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  • richardmitnick 12:50 pm on April 5, 2019 Permalink | Reply
    Tags: "Putting a New Spin on Majorana Fermions", , , , Majorana fermions are particle-like excitations called quasiparticles that emerge as a result of the fractionalization (splitting) of individual electrons into two halves., Material Sciences, , , , Spin ladders- crystals formed of atoms with a three-dimensional (3-D) structure subdivided into pairs of chains that look like ladders.   

    From Brookhaven National Lab: “Putting a New Spin on Majorana Fermions” 

    From Brookhaven National Lab

    April 1, 2019
    Ariana Tantillo
    atantillo@bnl.gov
    (631) 344-2347

    Peter Genzer
    genzer@bnl.gov
    (631) 344-3174

    Split electrons that emerge at the boundaries between different magnetic states in materials known as spin ladders could act as stable bits of information in next-generation quantum computers.

    2
    Theoretical calculations performed by (left to right) Neil Robinson, Robert Konik, Alexei Tsvelik, and Andreas Weichselbaum of Brookhaven Lab’s Condensed Matter Physics and Materials Science Department suggest that Majorana fermions exist in the boundaries of magnetic materials with different magnetic phases. Majorana fermions are particle-like excitations that emerge when single electrons fractionalize into two halves, and their unique properties are of interest for quantum applications.

    The combination of different phases of water—solid ice, liquid water, and water vapor—would require some effort to achieve experimentally. For instance, if you wanted to place ice next to vapor, you would have to continuously chill the water to maintain the solid phase while heating it to maintain the gas phase.

    For condensed matter physicists, this ability to create different conditions in the same system is desirable because interesting phenomena and properties often emerge at the interfaces between two phases. Of current interest is the conditions under which Majorana fermions might appear near these boundaries.

    Majorana fermions are particle-like excitations called quasiparticles that emerge as a result of the fractionalization (splitting) of individual electrons into two halves. In other words, an electron becomes an entangled (linked) pair of two Majorana quasiparticles, with the link persisting regardless of the distance between them. Scientists hope to use Majorana fermions that are physically separated in a material to reliably store information in the form of qubits, the building blocks of quantum computers. The exotic properties of Majoranas—including their high insensitivity to electromagnetic fields and other environmental “noise”—make them ideal candidates for carrying information over long distances without loss.

    However, to date, Majorana fermions have only been realized in materials at extreme conditions, including at frigid temperatures close to absolute zero (−459 degrees Fahrenheit) and under high magnetic fields. And though they are “topologically” protected from local atomic impurities, disorder, and defects that are present in all materials (i.e., their spatial properties remain the same even if the material is bent, twisted, stretched, or otherwise distorted), they do not survive under strong perturbations. In addition, the range of temperatures over which they can operate is very narrow. For these reasons, Majorana fermions are not yet ready for practical technological application.

    Now, a team of physicists led by the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory and including collaborators from China, Germany, and the Netherlands has proposed a novel theoretical method for producing more robust Majorana fermions. According to their calculations, as described in a paper published on Jan. 15 in Physical Review Letters, these Majoranas emerge at higher temperatures (by many orders of magnitude) and are largely unaffected by disorder and noise. Even though they are not topologically protected, they can persist if the perturbations change slowly from one point to another in space.

    “Our numerical and analytical calculations provide evidence that Majorana fermions exist in the boundaries of magnetic materials with different magnetic phases, or directions of electron spins, positioned next to one other,” said co-author Alexei Tsvelik, senior scientist and leader of the Condensed Matter Theory Group in Brookhaven Lab’s Condensed Matter Physics and Materials Science (CMPMS) Department. “We also determined the number of Majorana fermions you should expect to get if you combine certain magnetic phases.”

    For their theoretical study, the scientists focused on magnetic materials called spin ladders, which are crystals formed of atoms with a three-dimensional (3-D) structure subdivided into pairs of chains that look like ladders. Though the scientists have been studying the properties of spin ladder systems for many years and expected that they would produce Majorana fermions, they did not know how many. To perform their calculations, they applied the mathematical framework of quantum field theory for describing the fundamental physics of elementary particles, and a numerical method (density-matrix renormalization group) for simulating quantum systems whose electrons behave in a strongly correlated way.

    “We were surprised to learn that for certain configurations of magnetic phases we can generate more than one Majorana fermion at each boundary,” said co-author and CMPMS Department Chair Robert Konik.

    For Majorana fermions to be practically useful in quantum computing, they need to be generated in large numbers. Computing experts believe that the minimum threshold at which quantum computers will be able to solve problems that classical computers cannot is 100 qubits. The Majorana fermions also have to be moveable in such a way that they can become entangled.

    The team plans to follow up their theoretical study with experiments using engineered systems such as quantum dots (nanosized semiconducting particles) or trapped (confined) ions. Compared to the properties of real materials, those of engineered ones can be more easily tuned and manipulated to introduce the different phase boundaries where Majorana fermions may emerge.

    “What the next generation of quantum computers will be made of is unclear right now,” said Konik. “We’re trying to find better alternatives to the low-temperature superconductors of the current generation, similar to how silicon replaced germanium in transistors. We’re in such early stages that we need to explore every possibility available.”

    See the full article here .


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  • richardmitnick 3:54 pm on April 2, 2019 Permalink | Reply
    Tags: "Researchers tune material’s color and thermal properties separately", , Material Sciences, , , Polymers could be designed to reflect or trap heat regardless of hue   

    From MIT News: “Researchers tune material’s color and thermal properties separately” 

    MIT News
    MIT Widget

    From MIT News

    April 2, 2019
    Jennifer Chu

    1
    The visual and thermal properties of polyethylene can be tweaked to produce colorful films with a wide range of heat-radiating capabilities. Image: Felice Frankel

    Polymers could be designed to reflect or trap heat, regardless of hue.

    The color of a material can often tell you something about how it handles heat. Think of wearing a black shirt on a sweltering summer’s day — the darker the pigment, the warmer you’re likely to feel. Likewise, the more transparent a glass window, the more heat it can let through. A material’s responses to visible and infrared radiation are often naturally linked.

    Now MIT engineers have made samples of strong, tissue-like polymer material, the color and heat properties of which they can tailor independently of the other. For instance, they have fabricated samples of very thin black film designed to reflect heat and stay cool. They’ve also made films exhibiting a rainbow of other colors, each made to reflect or absorb infrared radiation regardless of the way they respond to visible light.

    The researchers can specifically tune the color and heat properties of this new material to fit the requirements for a host of wide-ranging applications, including colorful, heat-reflecting building facades, windows, and roofs; light-absorbing, heat-dissipating covers for solar panels; and lightweight fabric for clothing, outerwear, tents, and backpacks — all designed to either trap or reflect heat, depending on the environments in which they would be used.

    “With this material, everything could look more colorful, because then you wouldn’t be concerned with what color does to the thermal balance of, say, a building, or a window, or your clothing,” says Svetlana Boriskina, a research scientist in MIT’s Department of Mechanical Engineering.

    Boriskina is author of a study that appears today in the journal Optical Materials Express, outlining the new material-engineering technique. Her MIT co-authors are Luis Marcelo Lozano, Seongdon Hong, Yi Huang, Hadi Zandavi, Yoichiro Tsurimaki, Jiawei Zhou, Yanfei Xu, and Gang Chen, the Carl Richard Soderberg Professor of Power Engineering, along with Yassine Ait El Aoud and Richard Osgood III, both of the Combat Capabilities Development Command Soldier Center, in Natick, Massachusetts.

    Polymer conductors

    For this work, Boriskina was inspired by the vibrant colors in stained-glass windows, which for centuries have been made by adding particles of metals and other natural pigments to glass.

    “However, despite providing excellent visual transparency, glass has many limitations as a material,” Boriskina notes. “It is bulky, inflexible, fragile, does not spread heat well, and is obviously not suitable for wearable applications.”

    She says that while it’s relatively simple to tailor the color of glass, the material’s response to heat is difficult to tune. For instance, glass panels reflect room-temperature heat and trap it inside the room. Furthermore, if colored glass is exposed to incoming sunlight from a particular direction, the heat from the sun can create a hotspot, which is difficult to dissipate in glass. If a material like glass can’t conduct or dissipate heat well, that heat could damage the material.

    The same can be said for most plastics, which can be engineered in any color but for the most part are thermal absorbers and insulators, concentrating and trapping heat rather than reflecting it away.

    For the past several years, Chen’s lab has been looking into ways to manipulate flexible, lightweight polymer materials to conduct, rather than insulate, heat, mostly for applications in electronics. In previous work, the researchers found that by carefully stretching polymers like polyethylene, they could change the material’s internal structure in a way that also changed its heat-conducting properties.

    Boriskina thought this technique might be useful not just for fabricating polymer-based electronics, but also in architecture and apparel. She adapted this polymer-fabrication technique, adding a twist of color.

    “It’s very hard to develop a new material with all these different properties in it,” she says. “Usually if you tune one property, the other gets destroyed. Here, we started with one property that was discovered in this group, and then we added a new property creatively. All together it works as a multifunctional material.”

    Hotspots stretched away

    To fabricate the colorful films, the team started with a mixture of polyethylene powder and a chemical solvent, to which they added certain nanoparticles to give the film a desired color. For instance, to make black film, they added particles of silicon; other red, blue, green, and yellow films were made with the addition of various commercial dyes.

    The team then attached each nanoparticle-embedded film onto a roll-to-roll apparatus, which they heated up to soften the film, making it more pliable as the researchers carefully stretched the material.

    As they stretched each film, they found, unsurprisingly, that the material became more transparent. They also observed that polyethylene’s microscopic structure changed as it stretched. Where normally the material’s polymer chains resemble a disorganized tangle, similar to cooked spaghetti, when stretched these chains straighten out, forming parallel fibers.

    When the researchers placed each sample under a solar simulator — a lamp that mimics the visible and thermal radiation of the sun — they found the more stretched out a film, the more heat it was able to dissipate. The long, parallel polymer chains essentially provided a direct route along which heat could travel. Along these chains, heat, in the form of phonons, could then shoot away from its source, in a “ballistic” fashion, avoiding the formation of hotspots.

    The researchers also found that the less they stretched the material, the more insulating it was, trapping heat, and forming hotspots within polymer tangles.

    By controlling the degree to which the material is stretched, Boriskina could control polyethylene’s heat-conducting properties, regardless of the material’s color. She also carefully chose the nanoparticles, not just by their visual color, but also by their interactions with invisible radiative heat. She says researchers can potentially use this technique to produce thin, flexible, colorful polymer films, that can conduct or insulate heat, depending on the application.

    Going forward, she plans to launch a website that offers algorithms to calculate a material’s color and thermal properties, based on its dimensions and internal structure.

    In addition to films, her group is now working on fabricating nanoparticle-embedded polyethylene thread, which can be stitched together to form lightweight apparel, designed to be either insulating, or cooling.

    “This is in film factor now, but we’re working it into fibers and fabrics,” Boriskina says. “Polyethylene is produced by the billions of tons and could be recycled, too. I don’t see any significant impediments to large-scale production.”

    This research was supported, in part, by the Combat Capabilities Development Command Soldier Center.

    See the full article here .


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  • richardmitnick 12:27 pm on March 8, 2019 Permalink | Reply
    Tags: "Scientists Take a Deep Dive Into the Imperfect World of 2D Materials", (Raman/photoluminescence spectroscopy) to study how light interacts with the electrons at microscope scales was used, A form of AFM (atomic force microscopy) was used to view structural details approaching the atomic scale, Adam Schwartzberg: “Now that we know what defects we have and what effect they have on the properties of the material we can use this information to reduce or eliminate defects, , “It’s a very big advance to get this electronic structure on small length scales” said Eli Rotenberg, Because research of WS2 and related 2D materials is still in its infancy there are many unknowns about the roles specific types of defects play in these materials, For this study the defects were due to the sample-growth process, , Material Sciences, Most of the experiments focused on a single flake of tungsten disulfide, NanoARPES which researchers enlisted to probe the 2D samples with X-rays was used in this work, , Researchers from the Berkeley Lab Chemical Sciences Division Aarhus University in Denmark and Montana State University also participated in this study., Researchers hope to control the amount and kinds of atoms that are affected and the locations where these defects are concentrated in the flakes., The defects were largely concentrated around the edges of the flakes a signature of the growth process, The sample used in the study contained microscopic roughly triangular flakes each measuring about 1 to 5 microns (millionths of a meter) across, The team also enlisted a technique known as XPS (X-ray photoelectron spectroscopy) to study the chemical makeup of a sample at very small scales, The various techniques were applied at the Molecular Foundry where the material was synthesized and at the ALS, The X-rays knocked out electrons in the sample allowing researchers to measure their direction and energy, These 2D materials could also be incorporated in new forms of memory storage and data transfer such as spintronics and valleytronics, They were grown atop titanium dioxide crystals using a conventional layering process known as chemical vapor deposition, This revealed nanoscale defects and how the electrons interact with each other.,   

    From Lawrence Berkeley National Lab: “Scientists Take a Deep Dive Into the Imperfect World of 2D Materials” 

    Berkeley Logo

    From Lawrence Berkeley National Lab

    March 8, 2019

    Glenn Roberts Jr.
    geroberts@lbl.gov
    (510) 486-5582

    Berkeley Lab-led team combines several nanoscale techniques to gain new insights on the effects of defects in a well-studied monolayer material.

    1
    This animation displays a scan of arrow-shaped flakes of a 2D material. Samples were scanned across their electron energy, momentum, and horizontal and vertical coordinates using an X-ray-based technique known as nanoARPES at Berkeley Lab’s Advanced Light Source. Red represents the highest intensity measured, followed by orange, yellow, green, and blue, and purple (least intense). (Credit: Roland Koch/Berkeley Lab)

    Nothing is perfect, or so the saying goes, and that’s not always a bad thing. In a study at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), scientists learned how nanoscale defects can enhance the properties of an ultrathin, so-called 2D material.

    They combined a toolbox of techniques to home in on natural, nanoscale defects formed in the manufacture of tiny flakes of a monolayer material known as tungsten disulfide (WS2) and measured their electronic effects in detail not possible before.

    “Usually we say that defects are bad for a material,” said Christoph Kastl, a postdoctoral researcher at Berkeley Lab’s Molecular Foundry and the lead author of the study, published in the journal ACS Nano. “Here they provide functionality.”

    Tungsten disulfide is a well-studied 2D material that, like other 2D materials of its kind, exhibits special properties because of its atomic thinness. It is particularly well-known for its efficiency in absorbing and emitting light, and it is a semiconductor.

    Members of this family of 2D materials could serve as high-efficiency computer transistors and as other electronics components, and they also are prime candidates for use in ultrathin, high-efficiency solar cells and LED lighting, as well as in quantum computers.

    These 2D materials could also be incorporated in new forms of memory storage and data transfer, such as spintronics and valleytronics, that would revolutionize electronics by making use of materials in new ways to make smaller and more efficient devices.

    The latest result marks the first comprehensive study at the Lab’s Advanced Light Source (ALS) involving a technique called nanoARPES, which researchers enlisted to probe the 2D samples with X-rays.

    LBL ALS

    The X-rays knocked out electrons in the sample, allowing researchers to measure their direction and energy. This revealed nanoscale defects and how the electrons interact with each other.

    The nanoARPES capability is housed in an X-ray beamline, launched in 2016, known as MAESTRO (Microscopic and Electronic Structure Observatory). It is one of dozens of specialized beamlines at the ALS, which produces light in different forms – from infrared to X-rays – for a variety of simultaneous experiments.

    “It’s a very big advance to get this electronic structure on small length scales,” said Eli Rotenberg, a senior staff scientist at the ALS who was a driving force in developing MAESTRO and served as one of the study’s leaders. “That matters for real devices.”

    The team also enlisted a technique known as XPS (X-ray photoelectron spectroscopy) to study the chemical makeup of a sample at very small scales; a form of AFM (atomic force microscopy) to view structural details approaching the atomic scale; and a combined form of optical spectroscopy (Raman/photoluminescence spectroscopy) to study how light interacts with the electrons at microscope scales.

    The various techniques were applied at the Molecular Foundry, where the material was synthesized, and at the ALS.

    LBNL Molecular Foundry

    The sample used in the study contained microscopic, roughly triangular flakes, each measuring about 1 to 5 microns (millionths of a meter) across. They were grown atop titanium dioxide crystals using a conventional layering process known as chemical vapor deposition, and the defects were largely concentrated around the edges of the flakes, a signature of the growth process. Most of the experiments focused on a single flake of tungsten disulfide.

    2
    This image shows an illustration of the atomic structure of a 2D material called tungsten disulfide. Tungsten atoms are shown in blue and sulfur atoms are shown in yellow. The background image, taken by an electron microscope at Berkeley Lab’s Molecular Foundry, shows groupings of flakes of the material (dark gray) grown by a process called chemical vapor deposition on a titanium dioxide layer (light gray). (Credit: Katherine Cochrane/Berkeley Lab)

    Adam Schwartzberg, a staff scientist at the Molecular Foundry who served as a co-lead in the study, said, “It took a combination of multiple types of techniques to pin down what’s really going on.”

    He added, “Now that we know what defects we have and what effect they have on the properties of the material, we can use this information to reduce or eliminate defects – or if you want the defect, it gives us a way of knowing where the defects are,” and provides fresh insight about how to propagate and amplify the defects in the sample-production process.

    While the concentration of edge defects in the WS2 flakes was generally known before the latest study, Schwartzberg said that their effects on materials performance hadn’t previously been studied in such a comprehensive and detailed way.

    Researchers learned that a 10 percent deficiency in sulfur atoms was associated with the defective edge regions of the samples compared to other regions, and they identified a slighter, 3 percent sulfur deficiency toward the center of the flakes. Researchers also noted a change in the electronic structure and higher abundance of freely moving electrical charge-carriers associated with the high-defect edge areas.

    4
    This sequence of images shows a variety of energy intensities (white and yellow) at the edges of a 2D material known as tungsten disulfide, as measured via different techniques: photoluminescense intensity (far left); contact potential difference map (second from left); exciton emission intensity (third from left) – excitons are pairs consistent of an electrons and their quasiparticle counterpart, called a hole; trion emission intensity (far right) – trions are gropus of three charged quasiparticles consistening of either two electrons and a hole or two holes and an electron). (Credit: Christoph Kastl/Berkeley Lab)

    For this study, the defects were due to the sample-growth process. Future nanoARPES studies will focus on samples with defects that are induced through chemical processing or other treatments. Researchers hope to control the amount and kinds of atoms that are affected, and the locations where these defects are concentrated in the flakes.

    Such tiny tweaks could be important for processes like catalysis, which is used to enhance and accelerate many important industrial chemical production processes, and to explore quantum processes that rely on the production of individual particles that serve as information carriers in electronics.

    Because research of WS2 and related 2D materials is still in its infancy, there are many unknowns about the roles specific types of defects play in these materials, and Rotenberg noted that there is a world of possibilities for so-called “defect engineering” in these materials.

    In addition, MAESTRO’s nanoARPES has the ability to study the electronic structures of stacks of different types of 2D material layers. This can help researchers understand how their properties depend on their physical arrangement, and to explore working devices that incorporate 2D materials.

    “The unprecedented small scale of the measurements – currently approaching 50 nanometers – makes nanoARPES a great discovery tool that will be particularly useful to understand new materials as they are invented,” Rotenberg said.

    MAESTRO is one of the priority beamlines to be upgraded as part of the Lab’s ALS Upgrade (ALS-U) project, a major undertaking that will produce even brighter, more focused beams of light for experiments. “The ALS-U project will further improve the performance of the nanoARPES technique,” Rotenberg said, “making its measurements 10 to 30 times more efficient and significantly improving our ability to reach even shorter length scales.”

    NanoARPES could play an important role in the development of new solar technologies, because it allows researchers to see how nanoscale variations in chemical makeup, number of defects, and other structural features affect the electrons that ultimately govern their performance. These same issues are important for many other complex materials, such as superconductors, magnets, and thermoelectrics – which convert temperature to current and vice versa – so nanoARPES will also be very useful for these as well.

    The Molecular Foundry and ALS are both DOE Office of Science User Facilities.

    Researchers from the Berkeley Lab Chemical Sciences Division, Aarhus University in Denmark, and Montana State University also participated in this study. The work was supported by the U.S. Department of Energy’s Office of Basic Energy Sciences, the DOE Early Career Grant program, Berkeley Lab’s Laboratory Directed Research and Development program, the Villum Foundation, and the German Academic Exchange Service.

    See the full article here .

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    In the world of science, Lawrence Berkeley National Laboratory (Berkeley Lab) is synonymous with “excellence.” Thirteen Nobel prizes are associated with Berkeley Lab. Seventy Lab scientists are members of the National Academy of Sciences (NAS), one of the highest honors for a scientist in the United States. Thirteen of our scientists have won the National Medal of Science, our nation’s highest award for lifetime achievement in fields of scientific research. Eighteen of our engineers have been elected to the National Academy of Engineering, and three of our scientists have been elected into the Institute of Medicine. In addition, Berkeley Lab has trained thousands of university science and engineering students who are advancing technological innovations across the nation and around the world.

    Berkeley Lab is a member of the national laboratory system supported by the U.S. Department of Energy through its Office of Science. It is managed by the University of California (UC) and is charged with conducting unclassified research across a wide range of scientific disciplines. Located on a 202-acre site in the hills above the UC Berkeley campus that offers spectacular views of the San Francisco Bay, Berkeley Lab employs approximately 3,232 scientists, engineers and support staff. The Lab’s total costs for FY 2014 were $785 million. A recent study estimates the Laboratory’s overall economic impact through direct, indirect and induced spending on the nine counties that make up the San Francisco Bay Area to be nearly $700 million annually. The Lab was also responsible for creating 5,600 jobs locally and 12,000 nationally. The overall economic impact on the national economy is estimated at $1.6 billion a year. Technologies developed at Berkeley Lab have generated billions of dollars in revenues, and thousands of jobs. Savings as a result of Berkeley Lab developments in lighting and windows, and other energy-efficient technologies, have also been in the billions of dollars.

    Berkeley Lab was founded in 1931 by Ernest Orlando Lawrence, a UC Berkeley physicist who won the 1939 Nobel Prize in physics for his invention of the cyclotron, a circular particle accelerator that opened the door to high-energy physics. It was Lawrence’s belief that scientific research is best done through teams of individuals with different fields of expertise, working together. His teamwork concept is a Berkeley Lab legacy that continues today.

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  • richardmitnick 5:32 pm on March 7, 2019 Permalink | Reply
    Tags: a field that could extend the limits of Moore’s law by miniaturizing electronic components, A new study led by Berkeley Lab reveals how aligned layers of atomically thin semiconductors can yield an exotic new quantum material, A team of researchers led by the Department of Energy’s Lawrence Berkeley National Laboratory has developed a method that could turn ordinary semiconducting materials into quantum machines, Aiming Yan and Alex Zettl used a transmission electron microscope (TEM) at Berkeley Lab’s Molecular Foundry to take atomic-resolution images, Also contributing to the study were researchers from Arizona State University and the National Institute for Materials Science in Japan, Also valleytronics, and superconductivity which would allow electrons to flow in devices with virtually no resistance, , “This is an amazing discovery because we didn’t think of these semiconducting materials as strongly interacting” said Feng Wang, , Material Sciences, The researchers next plan to measure how this new quantum system could be applied to optoelectronics, The TEM images confirmed what they had suspected all along: the materials had indeed formed a moiré superlattice, Two-dimensional (2D) materials which are just one atom thick are like nanosized building blocks that can be stacked arbitrarily to form tiny devices, When the lattices of two 2D materials are similar and well-aligned a repeating pattern called a moiré superlattice can form   

    From Lawrence Berkeley National Lab: “When Semiconductors Stick Together, Materials Go Quantum” 

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    From Lawrence Berkeley National Lab

    March 7, 2019
    Theresa Duque
    tnduque@lbl.gov
    (510) 495-2418

    A new study led by Berkeley Lab reveals how aligned layers of atomically thin semiconductors can yield an exotic new quantum material.

    1
    A method developed by a Berkeley Lab-led research team may one day turn ordinary semiconducting materials into quantum electronic devices. (Credit: iStock.com/NiPlot)

    A team of researchers led by the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) has developed a simple method that could turn ordinary semiconducting materials into quantum machines – superthin devices marked by extraordinary electronic behavior. Such an advancement could help to revolutionize a number of industries aiming for energy-efficient electronic systems – and provide a platform for exotic new physics.

    The study describing the method, which stacks together 2D layers of tungsten disulfide and tungsten diselenide to create an intricately patterned material, or superlattice, was published online recently in the journal Nature.

    “This is an amazing discovery because we didn’t think of these semiconducting materials as strongly interacting,” said Feng Wang, a condensed matter physicist with Berkeley Lab’s Materials Sciences Division and professor of physics at UC Berkeley. “Now this work has brought these seemingly ordinary semiconductors into the quantum materials space.”

    2
    The twist angle formed between atomically thin layers of tungsten disulfide and tungsten diselenide acts as a “tuning knob,” transforming these semiconductors into an exotic quantum material. (Credit: Berkeley Lab) (Credit: Berkeley Lab)

    Two-dimensional (2D) materials, which are just one atom thick, are like nanosized building blocks that can be stacked arbitrarily to form tiny devices. When the lattices of two 2D materials are similar and well-aligned, a repeating pattern called a moiré superlattice can form.

    For the past decade, researchers have been studying ways to combine different 2D materials, often starting with graphene – a material known for its ability to efficiently conduct heat and electricity. Out of this body of work, other researchers had discovered that moiré superlattices formed with graphene exhibit exotic physics such as superconductivity when the layers are aligned at just the right angle.

    The new study, led by Wang, used 2D samples of semiconducting materials – tungsten disulfide and tungsten diselenide – to show that the twist angle between layers provides a “tuning knob” to turn a 2D semiconducting system into an exotic quantum material with highly interacting electrons.

    Entering a new realm of physics

    Co-lead authors Chenhao Jin, a postdoctoral scholar, and Emma Regan, a graduate student researcher, both of whom work under Wang in the Ultrafast Nano-Optics Group at UC Berkeley, fabricated the tungsten disulfide and tungsten diselenide samples using a polymer-based technique to pick up and transfer flakes of the materials, each measuring just tens of microns in diameter, into a stack.

    They had fabricated similar samples of the materials for a previous study [Science], but with the two layers stacked at no particular angle. When they measured the optical absorption of a new tungsten disulfide and tungsten diselenide sample for the current study, they were taken completely by surprise.

    The absorption of visible light in a tungsten disulfide/tungsten diselenide device is largest when the light has the same energy as the system’s exciton, a quasiparticle that consists of an electron bound to a hole that is common in 2D semiconductors. (In physics, a hole is a currently vacant state that an electron could occupy.)

    3
    The large potential energy of three distinct exciton states in a 2D tungsten disulfide/tungsten diselenide device could introduce exotic quantum phenomena into semiconducting materials. (Credit: Berkeley Lab)

    For light in the energy range that the researchers were considering, they expected to see one peak in the signal that corresponded to the energy of an exciton.

    Instead, they found that the original peak that they expected to see had split into three different peaks representing three distinct exciton states.

    What could have increased the number of exciton states in the tungsten disulfide/tungsten diselenide device from one to three? Was it the addition of a moiré superlattice?

    To find out, their collaborators Aiming Yan and Alex Zettl used a transmission electron microscope (TEM) at Berkeley Lab’s Molecular Foundry, a nanoscale science research facility, to take atomic-resolution images of the tungsten disulfide/tungsten diselenide device to check how the materials’ lattices were aligned.

    The TEM images confirmed what they had suspected all along: the materials had indeed formed a moiré superlattice. “We saw beautiful, repeating patterns over the entire sample,” said Regan. “After comparing this experimental observation with a theoretical model, we found that the moiré pattern introduces a large potential energy periodically over the device and could therefore introduce exotic quantum phenomena.”

    The researchers next plan to measure how this new quantum system could be applied to optoelectronics, which relates to the use of light in electronics; valleytronics, a field that could extend the limits of Moore’s law by miniaturizing electronic components; and superconductivity, which would allow electrons to flow in devices with virtually no resistance.

    Also contributing to the study were researchers from Arizona State University and the National Institute for Materials Science in Japan.

    The work was supported by the DOE Office of Science. Additional funding was provided by the National Science Foundation, the Department of Defense, and the Elemental Strategy Initiative conducted by MEXT, Japan, and JSPS KAKENHI. The Molecular Foundry is a DOE Office of Science user facility.

    See the full article here .

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    In the world of science, Lawrence Berkeley National Laboratory (Berkeley Lab) is synonymous with “excellence.” Thirteen Nobel prizes are associated with Berkeley Lab. Seventy Lab scientists are members of the National Academy of Sciences (NAS), one of the highest honors for a scientist in the United States. Thirteen of our scientists have won the National Medal of Science, our nation’s highest award for lifetime achievement in fields of scientific research. Eighteen of our engineers have been elected to the National Academy of Engineering, and three of our scientists have been elected into the Institute of Medicine. In addition, Berkeley Lab has trained thousands of university science and engineering students who are advancing technological innovations across the nation and around the world.

    Berkeley Lab is a member of the national laboratory system supported by the U.S. Department of Energy through its Office of Science. It is managed by the University of California (UC) and is charged with conducting unclassified research across a wide range of scientific disciplines. Located on a 202-acre site in the hills above the UC Berkeley campus that offers spectacular views of the San Francisco Bay, Berkeley Lab employs approximately 3,232 scientists, engineers and support staff. The Lab’s total costs for FY 2014 were $785 million. A recent study estimates the Laboratory’s overall economic impact through direct, indirect and induced spending on the nine counties that make up the San Francisco Bay Area to be nearly $700 million annually. The Lab was also responsible for creating 5,600 jobs locally and 12,000 nationally. The overall economic impact on the national economy is estimated at $1.6 billion a year. Technologies developed at Berkeley Lab have generated billions of dollars in revenues, and thousands of jobs. Savings as a result of Berkeley Lab developments in lighting and windows, and other energy-efficient technologies, have also been in the billions of dollars.

    Berkeley Lab was founded in 1931 by Ernest Orlando Lawrence, a UC Berkeley physicist who won the 1939 Nobel Prize in physics for his invention of the cyclotron, a circular particle accelerator that opened the door to high-energy physics. It was Lawrence’s belief that scientific research is best done through teams of individuals with different fields of expertise, working together. His teamwork concept is a Berkeley Lab legacy that continues today.

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

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  • richardmitnick 3:24 pm on February 22, 2019 Permalink | Reply
    Tags: , , , Material Sciences, Quantum condensed matter, Topological materials and insulators   

    From Discovery at Princeton University: “A quantum magnet with a topological twist” 

    Discovery at Princeton

    Princeton University
    From From Discovery at Princeton University

    February 22, 2019
    Catherine Zandonella

    1
    Taking their name from an intricate Japanese basket pattern, kagome magnets are thought to have electronic properties that could be valuable for future quantum devices and applications. Theories predict that some electrons in these materials have exotic, so-called topological behaviors and others behave somewhat like graphene, another material prized for its potential for new types of electronics.

    Now, an international team led by researchers at Princeton University has observed that some of the electrons in these magnets behave collectively, like an almost infinitely massive electron that is strangely magnetic, rather than like individual particles. The study was published in the journal Nature Physics this week. The science team is Jia-Xin Yin, Songtian S. Zhang, Guoqing Chang, Qi Wang, Stepan S. Tsirkin, Zurab Guguchia, Biao Lian, Huibin Zhou, Kun Jiang, Ilya Belopolski, Nana Shumiya, Daniel Multer, Maksim Litskevich, Tyler A. Cochran, Hsin Lin, Ziqiang Wang, Titus Neupert, Shuang Jia, Hechang Lei and M. Zahid Hasan.

    The team also showed that placing the kagome magnet in a high magnetic field causes the direction of magnetism to reverse. This “negative magnetism” is akin to having a compass that points south instead of north, or a refrigerator magnet that suddenly refuses to stick.

    “We have been searching for super-massive ‘flat band’ electrons that can still conduct electricity for a long time, and finally we have found them,” said M. Zahid Hasan, the Eugene Higgins Professor of Physics at Princeton University, who led the team. “In this system, we also found that due to an internal quantum phase effect, some electrons line up opposite to the magnetic field, producing negative magnetism.”

    The team explored how atoms arranged in a kagome pattern in a crystal give rise to strange electronic properties that can have real-world benefits, such as superconductivity, which allows electricity to flow without loss as heat, or magnetism that can be controlled at the quantum level for use in future electronics.

    The researchers used state-of-the-art scanning tunneling microscopy and spectroscopy (STM/S) to look at the behavior of electrons in a kagome-patterned crystal made from cobalt and tin, sandwiched between two layers of sulfur atoms, which are further sandwiched between two layers of tin.

    2
    Researchers explored a material that has an internal structure, shown in 3D in left panel, that consists of triangles and hexagons arranged in a pattern similar to that of a Japanese (kagome) basket. Credit: Hasan, et al.

    In the kagome layer, the cobalt atoms form triangles around a hexagon with a tin atom in the center. This geometry forces the electrons into some uncomfortable positions – leading this type of material to be called a “frustrated magnet.”

    To explore the electron behavior in this structure, the researchers nicked the top layers to reveal the kagome layer beneath.

    They then used the STM/S technique to detect each electron’s energy profile, or band structure. The band structure describes the range of energies an electron can have within a crystal, and explains, for example, why some materials conduct electricity and others are insulators. The researchers found that some of electrons in the kagome layer have a band structure that, rather than being curved as in most materials, is flat.

    A flat band structure indicates that the electrons have an effective mass that is so large as to be almost infinite. In such a state, the particles act collectively rather than as individual particles.

    Theories have long predicted that the kagome pattern would create a flat band structure, but this study is the first experimental detection of a flat band electron in such a system.
    diagrams

    3
    (Left) Although it is expected that a magnet pointing North would move up when the field is pointing up, it actually moves down. (Middle) Application of a magnetic field shifts the energy levels of electrons. (right, up) Energy shifts of Kagome electrons show a large negative magnetic moment (right, down) Orbital arrangements of Kagome electrons give rise to a geometrical quantum phase factor (Berry phase) which creates an unusual magnetic state.

    One of the general predictions that follows is that a material with a flat band may exhibit negative magnetism.

    Indeed, in the current study, when the researchers applied a strong magnetic field, some of the kagome magnet’s electrons pointed in the opposite direction.

    “Whether the field was applied up or down, the electrons’ energy flipped in the same direction, that was the first thing that was strange in terms of the experiments,” said Songtian Sonia Zhang, a graduate student in physics and one of three co-first-authors on the paper.

    “That puzzled us for about three months,” said Jia-Xin Yin, a postdoctoral research associate and another co-first author on the study. “We were searching for the reason, and with our collaborators we realized that this was the first experimental evidence that this flat band peak in the kagome lattice has a negative magnetic moment.”

    The researchers found that the negative magnetism arises due to the relationship between the kagome flat band, a quantum phenomenon called spin–orbit coupling, magnetism and a quantum factor called the Berry curvature field. Spin-orbit coupling refers to a situation where an electron’s spin, which itself is a quantum property of electrons, becomes linked to the electron’s orbital rotation. The combination of spin-orbital coupling and the magnetic nature of the material leads all the electrons to behave in lock step, like a giant single particle.

    Another intriguing behavior that arises from the tightly coupled spin-orbit interactions is the emergence of topological behaviors. The subject of the 2016 Nobel Prize in Physics, topological materials can have electrons that flow without resistance on their surfaces and are an active area of research. The cobalt-tin-sulfur material is an example of a topological system.

    Two-dimensional patterned lattices can have other desirable types of electron conductance. For example, graphene is a pattern of carbon atoms that has generated considerable interest for its electronic applications over the past two decades. The kagome lattice’s band structure gives rise to electrons that behave similarly to those in graphene.

    4
    Funding for this study was provided as follows: The STM experimental and theoretical work at Princeton University was supported by the Gordon and Betty Moore Foundation (GBMF4547). The ARPES characterization of the sample is supported by the United States Department of Energy under the Basic Energy Sciences program (DE-FG-02–05ER46200). Support was also provided through the Princeton Center for Theoretical Science and the Princeton Institute for the Science and Technology of Materials Imaging and Analysis Center at Princeton University, Lawrence Berkeley National Laboratory and the University of California, Berkeley.

    Authors and affiliations:

    Jia-Xin Yin, Songtian S. Zhang, Guoqing Chang, Zurab Guguchia, Ilya Belopolski, Nana Shumiya, Daniel Multer, Maksim Litskevich, Tyler A. Cochran, Biao Lian and M. Zahid Hasan: Department of Physics, Princeton University
    Qi Wang and Hechang Lei: Renmin University of China
    Stepan S. Tsirkin and Titus Neupert: University of Zurich
    Zurab Guguchia: Paul Scherrer Institute, Villigen PSI, Switzerland
    Huibin Zhou and Shuang Jia: Peking University and Chinese Academy of Sciences
    Kun Jiang and Ziqiang Wang: Boston College
    Hsin Lin: Institute of Physics, Academia Sinica, Taipei
    Zahid Hasan is also affiliated with the Princeton Institute for the Science and Technology of Materials and Lawrence Berkeley National Laboratory.

    See the full article here .

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    Princeton University Campus

    Discovery at Princeton University

    Whatever the controversy of the day, the way forward in science relies on following the evidence, wherever it may lead. Princeton researchers are at the forefront of this path, both through theoretical advances in artificial intelligence and machine learning, and through innovations in data science that are helping to address societal challenges, such as eviction and its impacts, energy-efficient transportation, marine “dead zones,” attitudes on immigration, and many more.

    The search for understanding is at the heart of University research, whether the quest leads to beautiful theorems, practical inventions or a new interpretation of art (page 26). Princeton is a place where all of these aspects of research coexist, cross-fertilize and intermingle. But why take my word for it? Let the pages of Discovery be the data that convince you.

    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 10:32 am on February 9, 2019 Permalink | Reply
    Tags: Condenced-Matter Physics, , Material Sciences, , ,   

    From SLAC National Accelerator Lab: “First direct view of an electron’s short, speedy trip across a border” 

    From SLAC National Accelerator Lab

    February 8, 2019
    Glennda Chui

    1
    Electrons traveling between two layers of atomically thin material give off tiny bursts of electromagnetic waves in the terahertz spectral range. This glow, shown in red and blue, allowed researchers at SLAC and Stanford to observe and track the electrons’ ultrafast movements. (Greg Stewart/SLAC National Accelerator Laboratory)

    Watching electrons sprint between atomically thin layers of material will shed light on the fundamental workings of semiconductors, solar cells and other key technologies.

    Electrons flowing across the boundary between two materials are the foundation of many key technologies, from flash memories to batteries and solar cells. Now researchers have directly observed and clocked these tiny cross-border movements for the first time, watching as electrons raced seven-tenths of a nanometer – about the width of seven hydrogen atoms – in 100 millionths of a billionth of a second.

    Led by scientists at the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University, the team made these observations by measuring tiny bursts of electromagnetic waves given off by the traveling electrons – a phenomenon described more than a century ago by Maxwell’s equations, but only now applied to this important measurement.

    “To make something useful, generally you need to put different materials together and transfer charge or heat or light between them,” said Eric Yue Ma, a postdoctoral researcher in the laboratory of SLAC/Stanford Professor Tony Heinz and lead author of a report in Science Advances.

    “This opens up a new way to measure how charge – in this case, electrons and holes – travels across the abrupt interface between two materials,” he said. “It doesn’t just apply to layered materials. For instance, it can also be used to look at electrons flowing between a solid surface and molecules that are attached to it, or even, in principle, between a liquid and a solid.”

    Too short, too fast – or were they?

    The materials used in this experiment are transition metal dichalcogenides, or TMDCs – an emerging class of semiconducting materials that consist of layers just a few atoms thick. There’s been an explosion of interest in TMDCs over the past few years as scientists explore their fundamental properties and potential uses in nanoelectronics and photonics.

    When two types of TMDC are stacked in alternating layers, electrons can flow from one layer to the next in a controllable way that people would like to harness for various applications.

    But until now, researchers who wanted to observe and study that flow had only been able to do it indirectly, by probing the material before and after the electrons had moved. The distances involved were just too short, and the electron speeds too fast, for today’s instruments to catch the flow of charge directly.

    At least that’s what they thought.

    Maxwell leads the way

    According to a famous set of equations named after physicist James Clerk Maxwell, pulses of current give off electromagnetic waves, which can vary from radio waves and microwaves to visible light and X-rays. In this case, the team realized that an electron’s journey from one TMDC layer to another should generate blips of terahertz waves – which fall between microwaves and infrared light on the electromagnetic spectrum – and that those blips could be detected with today’s state-of-the-art tools.

    “People had probably thought of this before, but dismissed the idea because they thought there was no way you could measure the current from electrons traveling such a small distance in such a small amount of material,” Ma said. “But if you do a back-of-the-envelope calculation, you see that if a current is really that fast you should be able to measure the emitted light, so we just tried.”

    Nudges from a laser

    The researchers, all investigators with the Stanford Institute for Materials and Energy Sciences (SIMES) at SLAC, tested their idea on a TMDC material made of molybdenum disulfide and tungsten disulfide.

    Working with SLAC/Stanford Professor Aaron Lindenberg, Ma and fellow postdoc Burak Guzelturk hit the material with ultrashort pulses of optical laser light to get the electrons moving and recorded the terahertz waves they gave off with a technique called time-domain terahertz emission spectroscopy. Those measurements not only revealed how far and fast the electric current traveled between layers, Ma said, but also the direction it traveled in. When the same two materials were stacked in reverse order, the current flowed in exactly the same way but in the opposite direction.

    “With the demonstration of this new technique, many exciting problems can now be addressed,” said Heinz, who led the team’s investigation. “For example, rotating one of the two crystal layers with respect to the other is known to dramatically change the electronic and optical properties of the combined layers. This method will allow us to directly follow the rapid motion of electrons from one layer to the other and see how this motion is affected by the relative positioning of the atoms.”

    Major funding for this work came from the DOE Office of Science and the Gordon and Betty Moore Foundation. The samples of material the team studied were grown at North Carolina State University.

    See the full article here .


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

     
  • richardmitnick 12:20 pm on January 28, 2019 Permalink | Reply
    Tags: , , Converting Wi-Fi signals to electricity with new 2-D materials, Material Sciences, ,   

    From MIT News: “Converting Wi-Fi signals to electricity with new 2-D materials” 

    MIT News
    MIT Widget

    From MIT News

    January 28, 2019
    Rob Matheson

    1
    Researchers from MIT and elsewhere have designed the first fully flexible, battery-free “rectenna” — a device that converts energy from Wi-Fi signals into electricity — that could be used to power flexible and wearable electronics, medical devices, and sensors for the “internet of things.” Image: Christine Daniloff

    Device made from flexible, inexpensive materials could power large-area electronics, wearables, medical devices, and more.

    Imagine a world where smartphones, laptops, wearables, and other electronics are powered without batteries. Researchers from MIT and elsewhere have taken a step in that direction, with the first fully flexible device that can convert energy from Wi-Fi signals into electricity that could power electronics.

    Devices that convert AC electromagnetic waves into DC electricity are known as “rectennas.” The researchers demonstrate a new kind of rectenna, described in a study appearing in Nature today, that uses a flexible radio-frequency (RF) antenna that captures electromagnetic waves — including those carrying Wi-Fi — as AC waveforms.

    The antenna is then connected to a novel device made out of a two-dimensional semiconductor just a few atoms thick. The AC signal travels into the semiconductor, which converts it into a DC voltage that could be used to power electronic circuits or recharge batteries.

    In this way, the battery-free device passively captures and transforms ubiquitous Wi-Fi signals into useful DC power. Moreover, the device is flexible and can be fabricated in a roll-to-roll process to cover very large areas.

    “What if we could develop electronic systems that we wrap around a bridge or cover an entire highway, or the walls of our office and bring electronic intelligence to everything around us? How do you provide energy for those electronics?” says paper co-author Tomás Palacios, a professor in the Department of Electrical Engineering and Computer Science and director of the MIT/MTL Center for Graphene Devices and 2D Systems in the Microsystems Technology Laboratories. “We have come up with a new way to power the electronics systems of the future — by harvesting Wi-Fi energy in a way that’s easily integrated in large areas — to bring intelligence to every object around us.”

    Promising early applications for the proposed rectenna include powering flexible and wearable electronics, medical devices, and sensors for the “internet of things.” Flexible smartphones, for instance, are a hot new market for major tech firms. In experiments, the researchers’ device can produce about 40 microwatts of power when exposed to the typical power levels of Wi-Fi signals (around 150 microwatts). That’s more than enough power to light up an LED or drive silicon chips.

    Another possible application is powering the data communications of implantable medical devices, says co-author Jesús Grajal, a researcher at the Technical University of Madrid. For example, researchers are beginning to develop pills that can be swallowed by patients and stream health data back to a computer for diagnostics.

    “Ideally you don’t want to use batteries to power these systems, because if they leak lithium, the patient could die,” Grajal says. “It is much better to harvest energy from the environment to power up these small labs inside the body and communicate data to external computers.”

    All rectennas rely on a component known as a “rectifier,” which converts the AC input signal into DC power. Traditional rectennas use either silicon or gallium arsenide for the rectifier. These materials can cover the Wi-Fi band, but they are rigid. And, although using these materials to fabricate small devices is relatively inexpensive, using them to cover vast areas, such as the surfaces of buildings and walls, would be cost-prohibitive. Researchers have been trying to fix these problems for a long time. But the few flexible rectennas reported so far operate at low frequencies and can’t capture and convert signals in gigahertz frequencies, where most of the relevant cell phone and Wi-Fi signals are.

    To build their rectifier, the researchers used a novel 2-D material called molybdenum disulfide (MoS2), which at three atoms thick is one of the thinnest semiconductors in the world. In doing so, the team leveraged a singular behavior of MoS2: When exposed to certain chemicals, the material’s atoms rearrange in a way that acts like a switch, forcing a phase transition from a semiconductor to a metallic material. The resulting structure is known as a Schottky diode, which is the junction of a semiconductor with a metal.

    “By engineering MoS2 into a 2-D semiconducting-metallic phase junction, we built an atomically thin, ultrafast Schottky diode that simultaneously minimizes the series resistance and parasitic capacitance,” says first author and EECS postdoc Xu Zhang, who will soon join Carnegie Mellon University as an assistant professor.

    Parasitic capacitance is an unavoidable situation in electronics where certain materials store a little electrical charge, which slows down the circuit. Lower capacitance, therefore, means increased rectifier speeds and higher operating frequencies. The parasitic capacitance of the researchers’ Schottky diode is an order of magnitude smaller than today’s state-of-the-art flexible rectifiers, so it is much faster at signal conversion and allows it to capture and convert up to 10 gigahertz of wireless signals.

    “Such a design has allowed a fully flexible device that is fast enough to cover most of the radio-frequency bands used by our daily electronics, including Wi-Fi, Bluetooth, cellular LTE, and many others,” Zhang says.

    The reported work provides blueprints for other flexible Wi-Fi-to-electricity devices with substantial output and efficiency. The maximum output efficiency for the current device stands at 40 percent, depending on the input power of the Wi-Fi input. At the typical Wi-Fi power level, the power efficiency of the MoS2 rectifier is about 30 percent. For reference, today’s rectennas made from rigid, more expensive silicon or gallium arsenide achieve around 50 to 60 percent.

    There are 15 other paper co-authors from MIT, Technical University of Madrid, the Army Research Laboratory, Charles III University of Madrid, Boston University, and the University of Southern California.

    The team is now planning to build more complex systems and improve efficiency. The work was made possible, in part, by a collaboration with the Technical University of Madrid through the MIT International Science and Technology Initiatives (MISTI). It was also partially supported by the Institute for Soldier Nanotechnologies, the Army Research Laboratory, the National Science Foundation’s Center for Integrated Quantum Materials, and the Air Force Office of Scientific Research.

    See the full article here .


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  • richardmitnick 11:04 am on January 2, 2019 Permalink | Reply
    Tags: , Material Sciences, , , Physicists record “lifetime” of graphene qubits, , ,   

    From MIT News: “Physicists record ‘lifetime’ of graphene qubits” 

    MIT News
    MIT Widget

    From MIT News

    December 31, 2018
    Rob Matheson

    1
    Researchers from MIT and elsewhere have recorded the “temporal coherence” of a graphene qubit — how long it maintains a special state that lets it represent two logical states simultaneously — marking a critical step forward for practical quantum computing. Stock image

    First measurement of its kind could provide stepping stone to practical quantum computing.

    Researchers from MIT and elsewhere have recorded, for the first time, the “temporal coherence” of a graphene qubit — meaning how long it can maintain a special state that allows it to represent two logical states simultaneously. The demonstration, which used a new kind of graphene-based qubit, represents a critical step forward for practical quantum computing, the researchers say.

    Superconducting quantum bits (simply, qubits) are artificial atoms that use various methods to produce bits of quantum information, the fundamental component of quantum computers. Similar to traditional binary circuits in computers, qubits can maintain one of two states corresponding to the classic binary bits, a 0 or 1. But these qubits can also be a superposition of both states simultaneously, which could allow quantum computers to solve complex problems that are practically impossible for traditional computers.

    The amount of time that these qubits stay in this superposition state is referred to as their “coherence time.” The longer the coherence time, the greater the ability for the qubit to compute complex problems.

    Recently, researchers have been incorporating graphene-based materials into superconducting quantum computing devices, which promise faster, more efficient computing, among other perks. Until now, however, there’s been no recorded coherence for these advanced qubits, so there’s no knowing if they’re feasible for practical quantum computing.

    In a paper published today in Nature Nanotechnology, the researchers demonstrate, for the first time, a coherent qubit made from graphene and exotic materials. These materials enable the qubit to change states through voltage, much like transistors in today’s traditional computer chips — and unlike most other types of superconducting qubits. Moreover, the researchers put a number to that coherence, clocking it at 55 nanoseconds, before the qubit returns to its ground state.

    The work combined expertise from co-authors William D. Oliver, a physics professor of the practice and Lincoln Laboratory Fellow whose work focuses on quantum computing systems, and Pablo Jarillo-Herrero, the Cecil and Ida Green Professor of Physics at MIT who researches innovations in graphene.

    “Our motivation is to use the unique properties of graphene to improve the performance of superconducting qubits,” says first author Joel I-Jan Wang, a postdoc in Oliver’s group in the Research Laboratory of Electronics (RLE) at MIT. “In this work, we show for the first time that a superconducting qubit made from graphene is temporally quantum coherent, a key requisite for building more sophisticated quantum circuits. Ours is the first device to show a measurable coherence time — a primary metric of a qubit — that’s long enough for humans to control.”

    There are 14 other co-authors, including Daniel Rodan-Legrain, a graduate student in Jarillo-Herrero’s group who contributed equally to the work with Wang; MIT researchers from RLE, the Department of Physics, the Department of Electrical Engineering and Computer Science, and Lincoln Laboratory; and researchers from the Laboratory of Irradiated Solids at the École Polytechnique and the Advanced Materials Laboratory of the National Institute for Materials Science.

    A pristine graphene sandwich

    Superconducting qubits rely on a structure known as a “Josephson junction,” where an insulator (usually an oxide) is sandwiched between two superconducting materials (usually aluminum). In traditional tunable qubit designs, a current loop creates a small magnetic field that causes electrons to hop back and forth between the superconducting materials, causing the qubit to switch states.

    But this flowing current consumes a lot of energy and causes other issues. Recently, a few research groups have replaced the insulator with graphene, an atom-thick layer of carbon that’s inexpensive to mass produce and has unique properties that might enable faster, more efficient computation.

    To fabricate their qubit, the researchers turned to a class of materials, called van der Waals materials — atomic-thin materials that can be stacked like Legos on top of one another, with little to no resistance or damage. These materials can be stacked in specific ways to create various electronic systems. Despite their near-flawless surface quality, only a few research groups have ever applied van der Waals materials to quantum circuits, and none have previously been shown to exhibit temporal coherence.

    For their Josephson junction, the researchers sandwiched a sheet of graphene in between the two layers of a van der Waals insulator called hexagonal boron nitride (hBN). Importantly, graphene takes on the superconductivity of the superconducting materials it touches. The selected van der Waals materials can be made to usher electrons around using voltage, instead of the traditional current-based magnetic field. Therefore, so can the graphene — and so can the entire qubit.

    When voltage gets applied to the qubit, electrons bounce back and forth between two superconducting leads connected by graphene, changing the qubit from ground (0) to excited or superposition state (1). The bottom hBN layer serves as a substrate to host the graphene. The top hBN layer encapsulates the graphene, protecting it from any contamination. Because the materials are so pristine, the traveling electrons never interact with defects. This represents the ideal “ballistic transport” for qubits, where a majority of electrons move from one superconducting lead to another without scattering with impurities, making a quick, precise change of states.

    How voltage helps

    The work can help tackle the qubit “scaling problem,” Wang says. Currently, only about 1,000 qubits can fit on a single chip. Having qubits controlled by voltage will be especially important as millions of qubits start being crammed on a single chip. “Without voltage control, you’ll also need thousands or millions of current loops too, and that takes up a lot of space and leads to energy dissipation,” he says.

    Additionally, voltage control means greater efficiency and a more localized, precise targeting of individual qubits on a chip, without “cross talk.” That happens when a little bit of the magnetic field created by the current interferes with a qubit it’s not targeting, causing computation problems.

    For now, the researchers’ qubit has a brief lifetime. For reference, conventional superconducting qubits that hold promise for practical application have documented coherence times of a few tens of microseconds, a few hundred times greater than the researchers’ qubit.

    But the researchers are already addressing several issues that cause this short lifetime, most of which require structural modifications. They’re also using their new coherence-probing method to further investigate how electrons move ballistically around the qubits, with aims of extending the coherence of qubits in general.

    See the full article here .


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  • richardmitnick 11:39 am on November 7, 2018 Permalink | Reply
    Tags: , , , , Dancing atoms in perovskite materials provide insight into how solar cells work, Material Sciences, , ,   

    From SLAC National Accelerator Lab: “Dancing atoms in perovskite materials provide insight into how solar cells work” 

    From SLAC National Accelerator Lab

    November 6, 2018
    Ali Sundermier

    1
    When the researchers scattered neutrons off the perovskite material (red beam) they were able to measure the energy the neutrons lost or gained (white and blue lines). Using this information, they were able to see the structure and motion of the atoms and molecules within the material (arrangement of blue and purple spheres). (Greg Stewart/SLAC National Accelerator Laboratory)

    A new study is a step forward in understanding why perovskite materials work so well in energy devices and potentially leads the way toward a theorized “hot” technology that would significantly improve the efficiency of today’s solar cells.

    A closer look at materials that make up conventional solar cells reveals a nearly rigid arrangement of atoms with little movement. But in hybrid perovskites, a promising class of solar cell materials, the arrangements are more flexible and atoms dance wildly around, an effect that impacts the performance of the solar cells but has been difficult to measure.

    In a paper published in the PNAS, an international team of researchers led by the U.S. Department of Energy’s SLAC National Accelerator Laboratory has developed a new understanding of those wild dances and how they affect the functioning of perovskite materials. The results could explain why perovskite solar cells are so efficient and aid the quest to design hot-carrier solar cells, a theorized technology that would almost double the efficiency limits of conventional solar cells by converting more sunlight into usable electrical energy.

    Piece of the puzzle

    Perovskite solar cells, which can be produced at room temperature, offer a less expensive and potentially better performing alternative to conventional solar cell materials like silicon, which have to be manufactured at extremely high temperatures to eliminate defects. But a lack of understanding about what makes perovskite materials so efficient at converting sunlight into electricity has been a major hurdle to producing even higher efficiency perovskite solar cells.

    “It’s really only been over the last five or six years that people have developed this intense interest in solar perovskite materials,” says Mike Toney, a distinguished staff scientist at SLAC’s Stanford Synchrotron Radiation Light Source (SSRL) who led the study.

    SLAC/SSRL

    “As a consequence, a lot of the foundational knowledge about what makes the materials work is missing. In this research, we provided an important piece of this puzzle by showing what sets them apart from more conventional solar cell materials. This provides us with scientific underpinnings that will allow us to start engineering these materials in a rational way.”

    Keeping it hot

    When sunlight hits a solar cell, some of the energy can be used to kick electrons in the material up to higher energy states. These higher-energy electrons are funneled out of the material, producing electricity.

    But before this happens, a majority of the sun’s energy is lost to heat with some fraction also lost during the extraction of usable energy or due to inefficient light collection. In many conventional solar cells, such as those made with silicon, acoustic phonons – a sort of sound wave that propagates through material – are the primary way that this heat is carried through the material. The energy lost by the electron as heat limits the efficiency of the solar cell.

    In this study, theorists from the United Kingdom, led by Imperial College Professor Aron Walsh and electronic structure theorists Jonathan Skelton and Jarvist Frost, provided a theoretical framework for interpreting the experimental results. They predicted that acoustic phonons traveling through perovskites would have short lifetimes as a result of the flexible arrangements of dancing atoms and molecules in the material.

    Stanford chemists Hema Karunadasa and Ian Smith were able to grow the large, specialized single crystals that were essential for this work. With the help of Peter Gehring, a physicist at the NIST Center for Neutron Research, the team scattered neutrons off these perovskite single crystals in a way that allowed them to retrace the motion of the atoms and molecules within the material. This allowed them to precisely measure the lifetime of the acoustic phonons.

    The research team found that in perovskites, acoustic phonons are incredibly short-lived, surviving for only 10 to 20 trillionths of a second. Without these phonons trucking heat through the material, the electrons might stay hot and hold onto their energy as they’re pulled out of the material. Harnessing this effect could potentially lead to hot-carrier solar cells with efficiencies that are nearly twice as high as conventional solar cells.

    In addition, this phenomenon could explain how perovskite solar cells work so well despite the material being riddled with defects that would trap electrons and dampen performance in other materials.

    “Since phonons in perovskites don’t travel very far, they end up heating the area surrounding the electrons, which might provide the boost the electrons need to escape the traps and continue on their merry way,” Toney says.

    Transforming energy production

    To follow up on this study, researchers at the Center for Hybrid Organic-Inorganic Semiconductors for Energy (CHOISE) Energy Frontier Research Center led by DOE’s National Renewable Energy Laboratory will investigate this phenomenon in more complicated perovskite materials that are shown to be more efficient in energy devices. They would like to figure out how changing the chemical make-up of the material affects acoustic phonon lifetimes.

    “We need to fundamentally transform our energy system as quickly as possible,” says Aryeh Gold-Parker, who co-led the study as a PhD student at Stanford University and SLAC. “As we move toward a low-carbon future, a very important piece is having cheap and efficient solar cells. The hope in perovskites is that they’ll lead to commercial solar panels that are more efficient and cheaper than the ones on the market today.”

    The research team also included scientists from NIST; the University of Bath and Kings College London, both in the UK; and Yonsei University in Korea.

    SSRL is a DOE Office of Science user facility. This work was supported by the DOE’s Office of Science and the Solar Energy Technologies Office; the Engineering and Physical Sciences Research Council; the Royal Society; and the Leverhulme Trust.

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings
    Please help promote STEM in your local schools.

    Stem Education Coalition

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

     
  • richardmitnick 10:37 am on November 1, 2018 Permalink | Reply
    Tags: , , Material Sciences, , , ,   

    From SLAC National Accelerator Lab: “Scientists make first detailed measurements of key factors related to high-temperature superconductivity” 

    From SLAC National Accelerator Lab

    October 31, 2018
    Glennda Chui

    1
    A new study reveals how coordinated motions of copper (red) and oxygen (grey) atoms in a high-temperature superconductor boost the superconducting strength of pairs of electrons (white glow), allowing the material to conduct electricity without any loss at much higher temperatures. The discovery opens a new path to engineering higher-temperature superconductors. (Greg Stewart/SLAC National Accelerator Laboratory)

    2
    An illustration depicts the repulsive energy (yellow flashes) generated by electrons in one layer of a cuprate material repelling electrons in the next layer. Theorists think this energy could play a critical role in creating the superconducting state, leading electrons to form a distinctive form of “sound wave” that could boost superconducting temperatures. Scientists have now observed and measured those sound waves for the first time. (Greg Stewart/SLAC National Accelerator Laboratory)

    In superconducting materials, electrons pair up and condense into a quantum state that carries electrical current with no loss. This usually happens at very low temperatures. Scientists have mounted an all-out effort to develop new types of superconductors that work at close to room temperature, which would save huge amounts of energy and open a new route for designing quantum electronics. To get there, they need to figure out what triggers this high-temperature form of superconductivity and how to make it happen on demand.

    Now, in independent studies reported in Science and Nature, scientists from the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University report two important advances: They measured collective vibrations of electrons for the first time and showed how collective interactions of the electrons with other factors appear to boost superconductivity.

    Carried out with different copper-based materials and with different cutting-edge techniques, the experiments lay out new approaches for investigating how unconventional superconductors operate.

    “Basically, what we’re trying to do is understand what makes a good superconductor,” said co-author Thomas Devereaux, a professor at SLAC and Stanford and director of SIMES, the Stanford Institute for Materials and Energy Sciences, whose investigators led both studies.

    “What are the ingredients that could give rise to superconductivity at temperatures well above what they are today?” he said. “These and other recent studies indicate that the atomic lattice plays an important role, giving us hope that we are gaining ground in answering that question.”

    The high-temperature puzzle

    Conventional superconductors were discovered in 1911, and scientists know how they work: Free-floating electrons are attracted to a material’s lattice of atoms, which has a positive charge, in a way that lets them pair up and flow as electric current with 100 percent efficiency. Today, superconducting technology is used in MRI machines, maglev trains and particle accelerators.

    But these superconductors work only when chilled to temperatures as cold as outer space. So when scientists discovered in 1986 that a family of copper-based materials known as cuprates can superconduct at much higher, although still quite chilly, temperatures, they were elated.

    The operating temperature of cuprates has been inching up ever since – the current record is about 120 degrees Celsius below the freezing point of water – as scientists explore a number of factors that could either boost or interfere with their superconductivity. But there’s still no consensus about how the cuprates function.

    “The key question is how can we make all these electrons, which very much behave as individuals and do not want to cooperate with others, condense into a collective state where all the parties participate and give rise to this remarkable collective behavior?” said Zhi-Xun Shen, a SLAC/Stanford professor and SIMES investigator who participated in both studies.

    Behind-the-scenes boost

    One of the new studies, at SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL), took a systematic look at how “doping” – adding a chemical that changes the density of electrons in a material – affects the superconductivity and other properties of a cuprate called Bi2212.

    SLAC/SSRL


    SLAC/SSRL

    Collaborating researchers at the National Institute of Advanced Industrial Science and Technology (AIST) in Japan prepared samples of the material with slightly different levels of doping. Then a team led by SIMES researcher Yu He and SSRL staff scientist Makoto Hashimoto examined the samples at SSRL with angle-resolved photoemission spectroscopy, or ARPES. It uses a powerful beam of X-ray light to kick individual electrons out of a sample material so their momentum and energy can be measured. This reveals what the electrons in the material are doing.

    In this case, as the level of doping increased, the maximum superconducting temperature of the material peaked and fell off again, He said.

    The team focused in on samples with particularly robust superconducting properties. They discovered that three interwoven effects – interactions of electrons with each other, with lattice vibrations and with superconductivity itself – reinforce each other in a positive feedback loop when conditions are right, boosting superconductivity and raising the superconducting temperature of the material.

    Small changes in doping produced big changes in superconductivity and in the electrons’ interaction with lattice vibrations, Devereaux said. The next step is to figure out why this particular level of doping is so important.

    “One popular theory has been that rather than the atomic lattice being the source of the electron pairing, as in conventional superconductors, the electrons in high-temperature superconductors form some kind of conspiracy by themselves. This is called electronic correlation,” Yu He said. “For instance, if you had a room full of electrons, they would spread out. But if some of them demand more individual space, others will have to squeeze closer to accommodate them.”

    In this study, He said, “What we find is that the lattice has a behind-the-scenes role after all, and we may have overlooked an important ingredient for high-temperature superconductivity for the past three decades,” a conclusion that ties into the results of earlier research by the SIMES group Science.

    Electron ‘Sound Waves’

    The other study, performed at the European Synchrotron Radiation Facility (ESRF) in France, used a technique called resonant inelastic X-ray scattering, or RIXS, to observe the collective behavior of electrons in layered cuprates known as LCCO and NCCO.


    ESRF. Grenoble, France

    RIXS excites electrons deep inside atoms with X-rays, and then measures the light they give off as they settle back down into their original spots.

    In the past, most studies have focused only on the behavior of electrons within a single layer of cuprate material, where electrons are known to be much more mobile than they are between layers, said SIMES staff scientist Wei-Sheng Lee. He led the study with Matthias Hepting, who is now at the Max Planck Institute for Solid State Research in Germany.

    But in this case, the team wanted to test an idea raised by theorists – that the energy generated by electrons in one layer repelling electrons in the next one plays a critical role in forming the superconducting state.

    When excited by light, this repulsion energy leads electrons to form a distinctive sound wave known as an acoustic plasmon, which theorists predict could account for as much as 20 percent of the increase in superconducting temperature seen in cuprates.

    With the latest in RIXS technology, the SIMES team was able to observe and measure those acoustic plasmons.

    “Here we see for the first time how acoustic plasmons propagate through the whole lattice,” Lee said. “While this doesn’t settle the question of where the energy needed to form the superconducting state comes from, it does tell us that the layered structure itself affects how the electrons behave in a very profound way.”

    This observation sets the stage for future studies that manipulate the sound waves with light, for instance, in a way that enhances superconductivity, Lee said. The results are also relevant for developing future plasmonic technology, he said, with a range of applications from sensors to photonic and electronic devices for communications.

    SSRL is a DOE Office of Science user facility, and SIMES is a joint institute of SLAC and Stanford.

    In addition to researchers from SLAC, Stanford and AIST, the study carried out at SSRL involved scientists from University of Tokyo; University of California, Berkeley; and Lorentz Institute for Theoretical Physics in the Netherlands.

    The study conducted at ESRF also involved researchers from SSRL; Polytechnic University of Milan in Italy; ESRF; Binghamton University in New York; and the University of Maryland.

    Both studies were funded by the DOE Office of Science.

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings
    Please help promote STEM in your local schools.

    Stem Education Coalition

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

     
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