Tagged: Superconductivity Toggle Comment Threads | Keyboard Shortcuts

  • richardmitnick 4:36 pm on March 26, 2015 Permalink | Reply
    Tags: , , Superconductivity   

    From LANL: “Using magnetic fields to understand high-temperature superconductivity “ 

    LANL bloc

    LANL Sign
    Los Alamos National Laboratory

    March 26, 2015
    Nancy Ambrosiano

    Los Alamos explores experimental path to potential ‘next theory of superconductivity’

    1
    Los Alamos National Laboratory scientist Brad Ramshaw conducts an experiment at the Pulsed Field Facility of the National High Magnetic Field Lab, exposing high-temperature superconductors to very high magnetic fields, changing the temperature at which the materials become perfectly conducting and revealing unique properties of these substances.

    Taking our understanding of quantum matter to new levels, scientists at Los Alamos National Laboratory are exposing high-temperature superconductors to very high magnetic fields, changing the temperature at which the materials become perfectly conducting and revealing unique properties of these substances.

    “High magnetic-field measurements of doped copper-oxide superconductors are paving the way to a new theory of superconductivity,” said Brad Ramshaw, a Los Alamos scientist and lead researcher on the project. Using world-record high magnetic fields available at the National High Magnetic Field Laboratory (NHMFL) Pulsed Field Facility, based in Los Alamos, Ramshaw and his coworkers are pushing the boundaries of how matter can conduct electricity without the resistance that plagues normal materials carrying an electrical current.

    LANL National High Magnetic Field Lab
    NHMFL

    The eventual goal of the research would be to create a superconductor that operates at room temperature and needs no cooling at all. At this point, all devices that make use of superconductors, such as the MRI magnets found in hospitals, must be cooled to temperatures far below zero with liquid nitrogen or helium, adding to the cost and complexity of the enterprise.

    “This is a truly landmark experiment that illuminates a problem of central importance to condensed matter physics,” said MagLab Director Gregory Boebinger, who is also chief scientist for Condensed Matter Science at the National High Magnetic Field Laboratory’s headquarters in Florida. “The success of this quintessential MagLab work relied on having the best samples, the highest magnetic fields, the most sensitive techniques, and the inspired creativity of a multi-institutional research team.”

    High-temperature superconductors have been a thriving field of research for almost 30 years, not just because they can conduct electricity with no losses—one hundred degrees higher than any other material—but also because they represent a very difficult and interesting “correlated-electron” physics problem in their own right.

    The theory of traditional, low-temperature superconductors was constructed by Bardeen, Cooper, and Schrieffer in 1957, winning them the Nobel prize; this theory (known as the BCS theory) had a far-reaching impact, laying the foundation for the Higgs mechanism in particle physics, and it represents one of the greatest triumphs of 20th century physics.

    On the other hand, high-temperature superconductors, such as yttrium barium copper oxide (YBa2Cu3O6+x), cannot be explained with BCS theory, and so researchers need a new theory for these materials. One particularly interesting aspect of high-temperature superconductors, such as YBa2Cu3O6+x, is that one can change the superconducting transition temperature (Tc, where the material becomes perfectly conducting) by “doping” it, : changing the number of electrons that participate in superconductivity.

    The Los Alamos team’s research in the 100-T magnet found that if one dopes YBa2Cu3O6+x to the point where Tc is highest (“optimal doping”), the electrons become very heavy and move around in a correlated way.

    “This tells us that the electrons are interacting very strongly when the material is an optimal superconductor,” said Ramshaw. “This is a vital piece of information for building the next theory of superconductivity.”

    “An outstanding problem in the field of high-transition-temperature (high-Tc) superconductivity has been the issue as to whether a quantum critical point—a special doping value where quantum fluctuations lead to strong electron-electron interactions—is driving the remarkably high Tc’s in these materials,” he said.

    Proof of its existence has previously not been found due to the robust nature of the superconductivity in the copper oxide materials, yet if scientists can show that there is a quantum critical point, it would constitute a significant milestone toward resolving the superconducting pairing mechanism, Ramshaw explained.

    “Assembling the pieces of this complex superconductivity puzzle is a daunting task that has involved scientists from around the world for decades,” said Charles H. Mielke, NHMFL-Pulsed Field Facility director at Los Alamos. “Though the puzzle is unfinished, this essential piece links unquestionable experimental results to fundamental condensed matter physics — a connection made possible by an exceptional team, strong partner support and unsurpassed capabilities.”

    In a paper this week in the journal Science, the team addresses this longstanding problem by measuring magnetic quantum oscillations as a function of hole doping in very strong magnetic fields in excess of 90 tesla.

    Strong magnetic fields such as the world-record field accessible at the NHMFL site at Los Alamos enable the normal metallic state to be accessed by suppressing superconductivity. Fields approaching 100 tesla, in particular, enable quantum oscillations to be measured very close to the maximum in the transition temperature Tc ~ 94 kelvin. These quantum oscillations give scientists a picture of how the electrons are interacting with each other before they become superconducting.

    By accessing a very broad range of dopings, the authors show that there is a strong enhancement of the effective mass at optimal doping. A strong enhancement of the effective mass is the signature of increasing electron interaction strength, and the signature of a quantum critical point. The broken symmetry responsible for this point has yet to be pinned down, although a connection with charge ordering appears to be likely, Ramshaw notes.

    Funding: Work carried out at the National High Magnetic Field Laboratory—Pulsed Field Facility at Los Alamos National Laboratory was provided through funding from the National Science Foundation Division of Materials Research through Grant No. DMR-1157490 and from the US Department of Energy’s Office of Science, Florida State University, the State of Florida, and Los Alamos National Laboratory through the LDRD program.

    See the full article here.

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    Los Alamos National Laboratory’s mission is to solve national security challenges through scientific excellence.

    LANL Campus

    Los Alamos National Laboratory, a multidisciplinary research institution engaged in strategic science on behalf of national security, is operated by Los Alamos National Security, LLC, a team composed of Bechtel National, the University of California, The Babcock & Wilcox Company, and URS for the Department of Energy’s National Nuclear Security Administration.

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

    Operated by Los Alamos National Security, LLC for the U.S. Dept. of Energy’s NNSA

    DOE Main

    NNSA

     
  • richardmitnick 8:55 am on March 13, 2015 Permalink | Reply
    Tags: , , Superconductivity   

    From Argonne: “Study proposes new way to measure superconducting fluctuations” 

    News from Argonne National Laboratory

    March 10, 2015
    Louise Lerner

    1
    Scientists at Argonne proposed theoretical evidence for a new superconducting fluctuation, which may lead to a way of measuring the exact temperature at which superconductivity kicks in and shed light on the poorly understood properties of superconducting materials above this temperature. Above: Sharp peaks are visible as the temperature nears Tc, the temperature at which superconductivity kicks in. Credit: Alexey Galda

    A study published last month by researchers at the U.S. Department of Energy’s Argonne National Laboratory provides theoretical evidence for a new effect that may lead to a way of measuring the exact temperature at which superconductivity kicks in and shed light on the poorly understood properties of superconducting materials above this temperature.

    Superconductors are an old puzzle in physics, made all the more tantalizing because their technological applications are so valuable. Electricity is being lost all around you; very few electric systems use power completely efficiently, and some is always lost—generally as heat, which you can feel as your laptop or phone gets warm. That’s because even our best conductors, like copper, are always losing a little bit of electricity to resistance. Superconductors don’t. When cooled down to operating temperature, they never lose any electricity.

    This is the kind of unique property that can spur entire new fields of invention, and they have—MRIs, cell phone towers and Maglev trains all use superconductors. But they’re not in every engine or transmission line because of a serious logistical issue: their operating temperature is -270°F or lower, so they have to be cooled with liquid helium or nitrogen.

    Superconducting materials have a number of other fascinating properties. For example, scientists found that the electricity flow between two superconductors separated by a thin non-conducting material (called a Josephson junction) can be extremely sensitive to external microwave radiation. As little as a single photon can trigger electricity to flow through such a device when just the right voltage is applied. This unique effect, called resonant tunneling, allows such a high precision of measurement that it is used for DNA sequencing and quantum encryption. The same phenomenon has determined the international standard of voltage for decades.

    The problem is that we still don’t fully understand how superconductors work, and if we want to realize their full potential, we need to.

    To explore superconductors, one of the things scientists do is rearrange them in all sorts of new ways—stacking them in layers, punching holes in them and trimming them down to wires just 50 nanometers across, for example.

    These new arrangements change the way materials behave, including essential properties like the exact temperature at which they become superconducting—called the “critical temperature” or Tc .

    “Until now,” said Valerii Vinokur, Argonne Distinguished Fellow and a coauthor on the paper, “the field hasn’t had a standard, precise way to measure Tc.”

    One of the things we do know is that short-lived islands of superconductivity can form in a material just above Tc. These sporadically emerging and rapidly vanishing regions, called superconducting fluctuations, mirror in one way or another most of the superconducting properties of the material at temperatures below Tc. Despite this, superconducting fluctuations remain poorly understood—so much so that even measuring their lifetime has been a challenge. In the paper, Vinokur and Argonne postdoctoral fellow Alexey Galda proposed an effect that mirrors resonant tunneling above Tc that is strong enough to measure, and—most importantly—gets sharper as the temperature approaches Tc.

    If verified by experiment, this would be a new high-precision tool for measuring fundamental properties of superconducting fluctuations, such as their lifetime, and provide a way to measure more precisely where Tc lies for each material.

    “Every new tool in studying superconductivity is absolutely invaluable—it brings more precision to the field,” Galda said.

    “This would also let us study fluctuations more widely,” he said.

    The fluctuations, Galda said, are interesting because they can help researchers map the microscopic behaviors of materials, which are likely key to why and how materials act the way they do. Fluctuations are influenced by a number of different phenomena; a tool to untangle at least one variable from the set would help researchers tease out the contributions of others.

    “To know how long fluctuations live is very important and has been difficult to determine experimentally,” Vinokur said.

    Researchers in Argonne’s Materials Science division, led by Argonne physicist Wai Kwok, are planning to verify the results experimentally.

    The paper, “Resonant tunneling of fluctuation Cooper pairs,” was published by Nature’s Scientific Reports. The other author on the paper was A. S. Mel’nikov of the Russian Academy of Sciences.

    The study was supported by the U.S. Department of Energy’s Office of Science, as well as the Russian Foundation for Basic Research and the Russian Ministry of Science and Education.

    See the full article here.

    Please help promote STEM in your local schools.
    STEM Icon
    Stem Education Coalition
    Argonne National Laboratory seeks solutions to pressing national problems in science and technology. The nation’s first national laboratory, Argonne conducts leading-edge basic and applied scientific research in virtually every scientific discipline. Argonne researchers work closely with researchers from hundreds of companies, universities, and federal, state and municipal agencies to help them solve their specific problems, advance America’s scientific leadership and prepare the nation for a better future. With employees from more than 60 nations, Argonne is managed by UChicago Argonne, LLC for the U.S. Department of Energy’s Office of Science. For more visit http://www.anl.gov.

    The Advanced Photon Source at Argonne National Laboratory is one of five national synchrotron radiation light sources supported by the U.S. Department of Energy’s Office of Science to carry out applied and basic research to understand, predict, and ultimately control matter and energy at the electronic, atomic, and molecular levels, provide the foundations for new energy technologies, and support DOE missions in energy, environment, and national security. To learn more about the Office of Science X-ray user facilities, visit http://science.energy.gov/user-facilities/basic-energy-sciences/.

    Argonne is managed by UChicago Argonne, LLC for the U.S. Department of Energy’s Office of Science

    Argonne Lab Campus

     
  • richardmitnick 8:39 am on March 3, 2015 Permalink | Reply
    Tags: , , Superconductivity   

    From AAAS: “A step closer to explaining high-temperature superconductivity?” 

    AAAS

    AAAS

    27 February 2015
    Adrian Cho

    1
    In the new experiment, scientists glimpsed a pattern of up- and down-spinning atoms, which mimics the up-and-down pattern of magnetism seen in high-temperature superconductors. R. A. Hart et al., Nature (2015)

    For years some physicists have been hoping to crack the mystery of high-temperature superconductivity—the ability of some complex materials to carry electricity without resistance at temperatures high above absolute zero—by simulating crystals with patterns of laser light and individual atoms. Now, a team has taken—almost—the next-to-last step in such “optical lattice” simulation by reproducing the pattern of magnetism seen in high-temperature superconductors from which the resistance-free flow of electricity emerges.

    “It’s a very big improvement over previous results,” says Tilman Esslinger, an experimentalist at the Swiss Federal Institute of Technology in Zurich, who was not involved in the work. “It’s very exciting to see steady progress.”

    An optical lattice simulation is essentially a crystal made of light. A real crystal contains a repeating 3D pattern of ions, and electrons flow from ion to ion. In the simulation, spots of laser light replace the ions, and ultracold atoms moving among spots replace the electrons. Physicists can adjust the pattern of spots, how strongly the spots attract the atoms, and how strongly the atoms repel one another. That makes the experiments ideal for probing physics such as high-temperature superconductivity, in which materials such as mercury barium calcium copper oxide carry electricity without resistance at temperatures up to 138 K, far higher above absolute zero than ordinary superconductors such as niobium can.

    Just how the copper-and-oxygen, or cuprate, superconductors work remains unclear. The materials contain planes of copper and oxygen ions with the coppers arranged in a square pattern. Repelling one another, the electrons get stuck in a one-to-a-copper traffic jam called a Mott insulator state. They also spin like tops, and at low temperatures neighboring electrons spin in opposite directions, creating an up-down-up-down pattern of magnetism called antiferromagnetism. Superconductivity sets in when impurities soak up a few electrons and ease the traffic jam. The remaining electrons then pair to glide freely along the planes.

    Theorists do not yet agree how that pairing occurs. Some think that wavelike ripples in the antiferromagnetic pattern act as a glue to attract one electron to the other. Others argue that the pairing arises, paradoxically, from the repulsion among the electrons alone. Theorists can write down a mathematical model of electrons on a checkerboard plane, known as the Fermi-Hubbard model, but it is so hard to “solve” that nobody has been able to show whether it produces superconductivity.

    Experimentalists hope to reproduce the Fermi-Hubbard model in laser light and cold atoms to see if it yields superconductivity. In 2002, Immanuel Bloch, a physicist at the Max Planck Institute for Quantum Optics (MPQ) in Garching, Germany, and colleagues realized a Mott insulator state in an optical lattice. Six years later, Esslinger and colleagues achieved the Mott state with atoms with the right amount of spin to mimic electrons. Now, Randall Hulet, a physicist at Rice University in Houston, Texas, and colleagues have nearly achieved the next-to-last step along the way: antiferromagnetism.

    Hulet and colleagues trapped between 100,000 and 250,000 lithium-6 atoms in laser light. They then ramped up the optical lattice and ramped it back down to put them in order. Shining laser light of a specific wavelength on the atoms, they observed evidence of an emerging up-down-up-down spin pattern. The laser light was redirected, or diffracted, at a particular angle by the rows of atoms—just as x-rays diffract off the ions in a real crystal. Crucially, the light probed the spin of the atoms: The light wave flipped if it bounced off an atom spinning one way but not the other. Without that flipping, the diffraction wouldn’t have occurred, so observation confirms the emergence of the up-down-up-down pattern, Hulet says.

    Hulet’s team solved a problem that has plagued other efforts. Usually, turning the optical lattice on heats the atoms. To avoid that, the researchers added another laser that slightly repelled the atoms, so that the most energetic ones were just barely held by the trap. Then, as the atoms heated, the most energetic ones “evaporated” like steam from hot soup to keep the other ones cool, the researchers report online this week in Nature. They didn’t quite reach a full stable antiferromagnetic pattern: The temperature was 40% too high. But the technique might get there and further, Hulet says. “We don’t have a good sense of what the limit of this method is,” he says. “We could get a factor of two lower, we could get a factor of 10 lower.”

    “It is indeed very promising,” says Tin-Lun “Jason” Ho, a theorist at Ohio State University, Columbus. Reducing the temperature by a factor of two or three might be enough to reach the superconducting state, he says. However, MPQ’s Bloch cautions that it may take still other techniques to get that cold. “There are several cooling techniques that people are developing and interesting experiments coming up,” he says.

    Physicists are also exploring other systems and problems with optical lattices. The approach is still gaining steam, Hulet says: “It’s an exciting time.”

    See the full article here.

    The American Association for the Advancement of Science is an international non-profit organization dedicated to advancing science for the benefit of all people.

    Please help promote STEM in your local schools.
    STEM Icon
    Stem Education Coalition

     
  • 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.

    1
    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.

    2
    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.

    3
    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.

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    Rice U campus

    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
    Tags: , , , , Superconductivity   

    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).

    1
    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.

    2
    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.

    3
    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.

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    Rutgers, The State University of New Jersey, is a leading national research university and the state’s preeminent, comprehensive public institution of higher education. Rutgers is dedicated to teaching that meets the highest standards of excellence; to conducting research that breaks new ground; and to providing services, solutions, and clinical care that help individuals and the local, national, and global communities where they live.

    Founded in 1766, Rutgers teaches across the full educational spectrum: preschool to precollege; undergraduate to graduate; postdoctoral fellowships to residencies; and continuing education for professional and personal advancement.

    Rutgers Seal

     
  • 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.

    1
    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.”

    See the full article here.

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

     
  • richardmitnick 6:57 pm on December 8, 2014 Permalink | Reply
    Tags: , , , Superconductivity, ,   

    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.

    t
    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.”

    a
    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.

    BNL Campus

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    One of ten national laboratories overseen and primarily funded by the Office of Science of the U.S. Department of Energy (DOE), Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. The Laboratory’s almost 3,000 scientists, engineers, and support staff are joined each year by more than 5,000 visiting researchers from around the world.Brookhaven is operated and managed for DOE’s Office of Science by Brookhaven Science Associates, a limited-liability company founded by Stony Brook University, the largest academic user of Laboratory facilities, and Battelle, a nonprofit, applied science and technology organization.
    i1

     
  • richardmitnick 7:51 pm on December 4, 2014 Permalink | Reply
    Tags: , , , Superconductivity,   

    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.

    a
    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
    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.

    See the full article here.

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    SLAC Campus
    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.
    i1

     
  • richardmitnick 8:20 am on October 17, 2014 Permalink | Reply
    Tags: , , , Superconductivity   

    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.

    chip
    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.

    ScienceSprings relies on technology from

    MAINGEAR computers

    Lenovo
    Lenovo

    Dell
    Dell

     
  • 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.

    two
    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.

    cry.
    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.

    jt
    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.
    i1

    ScienceSprings relies on technology from

    MAINGEAR computers

    Lenovo
    Lenovo

    Dell
    Dell

     
c
Compose new post
j
Next post/Next comment
k
Previous post/Previous comment
r
Reply
e
Edit
o
Show/Hide comments
t
Go to top
l
Go to login
h
Show/Hide help
shift + esc
Cancel
Follow

Get every new post delivered to your Inbox.

Join 434 other followers

%d bloggers like this: