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  • richardmitnick 11:55 am on June 11, 2017 Permalink | Reply
    Tags: , Dr. Binghai Yan, Spintronics, Topological materials,   

    From Weizmann: “Physics on the edge” 

    Weizmann Institute of Science logo

    Weizmann Institute of Science

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    Dr. Binghai Yan

    Dr. Binghai Yan is taking topological materials higher.

    Creating new materials for everyday life—think bendable electronics, quantum computers, life-saving medical devices and things we haven’t yet dreamed of—requires understanding and creatively brainstorming new possibilities at the atomic level.

    This is the essence Dr. Binghai Yan’s research. His field is topological materials, which is fusing theoretical science with practical engineering and taking the physics world by storm. And yes, his name gives away the other special news: he is the first principal investigator from China hired by the Weizmann Institute.

    Topological materials and states involve a kind of order very different from conventional bulk materials in that electrons (and their lattices of atoms and molecules) on the surface of a crystal or other material behave differently than those in the material itself. In is the special nature of such topological materials and states that can be leveraged for the creation of new materials. He straddles the world of theory—how such states could work—and experimentation—trying out the materials to synthesize new materials and devices such as quantum computers.

    From rural fields to topology

    So how did a Chinese physicist who grew up in a remote farming village in Shandong Province in eastern China make his way to the Weizmann Institute?

    After completing his BSc at Xi’an Jiatong University in Xi’an in 2003, he earned a PhD in physics at the Tsinghua University in Beijing in 2008. He did postdoctoral research at the University of Bremen in Germany, when the field of topological research was beginning to take off. But it was still a relatively niche subject in which few physicists were working. Thanks to a flexible postdoc grant, the prestigious Humboldt Research Fellowship, which allowed him to spend time at other institutions, he spent eight months at Stanford University learning from a leading expert in the field.

    He returned to Germany to become a group leader (the equivalent of a principal investigator) at the Max Planck Institute for Chemical Physics and Solids in Dresden. It was then that he began collaborating with Weizmann Institute colleagues—thanks to an introduction by Prof. Ady Stern at a conference in Germany—including Prof. Erez Berg and Dr. Haim Beidenkopf, all from the Department of Condensed Matter Physics. The collaboration was enabled by an ARCHES Award given by Germany’s Minerva Foundation, which stimulates collaborative projects by German and Israeli scientists. He visited the Weizmann Institute for the first time in 2013 to advance this work.

    The project and the visit were a “fantastic opportunity,” he says, because his Weizmann collaborators were both theoreticians and experimentalists who were eager to learn about the material he was working on—and Dr. Yan needed feedback from theory to advance his investigations by predicting possible new materials and actualize his ideas in experiments. “I immediately realized that we have lots to do,” he says. “Together, we are able to bridge fundamental physics and experimentation.”

    Last year, he received a competing offer from a university in China, but took the Weizmann offer “because of my existing collaborations and potential collaborations, the depth of theory and experiment work here, and the fact that Weizmann is one of the few places that is advancing this field,” he says.

    Dr. Yan has already discovered a new class of topological materials: a three-dimensional, layered, metallic insulating material which he grows in the lab. He has done so by way of his expertise in electron charge and spin, and so this research has implications for the new, hot field of “spintronics”. Spintronics differs from traditional electronics in that it leverages the way in which electrons spin—not only their charge—to find better efficiency with data storage and transfer. This, in turn, has relevance for the new age of quantum computing, and he hopes to collaborate with quantum computing pioneers at the Institute.

    For his wife, Huanhuan Wang, the decision to make a potentially permanent move to Israel—a country she’d never before visited and about which she had little knowledge—was not as obvious as it was for Dr. Yan. “It took a little bit of convincing my wife to come; if you’ve never been here, all you think is political strife,” says Dr. Yan. “But the reality is different. We are really happy here and it is quickly starting to feel like home.”

    The family arrived in February and moved into campus housing. His wife is now pursuing a PhD under the guidance of Prof. Dan Yakir in the Department of Plant and Environmental Sciences. They have two kids, a boy and a girl, who just began learning German, and now are getting used to Hebrew—and they speak Chinese at home.

    Dr. Yan is finding opportunities to collaborate with scientists in Germany and China, and has already begun organizing a workshop on topological systems at the Weizmann Institute (together with Dr. Haim Beidenkopf and Dr. Nurit Avraham, also of the Department of Physics of Condensed Matter Physics), to which he has invited leading European and Chinese physicists and other leaders in the field.

    “Being in Israel, at Weizmann, is not something that I would have anticipated five or 10 years ago,” he says. “But life—like the materials of the future—holds many mysteries.”

    Dr. Yan is supported by the Ruth and Herman Albert Scholars Program for New Scientists.

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    Weizmann Institute Campus

    The Weizmann Institute of Science is one of the world’s leading multidisciplinary research institutions. Hundreds of scientists, laboratory technicians and research students working on its lushly landscaped campus embark daily on fascinating journeys into the unknown, seeking to improve our understanding of nature and our place within it.

    Guiding these scientists is the spirit of inquiry so characteristic of the human race. It is this spirit that propelled humans upward along the evolutionary ladder, helping them reach their utmost heights. It prompted humankind to pursue agriculture, learn to build lodgings, invent writing, harness electricity to power emerging technologies, observe distant galaxies, design drugs to combat various diseases, develop new materials and decipher the genetic code embedded in all the plants and animals on Earth.

    The quest to maintain this increasing momentum compels Weizmann Institute scientists to seek out places that have not yet been reached by the human mind. What awaits us in these places? No one has the answer to this question. But one thing is certain – the journey fired by curiosity will lead onward to a better future.

     
  • richardmitnick 4:33 pm on June 2, 2017 Permalink | Reply
    Tags: , , Magnetocapacitance, , Spintronics   

    From Brown: “Researchers flip the script on magnetocapacitance” 

    Brown University
    Brown University

    June 1, 2017
    Kevin Stacey
    kevin_stacey@brown.edu

    1
    A new study shows that anti-parallel electron spins between two electrodes create more capacitance than parallel spins, which is opposite of what is normally observed.

    The study demonstrates for the first time a new type of magnetocapacitance, a phenomenon that could be useful in the next generation of ‘spintronic’ devices.

    Capacitors, electronic components that store and quickly release a charge, play an important role in many types of electrical circuits. They’ll play an equally important role in next-generation spintronic devices, which take advantage of not only electron charge but also spin — the tiny magnetic moment of each electron.

    Two years ago, an international team of researchers showed that by manipulating electron spin at a quantum magnetic tunneling junction — a nanoscale sandwich made of two metal electrodes with an insulator in the middle — they could induce a large increase in the junction’s capacitance.

    Now, that same research team has flipped the script on the phenomenon, known as magnetocapacitance. In a paper published in the journal Scientific Reports, they show that by using different materials to build a quantum tunneling junction, they were able to alter capacitance by manipulating spins in the opposite way from “normal” magnetocapacitance. This inverse effect, the researchers say, adds one more potentially useful phenomenon to the spintronics toolkit.

    “It gives us more parameter space to design devices,” said Gang Xiao, chair of the physics department at Brown and one of the paper’s coauthors. “Sometimes normal capacitance might be better; sometimes the inverse might be better, depending on the application. This gives us a bit more flexibility.”

    Magnetocapacitors could be especially useful, Xiao says, in making magnetic sensors for a range of different spintronic devices, including computer hard drives and next-generation random access memory chips.

    The research was a collaboration between Xiao’s lab at Brown, the lab of Hideo Kaiju and Taro Nagahama at Japan’s Hokkaido University and the lab of Osamu Kitakami at Tohoku University.

    Xiao has been investigating magnetic tunneling junctions for several years. The tiny junctions can work in much the same way as capacitors in standard circuits. The insulator between the two conducting electrodes slows the free flow of current across the junction, creating resistance and another phenomenon, capacitance.

    But what makes tunneling junctions especially interesting is that the amount of capacitance can be changed dynamically by manipulating the spins of the electrons within the two metal electrodes. The electrodes are magnetic, meaning that electrons spinning within each electrode are pointed in one particular direction. The relative spin direction between two electrodes determines how much capacitance is present at the junction.

    In their initial work on this phenomenon, Xiao and the research team showed just how large the change in capacitance could be. Using electrodes made of iron-cobalt-boron, they showed that by flipping spins from anti-parallel to parallel, they could increase capacitance in experiments by 150 percent. Based on those results, the team developed a theory predicting that, under ideal conditions, the change in capacitance could actually go as high as 1,000 percent.

    The theory also suggested that using electrodes made from different types of metals would create an inverse magnetocapacitance effect, one in which anti-parallel spins create more capacitance than parallel spins. That’s exactly what they showed in this latest study.

    “We used iron for one electrode and iron oxide for the other,” Xiao said. “The electrical properties of the two are mirror images of each other, which is why we observed this inverse magnetocapacitance effect.”

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    Iron oxide and iron have different cation orientations in their crystalline structure, which causes them to have inverse electrical properties.

    Xiao says the findings not only suggest a larger parameter space for the use of magnetocapacitance in spintronic devices, they also provide important verification for the theory scientists use to explain the phenomenon.

    “Now we see that the theories fit well with the experiment, so we can be confident in using our theoretical models to maximize these effects, either the ‘normal’ effect or the inverse effect that we have demonstrated here,” Xiao said.

    The work was supported by the National Science Foundation (DMR-1307056), the Japan Society for the Promotion of Science (Grant-in-Aid for Scientific Research (B), 15H03981), the Japanese Ministry of Education, Culture, Sports, Science and Technology (Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials) and the Center for Spintronics Research Network at Tohoku University.

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    Welcome to Brown

    Brown U Robinson Hall
    Located in historic Providence, Rhode Island and founded in 1764, Brown University is the seventh-oldest college in the United States. Brown is an independent, coeducational Ivy League institution comprising undergraduate and graduate programs, plus the Alpert Medical School, School of Public Health, School of Engineering, and the School of Professional Studies.

    With its talented and motivated student body and accomplished faculty, Brown is a leading research university that maintains a particular commitment to exceptional undergraduate instruction.

    Brown’s vibrant, diverse community consists of 6,000 undergraduates, 2,000 graduate students, 400 medical school students, more than 5,000 summer, visiting and online students, and nearly 700 faculty members. Brown students come from all 50 states and more than 100 countries.

    Undergraduates pursue bachelor’s degrees in more than 70 concentrations, ranging from Egyptology to cognitive neuroscience. Anything’s possible at Brown—the university’s commitment to undergraduate freedom means students must take responsibility as architects of their courses of study.

     
  • richardmitnick 3:26 pm on May 30, 2017 Permalink | Reply
    Tags: , Organic-inorganic hybrid perovskites, Spintronics, U Utah   

    From U Utah: “A new spin on electronics” 

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    University of Utah

    A University of Utah-led team has discovered that a class of “miracle materials” called organic-inorganic hybrid perovskites could be a game changer for future spintronic devices.

    Spintronics uses the direction of the electron spin — either up or down — to carry information in ones and zeros. A spintronic device can process exponentially more data than traditional electronics that use the ebb and flow of electrical current to generate digital instructions. But physicists have struggled to make spintronic devices a reality.

    The new study, published online today in Nature Physics, is the first to show that organic-inorganic hybrid perovskites are a promising material class for spintronics. The researchers discovered that the perovskites possess two contradictory properties necessary to make spintronic devices work — the electrons’ spin can be easily controlled, and can also maintain the spin direction long enough to transport information, a property known as spin lifetime.

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    Sarah Li (left) and Z. Valy Vardeny (right) stand behind the area where they prepared the film sample of the hybrid perovskite methyl-ammonium lead iodine (CH3NH3PbI3). The researchers’ new study is the first to show that the material is a promising candidate for spintronics, an alternative to conventional electronics. Spintronics uses the spin of the electron itself to carry information, rather than the electron’s charge. Photo credit: University of Utah

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    The ultrafast laser shoots very short light pulses 80 million times a second at the hybrid perovskite material to determine whether its electrons could be used to carry information in future devices. They split the laser into two beams; the first one hits the film to set the electron spin in the desired direction. The second beam bends through a series of mirrors like a pin ball machine before hitting the perovskite film at increasing time intervals to measure how long the electron held the spin in the prepared direction. Photo credit: University of Utah

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    The University of Utah (also referred to as the U, the U of U, or Utah) is a public coeducational space-grant research university in Salt Lake City, Utah, United States. As the state’s flagship university, the university offers more than 100 undergraduate majors and more than 92 graduate degree programs. The university is classified in the highest ranking: “R-1: Doctoral Universities – Highest Research Activity” by the Carnegie Classification of Institutions of Higher Education. The Carnegie Classification also considers the university as “selective”, which is its second most selective admissions category. Graduate studies include the S.J. Quinney College of Law and the School of Medicine, Utah’s only medical school. As of Fall 2015, there are 23,909 undergraduate students and 7,764 graduate students, for an enrollment total of 31,673.

    The university was established in 1850 as the University of Deseret (Listeni/dɛz.əˈrɛt./[12]) by the General Assembly of the provisional State of Deseret, making it Utah’s oldest institution of higher education.It received its current name in 1892, four years before Utah attained statehood, and moved to its current location in 1900.

    The university ranks among the top 50 U.S. universities by total research expenditures with over $486 million spent in 2014. 22 Rhodes Scholars,[14] three Nobel Prize winners, two Turing Award winners, three MacArthur Fellows, various Pulitzer Prize winners, two astronauts, Gates Cambridge Scholars, and Churchill Scholars have been affiliated with the university as students, researchers, or faculty members in its history. In addition, the university’s Honors College has been reviewed among 50 leading national Honors Colleges in the U.S. The university has also been ranked the 12th most ideologically diverse university in the country.

     
  • richardmitnick 7:27 am on May 23, 2017 Permalink | Reply
    Tags: , Dirac fermions, , Force-carrying bosons, , , , Spintronics, Universiteit Leiden,   

    From Universiteit Leiden via phys.org: “Weyl fermions exhibit paradoxical behavior” 

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    Universiteit Leiden

    phys.org

    May 23, 2017
    No writer credit found

    1
    Credit: Leiden Institute of Physics

    Theoretical physicists have found Weyl fermions to exhibit paradoxical behavior in contradiction to a 30-year-old fundamental theory of electromagnetism. The discovery has possible applications in spintronics. The study has been published in Physical Review Letters.

    Physicists divide the world of elementary particles into two groups. On one side are force-carrying bosons, and on the other there are so-called fermions. The latter group comes in three different flavors. Dirac fermions are the most famous, comprising all matter. Physicists recently discovered Majorana fermions, which might form the basis of future quantum computers. Lastly, Weyl fermions exhibit weird behavior in, for example, electromagnets, which has sparked the interest of Prof. Carlo Beenakker’s theoretical physics group.

    Electromagnets

    Conventional electromagnets work on the interplay between electrical currents and magnetic fields. Inside a dynamo, a rotating magnet generates electricity, and vice versa: Moving electrical charges in a wire wrapped around a metal bar will induce a magnetic field. Paradoxically, an electric current produced within the bar in the same direction would produce a magnetic field around it, in turn generating a current in the opposite direction, and the whole system would collapse.

    Oddly enough, Beenakker and his group have found cases where this does actually happen. Following an idea from collaborator Prof. İnanç Adagideli (Sabanci University), Ph.D. student Thomas O’Brien built a computer simulation showing that materials harboring Weyl fermions actually exhibit this weird behavior. This has been observed before, but only at artificially short timescales, when the system didn’t get time to correct for the anomaly. The Leiden/Sabanci collaboration showed that in special circumstances—at temperatures close to absolute zero when materials become superconducting—the strange scenario occurs indefinitely.

    Until now, physicists considered this to be impossible due to underlying symmetries in the models used. That gives the discovery fundamental significance. “We study Weyl fermions mainly out of a fundamental interest,” says O’Brien. “Still, this research gives more freedom in the use of magnetism and materials. Perhaps the additional flexibility in a Weyl semimetal will be of use in future electronics design.”

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    Leiden University was founded in 1575 and is one of Europe’s leading international research universities. It has seven faculties in the arts, sciences and social sciences, spread over locations in Leiden and The Hague. The University has over 6,500 staff members and 26,900 students. The motto of the University is ‘Praesidium Libertatis’ – Bastion of Freedom.

     
  • richardmitnick 10:19 am on October 17, 2016 Permalink | Reply
    Tags: , , , , Spintronics,   

    From John A Paulson School of Engineering and Applied Sciences: “A new spin on superconductivity” 

    Harvard School of Engineering and Applied Sciences
    John A Paulson School of Engineering and Applied Sciences

    October 14, 2016
    Leah Burrows

    1

    Researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have made a discovery that could lay the foundation for quantum superconducting devices. Their breakthrough solves one the main challenges to quantum computing: how to transmit spin information through superconducting materials.

    Every electronic device — from a supercomputer to a dishwasher — works by controlling the flow of charged electrons. But electrons can carry so much more information than just charge; electrons also spin, like a gyroscope on axis.

    Harnessing electron spin is really exciting for quantum information processing because not only can an electron spin up or down — one or zero — but it can also spin any direction between the two poles. Because it follows the rules of quantum mechanics, an electron can occupy all of those positions at once. Imagine the power of a computer that could calculate all of those positions simultaneously.

    A whole field of applied physics, called spintronics, focuses on how to harness and measure electron spin and build spin equivalents of electronic gates and circuits.

    By using superconducting materials through which electrons can move without any loss of energy, physicists hope to build quantum devices that would require significantly less power.

    But there’s a problem.

    According to a fundamental property of superconductivity, superconductors can’t transmit spin. Any electron pairs that pass through a superconductor will have the combined spin of zero.

    In work published recently in Nature Physics, the Harvard researchers found a way to transmit spin information through superconducting materials.

    “We now have a way to control the spin of the transmitted electrons in simple superconducting devices,” said Amir Yacoby, Professor of Physics and of Applied Physics at SEAS and senior author of the paper.

    It’s easy to think of superconductors as particle super highways but a better analogy would be a super carpool lane as only paired electrons can move through a superconductor without resistance.

    These pairs are called Cooper Pairs and they interact in a very particular way. If the way they move in relation to each other (physicists call this momentum) is symmetric, then the pair’s spin has to be asymmetric — for example, one negative and one positive for a combined spin of zero. When they travel through a conventional superconductor, Cooper Pairs’ momentum has to be zero and their orbit perfectly symmetrical.

    But if you can change the momentum to asymmetric — leaning toward one direction — then the spin can be symmetric. To do that, you need the help of some exotic (aka weird) physics.

    Superconducting materials can imbue non-superconducting materials with their conductive powers simply by being in close proximity. Using this principle, the researchers built a superconducting sandwich, with superconductors on the outside and mercury telluride in the middle. The atoms in mercury telluride are so heavy and the electrons move so quickly, that the rules of relativity start to apply.

    “Because the atoms are so heavy, you have electrons that occupy high-speed orbits,” said Hechen Ren, coauthor of the study and graduate student at SEAS. “When an electron is moving this fast, its electric field turns into a magnetic field which then couples with the spin of the electron. This magnetic field acts on the spin and gives one spin a higher energy than another.”

    So, when the Cooper Pairs hit this material, their spin begins to rotate.

    “The Cooper Pairs jump into the mercury telluride and they see this strong spin orbit effect and start to couple differently,” said Ren. “The homogenous breed of zero momentum and zero combined spin is still there but now there is also a breed of pairs that gains momentum, breaking the symmetry of the orbit. The most important part of that is that the spin is now free to be something other than zero.”

    The team could measure the spin at various points as the electron waves moved through the material. By using an external magnet, the researchers could tune the total spin of the pairs.

    “This discovery opens up new possibilities for storing quantum information. Using the underlying physics behind this discovery provides also new possibilities for exploring the underlying nature of superconductivity in novel quantum materials,” said Yacoby.

    This research was coauthored by Sean Hart, Michael Kosowsky, Gilad Ben-Shach, Philipp Leubner, Christoph Brüne, Hartmut Buhmann, Laurens W. Molenkamp and Bertrand I. Halperin.

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    Through research and scholarship, the Harvard School of Engineering and Applied Sciences (SEAS) will create collaborative bridges across Harvard and educate the next generation of global leaders. By harnessing the power of engineering and applied sciences we will address the greatest challenges facing our society.

    Specifically, that means that SEAS will provide to all Harvard College students an introduction to and familiarity with engineering and technology as this is essential knowledge in the 21st century.

    Moreover, our concentrators will be immersed in the liberal arts environment and be able to understand the societal context for their problem solving, capable of working seamlessly withothers, including those in the arts, the sciences, and the professional schools. They will focus on the fundamental engineering and applied science disciplines for the 21st century; as we will not teach legacy 20th century engineering disciplines.

    Instead, our curriculum will be rigorous but inviting to students, and be infused with active learning, interdisciplinary research, entrepreneurship and engineering design experiences. For our concentrators and graduate students, we will educate “T-shaped” individuals – with depth in one discipline but capable of working seamlessly with others, including arts, humanities, natural science and social science.

    To address current and future societal challenges, knowledge from fundamental science, art, and the humanities must all be linked through the application of engineering principles with the professions of law, medicine, public policy, design and business practice.

    In other words, solving important issues requires a multidisciplinary approach.

    With the combined strengths of SEAS, the Faculty of Arts and Sciences, and the professional schools, Harvard is ideally positioned to both broadly educate the next generation of leaders who understand the complexities of technology and society and to use its intellectual resources and innovative thinking to meet the challenges of the 21st century.

    Ultimately, we will provide to our graduates a rigorous quantitative liberal arts education that is an excellent launching point for any career and profession.

     
  • richardmitnick 4:17 pm on May 15, 2016 Permalink | Reply
    Tags: , Data storage, , Spintronics   

    From Science Alert: “New ‘spintronics’ technology is set to offer a huge leap forward in data storage” 

    ScienceAlert

    Science Alert

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    Oleksiy Mark/Shutterstock.com

    14 MAY 2016
    DAVID NIELD

    When it comes to digital data storage, our thirst is unquenchable: today’s most advanced software programs, from neural network simulators to weather forecasting applications, need more room than ever before, and that’s before you get to all the photos, music, and videos we’re generating every single day.

    But now an international team of researchers working at the Diamond Light Source in the UK think they might have found the answer to our ongoing storage needs in a technology called spintronics, which offers some useful properties for recording our 1s and 0s, including significant boosts in data storage density.

    For the first time, the researchers looked at the potential of using antiferromagnetic materials as magnetic storage media.

    Each atom has a property called ‘spin’, which you can think of as a microscopic bar magnet, with a north and south pole. In antiferromagnetic materials, these spins are arranged head-to-toe, cancelling out any external magnetic field.

    Normally, data bits are stored by using an electric current to flip the orientation of the spins. As the arrangement in antiferromagnetic materials does not allow this, the researchers have come up with a novel way of rotating pairs of spins rather than flipping them to store data.

    Basically, the spin of electrons rather than their charge is used to map out the 1s and 0s of our data.

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    Diamond Light Source

    That’s the science, but here are the main benefits. First of all, there’s no density change in the current compared with the data storage devices we use today, so upgrading should become easier. Secondly, the antiferromagnetic materials don’t emit an external magnetic field, so there’s less chance of devices interfering with other equipment or being spied on.

    A lack of magnetic field also means individual regions can be packed very close together, giving us more data in less space. Information can also remain intact without power, and can be written and read at room temperature – all boxes that need ticking when you’re investigating the potential of a next-generation storage technology.

    The spintronics approach has the potential to increase read and write times by a factor of up to 1,000, according to the team, and the materials we’re talking about are pretty straightforward to make. The team has so far been experimenting with a thin film of antiferromagnetic copper manganese arsenide (CuMnAs) but think more suitable materials are also out there.

    It’s going to be a good while before spintronics arrive in laptops and phones, but the potential is definitely there. “The physics is beautiful but complex yet the practice is relatively simple,” said the lead researcher on the project, Peter Wadley.

    The team’s work has been published in Science.

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  • richardmitnick 7:16 pm on December 21, 2015 Permalink | Reply
    Tags: , , New topological insulators, Spintronics   

    From EPFL: “Spintronics, low-energy electricity take a step closer 

    EPFL bloc

    École Polytechnique Fédérale de Lausanne EPFL

    18.12.15
    Nik Papageorgiou

    1
    EPFL scientists have discovered a new topological insulator that could be used in future electronic technologies.

    Topological insulators are recently discovered materials that differ from the familiar insulators and semiconductors in many ways. While topological insulators are fascinating for fundamental physics, they could one day enable electricity with less energy loss, spintronics, and perhaps even quantum computing. Combining theory with experiment, EPFL scientists have now identified bismuth iodide as a topological insulator and the first representative of a whole new structural class of materials that could propel topological insulators into applications. The work, which was carried out within the framework of the EPFL-led NCCR Marvel project, is published in Nature Materials.

    The novel physical properties of topological insulators make them interesting as a conceptually new component in electronic devices. Most ideas for future technologies involve dissipation-less currents: if they are ever integrated into electrical circuits, topological insulators could greatly reduce energy losses. Added to this is the potential for faster, “spintronics” technologies that run on electron spin rather than charge. And finally, topological insulators might one day become the cornerstone of quantum computing.

    All this has lead to a great search for optimal topological insulators, including both natural and man-made materials. Such research, as the kind performed within the NCCR Marvel project, combines theoretical work that predicts what properties the structure of a particular material would have. The “candidate” materials that are identified with computer simulations are then passed for experimental examination to see if their topological insulating properties match the theoretical predictions.

    This is what the lab of Oleg Yazyev at EPFL’s Institute of Theoretical Physics has accomplished, working with experimentalist colleagues from around the world. By theoretically testing potential candidates from the database of previously described materials, the team has identified a material, described as a “crystalline phase” of bismuth iodide, as the first of a new class of topological insulators. What makes this material particularly exciting is the fact that its atomic structure does not resemble any other topological insulator known to date, which makes its properties very different as well.

    One clear advantage of bismuth iodide is that its structure is more ordered than that of previously known topological insulators, and with fewer natural defects. In order to have an insulating interior, a material must have as few defects in its structure as possible. “What we want is to pass current across the surface but not the interior,” explains Oleg Yazyev. “In theory, this sounds like an easy task, but in practice you’ll always have defects. So you need to find a new material with as few of them as possible.” The study shows that even these early samples of bismuth iodide appear to be very clean with very small concentration of structural imperfections.

    After characterizing bismuth iodide with theoretical tools, the scientists tested it experimentally with an array of methods. The main evidence came from a direct experimental technique called angle-resolved photoemission spectroscopy or ARPES. This method allows researchers to “see” electronic states on the surface of a solid material. ARPES turns out to be the crucial technique for proving the topological nature of electronic states at the surface.

    The ARPES measurements, carried out at the Lawrence Berkeley National Lab, proved to be fully consistent with the theoretical predictions made by Gabriel Autès, a postdoc at Yazyev’s lab and lead author of the study. The actual electron structure calculations were performed at the Swiss National Supercomputing Centre, while data analysis included a number of scientists from EPFL and other institutions.

    “This study began as theory and went through the entire chain of experimental verification,” says Yazyev. “For us is a very important collaborative effort.” His lab is now exploring further the properties of bismuth iodide, as well materials with similar structures. Meanwhile, other labs are joining the effort to support the theory behind the new class of topological insulators and propagate the experimental efforts.

    This study was carried out within the framework of NCCR Marvel, a research effort on Computational Design and Discovery of Novel Materials, created by the Swiss National Science Foundation and led by EPFL. It currently includes 33 labs across 11 Swiss institutions. The work presented here involved a collaboration of EPFL’s Institute of Theoretical Physics and Institute of Condensed Matter Physics with TU Dresden; the Lawrence Berkeley National Laboratory; the University of California, Berkeley; Lomonosov Moscow State University; Ulm University; Yonsei University; Pohang University of Science and Technology; and the Institute for Basic Science, Pohang. The study was funded by the Swiss National Science Foundation, the ERC, NCCR-MARVEL, the Deutsche Forschungsgemeinschaft, the U.S. Department of Energy, and the Carl-Zeiss Foundation.

    Reference

    Autès G, Isaeva A, Moreschini L, Johannsen JC, Pisoni A, Mori R, Zhang W, Filatova TG, Kuznetsov AN, Forró L, Van den Broek W, Kim Y, Kim KS, Lanzara A, Denlinger JD, Rotenberg E, Bostwick A, Grioni M, Yazyev OV. A Novel Quasi-One-Dimensional Topological Insulator in Bismuth Iodide β-Bi4I4.Nature Materials 14 December 2015. DOI: 10.1038/nmat4488
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  • richardmitnick 5:00 pm on January 7, 2015 Permalink | Reply
    Tags: , Spintronics   

    From M.I.T.: “Spin designers” 


    MIT News

    January 7, 2015
    Mike Lotti | Department of Materials Science and Engineering

    Caroline Ross and Geoffrey Beach are studying how the “spin” of electrons on nanomagnets could be manipulated to create faster, more energy-efficient computers.

    temp
    Magnetic tunnel junctions hidden under cones of tungsten used as an etch mask. The tunnel junctions were made from a film deposited by Weigang Wang’s group at the University of Arizona, and patterned using self-assembled block copolymer lithography in Professor Caroline Ross’ group. Image courtesy of Caroline Ross.

    Computers are basically machines that process information in the form of electronic zeros and ones. But two MIT professors of materials science and engineering are trying to change that.

    Caroline Ross and Geoffrey Beach are members of the Center for Spintronic Materials, Interfaces, and Novel Architectures (C-SPIN), a University of Minnesota-led team of 32 professors (and over 100 graduate students and postdocs) from 18 universities trying to restructure computers from the bottom up. C-SPIN researchers want to use the “spin” of electrons on nanomagnets — rather than electric charge — to encode zeros and ones. If they are successful, the computers of 2025 could be 10 times faster than today’s computers, while using only 1 percent of their energy.

    Before C-SPIN began in 2013, spintronics research was carried out in many corners of American academia. The center, which is funded by a consortium of defense and industry sponsors, has helped researchers like Ross and Beach work directly on specified projects with colleagues around the country. “I appreciate the diverse group of students, faculty, and industrial researchers that C-SPIN brings together,” says Ross. “I’m part of a work flow that includes researchers from Arizona, California-Riverside, Johns Hopkins, Carnegie Mellon, Minnesota, and Penn State. With the Center’s coordinated funding, we are making significant progress.”

    Ross, the Toyota Professor of Materials Science and Engineering and associate head of the Department of Materials Science and Engineering, is developing methods to pattern ultra-small magnetic structures, and she is also working on magnetic “insulators” that help control the way “spin” is shared with neighboring magnets and other devices. One such magnetic structure is pictured at right.

    Beach, the Class of ’58 Associate Professor of Materials Science and Engineering, is investigating ways to reduce the power required to “switch” magnetic spin — that is, to make an “up” magnet “down,” and vice versa. This process basically translates into changing zeros to ones and ones to zeros, something computers do billions of times per second. He recently discovered a new way to perform low-energy spin-switching (published in the prestigious Nature Materials and reported on here at MIT) which has led fellow C-SPIN researchers to develop new theoretical and experimental spin devices.

    Spin-based computers aren’t on the near horizon, notes Beach, but C-SPIN researchers have moved much closer to that goal over the past two years. “Hybrids are also a possibility,” says Beach. “It’s not hard to imagine a computer in 2025 with spin-based RAM and some spin-based processing.” Given what the center has accomplished in the past two years, the computing world could be much different by the time Ross, Beach, and their colleagues are done.

    See the full article here.

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  • richardmitnick 3:20 pm on June 25, 2014 Permalink | Reply
    Tags: , Spintronics   

    From Berkeley Lab- “Advanced Light Source Provides New Look at Skyrmions: Results Hold Promise for Spintronics” 

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    Berkeley Lab

    June 25, 2014
    Lynn Yarris

    Skyrmions, subatomic quasiparticles that could play a key role in future spintronic technologies, have been observed for the first time using x-rays. An international collaboration of researchers working at Berkeley Lab’s Advanced Light Source (ALS) observed skyrmions in copper selenite (Cu2SeO3) an insulator with multiferroic properties. The results not only hold promise for ultracompact data storage and processing, but may also open up entire new areas of study in the emerging field of quantum topology.

    sky
    Advanced Light Source images of a Cu2SeO3 sample show five sets of dual-peak skyrmion structures, highlighted by the white ovals. The dual peaks represent the two skyrmion sub-lattices that rotate with respect to each other. All peaks fall on an arc (dotted line) representing the constant amplitude of the skyrmion wave vector. No image credit

    “Using resonant x-ray scattering, we were able to gather unique element-specific, orbital-sensitive electronic and magnetic structural information not available by any other method,” says Sujoy Roy, a physicist who oversees research at ALS Beamline 12.0.2 where the study was carried out, and the corresponding author of a paper describing this research in Physical Review Letters titled Coupled Skyrmion Sublattices in Cu2OSeO3.

    A skyrmion is an atom-sized whirlwind of magnetism, in which the spins of charged particles form a vortex. In this image the color scale – red for longer and blue for shorter vectors – shows that the magnetization is highest at the center of the skyrmion. (Image by Matthew Langner)

    “We found the unexpected existence of two distinct skyrmion sub-lattices that rotate with respect to each other, creating a moiré-like pattern,” Roy says. “Compared to materials with a simpler magnetic structure, the sub-lattices provide for an extra degree of freedom to minimize the free energy. This leads to magnetic excitations that can’t exist in materials with a single magnetic lattice structure.”

    ski
    A skyrmion is an atom-sized whirlwind of magnetism, in which the spins of charged particles form a vortex. In this image the color scale – red for longer and blue for shorter vectors – shows that the magnetization is highest at the center of the skyrmion. (Image by Matthew Langner)

    Although skyrmions act like baryons, they are actually magnetic vortices – discrete swirls of magnetism – formed from the spins of charged particles. Spin is a quantum property in which the charged particles act as if they were bar magnets rotating about an axis and pointing in either an “up” or “down” direction. The discovery of skyrmions – named for Tony Skyrme, a British physicist who first theorized their existence – in manganese silicide generated much excitement in the materials sciences world because their exotic hedgehog-like spin texture is topologically protected – meaning it can’t be perturbed. Add to this the discovery that skyrmions can be moved coherently over macroscopic distances with a tiny electrical current and you have a strong spintronic candidate.

    “A major breakthrough came with the discovery of skyrmions in copper selenite because its magnetic properties can be controlled with an electric field,” says Roy. “To achieve this control, however, we must understand how different electron orbitals stabilize the skyrmionic phase. Until our study, the copper selenite skyrmions had only been observed with neutron scattering and transmission electron microscopy, techniques that are insensitive to electron orbitals.”

    ALS Beamline 12.0.2 is an undulator beamline with experimental facilities optimized for coherent x-ray scattering studies of magnetic materials. The collaboration, which included researchers from Berkeley Lab’s Materials Sciences Division and Japan’s RIKEN Institute, used these facilities to first identify the magnetic vortex. Then, at a certain applied electric field and temperature, they saw x-ray signals due to the formation of a skyrmion lattice.

    “We were able to show that although the skyrmions act like magnetic particles, their origin in copper selenite is electronic,” says Matthew Langner, lead author of the Physical Review Letters paper. “We also found that temperature can be used to move the skyrmions in copper selenite in either a clockwise or counter-clockwise direction.”

    five
    From left, Matthew Langner, Stephan Kevan, Sujoy Roy, Robert Schoenlein and Xiaowen Shi were part of an international team of researchers that used the Advanced Light Source to provide new information on the quasiparticles known as skyrmions. (Photo by Roy Kaltschmidt)

    Controlling the movement of skyrmions in a multiferroic compound suggests these magnetic vortices could be used to read and write data. Skyrmions are considered especially promising for the holographic information storage concept known as magnetic race-track memory.

    “The skyrmion is topologically distinct from the other ground-state magnetic structures, meaning it can be moved around the sample without losing its shape,” Langner says. “The combination of this stability and the low magnetic and electric fields required for manipulating the skyrmions is what makes them potentially useful for spintronic applications.”

    In addition to device applications, the collaboration’s findings show that is now possible to use x-rays to study spectroscopic and electronic aspects of the skyrmion, and to study skyrmion dynamics on the time-scale of fundamental interactions.

    Co-authors of the Physical Review Letters paper, in addition to Roy and Langner, are Shrawan Mishra, Jason Lee, Xiaowen Shi, Muhammad Hossain, Yi-De Chuang, Shinichiro Seki, Yoshinori Tokura, Stephen Kevan and Robert Schoenlein.

    This research was supported by the U.S. Department of Energy’s Office of Science.

    See the full article here.

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  • richardmitnick 3:09 pm on January 16, 2014 Permalink | Reply
    Tags: , , , Spintronics, superconductors   

    From Berkeley Lab: “Natural 3D Counterpart to Graphene Discovered” 


    Berkeley Lab

    Researchers at Berkeley Lab’s Advanced Light Source Find New Form of Quantum Matter

    January 16, 2014
    Lynn Yarris (510) 486-5375 lcyarris@lbl.gov

    The discovery of what is essentially a 3D version of graphene – the 2D sheets of carbon through which electrons race at many times the speed at which they move through silicon – promises exciting new things to come for the high-tech industry, including much faster transistors and far more compact hard drives. A collaboration of researchers at the U.S Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) has discovered that sodium bismuthate can exist as a form of quantum matter called a three-dimensional topological Dirac semi-metal (3DTDS). This is the first experimental confirmation of 3D Dirac fermions in the interior or bulk of a material, a novel state that was only recently proposed by theorists.

    graph
    A topological Dirac semi-metal state is realized at the critical point in the phase transition from a normal insulator to a topological insulator. The + and – signs denote the even and odd parity of the energy bands.

    “A 3DTDS is a natural three-dimensional counterpart to graphene with similar or even better electron mobility and velocity,” says Yulin Chen, a physicist with Berkeley Lab’s Advanced Light Source (ALS) when he initiated the study that led to this discovery, and now with the University of Oxford. “Because of its 3D Dirac fermions in the bulk, a 3DTDS also features intriguing non-saturating linear magnetoresistance that can be orders of magnitude higher than the materials now used in hard drives, and it opens the door to more efficient optical sensors.”

    Chen is the corresponding author of a paper in Science reporting the discovery. The paper is titled Discovery of a Three-dimensional Topological Dirac Semimetal, Na3Bi. Co-authors were Zhongkai Liu, Bo Zhou, Yi Zhang, Zhijun Wang, Hongming Weng, Dharmalingam Prabhakaran, Sung-Kwan Mo, Zhi-Xun Shen, Zhong Fang, Xi Dai and Zahid Hussain.

    See the full article and all of the excitment here.

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

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