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

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


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    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.


    Elettra Synchrotron Italy

    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.

    „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

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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.

  • richardmitnick 1:30 pm on September 13, 2015 Permalink | Reply
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    From UBC: “First superconducting graphene created by UBC researchers” 

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    University of British Columbia

    September 8, 2015
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    Researchers add lithium to graphene to create superconductivity. Credit: Andrea Damascelli.

    Graphene, the ultra-thin, ultra-strong material made from a single layer of carbon atoms, just got a little more extreme. UBC physicists have been able to create the first ever superconducting graphene sample by coating it with lithium atoms.

    Although superconductivity has already been observed in intercalated bulk graphite—three-dimensional crystals layered with alkali metal atoms, based on the graphite used in pencils—inducing superconductivity in single-layer graphene has until now eluded scientists.

    Andrea Damascelli

    “This first experimental realization of superconductivity in graphene promises to usher us in a new era of graphene electronics and nanoscale quantum devices,” says Andrea Damascelli, director of UBC’s Quantum Matter Institute and leading scientist of the Proceedings of the National Academy of Sciences study outlining the discovery.

    Graphene, roughly 200 times stronger than steel by weight, is a single layer of carbon atoms arranged in a honeycomb pattern. Along with studying its extreme physical properties, scientists eventually hope to make very fast transistors, semiconductors, sensors and transparent electrodes using graphene.

    “This is an amazing material,’” says Bart Ludbrook, first author on the PNAS paper and a former PhD researcher in Damascelli’s group at UBC. “Decorating monolayer graphene with a layer of lithium atoms enhances the graphene’s electron–phonon coupling to the point where superconductivity can be stabilized.”

    Given the massive scientific and technological interest, the ability to induce superconductivity in single-layer graphene promises to have significant cross-disciplinary impacts. According to financial reports, the global market for graphene reached $9 million in 2014 with most sales in the semiconductor, electronics, battery, energy, and composites industries.

    The researchers, which include colleagues at the Max Planck Institute for Solid State Research through the joint Max-Planck-UBC Centre for Quantum Materials, prepared the lithium-decorated graphene in ultra-high vacuum conditions and at ultra-low temperatures (-267 degrees Celsius or 5 Kelvin), to achieve this breakthrough.

    UBC’s Quantum Matter Institute

    UBC’s Quantum Matter Institute (QMI) is internationally recognized for its research and discoveries in quantum structures, quantum materials, and applications towards quantum devices. A recent $66.5-million investment from the Canada First Research Excellence Fund will broaden the scope of QMI’s research and support the discovery of practical applications for computing, electronics, medicine and sustainable energy technologies.

    Study: Evidence for superconductivity in Li-decorated monolayer graphene in Proceedings of the National Academy of Sciences. Tracking number: 2015-10435R

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    The University of British Columbia is a global centre for research and teaching, consistently ranked among the 40 best universities in the world. Since 1915, UBC’s West Coast spirit has embraced innovation and challenged the status quo. Its entrepreneurial perspective encourages students, staff and faculty to challenge convention, lead discovery and explore new ways of learning. At UBC, bold thinking is given a place to develop into ideas that can change the world.

    • flowerpoet 3:47 pm on September 13, 2015 Permalink | Reply

      an amazing breakthrough with ever-increasing possibilities…thanks for sharing this info


  • richardmitnick 11:47 am on September 11, 2015 Permalink | Reply
    Tags: , , Mott transition, , , Superconductivity   

    From phys.org: “Team announces breakthrough observation of Mott transition in a superconductor” 


    September 11, 2015
    Joost Bruysters


    An international team of researchers, including the MESA+ Institute for Nanotechnology at the University of Twente in The Netherlands and the U.S. Department of Energy’s Argonne National Laboratory, announced today in Science the observation of a dynamic Mott transition in a superconductor.

    The discovery experimentally connects the worlds of classical and quantum mechanics and illuminates the mysterious nature of the Mott transition. It also could shed light on non-equilibrium physics, which is poorly understood but governs most of what occurs in our world. The finding may also represent a step towards more efficient electronics based on the Mott transition.

    Since its foundations were laid in the early part of the 20th century, scientists have been trying to reconcile quantum mechanics with the rules of classical or Newtonian physics (like how you describe the path of an apple thrown into the air—or dropped from a tree). Physicists have made strides in linking the two approaches, but experiments that connect the two are still few and far between; physics phenomena are usually classified as either quantum or classical, but not both.

    One system that unites the two is found in superconductors, certain materials that conduct electricity perfectly when cooled to very low temperatures. Magnetic fields penetrate the superconducting material in the form of tiny filaments called vortices, which control the electronic and magnetic properties of the materials.

    These vortices display both classical and quantum properties, which led researchers to study them for access to one of the most enigmatic phenomena of modern condensed matter physics: the Mott insulator-to-metal transition.

    The Mott transition occurs in certain materials that according to textbook quantum mechanics should be metals, but in reality turn insulators. A complex phenomenon controlled by the interactions of many quantum particles, the Mott transition remains mysterious—even whether or not it’s a classical or quantum phenomenon is not quite clear. Moreover, scientists have never directly observed a dynamic Mott transition, in which a phase transition from an insulating to a metallic state is induced by driving an electrical current through the system; the disorder inherent in real systems disguises Mott properties.

    At the University of Twente, researchers built a system containing 90,000 superconducting niobium nano-sized islands on top of a gold film. In this configuration, the vortices find it energetically easiest to settle into energy dimples in an arrangement like an egg crate—and make the material act as a Mott insulator, since the vortices won’t move if the applied electric current is small.


    When they applied a large enough electric current, however, the scientists saw a dynamic Mott transition as the system flipped to become a conducting metal; the properties of the material had changed as the current pushed it out of equilibrium.

    The vortex system behaved exactly like an electronic Mott transition driven by temperature, said Valerii Vinokur, an Argonne Distinguished Fellow and corresponding author on the study. He and study co-author Tatyana Baturina, then at Argonne, analyzed the data and recognized the Mott behavior.

    “This experimentally materializes the correspondence between quantum and classical physics,” Vinokur said. “We can controllably induce a phase transition between a state of locked vortices to itinerant vortices by applying an electric current to the system,” said Hans Hilgenkamp, head of the University of Twente research group. “Studying these phase transitions in our artificial systems is interesting in its own right, but may also provide further insight in the electronic transitions in real materials.”

    The system could further provide scientists with insight into two categories of physics that have been hard to understand: many-body systems and out-of-equilibrium systems.

    “This is a classical system that which is easy to experiment with and provides what looks like access to very complicated many-body systems,” said Vinokur. “It looks a bit like magic.”

    As the name implies, many-body problems involve a large number of particles interacting; with current theory they are very difficult to model or understand.


    “Furthermore, this system will be key to building a general understanding of out-of-equilibrium physics, which would be a major breakthrough in physics,” Vinokur said.

    The Department of Energy named five great basic energy scientific challenges of our time; one of them is understanding and controlling out-of-equilibrium phenomena. Equilibrium systems—where there’s no energy moving around—are now understood quite well. But nearly everything in our lives involves energy flow, from photosynthesis to digestion to tropical cyclones, and we don’t yet have the physics to describe it well. Scientists think a better understanding could lead to huge improvements in energy capture, batteries and energy storage, electronics and more.

    As we seek to make electronics faster and smaller, Mott systems also offer a possible alternative to the silicon transistor. Since they can be flipped between conducting and insulating with small changes in voltage, they may be able to encode 1s and 0s at smaller scales and higher accuracy than silicon transistors.

    ‘Initially, we were studying the structures for completely different reasons, namely to investigate the effects of inhomogeneities on superconductivity,” Hilgenkamp said. “After discussing with Valerii Vinokur at Argonne, we looked more specifically into our data and were quite amazed to see that it revealed so nicely the details of the transition between the state of locked and moving vortices. There are many ideas for follow up studies, and we look forward to our continued collaboration.”

    The results were printed in the study Critical behavior at a dynamic vortex insulator-to-metal transition, released today in Science. Other co-authors are associated with the Siberian Branch of Russian Academy of Science, the Rome International Center for Materials Science Superstripes, Novosibirsk State University, the Moscow Institute of Physics and Technology and Queen Mary University of London.

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  • richardmitnick 12:21 pm on May 28, 2015 Permalink | Reply
    Tags: , , , Superconductivity   

    From Carnegie: “Linking superconductivity and structure” 

    Carnegie Institution for Science
    Carnegie Institution for Science

    May 26, 2015

    The collapsed tetragonal crystal structure of , with arsenic (As) atoms in a 5-fold coordination, courtesy of Alexander Goncharov.

    Superconductivity is a rare physical state in which matter is able to conduct electricity—maintain a flow of electrons—without any resistance. It can only be found in certain materials, and even then it can only be achieved under controlled conditions of low temperatures and high pressures. New research from a team including Carnegie’s Elissaios Stavrou, Xiao-Jia Chen, and Alexander Goncharov hones in on the structural changes underlying superconductivity in iron arsenide compounds—those containing iron and arsenic. It is published by Scientific Reports.

    Although superconductivity has many practical applications for electronics (including scientific research instruments), medical engineering (MRI machines), and potential future applications including high-performance power transmission and storage, and very fast train travel, the difficulty of creating superconducting materials prevents it from being used to its full potential. As such, any newly discovered superconducting ability is of great interest to scientists and engineers.

    Iron arsenides are relatively recently discovered superconductors. The nature of superconductivity in these particular materials remains a challenge for modern solid state physics. If the complex links between superconductivity, structure, and magnetism in these materials are unlocked, then iron arsenides could potentially be used to reveal superconductivity at much higher temperatures than previously seen, which would vastly increase the ease of practical applications.

    When iron arsenide is combined with a metal—such as in the sodium-containing NaFe2As2 compound studied here—it was known that the ensuing compound is crystallized in a tetrahedral structure. But until now, a detailed structure of the atomic positions involved and how they change under pressure had not been determined.

    The layering of arsenic and iron (As-Fe-As) in this structure is believed to be key to the compound’s superconductivity. However, under pressure, this structure is thought to be partially misshapen into a so-called collapsed tetragonal lattice, which is no longer capable of superconducting, or has diminished superconducting ability.

    The team used experimental evidence and modeling under pressure to actually demonstrate these previously theorized structural changes—tetragonal to collapsed tetragonal—on the atomic level. This is just the first step toward definitively determining the link between structure and superconductivity, which could potentially make higher-temperature superconductivity a real possibility.

    They showed that at about 40,000 times normal atmospheric pressure (4 gigapascals), NaFe2As2 takes on the collapsed tetragonal structure. This changes the angles in the arsenic-iron-arsenic layers and is coincident with the loss in superconductivity. Moreover, they found that this transition is accompanied by a major change in bonding coordination in the formation of the interlayer arsenic-arsenic bonds. A direct consequence of this new coordination is that the system loses its two-dimensionality, and with it, superconductivity.

    “Our findings are an important step in identifying the hypothesized connection between structure and superconductivity in iron-containing compounds,” Goncharov said. “Understanding the loss of superconductivity on an atomic level could enhance our ease of manufacturing such compounds for practical applications, as well as improving our understanding of condensed matter physics.”

    The paper’s other co-authors are: Artem Oganov of Stony Brook University, the Moscow Institute of Physics and Technology, and Northwestern Polytechnical University Xi’an; and Ai-Feng Wang, Ya-Jun Yan, Xi-Gang Luo, and Xian-Hui Chen of the University of Science and Technology of China, Hefei, Anhui.

    The tetragonal crystal structure of NaFe2As2, courtesy of Alexander Goncharov.

    This work was supported by DARPA, the Carnegie Institution of Canada, EFree (the DOE EFRC center at the Carnegie Institution for Science), the government of the Russian Federation, the Ministry of Education and Science of the Russian Federation.

    GSECARS is supported by the U.S. NSF and DOE Geosciences. Use of the APS was supported by the DOE-BES. Calculations were performed on XSEDE facilities and on the cluster of the Center for Functional Nanomaterials, BNL, which is supported by the DOE-BES. Sample growth was supported by the Natural Science Foundation of China, the ‘‘Strategic Priority Research Program (B)’’ of the Chinese Academy of Sciences, and the National Basic Research Program of China.

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    Andrew Carnegie established a unique organization dedicated to scientific discovery “to encourage, in the broadest and most liberal manner, investigation, research, and discovery and the application of knowledge to the improvement of mankind…” The philosophy was and is to devote the institution’s resources to “exceptional” individuals so that they can explore the most intriguing scientific questions in an atmosphere of complete freedom. Carnegie and his trustees realized that flexibility and freedom were essential to the institution’s success and that tradition is the foundation of the institution today as it supports research in the Earth, space, and life sciences.

  • richardmitnick 4:36 pm on March 26, 2015 Permalink | Reply
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    From LANL: “Using magnetic fields to understand high-temperature superconductivity “ 

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    March 26, 2015
    Nancy Ambrosiano

    Los Alamos explores experimental path to potential ‘next theory of superconductivity’

    Los Alamos National Laboratory scientist Brad Ramshaw conducts an experiment at the Pulsed Field Facility of the National High Magnetic Field Lab, exposing high-temperature superconductors to very high magnetic fields, changing the temperature at which the materials become perfectly conducting and revealing unique properties of these substances.

    Taking our understanding of quantum matter to new levels, scientists at Los Alamos National Laboratory are exposing high-temperature superconductors to very high magnetic fields, changing the temperature at which the materials become perfectly conducting and revealing unique properties of these substances.

    “High magnetic-field measurements of doped copper-oxide superconductors are paving the way to a new theory of superconductivity,” said Brad Ramshaw, a Los Alamos scientist and lead researcher on the project. Using world-record high magnetic fields available at the National High Magnetic Field Laboratory (NHMFL) Pulsed Field Facility, based in Los Alamos, Ramshaw and his coworkers are pushing the boundaries of how matter can conduct electricity without the resistance that plagues normal materials carrying an electrical current.

    LANL National High Magnetic Field Lab

    The eventual goal of the research would be to create a superconductor that operates at room temperature and needs no cooling at all. At this point, all devices that make use of superconductors, such as the MRI magnets found in hospitals, must be cooled to temperatures far below zero with liquid nitrogen or helium, adding to the cost and complexity of the enterprise.

    “This is a truly landmark experiment that illuminates a problem of central importance to condensed matter physics,” said MagLab Director Gregory Boebinger, who is also chief scientist for Condensed Matter Science at the National High Magnetic Field Laboratory’s headquarters in Florida. “The success of this quintessential MagLab work relied on having the best samples, the highest magnetic fields, the most sensitive techniques, and the inspired creativity of a multi-institutional research team.”

    High-temperature superconductors have been a thriving field of research for almost 30 years, not just because they can conduct electricity with no losses—one hundred degrees higher than any other material—but also because they represent a very difficult and interesting “correlated-electron” physics problem in their own right.

    The theory of traditional, low-temperature superconductors was constructed by Bardeen, Cooper, and Schrieffer in 1957, winning them the Nobel prize; this theory (known as the BCS theory) had a far-reaching impact, laying the foundation for the Higgs mechanism in particle physics, and it represents one of the greatest triumphs of 20th century physics.

    On the other hand, high-temperature superconductors, such as yttrium barium copper oxide (YBa2Cu3O6+x), cannot be explained with BCS theory, and so researchers need a new theory for these materials. One particularly interesting aspect of high-temperature superconductors, such as YBa2Cu3O6+x, is that one can change the superconducting transition temperature (Tc, where the material becomes perfectly conducting) by “doping” it, : changing the number of electrons that participate in superconductivity.

    The Los Alamos team’s research in the 100-T magnet found that if one dopes YBa2Cu3O6+x to the point where Tc is highest (“optimal doping”), the electrons become very heavy and move around in a correlated way.

    “This tells us that the electrons are interacting very strongly when the material is an optimal superconductor,” said Ramshaw. “This is a vital piece of information for building the next theory of superconductivity.”

    “An outstanding problem in the field of high-transition-temperature (high-Tc) superconductivity has been the issue as to whether a quantum critical point—a special doping value where quantum fluctuations lead to strong electron-electron interactions—is driving the remarkably high Tc’s in these materials,” he said.

    Proof of its existence has previously not been found due to the robust nature of the superconductivity in the copper oxide materials, yet if scientists can show that there is a quantum critical point, it would constitute a significant milestone toward resolving the superconducting pairing mechanism, Ramshaw explained.

    “Assembling the pieces of this complex superconductivity puzzle is a daunting task that has involved scientists from around the world for decades,” said Charles H. Mielke, NHMFL-Pulsed Field Facility director at Los Alamos. “Though the puzzle is unfinished, this essential piece links unquestionable experimental results to fundamental condensed matter physics — a connection made possible by an exceptional team, strong partner support and unsurpassed capabilities.”

    In a paper this week in the journal Science, the team addresses this longstanding problem by measuring magnetic quantum oscillations as a function of hole doping in very strong magnetic fields in excess of 90 tesla.

    Strong magnetic fields such as the world-record field accessible at the NHMFL site at Los Alamos enable the normal metallic state to be accessed by suppressing superconductivity. Fields approaching 100 tesla, in particular, enable quantum oscillations to be measured very close to the maximum in the transition temperature Tc ~ 94 kelvin. These quantum oscillations give scientists a picture of how the electrons are interacting with each other before they become superconducting.

    By accessing a very broad range of dopings, the authors show that there is a strong enhancement of the effective mass at optimal doping. A strong enhancement of the effective mass is the signature of increasing electron interaction strength, and the signature of a quantum critical point. The broken symmetry responsible for this point has yet to be pinned down, although a connection with charge ordering appears to be likely, Ramshaw notes.

    Funding: Work carried out at the National High Magnetic Field Laboratory—Pulsed Field Facility at Los Alamos National Laboratory was provided through funding from the National Science Foundation Division of Materials Research through Grant No. DMR-1157490 and from the US Department of Energy’s Office of Science, Florida State University, the State of Florida, and Los Alamos National Laboratory through the LDRD program.

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  • richardmitnick 8:55 am on March 13, 2015 Permalink | Reply
    Tags: , , Superconductivity   

    From Argonne: “Study proposes new way to measure superconducting fluctuations” 

    News from Argonne National Laboratory

    March 10, 2015
    Louise Lerner

    Scientists at Argonne proposed theoretical evidence for a new superconducting fluctuation, which may lead to a way of measuring the exact temperature at which superconductivity kicks in and shed light on the poorly understood properties of superconducting materials above this temperature. Above: Sharp peaks are visible as the temperature nears Tc, the temperature at which superconductivity kicks in. Credit: Alexey Galda

    A study published last month by researchers at the U.S. Department of Energy’s Argonne National Laboratory provides theoretical evidence for a new effect that may lead to a way of measuring the exact temperature at which superconductivity kicks in and shed light on the poorly understood properties of superconducting materials above this temperature.

    Superconductors are an old puzzle in physics, made all the more tantalizing because their technological applications are so valuable. Electricity is being lost all around you; very few electric systems use power completely efficiently, and some is always lost—generally as heat, which you can feel as your laptop or phone gets warm. That’s because even our best conductors, like copper, are always losing a little bit of electricity to resistance. Superconductors don’t. When cooled down to operating temperature, they never lose any electricity.

    This is the kind of unique property that can spur entire new fields of invention, and they have—MRIs, cell phone towers and Maglev trains all use superconductors. But they’re not in every engine or transmission line because of a serious logistical issue: their operating temperature is -270°F or lower, so they have to be cooled with liquid helium or nitrogen.

    Superconducting materials have a number of other fascinating properties. For example, scientists found that the electricity flow between two superconductors separated by a thin non-conducting material (called a Josephson junction) can be extremely sensitive to external microwave radiation. As little as a single photon can trigger electricity to flow through such a device when just the right voltage is applied. This unique effect, called resonant tunneling, allows such a high precision of measurement that it is used for DNA sequencing and quantum encryption. The same phenomenon has determined the international standard of voltage for decades.

    The problem is that we still don’t fully understand how superconductors work, and if we want to realize their full potential, we need to.

    To explore superconductors, one of the things scientists do is rearrange them in all sorts of new ways—stacking them in layers, punching holes in them and trimming them down to wires just 50 nanometers across, for example.

    These new arrangements change the way materials behave, including essential properties like the exact temperature at which they become superconducting—called the “critical temperature” or Tc .

    “Until now,” said Valerii Vinokur, Argonne Distinguished Fellow and a coauthor on the paper, “the field hasn’t had a standard, precise way to measure Tc.”

    One of the things we do know is that short-lived islands of superconductivity can form in a material just above Tc. These sporadically emerging and rapidly vanishing regions, called superconducting fluctuations, mirror in one way or another most of the superconducting properties of the material at temperatures below Tc. Despite this, superconducting fluctuations remain poorly understood—so much so that even measuring their lifetime has been a challenge. In the paper, Vinokur and Argonne postdoctoral fellow Alexey Galda proposed an effect that mirrors resonant tunneling above Tc that is strong enough to measure, and—most importantly—gets sharper as the temperature approaches Tc.

    If verified by experiment, this would be a new high-precision tool for measuring fundamental properties of superconducting fluctuations, such as their lifetime, and provide a way to measure more precisely where Tc lies for each material.

    “Every new tool in studying superconductivity is absolutely invaluable—it brings more precision to the field,” Galda said.

    “This would also let us study fluctuations more widely,” he said.

    The fluctuations, Galda said, are interesting because they can help researchers map the microscopic behaviors of materials, which are likely key to why and how materials act the way they do. Fluctuations are influenced by a number of different phenomena; a tool to untangle at least one variable from the set would help researchers tease out the contributions of others.

    “To know how long fluctuations live is very important and has been difficult to determine experimentally,” Vinokur said.

    Researchers in Argonne’s Materials Science division, led by Argonne physicist Wai Kwok, are planning to verify the results experimentally.

    The paper, “Resonant tunneling of fluctuation Cooper pairs,” was published by Nature’s Scientific Reports. The other author on the paper was A. S. Mel’nikov of the Russian Academy of Sciences.

    The study was supported by the U.S. Department of Energy’s Office of Science, as well as the Russian Foundation for Basic Research and the Russian Ministry of Science and Education.

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  • richardmitnick 8:39 am on March 3, 2015 Permalink | Reply
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    From AAAS: “A step closer to explaining high-temperature superconductivity?” 



    27 February 2015
    Adrian Cho

    In the new experiment, scientists glimpsed a pattern of up- and down-spinning atoms, which mimics the up-and-down pattern of magnetism seen in high-temperature superconductors. R. A. Hart et al., Nature (2015)

    For years some physicists have been hoping to crack the mystery of high-temperature superconductivity—the ability of some complex materials to carry electricity without resistance at temperatures high above absolute zero—by simulating crystals with patterns of laser light and individual atoms. Now, a team has taken—almost—the next-to-last step in such “optical lattice” simulation by reproducing the pattern of magnetism seen in high-temperature superconductors from which the resistance-free flow of electricity emerges.

    “It’s a very big improvement over previous results,” says Tilman Esslinger, an experimentalist at the Swiss Federal Institute of Technology in Zurich, who was not involved in the work. “It’s very exciting to see steady progress.”

    An optical lattice simulation is essentially a crystal made of light. A real crystal contains a repeating 3D pattern of ions, and electrons flow from ion to ion. In the simulation, spots of laser light replace the ions, and ultracold atoms moving among spots replace the electrons. Physicists can adjust the pattern of spots, how strongly the spots attract the atoms, and how strongly the atoms repel one another. That makes the experiments ideal for probing physics such as high-temperature superconductivity, in which materials such as mercury barium calcium copper oxide carry electricity without resistance at temperatures up to 138 K, far higher above absolute zero than ordinary superconductors such as niobium can.

    Just how the copper-and-oxygen, or cuprate, superconductors work remains unclear. The materials contain planes of copper and oxygen ions with the coppers arranged in a square pattern. Repelling one another, the electrons get stuck in a one-to-a-copper traffic jam called a Mott insulator state. They also spin like tops, and at low temperatures neighboring electrons spin in opposite directions, creating an up-down-up-down pattern of magnetism called antiferromagnetism. Superconductivity sets in when impurities soak up a few electrons and ease the traffic jam. The remaining electrons then pair to glide freely along the planes.

    Theorists do not yet agree how that pairing occurs. Some think that wavelike ripples in the antiferromagnetic pattern act as a glue to attract one electron to the other. Others argue that the pairing arises, paradoxically, from the repulsion among the electrons alone. Theorists can write down a mathematical model of electrons on a checkerboard plane, known as the Fermi-Hubbard model, but it is so hard to “solve” that nobody has been able to show whether it produces superconductivity.

    Experimentalists hope to reproduce the Fermi-Hubbard model in laser light and cold atoms to see if it yields superconductivity. In 2002, Immanuel Bloch, a physicist at the Max Planck Institute for Quantum Optics (MPQ) in Garching, Germany, and colleagues realized a Mott insulator state in an optical lattice. Six years later, Esslinger and colleagues achieved the Mott state with atoms with the right amount of spin to mimic electrons. Now, Randall Hulet, a physicist at Rice University in Houston, Texas, and colleagues have nearly achieved the next-to-last step along the way: antiferromagnetism.

    Hulet and colleagues trapped between 100,000 and 250,000 lithium-6 atoms in laser light. They then ramped up the optical lattice and ramped it back down to put them in order. Shining laser light of a specific wavelength on the atoms, they observed evidence of an emerging up-down-up-down spin pattern. The laser light was redirected, or diffracted, at a particular angle by the rows of atoms—just as x-rays diffract off the ions in a real crystal. Crucially, the light probed the spin of the atoms: The light wave flipped if it bounced off an atom spinning one way but not the other. Without that flipping, the diffraction wouldn’t have occurred, so observation confirms the emergence of the up-down-up-down pattern, Hulet says.

    Hulet’s team solved a problem that has plagued other efforts. Usually, turning the optical lattice on heats the atoms. To avoid that, the researchers added another laser that slightly repelled the atoms, so that the most energetic ones were just barely held by the trap. Then, as the atoms heated, the most energetic ones “evaporated” like steam from hot soup to keep the other ones cool, the researchers report online this week in Nature. They didn’t quite reach a full stable antiferromagnetic pattern: The temperature was 40% too high. But the technique might get there and further, Hulet says. “We don’t have a good sense of what the limit of this method is,” he says. “We could get a factor of two lower, we could get a factor of 10 lower.”

    “It is indeed very promising,” says Tin-Lun “Jason” Ho, a theorist at Ohio State University, Columbus. Reducing the temperature by a factor of two or three might be enough to reach the superconducting state, he says. However, MPQ’s Bloch cautions that it may take still other techniques to get that cold. “There are several cooling techniques that people are developing and interesting experiments coming up,” he says.

    Physicists are also exploring other systems and problems with optical lattices. The approach is still gaining steam, Hulet says: “It’s an exciting time.”

    See the full article here.

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  • richardmitnick 5:40 pm on February 23, 2015 Permalink | Reply
    Tags: , , Superconductivity   

    From Rice: “Simulating superconducting materials with ultracold atoms” 

    Rice U bloc

    Rice University

    February 23, 2015
    Jade Boyd

    Using ultracold atoms as a stand-in for electrons, a Rice University-based team of physicists has simulated superconducting materials and made headway on a problem that’s vexed physicists for nearly three decades.

    The research was carried out by an international team of experimental and theoretical physicists and appears online this week in the journal Nature. Team leader Randy Hulet, an experimental physicist at Rice, said the work could open up a new realm of unexplored science.

    Randy Hulet

    Nearly 30 years have passed since physicists discovered that electrons can flow freely through certain materials — superconductors — at relatively elevated temperatures. The reasons for this high-temperature, or “unconventional” superconductivity are still largely unknown. One of the most promising theories to explain unconventional superconductivity — called the Hubbard model — is simple to express mathematically but is impossible to solve with digital computers.

    “The Hubbard model is a set of mathematical equations that could hold the key to explaining high-temperature superconductivity, but they are too complex to solve — even with the fastest supercomputer,” said Hulet, Rice’s Fayez Sarofim Professor of Physics and Astronomy. “That’s where we come in.”

    Hulet’s lab specializes in cooling atoms to such low temperatures that their behavior is dictated by the rules of quantum mechanics — the same rules that electrons follow when they flow through superconductors.

    “Using our cold atoms as stand-ins for electrons and beams of laser light to mimic the crystal lattice in a real material, we were able to simulate the Hubbard model,” Hulet said. “When we did that, we were able to produce antiferromagnetism in exactly the way the Hubbard model predicts. That’s exciting because it’s the first ultracold atomic system that’s able to probe the Hubbard model in this way, and also because antiferromagnetism is known to exist in nearly all of the parent compounds of unconventional superconductors.”

    Hulet’s team is one of many that are racing to use ultracold atomic systems to simulate the physics of high-temperature superconductors.

    “Despite 30 years of effort, people have yet to develop a complete theory for high-temperature superconductivity,” Hulet said. “Real electronic materials are extraordinarily complex, with impurities and lattice defects that are difficult to fully control. In fact, it has been so difficult to study the phenomenon in these materials that physicists still don’t know the essential ingredients that are required to make an unconventional superconductor or how to make a material that superconducts at even greater temperature.”

    Hulet’s system mimics the actual electronic material, but with no lattice defects or disorder.

    Rice University physicists trapped ultracold atomic gas in grids of intersecting laser beams to mimic the antiferromagnetic order observed in the parent compounds of nearly all high-temperature superconductors. Credit: P. Duarte/Rice University

    “We believe that magnetism plays a role in this process, and we know that each electron in these materials correlates with every other, in a highly complex way,” he said. “With our latest findings, we’ve confirmed that we can cool our system to the point where we can simulate short-range magnetic correlations between electrons just as they begin to develop.

    “That’s significant because our theoretical colleagues — there were five on this paper — were able to use a mathematical technique known as the Quantum Monte Carlo method to verify that our results match the Hubbard model,” Hulet said. “It was a heroic effort, and they pushed their computer simulations as far as they could go. From here on out, as we get colder still, we’ll be extending the boundaries of known physics.”

    Nandini Trivedi, professor of physics at Ohio State University, explained that she and her colleagues at the University of California-Davis, who formed the theoretical side of the effort, had the task of identifying just how cold the atoms had to be in the experiment.

    “Some of the big questions we ask are related to the new kinds of ways in which atoms get organized at low temperatures,” she said. “Because going to such low temperatures is a challenge, theory helped determine the highest temperature at which we might expect the atoms to order themselves like those of an antiferromagnet.”

    After high-temperature superconductivity was discovered in the 1980s, some theoretical physicists proposed that the underlying physics could be explained with the Hubbard model, a set of equations invented in the early 1960s by physicist John Hubbard to describe the magnetic and conduction properties of electrons in transition metals and transition metal oxides.

    Every electron has a “spin” that behaves as a tiny magnet. Scientists in the 1950s and 1960s noticed that the spins of electrons in transition metals and transition metal oxides could become aligned in ordered patterns. In creating his model, Hubbard sought to create the simplest possible system for explaining how the electrons in these materials responded to one another.

    The Hubbard model features electrons that can hop between sites in an ordered grid, or lattice. Each site in the lattice represents an ion in the crystal lattice of a material, and the electrons’ behavior is dictated by just a handful of variables. First, electrons are disallowed from sharing an energy level, due to a rule known as the Pauli Exclusion Principle. Second, electrons repel one another and must pay an energy penalty when they occupy the same site.

    “The Hubbard model is remarkably simple to express mathematically,” Hulet said. “But because of the complexity of the solutions, we cannot calculate its properties for anything but a very small number of electrons on the lattice. There is simply too much quantum entanglement among the system’s degrees of freedom.”

    Correlated electron behaviors — like antiferromagnetism and superconductivity — result from feedback, as the action of every electron causes a cascade that affects all of its neighbors. Running the calculations becomes exponentially more time-consuming as the number of sites increases. To date, the best efforts to produce computer simulations of two- and three-dimensional Hubbard models involve systems with no more than a few hundred sites.

    Because of these computational difficulties, it has been impossible for physicists to determine whether the Hubbard model contains the essence of unconventional superconductivity. Studies have confirmed that the model’s solutions show antiferromagnetism, but it is unknown whether they also exhibit superconductivity.

    Researchers used the optical technique called Bragg scattering to observe the symmetry planes that are characteristic of anti-ferromagnetic order. Credit: P. Duarte/Rice University

    In the new study, Hulet and colleagues, including postdoctoral researcher Russell Hart and graduate student Pedro Duarte, created a new experimental technique to cool the atoms in their lab to sufficiently low temperatures to begin to observe antiferromagnetic order in an optical lattice with approximately 100,000 sites. This new technique results in temperatures on the lattice that are about half of that obtained in previous experiments.

    “The standard technique is to create the cold atomic gas, load it into the lattice and take measurements,” Hart said. “We developed the first method for evaporative cooling of atoms that had already been loaded in a lattice. That technique, which uses what we call a ‘compensated optical lattice,’ also helped control the density of the sample, which becomes critical for forming antiferromagnetic order.”

    Hulet said a second innovation was the team’s use of the optical technique called Bragg scattering to observe the symmetry planes that are characteristic of antiferromagnetic order.

    He said the team will need to develop an entirely new technique to measure the electron pair correlations that cause superconductivity. And they’ll also need colder samples, about 10 times colder than those used in the current study.

    “We have some things in mind,” Hulet said. “I am confident we can achieve lower temperatures both by refining what we’ve already done and by developing new techniques. Our immediate goal is to get cold enough to get fully into the antiferromagnetic regime, and from there we’d hope to get into the d-wave pairing regime and confirm whether or not it exists in the Hubbard model.”

    Additional co-authors include Tsung-lin Yang and Xinxing Liu, all of Rice; Thereza Paiva of Universidade Federal do Rio de Janeiro; Ehsan Khatami of both the University of California-Davis (UC-Davis) and San Jose State University; Richard Scalettar of UC-Davis; and David Huse of Princeton University. The research at Rice was supported by the Defense Advanced Research Projects Agency, the National Science Foundation, the Robert Welch Foundation and the Office of Naval Research.

    See the full article here.

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    In his 1912 inaugural address, Rice University president Edgar Odell Lovett set forth an ambitious vision for a great research university in Houston, Texas; one dedicated to excellence across the range of human endeavor. With this bold beginning in mind, and with Rice’s centennial approaching, it is time to ask again what we aspire to in a dynamic and shrinking world in which education and the production of knowledge will play an even greater role. What shall our vision be for Rice as we prepare for its second century, and how ought we to advance over the next decade?

    This was the fundamental question posed in the Call to Conversation, a document released to the Rice community in summer 2005. The Call to Conversation asked us to reexamine many aspects of our enterprise, from our fundamental mission and aspirations to the manner in which we define and achieve excellence. It identified the pressures of a constantly changing and increasingly competitive landscape; it asked us to assess honestly Rice’s comparative strengths and weaknesses; and it called on us to define strategic priorities for the future, an effort that will be a focus of the next phase of this process.

  • richardmitnick 1:44 pm on February 14, 2015 Permalink | Reply
    Tags: , , , , Superconductivity   

    From Rutgers: “Rutgers-Led Research Team Makes Major Stride in Explaining 30-Year-Old ‘Hidden Order’ Physics Mystery” 

    Rutgers University
    Rutgers University

    February 12, 2015

    Findings may lead to new kinds of materials for electronics and superconducting magnets.

    A new explanation for a type of order, or symmetry, in an exotic material made with uranium may lead to enhanced computer displays and data storage systems, and more powerful superconducting magnets for medical imaging and levitating high-speed trains, according to a Rutgers-led team of research physicists.

    The team’s findings are a major step toward explaining a puzzle that physicists worldwide have been struggling with for 30 years, when scientists first noticed a change in the material’s electrical and magnetic properties but were unable to describe it fully. This subtle change occurs when the material is cooled to 17.5 degrees above absolute zero or lower (a bone-chilling minus 428 degrees Fahrenheit).

    Physicists Hsiang-Hsi Kung and Girsh Blumberg with instrumentation they used to examine hidden order. Photo: Carl Blesch

    “This ‘hidden order’ has been the subject of nearly a thousand scientific papers since it was first reported in 1985 at Leiden University in the Netherlands,” said Girsh Blumberg, professor in the Department of Physics and Astronomy in the School of Arts and Sciences.

    Collaborators from Rutgers University, the Los Alamos National Laboratory in New Mexico, and Leiden University published their findings this week in the web-based journal Science Express, which features selected research papers in advance of their appearance in the journal Science. Blumberg and two Rutgers colleagues, graduate student Hsiang-Hsi Kung and professor Kristjan Haule, led the collaboration.

    Changes in order are what make liquid crystals, magnetic materials and superconductors work and perform useful functions. While the Rutgers-led discovery won’t transform high-tech products overnight, this kind of knowledge is vital to ongoing advances in electronic technology.

    “The Los Alamos collaborators produced a crystalline sample of the uranium, ruthenium and silicon compound with unprecedented purity, a breakthrough we needed to make progress in solving the puzzle of hidden order,” said Blumberg. Uranium is commonly known as an element in nuclear reactor fuel or weapons material, but in this case, physicists value it as a heavy metal with electrons that behave differently than those in common metals.

    Below the hidden order temperature of 17.5 degrees Kelvin, uranium electron orbital patterns in adjacent crystal layers become mirror images of each other (right side of illustration). Above that temperature, uranium electron orbitals are the same (left side of illustration).Image: Hsiang-Hsi Kung

    Under these cold conditions, the orbital patterns made by electrons in uranium atoms from adjacent crystal layers become mirror images of each other. Above the hidden order temperature, these electron orbitals are the same. The Rutgers researchers discovered this so-called “broken mirror symmetry” using instrumentation they developed – based on a principle known as Raman scattering – to distinguish the pattern of the mirror images in the electron orbitals.

    Blumberg also credits two theoretical physics professors at Rutgers for predicting the phenomenon that his team discovered.

    “In this field, it’s rare to have such predictive power,” he said, noting that Gabriel Kotliar developed a computational technique that led to the prediction of the hidden order symmetry. Haule and Kotliar applied this technique to predict the changes in electron orbitals that Kung and Blumberg detected.

    At still colder temperatures of 1.5 degrees above absolute zero, the material becomes superconducting – losing all resistance to the flow of electricity. While not practical for today’s products and systems that rely on superconductivity, the material provides new insights into ways that materials can become superconducting.

    Kristjan Haule, left, reviews prediction of hidden order symmetry with Hsiang-Hsi Kung and Girsh Blumberg. Photo: Carl Blesch

    The hidden order puzzle has also been a focus of other Rutgers researchers. Two years ago, professors Premala Chandra and Piers Coleman, along with alumna Rebecca Flint, published another theoretical explanation of the phenomenon in the journal Nature.

    The Leiden University collaborator, John Mydosh, is a member of the laboratory that discovered hidden order in 1985.

    “The work of Blumberg and his team is an important and viable step towards the understanding of hidden order,” Mydosh said. “We are well on our way after 30 years towards the final solution.”

    Working with Kung, Blumberg and Haule at Rutgers were Verner Thorsmølle and Weilu Zhang. The Los Alamos National Laboratory collaborators are Ryan Baumbach and Eric Bauer.

    The research was funded by the National Science Foundation and the U.S. Department of Energy’s Office of Basic Energy Sciences, Division of Materials Sciences and Engineering.

    See the full article here.

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  • richardmitnick 4:33 pm on December 16, 2014 Permalink | Reply
    Tags: , Superconductivity   

    From MIT: “New law for superconductors” 

    MIT News

    December 16, 2014
    Larry Hardesty | MIT News Office

    Mathematical description of relationship between thickness, temperature, and resistivity could spur advances.

    MIT researchers have discovered a new mathematical relationship — between material thickness, temperature, and electrical resistance — that appears to hold in all superconductors. They describe their findings in the latest issue of Physical Review B.

    Atoms of niobium and nitrogen in an ultrathin superconducting film that helped MIT researchers discover a universal law of superconductivity. Image: Yachin Ivry

    The result could shed light on the nature of superconductivity and could also lead to better-engineered superconducting circuits for applications like quantum computing and ultralow-power computing.

    “We were able to use this knowledge to make larger-area devices, which were not really possible to do previously, and the yield of the devices increased significantly,” says Yachin Ivry, a postdoc in MIT’s Research Laboratory of Electronics, and the first author on the paper.

    Ivry works in the Quantum Nanostructures and Nanofabrication Group, which is led by Karl Berggren, a professor of electrical engineering and one of Ivry’s co-authors on the paper. Among other things, the group studies thin films of superconductors.

    Superconductors are materials that, at temperatures near absolute zero, exhibit no electrical resistance; this means that it takes very little energy to induce an electrical current in them. A single photon will do the trick, which is why they’re useful as quantum photodetectors. And a computer chip built from superconducting circuits would, in principle, consume about one-hundredth as much energy as a conventional chip.

    “Thin films are interesting scientifically because they allow you to get closer to what we call the superconducting-to-insulating transition,” Ivry says. “Superconductivity is a phenomenon that relies on the collective behavior of the electrons. So if you go to smaller and smaller dimensions, you get to the onset of the collective behavior.”

    Vexing variation

    Specifically, Ivry studied niobium nitride, a material favored by researchers because, in its bulk form, it has a relatively high “critical temperature” — the temperature at which it switches from an ordinary metal to a superconductor. But like most superconductors, it has a lower critical temperature when it’s deposited in the thin films on which nanodevices rely.

    Previous theoretical work had characterized niobium nitride’s critical temperature as a function of either the thickness of the film or its measured resistivity at room temperature. But neither theory seemed to explain the results Ivry was getting. “We saw large scatter and no clear trend,” he says. “It made no sense, because we grew them in the lab under the same conditions.”

    So the researchers conducted a series of experiments in which they held constant either thickness or “sheet resistance,” the material’s resistance per unit area, while varying the other parameter; they then measured the ensuing changes in critical temperature. A clear pattern emerged: Thickness times critical temperature equaled a constant — call it A — divided by sheet resistance raised to a particular power — call it B.

    After deriving that formula, Ivry checked it against other results reported in the superconductor literature. His initial excitement evaporated, however, with the first outside paper he consulted. Though most of the results it reported fit his formula perfectly, two of them were dramatically awry. Then a colleague who was familiar with the paper pointed out that its authors had acknowledged in a footnote that those two measurements might reflect experimental error: When building their test device, the researchers had forgotten to turn on one of the gases they used to deposit their films.

    Broadening the scope

    The other niobium nitride papers Ivry consulted bore out his predictions, so he began to expand to other superconductors. Each new material he investigated required him to adjust the formula’s constants — A and B. But the general form of the equation held across results reported for roughly three dozen different superconductors.

    It wasn’t necessarily surprising that each superconductor should have its own associated constant, but Ivry and Berggren weren’t happy that their equation required two of them. When Ivry graphed A against B for all the materials he’d investigated, however, the results fell on a straight line.

    Finding a direct relationship between the constants allowed him to rely on only one of them in the general form of his equation. But perhaps more interestingly, the materials at either end of the line had distinct physical properties. Those at the top had highly disordered — or, technically, “amorphous” — crystalline structures; those at the bottom were more orderly, or “granular.” So Ivry’s initial attempt to banish an inelegance in his equation may already provide some insight into the physics of superconductors at small scales.

    “None of the admitted theory up to now explains with such a broad class of materials the relation of critical temperature with sheet resistance and thickness,” says Claude Chapelier, a superconductivity researcher at France’s Alternative Energies and Atomic Energy Commission. “There are several models that do not predict the same things.”

    Chapelier says he would like to see a theoretical explanation for that relationship. But in the meantime, “this is very convenient for technical applications,” he says, “because there is a lot of spreading of the results, and nobody knows whether they will get good films for superconducting devices. By putting a material into this law, you know already whether it’s a good superconducting film or not.”

    See the full article here.

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