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

    From Rice: “Simulating superconducting materials with ultracold atoms” 

    Rice U bloc

    Rice University

    February 23, 2015
    Jade Boyd

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

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

    Randy Hulet

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

    See the full article here.

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

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

  • richardmitnick 1:44 pm on February 14, 2015 Permalink | Reply
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    From Rutgers: “Rutgers-Led Research Team Makes Major Stride in Explaining 30-Year-Old ‘Hidden Order’ Physics Mystery” 

    Rutgers University
    Rutgers University

    February 12, 2015

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

    See the full article here.

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

    From MIT: “New law for superconductors” 

    MIT News

    December 16, 2014
    Larry Hardesty | MIT News Office

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

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

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

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

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

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

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

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

    Vexing variation

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

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

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

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

    Broadening the scope

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

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

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

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

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

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  • richardmitnick 6:57 pm on December 8, 2014 Permalink | Reply
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    From BNL: “Unusual Electronic State Found in New Class of Unconventional Superconductors” 

    Brookhaven Lab

    December 8, 2014
    Karen McNulty Walsh, (631) 344-8350 or Peter Genzer, (631) 344-3174

    Finding gives scientists a new group of materials to explore to unlock secrets of some materials’ ability to carry current with no energy loss

    A team of scientists from the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory, Columbia Engineering, Columbia Physics and Kyoto University has discovered an unusual form of electronic order in a new family of unconventional superconductors. The finding, described in the journal Nature Communications, establishes an unexpected connection between this new group of titanium-oxypnictide superconductors and the more familiar cuprates and iron-pnictides, providing scientists with a whole new family of materials from which they can gain deeper insights into the mysteries of high-temperature superconductivity.

    Team members conducting research at Brookhaven Lab, led by Simon Billinge of Brookhaven and Columbia Engineering (seated), included (l to r) Columbia U graduate student Ben Frandsen and Weiguo Yin, Yimei Zhu, and Emil Bozin of Brookhaven’s Condensed Matter Physics and Materials Science Department. They used the aberation-corrected electron microscope in Zhu’s lab to conduct electron diffraction experiments that were a key component of this study. Collaborators not shown: Hefei Hu, formerly of Brookhaven Lab and now at Intel, Yasumasa Nozaki and Hiroshi Kageyama of Kyoto University, and Yasutomo Uemura of Columbia.

    “Finding this new material is a bit like an archeologist finding a new Egyptian pharaoh’s tomb,” said Simon Billinge, a physicist at Brookhaven Lab and Columbia University’s School of Engineering and Applied Science, who led the research team. “As we try and solve the mysteries behind unconventional superconductivity, we need to discover different but related systems to give us a more complete picture of what is going on—just as a new tomb will turn up treasures not found before, giving a more complete picture of ancient Egyptian society.”

    Harnessing the power of superconductivity, or the ability of certain materials to conduct electricity with zero energy loss, is one of the most exciting possibilities for creating a more energy-efficient future. But because most superconductors only work at very low temperatures—just a few degrees above absolute zero, or -273 degrees Celsius—they are not yet useful for everyday life. The discovery in the 1980s of “high-temperature” superconductors that work at warmer temperatures (though still not room temperature) was a giant step forward, offering scientists the hope that a complete understanding of what enables these materials to carry loss-free current would help them design new materials for everyday applications. Each new discovery of a common theme among these materials is helping scientists unlock pieces of the puzzle.

    One of the greatest mysteries is seeking to understand how the electrons in high-temperature superconductors interact, sometimes trying to avoid each other and at other times pairing up—the crucial characteristic enabling them to carry current with no resistance. Scientists studying these materials at Brookhaven and elsewhere have discovered special types of electronic states, such as “charge density waves,” where charges huddle to form stripes, and checkerboard patterns of charge. Both of these break the “translational symmetry” of the material—the repetition of sameness as you move across the surface (e.g., moving across a checkerboard you move from white squares to black squares).

    Another pattern scientists have observed in the two most famous classes of high-temperature superconductors is broken rotational symmetry without a change in translational symmetry. In this case, called nematic order, every space on the checkerboard is white, but the shapes of the spaces are distorted from a square to a rectangle; as you turn round and round on one space, your neighboring space is nearer or farther depending on the direction you are facing. Having observed this unexpected state in the cuprates and iron-pnictides, scientists were eager to see whether this unusual electronic order would also be observed in a new class of titanium-oxypnictide high-temperature superconductors discovered in 2013.

    “These titanium-oxypnictide compounds are structurally similar to the other exotic superconductor systems, and they had all the telltale signs of a broken symmetry, such as anomalies in resistivity and thermodynamic measurements. But there was no sign of any kind of charge density wave in any previous measurement. It was a mystery,” said Emil Bozin, whose group at Brookhaven specializes in searching for hidden local broken symmetries. “It was a natural for us to jump on this problem.”

    Top: Ripples extending down the chain of atoms breaks translational symmetry (like a checkerboard with black and white squares), which would cause extra spots in the diffraction pattern (shown as red dots in the underlying diffraction pattern). Bottom: Stretching along one direction breaks rotational symmetry but not translational symmetry (like a checkerboard with identical squares but stretched in one of the directions), causing no additional diffraction spots. The experiments proved these new superconductors have the second type of electron density distribution, called a nematic. Image credit: Ben Frandsen

    The team searched for the broken rotational symmetry effect, a research question that had been raised by Tomo Uemura of Columbia, using samples provided by his collaborators in the group of Hiroshi Kageyama at Kyoto University. They conducted two kinds of diffraction studies: neutron scattering experiments at the Los Alamos Neutron Science Center (LANSCE) at DOE’s Los Alamos National Laboratory, and electron diffraction experiments using a transmission electron microscope at Brookhaven Lab.

    “We used these techniques to observe the pattern formed by beams of particles shot through powder samples of the superconductors under a range of temperatures and other conditions to see if there’s a structural change that corresponds to the formation of this special type of nematic state,” said Ben Frandsen, a graduate student in physics at Columbia and first author on the paper.

    The experiments revealed a telltale symmetry breaking distortion at low temperature. A collaborative effort among experimentalists and theorists established the particular nematic nature of the order.

    “Critical in this study was the fact that we could rapidly bring to bear multiple complementary experimental methods, together with crucial theoretical insights—something made easy by having most of the expertise in residence at Brookhaven Lab and wonderfully strong collaborations with colleagues at Columbia and beyond,” Billinge said.

    The discovery of nematicity in titanium-oxypnictides, together with the fact that their structural and chemical properties bridge those of the cuprate and iron-pnictide high-temperature superconductors, render these materials an important new system to help understand the role of electronic symmetry breaking in superconductivity.

    As Billinge noted, “This new pharaoh’s tomb indeed contained a treasure: nematicity.”

    This work was supported by the DOE Office of Science, the U.S. National Science Foundation (NSF, OISE-0968226), the Japan Society of the Promotion of Science, the Japan Atomic Energy Agency, and the Friends of Todai Inc.

    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 7:51 pm on December 4, 2014 Permalink | Reply
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    From SLAC: “Rattled Atoms Mimic High-temperature Superconductivity” 

    SLAC Lab

    December 4, 2014

    X-ray Laser Experiment Provides First Look at Changes in Atomic Structure that Support Superconductivity

    An experiment at the Department of Energy’s SLAC National Accelerator Laboratory provided the first fleeting glimpse of the atomic structure of a material as it entered a state resembling room-temperature superconductivity – a long-sought phenomenon in which materials might conduct electricity with 100 percent efficiency under everyday conditions.

    In a high-temperature superconducting material known as YBCO, light from a laser causes oxygen atoms (red) to vibrate between layers of copper oxide that are just two molecules thick. (The copper atoms are shown in blue.) This jars atoms in those layers out of their normal positions in a way that likely favors superconductivity. In this short-lived state, the distance between copper oxide planes within a layer increases, while the distance between the layers decreases. (Jörg Harms/Max Planck Institute for the Structure and Dynamics of Matter)

    Researchers used a specific wavelength of laser light to rattle the atomic structure of a material called yttrium barium copper oxide, or YBCO. Then they probed the resulting changes in the structure with an X-ray laser beam from the Linac Coherent Light Source (LCLS), a DOE Office of Science User Facility.

    SLAC LCLS Inside
    LCLS at SLAC

    They discovered that the initial exposure to laser light triggered specific shifts in copper and oxygen atoms that squeezed and stretched the distances between them, creating a temporary alignment that exhibited signs of superconductivity for a few trillionths of a second at well above room temperature – up to 60 degrees Celsius (140 degrees Fahrenheit). The scientists coupled data from the experiment with theory to show how these changes in atomic positions allow a transfer of electrons that drives the superconductivity.

    New Views of Atoms in Motion

    “This is a highly interesting state, even though it only exists for a short period of time,” said Roman Mankowsky of the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg, Germany, who was lead author of a report on the experiment in the Dec. 4 print issue of Nature. “When the laser excites the material, it shifts the atoms and changes the structure. We hope these results will ultimately help in the design of new materials to enhance superconductivity.”

    Sustaining such a state at room temperature would revolutionize many fields, making the electrical grid more efficient and enabling more powerful and compact computers. Traditional superconductors operate only at temperatures close to absolute zero. YBCO is one of a handful of materials discovered since 1986 that superconduct at somewhat higher temperatures; but they still have to be chilled to at least minus 135 degrees Celsius in order to sustain superconductivity, and scientists still don’t know what allows these so-called high-temperature superconductors to carry electricity with zero resistance.

    A Powerful Tool for Exploring Superconductivity

    Josh Turner, a SLAC staff scientist who has led other studies of YBCO at the LCLS, said powerful tools such as X-ray lasers have excited new interest in superconductor research by allowing researchers to isolate a specific property that they want to learn more about. This is important because high-temperature superconductors can exhibit a tangle of magnetic, electronic and structural properties that may compete or cooperate as the material moves toward a superconducting state. For example, another recently published LCLS study found that exciting YBCO with the same optical laser light disrupts an electronic order that competes with superconductivity.

    “What LCLS is now showing us is how these different properties change over short times,” Turner said. “We can actually see how the electrons or atoms are moving.”

    Mankowsky said future experiments at LCLS could try to sustain the superconducting state for longer periods, use a combination of experimental techniques to study how other properties evolve in the transition into the superconducting state and explore whether the same structural changes are at work in other high-temperature superconductors.

    Researchers from the National Center for Scientific Research in France, Paul Scherrer Institute in Switzerland, Max Planck Institute for Solid State Research in Germany, Swiss Federal Institute of Technology, College of France, University of Geneva, Oxford University in the United Kingdom, the Center for Free-Electron Laser Science in Germany, and University of Hamburg in Germany also participated in the study. The work was supported by the European Research Council, German Science Foundation, Swiss National Superconducting Center and Swiss National Science Foundation.

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

  • richardmitnick 8:20 am on October 17, 2014 Permalink | Reply
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    From MIT: “Superconducting circuits, simplified” 

    MIT News

    October 17, 2014
    Larry Hardesty | MIT News Office

    Computer chips with superconducting circuits — circuits with zero electrical resistance — would be 50 to 100 times as energy-efficient as today’s chips, an attractive trait given the increasing power consumption of the massive data centers that power the Internet’s most popular sites.

    Shown here is a square-centimeter chip containing the nTron adder, which performed the first computation using the researchers’ new superconducting circuit. Photo: Adam N. McCaughan

    Superconducting chips also promise greater processing power: Superconducting circuits that use so-called Josephson junctions have been clocked at 770 gigahertz, or 500 times the speed of the chip in the iPhone 6.

    But Josephson-junction chips are big and hard to make; most problematic of all, they use such minute currents that the results of their computations are difficult to detect. For the most part, they’ve been relegated to a few custom-engineered signal-detection applications.

    In the latest issue of the journal Nano Letters, MIT researchers present a new circuit design that could make simple superconducting devices much cheaper to manufacture. And while the circuits’ speed probably wouldn’t top that of today’s chips, they could solve the problem of reading out the results of calculations performed with Josephson junctions.

    The MIT researchers — Adam McCaughan, a graduate student in electrical engineering, and his advisor, professor of electrical engineering and computer science Karl Berggren — call their device the nanocryotron, after the cryotron, an experimental computing circuit developed in the 1950s by MIT professor Dudley Buck. The cryotron was briefly the object of a great deal of interest — and federal funding — as the possible basis for a new generation of computers, but it was eclipsed by the integrated circuit.

    “The superconducting-electronics community has seen a lot of devices come and go, without any real-world application,” McCaughan says. “But in our paper, we have already applied our device to applications that will be highly relevant to future work in superconducting computing and quantum communications.”

    Superconducting circuits are used in light detectors that can register the arrival of a single light particle, or photon; that’s one of the applications in which the researchers tested the nanocryotron. McCaughan also wired together several of the circuits to produce a fundamental digital-arithmetic component called a half-adder.

    Resistance is futile

    Superconductors have no electrical resistance, meaning that electrons can travel through them completely unimpeded. Even the best standard conductors — like the copper wires in phone lines or conventional computer chips — have some resistance; overcoming it requires operational voltages much higher than those that can induce current in a superconductor. Once electrons start moving through an ordinary conductor, they still collide occasionally with its atoms, releasing energy as heat.

    Superconductors are ordinary materials cooled to extremely low temperatures, which damps the vibrations of their atoms, letting electrons zip past without collision. Berggren’s lab focuses on superconducting circuits made from niobium nitride, which has the relatively high operating temperature of 16 Kelvin, or minus 257 degrees Celsius. That’s achievable with liquid helium, which, in a superconducting chip, would probably circulate through a system of pipes inside an insulated housing, like Freon in a refrigerator.

    A liquid-helium cooling system would of course increase the power consumption of a superconducting chip. But given that the starting point is about 1 percent of the energy required by a conventional chip, the savings could still be enormous. Moreover, superconducting computation would let data centers dispense with the cooling systems they currently use to keep their banks of servers from overheating.

    Cheap superconducting circuits could also make it much more cost-effective to build single-photon detectors, an essential component of any information system that exploits the computational speedups promised by quantum computing.

    Engineered to a T

    The nanocryotron — or nTron — consists of a single layer of niobium nitride deposited on an insulator in a pattern that looks roughly like a capital “T.” But where the base of the T joins the crossbar, it tapers to only about one-tenth its width. Electrons sailing unimpeded through the base of the T are suddenly crushed together, producing heat, which radiates out into the crossbar and destroys the niobium nitride’s superconductivity.

    A current applied to the base of the T can thus turn off a current flowing through the crossbar. That makes the circuit a switch, the basic component of a digital computer.

    After the current in the base is turned off, the current in the crossbar will resume only after the junction cools back down. Since the superconductor is cooled by liquid helium, that doesn’t take long. But the circuits are unlikely to top the 1 gigahertz typical of today’s chips. Still, they could be useful for some lower-end applications where speed isn’t as important as energy efficiency.

    Their most promising application, however, could be in making calculations performed by Josephson junctions accessible to the outside world. Josephson junctions use tiny currents that until now have required sensitive lab equipment to detect. They’re not strong enough to move data to a local memory chip, let alone to send a visual signal to a computer monitor.

    In experiments, McCaughan demonstrated that currents even smaller than those found in Josephson-junction devices were adequate to switch the nTron from a conductive to a nonconductive state. And while the current in the base of the T can be small, the current passing through the crossbar could be much larger — large enough to carry information to other devices on a computer motherboard.

    “I think this is a great device,” says Oleg Mukhanov, chief technology officer of Hypres, a superconducting-electronics company whose products rely on Josephson junctions. “We are currently looking very seriously at the nTron for use in memory.”

    “There are several attractions of this device,” Mukhanov says. “First, it’s very compact, because after all, it’s a nanowire. One of the problems with Josephson junctions is that they are big. If you compare them with CMOS transistors, they’re just physically bigger. The second is that Josephson junctions are two-terminal devices. Semiconductor transistors are three-terminal, and that’s a big advantage. Similarly, nTrons are three-terminal devices.”

    “As far as memory is concerned,” Mukhanov adds, “one of the features that also attracts us is that we plan to integrate it with magnetoresistive spintronic devices, mRAM, magnetic random-access memories, at room temperature. And one of the features of these devices is that they are high-impedance. They are in the kilo-ohms range, and if you look at Josephson junctions, they are just a few ohms. So there is a big mismatch, which makes it very difficult from an electrical-engineering standpoint to match these two devices. NTrons are nanowire devices, so they’re high-impedance, too. They’re naturally compatible with the magnetoresistive elements.”

    McCaughan and Berggren’s research was funded by the National Science Foundation and by the Director of National Intelligence’s Intelligence Advanced Research Projects Activity.

    See the full article here.

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  • richardmitnick 2:41 pm on October 14, 2014 Permalink | Reply
    Tags: , , , Superconductivity   

    From BNL: “Unstoppable Magnetoresistance” 

    Brookhaven Lab

    October 14, 2014
    Tien Nguyen

    Mazhar Ali, a fifth-year graduate student in the laboratory of Bob Cava, the Russell Wellman Moore Professor of Chemistry at Princeton University, has spent his academic career discovering new superconductors, materials coveted for their ability to let electrons flow without resistance. While testing his latest candidate, the semimetal tungsten ditelluride (WTe2), he noticed a peculiar result.

    Ali applied a magnetic field to a sample of WTe2, one way to kill superconductivity if present, and saw that its resistance doubled. Intrigued, Ali worked with Jun Xiong, a student in the laboratory of Nai Phuan Ong, the Eugene Higgins Professor of Physics at Princeton, to re-measure the material’s magnetoresistance, which is the change in resistance as a material is exposed to stronger magnetic fields.

    Mazhar Ali (left) and Steven Flynn (right), co-authors on the Nature article
    Photo credit: C. Todd Reichart

    “They have unique capabilities at Brookhaven. One is that they can measure diffraction at 10 Kelvin (-441 °F).”
    — Bob Cava, Princeton University

    “He noticed the magnetoresistance kept going up and up and up—that never happens.” said Cava. The researchers then exposed WTe2 to a 60-tesla magnetic field, close to the strongest magnetic field mankind can create, and observed a magnetoresistance of 13 million percent. The material’s magnetoresistance displayed unlimited growth, making it the only known material without a saturation point. The results were published on September 14 in the journal Nature.

    Electronic information storage is dependent on the use of magnetic fields to switch between distinct resistivity values that correlate to either a one or a zero. The larger the magnetoresistance, the smaller the magnetic field needed to change from one state to another, Ali said. Today’s devices use layered materials with so-called “giant magnetoresistance,” with changes in resistance of 20,000 to 30,000 percent when a magnetic field is applied. “Colossal magnetoresistance” is close to 100,000 percent, so for a magnetoresistance percentage in the millions, the researchers hoped to coin a new term.

    Crystal Structure of WTe2. Image credit: Nature

    Their original choice was “ludicrous” magnetoresistance, which was inspired by “ludicrous speed,” the fictional form of fast-travel used in the comedy “Spaceballs.” They even included an acknowledgement to director Mel Brooks. After other lab members vetoed “ludicrous,” the researchers considered “titanic” before Nature editors ultimately steered them towards the term “large magnetoresistance.”

    Terminology aside, the fact remained that the magnetoresistance values were extraordinarily high, a phenomenon that might be understood through the structure of WTe2. To look at the structure with an electron microscope, the research team turned to Jing Tao, a researcher at Brookhaven National Laboratory.

    Jing Tao

    “Jing is a great microscopist. They have unique capabilities at Brookhaven,” Cava said. “One is that they can measure diffraction at 10 Kelvin (-441 °F). Not too many people on Earth can do that, but Jing can.”

    Electron microscopy experiments revealed the presence of tungsten dimers, paired tungsten atoms, arranged in chains responsible for the key distortion from the classic octahedral structure type. The research team proposed that WTe2 owes its lack of saturation to the nearly perfect balance of electrons and electron holes, which are empty docks for traveling electrons. Because of its structure, WTe2 only exhibits magnetoresistance when the magnetic field is applied in a certain direction. This could be very useful in scanners, where multiple WTe2 devices could be used to detect the position of magnetic fields, Ali said.

    “Aside from making devices from WTe2, the question to ask yourself as a scientist is: How can it be perfectly balanced, is there something more profound,” Cava said.

    See the full article here.

    BNL Campus

    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 9:09 am on August 15, 2014 Permalink | Reply
    Tags: , , Superconductivity   

    From Brookhaven Lab: “New Grant to Aid Search for the Secrets of Superconductivity” 

    Brookhaven Lab

    August 12, 2014
    Karen McNulty Walsh

    Research aimed at unlocking the secrets of high-temperature superconductivity at the U.S. Department of Energy’s Brookhaven National Laboratory will get a boost from a new grant awarded to Ivan Bozovic, a Brookhaven physicist and an Adjunct Professor at Yale University, by the Gordon and Betty Moore Foundation. Bozovic will receive $1.9 million over five years as part of the Moore Materials Synthesis Investigators program to continue the meticulous assembly and manipulation of superconducting thin films and the exploration of factors underlying these remarkable materials’ ability to carry electric current with no energy loss.

    “I am very grateful for this grant, which recognizes the importance of methodical work that slowly but steadily improves materials synthesis techniques and sample quality,” Bozovic said. Such quality is essential to uncover subtle effects in high-temperature superconductors, which, Bozovic notes, can be masked by impurities. “The better the samples, the more precise and revealing our experiments can be — and the greater their potential for new insights and discoveries,” he said.

    To achieve such precision, Bozovic uses a one-of-a-kind molecular-beam epitaxy (MBE) machine that he built and continues to improve to fabricate superconducting thin films one atomic layer at a time. He and collaborators have used the machine to assemble more than 2,000 thin film samples and conduct hundreds of scientific experiments. He also contributes to research at Brookhaven’s Center for Emergent Superconductivity, one of DOE’s Energy Frontier Research Centers, which recently received renewed funding.

    “I am very grateful for this grant, which recognizes the importance of methodical work that slowly but steadily improves materials synthesis techniques and sample quality.”
    — Brookhaven physicist Ivan Bozovic


    Leveraging his atomic-layer-by-layer synthesis technique, Bozovic made a series of discoveries related to interface superconductivity, bringing it to the forefront of research in Condensed Matter Physics. He showed that superfluid can be confined to a single atomic layer at the interface of two materials, neither of which is superconducting. In another important experiment, he proved that electron pairs exist on both sides of the superconductor-to-insulator transition an important insight into the mysterious nature of the high-temperature superconductivity phenomenon.

    Bozovic is one of only 12 scientists to be awarded funding through the Moore Materials Synthesis Investigators program, part of the foundation’s Emerging Phenomena in Quantum Systems (EPiQS) initiative. Quantum materials, the Foundation notes, are substances in which the collective behavior of electrons leads to many complex and unexpected emergent phenomena, superconductivity being a prominent example.

    In announcing the grantees, the Foundation stated:

    “Our approach is to focus on some of the field’s leading scientists; to allow these scientists the freedom to explore and the flexibility to change research directions; and to incentivize sample sharing within the EPiQS program and beyond…We believe that our programs will lead to discoveries of new quantum materials with emergent electronic properties as well as an increase in the availability of top-quality samples to the experimental community.”

    Bozovic earned a Ph.D. in physics from the University of Belgrade in Yugoslavia in 1975. He remained there until 1985 and served as a professor and the Head of the Physics Department. From 1986 until 1988, he worked at the Applied Physics Department at Stanford University. He was a senior research scientist at Varian Research Center in Palo Alto, California, 1989 to 1998, and the chief technical officer and principal scientist for Oxxel GmbH in Germany 1998 to 2002. He joined Brookhaven as a senior scientist and the leader of the Molecular Beam Epitaxy group in 2003. In 2012 he was a co-recipient of the Bernd T. Matthias Prize for Superconducting Materials, and in 2013 was chosen to give the Max Planck Lecture at MPI-Stuttgart, Germany. His research results have been published in more than 200 research papers and cited more than 6,500 times. Many of these were published in the highest-impact journals such as Nature, Science, and Nature Materials. Bozovic is a Fellow of APS and of SPIE, and a Foreign Member of Serbian Academy of Science and Arts.

    Bozovic’s research at Brookhaven is supported by the DOE Office of Science. The Moore Foundation grant will be awarded to him by way of his adjunct appointment at Yale University.

    See the full article here.

    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 6:07 pm on February 13, 2014 Permalink | Reply
    Tags: , , , , Superconductivity   

    From Brookhaven Lab: “Superconductivity in Orbit: Scientists Find New Path to Loss-Free Electricity” 

    Brookhaven Lab

    February 13, 2014
    Contacts: Justin Eure, (631) 344-2347 or Peter Genzer, (631) 344-3174

    Brookhaven Lab researchers captured the distribution of multiple orbital electrons to help explain the emergence of superconductivity in iron-based materials

    Armed with just the right atomic arrangements, superconductors allow electricity to flow without loss and radically enhance energy generation, delivery, and storage. Scientists tweak these superconductor recipes by swapping out elements or manipulating the valence electrons in an atom’s outermost orbital shell to strike the perfect conductive balance. Most high-temperature superconductors contain atoms with only one orbital impacting performance—but what about mixing those elements with more complex configurations?

    Brookhaven Lab scientists and study coauthors (from left) Lijun Wu, Yimei Zhu, Chris Homes, and Weiguo Yin stand by the electron microscope used to reveal the multi-orbital distributions with a technique called quantitative convergent beam electron diffraction (CBED).

    Now, researchers at the U.S. Department of Energy’s Brookhaven National Laboratory have combined atoms with multiple orbitals and precisely pinned down their electron distributions. Using advanced electron diffraction techniques, the scientists discovered that orbital fluctuations in iron-based compounds induce strongly coupled polarizations that can enhance electron pairing—the essential mechanism behind superconductivity. The study, set to publish soon in the journal Physical Review Letters, provides a breakthrough method for exploring and improving superconductivity in a wide range of new materials.

    “For the first time, we obtained direct experimental evidence of the subtle changes in electron orbitals by comparing an unaltered, non-superconducting material with its doped, superconducting twin,” said Brookhaven Lab physicist and project leader Yimei Zhu.

    While the effect of doping the multi-orbital barium iron arsenic—customizing its crucial outer electron count by adding cobalt—mirrors the emergence of high-temperature superconductivity in simpler systems, the mechanism itself may be entirely different.

    “Now superconductor theory can incorporate proof of strong coupling between iron and arsenic in these dense electron cloud interactions,” said Brookhaven Lab physicist and study coauthor Weiguo Yin. “This unexpected discovery brings together both orbital fluctuation theory and the 50-year-old ‘excitonic’ theory for high-temperature superconductivity, opening a new frontier for condensed matter physics.”

    Atomic Jungle Gym

    Imagine a child playing inside a jungle gym, weaving through holes in the multicolored metal matrix in much the same way that electricity flows through materials. This particular kid happens to be wearing a powerful magnetic belt that repels the metal bars as she climbs. This causes the jungle gym’s grid-like structure to transform into an open tunnel, allowing the child to slide along effortlessly. The real bonus, however, is that this action attracts any nearby belt-wearing children, who can then blaze through that perfect path.

    These images show the distribution of the valence electrons in the samples explored by the Brookhaven Lab collaboration—both feature a central iron layer sandwiched between arsenic atoms. The tiny red clouds (more electrons) in the undoped sample on the left (BaFe2As2) reveal the weak charge quadrupole of the iron atom, while the blue clouds (fewer electrons) around the outer arsenic ions show weak polarization. The superconducting sample on the right (doped with cobalt atoms), however, exhibits a strong quadrupole in the center and the pronounced polarization of the arsenic atoms, as evidenced by the large, red balloons.

    Flowing electricity can have a similar effect on the atomic lattices of superconductors, repelling the negatively charged valence electrons in the surrounding atoms. In the right material, that repulsion actually creates a positively charged pocket, drawing in other electrons as part of the pairing mechanism that enables the loss-free flow of current—the so-called excitonic mechanism. To design an atomic jungle gym that warps just enough to form a channel, scientists audition different combinations of elements and tweak their quantum properties.

    “High-temperature copper-oxide superconductors, or cuprates, contain in effect a single orbital and lack the degree of freedom to accommodate strong enough interactions between electricity and the lattice,” Yin said. “But the barium iron arsenic we tested has multi-orbital electrons that push and pull the lattice in much more flexible and complex ways, for example by inter-orbital electron redistribution. This feature is especially promising because electricity can shift arsenic’s electron cloud much more easily than oxygen’s.”

    In the case of the atomic jungle gym, this complexity demands new theoretical models and experimental data, considering that even a simple lattice made of north-south bar magnets can become a multidimensional dance of attraction and repulsion. To control the doping effects and flow of electricity, scientists needed a window into the orbital interactions.

    Tracking Orbits

    “Consider measuring waves crashing across the ocean’s surface,” Zhu said. “We needed to pinpoint those complex fluctuations without having the data obscured by the deep water underneath. The waves represent the all-important electrons in the outer orbital shells, which are barely distinguishable from the layers of inner electrons. For example, each barium atom alone has 56 electrons, but we’re only concerned with the two in the outermost layer.”

    The Brookhaven researchers used a technique called quantitative convergent beam electron diffraction (CBED) to reveal the orbital clouds with subatomic precision. After an electron beam strikes the sample, it bounces off the charged particles to reveal the configuration of the atomic lattice, or the exact arrays of nuclei orbited by electrons. The scientists took thousands of these measurements, subtracted the inner electrons, and converted the data into probabilities—balloon-shaped areas where the valence electrons were most likely to be found.

    Shape-Shifting Atoms

    The researchers first examined the electron clouds of non-superconducting samples of barium iron arsenic. The CBED data revealed that the arsenic atoms—placed above and below the iron in a sandwich-like shape (see image)—exhibited little shift or polarization of valence electrons. However, when the scientists transformed the compound into a superconductor by doping it with cobalt, the electron distribution radically changed.

    “Cobalt doping pushed the orbital electrons in the arsenic outward, concentrating the negative charge on the outside of the ‘sandwich’ and creating a positively charged pocket closer to the central layer of iron,” Zhu said. “We created very precise electronic and atomic displacement that might actually drive the critical temperature of these superconductors higher.”

    Added Yin, “What’s really exciting is that this electron polarization exhibits strong coupling. The quadrupole polarization of the iron, which indicates the orbital fluctuation, couples intimately with the arsenic dipole polarization—this mechanism may be key to the emergence of high-temperature superconductivity in these iron-based compounds. And our results may guide the design of new materials.”

    This study explored the orbital fluctuations at room temperature under static conditions, but future experiments will apply dynamic diffraction methods to super-cold samples and explore alternative material compositions.

    The experimental work at Brookhaven Lab was supported by DOE’s Office of Science. The materials synthesis was carried out at the Chinese Academy of Sciences’ Institute of Physics. Brookhaven Lab coauthors of the study also include Chao Ma, Lijun Wu, and Chris Homes.

    See the full article here.

    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 6:35 pm on November 18, 2013 Permalink | Reply
    Tags: , , , Superconductivity   

    From Berkeley Lab: “A Superconductor-Surrogate Earns Its Stripes” 

    Berkeley Lab

    November 18, 2013
    Berkeley Lab Study Reveals Origins of an Exotic Phase of Matter

    Alison Hatt 510-486-7154 ajhatt@lbl.gov

    Understanding superconductivity – whereby certain materials can conduct electricity without any loss of energy – has proved to be one of the most persistent problems in modern physics. Scientists have struggled for decades to develop a cohesive theory of superconductivity, largely spurred by the game-changing prospect of creating a superconductor that works at room temperature, but it has proved to be a tremendous tangle of complex physics.

    Now scientists at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have teased out another important tangle from this giant ball of string, bringing us a significant step closer to understanding how high- temperature superconductors work their magic. Working with a model compound, the team illuminated the origins of the so-called “stripe phase” in which electrons become concentrated in stripes throughout a material, and which appears to be linked to superconductivity.

    Ultrafast changes in the optical properties of strontium-doped lanthanum nickelate throughout the infrared spectrum expose a rapid dynamics of electronic localization in the nickel-oxide plane, shown at left. This process, illustrated on the right, comprises the first step in the formation of ordered charge patterns or “stripes.”

    “We’re trying to understand nanoscale order and how that determines material properties such as superconductivity,” said Robert Kaindl, a physicist in Berkeley Lab’s Materials Sciences Division. “Using ultrafast optical techniques, we are able to observe how charge stripes start to form on a time scale of hundreds of femtoseconds.” A femtosecond is just one millionth of one billionth of a second.

    Electrons in a solid material interact extremely quickly and on very short length scales, so to observe their behavior researchers have built extraordinarily powerful “microscopes” that zoom into fast events using short flashes of laser light. Kaindl and his team brought to bear the power of their ultrafast-optics expertise to understand the stripe phase in strontium-doped lanthanum nickelate (LSNO), a close cousin of high-temperature superconducting materials.

    “We chose to work with LSNO because it has essential similarities to the cuprates (an important class of high-temperature superconductors), but its lack of superconductivity lets us focus on understanding just the stripe phase,” said Giacomo Coslovich, a postdoctoral researcher at Berkeley Lab working with Kaindl.

    “With science, you have to simplify your problems,” Coslovich continued. “If you try to solve them all at once with their complicated interplay, you will never understand what’s going on.”

    Giacomo Coslovich (left) and Robert Kaindl (right) next to the laser setup that generates extremely short pulses of light at “mid-infrared” wavelengths, far beyond the spectrum perceptible by the human eye.

    Beyond the ultrafast measurements, the team also studied X-ray scattering and the infrared reflectance of the material at the neighboring Advanced Light Source, to develop a thorough, cohesive understanding of the stripe phase and why it forms.

    Said Kaindl, “We took advantage of our fortunate location in the national lab environment, where we have both these ultrafast techniques and the Advanced Light Source. This collaborative effort made this work possible.”

    See the full article here.

    A U.S. Department of Energy National Laboratory Operated by the University of California

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