New research led by scientists from the Department of Energy's SLAC National Accelerator Laboratory and Stanford University shows how individual atoms move in trillionths of a second to form wrinkles on a three-atom-thick material. Revealed by a brand new "electron camera," one of the world's speediest, this unprecedented level of detail could guide researchers in the development of efficient solar cells, fast and flexible electronics and high-performance chemical catalysts. The breakthrough, accepted for publication Aug. 31 in ("Dynamic Structural Response and Deformations of Monolayer MoS2 Visualized by Femtosecond Electron Diffraction"), could take materials science to a whole new level. It was made possible with SLAC's instrument for ultrafast electron diffraction (UED), which uses energetic electrons to take snapshots of atoms and molecules on timescales as fast as 100 quadrillionths of a second. Researchers have used SLAC's experiment for ultrafast electron diffraction (UED), one of the world's fastest 'electron cameras' to take snapshots of a three-atom-thick layer of a promising material as it wrinkles in response to a laser pulse. Understanding these dynamic ripples could provide crucial clues for the development of next-generation solar cells, electronics and catalysts. (Image: SLAC National Accelerator Laboratory) "This is the first published scientific result with our new instrument," said scientist Xijie Wang, SLAC's UED team lead. "It showcases the method's outstanding combination of atomic resolution, speed and sensitivity." SLAC Director Chi-Chang Kao said, "Together with complementary data from SLAC's X-ray laser Linac Coherent Light Source, UED creates unprecedented opportunities for ultrafast science in a broad range of disciplines, from materials science to chemistry to the biosciences." LCLS is a DOE Office of Science User Facility. Extraordinary Material Properties in Two Dimensions Monolayers, or 2-D materials, contain just a single layer of molecules. In this form they can take on new and exciting properties such as superior mechanical strength and an extraordinary ability to conduct electricity and heat. But how do these monolayers acquire their unique characteristics? Until now, researchers only had a limited view of the underlying mechanisms. "The functionality of 2-D materials critically depends on how their atoms move," said SLAC and Stanford researcher Aaron Lindenberg, who led the research team. "However, no one has ever been able to study these motions on the atomic level and in real time before. Our results are an important step toward engineering next-generation devices from single-layer materials." The research team looked at molybdenum disulfide, or MoS2, which is widely used as a lubricant but takes on a number of interesting behaviors when in single-layer form - more than 150,000 times thinner than a human hair. For example, the monolayer form is normally an insulator, but when stretched, it can become electrically conductive. This switching behavior could be used in thin, flexible electronics and to encode information in data storage devices. Thin films of MoS2 are also under study as possible catalysts that facilitate chemical reactions. In addition, they capture light very efficiently and could be used in future solar cells. Because of this strong interaction with light, researchers also think they may be able to manipulate the material's properties with light pulses. "To engineer future devices, control them with light and create new properties through systematic modifications, we first need to understand the structural transformations of monolayers on the atomic level," said Stanford researcher Ehren Mannebach, the study's lead author. Electron Camera Reveals Ultrafast Motions Previous analyses showed that single layers of molybdenum disulfide have a wrinkled surface. However, these studies only provided a static picture. The new study reveals for the first time how surface ripples form and evolve in response to laser light. Researchers at SLAC placed their monolayer samples, which were prepared by Linyou Cao's group at North Carolina State University, into a beam of very energetic electrons. The electrons, which come bundled in ultrashort pulses, scatter off the sample's atoms and produce a signal on a detector that scientists use to determine where atoms are located in the monolayer. This technique is called ultrafast electron diffraction. The team then used ultrashort laser pulses to excite motions in the material, which cause the scattering pattern to change over time. "Combined with theoretical calculations, these data show how the light pulses generate wrinkles that have large amplitudes - more than 15 percent of the layer's thickness - and develop extremely quickly, in about a trillionth of a second. This is the first time someone has visualized these ultrafast atomic motions," Lindenberg said. Once scientists better understand monolayers of different materials, they could begin putting them together and engineer mixed materials with completely new optical, mechanical, electronic and chemical properties.
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Team announces breakthrough observation of Mott transition in a superconductor
An international team of researchers, including the MESA+ Institute for Nanotechnology at the University of Twente in The Netherlands and the U.S. Department of Energy’s Argonne National Laboratory, announced today in the observation of a dynamic Mott transition in a superconductor ("Critical behavior at a dynamic vortex insulator-to-metal transition"). The discovery experimentally connects the worlds of classical and quantum mechanics and illuminates the mysterious nature of the Mott transition. It also could shed light on non-equilibrium physics, which is poorly understood but governs most of what occurs in our world. The finding may also represent a step towards more efficient electronics based on the Mott transition. Since its foundations were laid in the early part of the 20th century, scientists have been trying to reconcile quantum mechanics with the rules of classical or Newtonian physics (like how you describe the path of an apple thrown into the air—or dropped from a tree). Physicists have made strides in linking the two approaches, but experiments that connect the two are still few and far between; physics phenomena are usually classified as either quantum or classical, but not both. One system that unites the two is found in superconductors, certain materials that conduct electricity perfectly when cooled to very low temperatures. Magnetic fields penetrate the superconducting material in the form of tiny filaments called vortices, which control the electronic and magnetic properties of the materials. These vortices display both classical and quantum properties, which led researchers to study them for access to one of the most enigmatic phenomena of modern condensed matter physics: the Mott insulator-to-metal transition. The Mott transition occurs in certain materials that according to textbook quantum mechanics should be metals, but in reality turn insulators. A complex phenomenon controlled by the interactions of many quantum particles, the Mott transition remains mysterious—even whether or not it’s a classical or quantum phenomenon is not quite clear. Moreover, scientists have never directly observed a dynamic Mott transition, in which a phase transition from an insulating to a metallic state is induced by driving an electrical current through the system; the disorder inherent in real systems disguises Mott properties. At the University of Twente, researchers built a system containing 90,000 superconducting niobium nano-sized islands on top of a gold film. In this configuration, the vortices find it energetically easiest to settle into energy dimples in an arrangement like an egg crate—and make the material act as a Mott insulator, since the vortices won’t move if the applied electric current is small. When they applied a large enough electric current, however, the scientists saw a dynamic Mott transition as the system flipped to become a conducting metal; the properties of the material had changed as the current pushed it out of equilibrium. The vortex system behaved exactly like an electronic Mott transition driven by temperature, said Valerii Vinokur, an Argonne Distinguished Fellow and corresponding author on the study. He and study co-author Tatyana Baturina, then at Argonne, analyzed the data and recognized the Mott behavior. “This experimentally materializes the correspondence between quantum and classical physics,” Vinokur said. “We can controllably induce a phase transition between a state of locked vortices to itinerant vortices by applying an electric current to the system,” said Hans Hilgenkamp, head of the University of Twente research group. “Studying these phase transitions in our artificial systems is interesting in its own right, but may also provide further insight in the electronic transitions in real materials.” The system could further provide scientists with insight into two categories of physics that have been hard to understand: many-body systems and out-of-equilibrium systems. “This is a classical system that which is easy to experiment with and provides what looks like access to very complicated many-body systems,” said Vinokur. “It looks a bit like magic.” As the name implies, many-body problems involve a large number of particles interacting; with current theory they are very difficult to model or understand. “Furthermore, this system will be key to building a general understanding of out-of-equilibrium physics, which would be a major breakthrough in physics,” Vinokur said. The Department of Energy named five great basic energy scientific challenges of our time; one of them is understanding and controlling out-of-equilibrium phenomena. Equilibrium systems—where there’s no energy moving around—are now understood quite well. But nearly everything in our lives involves energy flow, from photosynthesis to digestion to tropical cyclones, and we don’t yet have the physics to describe it well. Scientists think a better understanding could lead to huge improvements in energy capture, batteries and energy storage, electronics and more. As we seek to make electronics faster and smaller, Mott systems also offer a possible alternative to the silicon transistor. Since they can be flipped between conducting and insulating with small changes in voltage, they may be able to encode 1s and 0s at smaller scales and higher accuracy than silicon transistors. ‘Initially, we were studying the structures for completely different reasons, namely to investigate the effects of inhomogeneities on superconductivity,” Hilgenkamp said. “After discussing with Valerii Vinokur at Argonne, we looked more specifically into our data and were quite amazed to see that it revealed so nicely the details of the transition between the state of locked and moving vortices. There are many ideas for follow up studies, and we look forward to our continued collaboration.”
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Ultrafast uncoupled magnetism in atoms
Future computers will require a magnetic material which can be manipulated ultra-rapidly by breaking the strong magnetic coupling. A study has been published in ("Disparate ultrafast dynamics of itinerant and localized magnetic moments in gadolinium metal") today in which Swedish and German scientists demonstrate that even the strongest magnetic coupling may be broken within picoseconds (10-12 s). This will open up an exciting new area of research. The element gadolinium is named after the Uppsala chemist Johan Gadolin who discovered the first rare-earth metal yttrium in the late 1700s. Gadolinium is in the same class of elements and it has unique magnetic properties which make it especially interesting for magnetic data storage. Its most useful property is that it has the greatest spin magnetic moment of any element since there are two different magnetic moments on every atom. These spin moments are coupled in parallel so strongly that no existing magnetic field on earth could break the coupling. An international collaboration between Karel Carva and Peter Oppeneer, two physicists from Uppsala University, and researchers from the Free University Berlin and Konstanz University in Germany has shown that it is possible to break the coupling between the spin moments. Researchers in Berlin used light pulses shorter than picoseconds to excite metallic gadolinium and then monitored the spin dynamics of both spin moments with ultra-short, high-energy x-ray flashes. The spin dynamics they revealed showed that the strong coupling was broken within picoseconds and it remained uncoupled for almost 100 picoseconds. The theoretical calculations of the Uppsala researchers provided a detailed explanation of how this fundamental magnetic interaction can be overcome. "Not too long ago it became clear that the weaker coupling between spin moments on different atoms of a material can be broken. We've now shown that even the stronger spin magnetic coupling within an individual atom can be overpowered. This provides new opportunities to manipulate magnetic materials and opens new paths to the data storage of the future," says professor Peter Oppeneer.
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