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  • richardmitnick 4:24 pm on December 19, 2014 Permalink | Reply
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    From SLAC: “First Direct Evidence that a Mysterious Phase of Matter Competes with High-Temperature Superconductivity” 


    SLAC Lab

    December 19, 2014

    SLAC Study Shows “Pseudogap” Phase Hoards Electrons that Might Otherwise Conduct Electricity with 100 Percent Efficiency

    Scientists have found the first direct evidence that a mysterious phase of matter known as the “pseudogap” competes with high-temperature superconductivity, robbing it of electrons that otherwise might pair up to carry current through a material with 100 percent efficiency.

    The result, led by researchers at Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory, is the culmination of 20 years of research aimed at finding out whether the pseudogap helps or hinders superconductivity, which could transform society by making electrical transmission, computing and other areas much more energy efficient.

    2
    This illustration shows the complex relationship between high-temperature superconductivity (SC) and a mysterious phase called the pseudogap (PG). Copper oxide materials become superconducting when an optimal number of electrons are removed, leaving positively charged “holes,” and the material is chilled below a transition temperature (blue curve). This causes remaining electrons (yellow) to pair up and conduct electricity with 100 percent efficiency. Experiments at SLAC have produced the first direct evidence that the pseudogap competes for electrons with superconductivity over a wide range of temperatures at lower hole concentrations (SC+PG). At lower temperatures and higher hole concentrations, superconductivity wins out. (SLAC National Accelerator Laboratory)

    The new study definitively shows that the pseudogap is one of the things that stands in the way of getting superconductors to work at higher temperatures for everyday uses, said lead author Makoto Hashimoto, a staff scientist at SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL), the DOE Office of Science User Facility where the experiments were carried out. The results were published in Nature Materials.

    SLAC SSRL
    SSRL

    “Now we have clear, smoking-gun evidence that the pseudogap phase competes with and suppresses superconductivity,” Hashimoto said. “If we can somehow remove this competition, or handle it better, we may be able to raise the operating temperatures of these superconductors.”

    Tracking Down Electrons

    In the experiments, researchers used a technique called angle-resolved photoemission spectroscopy, or ARPES, to knock electrons out of a copper oxide material, one of a handful of materials that superconduct at relatively high temperatures – although they still have to be chilled to at least minus 135 degrees Celsius.

    Plotting the energies and momenta of the ejected electrons tells researchers how they were behaving when they were inside the material. In metals, for instance, electrons freely flow around and between atoms. In insulators, they stick close to their home atoms. And in superconductors, electrons leave their usual positions and pair up to conduct electricity with zero resistance and 100 percent efficiency; the missing electrons leave a characteristic gap in the researchers’ plots.

    But in the mid-1990s, scientists discovered another, puzzling gap in their plots of copper oxide superconductors. This “pseudogap” looked like the one left by superconducting electrons, but it showed up at temperatures too warm for superconductivity to occur. Was it a lead-in to superconducting behavior? A rival state that held superconductivity at bay? Where did it come from? No one knew.

    “It’s a complex, intimate relationship. These two phenomena likely share the same roots but are ultimately antagonistic,” said Zhi-Xun Shen, a professor at SLAC and Stanford and senior author of the study. “When the pseudogap is winning, superconductivity is losing ground.”

    Evidence of Competition

    Shen and his colleagues have been using ARPES to investigate the pseudogap ever since it showed up, refining their techniques over the years to pry more information out of the flying electrons.

    In this latest study, Hashimoto was able to find out exactly what was happening at the moment the material transitioned into a superconducting state. He did this by measuring not only the energies and momenta of the electrons, but the number of electrons coming out of the material with particular energies over a wide range of temperatures, and after the electronic properties of the material had been altered in various ways.

    He discovered clear, strong evidence that at this crucial transition temperature, the pseudogap and superconductivity are competing for electrons. Theoretical calculations by members of the team were able to reproduce this complex relationship.

    “The pseudogap tends to eat away the electrons that want to go into the superconducting state,” explained Thomas Devereaux, a professor at Stanford and SLAC and co-author of the study. “The electrons are busy doing the dance of the pseudogap, and superconductivity is trying to cut in, but the electrons are not letting that happen. Then, as the material goes into the superconducting state, the pseudogap gives up and spits the electrons back out. That’s really the strongest evidence we have that this competition is occurring.”

    Remaining Mysteries

    Scientists still don’t know what causes the pseudogap, Devereaux said: “This remains one of the most important questions in the field, because it’s clearly preventing superconductors from working at even higher temperatures, and we don’t know why.”

    But the results pave new directions for further research, the scientists said.

    “Now we can model the competition between the pseudogap and superconductivity from the theoretical side, which was not possible before,” Hashimoto said. “We can use simulations to reproduce the kinds of features we have seen, and change the variables within those simulations to try to pin down what the pseudogap is.”

    He added, “Competition may be only one aspect of the relationship between the two states. There may be more profound questions – for example, whether the pseudogap is necessary for superconductivity to occur.”

    In addition to SLAC and Stanford, researchers from Lawrence Berkeley National Laboratory, Osaka University, the National Institute of Advanced Industrial Science and Technology in Japan, the Japan Atomic Energy Agency, Tokyo Institute of Technology, University of Tokyo and Cornell University contributed to the study. The research was supported by the DOE Office of Science.

    See the full article here.

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    SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the DOE’s Office of Science.
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  • richardmitnick 2:25 pm on November 12, 2014 Permalink | Reply
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    From SLAC: “Study at SLAC Explains Atomic Action in High-Temperature Superconductors “ 


    SLAC Lab

    November 12, 2014

    A study at the Department of Energy’s SLAC National Accelerator Laboratory suggests for the first time how scientists might deliberately engineer superconductors that work at higher temperatures.

    In their report, a team led by SLAC and Stanford University researchers explains why a thin layer of iron selenide superconducts — carries electricity with 100 percent efficiency — at much higher temperatures when placed atop another material, which is called STO for its main ingredients strontium, titanium and oxygen.

    bs
    In this illustration, a single layer of superconducting iron selenide (balls and sticks) has been placed stop another material known as STO for its main ingredients strontium, titanium and oxygen. The STO is shown as blue pyramids, which represent the arrangement of its atoms. A study at SLAC found that when natural vibrations (green glow) from the STO move up into the iron selenide film, electrons in the film (white spheres) can pair up and conduct electricity with 100 percent efficiency at much higher temperatures than before. The results suggest a way to deliberately engineer superconductors that work at even higher temperatures. (SLAC National Accelerator Laboratory)

    side
    This view from the side makes an important point: Putting iron selenide on top of STO enhances its superconductivity only if it’s applied in a single layer (left). When more than one layer is applied, the natural vibrations coming up from the STO layer don’t give electrons the boost of energy they need to pair up and superconduct (right). (SLAC National Accelerator Laboratory)

    These findings, described today in the journal Nature, open a new chapter in the 30-year quest to develop superconductors that operate at room temperature, which could revolutionize society by making virtually everything that runs on electricity much more efficient. Although today’s high-temperature superconductors operate at much warmer temperatures than conventional superconductors do, they still work only when chilled to minus 135 degrees Celsius or below.

    In the new study, the scientists concluded that natural trillion-times-per-second vibrations in the STO travel up into the iron selenide film in distinct packets, like volleys of water droplets shaken off by a wet dog. These vibrations give electrons the energy they need to pair up and superconduct at higher temperatures than they would on their own.

    “Our simulations indicate that this approach – using natural vibrations in one material to boost superconductivity in another – could be used to raise the operating temperature of iron-based superconductors by at least 50 percent,” said Zhi-Xun Shen, a professor at SLAC and Stanford University and senior author of the study.

    While that’s still nowhere close to room temperature, he added, “We now have the first example of a mechanism that could be used to engineer high-temperature superconductors with atom-by-atom control and make them better.”

    Spying on Electrons

    The study probed a happy combination of materials developed two years ago by scientists in China. They discovered that when a single layer of iron selenide film is placed atop STO, its maximum superconducting temperature shoots up from 8 degrees to nearly 77 degrees above absolute zero (minus 196 degrees Celsius).

    While this was a huge and welcome leap, it would be hard to build on this advance without understanding what, exactly, was going on.

    In the new study, SLAC Staff Scientist Rob Moore and Stanford graduate student J.J. Lee and postdoctoral researcher Felix Schmitt built a system for growing iron selenide films just one layer thick on a base of STO.

    The team examined the combined material at SLAC’s Stanford Synchrotron Radiation Lightsource, a DOE Office of Science User Facility. They used an exquisitely sensitive technique called ARPES to measure the energies and momenta of electrons ejected from samples hit with X-ray light. This tells scientists how the electrons inside the sample are behaving; in superconductors they pair up to conduct electricity without resistance. The researchers also got help from theorists who did simulations to help explain what they were seeing.

    SLAC SSRL
    SSRL at SLAC

    A Promising New Direction

    “This is a very impressive experiment, one that would have been very difficult to impossible to do anywhere else,” said Andrew Millis, a theoretical condensed matter physicist at Columbia University, who was not involved in the study. “And it’s clearly telling us something important about why putting one thin layer of iron selenide on this substrate, which everyone thought was inert and boring, changes things so dramatically. It opens lots of interesting questions, and it will definitely stimulate a lot of research.”

    Scientists still don’t know what holds electron pairs together so they can effortlessly carry current in high-temperature superconductors. With no way to deliberately invent new high-temperature superconductors or improve old ones, progress has been slow.

    The new results “point to a new direction that people have not considered before,” Moore said. “They have the potential to really break records in high-temperature superconductivity and give us a new understanding of things we’ve been struggling with for years.”

    He added that SLAC is developing a new X-ray beamline at SSRL with a more advanced ARPES system to create and study these and other exotic materials. “This paper predicts a new pathway to engineering superconductivity in these materials,” Moore said, “and we’re building the tools to do just that.”

    In addition to researchers from SLAC’s Materials Science Division and from Stanford, scientists from the University of British Columbia, the University of Tennessee, Lawrence Berkeley National Laboratory and the University of California, Berkeley contributed to this study. The work was funded by the DOE Office of Science.

    See the full article here.

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    SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the DOE’s Office of Science.
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  • richardmitnick 6:22 am on October 21, 2014 Permalink | Reply
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    From SLAC: “Puzzling New Behavior Found in High-Temperature Superconductors” 


    SLAC Lab

    October 20, 2014

    Ultimate Goal: A Super-efficient Way to Conduct Electricity at Room Temperature

    Research by an international team led by SLAC and Stanford scientists has uncovered a new, unpredicted behavior in a copper oxide material that becomes superconducting – conducting electricity without any loss – at relatively high temperatures.

    This new phenomenon – an unforeseen collective motion of electric charges coursing through the material – presents a challenge to scientists seeking to understand its origin and connection with high-temperature superconductivity. Their ultimate goal is to design a superconducting material that works at room temperature.

    “Making a room-temperature superconductor would save the world enormous amounts of energy,” said Thomas Devereaux, leader of the research team and director of the Stanford Institute for Materials and Energy Sciences (SIMES), which is jointly run with SLAC. “But to do that we must understand what’s happening inside the materials as they become superconducting. This result adds a new piece to this long-standing puzzle.”

    The results are published Oct. 19 in Nature Physics.

    Delving Into Doping Differences

    The researchers used an emerging X-ray technique called resonant inelastic X-ray scattering, or RIXS, to measure how the properties of a copper oxide change as extra electrons are added in a process known as doping. The team used the Swiss Light Source’s RIXS instrument, which currently has the world’s highest resolution and can reveal atomic-scale excitations – rapid changes in magnetism, electrical charge and other properties – as they move through the material.

    Copper oxide, a ceramic that normally doesn’t conduct electricity at all, becomes superconducting only when doped with other elements to add or remove electrons and chilled to low temperatures. Intriguingly, the electron-rich version loses its superconductivity when warmed to about 30 degrees above absolute zero (30 kelvins) while the electron-poor one remains superconducting up to 120 kelvins (minus 244 degrees Fahrenheit). One of the goals of the new research is to understand why they behave so differently.

    The experiments revealed a surprising increase of magnetic energy and the emergence of a new collective excitation in the electron-rich compounds, said Wei-sheng Lee, a SLAC staff scientist and lead author on the Nature Physics paper. “It’s very puzzling that these new electronic phenomena are not seen in the electron-poor material,” he said.

    wl
    SLAC Staff Scientist Wei-sheng Lee (SLAC National Accelerator Laboratory)

    Lee added that it’s unclear whether the new collective excitation is related to the ability of electrons to pair up and effortlessly conduct electricity – the hallmark of superconductivity – or whether it promotes or limits high-temperature superconductivity. Further insight can be provided by additional experiments using next-generation RIXS instruments that will become available in a few years at synchrotron light sources worldwide.

    A Long, Tortuous Path

    This discovery is the latest step in the long and tortuous path toward understanding high-temperature superconductivity.

    Scientists have known since the late 1950s why certain metals and simple alloys become superconducting when chilled within a few degrees of absolute zero: Their electrons pair up and ride waves of atomic vibrations that act like a virtual glue to hold the pairs together. Above a certain temperature, however, the glue fails as thermal vibrations increase, the electron pairs split up and superconductivity disappears.

    Starting in 1986, researchers discovered a number of materials that are superconducting at higher temperatures. By understanding and optimizing how these materials work, they hope to develop superconductors that work at room temperature and above.

    Until recently, the most likely glue holding superconducting electron pairs together at higher temperatures seemed to be strong magnetic excitations created by interactions between electron spins. But a recent theoretical simulation by SLAC and Stanford researchers concluded that these high-energy magnetic interactions are not the sole factor in copper oxide’s high-temperature superconductivity. The new results confirm that prediction, and also complement a 2012 report on the behavior of electron-poor copper oxides by a team that included Lee, Devereaux and several other SLAC/Stanford scientists.

    “Theorists must now incorporate this new ingredient into their explanations of how high-temperature superconductivity works,” said Thorsten Schmitt, leader of the RIXS team at the Paul Scherrer Institute in Switzerland, who collaborated on the study.

    Other researchers involved in the study were from Columbia University, University of Minnesota, AGH University of Science and Technology in Poland, National Synchrotron Radiation Research Center and National Tsing Hua University in Taiwan, and the Chinese Academy of Sciences. Funding for the research came from the DOE Office of Science, U.S. National Science Foundation and Swiss National Science Foundation.

    See the full article, with animation video, here.

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    SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the DOE’s Office of Science.
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  • richardmitnick 4:08 pm on April 25, 2014 Permalink | Reply
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    From SLAC Lab: “Scientists Watch High-temperature Superconductivity Emerge out of Magnetism” 

    SLAC Lab

    April 24, 2014
    Glennda Chui

    Like Dancers at a Party, Electrons Pair Up a Few at a Time to Effortlessly Conduct Electricity

    Scientists at SLAC National Accelerator Laboratory and Stanford University have shown for the first time how high-temperature superconductivity emerges out of magnetism in an iron pnictide, a class of materials with great potential for making devices that conduct electricity with 100 percent efficiency.

    In experiments at SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL), the team “doped” the material – one of two known types of high-temperature superconductor – by adding or subtracting electrons to enhance its superconducting abilities. Then they used a beam of ultraviolet light to measure changes in the material’s electronic behavior as it was chilled to a temperature where superconductivity becomes possible.

    levityation
    Superconducting materials expel magnetic fields, whose repulsive force can levitate a magnet, as shown here. Nevertheless, studies have shown superconductivity and magnetism can coexist in the same material. Now SLAC and Stanford researchers show that the two phases are interwoven at a very fine, microscopic level in a type of high-temperature superconductor known as an iron pnictide, and reveal how one phase gives way to the other. (Julien Bobroff, Frederic Bouquet and Jeffrey Quilliam/Laboratory of Solid State Physics, LPS, via Wikimedia Commons)

    The researchers saw the two states battle for dominance: At first the electrons in the material all lined up with their spins pointed in specific directions, a hallmark of magnetism. But as the temperature dropped, a few electrons paired up, like dancers at a party, to effortlessly conduct electricity; then a few more; until finally all the active electrons found partners and the material was fully superconducting, a much more complex behavior.

    The results, published April 25 in Nature Communications, are an important step toward understanding how high-temperature superconductors work – information scientists need to realize their dream of engineering superconductors with more useful properties that operate at close to room temperature for a variety of practical applications.

    Complexity Emerges from Simple Ingredients

    “For a while both magnetism and superconductivity co-exist; that’s not a surprise,” said Ming Yi, a graduate student with the Stanford Institute for Materials and Energy Sciences (SIMES) and lead author of the report. “But we wanted to see how just these two phases interact with each other. Now we finally have the high-resolution tools we need to see these changes at a microscopic level, and we find that the same electrons that were participating in the magnetic order have switched over to participate in the superconducting order. These two orders compete for the same electrons.’’

    Comparing their experimental data to the results of simulations, the researchers determined that the magnetism and superconductivity in iron pnictide were interwoven at a very fine, microscopic level, rather than occupying larger, separate puddles within the material. The simulations were led by theorists Lex Kemper of Lawrence Berkeley National Laboratory, Stanford graduate student Nachum Plonka and SIMES Director Thomas Devereaux.

    “This is a beautiful example of ‘emergence,’ in which simple ingredients give rise to complex behavior,” said co-author Zhi-Xun Shen, a professor at SLAC and Stanford and SLAC’s advisor for science and technology. “Emergence is a major theme of modern research on organizing principles of nature,” he said. “Our hope is that research on quantum systems like this one, which are very simple model systems, will eventually give us insights into such organizing principles.”

    Exploring a Mystery Material

    Discovered in 1986, high-temperature superconductors carry electricity without any loss at much warmer temperatures than conventional superconductors, which have to be chilled to at least 30 kelvins (minus 243 degrees Celsius). Still, scientists have not been able to get high-temperature superconductors to operate above minus 138 degrees Celsius.

    While these materials have the potential to save money and energy in a number of applications, from carrying electricity over long-distance power lines to operating maglev trains, the high cost and logistics of keeping them cold and their difficult-to-handle properties have held them back.

    As in regular superconductors, electrons in high-temperature superconductors form pairs to conduct current. But the mechanism behind this pairing in the high-temperature materials – the “glue” that holds the electrons together – is still unknown, said Donghui Lu, a senior staff scientist at SSRL and one of the principal investigators for the study.

    Another mystery: In theory, superconductivity and magnetism are not supposed to co-exist; the presence of one should drive out the other. But previous studies have shown they can in fact exist in the same material, and scientists have been eager to learn the details of how and why that happens.

    While this study doesn’t answer those burning questions, it does give scientists a closer look at the details of what happens as superconductivity emerges.

    The results may also shed light on the other known family of high-temperature superconductors, the copper-based cuprates, the scientists wrote, and comparing results from the two may lead to “an eventual understanding of the mechanism of unconventional superconductivity.”

    In addition to SLAC and SIMES, which is a joint SLAC/Stanford institute, researchers from Stanford University, Lawrence Berkeley National Laboratory, Nanjing University, and the University of California-Berkeley contributed to this work. Some measurements were carried out at Berkeley Lab’s Advanced Light Source. The work at Stanford, SLAC and the Advanced Light Source was funded by the U.S. Department of Energy Office of Science.

    See the full article here.

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    SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the DOE’s Office of Science.
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  • richardmitnick 12:53 pm on March 20, 2014 Permalink | Reply
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    From SLAC Lab: “Scientists Discover Potential Way to Make Graphene Superconducting” 

    March 20, 2014
    Press Office Contact:
    Andy Freeberg, afreeberg@slac.stanford.edu, (650) 926-4359

    Scientist Contact:
    Shuolong Yang, syang2@stanford.edu, (650) 725-0440

    Scientists at the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University have discovered a potential way to make graphene – a single layer of carbon atoms with great promise for future electronics – superconducting, a state in which it would carry electricity with 100 percent efficiency.

    graph

    Researchers used a beam of intense ultraviolet light to look deep into the electronic structure of a material made of alternating layers of graphene and calcium. While it’s been known for nearly a decade that this combined material is superconducting, the new study offers the first compelling evidence that the graphene layers are instrumental in this process, a discovery that could transform the engineering of materials for nanoscale electronic devices.

    “Our work points to a pathway to make graphene superconducting – something the scientific community has dreamed about for a long time, but failed to achieve,” said Shuolong Yang, a graduate student at the Stanford Institute of Materials and Energy Sciences (SIMES) who led the research at SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL).

    ssrl
    Stanford University / SLAC professor Zhi Xun Shen with a spectrometer at Stanford Synchrotron Radiation Lightsource (SSRL) Beamline 5-4.

    The researchers saw how electrons scatter back and forth between graphene and calcium, interact with natural vibrations in the material’s atomic structure and pair up to conduct electricity without resistance. They reported their findings March 20 in Nature Communications.

    Graphite Meets Calcium

    Graphene, a single layer of carbon atoms arranged in a honeycomb pattern, is the thinnest and strongest known material and a great conductor of electricity, among other remarkable properties. Scientists hope to eventually use it to make very fast transistors, sensors and even transparent electrodes.

    The classic way to make graphene is by peeling atomically thin sheets from a block of graphite, a form of pure carbon that’s familiar as the lead in pencils. But scientists can also isolate these carbon sheets by chemically interweaving graphite with crystals of pure calcium. The result, known as calcium intercalated graphite or CaC6, consists of alternating one-atom-thick layers of graphene and calcium.

    The discovery that CaC6 is superconducting set off a wave of excitement: Did this mean graphene could add superconductivity to its list of accomplishments? But in nearly a decade of trying, researchers were unable to tell whether CaC6’s superconductivity came from the calcium layer, the graphene layer or both.

    Observing Superconducting Electrons

    For this study, samples of CaC6 were made at University College London and brought to SSRL for analysis.

    “These are extremely difficult experiments,” said Patrick Kirchmann, a staff scientist at SLAC and SIMES. But the purity of the sample combined with the high quality of the ultraviolet light beam allowed them to see deep into the material and distinguish what the electrons in each layer were doing, he said, revealing details of their behavior that had not been seen before.

    “With this technique, we can show for the first time how the electrons living on the graphene planes actually superconduct,” said SIMES graduate student Jonathan Sobota, who carried out the experiments with Yang. “The calcium layer also makes crucial contributions. Finally we think we understand the superconducting mechanism in this material.”

    Although applications of superconducting graphene are speculative and far in the future, the scientists said, they could include ultra-high frequency analog transistors, nanoscale sensors and electromechanical devices and quantum computing devices.

    The research team was supervised by Zhi-Xun Shen, a professor at SLAC and Stanford and SLAC’s advisor for science and technology, and included other researchers from SLAC, Stanford, Lawrence Berkeley National Laboratory and University College London. The work was supported by the DOE’s Office of Science, the Engineering and Physical Sciences Research Council of UK and the Stanford Graduate Fellowship program.

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

    The Stanford Institute for Materials and Energy Sciences (SIMES) is a joint institute of SLAC National Accelerator Laboratory and Stanford University. SIMES studies the nature, properties and synthesis of complex and novel materials in the effort to create clean, renewable energy technologies. For more information, visit simes.slac.stanford.edu.

    SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL) is a third-generation light source producing extremely bright X-rays for basic and applied science. A DOE national user facility, SSRL attracts and supports scientists from around the world who use its state-of-the-art capabilities to make discoveries that benefit society. For more information, visit ssrl.slac.stanford.edu.

    DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, visit science.energy.gov.

    See the full article here.

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    SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the DOE’s Office of Science.
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  • richardmitnick 5:15 pm on February 16, 2014 Permalink | Reply
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    From SLAC: “New ‘Pomegranate-inspired’ Design Solves Problems for Lithium-Ion Batteries” 

    February 16, 2014
    No Writer Credit

    An electrode designed like a pomegranate – with silicon nanoparticles clustered like seeds in a tough carbon rind – overcomes several remaining obstacles to using silicon for a new generation of lithium-ion batteries, say its inventors at Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory.

    nano
    Transmission electron microscopy(a, b, and c) images of prepared mesoporous silica nanoparticles with mean outer diameter: (a) 20nm, (b) 45nm, and (c) 80nm. Scanning electron microscope(d) image corresponding to (b). The insets are a high magnification of mesoporous silica particle.

    “While a couple of challenges remain, this design brings us closer to using silicon anodes in smaller, lighter and more powerful batteries for products like cell phones, tablets and electric cars,” said Yi Cui, an associate professor at Stanford and SLAC who led the research, reported today in Nature Nanotechnology.

    “Experiments showed our pomegranate-inspired anode operates at 97 percent capacity even after 1,000 cycles of charging and discharging, which puts it well within the desired range for commercial operation.”

    The anode, or negative electrode, is where energy is stored when a battery charges. Silicon anodes could store 10 times more charge than the graphite anodes in today’s rechargeable lithium-ion batteries, but they also have major drawbacks: The brittle silicon swells and falls apart during battery charging, and it reacts with the battery’s electrolyte to form gunk that coats the anode and degrades its performance.

    Over the past eight years, Cui’s team has tackled the breakage problem by using silicon nanowires or nanoparticles that are too small to break into even smaller bits and encasing the nanoparticles in carbon yolk shells that give them room to swell and shrink during charging.

    The new study builds on that work. Graduate student Nian Liu and postdoctoral researcher Zhenda Lu used a microemulsion technique common in the oil, paint and cosmetic industries to gather silicon yolk shells into clusters, and coated each cluster with a second, thicker layer of carbon. These carbon rinds hold the pomegranate clusters together and provide a sturdy highway for electrical currents.

    And since each pomegranate cluster has just one-tenth the surface area of the individual particles inside it, a much smaller area is exposed to the electrolyte, thereby reducing the amount of gunk that forms to a manageable level.

    Although the clusters are too small to see individually, together they form a fine black powder that can be used to coat a piece of foil and form an anode. Lab tests showed that pomegranate anodes worked well when made in the thickness required for commercial battery performance.

    While these experiments show the technique works, Cui said, the team will have to solve two more problems to make it viable on a commercial scale: They need to simplify the process and find a cheaper source of silicon nanoparticles. One possible source is rice husks: They’re unfit for human food, produced by the millions of tons and 20 percent silicon dioxide by weight. According to Liu, they could be transformed into pure silicon nanoparticles relatively easily, as his team recently described in Scientific Reports.

    “To me it’s very exciting to see how much progress we’ve made in the last seven or eight years,” Cui said, “and how we have solved the problems one by one.”

    The research team also included Jie Zhao, Matthew T. McDowell, Hyun-Wook Lee and Wenting Zhao of Stanford. Cui is a member of the Stanford Institute for Materials and Energy Sciences [SIMES], a joint SLAC/Stanford institute. The research was funded by the DOE Office of Energy Efficiency and Renewable Energy.

    See the full article here.

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    SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the DOE’s Office of Science.
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  • richardmitnick 10:36 am on June 17, 2013 Permalink | Reply
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    From SLAC Lab: “SLAC/Stanford Scientists Make First Direct Images of Topological Insulator’s Edge Currents” 

    June 17, 2013
    Mike Ross

    “Researchers at a SLAC/Stanford institute have made the first direct images of electrical currents flowing along the edges of a topological insulator – a recently discovered state of matter with potential applications in information technology.

    grid
    This graphic depicts the tiny loop of a scanning SQUID, or superconducting quantum interference device (silver), which detects magnetic fields (red) created by an edge current (blue) in a topological insulator. (Greg Stewart)

    In these strange solid-state materials, currents flow only along the edges of a sample while avoiding the interior. Using an exquisitely sensitive detector they built, scientists from the Stanford Institute for Materials and Energy Sciences (SIMES) were able to sense the weak magnetic fields generated by the edge currents and tell exactly where the currents were flowing.

    ‘Now no one can doubt that they exist,’ said Kathryn A. “Kam” Moler, the SIMES and Stanford University physics professor who led the research, which was published Sunday in Nature Materials.

    The scientists surveyed a tiny rectangular piece of mercury telluride, a semiconductor that becomes a topological insulator when cooled to nearly absolute zero in the presence of an electric field.

    Post-doctoral researcher Katja Nowack and graduate student Eric Spanton, first and second authors on the team’s research report, scanned the sample surface with a SQUID, or superconducting quantum interference device – a microscope for detecting magnetic fields. The team’s custom-made SQUID was 100 million times more sensitive to magnetic moments than the best commercial version.”

    See the full article here.

    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.

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  • richardmitnick 9:33 am on March 20, 2013 Permalink | Reply
    Tags: , , , SLAC SIMES,   

    From SLAC: “X-ray Laser Explores How to Write Data with Light” 

    March 19, 2013
    Glenn Roberts Jr.

    “Using laser light to read and write magnetic data by quickly flipping tiny magnetic domains could help keep pace with the demand for faster computing devices.

    chamber
    A look inside the RCI sample chamber while researchers close up the chamber for vacuum for an experiment at LCLS. (Credit: Diling Zhu/SLAC)

    Now experiments with SLAC’s Linac Coherent Light Source (LCLS) X-ray laser have given scientists their first detailed look at how light controls the first trillionth of a second of this process, known as all-optical magnetic switching.

    The experiments show that the optically induced switching of the magnetic regions begins much faster than conventional switching and proceeds in a more complex way than scientists had thought – a level of detail long sought by the data storage industry, which is eager to learn more about the key drivers of optical switching. The new insight could help guide efforts to engineer materials that better control and speed this process.

    group
    Group photo of researchers who participated in an all-optical magnetic switching experiment at the Linac Coherent Light Source. (Credit: SLAC)

    image

    ‘This is really one of the first examples of new materials science that can be done with LCLS, which allows you to look at very short time scales and very small length scales,’ said Hermann Dürr, a staff scientist for the Stanford Institute for Materials and Energy Sciences (SIMES) and a principal investigator of the multinational team that performed the experiment, detailed in the March 17 issue of Nature Materials. SIMES is a joint institute of SLAC and Stanford.”

    See the full article here.

    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.
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    • Jaime 3:31 pm on May 18, 2013 Permalink | Reply

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  • richardmitnick 7:04 pm on March 19, 2013 Permalink | Reply
    Tags: , , , , SLAC SIMES   

    From SLAC: “Materials Scientists Make Solar Energy Chip 100 Times More Efficient” 

    March 19, 2013
    Mike Ross

    “Scientists working at the Stanford Institute for Materials and Energy Sciences (SIMES) have improved an innovative solar-energy device to be about 100 times more efficient than its previous design in converting the sun’s light and heat into electricity.

    ‘This is a major step toward making practical devices based on our technique for harnessing both the light and heat energy provided by the sun,’ said Nicholas Melosh, associate professor of materials science and engineering at Stanford and a researcher with SIMES, a joint SLAC/Stanford institute.

    two
    Nick Melosh (left), associate professor of materials science and engineering at Stanford and a researcher with SIMES, and graduate student Jared Schwede. (Credit: Brad Plummer / SLAC)

    The new device is based on the photon-enhanced thermionic emission (PETE) process first demonstrated in 2010 by a group led by Melosh and SIMES colleague Zhi-Xun Shen, who is SLAC’s advisor for science and technology. In a report last week in Nature Communications, the group described how they improved the device’s efficiency from a few hundredths of a percent to nearly 2 percent, and said they expect to achieve at least another 10-fold gain in the future.”

    chip
    Part of a 2-inch-diameter gallium-arsenide wafer used as a base for photon-enhanced thermionic emission chips. (Credit: Brad Plummer / SLAC)

    This is exciting news for Clean Energy. See the full article here.

    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.
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  • richardmitnick 11:14 am on July 20, 2012 Permalink | Reply
    Tags: , , , , SLAC SIMES   

    From SLAC Today: “Phrase of the Week: Thermionic Emission” 

    July 20, 2012
    Mike Ross

    If you heat materials to a high enough temperature, some of their electrons will gain enough kinetic energy to literally boil off the surface and into the air or vacuum beyond. Since net motion of electrons constitutes an electrical current, this phenomena, called thermionic emission, is one of the seven basic methods for producing electricity.

    image
    (Image courtesy tpub.com)

    Thermionic emission is at the heart of a new approach to solar energy harvesting pioneered by Stanford Institute of Materials and Energy Sciences [SIMES] researchers that promises unprecedented efficiency by taking advantage of the improved performances of thermionic and thermal processes at high temperatures.”

    See the full article here.

    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.
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