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  • richardmitnick 1:40 pm on January 3, 2019 Permalink | Reply
    Tags: A fundamental characteristic of electrons is their spin which points either up or down. A skyrmion is a circular cluster of electrons whose spins are opposite to the orientation of surrounding electro, , Dzyaloshinskii-Moriya interaction (DMI), Ferromagnets, , , , Spintronics, tiny magnetic bits"   

    From MIT News: “Controllable fast, tiny magnetic bits” 

    MIT News
    MIT Widget

    From MIT News

    January 3, 2019
    Denis Paiste

    MIT researchers show how to make and drive nanoscale magnetic quasi-particles known as skyrmions for spintronic memory devices.

    1
    Work by researchers in the group of MIT materials science and engineering Professor Geoffrey Beach and colleagues in California, Germany, Switzerland and Korea, was featured on the covers of Nature Nanotechnology and Advanced Materials. Cover images reproduced with permission of the publishers.

    2
    Lucas Caretta (left) and Ivan Lemesh, graduate students in the lab of MIT professor of materials science and engineering Geoffrey Beach, each had a cover article in a peer-reviewed journal article in December. Their work is pioneering new directions for spintronic devices based on quasi-particles known as skyrmions. Photo: Denis Paiste/Materials Research Laboratory.

    For many modern technical applications, such as superconducting wires for magnetic resonance imaging, engineers want as much as possible to get rid of electrical resistance and its accompanying production of heat.

    It turns out, however, that a bit of heat production from resistance is a desirable characteristic in metallic thin films for spintronic applications such as solid-state computer memory. Similarly, while defects are often undesirable in materials science, they can be used to control creation of magnetic quasi-particles known as skyrmions.

    In separate papers published this month in the journals Nature Nanotechnology and Advanced Materials, researchers in the group of MIT Professor Geoffrey S.D. Beach and colleagues in California, Germany, Switzerland, and Korea, showed that they can generate stable and fast moving skyrmions in specially formulated layered materials at room temperature, setting world records for size and speed. Each paper was featured on the cover of its respective journal.

    For the research published in Advanced Materials [link is above], the researchers created a wire that stacks 15 repeating layers of a specially fabricated metal alloy made up of platinum, which is a heavy metal, cobalt-iron-boron, which is a magnetic material, and magnesium-oxygen. In these layered materials, the interface between the platinum metal layer and cobalt-iron-boron creates an environment in which skyrmions can be formed by applying an external magnetic field perpendicular to the film and electric current pulses that travel along the length of the wire.

    Notably, under a 20 milliTesla field, a measure of the magnetic field strength, the wire forms skyrmions at room temperature. At temperatures above 349 kelvins (168 degrees Fahrenheit), the skyrmions form without an external magnetic field, an effect caused by the material heating up, and the skyrmions remain stable even after the material is cooled back to room temperature. Previously, results like this had been seen only at low temperature and with large applied magnetic fields, Beach says.

    Predictable structure

    “After developing a number of theoretical tools, we now can not only predict the internal skyrmion structure and size, but we also can do a reverse engineering problem, we can say, for instance, we want to have a skyrmion of that size, and we’ll be able to generate the multi-layer, or the material, parameters, that would lead to the size of that skyrmion,” says Ivan Lemesh, first author of the Advanced Materials paper and a graduate student in materials science and engineering at MIT. Co-authors include senior author Beach and 17 others.

    A fundamental characteristic of electrons is their spin, which points either up or down. A skyrmion is a circular cluster of electrons whose spins are opposite to the orientation of surrounding electrons, and the skyrmions maintain a clockwise or counter-clockwise direction.

    “However, on top of that, we have also discovered that skyrmions in magnetic multilayers develop a complex through-thickness dependent twisted nature,” Lemesh said during a presentation on his work at the Materials Research Society (MRS) fall meeting in Boston on Nov. 30. Those findings were published in a separate theoretical study in Physical Review B in September.

    The current research shows that while this twisted structure of skyrmions has a minor impact on the ability to calculate the average size of the skyrmion, it significantly affects their current-induced behavior.

    Fundamental limits

    For the paper in Nature Nanotechnology [link is above], the researchers studied a different magnetic material, layering platinum with a magnetic layer of a gadolinium cobalt alloy, and tantalum oxide. In this material, the researchers showed they could produce skyrmions as small as 10 nanometers and established that they could move at a fast speed in the material.

    “What we discovered in this paper is that ferromagnets have fundamental limits for the size of the quasi-particle you can make and how fast you can drive them using currents,” says first author Lucas Caretta, a graduate student in materials science and engineering.

    In a ferromagnet, such as cobalt-iron-boron, neighboring spins are aligned parallel to one another and develop a strong directional magnetic moment. To overcome the fundamental limits of ferromagnets, the researchers turned to gadolinium-cobalt, which is a ferrimagnet, in which neighboring spins alternate up and down so they can cancel each other out and result in an overall zero magnetic moment.

    “One can engineer a ferrimagnet such that the net magnetization is zero, allowing ultrasmall spin textures, or tune it such that the net angular momentum is zero, enabling ultrafast spin textures. These properties can be engineered by material composition or temperature,” Caretta explains.

    In 2017, researchers in Beach’s group and their collaborators demonstrated experimentally that they could create these quasi-particles at will in specific locations by introducing a particular kind of defect in the magnetic layer.

    “You can change the properties of a material by using different local techniques such as ion bombardment, for instance, and by doing that you change its magnetic properties,” Lemesh says, “and then if you inject a current into the wire, the skyrmion will be born in that location.”

    Adds Caretta: “It was originally discovered with natural defects in the material, then they became engineered defects through the geometry of the wire.”

    They used this method to create skyrmions in the new Nature Nanotechnology [link is above] paper.

    The researchers made images of the skyrmions in the cobalt-gadolinium mixture at room temperature at synchrotron centers in Germany, using X-ray holography. Felix Büttner, a postdoc in the Beach lab, was one of the developers of this X-ray holography technique. “It’s one of the only techniques that can allow for such highly resolved images where you make out skyrmions of this size,” Caretta says.

    These skyrmions are as small as 10 nanometers, which is the current world record for room temperature skyrmions. The researchers demonstrated current driven domain wall motion of 1.3 kilometers per second, using a mechanism that can also be used to move skyrmions, which also sets a new world record.

    Except for the synchrotron work, all the research was done at MIT. “We grow the materials, do the fabrication and characterize the materials here at MIT,” Caretta says.

    Magnetic modeling

    These skyrmions are one type of spin configuration of electron spins in these materials, while domain walls are another. Domain walls are the boundary between domains of opposing spin orientation. In the field of spintronics, these configurations are known as solitons, or spin textures. Since skyrmions are a fundamental property of materials, mathematical characterization of their energy of formation and motion involves a complex set of equations incorporating their circular size, spin angular momentum, orbital angular momentum, electronic charge, magnetic strength, layer thickness, and several special physics terms that capture the energy of interactions between neighboring spins and neighboring layers, such as the exchange interaction.

    One of these interactions, which is called the Dzyaloshinskii-Moriya interaction (DMI), is of special significance to forming skyrmions and arises from the interplay between electrons in the platinum layer and the magnetic layer. In the Dzyaloshinskii-Moriya interaction, spins align perpendicular to each other, which stabilizes the skyrmion, Lemesh says. The DMI interaction allows for these skyrmions to be topological, giving rise to fascinating physics phenomena, making them stable, and allowing for them to be moved with a current.

    “The platinum itself is what provides what’s called a spin current which is what drives the spin textures into motion,” Caretta says. “The spin current provides a torque on the magnetization of the ferro or ferrimagnet adjacent to it, and this torque is what ultimately causes the motion of the spin texture. We’re basically using simple materials to realize complicated phenomena at interfaces.”

    In both papers, the researchers performed a mix of micromagnetic and atomistic spin calculations to determine the energy required to form skyrmions and to move them.

    “It turns out that by changing the fraction of a magnetic layer, you can change the average magnetic properties of the whole system, so now we don’t need to go to a different material to generate other properties,” Lemesh says. “You can just dilute the magnetic layer with a spacer layer of different thickness, and you will wind up with different magnetic properties, and that gives you an infinite number of opportunities to fabricate your system.”

    Precise control

    “Precise control of creating magnetic skyrmions is a central topic of the field,” says Jiadong Zang, an assistant professor of physics at the University of New Hampshire, who was not involved in this research, regarding the Advanced Materials paper. “This work has presented a new way of generating zero field skyrmions via current pulse. This is definitely a solid step towards skyrmion manipulations in nanosecond regime.”

    Commenting on the Nature Nanotechnology report, Christopher Marrows, a professor of condensed matter physics at the University of Leeds in the United Kingdom says: “The fact that the skyrmions are so small but can be stabilized at room temperature makes it very significant.”

    Marrows, who also was not involved in this research, noted that the Beach group had predicted room temperature skyrmions in a Scientific Reports paper earlier this year and said the new results are work of the highest quality. “But they made the prediction and real life does not always live up to theoretical expectations, so they deserve all the credit for this breakthrough,” Marrows says.

    Zang, commenting on the Nature Nanotechnology paper, adds: “A bottleneck of skyrmion study is to reach a size of smaller than 20 nanometers [the size of state-of-art memory unit], and drive its motion with speed beyond one kilometer per second. Both challenges have been tackled in this seminal work.

    “A key innovation is to use ferrimagnet, instead of commonly used ferromagnet, to host skyrmions,” Zang says. “This work greatly stimulates the design of skyrmion-based memory and logic devices. This is definitely a star paper in the skyrmion field.”

    Racetrack systems

    Solid-state devices built on these skyrmions could someday replace current magnetic storage hard drives. Streams of magnetic skyrmions can act as bits for computer applications. “In these materials, we can readily pattern magnetic tracks,” Beach said during a presentation at MRS.

    These new findings could be applied to racetrack memory devices, which were developed by Stuart Parkin at IBM. A key to engineering these materials for use in racetrack devices is engineering deliberate defects into the material where skyrmions can form, because skyrmions form where there are defects in the material.

    “One can engineer by putting notches in this type of system,” said Beach, who also is co-director of the Materials Research Laboratory (MRL) at MIT. A current pulse injected into the material forms the skyrmions at a notch. “The same current pulse can be used to write and delete,” he said. These skyrmions form extremely quickly, in less than a billionth of a second, Beach says.

    Says Caretta: “To be able to have a practical operating logic or memory racetrack device, you have to write the bit, so that’s what we talk about in creating the magnetic quasi particle, and you have to make sure that the written bit is very small and you have to translate that bit through the material at a very fast rate,” Caretta says.

    Marrows, the Leeds professor, adds: “Applications in skyrmion-based spintronics, will benefit, although again it’s a bit early to say for sure what will be the winners among the various proposals, which include memories, logic devices, oscillators and neuromorphic devices,”

    A remaining challenge is the best way to read these skyrmion bits. Work in the Beach group is continuing in this area, Lemesh says, noting that the current challenge is to discover a way to detect these skyrmions electrically in order to use them in computers or phones.

    “Yea, so you don’t have to take your phone to a synchrotron to read a bit,” Caretta says. “As a result of some of the work done on ferrimagnets and similar systems called anti-ferromagnets, I think the majority of the field will actually start to shift toward these types of materials because of the huge promise that they hold.”

    See the full article here .


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  • richardmitnick 12:18 pm on August 3, 2018 Permalink | Reply
    Tags: , , Gears in a quantum clock, , Spintronics, X-ray scattering,   

    From Brookhaven Lab: “New Magnetic Materials Overcome Key Barrier to Spintronic Devices” 

    From Brookhaven National Lab

    August 1, 2018
    Justin Eure
    justin.eure@gmail.com

    Custom-engineered structure enables unprecedented control and efficiency in otherwise impervious antiferromagnetic materials.

    1
    Brookhaven scientists Derek Meyers (left) and Mark Dean (right) using their x-ray diffractometer to characterize the atomic structure of the samples for the experiment.

    Consider the classic, permanent magnet: it both clings to the refrigerator and drives data storage in most devices. But another kind of impervious magnetism hides deep within many materials—a phenomenon called antiferromagnetism (AFM)—and is nearly imperceptible beyond the atomic scale.

    Now, a team of scientists just developed an unprecedented material that cracks open this hermetic magnetism, confirming a decades-old theory and creating new engineering possibilities. The team, led by the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory and the University of Tennessee, designed AFM materials with spin—the quantum mechanism behind all magnetism—that can be easily controlled with minimal energy.

    “Material synthesis finally caught up to theory, and we found a way around the most prohibitive quantum quirks of exploiting antiferromagnetism,” said Brookhaven Lab physicist and study corresponding author Mark Dean. “This work dives deeper into the underpinnings of magnetism and creates new possibilities for spin-based technologies.”

    The results, published this summer in Nature Physics, could dramatically enhance the emerging field of spintronics, where information is coded into the directional spin of electrons.

    “The real surprise was just how well this synthetic material functioned right out of the gate,” said coauthor and Brookhaven Lab scientist Derek Meyers. “Not only can we manipulate this remarkable spin, but we can do it with extreme efficiency.”

    Twisting electron spins

    The spin orientation of electrons within atoms and can be visualized as simple arrows pointing in well-defined directions.

    “In ferromagnets, these spins are all aligned,” said University of Tennessee professor and corresponding author Jian Liu. “They all point up or down, creating an external magnetic effect—like refrigerator magnets—that can be flipped when an external field is applied.”

    This flipping process powers the writing of digital information on most data storage devices, among other things.

    “Antiferromagnets are much stranger,” Meyers said. “Every arrow points in the opposite direction of its nearest neighbor, alternating up-down-up-down across the material. And it stays synchronized, such that one flip reverses all the others. That means, essentially, they all cancel each other out.”

    This perfect balance makes AFM spin notoriously impervious to manipulation, requiring too much energy to make the process useful. So the scientists introduced a little imperfection.

    “If we tilt, or cant, the spins, we create asymmetry and make the material more susceptible to influence,” Dean said. “External magnets can couple with the spin. But prior to this work, there was a built-in compromise to this approach.”

    While the canted spin can “feel” magnetic fields, the directional freedom is lost—the spins can no longer change direction.

    Gears in a quantum clock

    Imagine adjacent electrons as gears in a clock: the teeth all fit together to move in tandem and preserve precise relationships. Tilting the spin realigns those gears, almost as if they abruptly began to rotate in opposite directions and locked in place. How, then, to set those gears back in motion?

    “We followed a long-standing theory to create an unprecedented material that both cants the spin and keeps it free to rotate, which we would call preserving isotropy,” said first author Lin Hao of the University of Tennessee. “To do this, we designed a structure that cancels out those competing anisotropies, or directional asymmetries.”

    In a way, they built another gear into their antiferromagnetic clock. The extra gear slots in between the jammed electron spins, giving them a balance and space that would never naturally occur. The “gear” is actually a hidden symmetry called SU(2), a mathematical term describing the isotropic freedom.

    Layered crystalline lattice

    “The extreme sophistication of two-dimensional materials synthesis made this possible,” Liu said. “We grew a crystalline lattice with fully customized geometries to prevent the spins from locking—this is engineering with almost quantum precision.”

    The team used pulsed laser deposition to create a lattice composed of strontium, iridium, titanium, and oxygen. In this way, atomically thin layers could be stacked in different configurations to induce artificial and much desired properties.

    In this work, the team exploited special “gearing” properties of the iridium oxide layers in which the spins can be tilted, but remain free to respond to an applied magnetic field.

    The collaboration turned to the Advanced Photon Source (APS)—a DOE Office of Science User Facility at DOE’s Argonne National Laboratory—to confirm the crystal structure of the material. Using advanced resonant x-ray diffraction, the scientists revealed details of both the lattice and the electron configuration.


    ANL/APS

    “Because of the precision possible at the APS, we were able to see the fruits of the difficult synthesis process,” Meyers said. “We saw the precise layered structure we wanted, but the real test was in the magnetic function.”

    Again turning to APS, the team used x-ray scattering to measure the antiferromagnetic order, the alignment of the spins within the material.

    “We were pleased to see our canted spins retain the freedom of motion we expected,” Dean said. “It’s rare and thrilling to see things come together so seamlessly. And crucially, we proved that manipulating that AFM spin required very little energy—a must for spintronic applications.”

    Toward superior storage

    Traditional magnetic devices have an intrinsic limit: packed too closely together, ferromagnetic materials affect each other. This translates into a functional cap on data density beyond which the spins become corrupted. However, AFM materials—or discrete AFM crystals in this instance—exert no external influence.

    “We can, in theory, pack much more information into devices by manipulating antiferromagnetic spin,” Dean said. “That’s part of the promise of spintronics.”

    The combination of low energy input—think efficient writing of data—and density make the new material an ideal candidate for investment.

    “The obstacle right now has to do with scale,” Liu said. “This is a first-of-its-kind material, so no industrial-scale process exists. But this is how it starts, and the demand for this kind of functionality might rapidly move this innovation into applications.”

    Additional collaborating institutions include Charles University in Prague.

    See the full article here .


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  • richardmitnick 12:35 pm on May 28, 2018 Permalink | Reply
    Tags: , , Graphene Layered with Magnetic Materials Could Drive Ultrathin Spintronics, , , , Spintronics   

    From Lawrence Berkeley National Lab: “Graphene Layered with Magnetic Materials Could Drive Ultrathin Spintronics” 

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    From Lawrence Berkeley National Lab

    May 28, 2018
    Glenn Roberts Jr.
    GERoberts@lbl.gov
    (510) 486-5582

    Measurements at Berkeley Lab’s Molecular Foundry reveal exotic spin properties that could lead to new form of data storage.

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    Andreas Schmid, left, and Gong Chen are pictured here with the spin-polarized low-energy electron microscopy (SPLEEM) instrument at Berkeley Lab. The insturment was integral to measurements of ultrathin samples that included graphene and magnetic materials. (Credit: Roy Kaltschmidt/Berkeley Lab)

    Researchers working at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) coupled graphene, a monolayer form of carbon, with thin layers of magnetic materials like cobalt and nickel to produce exotic behavior in electrons that could be useful for next-generation computing applications.

    The work was performed in collaboration with French scientists including Nobel Laureate Albert Fert, an emeritus professor at Paris-Sud University and scientific director for a research laboratory in France. The team performed key measurements at Berkeley Lab’s Molecular Foundry, a DOE Office of Science User Facility focused on nanoscience research.

    LBNL Molecular Foundry – No image credits found

    Fert shared the Nobel Prize in Physics in 2007 for his work in understanding a magnetic effect in multilayer materials that led to new technology for reading data in hard drives, for example, and gave rise to a new field studying how to exploit and control a fundamental property known as “spin” in electrons to drive a new type of low-energy, high-speed computer memory and logic technology known as spintronics.

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    A view from above (top) and the side (bottom) of materials composed of a layer of graphene (top) with cobalt (bottom left) and with nickel (bottom right). The spin configurations are represented by arrows. (Credit: Nature Materials, May 28, 2018; DOI: 10.1038/s41563-018-0079-4)

    In this latest work, published online May 28 in the journal Nature Materials, the research team showed how that spin property – analogous to a compass needle that can be tuned to face either north or south – is affected by the interaction of graphene with the magnetic layers.

    The researchers found that the material’s electronic and magnetic properties create tiny swirling patterns where the layers meet, and this effect gives scientists hope for controlling the direction of these swirls and tapping this effect for a form of spintronics applications known as “spin-orbitronics” in ultrathin materials. The ultimate goal is to quickly and efficiently store and manipulate data at very small scales, and without the heat buildup that is a common hiccup for miniaturizing computing devices.

    Typically, researchers working to produce this behavior for electrons in materials have coupled heavy and expensive metals like platinum and tantalum with magnetic materials to achieve such effects, but graphene offers a potentially revolutionary alternative since it is ultrathin, lightweight, has very high electrical conductivity, and can also serve as a protective layer for corrosion-prone magnetic materials.

    “You could think about replacing computer hard disks with all solid state devices – no moving parts – using electrical signals alone,” said Andreas Schmid, a staff scientist at the Molecular Foundry who participated in the research. “Part of the goal is to get lower power-consumption and non-volatile data storage.”

    The latest research represents an early step toward this goal, Schmid noted, and a next step is to control nanoscale magnetic features, called skyrmions, which can exhibit a property known as chirality that makes them swirl in either a clockwise or counterclockwise direction.

    In more conventional layered materials, electrons traveling through the materials can act like an “electron wind” that changes magnetic structures like a pile of leaves blown by a strong wind, Schmid said.

    But with the new graphene-layered material, its strong electron spin effects can drive magnetic textures of opposite chirality in different directions as a result of the “spin Hall effect,” which explains how electrical currents can affect spin and vice versa. If that chirality can be universally aligned across a material and flipped in a controlled way, researchers could use it to process data.

    “Calculations by other team members show that if you take different magnetic materials and graphene and build a multilayer stack of many repeating structures, then this phenomenon and effect could possibly be very powerfully amplified,” Schmid said.

    3
    In these images developed using the SPLEEM instrument at Berkeley Lab, the orientation of magnetization in samples containing cobalt (Co) and ruthenium (Ru) is represented with white arrows. The image at left shows how the orientation is altered when a layer of graphene (“Gr”) is added. The scale bar at the lower right of both images is 1 micron, or 1 millionths of a meter. (Credit: Berkeley Lab)

    To measure the layered material, scientists applied spin-polarized low-energy electron microscopy (SPLEEM) using an instrument at the Molecular Foundry’s National Center for Electron Microscopy. It is one of just a handful of specialized devices around the world that allow scientists to combine different images to essentially map the orientations of a sample’s 3-D magnetization profile (or vector), revealing a its “spin textures.”

    The research team also created the samples using the same SPLEEM instrument through a precise process known as molecular beam epitaxy, and separately studied the samples using other forms of electron-beam probing techniques.

    Gong Chen, a co-lead author who participated in the study as a postdoctoral researcher at the Molecular Foundry and is now an assistant project scientist in the UC Davis Physics Department, said the collaboration sprang out of a discussion with French scientists at a conference in 2016 – both groups had independently been working on similar research and realized the synergy of working together.

    While the effects that researchers have now observed in the latest experiments had been discussed decades ago in previous journal articles, Chen noted that the concept of using an atomically thin material like graphene in place of heavy elements to generate those effects was a new concept.

    “It has only recently become a hot topic,” Chen said. “This effect in thin films had been ignored for a long time. This type of multilayer stacking is really stable and robust.”

    Using skyrmions could be revolutionary for data processing, he said, because information can potentially be stored at much higher densities than is possible with conventional technologies, and with much lower power usage.

    Molecular Foundry researchers are now working to form the graphene-magnetic multilayer material on an insulator or semiconductor to bring it closer to potential applications, Schmid said.

    Researchers from Grenoble Alps University; Paris-Sud University; a joint center that includes the French National Center for Scientific Research, Thales Physics Lab, Paris-Sud University, and Paris-Saclay University in France; University of California, Davis; the Chinese Academy of Sciences; Nuclear Technology Development Center (CDTN), Federal University of Minas Gerais, and Federal University de Lavras in Brazil participated in the study.

    The work was supported by the U.S. Department of Energy Office of Science; the European Union’s Horizon 2020 Research and Innovation Program; the U.S. National Science Foundation; the University of California Office of the President Multicampus Research Programs and Initiatives; Brazil’s CAPES, CNPq and FAPEMIG programs; and the 1000 Talents Program for Young Scientists of China and Ningbo Program.

    See the full article here .


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  • richardmitnick 1:44 pm on April 27, 2018 Permalink | Reply
    Tags: , , Majorana fermion science, , , Spintronics, , , 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 9:29 am on January 31, 2018 Permalink | Reply
    Tags: , , Spintronics, ,   

    From University of Arizona: “UA Researchers Observe Electrons Zipping Around in Crystals” 

    U Arizona bloc

    University of Arizona

    Jan. 29, 2018
    Daniel Stolte

    For the first time, scientists have tracked electrons moving through exotic materials that may make up the next generation of computing hardware, revealing intriguing properties not found in conventional, silicon-based semiconductors.

    1
    Extreme conditions are used to protect and preserve the TMDs during the experiments. As shown here, all samples are stored and manipulated in a vacuum that is close to the conditions in space. (Photo: Kyle Mittan/UANews.)

    The end of the silicon age has begun. As computer chips approach the physical limits of miniaturization and power-hungry processors drive up energy costs, scientists are looking to a new crop of exotic materials that could foster a new generation of computing devices that promise to push performance to new heights while skimping on energy consumption.

    Unlike current silicon-based electronics, which shed most of the energy they consume as waste heat, the future is all about low-power computing. Known as spintronics, this technology relies on a quantum physical property of electrons — up or down spin — to process and store information, rather than moving them around with electricity as conventional computing does.

    On the quest to making spintronic devices a reality, scientists at the University of Arizona are studying an exotic crop of materials known as transition metal dichalcogenides, or TMDs. TMDs have exciting properties lending themselves to new ways of processing and storing information and could provide the basis of future transistors and photovoltaics — and potentially even offer an avenue toward quantum computing.

    2
    Calley Eads, a fifth-year doctoral student in the UA’s Department of Chemistry and Biochemistry, aligns a laser system used to track electrons on time-scales at the limits of what can be measured. In her research, she investigates materials that could one day bring faster computing and more efficient solar cells. (Photo: Kyle Mittan/UANews.

    For example, current silicon-based solar cells convert realistically only about 25 percent of sunlight into electricity, so efficiency is an issue, says Calley Eads, a fifth-year doctoral student in the UA’s Department of Chemistry and Biochemistry who studies some of the properties of these new materials. “There could be a huge improvement there to harvest energy, and these materials could potentially do this,” she says.

    There is a catch, however: Most TMDs show their magic only in the form of sheets that are very large, but only one to three atoms thin. Such atomic layers are challenging enough to manufacture on a laboratory scale, let alone in industrial mass production.

    Many efforts are underway to design atomically thin materials for quantum communication, low-power electronics and solar cells, according to Oliver Monti, a professor in the department and Eads’ adviser. Studying a TMD consisting of alternating layers of tin and sulfur, his research team recently discovered a possible shortcut, published in the journal Nature Communications.

    “We show that for some of these properties, you don’t need to go to the atomically thin sheets,” he says. “You can go to the much more readily accessible crystalline form that’s available off the shelf. Some of the properties are saved and survive.”

    Understanding Electron Movement

    This, of course, could dramatically simplify device design.

    “These materials are so unusual that we keep discovering more and more about them, and they are revealing some incredible features that we think we can use, but how do we know for sure?” Monti says. “One way to know is by understanding how electrons move around in these materials so we can develop new ways of manipulating them — for example, with light instead of electrical current as conventional computers do.”

    To do this research, the team had to overcome a hurdle that never had been cleared before: figure out a way to “watch” individual electrons as they flow through the crystals.

    “We built what is essentially a clock that can time moving electrons like a stopwatch,” Monti says. “This allowed us to make the first direct observations of electrons move in crystals in real time. Until now, that had only been done indirectly, using theoretical models.”

    The work is an important step toward harnessing the unusual features that make TMDs intriguing candidates for future processing technology, because that requires a better understanding of how electrons behave and move around in them.

    Monti’s “stopwatch” makes it possible to track moving electrons at a resolution of a mere attosecond — a billionth of a billionth of a second. Tracking electrons inside the crystals, the team made another discovery: The charge flow depends on direction, an observation that seems to fly in the face of physics.

    Collaborating with Mahesh Neupane, a computational physicist at Army Research Laboratories, and Dennis Nordlund, an X-ray spectroscopy expert at Stanford University’s SLAC National Accelerator Laboratory, Monti’s team used a tunable, high-intensity X-ray source to excite individual electrons in their test samples and elevate them to very high energy levels.

    “When an electron is excited in that way, it’s the equivalent of a car that is being pushed from going 10 miles per hour to thousands of miles per hour,” Monti explains. “It wants to get rid of that enormous energy and fall back down to its original energy level. That process is extremely short, and when that happens, it gives off a specific signature that we can pick up with our instruments.”

    The researchers were able to do this in a way that allowed them to distinguish whether the excited electrons stayed within the same layer of the material, or spread into adjacent layers across the crystal.

    “We saw that electrons excited in this way scattered within the same layer and did so extremely fast, on the order of a few hundred attoseconds,” Monti says.

    In contrast, electrons that did cross into adjacent layers took more than 10 times longer to return to their ground energy state. The difference allowed the researchers to distinguish between the two populations.

    “I was very excited to find that directional mechanism of charge distribution occurring within a layer, as opposed to across layers,” says Eads, the paper’s lead author. “That had never been observed before.”

    Closer to Mass Manufacturing

    The X-ray “clock” used to track electrons is not part of the envisioned applications but a means to study the behavior of electrons inside them, Monti explains, a necessary first step in getting closer toward technology with the desired properties that could be mass-manufactured.

    “One example of the unusual behavior we see in these materials is that an electron going to the right is not the same as an electron going to the left,” he says. “That shouldn’t happen — according to physics of standard materials, going to the left or the right is the exact same thing. However, for these materials that is not true.”

    This directionality is an example of what makes TMDs intriguing to scientists, because it could be used to encode information.

    “Moving to the right could be encoded as ‘one’ and going to the left as ‘zero,'” Monti says. “So if I can generate electrons that neatly go to the right, I’ve written a bunch of ones, and if I can generate electrons that neatly go to the left, I have generated a bunch of zeroes.”

    Instead of applying electrical current, engineers could manipulate electrons in this way using light such as a laser, to optically write, read and process information. And perhaps someday it may even become possible to optically entangle information, clearing the way to quantum computing.

    “Every year, more and more discoveries are occurring in these materials,” Eads says. “They are exploding in terms of what kinds of electronic properties you can observe in them. There is a whole spectrum of ways in which they can function, from superconducting, semiconducting to insulating, and possibly more.”

    The research described here is just one way of probing the unexpected, exciting properties of layered TMD crystals, according to Monti.

    “If you did this experiment in silicon, you wouldn’t see any of this,” he says. “Silicon will always behave like a three-dimensional crystal, no matter what you do. It’s all about the layering.”

    See the full article here .

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    U Arizona campus

    The University of Arizona (UA) is a place without limits-where teaching, research, service and innovation merge to improve lives in Arizona and beyond. We aren’t afraid to ask big questions, and find even better answers.

    In 1885, establishing Arizona’s first university in the middle of the Sonoran Desert was a bold move. But our founders were fearless, and we have never lost that spirit. To this day, we’re revolutionizing the fields of space sciences, optics, biosciences, medicine, arts and humanities, business, technology transfer and many others. Since it was founded, the UA has grown to cover more than 380 acres in central Tucson, a rich breeding ground for discovery.

    Where else in the world can you find an astronomical observatory mirror lab under a football stadium? An entire ecosystem under a glass dome? Visit our campus, just once, and you’ll quickly understand why the UA is a university unlike any other.

     
  • richardmitnick 9:46 am on September 28, 2017 Permalink | Reply
    Tags: , , , , , Spintronics, Technical University of Dresden, The spin Hall effect, The spin Nernst effect, The spin Peltier effect, The spin Seebeck effect, Turning up the heat on electrons reveals an elusive physics phenomenon, When things heat up spinning electrons go their separate ways   

    From ScienceNews: “Turning up the heat on electrons reveals an elusive physics phenomenon” 

    ScienceNews bloc

    ScienceNews

    September 26, 2017
    Emily Conover

    Spin Nernst effect could help scientists design new gadgets that store data using quantum property of spin.

    1
    WHIRL AWAY Electrons in platinum move in different directions depending on their spin when the metal is heated at one end. Scientists have observed this phenomenon, called the spin Nernst effect, for the first time. Creativity103/Flickr (CC BY 2.0)

    When things heat up, spinning electrons go their separate ways.

    Warming one end of a strip of platinum shuttles electrons around according to their spin, a quantum property that makes them behave as if they are twirling around. Known as the spin Nernst effect, the newly detected phenomenon was the only one in a cadre of related spin effects that hadn’t previously been spotted, researchers report online September 11 in Nature Materials.

    “The last missing piece in the puzzle was spin Nernst and that’s why we set out to search for this,” says study coauthor Sebastian Goennenwein, a physicist at the Technical University of Dresden in Germany.

    The effect and its brethren — with names like the spin Hall effect, the spin Seebeck effect and the spin Peltier effect — allow scientists to create flows of electron spins, or spin currents. Such research could lead to smaller and more efficient electronic gadgets that use electrons’ spins to store and transmit information instead of electric charge, a technique known as “spintronics.”

    In the spin Nernst effect, named after Nobel laureate chemist Walther Nernst, heating one end of a metal causes electrons to flow toward the other end, bouncing around inside the material as they go. Within certain materials, that bouncing has a preferred direction: Electrons with spins pointing up (as if twirling counterclockwise) go to the right and electrons with spins pointing down (as if twirling clockwise) go to the left, creating an overall spin current. Although the effect had been predicted, no one had yet observed it.

    Finding evidence of the effect required disentangling it from other heat- and charge-related effects that occur in materials. To do so, the researchers coupled the platinum to a layer of a magnetic insulator, a material known as yttrium iron garnet. Then, they altered the direction of the insulator’s magnetization, which changed whether the spin current could flow through the insulator. That change slightly altered a voltage measured along the strip of platinum. The scientists measured how this voltage changed with the direction of the magnetization to isolate the fingerprints of the spin Nernst effect.

    “The measurement was a tour de force; the measurement was ridiculously hard,” says physicist Joseph Heremans of Ohio State University in Columbus, who was not involved with the research. The effect could help scientists to better understand materials that may be useful for building spintronic devices, he says. “It’s really a new set of eyes on the physics of what’s going on inside these devices.”

    A relative of the spin Nernst effect called the spin Hall effect is much studied for its potential use in spintronic devices. In the spin Hall effect, an electric field pushes electrons through a material, and the particles veer off to the left and right depending on their spin. The spin Nernst effect relies on the same basic physics, but uses heat instead of an electric field to get the particles moving.

    “It’s a beautiful experiment. It shows very nicely the spin Nernst effect,” says physicist Greg Fuchs of Cornell University. “It beautifully unifies our understanding of the interrelation between charge, heat and spin transport.”

    See the full article here .

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

    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.

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

    From Weizmann: “Physics on the edge” 

    Weizmann Institute of Science logo

    Weizmann Institute of Science

    1
    Dr. Binghai Yan

    Dr. Binghai Yan is taking topological materials higher.

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

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

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

    From rural fields to topology

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

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

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

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

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

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

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

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

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

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

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

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    Weizmann Institute Campus

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

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

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

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

    From Brown: “Researchers flip the script on magnetocapacitance” 

    Brown University
    Brown University

    June 1, 2017
    Kevin Stacey
    kevin_stacey@brown.edu

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

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

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

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

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

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

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

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

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

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

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

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

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

    2
    Iron oxide and iron have different cation orientations in their crystalline structure, which causes them to have inverse electrical properties.

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

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

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

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

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

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

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

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

    From U Utah: “A new spin on electronics” 

    1

    University of Utah

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

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

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

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

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

    See the full article here .

    Please help promote STEM in your local schools.

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    Stem Education Coalition

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

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

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

     
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