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  • richardmitnick 9:05 am on September 27, 2019 Permalink | Reply
    Tags: Cuprates, DMRG-density matrix renormalization group, , , The Hubbard model   

    From SLAC National Accelerator Lab: “Scientists finally find superconductivity in exactly the place they’ve been looking for decades” 

    From SLAC National Accelerator Lab

    September 26, 2019
    Glennda Chui

    1
    Computer simulations at SLAC and Stanford suggest a way to turn superconductivity on and off in copper-based materials called cuprates: Tweak the chemistry of the materials so electrons hop from atom to atom in a particular pattern – as if hopping to the atom diagonally across the street rather than to the one next door. This grid of simulated atoms illustrates the idea. Copper atoms are in orange, oxygen atoms are in red and electrons are in blue. (Greg Stewart/SLAC National Accelerator Laboratory)

    The Hubbard model, used to understand electron behavior in numerous quantum materials, now shows us its stripes, and superconductivity too, in simulations for cuprate superconductors.

    Researchers at Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory say they have found the first, long-sought proof that a decades-old scientific model of material behavior can be used to simulate and understand high-temperature superconductivity ­– an important step toward producing and controlling this puzzling phenomenon at will.

    The simulations they ran, published in Science today, suggest that researchers might be able to toggle superconductivity on and off in copper-based materials called cuprates by tweaking their chemistry so electrons hop from atom to atom in a particular pattern – as if hopping to the atom diagonally across the street rather than to the one next door.

    “The big thing you want to know is how to make superconductors operate at higher temperatures and how to make superconductivity more robust,” said study co-author Thomas Devereaux, director of the Stanford Institute for Materials and Energy Sciences (SIMES) at SLAC. “It’s about finding the knobs you can turn to tip the balance in your favor.”

    The biggest obstacle to doing that, he said, has been the lack of a model – a mathematical representation of how a system behaves – that describes this type of superconductivity, whose discovery in 1986 raised hopes that electricity might someday be transmitted with no loss for perfectly efficient power lines and maglev trains.

    While scientists thought the Hubbard model, used for decades to represent electron behavior in numerous materials, might apply to cuprate high-temperature superconductors, until now they had no proof, said Hong-Chen Jiang, a SIMES staff scientist and co-author of the report.

    “This has been a major unsolved problem in the field – does the Hubbard model describe high-temperature superconductivity in the cuprates, or is it missing some key ingredient?” he said. “Because there are a number of competing states in these materials, we have to rely on unbiased simulations to answer these questions, but the computational problems are very difficult, and so progress has been slow.”

    The many faces of quantum materials

    Why so difficult?

    While many materials behave in very predictable ways – copper is always a metal, and when you bust up a magnet the bits are still magnetic – high-temperature superconductors are quantum materials, where electrons cooperate to produce unexpected properties. In this case, they pair up to conduct electricity with no resistance or loss at much higher temperatures than established theories of superconductivity can explain.

    Unlike everyday materials, quantum materials can host a number of phases, or states of matter, at once, Devereaux said. For instance, a quantum material might be metallic under one set of conditions, but insulating under slightly different conditions. Scientists can tip the balance between phases by tinkering with the material’s chemistry or the way its electrons move around, for instance, and the goal is to do this in a deliberate way to create new materials with useful properties.

    One of the most powerful algorithms for modeling situations like this is known as density matrix renormalization group, or DMRG. But because these coexisting phases are so complex, using the DMRG to simulate them requires a lot of computation time and memory and typically takes quite a while, Jiang said.

    To reduce the computing time and reach a deeper level of analysis than would have been practical before, Jiang looked for ways to optimize the details of the simulation. “We have to carefully streamline each step,” he said, “making it as efficient as possible and even finding ways to do two separate things at once.” These efficiencies allowed the team to run DMRG simulations of the Hubbard model significantly faster than before, with about a year of computing time at Stanford’s Sherlock computing cluster and other facilities on the SLAC campus.

    Hopping electron neighbors

    This study focused on the delicate interplay between two phases that are known to exist in cuprates – high-temperature superconductivity and charge stripes, which are like a wave pattern of higher and lower electron density in the material. The relationship between these states is not clear, with some studies suggesting that charge stripes promote superconductivity and others suggesting they compete with it.

    For their analysis, Jiang and Devereaux created a virtual version of a cuprate on a square lattice, like a wire fence with square holes. The copper and oxygen atoms are confined to planes in the real material, but in the virtual version they become single, virtual atoms that sit at each of the intersections where wires meet. Each of these virtual atoms can accommodate at most two electrons that are free to jump or hop – either to their immediate neighbors on the square lattice or diagonally across each square.

    When the researchers used DMRG to simulate the Hubbard model as applied to this system, they discovered that changes in the electrons’ hopping patterns had a noticeable effect on the relationship between charge stripes and superconductivity.

    When electrons hopped only to their immediate neighbors on the square lattice, the pattern of charge stripes got stronger and the superconducting state never appeared. When electrons were allowed to hop diagonally, charge stripes eventually weakened, but did not go away, and the superconducting state finally emerged.

    “Until now we could not push far enough in our modeling to see if charge stripes and superconductivity can coexist when this material is in its lowest energy state. Now we know they do, at least for systems of this size,” Devereaux said.

    It’s still an open question whether the Hubbard model describes all of the incredibly complex behavior of real cuprates, he added. Even a small increase in the complexity of the system would require a huge leap in the power of the algorithm used to model it. “The time it takes to do your simulation goes up exponentially fast with the width of the system you want to study,” Devereaux said. “It’s exponentially more complicated and demanding.”

    But with these results, he said, “We now have a fully interacting model that describes high temperature superconductivity, at least for systems at the sizes we can study, and that’s a big step forward.”

    Funding for the study came from the DOE Office of Science.

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

    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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    In the world of science, Lawrence Berkeley National Laboratory (Berkeley Lab) is synonymous with “excellence.” Thirteen Nobel prizes are associated with Berkeley Lab. Seventy Lab scientists are members of the National Academy of Sciences (NAS), one of the highest honors for a scientist in the United States. Thirteen of our scientists have won the National Medal of Science, our nation’s highest award for lifetime achievement in fields of scientific research. Eighteen of our engineers have been elected to the National Academy of Engineering, and three of our scientists have been elected into the Institute of Medicine. In addition, Berkeley Lab has trained thousands of university science and engineering students who are advancing technological innovations across the nation and around the world.

    Berkeley Lab is a member of the national laboratory system supported by the U.S. Department of Energy through its Office of Science. It is managed by the University of California (UC) and is charged with conducting unclassified research across a wide range of scientific disciplines. Located on a 202-acre site in the hills above the UC Berkeley campus that offers spectacular views of the San Francisco Bay, Berkeley Lab employs approximately 3,232 scientists, engineers and support staff. The Lab’s total costs for FY 2014 were $785 million. A recent study estimates the Laboratory’s overall economic impact through direct, indirect and induced spending on the nine counties that make up the San Francisco Bay Area to be nearly $700 million annually. The Lab was also responsible for creating 5,600 jobs locally and 12,000 nationally. The overall economic impact on the national economy is estimated at $1.6 billion a year. Technologies developed at Berkeley Lab have generated billions of dollars in revenues, and thousands of jobs. Savings as a result of Berkeley Lab developments in lighting and windows, and other energy-efficient technologies, have also been in the billions of dollars.

    Berkeley Lab was founded in 1931 by Ernest Orlando Lawrence, a UC Berkeley physicist who won the 1939 Nobel Prize in physics for his invention of the cyclotron, a circular particle accelerator that opened the door to high-energy physics. It was Lawrence’s belief that scientific research is best done through teams of individuals with different fields of expertise, working together. His teamwork concept is a Berkeley Lab legacy that continues today.

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

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

    Brookhaven Lab

    October 14, 2016
    Ariana Tantillo
    atantillo@bnl.gov
    (631) 344-2347
    Peter Genzer
    genzer@bnl.gov
    (631) 344-3174

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

    1
    Members of the Brookhaven Lab research team—(clockwise from left) Stuart Wilkins, Xiaoqian Chen, Mark Dean, Vivek Thampy, and Andi Barbour—at the National Synchrotron Light Source II’s Coherent Soft X-ray Scattering beamline, where they studied the electronic order of “charge stripes” in a copper-oxide superconductor. No image credit.

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

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

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

    Charge stripes put to the test of time

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

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

    BNL NSLS-II Interior
    BNL NSLS-II

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

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

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

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

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

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

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

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

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

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

    See the full article here .

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