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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, , , , , Cuprates, , , OASIS- a new on-site experimental machine for growing and characterizing oxide thin films., ,   

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

    From Brookhaven National Lab

    February 3, 2020
    Ariana Manglaviti
    (631) 344-2347

    Peter Genzer
    (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.

    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, Cuprates, HTSCs-high-temperature superconductors, ,   

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

    From Brookhaven National Lab

    October 30, 2019
    Laura Mgrdichian

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

    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.”

    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.

    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.

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

    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

    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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  • 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” 

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    From Lawrence Berkeley National Lab

    January 3, 2019

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

    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.

    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.


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

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

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

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

    The work was supported by the DOE Office of Science.

    See the full article here .


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  • richardmitnick 10:42 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
    (631) 344-2347
    Peter Genzer
    (631) 344-3174

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

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

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

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

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

    Charge stripes put to the test of time

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

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

    BNL NSLS-II Interior

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

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

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

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

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

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

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

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

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

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

    See the full article here .

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

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