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  • richardmitnick 10:45 am on August 29, 2019 Permalink | Reply
    Tags: , Jenga chemistry, , , , Superconductivity   

    From SLAC National Accelerator Lab: “First report of superconductivity in a nickel oxide material” 

    August 28, 2019
    Glennda Chui
    glennda@slac.stanford.edu
    (650) 926-4897

    Made with ‘Jenga chemistry,’ the discovery could help crack the mystery of how high-temperature superconductors work.

    1
    An illustration depicts a key step in creating a new type of superconducting material: Much like pulling blocks from a tower in a Jenga game, scientists used chemistry to neatly remove a layer of oxygen atoms. This flipped the material into a new atomic structure – a nickelate – that can conduct electricity with 100 percent efficiency. (Greg Stewart/SLAC National Accelerator Laboratory)

    Scientists at the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University have made the first nickel oxide material that shows clear signs of superconductivity – the ability to transmit electrical current with no loss.

    Also known as a nickelate, it’s the first in a potential new family of unconventional superconductors that’s very similar to the copper oxides, or cuprates, whose discovery in 1986 raised hopes that superconductors could someday operate at close to room temperature and revolutionize electronic devices, power transmission and other technologies. Those similarities have scientists wondering if nickelates could also superconduct at relatively high temperatures.

    At the same time, the new material seems different from the cuprates in fundamental ways – for instance, it may not contain a type of magnetism that all the superconducting cuprates have – and this could overturn leading theories of how these unconventional superconductors work. After more than three decades of research, no one has pinned that down.

    The experiments were led by Danfeng Li, a postdoctoral researcher with the Stanford Institute for Materials and Energy Sciences (SIMES) at SLAC, and described today in Nature.

    2

    “This is a very important discovery that requires us to rethink the details of the electronic structure and possible mechanisms of superconductivity in these materials,” said George Sawatzky, a professor of physics and chemistry at the University of British Columbia who was not involved in the study but wrote a commentary that accompanied the paper in Nature. “This is going to cause an awful lot of people to jump into investigating this new class of materials, and all sorts of experimental and theoretical work will be done.”

    3
    To create a new type of superconducting material, scientists at SLAC and Stanford first made a thin film of a common material known as perovskite, left; “doped” it with strontium; and then exposed it to a chemical that yanked out a layer of oxygen atoms, much like removing a stick from a tower of Jenga blocks. This made the film flip into a different atomic structure known as a nickelate, right. Tests showed that this nickelate can conduct electricity with no resistance. (Danfeng Li/SLAC National Accelerator Laboratory and Stanford University)

    A difficult path

    Ever since the cuprate superconductors were discovered, scientists have dreamed of making similar oxide materials based on nickel, which is right next to copper on the periodic table of the elements.

    But making nickelates with an atomic structure that’s conducive to superconductivity turned out to be unexpectedly hard.

    “As far as we know, the nickelate we were trying to make is not stable at the very high temperatures – about 600 degrees Celsius – where these materials are normally grown,” Li said. “So we needed to start out with something we can stably grow at high temperatures and then transform it at lower temperatures into the form we wanted.”

    He started with a perovskite – a material defined by its unique, double-pyramid atomic structure – that contained neodymium, nickel and oxygen. Then he doped the perovskite by adding strontium; this is a common process that adds chemicals to a material to make more of its electrons flow freely.

    This stole electrons away from nickel atoms, leaving vacant “holes,” and the nickel atoms were not happy about it, Li said. The material was now unstable, making the next step – growing a thin film of it on a surface – really challenging; it took him half a year to get it to work.

    ‘Jenga chemistry’

    Once that was done, Li cut the film into tiny pieces, loosely wrapped it in aluminum foil and sealed it in a test tube with a chemical that neatly snatched away a layer of its oxygen atoms – much like removing a stick from a wobbly tower of Jenga blocks. This flipped the film into an entirely new atomic structure – a strontium-doped nickelate.


    SIMES researcher Danfeng Li explains the delicate ‘Jenga chemistry’ behind making a new nickel oxide material, the first in a potential new family of unconventional superconductors. (Linda McCulloch, SLAC National Accelerator Laboratory)

    “Each of these steps had been demonstrated before,” Li said, “but not in this combination.”

    He remembers the exact moment in the laboratory, around 2 a.m., when tests indicated that the doped nickelate might be superconducting. Li was so excited that he stayed up all night, and in the morning co-opted the regular meeting of his research group to show them what he’d found. Soon, many of the group members joined him in a round-the-clock effort to improve and study this material.

    Further testing would reveal that the nickelate was indeed superconducting in a temperature range from 9-15 kelvins – incredibly cold, but a first start, with possibilities of higher temperatures ahead.

    More work ahead

    Research on the new material is in a “very, very early stage, and there’s a lot of work ahead,” cautioned Harold Hwang, a SIMES investigator, professor at SLAC and Stanford and senior author of the report. “We have just seen the first basic experiments, and now we need to do the whole battery of investigations that are still going on with cuprates.”

    Among other things, he said, scientists will want to dope the nickelate material in various ways to see how this affects its superconductivity across a range of temperatures, and determine whether other nickelates can become superconducting. Other studies will explore the material’s magnetic structure and its relationship to superconductivity.

    SIMES researchers from the Stanford departments of Physics, Applied Physics and Materials Science and Engineering also contributed to the study, which was funded by the DOE Office of Science and the Gordon and Betty Moore Foundation’s Emergent Phenomena in Quantum Systems Initiative.

    See the full article here .


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    SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the DOE’s Office of Science.

     
  • richardmitnick 8:28 am on August 18, 2019 Permalink | Reply
    Tags: , , LCLS, , , , , SSRL-Stanford Synchrotron Light Source, Superconductivity,   

    From SLAC National Accelerator Lab: “Scientists report two advances in understanding the role of ‘charge stripes’ in superconducting materials” 

    From SLAC National Accelerator Lab

    Ali Sundermier
    Glennda Chui

    The studies could lead to a new understanding of how high-temperature superconductors operate.

    High-temperature superconductors, which carry electricity with zero resistance at much higher temperatures than conventional superconducting materials, have generated a lot of excitement since their discovery more than 30 years ago because of their potential for revolutionizing technologies such as maglev trains and long-distance power lines. But scientists still don’t understand how they work.

    One piece of the puzzle is the fact that charge density waves – static stripes of higher and lower electron density running through a material – have been found in one of the major families of high-temperature superconductors, the copper-based cuprates. But do these charge stripes enhance superconductivity, suppress it or play some other role?

    In independent studies, two research teams report important advances in understanding how charge stripes might interact with superconductivity. Both studies were carried out with X-rays at the Department of Energy’s SLAC National Accelerator Laboratory.

    Exquisite detail

    In a paper published today in Science Advances, researchers from the University of Illinois at Urbana-Champaign (UIUC) used SLAC’s Linac Coherent Light Source (LCLS) X-ray free-electron laser [below] to observe fluctuations in charge density waves in a cuprate superconductor.

    1
    This cutaway view shows stripes of higher and lower electron density – “charge stripes” – within a copper-based superconducting material. Experiments with SLAC’s X-ray laser directly observed how those stripes fluctuate when hit with a pulse of light, a step toward understanding how they interact with high-temperature superconductivity. (Greg Stewart/SLAC National Accelerator Laboratory)

    They disturbed the charge density waves with pulses from a conventional laser and then used RIXS, or resonant inelastic X-ray scattering, to watch the waves recover over a period of a few trillionths of a second. This recovery process behaved according to a universal dynamical scaling law: It was the same at all scales, much as a fractal pattern looks the same whether you zoom in or zoom out.

    With LCLS, the scientists were able to measure, for the first time and in exquisite detail, exactly how far and how fast the charge density waves fluctuated. To their surprise, the team discovered that the fluctuations were not like the ringing of a bell or the bouncing of a trampoline; instead, they were more like the slow diffusion of a syrup – a quantum analog of liquid crystal behavior, which had never been seen before in a solid.

    “Our experiments at LCLS establish a new way to study fluctuations in charge density waves, which could lead to a new understanding of how high-temperature superconductors operate,” says Matteo Mitrano, a postdoctoral researcher in professor Peter Abbamonte’s group at UIUC.

    This team also included researchers from Stanford University, the National Institute of Standards and Technology and Brookhaven National Laboratory.

    Hidden arrangements

    Another study, reported last month in Nature Communications, used X-rays from SLAC’S Stanford Synchrotron Radiation Lightsource (SSRL) to discover two types of charge density wave arrangements, making a new link between these waves and high-temperature superconductivity.

    SLAC/SSRL

    Led by SLAC scientist Jun-Sik Lee, the research team used RSXS, or resonant soft X-ray scattering, to watch how temperature affected the charge density waves in a cuprate superconductor.

    “This resolves a mismatch in data from previous experiments and charts a new course for fully mapping the behaviors of electrons in these exotic superconducting materials,” Lee says.

    “I believe that exploring new or hidden arrangements, as well as their intertwining phenomena, will contribute to our understanding of high-temperature superconductivity in cuprates, which will inform researchers in their quest to design and develop new superconductors that work at warmer temperatures.”

    The team also included researchers from Stanford, Pohang Accelerator Laboratory in South Korea and Tohoku University in Japan.

    SSRL and LCLS are DOE Office of Science user facilities. Both studies were supported by the Office of Science.

    See the full article here .


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    SLAC/LCLS II projected view


    SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the DOE’s Office of Science.

     
  • richardmitnick 1:41 pm on June 22, 2019 Permalink | Reply
    Tags: , , LBCO (lanthanum barium copper oxide) was the first high-temperature (high-Tc) superconductor discovered some 33 years ago., , Superconductivity   

    From Brookhaven National Lab: “Electron (or ‘Hole’) Pairs May Survive Effort to Kill Superconductivity” 

    From Brookhaven National Lab

    June 14, 2019
    Karen McNulty Walsh,
    kmcnulty@bnl.gov
    (631) 344-8350

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

    Emergence of unusual metallic state supports role of charge stripes in formation of charge-carrier pairs essential to resistance-free flow of electrical current.

    1
    Showing their stripes: Brookhaven Lab physicists present new evidence that stripes—alternating areas of charge and magnetism in certain copper-oxide materials—are good for forming the charge-carrier pairs needed for electrical current to flow with no resistance. Left to right: Qiang Li, Genda Gu, John Tranquada, Alexei Tsvelik, and Yangmu Li in front of an image of wind-blown ripples in desert sand.

    Scientists seeking to understand the mechanism underlying superconductivity in “stripe-ordered” cuprates—copper-oxide materials with alternating areas of electric charge and magnetism—discovered an unusual metallic state when attempting to turn superconductivity off. They found that under the conditions of their experiment, even after the material loses its ability to carry electrical current with no energy loss, it retains some conductivity—and possibly the electron (or hole) pairs required for its superconducting superpower.

    “This work provides circumstantial evidence that the stripe-ordered arrangement of charges and magnetism is good for forming the charge-carrier pairs required for superconductivity to emerge,” said John Tranquada, a physicist at the U.S. Department of Energy’s Brookhaven National Laboratory.

    Tranquada and his co-authors from Brookhaven Lab and the National High Magnetic Field Laboratory at Florida State University, where some of the work was done, describe their findings in a paper just published in Science Advances. A related paper in the Proceedings of the National Academy of Sciences by co-author Alexei Tsvelik, a theorist at Brookhaven Lab, provides insight into the theoretical underpinnings for the observations.

    2
    This image represents the stripes of magnetism and charge in the cuprate (copper and oxygen) layers of the superconductor LBCO. Gray shading represents the modulation of the charge (“holes,” or electron vacancies), which is maximized in stripes that separate areas of magnetism, indicated by arrows representing alternating magnetic orientations on adjacent copper atoms.

    The scientists were studying a particular formulation of lanthanum barium copper oxide (LBCO) that exhibits an unusual form of superconductivity at a temperature of 40 Kelvin (-233 degrees Celsius). That’s relatively warm in the realm of superconductors. Conventional superconductors must be cooled with liquid helium to temperatures near -273°C (0 Kelvin or absolute zero) to carry current without energy loss. Understanding the mechanism behind such “high-temperature” superconductivity might guide the discovery or strategic design of superconductors that operate at higher temperatures.

    “In principle, such superconductors could improve the electrical power infrastructure with zero-energy-loss power transmission lines,” Tranquada said, “or be used in powerful electromagnets for applications like magnetic resonance imaging (MRI) without the need for costly cooling.”

    The mystery of high-Tc

    LBCO was the first high-temperature (high-Tc) superconductor discovered, some 33 years ago. It consists of layers of copper-oxide separated by layers composed of lanthanum and barium. Barium contributes fewer electrons than lanthanum to the copper-oxide layers, so at a particular ratio, the imbalance leaves vacancies of electrons, known as holes, in the cuprate planes. Those holes can act as charge carriers and pair up, just like electrons, and at temperatures below 30K, current can move through the material with no resistance in three dimensions—both within and between the layers.

    3
    Copper-oxide layers of LBCO (the lanthanum-barium layers would be between these). 3-D superconductivity occurs when current can flow freely in any direction within and between the copper-oxide layers, while 2-D superconductivity exists when current moves freely only within the layers (not perpendicular). The perpendicular orientations of stripe patterns from one layer to the next may be part of what inhibits movement of current between layers.

    An odd characteristic of this material is that, in the copper-oxide layers, at the particular barium concentration, the holes segregate into “stripes” that alternate with areas of magnetic alignment. Since this discovery, in 1995, there has been much debate about the role these stripes play in inducing or inhibiting superconductivity.

    In 2007, Tranquada and his team discovered the most unusual form of superconductivity in this material at the higher temperature of 40K. If they altered the amount of barium to be just under the amount that allowed 3-D superconductivity, they observed 2-D superconductivity—meaning just within the copper-oxide layers but not between them.

    “The superconducting layers seem to decouple from one another,” Tsvelik, the theorist, said. The current can still flow without loss in any direction within the layers, but there is resistivity in the direction perpendicular to the layers. This observation was interpreted as a sign that charge-carrier pairs were forming “pair density waves” with orientations perpendicular to one another in neighboring layers. “That’s why the pairs can’t jump from layer to another. It would be like trying to merge into traffic moving in a perpendicular direction. They can’t merge,” Tsvelik said.

    Superconducting stripes are hard to kill

    In the new experiment, the scientists dove deeper into exploring the origins of the unusual superconductivity in the special formulation of LBCO by trying to destroy it. “Often times we test things by pushing them to failure,” Tranquada said. Their method of destruction was exposing the material to powerful magnetic fields generated at Florida State.

    “As the external field gets bigger, the current in the superconductor grows larger and larger to try to cancel out the magnetic field,” Tranquada explained. “But there’s a limit to the current that can flow without resistance. Finding that limit should tell us something about how strong the superconductor is.”

    4
    A phase diagram of LBCO at different temperatures and magnetic field strengths. Colors represent how resistant the material is to the flow of electrical current, with purple being a superconductor with no resistance. When cooled to near absolute zero with no magnetic field, the material acts as a 3-D superconductor. As the magnetic field strength goes up, 3-D superconductivity disappears, but 2-D superconductivity reappears at higher field strength, then disappears again. At the highest fields, resistance grew, but the material retained some unusual metallic conductivity, which the scientists interpreted as an indication that charge-carrier pairs might persist even after superconductivity is destroyed.

    For example, if the stripes of charge order and magnetism in LBCO are bad for superconductivity, a modest magnetic field should destroy it. “We thought maybe the charge would get frozen in the stripes so that the material would become an insulator,” Tranquada said.

    But the superconductivity turned out to be a lot more robust.

    Using perfect crystals of LBCO grown by Brookhaven physicist Genda Gu, Yangmu Li, a postdoctoral fellow who works in Tranquada’s lab, took measurements of the material’s resistance and conductivity under various conditions at the National High Magnetic Field Laboratory. At a temperature just above absolute zero with no magnetic field present, the material exhibited full, 3-D superconductivity. Keeping the temperature constant, the scientists had to ramp up the external magnetic field significantly to make the 3-D superconductivity disappear. Even more surprising, when they increased the field strength further, the resistance within the copper-oxide planes went down to zero again!

    “We saw the same 2-D superconductivity we’d discovered at 40K,” Tranquada said.

    Ramping up the field further destroyed the 2-D superconductivity, but it never completely destroyed the material’s ability to carry ordinary current.

    “The resistance grew but then leveled off,” Tranquada noted.

    Signs of persistent pairs?

    Additional measurements made under the highest-magnetic-field indicated that the charge-carriers in the material, though no longer superconducting, may still exist as pairs, Tranquada said.

    “The material becomes a metal that no longer deflects the flow of current,” Tsvelik said. “Whenever you have a current in a magnetic field, you would expect some deflection of the charges—electrons or holes—in the direction perpendicular to the current [what scientists call the Hall effect]. But that’s not what happens. There is no deflection.”

    In other words, even after the superconductivity is destroyed, the material keeps one of the key signatures of the “pair density wave” that is characteristic of the superconducting state.

    “My theory relates the presence of the charge-rich stripes with the existence of magnetic moments between them to the formation of the pair density wave state,” Tsvelik said. “The observation of no charge deflection at high field shows that the magnetic field can destroy the coherence needed for superconductivity without necessarily destroying the pair density wave.”

    “Together these observations provide additional evidence that the stripes are good for pairing,” Tranquada said. “We see the 2-D superconductivity reappear at high field and then, at an even higher field, when we lose the 2-D superconductivity, the material doesn’t just become an insulator. There’s still some current flowing. We may have lost coherent motion of pairs between the stripes, but we may still have pairs within the stripes that can move incoherently and give us an unusual metallic behavior.”

    See the full article here .


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  • richardmitnick 3:38 pm on January 3, 2019 Permalink | Reply
    Tags: , , , Electron spin, , SARPES detector, , Superconductivity   

    From Lawrence Berkeley National Lab: “Revealing Hidden Spin: Unlocking New Paths Toward High-Temperature Superconductors” 

    Berkeley Logo

    From Lawrence Berkeley National Lab

    January 3, 2019

    Theresa Duque
    tnduque@lbl.gov
    (510) 495-2418

    Berkeley Lab researchers uncover insights into superconductivity, leading potentially to more efficient power transmission.

    1
    A research team led by Berkeley Lab’s Alessandra Lanzara (second from left) used a SARPES (spin- and angle-resolved photoemission spectroscopy) detector to uncover a distinct pattern of electron spins within the material. Co-lead authors are Kenneth Gotlieb (second from right) and Chiu-Yun Lin (right). The study’s co-authors include Chris Jozwiak of Berkeley Lab’s Advanced Light Source (left). (Credit: Peter DaSilva/Berkeley Lab)

    In the 1980s, the discovery of high-temperature superconductors known as cuprates upended a widely held theory that superconductor materials carry electrical current without resistance only at very low temperatures of around 30 Kelvin (or minus 406 degrees Fahrenheit). For decades since, researchers have been mystified by the ability of some cuprates to superconduct at temperatures of more than 100 Kelvin (minus 280 degrees Fahrenheit).

    Now, researchers at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have unveiled a clue into the cuprates’ unusual properties – and the answer lies within an unexpected source: the electron spin. Their paper describing the research behind this discovery was published on Dec. 13 in the journal Science.

    Adding electron spin to the equation

    Every electron is like a tiny magnet that points in a certain direction. And electrons within most superconductor materials seem to follow their own inner compass. Rather than pointing in the same direction, their electron spins haphazardly point every which way – some up, some down, others left or right.

    2
    With the spin resolution enabled by SARPES, Berkeley Lab researchers revealed magnetic properties of Bi-2212 that have gone unnoticed in previous studies. (Credit: Kenneth Gotlieb, Chiu-Yun Lin, et al./Berkeley Lab)

    When scientists are developing new kinds of materials, they usually look at the materials’ electron spin, or the direction in which the electrons are pointing. But when it comes to making superconductors, condensed matter physicists haven’t traditionally focused on spin, because the conventionally held view was that all of the properties that make these materials unique were shaped only by the way in which two electrons interact with each other through what’s known as “electron correlation.”

    But when a research team led by Alessandra Lanzara, a faculty scientist in Berkeley Lab’s Materials Sciences Division and a Charles Kittel Professor of Physics at UC Berkeley, used a unique detector to measure samples of an exotic cuprate superconductor, Bi-2212 (bismuth strontium calcium copper oxide), with a powerful technique called SARPES (spin- and angle-resolved photoemission spectroscopy), they uncovered something that defied everything they had ever known about superconductors: a distinct pattern of electron spins within the material.

    “In other words, we discovered that there was a well-defined direction in which each electron was pointing given its momentum, a property also known as spin-momentum locking,” said Lanzara. “Finding it in high-temperature superconductors was a big surprise.”

    A new map for high-temperature superconductors

    In the world of superconductors, “high temperature” means that the material can conduct electricity without resistance at temperatures higher than expected but still in extremely cold temperatures far below zero degrees Fahrenheit. That’s because superconductors need to be extraordinarily cold to carry electricity without any resistance. At those low temperatures, electrons are able to move in sync with each other and not get knocked by jiggling atoms, causing electrical resistance.

    And within this special class of high-temperature superconductor materials, cuprates are some of the best performers, leading some researchers to believe that they have potential use as a new material for building super-efficient electrical wires that can carry power without any loss of electron momentum, said co-lead author Kenneth Gotlieb, who was a Ph.D. student in Lanzara’s lab at the time of the discovery. Understanding what makes some exotic cuprate superconductors such as Bi-2212 work at temperatures as high as 133 Kelvin (about -220 degrees Fahrenheit) could make it easier to realize a practical device.

    Among the very exotic materials that condensed matter physicists study, there are two kinds of electron interactions that give rise to novel properties for new materials, including superconductors, said Gotlieb. Scientists who have been studying cuprate superconductors have focused on just one of those interactions: electron correlation.

    The other kind of electron interaction found in exotic materials is “spin-orbit coupling” – the way in which the electron’s magnetic moment interacts with atoms in the material.

    Spin-orbit coupling was often neglected in the studies of cuprate superconductors, because many assumed that this kind of electron interaction would be weak when compared to electron correlation, said co-lead author Chiu-Yun Lin, a researcher in the Lab’s Materials Sciences Division and a Ph.D. student in the Department of Physics at UC Berkeley. So when they found the unusual spin pattern, Lin said that although they were pleasantly surprised by this initial finding, they still weren’t sure whether it was a “true” intrinsic property of the Bi-2212 material, or an external effect caused by the way the laser light interacted with the material in the experiment.

    Shining a light on electron spin with SARPES

    Over the course of nearly three years, Gotlieb and Lin used the SARPES detector to thoroughly map out the spin pattern at Lanzara’s lab. When they needed higher photon energies to excite a wider range of electrons within a sample, the researchers moved the detector next door to Berkeley Lab’s synchrotron, the Advanced Light Source (ALS), a U.S. DOE Office of Science User Facility that specializes in lower energy, “soft” X-ray light for studying the properties of materials.

    LBNL/ALS

    The SARPES detector was developed by Lanzara, along with co-authors Zahid Hussain, the former ALS Division Deputy, and Chris Jozwiak, an ALS staff scientist. The detector allowed the scientists to probe key electronic properties of the electrons such as valence band structure.

    After tens of experiments at the ALS, where the team of researchers connected the SARPES detector to Beamline 10.0.1 so they could access this powerful light to explore the spin of the electrons moving with much higher momentum through the superconductor than those they could access in the lab, they found that Bi-2212’s distinct spin pattern – called “nonzero spin – was a true result, inspiring them to ask even more questions. “There remains many unsolved questions in the field of high-temperature superconductivity,” said Lin. “Our work provides new knowledge to better understand the cuprate superconductors, which can be a building block to resolve these questions.”

    Lanzara added that their discovery couldn’t have happened without the collaborative “team science” of Berkeley Lab, a DOE national lab with historic ties to nearby UC Berkeley. “This work is a typical example of where science can go when people with expertise across the scientific disciplines come together, and how new instrumentation can push the boundaries of science,” she said.

    Co-authors with Gotlieb, Lin, and Lanzara are Maksym Serbyn of the Institute of Science and Technology Austria, Wentao Zhang of Shanghai Jiao Tong University, Christopher L. Smallwood of San Jose State University, Christopher Jozwiak of Berkeley Lab, Hiroshi Eisaki of the National Institute of Advanced Industrial Science and Technology of Japan, Zahid Hussain of Berkeley Lab, and Ashvin Vishwanath, formerly of UC Berkeley and now with Harvard University and a Faculty Scientist in Berkeley Lab’s Materials Sciences Division.

    The work was supported by the DOE Office of Science.

    See the full article here .

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  • richardmitnick 10:37 am on November 1, 2018 Permalink | Reply
    Tags: , , , , , Superconductivity,   

    From SLAC National Accelerator Lab: “Scientists make first detailed measurements of key factors related to high-temperature superconductivity” 

    From SLAC National Accelerator Lab

    October 31, 2018
    Glennda Chui

    1
    A new study reveals how coordinated motions of copper (red) and oxygen (grey) atoms in a high-temperature superconductor boost the superconducting strength of pairs of electrons (white glow), allowing the material to conduct electricity without any loss at much higher temperatures. The discovery opens a new path to engineering higher-temperature superconductors. (Greg Stewart/SLAC National Accelerator Laboratory)

    2
    An illustration depicts the repulsive energy (yellow flashes) generated by electrons in one layer of a cuprate material repelling electrons in the next layer. Theorists think this energy could play a critical role in creating the superconducting state, leading electrons to form a distinctive form of “sound wave” that could boost superconducting temperatures. Scientists have now observed and measured those sound waves for the first time. (Greg Stewart/SLAC National Accelerator Laboratory)

    In superconducting materials, electrons pair up and condense into a quantum state that carries electrical current with no loss. This usually happens at very low temperatures. Scientists have mounted an all-out effort to develop new types of superconductors that work at close to room temperature, which would save huge amounts of energy and open a new route for designing quantum electronics. To get there, they need to figure out what triggers this high-temperature form of superconductivity and how to make it happen on demand.

    Now, in independent studies reported in Science and Nature, scientists from the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University report two important advances: They measured collective vibrations of electrons for the first time and showed how collective interactions of the electrons with other factors appear to boost superconductivity.

    Carried out with different copper-based materials and with different cutting-edge techniques, the experiments lay out new approaches for investigating how unconventional superconductors operate.

    “Basically, what we’re trying to do is understand what makes a good superconductor,” said co-author Thomas Devereaux, a professor at SLAC and Stanford and director of SIMES, the Stanford Institute for Materials and Energy Sciences, whose investigators led both studies.

    “What are the ingredients that could give rise to superconductivity at temperatures well above what they are today?” he said. “These and other recent studies indicate that the atomic lattice plays an important role, giving us hope that we are gaining ground in answering that question.”

    The high-temperature puzzle

    Conventional superconductors were discovered in 1911, and scientists know how they work: Free-floating electrons are attracted to a material’s lattice of atoms, which has a positive charge, in a way that lets them pair up and flow as electric current with 100 percent efficiency. Today, superconducting technology is used in MRI machines, maglev trains and particle accelerators.

    But these superconductors work only when chilled to temperatures as cold as outer space. So when scientists discovered in 1986 that a family of copper-based materials known as cuprates can superconduct at much higher, although still quite chilly, temperatures, they were elated.

    The operating temperature of cuprates has been inching up ever since – the current record is about 120 degrees Celsius below the freezing point of water – as scientists explore a number of factors that could either boost or interfere with their superconductivity. But there’s still no consensus about how the cuprates function.

    “The key question is how can we make all these electrons, which very much behave as individuals and do not want to cooperate with others, condense into a collective state where all the parties participate and give rise to this remarkable collective behavior?” said Zhi-Xun Shen, a SLAC/Stanford professor and SIMES investigator who participated in both studies.

    Behind-the-scenes boost

    One of the new studies, at SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL), took a systematic look at how “doping” – adding a chemical that changes the density of electrons in a material – affects the superconductivity and other properties of a cuprate called Bi2212.

    SLAC/SSRL


    SLAC/SSRL

    Collaborating researchers at the National Institute of Advanced Industrial Science and Technology (AIST) in Japan prepared samples of the material with slightly different levels of doping. Then a team led by SIMES researcher Yu He and SSRL staff scientist Makoto Hashimoto examined the samples at SSRL with angle-resolved photoemission spectroscopy, or ARPES. It uses a powerful beam of X-ray light to kick individual electrons out of a sample material so their momentum and energy can be measured. This reveals what the electrons in the material are doing.

    In this case, as the level of doping increased, the maximum superconducting temperature of the material peaked and fell off again, He said.

    The team focused in on samples with particularly robust superconducting properties. They discovered that three interwoven effects – interactions of electrons with each other, with lattice vibrations and with superconductivity itself – reinforce each other in a positive feedback loop when conditions are right, boosting superconductivity and raising the superconducting temperature of the material.

    Small changes in doping produced big changes in superconductivity and in the electrons’ interaction with lattice vibrations, Devereaux said. The next step is to figure out why this particular level of doping is so important.

    “One popular theory has been that rather than the atomic lattice being the source of the electron pairing, as in conventional superconductors, the electrons in high-temperature superconductors form some kind of conspiracy by themselves. This is called electronic correlation,” Yu He said. “For instance, if you had a room full of electrons, they would spread out. But if some of them demand more individual space, others will have to squeeze closer to accommodate them.”

    In this study, He said, “What we find is that the lattice has a behind-the-scenes role after all, and we may have overlooked an important ingredient for high-temperature superconductivity for the past three decades,” a conclusion that ties into the results of earlier research by the SIMES group Science.

    Electron ‘Sound Waves’

    The other study, performed at the European Synchrotron Radiation Facility (ESRF) in France, used a technique called resonant inelastic X-ray scattering, or RIXS, to observe the collective behavior of electrons in layered cuprates known as LCCO and NCCO.


    ESRF. Grenoble, France

    RIXS excites electrons deep inside atoms with X-rays, and then measures the light they give off as they settle back down into their original spots.

    In the past, most studies have focused only on the behavior of electrons within a single layer of cuprate material, where electrons are known to be much more mobile than they are between layers, said SIMES staff scientist Wei-Sheng Lee. He led the study with Matthias Hepting, who is now at the Max Planck Institute for Solid State Research in Germany.

    But in this case, the team wanted to test an idea raised by theorists – that the energy generated by electrons in one layer repelling electrons in the next one plays a critical role in forming the superconducting state.

    When excited by light, this repulsion energy leads electrons to form a distinctive sound wave known as an acoustic plasmon, which theorists predict could account for as much as 20 percent of the increase in superconducting temperature seen in cuprates.

    With the latest in RIXS technology, the SIMES team was able to observe and measure those acoustic plasmons.

    “Here we see for the first time how acoustic plasmons propagate through the whole lattice,” Lee said. “While this doesn’t settle the question of where the energy needed to form the superconducting state comes from, it does tell us that the layered structure itself affects how the electrons behave in a very profound way.”

    This observation sets the stage for future studies that manipulate the sound waves with light, for instance, in a way that enhances superconductivity, Lee said. The results are also relevant for developing future plasmonic technology, he said, with a range of applications from sensors to photonic and electronic devices for communications.

    SSRL is a DOE Office of Science user facility, and SIMES is a joint institute of SLAC and Stanford.

    In addition to researchers from SLAC, Stanford and AIST, the study carried out at SSRL involved scientists from University of Tokyo; University of California, Berkeley; and Lorentz Institute for Theoretical Physics in the Netherlands.

    The study conducted at ESRF also involved researchers from SSRL; Polytechnic University of Milan in Italy; ESRF; Binghamton University in New York; and the University of Maryland.

    Both studies were funded by the DOE Office of Science.

    See the full article here .


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  • richardmitnick 12:05 pm on August 10, 2018 Permalink | Reply
    Tags: , , , Lining Up the Surprising Behaviors of a Superconductor with One of the World's Strongest Magnets, , , National High Magnetic Field Laboratory, Pulsed Field Facility at Los Alamos National Laboratory, Superconductivity   

    From Brookhaven National Lab: “Lining Up the Surprising Behaviors of a Superconductor with One of the World’s Strongest Magnets” 

    From Brookhaven National Lab

    August 8, 2018

    atantillo@bnl.gov
    Ariana Tantillo
    (631) 344-2347

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

    Scientists have discovered that the electrical resistance of a copper-oxide compound depends on the magnetic field in a very unusual way—a finding that could help direct the search for materials that can perfectly conduct electricity at room temperature.

    1
    (Clockwise from back left) Brookhaven Lab physicists Ivan Bozovic, Anthony Bollinger, and Jie Wu, and postdoctoral researcher Xi He used the molecular beam epitaxy system seen above to synthesize perfect single-crystal thin films made of lanthanum, strontium, oxygen, and copper (LSCO). They brought these superconducting films to the National High Magnetic Field Laboratory to see how the electrical resistance of LSCO in its “strange” metallic state changes under extremely strong magnetic fields.

    What happens when really powerful magnets—capable of producing magnetic fields nearly two million times stronger than Earth’s—are applied to materials that have a “super” ability to conduct electricity when chilled by liquid nitrogen? A team of scientists set out to answer this question in one such superconductor made of the elements lanthanum, strontium, copper, and oxygen (LSCO). They discovered that the electrical resistance of this copper-oxide compound, or cuprate, changes in an unusual way when very high magnetic fields suppress its superconductivity at low temperatures.

    “The most pressing problem in condensed matter physics is understanding the mechanism of superconductivity in cuprates because at ambient pressure they become superconducting at the highest temperature of any currently known material,” said physicist Ivan Bozovic, who leads the Oxide Molecular Beam Epitaxy Group at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory and who is a coauthor of the Aug. 3 Science paper reporting the discovery. “This new result—that the electrical resistivity of LSCO scales linearly with magnetic field strength at low temperatures—provides further evidence that high-temperature superconductors do not behave like ordinary metals or superconductors. Once we can come up with a theory to explain their unusual behavior, we will know whether and where to search for superconductors that can carry large amounts of electrical current at higher temperatures, and perhaps even at room temperature.”

    Cuprates such as LSCO are normally insulators. Only when they are cooled to some hundred degrees below zero and the concentrations of their chemical composition are modified (a process called doping) to a make them metallic can their mobile electrons pair up to form a “superfluid” that flows without resistance. Scientists hope that understanding how cuprates achieve this amazing feat will enable them to develop room-temperature superconductors, which would make energy generation and delivery significantly more efficient and less expensive.

    In 2016, Bozovic’s group reported that LSCO’s superconducting state is nothing like the one explained by the generally accepted theory of classical superconductivity; it depends on the number of electron pairs in a given volume rather than the strength of the electron pairing interaction. In a follow-up experiment published the following year, they obtained another puzzling result: when LSCO is in its non-superconducting (normal, or “metallic”) state, its electrons do not behave as a liquid, as would be expected from the standard understanding of metals.

    “The condensed matter physics community has been divided about this most basic question: do the behaviors of cuprates fall within existing theories for superconductors and metals, or are there profoundly different physical principles involved?” said Bozovic.

    Continuing this comprehensive multipart study that began in 2005, Bozovic’s group and collaborators have now found additional evidence to support the latter idea that the existing theories are incomplete. In other words, it is possible that these theories do not encompass every known material. Maybe there are two different types of metals and superconductors, for example.

    “This study points to another property of the strange metallic state in the cuprates that is not typical of metals: linear magnetoresistance at very high magnetic fields,” said Bozovic. “At low temperatures where the superconducting state is suppressed, the electrical resistivity of LSCO scales linearly (in a straight line) with the magnetic field; in metals, this relationship is quadratic (forms a parabola).”

    2
    This composite image offers a glimpse inside the custom-designed molecular beam epitaxy system that the Brookhaven physicists use to create single-crystal thin films for studying the properties of superconducting cuprates.

    In order to study magneto resistance, Bozovic and group members Anthony Bollinger, Xi He, and Jie Wu first had to create flawless single-crystal thin films of LSCO near its optimal doping level. They used a technique called molecular beam epitaxy, in which separate beams containing atoms of the different chemical elements are fired onto a heated single-crystal substrate. When the atoms land on the substrate surface, they condense and slowly grow into ultra-thin layers, building a single atomic layer at a time. The growth of the crystal occurs in highly controlled conditions of ultra-high vacuum to ensure that the samples do not get contaminated.

    “Brookhaven Lab’s key contribution to this study is this material synthesis platform,” said Bozovic. “It allows us to tailor the chemical composition of the films for different studies and provides the foundation for us to observe the true properties of superconducting materials, as opposed to properties induced by sample defects or impurities.”

    The scientists then patterned the thin films onto strips containing voltage leads so that the amount of electrical current flowing through LSCO under an applied magnetic field could be measured.

    They conducted initial magneto resistivity measurements with two 9 Tesla magnets at Brookhaven Lab—for reference, the strength of the magnets used in today’s magnetic resonance imaging (MRI) machines are typically up to 3 Tesla. Then, they brought their best samples (those with the best structural and transport qualities) to the Pulsed Field Facility. Located at DOE’s Los Alamos National Laboratory, this international user facility is part of the National High Magnetic Field Laboratory, which houses some of the strongest magnets in the world. Scientists at the Pulsed Field Facility placed the samples in an 80 Tesla pulsed magnet, powered by quick pulses, or shots, of electrical current. The magnet produces such large magnetic fields that it cannot be energized for more than a very short period of time (microseconds to a fraction of a second) without destroying itself.

    “This large magnet, which is the size of a room and draws the electricity of a small city, is the only such installation on this continent,” said Bozovic. “We only get access to it once a year if we are lucky, so we chose our best samples to study.”

    In October, the scientists will get access to a stronger (90 Tesla) magnet, which they will use to collect additional magneto resistance data to see if the linear relationship still holds.

    3
    An example of a typical device that the scientists use to measure electrical resistivity as a function of temperature and magnetic field. The scientists grew the film via atomic layer-by-layer molecular beam epitaxy, patterned it into a device, and wire bonded it to a chip carrier.

    “While I do not expect to see something different, this higher field strength will allow us to expand the range of doping levels at which we can suppress superconductivity,” said Bozovic. “Collecting more data over a broader range of chemical compositions will help theorists formulate the ultimate theory of high-temperature superconductivity in cuprates.”

    In the next year, Bozovic and the other physicists will collaborate with theorists to interpret the experimental data.

    “It appears that the strongly correlated motion of electrons is behind the linear relationship we observed,” said Bozovic. “There are various ideas of how to explain this behavior, but at this point, I would not single out any of them.”

    See the full article here .


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  • richardmitnick 1:40 pm on July 28, 2018 Permalink | Reply
    Tags: , Electronic symmetry breaking, , , Superconductivity   

    From Los Alamos National Laboratory: “Superconductivity research reveals potential new state of matter” 

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    From Los Alamos National Laboratory

    Aug. 16, 2017 [Just showed up in social media]

    Research is showing that among superconducting materials in high magnetic fields, the phenomenon of electronic symmetry breaking is common.

    1
    Filip Ronning. No image credit.

    Common phenomenon could be key to understanding mechanism of unconventional superconductivity.

    A potential new state of matter is being reported in the journal Nature, with research showing that among superconducting materials in high magnetic fields, the phenomenon of electronic symmetry breaking is common. The ability to find similarities and differences among classes of materials with phenomena such as this helps researchers establish the essential ingredients that cause novel functionalities such as superconductivity.

    The high-magnetic-field state of the heavy fermion superconductor CeRhIn5 revealed a so-called electronic nematic state, in which the material’s electrons aligned in a way to reduce the symmetry of the original crystal, something that now appears to be universal among unconventional superconductors. Unconventional superconductivity develops near a phase boundary separating magnetically ordered and magnetically disordered phases of a material.

    “The appearance of the electronic alignment, called nematic behavior, in a prototypical heavy-fermion superconductor highlights the interrelation of nematicity and unconventional superconductivity, suggesting nematicity to be common among correlated superconducting materials,” said Filip Ronning of Los Alamos National Laboratory, lead author on the paper. Heavy fermions are intermetallic compounds, containing rare earth or actinide elements.

    “These heavy fermion materials have a different hierarchy of energy scales than is found in transition metal and organic materials, but they often have similar complex and intertwined physics coupling spin, charge and lattice degrees of freedom,” he said.

    The work was reported in Nature by staff from the Los Alamos Condensed Matter and Magnet Science group and collaborators.

    Using transport measurements near the field-tuned quantum critical point of CeRhIn5 at 50 Tesla, the researchers observed a fluctuating nematic-like state. A nematic state is most well known in liquid crystals, wherein the molecules of the liquid are parallel but not arranged in a periodic array. Nematic-like states have been observed in transition metal systems near magnetic and superconducting phase transitions. The occurrence of this property points to nematicity’s correlation with unconventional superconductivity. The difference, however, of the new nematic state found in CeRhIn5 relative to other systems is that it can be easily rotated by the magnetic field direction.

    The use of the National High Magnetic Field Laboratory’s pulsed field magnet facility at Los Alamos was essential, Ronning noted, due to the large magnetic fields required to access this state. In addition, another essential contribution was the fabrication of micron-sized devices using focused ion-beam milling performed in Germany, which enabled the transport measurements in large magnetic fields.

    Superconductivity is extensively used in magnetic resonance imaging (MRI) and in particle accelerators, magnetic fusion devices, and RF and microwave filters, among other uses.

    Researchers: Filip Ronning, Mun K. Chan, Brad J. Ramshaw, Ross D. McDonald, Fedor F. Balakirev, Marcelo Jaime, and Eric D. Bauer (Los Alamos National Laboratory); Luis Balicas (Florida State University); Toni Helm, Kent Shirer, Maya Bachmann, and Philip J.W. Moll (Max-Planck-Institut for Chemical Physics of Solids – Dresden).

    See the full article here .

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    Los Alamos enhances national security by ensuring the safety and reliability of the U.S. nuclear stockpile, developing technologies to reduce threats from weapons of mass destruction, and solving problems related to energy, environment, infrastructure, health, and global security concerns.

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  • richardmitnick 1:44 pm on April 27, 2018 Permalink | Reply
    Tags: , , Majorana fermion science, , , , Superconductivity, , Topological quantum computation,   

    From Physics Illinois: “Topological insulator �flips� for superconductivity” 

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    Physics Illinois

    U Illinois Physics bloc

    4/27/2018
    Siv Schwink

    Topology meets superconductivity through innovative reverse-order sample preparation.

    1
    (L-R) Professor of Physics James Eckstein, his graduate student Yang Bai, and Professor of Physics Tai-Chang Chiang pose in front of the atomic layer by layer molecular beam epitaxy system used to grow the topological insulator thin-film samples for this study, in the Eckstein laboratory at the University of Illinois. Photo by L. Brian Stauffer, University of Illinois at Urbana-Champaign.

    A groundbreaking sample preparation technique has enabled researchers at the University of Illinois at Urbana-Champaign and the University of Tokyo to perform the most controlled and sensitive study to date of a topological insulator (TI) closely coupled to a superconductor (SC). The scientists observed the superconducting proximity effect—induced superconductivity in the TI due to its proximity to the SC—and measured its relationship to temperature and the thickness of the TI.

    TIs with induced superconductivity are of paramount interest to physicists because they have the potential to host exotic physical phenomena, including the elusive Majorana fermion—an elementary particle theorized to be its own antiparticle—and to exhibit supersymmetry—a phenomenon reaching beyond the standard model that would shed light on many outstanding problems in physics. Superconducting TIs also hold tremendous promise for technological applications, including topological quantum computation and spintronics.

    Naturally occurring topological superconductors are rare, and those that have been investigated have exhibited extremely small superconducting gaps and very low transition temperatures, limiting their usefulness for uncovering the interesting physical properties and behaviors that have been theorized.

    TIs have been used in engineering superconducting topological superconductors (TI/SC), by growing TIs on a superconducting substrate. Since their experimental discovery in 2007, TIs have intrigued condensed matter physicists, and a flurry of theoretical and experimental research taking place around the globe has explored the quantum-mechanical properties of this extraordinary class of materials. These 2D and 3D materials are insulating in their bulk, but conduct electricity on their edges or outer surfaces via special surface electronic states which are topologically protected, meaning they can’t be easily destroyed by impurities or imperfections in the material.

    But engineering such TI/SC systems via growing TI thin films on superconducting substrates has also proven challenging, given several obstacles, including lattice structure mismatch, chemical reactions and structural defects at the interface, and other as-yet poorly understood factors.

    2
    The �flip-chip� cleavage-based sample preparation: (A) A photo and a schematic diagram of assembled Bi2Se3(0001)/Nb sample structure before cleavage. (B) Same sample structure after cleavage exposing a �fresh� surface of the Bi2Se3 film with a pre-determined thickness. Image courtesy of James Eckstein and Tai-Chang-Chiang, U. of I. Department of Physics and Frederick Seitz Materials Research Laboratory.

    Now, a novel sample-growing technique developed at the U. of I. has overcome these obstacles. Developed by physics professor James Eckstein in collaboration with physics professor Tai-Chang Chiang, the new “flip-chip” TI/SC sample-growing technique allowed the scientists to produce layered thin-films of the well-studied TI bismuth selenide on top of the prototypical SC niobium—despite their incompatible crystalline lattice structures and the highly reactive nature of niobium.

    These two materials taken together are ideal for probing fundamental aspects of the TI/SC physics, according to Chiang: “This is arguably the simplest example of a TI/SC in terms of the electronic and chemical structures. And the SC we used has the highest transition temperature among all elements in the periodic table, which makes the physics more accessible. This is really ideal; it provides a simpler, more accessible basis for exploring the basics of topological superconductivity,” Chiang comments.

    The method allows for very precise control over sample thickness, and the scientists looked at a range of 3 to 10 TI layers, with 5 atomic layers per TI layer. The team’s measurements showed that the proximity effect induces superconductivity into both the bulk states and the topological surface states of the TI films. Chiang stresses, what they saw gives new insights into superconducting pairing of the spin-polarized topological surface states.

    “The results of this research are unambiguous. We see the signal clearly,” Chiang sums up. “We investigated the superconducting gap as a function of TI film thickness and also as a function of temperature. The results are pretty simple: the gap disappears as you go above niobium’s transition temperature. That’s good—it’s simple. It shows the physics works. More interesting is the dependence on the thickness of the film. Not surprisingly, we see the superconducting gap reduces for increasing TI film thickness, but the reduction is surprisingly slow. This observation raises an intriguing question regarding how the pairing at the film surface is induced by coupling at the interface.”

    Chiang credits Eckstein with developing the ingenious sample preparation method. It involves assembling the sample in reverse order, on top of a sacrificial substrate of aluminum oxide, commonly known as the mineral sapphire. The scientists are able to control the specific number of layers of TI crystals grown, each of quintuple atomic thickness. Then a polycrystalline superconducting layer of niobium is sputter-deposited on top of the TI film. The sample is then flipped over and the sacrificial layer that had served as the substrate is dislodged by striking a “cleavage pin.” The layers are cleaved precisely at the interface of the TI and aluminum oxide.

    3
    A close-up shot of the atomic layer by layer molecular beam epitaxy system used to grow the topological insulator thin-film samples for this study, located in the Eckstein laboratory at the University of Illinois. Photo by L. Brian Stauffer, University of Illinois at Urbana-Champaign.

    Eckstein explains, “The ‘flip-chip’ technique works because the layers aren’t strongly bonded—they are like a stack of paper, where there is strength in the stack, but you can pull apart the layers easily. Here, we have a triangular lattice of atoms, which comes in packages of five—these layers are strongly bonded. The next five layers sit on top, but are weakly bonded to the first five. It turns out, the weakest link is right at the substrate-TI interface. When cleaved, this method gives a pure surface, with no contamination from air exposure.”

    The cleavage was performed in an ultrahigh vacuum, within a highly sensitive instrument at the Institute for Solid State Physics at the University of Tokyo capable of angle-resolved photoemission spectroscopy (ARPES) at a range of temperatures.

    Chiang acknowledges, “The superconducting features occur at very small energy scales—it requires a very high energy resolution and very low temperatures. This portion of the experiment was completed by our colleagues in the University of Tokyo, where they have the instruments with the sensitivity to get the resolution we need for this kind of study. We couldn’t have done this without this international collaboration.”

    “This new sample preparation method opens up many new avenues in research, in terms of exotic physics, and, in the long term, in terms of possible useful applications—potentially even including building a better superconductor. It will allow preparation of samples using a wide range of other TIs and SCs. It could also be useful in miniaturization of electronic devices, and in spintronic computing, which would require less energy in terms of heat dissipation,” Chiang concludes.

    Eckstein adds, “There is a lot of excitement about this. If we can make a superconducting TI, theoretical predictions tell us that we could find a new elementary excitation that would make an ideal topological quantum bit, or qubit. We’re not there yet, and there are still many things to worry about. But it would be a qubit whose quantum mechanical wave function would be less susceptible to local perturbations that might cause dephasing, messing up calculations.”

    These findings were published online on 27 April 2018 in the journal Science Advances.

    See the full article here .

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  • richardmitnick 1:03 pm on April 9, 2018 Permalink | Reply
    Tags: , Physicists Just Discovered an Entirely New Type of Superconductivity, , , Superconductivity,   

    From University of Maryland via Science Alert: “Physicists Just Discovered an Entirely New Type of Superconductivity “ 

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    University of Maryland

    Science Alert

    9 APR 2018
    FIONA MACDONALD

    “No one thought this was possible in solid materials.”

    1
    (Emily Edwards, University of Maryland)

    One of the ultimate goals of modern physics is to unlock the power of superconductivity, where electricity flows with zero resistance at room temperature.

    Progress has been slow, but physicists have just made an unexpected breakthrough. They’ve discovered a superconductor that works in a way no one’s ever seen before – and it opens the door to a whole world of possibilities not considered until now.

    In other words, they’ve identified a brand new type of superconductivity.

    Why does that matter? Well, when electricity normally flows through a material – for example, the way it travels through wires in the wall when we switch on a light – it’s fast, but surprisingly ineffective.

    Electricity is carried by electrons, which bump into atoms in the material along the way, losing some of their energy each time they have one of these collisions. Known as resistance, it’s the reason why electricity grids lose up to 7 percent of their electricity.

    But when some materials are chilled to ridiculously cold temperatures, something else happens – the electrons pair up, and begin to flow orderly without resistance.

    This is known as superconductivity, and it has incredible potential to revolutionise our world, making our electronics unimaginably more efficient.

    The good news is we’ve found the phenomenon in many materials so far. In fact, superconductivity is already used to create the strong magnetic fields in MRI machines and maglev trains.

    The bad news is that it currently requires expensive and bulky equipment to keep the superconductors cold enough to achieve this phenomenon – so it remains impractical for broader use.

    Now researchers led by the University of Maryland have observed a new type of superconductivity when probing an exotic material at super cool temperatures.

    Not only does this type of superconductivity appear in an unexpected material, the phenomenon actually seems to rely on electron interactions that are profoundly different from the pairings we’ve seen to date. And that means we have no idea what kind of potential it might have.

    To understand the difference, you need to know that the way electrons interact is dictated by a quantum property called spin.

    In regular superconductors, electrons carry a spin referred to as 1/2.

    But in this particular material, known as YPtBi, the team found that something else was going on – the electrons appear to have a spin of 3/2.

    “No one had really thought that this was possible in solid materials,” explains physicist and senior author Johnpierre Paglione.

    “High-spin states in individual atoms are possible but once you put the atoms together in a solid, these states usually break apart and you end up with spin one-half. ”

    YPtBi was first discovered to be a superconductor a couple of years ago, and that in itself was a surprise, because the material doesn’t actually fit one of the main criteria – being a relatively good conductor, with a lot of mobile electrons, at normal temperatures.

    According to conventional theory, YPtBi would need about a thousand times more mobile electrons in order to become superconducting at temperatures below 0.8 Kelvin.

    But when researchers cooled the material down, they saw superconductivity happening anyway.

    To figure out what was going on, the latest study looked at the way the material interacted with magnetic fields to get a sense of exactly what was going on inside.

    Usually as a material undergoes the transition to a superconductor, it will try to expel any added magnetic field from its surface – but a magnetic field can still enter near, before quickly decaying away. How far they penetrate depends on the nature of the electron pairing happening within.

    The team used copper coils to detect changes in YPtBi’s magnetic properties as they changed its temperature.

    What they found was odd – as the material warmed up from absolute zero, the amount that a magnetic field could penetrate the material increased linearly instead of exponentially, which is what is normally seen with superconductors.

    After running a series of measurements and calculations, the researched concluded that the best explanation for what was going on was that the electrons must have been disguised as particles with higher spin – something that wasn’t even considered as a possibility for a superconductor before.

    While this new type of superconductivity still requires incredibly cold temperatures for now, the discovery gives the entire field a whole new direction.

    “We used to be confined to pairing with spin one-half particles,” says lead author Hyunsoo Kim.

    “But if we start considering higher spin, then the landscape of this superconducting research expands and just gets more interesting.”

    This is incredibly early days, and there’s still a lot we have to learn about exactly what’s going on here.

    But the fact that we have a brand new type of superconductivity to test and measure, adding a cool new breakthrough to the 100 years of this type of research, is pretty exciting.

    “When you have this high-spin pairing, what’s the glue that holds these pairs together?” says Paglione.

    “There are some ideas of what might be happening, but fundamental questions remain-which makes it even more fascinating.”

    The research has been published in Science Advances.

    See the full article here .

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  • richardmitnick 9:39 am on February 16, 2018 Permalink | Reply
    Tags: , , , , Superconductivity   

    From BNL: “Bringing a Hidden Superconducting State to Light” 

    Brookhaven Lab

    February 16, 2018
    Ariana Tantillo,
    atantillo@bnl.gov
    (631) 344-2347

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

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

    1
    Physicist Genda Gu holds a single-crystal rod of LBCO—a compound made of lanthanum, barium, copper, and oxygen—in Brookhaven’s state-of-the-art crystal growth lab. The infrared image furnace he used to synthesize these high-quality crystals is pictured in the background. No image credit.

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

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

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

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

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

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

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

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

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

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

    University of Hamburg theorists supported this experimental observation with analysis and numerical simulations of the reflectivity.

    This research provides a new technique to probe different types of electronic orders in high-temperature superconductors, and the new understanding may be helpful in explaining other strange behaviors in the cuprates.

    The work performed at Brookhaven was supported 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.
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