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  • richardmitnick 7:36 am on July 22, 2021 Permalink | Reply
    Tags: "'Magic-angle' trilayer graphene may be a rare magnet-proof superconductor", , “spin-singlet", Cooper Pairs, In spin-singlet superconductors if you kill superconductivity it never comes back — it’s gone for good. Here it reappeared again. So this definitely says this material is not spin-singlet., , , , Spin-singlet pairs happily speed through a superconductor-except under high magnetic fields-which can shift the energy of each electron in opposite directions pulling the pair apart., Spin-triplet superconductivity, Superconducting materials are defined by their super-efficient ability to conduct electricity without losing energy., When exposed to high magnetic fields the spin of both electrons in a Cooper pair shift in the same direction. They are not pulled apart- continuing superconducting regardless of the magnetic field.   

    From Massachusetts Institute of Technology (US) : “‘Magic-angle’ trilayer graphene may be a rare magnet-proof superconductor” 

    MIT News

    From Massachusetts Institute of Technology (US)

    July 21, 2021
    Jennifer Chu

    New findings might help inform the design of more powerful MRI machines or robust quantum computers.

    1
    MIT physicists have observed signs of a rare type of superconductivity in a material called “magic-angle” twisted trilayer graphene. Credit: the researchers.

    MIT physicists have observed signs of a rare type of superconductivity in a material called magic-angle twisted trilayer graphene. In a study appearing today in Nature, the researchers report that the material exhibits superconductivity at surprisingly high magnetic fields of up to 10 Tesla, which is three times higher than what the material is predicted to endure if it were a conventional superconductor.

    The results strongly imply that magic-angle trilayer graphene, which was initially discovered by the same group, is a very rare type of superconductor, known as a “spin-triplet,” that is impervious to high magnetic fields. Such exotic superconductors could vastly improve technologies such as magnetic resonance imaging, which uses superconducting wires under a magnetic field to resonate with and image biological tissue. MRI machines are currently limited to magnet fields of 1 to 3 Tesla. If they could be built with spin-triplet superconductors, MRI could operate under higher magnetic fields to produce sharper, deeper images of the human body.

    The new evidence of spin-triplet superconductivity in trilayer graphene could also help scientists design stronger superconductors for practical quantum computing.

    “The value of this experiment is what it teaches us about fundamental superconductivity, about how materials can behave, so that with those lessons learned, we can try to design principles for other materials which would be easier to manufacture, that could perhaps give you better superconductivity,” says Pablo Jarillo-Herrero, the Cecil and Ida Green Professor of Physics at MIT.

    His co-authors on the paper include postdoc Yuan Cao and graduate student Jeong Min Park at MIT, and Kenji Watanabe and Takashi Taniguchi of the NIMS-National Institute for Materials Science [物質・材料研究機構] (JP).

    Strange shift

    Superconducting materials are defined by their super-efficient ability to conduct electricity without losing energy. When exposed to an electric current, electrons in a superconductor couple up in “Cooper pairs” that then travel through the material without resistance, like passengers on an express train.

    In a vast majority of superconductors, these passenger pairs have opposite spins, with one electron spinning up, and the other down — a configuration known as a “spin-singlet.” These pairs happily speed through a superconductor-except under high magnetic fields-which can shift the energy of each electron in opposite directions pulling the pair apart. In this way-and through mechanisms-high magnetic fields can derail superconductivity in conventional spin-singlet superconductors.

    “That’s the ultimate reason why in a large-enough magnetic field, superconductivity disappears,” Park says.

    But there exists a handful of exotic superconductors that are impervious to magnetic fields, up to very large strengths. These materials superconduct through pairs of electrons with the same spin — a property known as “spin-triplet.” When exposed to high magnetic fields, the energy of both electrons in a Cooper pair shift in the same direction, in a way that they are not pulled apart but continue superconducting unperturbed, regardless of the magnetic field strength.

    Jarillo-Herrero’s group was curious whether magic-angle trilayer graphene might harbor signs of this more unusual spin-triplet superconductivity. The team has produced pioneering work in the study of graphene moiré structures — layers of atom-thin carbon lattices that, when stacked at specific angles, can give rise to surprising electronic behaviors.

    The researchers initially reported such curious properties in two angled sheets of graphene, which they dubbed magic-angle bilayer graphene ([Nature] and [Nature]) . They soon followed up with tests of trilayer graphene [Nature], a sandwich configuration of three graphene sheets that turned out to be even stronger than its bilayer counterpart, retaining superconductivity at higher temperatures. When the researchers applied a modest magnetic field, they noticed that trilayer graphene was able to superconduct at field strengths that would destroy superconductivity in bilayer graphene.

    “We thought, this is something very strange,” Jarillo-Herrero says.

    A super comeback

    In their new study, the physicists tested trilayer graphene’s superconductivity under increasingly higher magnetic fields. They fabricated the material by peeling away atom-thin layers of carbon from a block of graphite, stacking three layers together, and rotating the middle one by 1.56 degrees with respect to the outer layers. They attached an electrode to either end of the material to run a current through and measure any energy lost in the process. Then they turned on a large magnet in the lab, with a field which they oriented parallel to the material.

    As they increased the magnetic field around trilayer graphene, they observed that superconductivity held strong up to a point before disappearing, but then curiously reappeared at higher field strengths — a comeback that is highly unusual and not known to occur in conventional spin-singlet superconductors.

    “,” Cao says. “Here it reappeared again. So this definitely says this material is not spin-singlet.”

    They also observed that after “re-entry,” superconductivity persisted up to 10 Tesla, the maximum field strength that the lab’s magnet could produce. This is about three times higher than what the superconductor should withstand if it were a conventional spin-singlet, according to Pauli’s limit, a theory that predicts the maximum magnetic field at which a material can retain superconductivity.

    Trilayer graphene’s reappearance of superconductivity, paired with its persistence at higher magnetic fields than predicted, rules out the possibility that the material is a run-of-the-mill superconductor. Instead, it is likely a very rare type, possibly a spin-triplet, hosting Cooper pairs that speed through the material, impervious to high magnetic fields. The team plans to drill down on the material to confirm its exact spin state, which could help to inform the design of more powerful MRI machines, and also more robust quantum computers.

    “Regular quantum computing is super fragile,” Jarillo-Herrero says. “You look at it and, poof, it disappears. About 20 years ago, theorists proposed a type of topological superconductivity that, if realized in any material, could [enable] a quantum computer where states responsible for computation are very robust. That would give infinite more power to do computing. The key ingredient to realize that would be spin-triplet superconductors, of a certain type. We have no idea if our type is of that type. But even if it’s not, this could make it easier to put trilayer graphene with other materials to engineer that kind of superconductivity. That could be a major breakthrough. But it’s still super early.”

    This research was supported by the U.S. Department of Energy, the National Science Foundation, the Gordon and Betty Moore Foundation, the Fundacion Ramon Areces, and the CIFAR Quantum Materials Program.

    See the full article here .


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    Please help promote STEM in your local schools.

    Stem Education Coalition

    MIT Seal

    USPS “Forever” postage stamps celebrating Innovation at MIT.

    MIT Campus

    Massachusetts Institute of Technology (US) is a private land-grant research university in Cambridge, Massachusetts. The institute has an urban campus that extends more than a mile (1.6 km) alongside the Charles River. The institute also encompasses a number of major off-campus facilities such as the MIT Lincoln Laboratory (US), the MIT Bates Research and Engineering Center (US), and the Haystack Observatory (US), as well as affiliated laboratories such as the Broad Institute of MIT and Harvard(US) and Whitehead Institute (US).

    Founded in 1861 in response to the increasing industrialization of the United States, Massachusetts Institute of Technology (US) adopted a European polytechnic university model and stressed laboratory instruction in applied science and engineering. It has since played a key role in the development of many aspects of modern science, engineering, mathematics, and technology, and is widely known for its innovation and academic strength. It is frequently regarded as one of the most prestigious universities in the world.

    As of December 2020, 97 Nobel laureates, 26 Turing Award winners, and 8 Fields Medalists have been affiliated with MIT as alumni, faculty members, or researchers. In addition, 58 National Medal of Science recipients, 29 National Medals of Technology and Innovation recipients, 50 MacArthur Fellows, 80 Marshall Scholars, 3 Mitchell Scholars, 22 Schwarzman Scholars, 41 astronauts, and 16 Chief Scientists of the U.S. Air Force have been affiliated with Massachusetts Institute of Technology (US) . The university also has a strong entrepreneurial culture and MIT alumni have founded or co-founded many notable companies. Massachusetts Institute of Technology (US) is a member of the Association of American Universities (AAU).

    Foundation and vision

    In 1859, a proposal was submitted to the Massachusetts General Court to use newly filled lands in Back Bay, Boston for a “Conservatory of Art and Science”, but the proposal failed. A charter for the incorporation of the Massachusetts Institute of Technology, proposed by William Barton Rogers, was signed by John Albion Andrew, the governor of Massachusetts, on April 10, 1861.

    Rogers, a professor from the University of Virginia (US), wanted to establish an institution to address rapid scientific and technological advances. He did not wish to found a professional school, but a combination with elements of both professional and liberal education, proposing that:

    “The true and only practicable object of a polytechnic school is, as I conceive, the teaching, not of the minute details and manipulations of the arts, which can be done only in the workshop, but the inculcation of those scientific principles which form the basis and explanation of them, and along with this, a full and methodical review of all their leading processes and operations in connection with physical laws.”

    The Rogers Plan reflected the German research university model, emphasizing an independent faculty engaged in research, as well as instruction oriented around seminars and laboratories.

    Early developments

    Two days after Massachusetts Institute of Technology (US) was chartered, the first battle of the Civil War broke out. After a long delay through the war years, MIT’s first classes were held in the Mercantile Building in Boston in 1865. The new institute was founded as part of the Morrill Land-Grant Colleges Act to fund institutions “to promote the liberal and practical education of the industrial classes” and was a land-grant school. In 1863 under the same act, the Commonwealth of Massachusetts founded the Massachusetts Agricultural College, which developed as the University of Massachusetts Amherst (US)). In 1866, the proceeds from land sales went toward new buildings in the Back Bay.

    Massachusetts Institute of Technology (US) was informally called “Boston Tech”. The institute adopted the European polytechnic university model and emphasized laboratory instruction from an early date. Despite chronic financial problems, the institute saw growth in the last two decades of the 19th century under President Francis Amasa Walker. Programs in electrical, chemical, marine, and sanitary engineering were introduced, new buildings were built, and the size of the student body increased to more than one thousand.

    The curriculum drifted to a vocational emphasis, with less focus on theoretical science. The fledgling school still suffered from chronic financial shortages which diverted the attention of the MIT leadership. During these “Boston Tech” years, Massachusetts Institute of Technology (US) faculty and alumni rebuffed Harvard University (US) president (and former MIT faculty) Charles W. Eliot’s repeated attempts to merge MIT with Harvard College’s Lawrence Scientific School. There would be at least six attempts to absorb MIT into Harvard. In its cramped Back Bay location, MIT could not afford to expand its overcrowded facilities, driving a desperate search for a new campus and funding. Eventually, the MIT Corporation approved a formal agreement to merge with Harvard, over the vehement objections of MIT faculty, students, and alumni. However, a 1917 decision by the Massachusetts Supreme Judicial Court effectively put an end to the merger scheme.

    In 1916, the Massachusetts Institute of Technology (US) administration and the MIT charter crossed the Charles River on the ceremonial barge Bucentaur built for the occasion, to signify MIT’s move to a spacious new campus largely consisting of filled land on a one-mile-long (1.6 km) tract along the Cambridge side of the Charles River. The neoclassical “New Technology” campus was designed by William W. Bosworth and had been funded largely by anonymous donations from a mysterious “Mr. Smith”, starting in 1912. In January 1920, the donor was revealed to be the industrialist George Eastman of Rochester, New York, who had invented methods of film production and processing, and founded Eastman Kodak. Between 1912 and 1920, Eastman donated $20 million ($236.6 million in 2015 dollars) in cash and Kodak stock to MIT.

    Curricular reforms

    In the 1930s, President Karl Taylor Compton and Vice-President (effectively Provost) Vannevar Bush emphasized the importance of pure sciences like physics and chemistry and reduced the vocational practice required in shops and drafting studios. The Compton reforms “renewed confidence in the ability of the Institute to develop leadership in science as well as in engineering”. Unlike Ivy League schools, Massachusetts Institute of Technology (US) catered more to middle-class families, and depended more on tuition than on endowments or grants for its funding. The school was elected to the Association of American Universities (US)in 1934.

    Still, as late as 1949, the Lewis Committee lamented in its report on the state of education at Massachusetts Institute of Technology (US) that “the Institute is widely conceived as basically a vocational school”, a “partly unjustified” perception the committee sought to change. The report comprehensively reviewed the undergraduate curriculum, recommended offering a broader education, and warned against letting engineering and government-sponsored research detract from the sciences and humanities. The School of Humanities, Arts, and Social Sciences and the MIT Sloan School of Management were formed in 1950 to compete with the powerful Schools of Science and Engineering. Previously marginalized faculties in the areas of economics, management, political science, and linguistics emerged into cohesive and assertive departments by attracting respected professors and launching competitive graduate programs. The School of Humanities, Arts, and Social Sciences continued to develop under the successive terms of the more humanistically oriented presidents Howard W. Johnson and Jerome Wiesner between 1966 and 1980.

    Massachusetts Institute of Technology (US)‘s involvement in military science surged during World War II. In 1941, Vannevar Bush was appointed head of the federal Office of Scientific Research and Development and directed funding to only a select group of universities, including MIT. Engineers and scientists from across the country gathered at Massachusetts Institute of Technology (US)’s Radiation Laboratory, established in 1940 to assist the British military in developing microwave radar. The work done there significantly affected both the war and subsequent research in the area. Other defense projects included gyroscope-based and other complex control systems for gunsight, bombsight, and inertial navigation under Charles Stark Draper’s Instrumentation Laboratory; the development of a digital computer for flight simulations under Project Whirlwind; and high-speed and high-altitude photography under Harold Edgerton. By the end of the war, Massachusetts Institute of Technology (US) became the nation’s largest wartime R&D contractor (attracting some criticism of Bush), employing nearly 4000 in the Radiation Laboratory alone and receiving in excess of $100 million ($1.2 billion in 2015 dollars) before 1946. Work on defense projects continued even after then. Post-war government-sponsored research at MIT included SAGE and guidance systems for ballistic missiles and Project Apollo.

    These activities affected Massachusetts Institute of Technology (US) profoundly. A 1949 report noted the lack of “any great slackening in the pace of life at the Institute” to match the return to peacetime, remembering the “academic tranquility of the prewar years”, though acknowledging the significant contributions of military research to the increased emphasis on graduate education and rapid growth of personnel and facilities. The faculty doubled and the graduate student body quintupled during the terms of Karl Taylor Compton, president of Massachusetts Institute of Technology (US) between 1930 and 1948; James Rhyne Killian, president from 1948 to 1957; and Julius Adams Stratton, chancellor from 1952 to 1957, whose institution-building strategies shaped the expanding university. By the 1950s, Massachusetts Institute of Technology (US) no longer simply benefited the industries with which it had worked for three decades, and it had developed closer working relationships with new patrons, philanthropic foundations and the federal government.

    In late 1960s and early 1970s, student and faculty activists protested against the Vietnam War and Massachusetts Institute of Technology (US)’s defense research. In this period Massachusetts Institute of Technology (US)’s various departments were researching helicopters, smart bombs and counterinsurgency techniques for the war in Vietnam as well as guidance systems for nuclear missiles. The Union of Concerned Scientists was founded on March 4, 1969 during a meeting of faculty members and students seeking to shift the emphasis on military research toward environmental and social problems. Massachusetts Institute of Technology (US) ultimately divested itself from the Instrumentation Laboratory and moved all classified research off-campus to the MIT (US) Lincoln Laboratory facility in 1973 in response to the protests. The student body, faculty, and administration remained comparatively unpolarized during what was a tumultuous time for many other universities. Johnson was seen to be highly successful in leading his institution to “greater strength and unity” after these times of turmoil. However six Massachusetts Institute of Technology (US) students were sentenced to prison terms at this time and some former student leaders, such as Michael Albert and George Katsiaficas, are still indignant about MIT’s role in military research and its suppression of these protests. (Richard Leacock’s film, November Actions, records some of these tumultuous events.)

    In the 1980s, there was more controversy at Massachusetts Institute of Technology (US) over its involvement in SDI (space weaponry) and CBW (chemical and biological warfare) research. More recently, Massachusetts Institute of Technology (US)’s research for the military has included work on robots, drones and ‘battle suits’.

    Recent history

    Massachusetts Institute of Technology (US) has kept pace with and helped to advance the digital age. In addition to developing the predecessors to modern computing and networking technologies, students, staff, and faculty members at Project MAC, the Artificial Intelligence Laboratory, and the Tech Model Railroad Club wrote some of the earliest interactive computer video games like Spacewar! and created much of modern hacker slang and culture. Several major computer-related organizations have originated at MIT since the 1980s: Richard Stallman’s GNU Project and the subsequent Free Software Foundation were founded in the mid-1980s at the AI Lab; the MIT Media Lab was founded in 1985 by Nicholas Negroponte and Jerome Wiesner to promote research into novel uses of computer technology; the World Wide Web Consortium standards organization was founded at the Laboratory for Computer Science in 1994 by Tim Berners-Lee; the MIT OpenCourseWare project has made course materials for over 2,000 Massachusetts Institute of Technology (US) classes available online free of charge since 2002; and the One Laptop per Child initiative to expand computer education and connectivity to children worldwide was launched in 2005.

    Massachusetts Institute of Technology (US) was named a sea-grant college in 1976 to support its programs in oceanography and marine sciences and was named a space-grant college in 1989 to support its aeronautics and astronautics programs. Despite diminishing government financial support over the past quarter century, MIT launched several successful development campaigns to significantly expand the campus: new dormitories and athletics buildings on west campus; the Tang Center for Management Education; several buildings in the northeast corner of campus supporting research into biology, brain and cognitive sciences, genomics, biotechnology, and cancer research; and a number of new “backlot” buildings on Vassar Street including the Stata Center. Construction on campus in the 2000s included expansions of the Media Lab, the Sloan School’s eastern campus, and graduate residences in the northwest. In 2006, President Hockfield launched the MIT Energy Research Council to investigate the interdisciplinary challenges posed by increasing global energy consumption.

    In 2001, inspired by the open source and open access movements, Massachusetts Institute of Technology (US) launched OpenCourseWare to make the lecture notes, problem sets, syllabi, exams, and lectures from the great majority of its courses available online for no charge, though without any formal accreditation for coursework completed. While the cost of supporting and hosting the project is high, OCW expanded in 2005 to include other universities as a part of the OpenCourseWare Consortium, which currently includes more than 250 academic institutions with content available in at least six languages. In 2011, Massachusetts Institute of Technology (US) announced it would offer formal certification (but not credits or degrees) to online participants completing coursework in its “MITx” program, for a modest fee. The “edX” online platform supporting MITx was initially developed in partnership with Harvard and its analogous “Harvardx” initiative. The courseware platform is open source, and other universities have already joined and added their own course content. In March 2009 the Massachusetts Institute of Technology (US) faculty adopted an open-access policy to make its scholarship publicly accessible online.

    Massachusetts Institute of Technology (US) has its own police force. Three days after the Boston Marathon bombing of April 2013, MIT Police patrol officer Sean Collier was fatally shot by the suspects Dzhokhar and Tamerlan Tsarnaev, setting off a violent manhunt that shut down the campus and much of the Boston metropolitan area for a day. One week later, Collier’s memorial service was attended by more than 10,000 people, in a ceremony hosted by the Massachusetts Institute of Technology (US) community with thousands of police officers from the New England region and Canada. On November 25, 2013, Massachusetts Institute of Technology (US) announced the creation of the Collier Medal, to be awarded annually to “an individual or group that embodies the character and qualities that Officer Collier exhibited as a member of the Massachusetts Institute of Technology (US) community and in all aspects of his life”. The announcement further stated that “Future recipients of the award will include those whose contributions exceed the boundaries of their profession, those who have contributed to building bridges across the community, and those who consistently and selflessly perform acts of kindness”.

    In September 2017, the school announced the creation of an artificial intelligence research lab called the MIT-IBM Watson AI Lab. IBM will spend $240 million over the next decade, and the lab will be staffed by MIT and IBM scientists. In October 2018 MIT announced that it would open a new Schwarzman College of Computing dedicated to the study of artificial intelligence, named after lead donor and The Blackstone Group CEO Stephen Schwarzman. The focus of the new college is to study not just AI, but interdisciplinary AI education, and how AI can be used in fields as diverse as history and biology. The cost of buildings and new faculty for the new college is expected to be $1 billion upon completion.

    The Caltech/MIT Advanced aLIGO (US) was designed and constructed by a team of scientists from California Institute of Technology (US), Massachusetts Institute of Technology (US), and industrial contractors, and funded by the National Science Foundation (US) .

    MIT/Caltech Advanced aLigo .

    It was designed to open the field of gravitational-wave astronomy through the detection of gravitational waves predicted by general relativity. Gravitational waves were detected for the first time by the LIGO detector in 2015. For contributions to the LIGO detector and the observation of gravitational waves, two Caltech physicists, Kip Thorne and Barry Barish, and Massachusetts Institute of Technology (US) physicist Rainer Weiss won the Nobel Prize in physics in 2017. Weiss, who is also an Massachusetts Institute of Technology (US) graduate, designed the laser interferometric technique, which served as the essential blueprint for the LIGO.

    The mission of Massachusetts Institute of Technology (US) is to advance knowledge and educate students in science, technology, and other areas of scholarship that will best serve the nation and the world in the twenty-first century. We seek to develop in each member of the Massachusetts Institute of Technology (US) community the ability and passion to work wisely, creatively, and effectively for the betterment of humankind.

     
  • richardmitnick 9:30 am on September 22, 2020 Permalink | Reply
    Tags: "Researchers identify new type of superconductor", , Cooper Pairs, , G-wave superconductors proposed., No electrical resistance   

    From Cornell Chronicle: “Researchers identify new type of superconductor” 

    From Cornell Chronicle

    September 21, 2020
    David Nutt
    cunews@cornell.edu

    1
    This illustration shows a crystal lattice of strontium ruthenate responding to various sound waves sent via resonant ultrasound spectroscopy as the material cools through its superconducting transition at 1.4 kelvin (minus 457 degrees Fahrenheit). The highlighted deformation suggests the material may be a new type of superconductor. Provided.

    Until now, the history of superconducting materials has been a tale of two types: s-wave and d-wave.

    Now, Cornell researchers – led by Brad Ramshaw, the Dick & Dale Reis Johnson Assistant Professor in the College of Arts and Sciences – have discovered a possible third type: g-wave.

    Their paper, Thermodynamic Evidence for a Two-Component Superconducting Order Parameter in Sr2RuO4, published Sept. 21 in Nature Physics. The lead author is doctoral student Sayak Ghosh, M.S. ’19.

    Electrons in superconductors move together in what are known as Cooper pairs. This “pairing” endows superconductors with their most famous property – no electrical resistance – because, in order to generate resistance, the Cooper pairs have to be broken apart, and this takes energy.

    In s-wave superconductors – generally conventional materials, such as lead, tin and mercury – the Cooper pairs are made of one electron pointing up and one pointing down, both moving head-on toward each other, with no net angular momentum. In recent decades, a new class of exotic materials has exhibited what’s called d-wave superconductivity, whereby the Cooper pairs have two quanta of angular momentum.

    Physicists have theorized the existence of a third type of superconductor between these two so-called “singlet” states: a p-wave superconductor, with one quanta of angular momentum and the electrons pairing with parallel rather than antiparallel spins. This spin-triplet superconductor would be a major breakthrough for quantum computing because it can be used to create Majorana fermions, a unique particle which is its own antiparticle.

    For more than 20 years, one of the leading candidates for a p-wave superconductor has been strontium ruthenate (Sr2RuO4­), although recent research has started to poke holes in the idea.

    Ramshaw and his team set out to determine once and for all whether strontium ruthenate is a highly desired p-wave superconductor. Using high-resolution resonant ultrasound spectroscopy, they discovered that the material is potentially an entirely new kind of superconductor altogether: g-wave.

    “This experiment really shows the possibility of this new type of superconductor that we had never thought about before,” Ramshaw said. “It really opens up the space of possibilities for what a superconductor can be and how it can manifest itself. If we’re ever going to get a handle on controlling superconductors and using them in technology with the kind of fine-tuned control we have with semiconductors, we really want to know how they work and what varieties and flavors they come in.”

    As with previous projects, Ramshaw and Ghosh used resonant ultrasound spectroscopy to study the symmetry properties of the superconductivity in a crystal of strontium ruthenate that was grown and precision-cut by collaborators at the Max Planck Institute for Chemical Physics of Solids in Germany.

    However, unlike previous attempts, Ramshaw and Ghosh encountered a significant problem when trying to conduct the experiment.

    “Cooling down resonant ultrasound to 1 kelvin (minus 457.87 degrees Fahrenheit) is difficult, and we had to build a completely new apparatus to achieve this,” Ghosh said.

    With their new setup, the Cornell team measured the response of the crystal’s elastic constants – essentially the speed of sound in the material – to a variety of sound waves as the material cooled through its superconducting transition at 1.4 kelvin (minus 457 degrees Fahrenheit).

    “This is by far the highest-precision resonant ultrasound spectroscopy data ever taken at these low temperatures,” Ramshaw said.

    Based on the data, they determined that strontium ruthenate is what’s called a two-component superconductor, meaning the way electrons bind together is so complex, it can’t be described by a single number; it needs a direction as well.

    Previous studies [Science Advances] had used nuclear magnetic resonance (NMR) spectroscopy to narrow the possibilities of what kind of wave material strontium ruthenate might be, effectively eliminating p-wave as an option.

    By determining that the material was two-component, Ramshaw’s team not only confirmed those findings, but also showed strontium ruthenate wasn’t a conventional s- or d-wave superconductor, either.

    “Resonant ultrasound really lets you go in and even if you can’t identify all the microscopic details, you can make broad statements about which ones are ruled out,” Ramshaw said. “So then the only things that the experiments are consistent with are these very, very weird things that nobody has ever seen before. One of which is g-wave, which means angular momentum 4. No one has ever even thought that there would be a g-wave superconductor.”

    Now the researchers can use the technique to examine other materials to find out if they are potential p-wave candidates.

    However, the work on strontium ruthenate isn’t finished.

    “This material is extremely well studied in a lot of different contexts, not just for its superconductivity,” Ramshaw said. “We understand what kind of metal it is, why it’s a metal, how it behaves when you change temperature, how it behaves when you change the magnetic field. So you should be able to construct a theory of why it becomes a superconductor better here than just about anywhere else.”

    Co-authors include researchers from the Max Planck Institute for Chemical Physics of Solids; the National High Magnetic Field Laboratory at Florida State University; and the National Institute for Materials Science in Tsukuba, Japan.

    The Cornell research was supported by U.S. Department of Energy’s Office of Basic Energy Sciences and the Cornell Center for Materials Research, which is supported by the National Science Foundation’s Materials Research Science and Engineering Center program.

    See the full article here .


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    Please help promote STEM in your local schools.

    Stem Education Coalition

    Once called “the first American university” by educational historian Frederick Rudolph, Cornell University represents a distinctive mix of eminent scholarship and democratic ideals. Adding practical subjects to the classics and admitting qualified students regardless of nationality, race, social circumstance, gender, or religion was quite a departure when Cornell was founded in 1865.

    Today’s Cornell reflects this heritage of egalitarian excellence. It is home to the nation’s first colleges devoted to hotel administration, industrial and labor relations, and veterinary medicine. Both a private university and the land-grant institution of New York State, Cornell University is the most educationally diverse member of the Ivy League.

    On the Ithaca campus alone nearly 20,000 students representing every state and 120 countries choose from among 4,000 courses in 11 undergraduate, graduate, and professional schools. Many undergraduates participate in a wide range of interdisciplinary programs, play meaningful roles in original research, and study in Cornell programs in Washington, New York City, and the world over.

     
  • richardmitnick 8:48 am on March 20, 2017 Permalink | Reply
    Tags: Anderson limit, Cooper Pairs, How small can superconductors be?, , P.W. Anderson, Parity effect, , , Richardson-Gaudin models   

    From phys.org: “How small can superconductors be?” 

    physdotorg
    phys.org

    March 20, 2017
    Lisa Zyga

    1
    Topographic image of a lead nanocrystal used in the study. Scale bar: 10 nm. Credit: Vlaic et al. Nature Communications

    For the first time, physicists have experimentally validated a 1959 conjecture that places limits on how small superconductors can be. Understanding superconductivity (or the lack thereof) on the nanoscale is expected to be important for designing future quantum computers, among other applications.

    In 1959, physicist P.W. Anderson conjectured that superconductivity can exist only in objects that are large enough to meet certain criteria. Namely, the object’s superconducting gap energy must be larger than its electronic energy level spacing—and this spacing increases as size decreases. The cutoff point (where the two values are equal) corresponds to a volume of about 100 nm3. Until now it has not been possible to experimentally test the Anderson limit due to the challenges in observing superconducting effects at this scale.

    In the new study published in Nature Communications, Sergio Vlaic and coauthors at the University Paris Sciences et Lettres and French National Centre for Scientific Research (CNRS) designed a nanosystem that allowed them to experimentally investigate the Anderson limit for the first time.

    The Anderson limit arises because, at very small scales, the mechanisms underlying superconductivity essentially stop working. In general, superconductivity occurs when electrons bind together to form Cooper pairs. Cooper pairs have a slightly lower energy than individual electrons, and this difference in energy is the superconducting gap energy. The Cooper pairs’ lower energy inhibits electron collisions that normally create resistance. If the superconducting gap energy gets too small and vanishes—which can occur, for example, when the temperature increases—then the electron collisions resume and the object stops being a superconductor.

    The Anderson limit shows that small size is another way that an object may stop being a superconductor. However, unlike the effects of increasing the temperature, this is not because smaller objects have a smaller superconducting gap energy. Instead, it arises because smaller crystals have fewer electrons, and therefore fewer electron energy levels, than larger crystals do. Since the total possible electron energy of an element stays the same, regardless of size, smaller crystals have larger spacings between their electron energy levels than larger crystals do.

    According to Anderson, this large electronic energy level spacing should pose a problem, and he expected superconductivity to disappear when the spacing becomes larger than the superconducting gap energy. The reason for this, generally speaking, is that one consequence of increased spacing is a decrease in potential energy, which interferes with the competition between kinetic and potential energy that is necessary for superconductivity to occur.

    To investigate what happens to the superconductivity of objects around the Anderson limit, the scientists in the new study prepared large quantities of isolated lead nanocrystals ranging in volume from 20 to 800 nm3.

    Although they could not directly measure the superconductivity of such tiny objects, the researchers could measure something called the parity effect, which results from superconductivity. When an electron is added to a superconductor, the additional energy is partly affected by whether there is an even or odd number of electrons (the parity), which is due to the electrons forming Cooper pairs. If the electrons don’t form Cooper pairs, there is no parity effect, indicating no superconductivity.

    Although the parity effect has previously been observed in large superconductors, this study is the first time that it has been observed in small nanocrystals approaching the Anderson limit. In accordance with Anderson’s predictions from more than 50 years ago, the researchers observed the parity effect for larger nanocrystals, but not for the smallest nanocrystals below approximately 100 nm3.

    The results not only validate the Anderson conjecture, but also extend to a more general area, the Richardson-Gaudin models. These models are equivalent to the conventional theory of superconductivity, the Bardeen Cooper Schrieffer theory, for very small objects.

    “Our experimental demonstration of the Anderson conjecture is also a demonstration of the validity of the Richardson-Gaudin models,” coauthor Hervé Aubin at the University Paris Sciences et Lettres and CNRS told Phys.org. “The Richardson-Gaudin models are an important piece of theoretical works because they can be solved exactly and apply to a wide range of systems; not only to superconducting nanocrystals but also to atomic nuclei and cold fermionic atomic gas, where protons and neutrons, which are fermions like electrons, can also form Cooper pairs.”

    On the more practical side, the researchers expect the results to have applications in future quantum computers.

    “One of the most interesting applications of superconducting islands is their use as Cooper pair boxes employed in quantum bits, the elemental unit of a hypothetical quantum computer,” Aubin said. “So far, Cooper pair boxes used in qubits are much larger than the Anderson limit. Upon reducing the size of the Cooper pair box, quantum computer engineers will eventually have to cope with superconductivity at the Anderson limit.”

    See the full article here .

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    Phys.org™ (formerly Physorg.com) is a leading web-based science, research and technology news service which covers a full range of topics. These include physics, earth science, medicine, nanotechnology, electronics, space, biology, chemistry, computer sciences, engineering, mathematics and other sciences and technologies. Launched in 2004, Phys.org’s readership has grown steadily to include 1.75 million scientists, researchers, and engineers every month. Phys.org publishes approximately 100 quality articles every day, offering some of the most comprehensive coverage of sci-tech developments world-wide. Quancast 2009 includes Phys.org in its list of the Global Top 2,000 Websites. Phys.org community members enjoy access to many personalized features such as social networking, a personal home page set-up, RSS/XML feeds, article comments and ranking, the ability to save favorite articles, a daily newsletter, and other options.

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

    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.

    See the full article here .

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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.

     
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