while educating outstanding students to become creative members of society.”
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  • richardmitnick 10:19 am on October 17, 2016 Permalink | Reply
    Tags: , , , , , Superconductivity   

    From John A Paulson School of Engineering and Applied Sciences: “A new spin on superconductivity” 

    Harvard School of Engineering and Applied Sciences
    John A Paulson School of Engineering and Applied Sciences

    October 14, 2016
    Leah Burrows


    Researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have made a discovery that could lay the foundation for quantum superconducting devices. Their breakthrough solves one the main challenges to quantum computing: how to transmit spin information through superconducting materials.

    Every electronic device — from a supercomputer to a dishwasher — works by controlling the flow of charged electrons. But electrons can carry so much more information than just charge; electrons also spin, like a gyroscope on axis.

    Harnessing electron spin is really exciting for quantum information processing because not only can an electron spin up or down — one or zero — but it can also spin any direction between the two poles. Because it follows the rules of quantum mechanics, an electron can occupy all of those positions at once. Imagine the power of a computer that could calculate all of those positions simultaneously.

    A whole field of applied physics, called spintronics, focuses on how to harness and measure electron spin and build spin equivalents of electronic gates and circuits.

    By using superconducting materials through which electrons can move without any loss of energy, physicists hope to build quantum devices that would require significantly less power.

    But there’s a problem.

    According to a fundamental property of superconductivity, superconductors can’t transmit spin. Any electron pairs that pass through a superconductor will have the combined spin of zero.

    In work published recently in Nature Physics, the Harvard researchers found a way to transmit spin information through superconducting materials.

    “We now have a way to control the spin of the transmitted electrons in simple superconducting devices,” said Amir Yacoby, Professor of Physics and of Applied Physics at SEAS and senior author of the paper.

    It’s easy to think of superconductors as particle super highways but a better analogy would be a super carpool lane as only paired electrons can move through a superconductor without resistance.

    These pairs are called Cooper Pairs and they interact in a very particular way. If the way they move in relation to each other (physicists call this momentum) is symmetric, then the pair’s spin has to be asymmetric — for example, one negative and one positive for a combined spin of zero. When they travel through a conventional superconductor, Cooper Pairs’ momentum has to be zero and their orbit perfectly symmetrical.

    But if you can change the momentum to asymmetric — leaning toward one direction — then the spin can be symmetric. To do that, you need the help of some exotic (aka weird) physics.

    Superconducting materials can imbue non-superconducting materials with their conductive powers simply by being in close proximity. Using this principle, the researchers built a superconducting sandwich, with superconductors on the outside and mercury telluride in the middle. The atoms in mercury telluride are so heavy and the electrons move so quickly, that the rules of relativity start to apply.

    “Because the atoms are so heavy, you have electrons that occupy high-speed orbits,” said Hechen Ren, coauthor of the study and graduate student at SEAS. “When an electron is moving this fast, its electric field turns into a magnetic field which then couples with the spin of the electron. This magnetic field acts on the spin and gives one spin a higher energy than another.”

    So, when the Cooper Pairs hit this material, their spin begins to rotate.

    “The Cooper Pairs jump into the mercury telluride and they see this strong spin orbit effect and start to couple differently,” said Ren. “The homogenous breed of zero momentum and zero combined spin is still there but now there is also a breed of pairs that gains momentum, breaking the symmetry of the orbit. The most important part of that is that the spin is now free to be something other than zero.”

    The team could measure the spin at various points as the electron waves moved through the material. By using an external magnet, the researchers could tune the total spin of the pairs.

    “This discovery opens up new possibilities for storing quantum information. Using the underlying physics behind this discovery provides also new possibilities for exploring the underlying nature of superconductivity in novel quantum materials,” said Yacoby.

    This research was coauthored by Sean Hart, Michael Kosowsky, Gilad Ben-Shach, Philipp Leubner, Christoph Brüne, Hartmut Buhmann, Laurens W. Molenkamp and Bertrand I. Halperin.

    See the full article here .

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    Through research and scholarship, the Harvard School of Engineering and Applied Sciences (SEAS) will create collaborative bridges across Harvard and educate the next generation of global leaders. By harnessing the power of engineering and applied sciences we will address the greatest challenges facing our society.

    Specifically, that means that SEAS will provide to all Harvard College students an introduction to and familiarity with engineering and technology as this is essential knowledge in the 21st century.

    Moreover, our concentrators will be immersed in the liberal arts environment and be able to understand the societal context for their problem solving, capable of working seamlessly withothers, including those in the arts, the sciences, and the professional schools. They will focus on the fundamental engineering and applied science disciplines for the 21st century; as we will not teach legacy 20th century engineering disciplines.

    Instead, our curriculum will be rigorous but inviting to students, and be infused with active learning, interdisciplinary research, entrepreneurship and engineering design experiences. For our concentrators and graduate students, we will educate “T-shaped” individuals – with depth in one discipline but capable of working seamlessly with others, including arts, humanities, natural science and social science.

    To address current and future societal challenges, knowledge from fundamental science, art, and the humanities must all be linked through the application of engineering principles with the professions of law, medicine, public policy, design and business practice.

    In other words, solving important issues requires a multidisciplinary approach.

    With the combined strengths of SEAS, the Faculty of Arts and Sciences, and the professional schools, Harvard is ideally positioned to both broadly educate the next generation of leaders who understand the complexities of technology and society and to use its intellectual resources and innovative thinking to meet the challenges of the 21st century.

    Ultimately, we will provide to our graduates a rigorous quantitative liberal arts education that is an excellent launching point for any career and profession.

  • richardmitnick 10:42 am on October 14, 2016 Permalink | Reply
    Tags: , , Cuprates, , Superconductivity, X-ray photon correlation spectroscopy   

    From BNL: “Scientists Find Static “Stripes” of Electrical Charge in Copper-Oxide Superconductor” 

    Brookhaven Lab

    October 14, 2016
    Ariana Tantillo
    (631) 344-2347
    Peter Genzer
    (631) 344-3174

    Fixed arrangement of charges coexists with material’s ability to conduct electricity without resistance

    Members of the Brookhaven Lab research team—(clockwise from left) Stuart Wilkins, Xiaoqian Chen, Mark Dean, Vivek Thampy, and Andi Barbour—at the National Synchrotron Light Source II’s Coherent Soft X-ray Scattering beamline, where they studied the electronic order of “charge stripes” in a copper-oxide superconductor. No image credit.

    Cuprates, or compounds made of copper and oxygen, can conduct electricity without resistance by being “doped” with other chemical elements and cooled to temperatures below minus 210 degrees Fahrenheit. Despite extensive research on this phenomenon—called high-temperature superconductivity—scientists still aren’t sure how it works. Previous experiments have established that ordered arrangements of electrical charges known as “charge stripes” coexist with superconductivity in many forms of cuprates. However, the exact nature of these stripes—specifically, whether they fluctuate over time—and their relationship to superconductivity—whether they work together with or against the electrons that pair up and flow without energy loss—have remained a mystery.

    Now, scientists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory have demonstrated that static, as opposed to fluctuating, charge stripes coexist with superconductivity in a cuprate when lanthanum and barium are added in certain amounts. Their research, described in a paper published on October 11 in Physical Review Letters, suggests that this static ordering of electrical charges may cooperate rather than compete with superconductivity. If this is the case, then the electrons that periodically bunch together to form the static charge stripes may be separated in space from the free-moving electron pairs required for superconductivity.

    “Understanding the detailed physics of how these compounds work helps us validate or rule out existing theories and should point the way toward a recipe for how to raise the superconducting temperature,” said paper co-author Mark Dean, a physicist in the X-Ray Scattering Group of the Condensed Matter Physics and Materials Science Department at Brookhaven Lab. “Raising this temperature is crucial for the application of superconductivity to lossless power transmission.”

    Charge stripes put to the test of time

    To see whether the charge stripes were static or fluctuating in their compound, the scientists used a technique called x-ray photon correlation spectroscopy. In this technique, a beam of coherent x-rays is fired at a sample, causing the x-ray photons, or light particles, to scatter off the sample’s electrons. These photons fall onto a specialized, high-speed x-ray camera, where they generate electrical signals that are converted to a digital image of the scattering pattern. Based on how the light interacts with the electrons in the sample, the pattern contains grainy dark and bright spots called speckles. By studying this “speckle pattern” over time, scientists can tell if and how the charge stripes change.

    In this study, the source of the x-rays was the Coherent Soft X-ray Scattering (CSX-1) beamline at the National Synchrotron Light Source II (NSLS-II), a DOE Office of Science User Facility at Brookhaven.

    BNL NSLS-II Interior

    “It would be very difficult to do this experiment anywhere else in the world,” said co-author Stuart Wilkins, manager of the soft x-ray scattering and spectroscopy program at NSLS-II and lead scientist for the CSX-1 beamline. “Only a small fraction of the total electrons in the cuprate participate in the charge stripe order, so the intensity of the scattered x-rays from this cuprate is extremely small. As a result, we need a very intense, highly coherent x-ray beam to see the speckles. NSLS-II’s unprecedented brightness and coherent photon flux allowed us to achieve this beam. Without it, we wouldn’t be able to discern the very subtle electronic order of the charge stripes.”

    The team’s speckle pattern was consistent throughout a nearly three-hour measurement period, suggesting that the compound has a highly static charge stripe order. Previous studies had only been able to confirm this static order up to a timescale of microseconds, so scientists were unsure if any fluctuations would emerge beyond that point.

    X-ray photon correlation spectroscopy is one of the few techniques that scientists can use to test for these fluctuations on very long timescales. The team of Brookhaven scientists—representing a close collaboration between one of Brookhaven’s core departments and one of its user facilities—is the first to apply the technique to study the charge ordering in this particular cuprate. “Combining our expertise in high-temperature superconductivity and x-ray scattering with the capabilities at NSLS-II is a great way to approach these kind of studies,” said Wilkins.

    To make accurate measurements over such a long time, the team had to ensure the experimental setup was incredibly stable. “Maintaining the same x-ray intensity and sample position with respect to the x-ray beam are crucial, but these parameters become more difficult to control as time goes on and eventually impossible,” said Dean. “When the temperature of the building changes or there are vibrations from cars or other experiments, things can move. NSLS-II has been carefully engineered to counteract these factors, but not indefinitely.”

    “The x-ray beam at CSX-1 is stable within a very small fraction of the 10-micron beam size over our almost three-hour practical limit,” added Xiaoqian Chen, co-first author and a postdoc in the X-Ray Scattering Group at Brookhaven. CSX-1’s performance exceeds that of any other soft x-ray beamline currently operational in the United States.

    In part of the experiment, the scientists heated up the compound to test whether thermal energy might cause the charge stripes to fluctuate. They observed no fluctuations, even up to the temperature at which the compound is known to stop behaving as a superconductor.

    “We were surprised that the charge stripes were so remarkably static over such long timescales and temperature ranges,” said co-first author and postdoc Vivek Thampy of the X-Ray Scattering Group. “We thought we may see some fluctuations near the transition temperature where the charge stripe order disappears, but we didn’t.”

    In a final check, the team theoretically calculated the speckle patterns, which were consistent with their experimental data.

    Going forward, the team plans to use this technique to probe the nature of charges in cuprates with different chemical compositions.

    X-ray scattering measurements were supported by the Center for Emergent Superconductivity, an Energy Frontier Research Center funded by DOE’s Office of Science.

    See the full article here .

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    One of ten national laboratories overseen and primarily funded by the Office of Science of the U.S. Department of Energy (DOE), Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. The Laboratory’s almost 3,000 scientists, engineers, and support staff are joined each year by more than 5,000 visiting researchers from around the world.Brookhaven is operated and managed for DOE’s Office of Science by Brookhaven Science Associates, a limited-liability company founded by Stony Brook University, the largest academic user of Laboratory facilities, and Battelle, a nonprofit, applied science and technology organization.

  • 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

    “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

    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
    (631) 344-2347

    Peter Genzer,
    (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.

    (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

    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

    Peter Genzer
    (631) 344-3174

    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


    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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