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  • richardmitnick 10:26 am on October 21, 2021 Permalink | Reply
    Tags: "Quantum material to boost terahertz frequencies", , Helmholtz-Zentrum Dresden-Rossendorf (HZDR)(DE), , Topological insulators   

    From Helmholtz-Zentrum Dresden-Rossendorf (HZDR)(DE) : “Quantum material to boost terahertz frequencies” 

    From Helmholtz-Zentrum Dresden-Rossendorf (HZDR)(DE)

    HZDR is a member of the Helmholtz Association of German Research Centres [Helmholtz-Gemeinschaft Deutscher Forschungszentren] (DE)

    October 20, 2021

    Further information:

    Jan-Christoph Deinert
    Institute of Radiation Physics at HZDR
    Tel. +49 351 260 3626
    j.deinert@hzdr.de

    Media contact:

    Simon Schmitt |
    Head and Press Officer
    Department of Communication and Media Relations at HZDR
    Tel.: +49 351 260-3400
    s.schmitt@hzdr.de

    New study elucidates fundamental enigma of topological insulators.

    They are regarded as one of the most interesting materials for future electronics: Topological insulators conduct electricity in a special way and hold the promise of novel circuits and faster mobile communications. Under the leadership of the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), a research team from Germany, Spain and Russia has now unravelled a fundamental property of this new class of materials: How exactly do the electrons in the material respond when they are “startled” by short pulses of so-called terahertz radiation? The results are not just significant for our basic understanding of this novel quantum material, but could herald faster mobile data communication or high-sensitivity detector systems for exploring distant worlds in years to come, the team reports in NPJ Quantum Materials.

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    Terahertz pulses (from left) strike a topological insulator. They excite the electrons, whose possible states are confined to energy bands. The extended bands in the bulk of the material (blue) exhibit an energy gap, which makes the material an electrical insulator. The situation is quite different at the surface: Here, the bands bridge the energy gap and thereby induce metallic behavior. The experiment shows that the electrons in these surface states return to equilibrium very quickly (purple grid). In contrast, the electrons in the other bands need about ten times longer to come to rest (yellow grid). Photo: Juniks/HZDR.

    Topological insulators are a very recent class of materials which have a special quantum property: on their surface they can conduct electricity almost loss-free while their interior functions as an insulator – no current can flow there. Looking to the future, this opens up interesting prospects: Topological insulators could form the basis for high efficiency electronic components, which makes them an interesting research field for physicists.

    But a number of fundamental questions are still unanswered. What happens, for example, when you give the electrons in the material a “nudge” using specific electromagnetic waves – so-called terahertz radiation – thus generating an excited state? One thing is clear: the electrons want to rid themselves of the energy boost forced upon them as quickly as possible, such as by heating up the crystal lattice surrounding them. In the case of topological insulators, however, it was previously unclear whether getting rid of this energy happened faster in the conducting surface than in the insulating core. “So far, we simply didn’t have the appropriate experiments to find out,” explains study leader Dr. Sergey Kovalev from the Institute of Radiation Physics at HZDR. “Up to now, at room temperature, it was extremely difficult to differentiate the surface reaction from that in the interior of the material.”

    In order to overcome this hurdle, he and his international team developed an ingenious test set-up: intensive terahertz pulses hit a sample and excite the electrons. Immediately after, laser flashes illuminate the material and register how the sample responds to the terahertz stimulation. In a second test series, special detectors measure to what extent the sample exhibits an unusual non-linear effect and multiplies the frequency of the terahertz pulses applied. Kovalev and his colleagues conducted these experiments using the TELBE terahertz light source at HZDR’s ELBE Center for High-Power Radiation Sources.

    Researchers from The Catalan Institute of Nanoscience and Nanotechnology [Institut Català de Nanociència i Nanotecnologia -ICN2] at The Autonomous University of Barcelona [Universidad Autónoma de Barcelona](ES), Bielefeld University [Universität Bielefeld](DE), The DLR German Aerospace Center [Deutsches Zentrum für Luft- und Raumfahrt e.V.](DE), The Technical University of Berlin [Technische Universität Berlin](DE) , and Lomonosova Moscow State University[ Московский государственный университет имени](RU) and the Kotelnikov Institute of Institute of Radio-engineering and Electronics [Институт радиотехники и электроники]of Russian Academy of Sciences [ Росси́йская акаде́мия нау́к](RU). were involved.

    Rapid energy transfer

    The decisive thing was that the international team did not only investigate a single material. Instead, the Russian project partners produced three different topological insulators with different, precisely determined properties: in one case, only the electrons on the surface could directly absorb the terahertz pulses. In the others, the electrons were mainly excited in the interior of the sample. “By comparing these three experiments we were able to differentiate precisely between the behavior of the surface and the interior of the material,” Kovalev explains. “And it emerged that the electrons in the surface became excited significantly faster than those in the interior of the material.” Apparently, they were able to transfer their energy to the crystal lattice immediately.

    Put into figures: while the surface electrons reverted to their original energetic state in a few hundred femtoseconds, the “inner” electrons took approximately ten times as long, that is, a few picoseconds. “Topological insulators are highly-complex systems. The theory is anything but easy to understand,” emphasizes Michael Gensch, former head of the TELBE facility at HZDR and now head of department in the Institute of Optical Sensor Systems at the German Aerospace Center (DLR) and professor at TU Berlin. “Our results can help decide which of the theoretical ideas hold true.

    Highly effective multiplication

    But the experiment also augurs well for interesting developments in digital communication like WLAN and mobile communications. Today, technologies such as 5G function in the gigahertz range. If we could harness higher frequencies in the terahertz range, significantly more data could be transmitted by a single radio channel, whereby frequency multipliers could play an important role: They are able to translate relatively low radio frequencies into significantly higher ones.

    Some time ago, the research team had already realized that, under certain conditions, graphene – a two-dimensional, super thin carbon – can act as an efficient frequency multiplier. It is able to convert 300 gigahertz radiation into frequencies of some terahertz. The problem is that when the applied radiation is extremely intensive, there is a significant drop in the efficiency of the graphene. Topological insulators, on the other hand, even function with the most intensive stimulation, the new study discovered. “This might mean it’s possible to multiply frequencies from a few terahertz to several dozen terahertz,” surmises HZDR physicist Jan-Christoph Deinert, who heads the TELBE team together with Sergey Kovalev. “At the moment, there is no end in sight when it comes to topological insulators.”

    If such a development comes about, the new quantum materials could be used in a much wider frequency range than with graphene. “At DLR, we are very interested in using quantum materials of this kind in high-performance heterodyne receivers for astronomy, especially in space telescopes,” Gensch explains.

    See the full article here.

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    HIF_Hauptgebäude

    Helmholtz-Zentrum Dresden-Rossendorf (HZDR)(DE) is a Dresden-based research laboratory. It conducts research in three of the Helmholtz Association’s areas: materials, health, and energy. HZDR is a member of the Helmholtz Association of German Research Centres [Helmholtz-Gemeinschaft Deutscher Forschungszentren](DE).

    The Helmholtz Association of German Research Centres (DE) is the largest scientific organisation in Germany. It is a union of 18 scientific-technical and biological-medical research centers. The official mission of the Association is “solving the grand challenges of science, society and industry”. Scientists at Helmholtz therefore focus research on complex systems which affect human life and the environment. The namesake of the association is the German physiologist and physicist Hermann von Helmholtz.

    The annual budget of the Helmholtz Association amounts to €4.56 billion, of which about 72% is raised from public funds. The remaining 28% of the budget is acquired by the 19 individual Helmholtz Centres in the form of contract funding. The public funds are provided by the federal government (90%) and the rest by the States of Germany (10%).

    The Helmholtz Association was ranked #8 in 2015 and #7 in 2017 by the Nature Index, which measures the largest contributors to papers published in 82 leading journals.

     
  • richardmitnick 9:34 am on February 3, 2021 Permalink | Reply
    Tags: "A new hands-off probe uses light to explore the subtleties of electron behavior in a topological insulator", , , HHG-high harmonic generation, , , Topological insulators   

    From DOE’s SLAC National Accelerator Laboratory and : “A new hands-off probe uses light to explore the subtleties of electron behavior in a topological insulator” 

    From DOE’s SLAC National Accelerator Laboratory

    and

    Stanford University Name

    From Stanford University

    February 2, 2021
    Glennda Chui

    Just as pressing a guitar string produces a higher pitch, sending laser light through a material can shift it to higher energies and higher frequencies. Now scientists have discovered how to use this phenomenon to explore quantum materials in a new and much more detailed way.

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    Researchers at SLAC National Accelerator Laboratory and Stanford University discovered that focusing intense, circularly polarized laser light on a topological insulator generates harmonics that can be used to probe electron behavior in the material’s topological surface, a sort of electron superhighway where electrons flow with no loss. The technique should be applicable to a wide range of quantum materials. Credit: Greg Stewart/SLAC.

    2
    Laser light is usually linearly polarized, meaning that its waves oscillate in only one direction – up and down, in the example at left. But it can also be circularly polarized, at right, so its waves spiral like a corkscrew around the direction the light is traveling. A new study from SLAC and Stanford predicts that this circularly polarized light can be used to explore quantum materials in ways that were not possible before. Cedit: Greg Stewart/SLAC.

    Topological insulators are one of the most puzzling quantum materials – a class of materials whose electrons cooperate in surprising ways to produce unexpected properties. The edges of a TI are electron superhighways where electrons flow with no loss, ignoring any impurities or other obstacles in their path, while the bulk of the material blocks electron flow.

    Scientists have studied these puzzling materials since their discovery just over a decade ago with an eye to harnessing them for things like quantum computing and information processing.

    Now researchers at the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University have invented a new, hands-off way to probe the fastest and most ephemeral phenomena within a TI and clearly distinguish what its electrons are doing on the superhighway edges from what they’re doing everywhere else.

    The technique takes advantage of a phenomenon called high harmonic generation, or HHG, which shifts laser light to higher energies and higher frequencies – much like pressing a guitar string produces a higher note – by shining it through a material. ­­By varying the polarization of laser light going into a TI and analyzing the shifted light coming out, researchers got strong and separate signals that told them what was happening in each of the material’s two contrasting domains.

    “What we found out is that the light coming out gives us information about the properties of the superhighway surfaces,” said Shambhu Ghimire, a principal investigator with the Stanford PULSE Institute at SLAC, where the work was carried out.

    “This signal is quite remarkable, and its dependence on the polarization of the laser light is dramatically different from what we see in conventional materials. We think we have a potentially novel approach for initiating and probing quantum behaviors that are supposed to be present in a broad range of quantum materials.”

    The research team reported the results today in Physical Review A.

    Light in, light out

    Starting in 2010, a series of experiments led by Ghimire and PULSE Director David Reis showed HHG can be produced in ways that were previously thought unlikely or even impossible: by beaming laser light into a crystal, a frozen argon gas or an atomically thin semiconductor material. Another study described how to use HHG to generate attosecond laser pulses, which can be used to observe and control the movements of electrons, by shining a laser through ordinary glass.

    In 2018, Denitsa Baykusheva, a Swiss National Science Foundation Fellow with a background in HHG research, joined the PULSE group as a postdoctoral researcher. Her goal was to study the potential for generating HHG in topological insulators – the first such study in a quantum material. “We wanted to see what happens to the intense laser pulse used to generate HHG,” she said. “No one had actually focused such a strong laser light on these materials before.”

    But midway through those experiments, the COVID-19 pandemic hit and the lab shut down in March 2020 for all but essential research. So the team had to think of other ways to make progress, Baykusheva said.

    “In a new area of research like this one, theory and experiment have to go hand in hand,” she explained. “Theory is essential for explaining experimental results and also predicting the most promising avenues for future experiments. So we all turned ourselves into theorists” – first working with pen and paper and then writing code and doing calculations to feed into computer models.

    An illuminating result

    To their surprise, the results predicted that circularly polarized laser light, whose waves spiral around the beam like a corkscrew, could be used to trigger HHG in topological insulators [above].

    “One of the interesting things we observed is that circularly polarized laser light is very efficient at generating harmonics from the superhighway surfaces of the topological insulator, but not from the rest of it,” Baykusheva said. “This is something very unique and specific to this type of material. It can be used to get information about electrons that travel the superhighways and those that don’t, and it can also be used to explore other types of materials that can’t be probed with linearly polarized light.”

    The results lay out a recipe for continuing to explore HHG in quantum materials, said Reis, who is a co-author of the study.

    “It’s remarkable that a technique that generates strong and potentially disruptive fields, which takes electrons in the material and jostles them around and uses them to probe the properties of the material itself, can give you such a clear and robust signal about the material’s topological states,” he said.

    “The fact that we can see anything at all is amazing, not to mention the fact that we could potentially use that same light to change the material’s topological properties.”

    Experiments at SLAC have resumed on a limited basis, Reis added, and the results of the theoretical work have given the team new confidence that they know exactly what they are looking for.

    Researchers from the Max Planck POSTECH/KOREA Research Initiative also contributed to this report. Major funding for the study came from the DOE Office of Science and the Swiss National Science Foundation.

    See the full article here .


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    Stanford University campus. No image credit

    Stanford University

    Leland and Jane Stanford founded the University to “promote the public welfare by exercising an influence on behalf of humanity and civilization.” Stanford opened its doors in 1891, and more than a century later, it remains dedicated to finding solutions to the great challenges of the day and to preparing our students for leadership in today’s complex world. Stanford, is an American private research university located in Stanford, California on an 8,180-acre (3,310 ha) campus near Palo Alto. Since 1952, more than 54 Stanford faculty, staff, and alumni have won the Nobel Prize, including 19 current faculty members

    Stanford University Seal

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

    SLAC National Accelerator Lab


    SLAC/LCLS


    SLAC/LCLS II projected view

    SLAC LCLS-II Undulators The Linac Coherent Light Source’s new undulators each use an intricately tuned series of magnets to convert electron energy into intense bursts of X-rays. The “soft” X-ray undulator stretches for 100 meters on the left side of this hall, with the “hard” x-ray undulator on the right. Credit: Alberto Gamazo/SLAC National Accelerator Laboratory.

    SSRL and LCLS are DOE Office of Science user facilities.

    3

     
  • richardmitnick 8:26 am on September 23, 2020 Permalink | Reply
    Tags: "I focus on how electrons behave within solids.", "Think We Already Know Everything About Electrons? Think Again", , , , Emergent phenomena-in which groups of atoms or electrons act in unexpected ways., Groups of electrons collectively behave differently than we would expect from the way each individual electron acts on its own., , , , Songtian Sonia Zhang, Superconductivity is an example of an emergent phenomenon within physics., Topological insulators, ,   

    From Simons Foundation: Women in STEM-Songtian Sonia Zhang”Think We Already Know Everything About Electrons? Think Again” 

    From Simons Foundation

    1
    Songtian Sonia Zhang. Credit: Rick Soden, Princeton University
    Physicist Songtian Sonia Zhang explores how electrons work within the tiniest objects and finds that they sometimes do unexpected things.

    September 22, 2020
    Marcus Banks

    Songtian Sonia Zhang envisioned a life in finance, until she discovered that learning how electrons work is much more rewarding. A fundamental physicist with a bachelor’s degree from the University of Waterloo in Ontario, Canada, and a doctorate from Princeton University, Zhang has already discovered unexpected behaviors among electrons found in quantum materials like superconductors or magnets. But many mysteries remain about the behavior of these tiny particles. Now beginning a postdoctoral appointment at Columbia University in the physics lab of Dmitri N. Basov, Zhang already has lots of ideas about what she wants to explore next. Our conversation has been edited for clarity.

    You began as a dual major in economics and physics at the University of Waterloo. What prompted the sharper focus on physics instead?

    When I was an undergraduate at Waterloo, I planned to pursue a career in finance, perhaps even on Wall Street. I was interested in physics too, but I never imagined becoming a physicist. That all changed after I completed a physics research project in my third year of college, which happened to overlap with my first job at a financial services firm. This gave me the chance to directly compare finance to physics work — and physics won out handily.

    When I was doing physics, I felt like I was helping to bring new understanding into the world. I know that sounds corny, but it’s true. And it was far more rewarding to me than my work at the financial firm, where I essentially was moving money around. Don’t get me wrong, we need money! But I knew early that physics was for me.

    That sounds clarifying! But the study of physics is broad. How did you narrow your interests?

    During my last semester at Waterloo I researched a special kind of magnet known as ‘spin ice,’ in which the atoms are arranged in a complex lattice pattern. Most magnets have a north and south pole. And if you cut a magnet in two, each new magnet will then also have a north and south pole. But spin ice magnets have such a complex structure that we call them geometrically frustrated. Spin ice magnets can behave like monopoles — that is, a magnet with only one pole instead of the normal two. We still don’t know if monopoles even exist! But the spin ice magnet I studied sure seemed like a monopole, which was fascinating to me. When I first began to study physics, I assumed I would become an astrophysicist. Instead I decided to be more down-to-earth — literally. Today I study the physics of solids, not stars.

    This sounds like quantum physics.

    In many ways, yes. But quantum physics is an extremely broad term that can apply to many things, so in some ways it’s too general. The specific field I work in is condensed matter physics. I focus on how electrons behave within solids. I’m particularly interested in how groups of electrons behave, and especially how their collective behavior cannot be predicted by how each individual in the group acts.

    You’re saying that groups of electrons collectively behave differently than we would expect from the way each individual electron acts on its own?

    Exactly. We call this overall concept ‘emergent phenomena,’ in which groups of atoms or electrons act in unexpected ways. There are many examples of emergent phenomena in nature that go well beyond quantum physics. Think of individual birds migrating together as a cohesive flock, or a school of fish swimming upriver to spawn. Even though each individual bird or fish moves independently, they become entangled in the group and impossible to distinguish from one another.

    Superconductivity is an example of an emergent phenomenon within physics. Regular electrical conductors carry a current known as electricity; this is how we light lightbulbs, for example, by connecting an electricity source to an object that emits light. These kinds of everyday conductors come with inherent inefficiencies — energy is always lost because the electricity faces resistance as it travels. This is why lightbulbs eventually burn out.

    In contrast, a superconductor operating in extremely low temperatures (−450 F) can keep conducting electricity forever, because the electricity meets no resistance at all. Nobody could have predicted that superconductors could do this. It had to be discovered through observation, and it’s an example of how we are constantly learning about new types of emergent phenomena. Sometimes this is purely about developing knowledge for its own sake, but oftentimes this work has practical applications.

    We’ll loop back to the practical applications in a moment. First, though, what was your most exciting discovery at Princeton?

    At Princeton I studied kagome magnets. The atoms that comprise these magnets are arranged in a lattice which evokes the famous Japanese basket lattice pattern of the same name.

    4
    Scanning tunneling microscopy (STM) image of magnetic adatoms deposited on topological superconductor candidate PbTaSe2. Inset: a 2D enlarged view. Each magnetic adatom can host a Majorana zero mode acting as a topological qubit which has the potential to be used for robust quantum computation. Credit: Songtian Zhang.

    Like the spin ice magnets I previously mentioned, kagome magnets are geometrically frustrated. In our research, we did various things to these magnets, such as observing them within magnetic fields of various strengths or alternating their temperatures. This was all to see how the electrons behaved in different conditions. In high magnetic fields the kagome magnets started acting like negative magnets — meaning they exerted more energy when moving in the same direction as a magnetic field and not when going ‘against the wind,’ so to speak.

    We published these unexpected results in Nature Physics last year.

    The fact that we discovered something totally unexpected shows the importance of keeping an open mind, of not being locked into any one idea. Instead, we are always finding new questions to ask.

    As you begin your postdoctoral work at Columbia, what do you plan to focus on, at least initially, until you discover new questions?

    I’m interested in learning more about topological insulators: objects that, on the surface, conduct electricity but in their interior act as an insulator. When the material is cut, the new surface, which was previously the insulating bulk, becomes conductive and can now support surface currents. Besides topological insulators there are topological superconductors, which can superconduct currents of electricity. The physics community has made some headway in understanding these superconductors, but there’s a lot more work that needs to be done.

    And how do you hope this knowledge will inform our understanding of the natural and physical world?

    Topological superconductors come from the math concept of topology. A good way to think about topology is the relationship between a doughnut, with a hole in the middle, and a ball, which has no holes. In this comparison, the doughnut and the ball are topologically distinct.

    By comparison, the doughnut would be topologically identical to a ring, which also has a hole in the middle. In this example, the number of holes is a topological property that can’t be destroyed without changing the underlying nature of the object.

    In physics, we’re interested in electronic behaviors that are similarly robust, such as in topological superconductors. There’s great potential for topological superconductors to be used for powerful, reliable and robust quantum computation, which will be a giant leap past the computers we use today. I can’t say exactly how my work will contribute to this, but I do know I’m excited to be on the journey. And I know I will enjoy it more than working on Wall Street.

    See the full article here.

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    Mission and Model

    The Simons Foundation’s mission is to advance the frontiers of research in mathematics and the basic sciences.

    Co-founded in New York City by Jim and Marilyn Simons, the foundation exists to support basic — or discovery-driven — scientific research undertaken in the pursuit of understanding the phenomena of our world.

    The Simons Foundation’s support of science takes two forms: We support research by making grants to individual investigators and their projects through academic institutions, and, with the launch of the Flatiron Institute in 2016, we now conduct scientific research in-house, supporting teams of top computational scientists.

     
  • richardmitnick 11:29 am on October 2, 2019 Permalink | Reply
    Tags: "Penn Engineers’ New Topological Insulator Can Reroute Photonic ‘Traffic’ On the Fly Making for Faster Chips", , , Topological insulators, , Using photons instead of electrons   

    From University of Pennsylvania Engineering: “Penn Engineers’ New Topological Insulator Can Reroute Photonic ‘Traffic’ On the Fly, Making for Faster Chips” 

    From University of Pennsylvania Engineering

    Topological insulators are a game-changing class of materials; charged particles can flow freely on their edges and route themselves around defects, but can’t pass through their interiors. This perfect surface conduction holds promise for fast and efficient electronic circuits, though engineers must contend with the fact that the interiors of such materials are effectively wasted space.

    1
    The researchers’ chip features a tessellated grid of oval rings. By “pumping” individual rings with an external laser, they are able to dynamically redefine the path photons take. (Image: Penn Engineering)

    Now, researchers from the University of Pennsylvania, where topological insulators were first discovered in 2005, have shown a way to fulfill that promise in a field where physical space is at an even bigger premium: photonics. They have shown, for the first time, a way for a topological insulator to make use of its entire footprint.

    By using photons instead of electrons, photonic chips promise even faster data transfer speeds and information-dense applications, but the components necessary for building them remain considerably larger than their electronic counterparts, due to the lack of efficient data-routing architecture.

    A photonic topological insulator with edges that can be redefined on the fly, however, would help solve the footprint problem. Being able to route these “roads” around one another as needed means the entire interior bulk could be used to efficiently build data links.

    Researchers at Penn’s School of Engineering and Applied Science have built and tested such a device for the first time, publishing their findings in the journal Science.

    “This could have a big impact on large-information capacity applications, like 5G, or even 6G, cellphone networks,” says Liang Feng, assistant professor in Penn Engineering’s Departments of Materials Science and Engineering and Electrical and Systems Engineering.

    “We think this may be the first practical application of topological insulators,” he says.

    2
    Liang Feng and Han Zhao

    Feng led the study along with graduate student Han Zhao, a member of his lab. Fellow lab members Xingdu Qiao, Tianwei Wu and Bikashkali Midya, along with Stefano Longhi, professor at the Polytechnic University of Milan in Italy, also contributed to the research.

    The data centers that form the backbone of communication networks route calls, texts, email attachments and streaming movies to and between millions of cellular devices. But as the amount of data flowing through these data centers increases, so does the need for high-capacity data routing that can keep up with the demand.

    Switching from electrons to photons would speed up this process for the upcoming information explosion, but engineers must first design a whole new library of devices for getting those photons from input to output without mixing them up and losing them in the process.

    Advances in data-processing speed in electronics have relied on making their core components smaller and smaller, but photonics researchers have needed to take a different approach.

    Feng, Zhao and their colleagues set out to maximize the complexity of photonic waveguides — the prescribed paths individual photons take on their way from input to output — on a given chip.

    3
    Microscope details of the researchers’ photonic chip.

    The researchers’ prototype photonic chip is roughly 250 microns squared, and features a tessellated grid of oval rings. By “pumping” the chip with an external laser, targeted to alter the photonic properties of individual rings, they are able to alter which of those rings constitute the boundaries of a waveguide.

    The result is a reconfigurable topological insulator. By changing the pumping patterns, photons headed in different directions can be routed around each other, allowing photons from multiple data packets to travel through the chip simultaneously, like a complicated highway interchange.

    “We can define the edges such that photons can go from any input port to any output port, or even to multiple outputs at once,” Feng says. “That means the ports-to-footprint ratio is at least two orders of magnitude greater than current state-of-the-art photonic routers and switches.”

    Increased efficiency and speed is not the only advantage of the researchers’ approach.

    “Our system is also robust against unexpected defects,” Zhao says. “If one of the rings is damaged by a grain of dust, for example, that damage is just making a new set of edges that we can send photons along.”

    Since the system requires an off-chip laser source to redefine the shape of the waveguides, the researcher’s system is not yet small enough to be useful for data centers or other commercial applications. Next steps for the team will be to establish a fast reconfiguring scheme in an integrated fashion.

    Support for this research comes from the U.S. Army Research Office through grant W911NF-19–1–0249 and the National Science Foundation through grants ECCS-1846766 and CMMI-1635026 and University of Pennsylvania Materials Research Science and Engineering Center NSF MRSEC grant DMR-1720530. The work was carried out in part at the Singh Center for Nanotechnology, which is supported by the NSF National Nanotechnology Coordinated Infrastructure Program under grant NNCI-1542153.

    See the full article here .

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    U Penn campus

    Penn Engineering – Galway, Ireland

    Academic life at Penn is unparalleled, with 100 countries and every U.S. state represented in one of the Ivy League’s most diverse student bodies. Consistently ranked among the top 10 universities in the country, Penn enrolls 10,000 undergraduate students and welcomes an additional 10,000 students to our world-renowned graduate and professional schools.

    Penn’s award-winning educators and scholars encourage students to pursue inquiry and discovery, follow their passions, and address the world’s most challenging problems through an interdisciplinary approach.

     
  • richardmitnick 1:44 pm on April 27, 2018 Permalink | Reply
    Tags: , , Majorana fermion science, , , , , Topological insulators, Topological quantum computation,   

    From Physics Illinois: “Topological insulator �flips� for superconductivity” 

    U Illinois bloc

    Physics Illinois

    U Illinois Physics bloc

    4/27/2018
    Siv Schwink

    Topology meets superconductivity through innovative reverse-order sample preparation.

    1
    (L-R) Professor of Physics James Eckstein, his graduate student Yang Bai, and Professor of Physics Tai-Chang Chiang pose in front of the atomic layer by layer molecular beam epitaxy system used to grow the topological insulator thin-film samples for this study, in the Eckstein laboratory at the University of Illinois. Photo by L. Brian Stauffer, University of Illinois at Urbana-Champaign.

    A groundbreaking sample preparation technique has enabled researchers at the University of Illinois at Urbana-Champaign and the University of Tokyo to perform the most controlled and sensitive study to date of a topological insulator (TI) closely coupled to a superconductor (SC). The scientists observed the superconducting proximity effect—induced superconductivity in the TI due to its proximity to the SC—and measured its relationship to temperature and the thickness of the TI.

    TIs with induced superconductivity are of paramount interest to physicists because they have the potential to host exotic physical phenomena, including the elusive Majorana fermion—an elementary particle theorized to be its own antiparticle—and to exhibit supersymmetry—a phenomenon reaching beyond the standard model that would shed light on many outstanding problems in physics. Superconducting TIs also hold tremendous promise for technological applications, including topological quantum computation and spintronics.

    Naturally occurring topological superconductors are rare, and those that have been investigated have exhibited extremely small superconducting gaps and very low transition temperatures, limiting their usefulness for uncovering the interesting physical properties and behaviors that have been theorized.

    TIs have been used in engineering superconducting topological superconductors (TI/SC), by growing TIs on a superconducting substrate. Since their experimental discovery in 2007, TIs have intrigued condensed matter physicists, and a flurry of theoretical and experimental research taking place around the globe has explored the quantum-mechanical properties of this extraordinary class of materials. These 2D and 3D materials are insulating in their bulk, but conduct electricity on their edges or outer surfaces via special surface electronic states which are topologically protected, meaning they can’t be easily destroyed by impurities or imperfections in the material.

    But engineering such TI/SC systems via growing TI thin films on superconducting substrates has also proven challenging, given several obstacles, including lattice structure mismatch, chemical reactions and structural defects at the interface, and other as-yet poorly understood factors.

    2
    The �flip-chip� cleavage-based sample preparation: (A) A photo and a schematic diagram of assembled Bi2Se3(0001)/Nb sample structure before cleavage. (B) Same sample structure after cleavage exposing a �fresh� surface of the Bi2Se3 film with a pre-determined thickness. Image courtesy of James Eckstein and Tai-Chang-Chiang, U. of I. Department of Physics and Frederick Seitz Materials Research Laboratory.

    Now, a novel sample-growing technique developed at the U. of I. has overcome these obstacles. Developed by physics professor James Eckstein in collaboration with physics professor Tai-Chang Chiang, the new “flip-chip” TI/SC sample-growing technique allowed the scientists to produce layered thin-films of the well-studied TI bismuth selenide on top of the prototypical SC niobium—despite their incompatible crystalline lattice structures and the highly reactive nature of niobium.

    These two materials taken together are ideal for probing fundamental aspects of the TI/SC physics, according to Chiang: “This is arguably the simplest example of a TI/SC in terms of the electronic and chemical structures. And the SC we used has the highest transition temperature among all elements in the periodic table, which makes the physics more accessible. This is really ideal; it provides a simpler, more accessible basis for exploring the basics of topological superconductivity,” Chiang comments.

    The method allows for very precise control over sample thickness, and the scientists looked at a range of 3 to 10 TI layers, with 5 atomic layers per TI layer. The team’s measurements showed that the proximity effect induces superconductivity into both the bulk states and the topological surface states of the TI films. Chiang stresses, what they saw gives new insights into superconducting pairing of the spin-polarized topological surface states.

    “The results of this research are unambiguous. We see the signal clearly,” Chiang sums up. “We investigated the superconducting gap as a function of TI film thickness and also as a function of temperature. The results are pretty simple: the gap disappears as you go above niobium’s transition temperature. That’s good—it’s simple. It shows the physics works. More interesting is the dependence on the thickness of the film. Not surprisingly, we see the superconducting gap reduces for increasing TI film thickness, but the reduction is surprisingly slow. This observation raises an intriguing question regarding how the pairing at the film surface is induced by coupling at the interface.”

    Chiang credits Eckstein with developing the ingenious sample preparation method. It involves assembling the sample in reverse order, on top of a sacrificial substrate of aluminum oxide, commonly known as the mineral sapphire. The scientists are able to control the specific number of layers of TI crystals grown, each of quintuple atomic thickness. Then a polycrystalline superconducting layer of niobium is sputter-deposited on top of the TI film. The sample is then flipped over and the sacrificial layer that had served as the substrate is dislodged by striking a “cleavage pin.” The layers are cleaved precisely at the interface of the TI and aluminum oxide.

    3
    A close-up shot of the atomic layer by layer molecular beam epitaxy system used to grow the topological insulator thin-film samples for this study, located in the Eckstein laboratory at the University of Illinois. Photo by L. Brian Stauffer, University of Illinois at Urbana-Champaign.

    Eckstein explains, “The ‘flip-chip’ technique works because the layers aren’t strongly bonded—they are like a stack of paper, where there is strength in the stack, but you can pull apart the layers easily. Here, we have a triangular lattice of atoms, which comes in packages of five—these layers are strongly bonded. The next five layers sit on top, but are weakly bonded to the first five. It turns out, the weakest link is right at the substrate-TI interface. When cleaved, this method gives a pure surface, with no contamination from air exposure.”

    The cleavage was performed in an ultrahigh vacuum, within a highly sensitive instrument at the Institute for Solid State Physics at the University of Tokyo capable of angle-resolved photoemission spectroscopy (ARPES) at a range of temperatures.

    Chiang acknowledges, “The superconducting features occur at very small energy scales—it requires a very high energy resolution and very low temperatures. This portion of the experiment was completed by our colleagues in the University of Tokyo, where they have the instruments with the sensitivity to get the resolution we need for this kind of study. We couldn’t have done this without this international collaboration.”

    “This new sample preparation method opens up many new avenues in research, in terms of exotic physics, and, in the long term, in terms of possible useful applications—potentially even including building a better superconductor. It will allow preparation of samples using a wide range of other TIs and SCs. It could also be useful in miniaturization of electronic devices, and in spintronic computing, which would require less energy in terms of heat dissipation,” Chiang concludes.

    Eckstein adds, “There is a lot of excitement about this. If we can make a superconducting TI, theoretical predictions tell us that we could find a new elementary excitation that would make an ideal topological quantum bit, or qubit. We’re not there yet, and there are still many things to worry about. But it would be a qubit whose quantum mechanical wave function would be less susceptible to local perturbations that might cause dephasing, messing up calculations.”

    These findings were published online on 27 April 2018 in the journal Science Advances.

    See the full article here .

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    The University of Illinois at Urbana-Champaign community of students, scholars, and alumni is changing the world.

    With our land-grant heritage as a foundation, we pioneer innovative research that tackles global problems and expands the human experience. Our transformative learning experiences, in and out of the classroom, are designed to produce alumni who desire to make a significant, societal impact.

     
  • richardmitnick 1:05 pm on November 29, 2017 Permalink | Reply
    Tags: , Topological insulators,   

    From U Sidney via Science Alert: “Physicists Just Invented an Essential Component Needed For Quantum Computers” 

    U Sidney bloc

    University of Sidney

    Science Alert

    29 NOV 2017
    MIKE MCRAE

    1
    (The University of Sydney)

    They’re using a new state of matter for this.

    In 2016, the Nobel Prize in Physics went to three British scientists for their work on superconductors and superfluids, which included the explanation of a rather odd phase of matter.

    Now, for the first time, their discovery has a practical application – shrinking an electrical component to a size that will help quantum computers reach a scale that just might make them useful.

    In a collaboration with Stanford University in the US, a team of scientists from the University of Sydney and Microsoft have used the newly found phase of matter – topological insulator – in shrinking an electrical component called a circulator 1,000 times smaller.

    That’s super good news when it comes to squeezing more qubits into a small enough space.

    If you missed the fuss last year, a trio of physicists received the Nobel Prize for discovering that under certain conditions some materials could easily conduct electrons along their surface, but remain an insulator within.

    Most importantly, they discovered cases where matter transitioned between states without breaking something called symmetry, as happens when water atoms rearrange into ice or vapour.

    As we shrink electrical components down to virtually atomic scales, the way electrons move in different dimensions becomes increasingly important.

    Enter the qubit – a chunky piece of electronics that uses the probabilities of an unmeasured bit of matter to perform calculations classical computers can’t hope to match.

    We can make qubits in a variety of ways, and are getting pretty good at stringing them together in ever larger numbers.

    But shrinking qubits to sizes small enough that we can shove hundreds of thousands into a small-enough space is a challenge.

    “Even if we had millions of qubits today, it is not clear that we have the classical technology to control them,” says David Reilly, a physicist at the University of Sydney and Director of Microsoft Station Q.

    “Realising a scaled-up quantum computer will require the invention of new devices and techniques at the quantum-classical interface.”

    One such device is called a circulator, which is kind-of like a roundabout for electrical signals, ensuring information heads in one direction only.

    Until now, the smallest versions of this hardware could be held in the palm of your hand.

    This is now set to change as scientists have shown a magnetised wafer made of a particular topological insulator could do the job, and be made 1,000 times smaller than existing components.

    “Such compact circulators could be implemented in a variety of quantum hardware platforms, irrespective of the particular quantum system used,” says the study’s lead author, Alice Mahoney.

    In many respects, we’re still at the pre-vacuum-tube and magnetic tape phase of quantum computers – they’re more promise than practical.

    But if we keep seeing advances like this, it won’t be long before we’ll be bringing you news of quantum computers cracking problems which leave our best supercomputers gasping.

    This research was published in Nature Communications.

    See the full article here .

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    Our founding principle as Australia’s first university was that we would be a modern and progressive institution. It’s an ideal we still hold dear today.

    When Charles William Wentworth proposed the idea of Australia’s first university in 1850, he imagined “the opportunity for the child of every class to become great and useful in the destinies of this country”.

    We’ve stayed true to that original value and purpose by promoting inclusion and diversity for the past 160 years.

    It’s the reason that, as early as 1881, we admitted women on an equal footing to male students. Oxford University didn’t follow suit until 30 years later, and Jesus College at Cambridge University did not begin admitting female students until 1974.

    It’s also why, from the very start, talented students of all backgrounds were given the chance to access further education through bursaries and scholarships.

    Today we offer hundreds of scholarships to support and encourage talented students, and a range of grants and bursaries to those who need a financial helping hand.

     
  • richardmitnick 5:31 pm on July 20, 2017 Permalink | Reply
    Tags: , , If all the states in a group of bands are filled with electrons then the electrons cannot move and the material is an insulator, , , , The approach combined tools from such disparate fields as chemistry and mathematics also physics and materials science, The nearly century-old band theory of solids considered one of the early landmark achievements of quantum mechanics, The research shows that symmetry and topology also chemistry and physics all have a fundamental role to play in our understanding of materials, The theory describes the electrons in crystals as residing in specific energy levels known as bands, Topological insulators   

    From Princeton: “Researchers find path to discovering new topological materials, holding promise for technological applications” 

    Princeton University
    Princeton University

    July 20, 2017
    No writer credit

    1
    Researchers have discovered how to identify new examples of topological materials, which have unique and desirable electronic properties. The technique involves finding the connection between band theory, which describes the energy levels of electrons in a solid, with a material’s topological nature. The disconnected bands indicate the material is a topological insulator. Image courtesy of Nature.

    Researchers find path to discovering new topological materials, holding promise for technological applications.

    An international team of researchers has found a way to determine whether a crystal is a topological insulator — and to predict crystal structures and chemical compositions in which new ones can arise. The results, published July 20 in the journal Nature, show that topological insulators are much more common in nature than currently believed.

    Topological materials, which hold promise for a wide range of technological applications due to their exotic electronic properties, have attracted a great deal of theoretical and experimental interest over the past decade, culminating in the 2016 Nobel Prize in physics. The materials’ electronic properties include the ability of current to flow without resistance and to respond in unconventional ways to electric and magnetic fields.

    Until now, however, the discovery of new topological materials occurred mainly by trial and error. The new approach described this week [see above reference to the Nature article] allows researchers to identify a large series of potential new topological insulators. The research represents a fundamental advance in the physics of topological materials and changes the way topological properties are understood.

    The team included: at Princeton University, Barry Bradlyn and Jennifer Cano, both associate research scholars at the Princeton Center for Theoretical Science, Zhijun Wang, a postdoctoral research associate, and B. Andrei Bernevig, professor of physics; professors Luis Elcoro and Mois Aroyo at the University of the Basque Country in Bilbao; assistant professor Maia Garcia Vergniory of University of the Basque Country and Donostia International Physics Center (DIPC) in Spain; and Claudia Felser, professor at the Max Planck Institute for Chemical Physics of Solids in Germany.

    “Our approach allows for a much easier way to find topological materials, avoiding the need for detailed calculations,” Felser said. “For some special lattices, we can say that, regardless of whether a material is an insulator or a metal, something topological will be going on,” Bradlyn added.

    Until now, of the roughly 200,000 materials catalogued in materials databases, only around a few hundred are known to host topological behavior, according to the researchers. “This raised the question for the team: Are topological materials really that scarce, or does this merely reflect an incomplete understanding of solids?” Cano said.

    To find out, the researchers turned to the nearly century-old band theory of solids, considered one of the early landmark achievements of quantum mechanics. Pioneered by Swiss-born physicist Felix Bloch and others, the theory describes the electrons in crystals as residing in specific energy levels known as bands. If all the states in a group of bands are filled with electrons, then the electrons cannot move and the material is an insulator. If some of the states are unoccupied, then electrons can move from atom to atom and the material is capable of conducting an electrical current.

    Because of the symmetry properties of crystals, however, the quantum states of electrons in solids have special properties. These states can be described as a set of interconnected bands characterized by their momentum, energy and shape. The connections between these bands, which on a graph resemble tangled spaghetti strands, give rise to topological behaviors such as those of electrons that can travel on surfaces or edges without resistance.

    The team used a systematic search to identify many previously undiscovered families of candidate topological materials. The approach combined tools from such disparate fields as chemistry, mathematics, physics and materials science.

    First, the team characterized all the possible electronic band structures arising from electronic orbitals at all the possible atomic positions for all possible crystal patterns, or symmetry groups, that exist in nature, with the exception of magnetic crystals. To search for topological bands, the team first found a way to enumerate all allowed non-topological bands, with the understanding that anything left out of the list must be topological. Using tools from group theory, the team organized into classes all the possible non-topological band structures that can arise in nature.

    Next, by employing a branch of mathematics known as graph theory — the same approach used by search engines to determine links between websites — the team determined the allowed connectivity patterns for all of the band structures. The bands can either separate or connect together. The mathematical tools determine all the possible band structures in nature — both topological and non-topological. But having already enumerated the non-topological ones, the team was able to show which band structures are topological.

    By looking at the symmetry and connectivity properties of different crystals, the team identified several crystal structures that, by virtue of their band connectivity, must host topological bands. The team has made all of the data about non-topological bands and band connectivity available to the public through the Bilbao Crystallographic Server. “Using these tools, along with our results, researchers from around the world can quickly determine if a material of interest can potentially be topological,” Elcoro said.

    The research shows that symmetry, topology, chemistry and physics all have a fundamental role to play in our understanding of materials, Bernevig said. “The new theory embeds two previously missing ingredients, band topology and orbital hybridization, into Bloch’s theory and provides a prescriptive path for the discovery and characterization of metals and insulators with topological properties.”

    David Vanderbilt, a professor of physics and astronomy at Rutgers University who was not involved in the study, called the work remarkable. “Most of us thought it would be many years before the topological possibilities could be catalogued exhaustively in this enormous space of crystal classes,” Vanderbilt said. “This is why the work of Bradlyn and co-workers comes as such a surprise. They have developed a remarkable set of principles and algorithms that allow them to construct this catalogue at a single stroke. Moreover, they have combined their theoretical approach with materials database search methods to make concrete predictions of a wealth of new topological insulator materials.”

    The theoretical underpinnings for these materials, called “topological” because they are described by properties that remain intact when an object is stretched, twisted or deformed, led to the awarding of the Nobel Prize in physics in 2016 to F. Duncan M. Haldane, Princeton’s Sherman Fairchild University Professor of Physics; J. Michael Kosterlitz of Brown University; and David J. Thouless of the University of Washington.

    Chemistry and physics take different approaches to describing crystalline materials, in which atoms occur in regularly ordered patterns or symmetries. Chemists tend to focus on the atoms and their surrounding clouds of electrons, known as orbitals. Physicists tend to focus on the electrons themselves, which can carry electric current when they hop from atom to atom and are described by their momentum.

    “This simple fact — that the physics of electrons is usually described in terms of momentum, while the chemistry of electrons is usually described in terms of electronic orbitals — has left material discovery in this field at the mercy of chance,” Wang said.

    “We initially set out to better understand the chemistry of topological materials — to understand why some materials have to be topological,” Vergniory said.

    Aroyo added, “What came out was, however, much more interesting: a way to marry chemistry, physics and mathematics that adds the last missing ingredient in a century-old theory of electronics, and in the present-day search for topological materials.”

    Funding for the study was provided by the U.S. Department of Energy (DE-SC0016239), the U.S. National Science Foundation (EAGER DMR-1643312 and MRSEC DMR-1420541), and the U.S. Office of Naval Research (N00014-14-1-0330). Additional funding came from a Simons Investigator Award, the David & Lucile Packard Foundation, and Princeton University’s Eric and Wendy Schmidt Transformative Technology Fund. Funding was also provided by the Spanish Ministry of Economy and Competitiveness (FIS2016-75862-P and FIS2013-48286-C2-1-P), the Government of the Basque Country (project IT779-13), and the Spanish Ministry of Economy and Competitiveness and European Federation for Regional Development (MAT2015-66441-P).

    The study, Topological quantum chemistry, by Barry Bradlyn, Luis Elcoro, Jennifer Cano, Maia Garcia Vergniory, Zhijun Wang, Claudia Felser, Mois Aroyo and B. Andrei Bernevig, was published in the journal Nature on July 20, 2017.

    See the full article here .

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    About Princeton: Overview

    Princeton University is a vibrant community of scholarship and learning that stands in the nation’s service and in the service of all nations. Chartered in 1746, Princeton is the fourth-oldest college in the United States. Princeton is an independent, coeducational, nondenominational institution that provides undergraduate and graduate instruction in the humanities, social sciences, natural sciences and engineering.

    As a world-renowned research university, Princeton seeks to achieve the highest levels of distinction in the discovery and transmission of knowledge and understanding. At the same time, Princeton is distinctive among research universities in its commitment to undergraduate teaching.

    Today, more than 1,100 faculty members instruct approximately 5,200 undergraduate students and 2,600 graduate students. The University’s generous financial aid program ensures that talented students from all economic backgrounds can afford a Princeton education.

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  • richardmitnick 12:55 pm on June 26, 2017 Permalink | Reply
    Tags: 1T’-WTe2, , , , , , , , , Topological insulators   

    From LBNL: “2-D Material’s Traits Could Send Electronics R&D Spinning in New Directions” 

    Berkeley Logo

    Berkeley Lab

    June 26, 2017
    Glenn Roberts Jr
    geroberts@lbl.gov
    (510) 486-5582

    1
    This animated rendering shows the atomic structure of a 2-D material known as 1T’-WTe2 that was created and studied at Berkeley Lab’s Advanced Light Source. (Credit: Berkeley Lab.)

    An international team of researchers, working at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and UC Berkeley, fabricated an atomically thin material and measured its exotic and durable properties that make it a promising candidate for a budding branch of electronics known as “spintronics.”

    The material – known as 1T’-WTe2 – bridges two flourishing fields of research: that of so-called 2-D materials, which include monolayer materials such as graphene that behave in different ways than their thicker forms; and topological materials, in which electrons can zip around in predictable ways with next to no resistance and regardless of defects that would ordinarily impede their movement.

    At the edges of this material, the spin of electrons – a particle property that functions a bit like a compass needle pointing either north or south – and their momentum are closely tied and predictable.

    2
    A scanning tunneling microscopy image of a 2-D material created and studied at Berkeley Lab’s Advanced Light Source (orange, background). In the upper right corner, the blue dots represent the layout of tungsten atoms and the red dots represent tellurium atoms. (Credit: Berkeley Lab.)

    This latest experimental evidence could elevate the material’s use as a test subject for next-gen applications, such as a new breed of electronic devices that manipulate its spin property to carry and store data more efficiently than present-day devices. These traits are fundamental to spintronics.

    The material is called a topological insulator because its interior surface does not conduct electricity, and its electrical conductivity (the flow of electrons) is restricted to its edges.

    “This material should be very useful for spintronics studies,” said Sung-Kwan Mo, a physicist and staff scientist at Berkeley Lab’s Advanced Light Source (ALS) who co-led the study, published today in Nature Physics.

    LBNL/ALS

    “We’re excited about the fact that we have found another family of materials where we can both explore the physics of 2-D topological insulators and do experiments that may lead to future applications,” said Zhi-Xun Shen, a professor in Physical Sciences at Stanford University and the Advisor for Science and Technology at SLAC National Accelerator Laboratory who also co-led the research effort.

    “This general class of materials is known to be robust and to hold up well under various experimental conditions, and these qualities should allow the field to develop faster,” he added.

    The material was fabricated and studied at the ALS, an X-ray research facility known as a synchrotron. Shujie Tang, a visiting postdoctoral researcher at Berkeley Lab and Stanford University, and a co-lead author in the study, was instrumental in growing 3-atom-thick crystalline samples of the material in a highly purified, vacuum-sealed compartment at the ALS, using a process known as molecular beam epitaxy.

    The high-purity samples were then studied at the ALS using a technique known as ARPES (or angle-resolved photoemission spectroscopy), which provides a powerful probe of materials’ electron properties.

    3
    Beamline 10.0.1 at Berkeley Lab’s Advanced Light Source enables researchers to both create and study atomically thin materials. (Credit: Roy Kaltschmidt/Berkeley Lab.)

    “After we refined the growth recipe, we measured it with ARPES. We immediately recognized the characteristic electronic structure of a 2-D topological insulator,” Tang said, based on theory and predictions. “We were the first ones to perform this type of measurement on this material.”

    But because the conducting part of this material, at its outermost edge, measured only a few nanometers thin – thousands of times thinner than the X-ray beam’s focus – it was difficult to positively identify all of the material’s electronic properties.

    So collaborators at UC Berkeley performed additional measurements at the atomic scale using a technique known as STM, or scanning tunneling microscopy. “STM measured its edge state directly, so that was a really key contribution,” Tang said.

    The research effort, which began in 2015, involved more than two dozen researchers in a variety of disciplines. The research team also benefited from computational work at Berkeley Lab’s National Energy Research Scientific Computing Center (NERSC).

    NERSC Cray Cori II supercomputer

    LBL NERSC Cray XC30 Edison supercomputer

    Two-dimensional materials have unique electronic properties that are considered key to adapting them for spintronics applications, and there is a very active worldwide R&D effort focused on tailoring these materials for specific uses by selectively stacking different types.

    “Researchers are trying to sandwich them on top of each other to tweak the material as they wish – like Lego blocks,” Mo said. “Now that we have experimental proof of this material’s properties, we want to stack it up with other materials to see how these properties change.”

    A typical problem in creating such designer materials from atomically thin layers is that materials typically have nanoscale defects that can be difficult to eliminate and that can affect their performance. But because 1T’-WTe2 is a topological insulator, its electronic properties are by nature resilient.

    “At the nanoscale it may not be a perfect crystal,” Mo said, “but the beauty of topological materials is that even when you have less than perfect crystals, the edge states survive. The imperfections don’t break the key properties.”

    Going forward, researchers aim to develop larger samples of the material and to discover how to selectively tune and accentuate specific properties. Besides its topological properties, its “sister materials,” which have similar properties and were also studied by the research team, are known to be light-sensitive and have useful properties for solar cells and for optoelectronics, which control light for use in electronic devices.

    The ALS and NERSC are DOE Office of Science User Facilities. Researchers from Stanford University, the Chinese Academy of Sciences, Shanghai Tech University, POSTECH in Korea, and Pusan National University in Korea also participated in this study. This work was supported by the Department of Energy’s Office of Science, the National Science Foundation, the National Science Foundation of China, the National Research Foundation (NRF) of Korea, and the Basic Science Research Program in Korea.

    See the full article here .

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  • richardmitnick 12:19 pm on December 12, 2016 Permalink | Reply
    Tags: , , , , , Topological insulators   

    From Hopkins and Rutgers: “Between two worlds: Exotic insulator may hold clue to key mystery of modern physics” 

    Johns Hopkins
    Johns Hopkins University

    Rutgers smaller

    Rutgers University

    Dec 6, 2016
    Arthur Hirsch

    Scientists experiment with material that straddles world of classical physics and hidden quantum realm

    Experiments using laser light and pieces of gray material the size of fingernail clippings may offer clues to a fundamental scientific riddle: what is the relationship between the everyday world of classical physics and the hidden quantum realm that obeys entirely different rules?

    1
    N. Peter Armitage

    “We found a particular material that is straddling these two regimes,” said N. Peter Armitage, an associate professor of physics at Johns Hopkins University who led the research for the paper just published in the journal Science. Six scientists from Johns Hopkins and Rutgers University were involved in the work on materials called topological insulators, which can conduct electricity on their atoms-thin surface, but not in their insides.

    Topological insulators were predicted in the 1980s, first observed in 2007, and have been studied intensively since. Made from any number of hundreds of elements, these materials have the capacity to show quantum properties that usually appear only at the microscopic level, but here appear in a material visible to the naked eye.

    The experiments reported in Science establish these materials as a distinct state of matter “that exhibits macroscopic quantum mechanical effects,” Armitage said. “Usually we think of quantum mechanics as a theory of small things, but in this system quantum mechanics is appearing on macroscopic length scales. The experiments are made possible by unique instrumentation developed in my laboratory.”

    In the experiments reported in Science, the elements bismuth and selenium make up dark gray material samples—each a few millimeters long and of different thicknesses—that were hit with “THz” light beams that are invisible to the unaided eye. Researchers measured the reflected light as it moved through the material samples and found indicators of a quantum state of matter.

    Specifically, they found that as the light was transmitted through the material, the wave rotated a specific amount, which is related to physical constants that are usually only measurable in atomic scale experiments. The amount matched predictions of what would be possible in this quantum state.

    The results add to scientists’ understanding of topological insulators, but also may contribute to the larger subject that Armitage says is the central question of modern physics: what is the relationship between the macroscopic classical world, and the microscopic quantum world from which it arises?

    Scientists since the early 20th century have struggled with the question of how one set of physical laws governing objects above a certain size can co-exist alongside a different set of laws governing the atomic and subatomic scale. How does classical mechanics emerge from quantum mechanics, and where is the threshold that divides the realms?

    Those questions remain to be answered, but topological insulators could be part of the solution.

    “It’s a piece of the puzzle,” said Armitage, who worked on the experiments along with Liang Wu, who was a graduate student at Johns Hopkins when the work was done; Maryam Salehi of the Rutgers University Department of Material Science and Engineering; and Nikesh Koirala, Jisoo Moon, and Sean Oh of the Rutgers University Department of Physics and Astronomy.

    See the full article here .

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

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    The Johns Hopkins University opened in 1876, with the inauguration of its first president, Daniel Coit Gilman. “What are we aiming at?” Gilman asked in his installation address. “The encouragement of research … and the advancement of individual scholars, who by their excellence will advance the sciences they pursue, and the society where they dwell.”

    The mission laid out by Gilman remains the university’s mission today, summed up in a simple but powerful restatement of Gilman’s own words: “Knowledge for the world.”

    What Gilman created was a research university, dedicated to advancing both students’ knowledge and the state of human knowledge through research and scholarship. Gilman believed that teaching and research are interdependent, that success in one depends on success in the other. A modern university, he believed, must do both well. The realization of Gilman’s philosophy at Johns Hopkins, and at other institutions that later attracted Johns Hopkins-trained scholars, revolutionized higher education in America, leading to the research university system as it exists today.

     
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