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  • richardmitnick 12:40 pm on February 7, 2020 Permalink | Reply
    Tags: "Making High-Temperature Superconductivity Disappear to Understand Its Origin", (SI-STM)-spectroscopic imaging–scanning tunneling microscopy, , , , , , , , OASIS- a new on-site experimental machine for growing and characterizing oxide thin films., , Superconductivity   

    From Brookhaven National Lab: “Making High-Temperature Superconductivity Disappear to Understand Its Origin” 

    From Brookhaven National Lab

    February 3, 2020
    Ariana Manglaviti
    amanglaviti@bnl.gov
    (631) 344-2347

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

    Scientists have collected evidence suggesting that a purely electronic mechanism causes copper-oxygen compounds to conduct electricity without resistance at temperatures well above absolute zero.

    1
    Brookhaven Lab physicists (from left to right) Genda Gu, Tonica Valla, and Ilya Drozdov at OASIS, a new on-site experimental machine for growing and characterizing oxide thin films, such as those of a class of high-temperature superconductors (HTS) known as the cuprates. Compared to conventional superconductors, HTS become able to conduct electricity without resistance at much warmer temperatures. The team used the unique capabilities at OASIS to make superconductivity in a cuprate sample disappear and then reappear in order to understand the origin of the phenomenon.

    When there are several processes going on at once, establishing cause-and-effect relationships is difficult. This scenario holds true for a class of high-temperature superconductors known as the cuprates. Discovered nearly 35 years ago, these copper-oxygen compounds can conduct electricity without resistance under certain conditions. They must be chemically modified (“doped”) with additional atoms that introduce electrons or holes (electron vacancies) into the copper-oxide layers and cooled to temperatures below 100 Kelvin (−280 degrees Fahrenheit)—significantly warmer temperatures than those needed for conventional superconductors. But exactly how electrons overcome their mutual repulsion and pair up to flow freely in these materials remains one of the biggest questions in condensed matter physics. High-temperature superconductivity (HTS) is among many phenomena occurring due to strong interactions between electrons, making it difficult to determine where it comes from.

    That’s why physicists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory studying a well-known cuprate containing layers made of bismuth oxide, strontium oxide, calcium, and copper oxide (BSCCO) decided to focus on the less complicated “overdoped” side, doping the material so much so that superconductivity eventually disappears. As they reported in a paper published on Jan. 29 in Nature Communications, this approach enabled them to identify that purely electronic interactions likely lead to HTS.

    “Superconductivity in cuprates usually coexists with periodic arrangements of electric charge or spin and many other phenomena that can either compete with or aid superconductivity, complicating the picture,” explained first author Tonica Valla, a physicist in the Electron Spectroscopy Group of Brookhaven Lab’s Condensed Matter Physics and Materials Science Division. “But these phenomena weaken or completely vanish with overdoping, leaving nothing but superconductivity. Thus, this is the perfect region to study the origin of superconductivity. Our experiments have uncovered an interaction between electrons in BSCCO that correlates one to one with superconductivity. Superconductivity emerges exactly when this interaction first appears and becomes stronger as the interaction strengthens.”

    Only very recently has it become possible to overdope cuprate samples beyond the point where superconductivity vanishes. Previously, a bulk crystal of the material would be annealed (heated) in high-pressure oxygen gas to increase the concentration of oxygen (the dopant material). The new method—which Valla and other Brookhaven scientists first demonstrated about a year ago at OASIS, a new on-site instrument for sample preparation and characterization—uses ozone instead of oxygen to anneal cleaved samples. Cleaving refers to breaking the crystal in vacuum to create perfectly flat and clean surfaces.

    “The oxidation power of ozone, or its ability to accept electrons, is much stronger than that of molecular oxygen,” explained coauthor Ilya Drozdov, a physicist in the division’s Oxide Molecular Beam Epitaxy (OMBE) Group. “This means we can bring more oxygen into the crystal to create more holes in the copper-oxide planes, where superconductivity occurs. At OASIS, we can overdope surface layers of the material all the way to the nonsuperconducting region and study the resulting electronic excitations.”

    OASIS combines an OMBE system for growing oxide thin films with angle-resolved photoemission spectroscopy (ARPES) and spectroscopic imaging–scanning tunneling microscopy (SI-STM) instruments for studying the electronic structure of these films. Here, materials can be grown and studied using the same connected ultrahigh vacuum system to avoid oxidation and contamination by carbon dioxide, water, and other molecules in the atmosphere. Because ARPES and SI-STM are extremely surface-sensitive techniques, pristine surfaces are critical to obtaining accurate measurements.

    For this study, coauthor Genda Gu, a physicist in the division’s Neutron Scattering Group, grew bulk BSCCO crystals. Drozdov annealed the cleaved crystals in ozone in the OMBE chamber at OASIS to increase the doping until superconductivity was completely lost. The same sample was then annealed in vacuum in order to gradually reduce the doping and increase the transition temperature at which superconductivity emerges. Valla analyzed the electronic structure of BSCCO across this doping-temperature phase diagram through ARPES.

    “ARPES gives you the most direct picture of the electronic structure of any material,” said Valla. “Light excites electrons from a sample, and by measuring their energy and the angle at which they escape, you can recreate the energy and momentum of the electrons while they were still in the crystal.”

    In measuring this energy-versus-momentum relationship, Valla detected a kink (anomaly) in the electronic structure that follows the superconducting transition temperature. The kink becomes more pronounced and shifts to higher energies as this temperature increases and superconductivity gets stronger, but disappears outside of the superconducting state. On the basis of this information, he knew that the interaction creating the electron pairs required for superconductivity could not be electron-phonon coupling, as theorized for conventional superconductors. Under this theory, phonons, or vibrations of atoms in the crystal lattice, serve as an attractive force for otherwise repulsive electrons through the exchange of momentum and energy.

    “Our result allowed us to rule out electron-phonon coupling because atoms in the lattice can vibrate and electrons can interact with those vibrations, regardless of whether the material is superconducting or not,” said Valla. “If phonons were involved, we would expect to see the kink in both the superconducting and normal state, and the kink would not be changing with doping.”

    The team believes that something similar to electron-phonon coupling is going on in this case, but instead of phonons, another excitation gets exchanged between electrons. It appears that electrons are interacting through spin fluctuations, which are related to electrons themselves. Spin fluctuations are changes in electron spin, or the way that electrons point either up or down as tiny magnets.

    Moreover, the scientists found that the energy of the kink is less than that of a characteristic energy at which a sharp peak (resonance) in the spin fluctuation spectrum appears. Their finding suggests that the onset of spin fluctuations (instead of the resonance peak) is responsible for the observed kink and may be the “glue” that binds electrons into the pairs required for HTS.

    Next, the team plans to collect additional evidence showing that spin fluctuations are related to superconductivity by obtaining SI-STM measurements. They will also perform similar experiments on another well-known cuprate, lanthanum strontium copper oxide (LSCO).

    “For the first time, we are seeing something that strongly correlates with superconductivity,” said Valla. “After all these years, we now have a better grasp of what may be causing superconductivity in not only BSCCO but also other cuprates.”

    See the full article here .


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

     
  • richardmitnick 12:26 pm on November 1, 2019 Permalink | Reply
    Tags: "A Superconductor That "Remembers" its Electronic Charge Arrangement", A cuprate known as LBCO for the compounds it contains: lanthanum; barium; copper; and oxygen., , , CDW-charge density wave, , HTSCs-high-temperature superconductors, , Superconductivity   

    From Brookhaven National Lab: “A Superconductor That “Remembers” its Electronic Charge Arrangement” 

    From Brookhaven National Lab

    October 30, 2019
    Laura Mgrdichian
    mgrdichian@gmail.com

    New information on charge order in a high-temperature superconductor may lead to a fuller understanding of these materials’ electronic behavior.

    1
    The Coherent Soft X-ray Scattering (CSX) beamline at NSLS-II offers the researchers the right tools to probe charge ordering phenomena in quantum materials such as high-temperature superconductors with unprecedented precision. The researcher here aside the CSX scattering station are Stuart Wilkins (left) and Xiaoqian Chen, Mark Dean, Andi Barbour and Vivek Thampy (right from back to front). Other co-authors not shown include X-Ray Scattering Group Leader Ian Robinson and Neutron Scattering Group Leader John Tranquada.

    In the field of superconductivity – the ability of a material to conduct electricity with virtually zero resistance – the so-called high-temperature superconductors (HTSCs) are possible candidates for a new generation of advanced technologies. One subset of these, the “cuprates,” which are crystalline materials based on planes of copper oxide, are particularly promising. But scientists still need to learn much more about these materials before mainstream, room-temperature applications are possible. Currently, even the “high-temperature” superconductors must be chilled to very, very cold temperatures by everyday standards.

    Working at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory, researchers from Brookhaven and University College London recently discovered something new and very surprising about one type of periodic electric charge arrangement, which coexists with superconductivity in cuprates, known as a charge density wave (CDW). They found that the specific CDW order within their sample was “remembered” when the sample was repeatedly heated past the temperature where the CDW disappears. This discovery opens a new avenue of research into how these intriguing materials work, bringing scientists one step closer to a complete picture of electronic behavior in cuprates.

    “It would be like melting a pile of ice cubes and then refreezing them – and discovering that they refroze into an identical pile of cubes, even down at the microscopic level,” explained Brookhaven Lab physicist Claudio Mazzoli, one of the researchers involved in the study. “Nobody would expect to see that.”

    Mazzoli and his co-researchers describe their work in the March 29, 2019 online edition of Nature Communications.

    The electronic behavior of the cuprates, as with all HTSCs, is quite complex. As the name implies, the electrons that make up a CDW form a periodic standing-wave pattern. CDWs have been observed in nearly all the cuprates, but their role in superconductivity is still not fully understood. Do they compete with superconductivity? Do they participate in it? Do they hinder superconductivity in certain ways and possibly add to it in others? Scientists are still working this out.

    “In the HTSCs, any arrangement of electrons is of interest to researchers,” said Brookhaven physicist Mark Dean, another of the paper’s authors. “The goal is to investigate these arrangements and tune them – or perhaps remove them – so that the superconducting transition temperature of the material can approach, or maybe surpass, room temperature. To do this, we must learn as much as we can about the electrons’ behavior and their structures in HTSCs.”

    2
    Claudio Mazzoli (left) and Mark Dean (right) used the TARDIS experimental chamber at NSLS-II’s Coherent Soft X-ray Scattering (CSX) beamline to investigate the behavior of charge density waves in a specific high-temperature superconductor.

    One thing that researchers do know is that cuprates containing the same copper oxide planes – but arranged in a slightly different way – may have CDWs with dramatically different properties. It seems, then, that the part of the crystal lattice that hosts the CDW has an effect on the CDW.

    Here, the group set out to learn more about the relationship between the material’s lattice structure and CDW behavior. Their model system was a cuprate known as LBCO for the compounds it contains: lanthanum, barium, copper, and oxygen. LBCO has a transition temperature – the temperature below which it displays the CDW, and above which it does not – of 54 degrees Kelvin (K) (although equivalent to about -360 degrees Fahrenheit, this temperature is still relatively high in the superconductor world).

    The group wanted to find out how imperfections in the LBCO crystal lattice can stabilize the CDW. They were interested in a well-known lattice distortion that occurs in the material: a tilt in the octahedral shape formed by bound copper and oxygen atoms. This tilt tends to anchor the CDW to the lattice such that it orients in a certain direction; it appears that the CDW may be sensitive to the spatial inhomogeneities, or domains, of the lattice. This relationship between the CDW and the domains, as suggested by the temperature behavior uncovered in this study, may be unique to LBCO. It will be very important to understand whether this is a general feature of the cuprates.

    The group cycled their LBCO sample through a range of temperatures, repeatedly heating and cooling it, while probing it with x-rays at Brookhaven’s National Synchrotron Light Source II (NSLS-II), a DOE Office of Science User Facility. At the Coherent Soft X-Ray Scattering (CSX) beamline, they used a technique known as coherent resonant x-ray diffraction, in which x-rays scatter from different domains in the CDW spatial arrangement, interfere with each other, and form a “speckle” pattern that is captured by a special camera. Analyzing this pattern yields information on the CDW’s features.

    3
    The schematic shows how a speckle pattern is measured: first the coherent x-ray beam delivered by the beamline is focused onto the sample, then the x-rays are scattered by the sample at a specific angle (sensitive to the charge density wave presence)and captured by the CCD detector. The pinhole provides a mask, allowing the researchers to illuminate only a small, specific area of the sample.

    This task – directly observing a CDW while tracking its changes, over a range of temperatures – is collectively very challenging, in large part due to the very short distances that characterize the features of a CDW. NSLS-II is uniquely suited to this type of investigation due to the coherent nature of the light it produces, meaning the light waves travel in unison rather than out-of-sync and jumbled. Older light sources do not have such highly coherent beams.

    The speckle analysis revealed that the specific CDW order present below 54 K returned even when the sample was repeatedly cycled through much higher temperatures, up to about 240 K (about -28 °F). The researchers think that the structural changes that take place in the crystal below 240 K create a “pinning landscape” that anchors the CDW to the lattice.

    “Our work opens a new route for studying the complex interplay between charge and lattice degrees of freedom in superconducting cuprates,” said the paper’s lead author, Xiaoqian Chen, a researcher in Brookhaven’s Condensed Matter Physics and Materials Science Department at the time this study was performed (she is now working at Lawrence Berkeley National Laboratory). “It is also a great demonstration of how NSLS-II can be used to study quantum phases of materials and their spectacular, unexpected properties.”

    “This result emphasizes the vital importance of the role of nanoscale domains in high-temperature superconductivity. Without the domain pinning effects that have been observed, the CDW might carry current and further disrupt the superconductivity,” added co-author Ian Robinson, a physicist at Brookhaven as well as at University College London. “Imaging these subtle ‘phase’ domain structures is still in its infancy and this work highlights the need to develop better imaging techniques so that structural details can be seen directly.”

    The preparation of the sample used in this study was done at Brookhaven’s Center for Functional Nanomaterials. Additionally, a small portion of this work was performed at Argonne National Laboratory’s Advanced Photon Source. Both are DOE Office of Science User Facilities.

    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.
    i1

     
  • richardmitnick 12:32 pm on October 11, 2019 Permalink | Reply
    Tags: , Heavy fermion materials, Superconductivity, The challenge is to find new materials in which the superconducting state can be easily manipulated in a device.   

    From École Polytechnique Fédérale de Lausanne: “Controlling superconducting regions within an exotic metal” 

    EPFL bloc

    From École Polytechnique Fédérale de Lausanne

    11.10.19
    Laure-Anne Pessina

    1
    Researchers at EPFL have created a metallic microdevice in which they can define and tune patterns of superconductivity. Their discovery, which holds great promise for quantum technologies of the future, has just been published in Science.

    Superconductivity has fascinated scientists for many years since it offers the potential to revolutionize current technologies. Materials only become superconductors – meaning that electrons can travel in them with no resistance – at very low temperatures. These days, this unique zero resistance superconductivity is commonly found in a number of technologies, such as magnetic resonance imaging (MRI). Future technologies, however, will harness the total synchrony of electronic behavior in superconductors – a property called the phase. There is currently a race to build the world’s first quantum computer, which will use these phases to perform calculations. Conventional superconductors are very robust and hard to influence, and the challenge is to find new materials in which the superconducting state can be easily manipulated in a device.

    EPFL’s Laboratory of Quantum Materials (QMAT), headed by Philip Moll, professor at the School of Engineering, has been working on a specific group of unconventional superconductors known as heavy fermion materials. The QMAT scientists, as part of a broad international collaboration between EPFL, the Max Planck Institute for Chemical Physics of Solids, the Los Alamos National Laboratory and Cornell University, made a surprising discovery about one of these materials, CeIrIn5.

    CeIrIn5 is a metal that superconducts at a very low temperature, only 0.4°C above absolute zero (around -273°C). The QMAT scientists, together with Katja C. Nowack from Cornell University, have now shown that this material could be produced with superconducting regions coexisting alongside regions in a normal metallic state. Better still, they produced a model that allows researchers to design complex conducting patterns and, by varying the temperature, to distribute them within the material in a highly controlled way. Their research has just been published in Science.

    To achieve this feat, the scientists sliced very thin layers of CeIrIn5 – only around a thousandth of a millimeter thick – that they joined to a sapphire substrate. When cooled, the material contracts significantly whereas the sapphire contracts very little. The resulting interaction puts stress on the material, as if it were being pulled in all directions, thus slightly distorting the atomic bonds in the slice. As the superconductivity in CeIrIn5 is unusually sensitive to the material’s exact atomic configuration, engineering a distortion pattern is all it takes to achieve a complex pattern of superconductivity. This new approach allows researchers to “draw” superconducting circuitry on a single crystal bar, a step that paves the way for new quantum technologies.

    3
    2
    The image illustrates the temperature evolution of the spatially modulated superconducting state.

    This discovery represents a major step forward in controlling superconductivity in heavy fermion materials. But that’s not the end of the story. Following on from this project, a post-doc researcher has just begun exploring possible technological applications.

    “We could, for example, change the regions of superconductivity by modifying the material’s distortion using a microactuator,” says Moll. “The ability to isolate and connect superconducting regions on a chip could also create a kind of switch for future quantum technologies, a little like the transistors used in today’s computing.”

    See the full article here .

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    EPFL is Europe’s most cosmopolitan technical university. It receives students, professors and staff from over 120 nationalities. With both a Swiss and international calling, it is therefore guided by a constant wish to open up; its missions of teaching, research and partnership impact various circles: universities and engineering schools, developing and emerging countries, secondary schools and gymnasiums, industry and economy, political circles and the general public.

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

    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/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 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/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 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


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  • 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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    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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  • 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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    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 10:37 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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    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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