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  • richardmitnick 3:55 pm on April 17, 2017 Permalink | Reply
    Tags: , , , RIKEN SACLA, , , , ,   

    From SLAC: “SLAC’s X-ray Laser Glimpses How Electrons Dance with Atomic Nuclei in Materials” 


    SLAC Lab

    September 22, 2016

    Studies Could Help Design and Control Materials with Intriguing Properties, Including Novel Electronics, Solar Cells and Superconductors.

    From hard to malleable, from transparent to opaque, from channeling electricity to blocking it: Materials come in all types. A number of their intriguing properties originate in the way a material’s electrons “dance” with its lattice of atomic nuclei, which is also in constant motion due to vibrations known as phonons.

    This coupling between electrons and phonons determines how efficiently solar cells convert sunlight into electricity. It also plays key roles in superconductors that transfer electricity without losses, topological insulators that conduct electricity only on their surfaces, materials that drastically change their electrical resistance when exposed to a magnetic field, and more.

    At the Department of Energy’s SLAC National Accelerator Laboratory, scientists can study these coupled motions in unprecedented detail with the world’s most powerful X-ray laser, the Linac Coherent Light Source (LCLS). LCLS is a DOE Office of Science User Facility.

    SLAC/LCLS

    1
    An illustration shows how laser light excites electrons (white spheres) in a solid material, creating vibrations in its lattice of atomic nuclei (black and blue spheres). SLAC’s LCLS X-ray laser reveals the ultrafast “dance” between electrons and vibrations that accounts for many important properties of materials. (SLAC National Accelerator Laboratory)

    “It has been a long-standing goal to understand, initiate and control these unusual behaviors,” says LCLS Director Mike Dunne. “With LCLS we are now able to see what happens in these materials and to model complex electron-phonon interactions. This ability is central to the lab’s mission of developing new materials for next-generation electronics and energy solutions.”

    LCLS works like an extraordinary strobe light: Its ultrabright X-rays take snapshots of materials with atomic resolution and capture motions as fast as a few femtoseconds, or millionths of a billionth of a second. For comparison, one femtosecond is to a second what seven minutes is to the age of the universe.

    Two recent studies made use of these capabilities to study electron-phonon interactions in lead telluride, a material that excels at converting heat into electricity, and chromium, which at low temperatures has peculiar properties similar to those of high-temperature superconductors.

    Turning Heat into Electricity and Vice Versa

    Lead telluride, a compound of the chemical elements lead and tellurium, is of interest because it is a good thermoelectric: It generates an electrical voltage when two opposite sides of the material have different temperatures.

    “This property is used to power NASA space missions like the Mars rover Curiosity and to convert waste heat into electricity in high-end cars,” says Mariano Trigo, a staff scientist at the Stanford PULSE Institute and the Stanford Institute for Materials and Energy Sciences (SIMES), both joint institutes of Stanford University and SLAC. “The effect also works in the opposite direction: An electrical voltage applied across the material creates a temperature difference, which can be exploited in thermoelectric cooling devices.”

    Mason Jiang, a recent graduate student at Stanford, PULSE and SIMES, says, “Lead telluride is exceptionally good at this. It has two important qualities: It’s a bad thermal conductor, so it keeps heat from flowing from one side to the other, and it’s also a good electrical conductor, so it can turn the temperature difference into an electric current. The coupling between lattice vibrations, caused by heat, and electron motions is therefore very important in this system. With our study at LCLS, we wanted to understand what’s naturally going on in this material.”

    In their experiment, the researchers excited electrons in a lead telluride sample with a brief pulse of infrared laser light, and then used LCLS’s X-rays to determine how this burst of energy stimulated lattice vibrations.

    2
    This illustration shows the arrangement of lead and tellurium atoms in lead telluride, an excellent thermoelectric that efficiently converts heat into electricity and vice versa. In its normal state (left), lead telluride’s structure is distorted and has a relatively large degree of lattice vibrations (blurring). When scientists hit the sample with a laser pulse, the structure became more ordered (right). The results elucidate how electrons couple with these distortions – an interaction that is crucial for lead telluride’s thermoelectric properties. (SLAC National Accelerator Laboratory)

    “Lead telluride sits at the precipice of a coupled electronic and structural transformation,” says principal investigator David Reis from PULSE, SIMES and Stanford. “It has a tendency to distort without fully transforming – an instability that is thought to play an important role in its thermoelectric behavior. With our method we can study the forces involved and literally watch them change in response to the infrared laser pulse.”

    The scientists found that the light pulse excites particular electronic states that are responsible for this instability through electron-phonon coupling. The excited electrons stabilize the material by weakening certain long-range forces that were previously associated with the material’s low thermal conductivity.

    “The light pulse actually walks the material back from the brink of instability, making it a worse thermoelectric,” Reis says. “This implies that the reverse is also true – that stronger long-range forces lead to better thermoelectric behavior.”

    The researchers hope their results, published July 22 in Nature Communications, will help them find other thermoelectric materials that are more abundant and less toxic than lead telluride.

    Controlling Materials by Stimulating Charged Waves

    The second study looked at charge density waves – alternating areas of high and low electron density across the nuclear lattice – that occur in materials that abruptly change their behavior at a certain threshold. This includes transitions from insulator to conductor, normal conductor to superconductor, and from one magnetic state to another.

    These waves don’t actually travel through the material; they are stationary, like icy waves near the shoreline of a frozen lake.

    “Charge density waves have been observed in a number of interesting materials, and establishing their connection to material properties is a very hot research topic,” says Andrej Singer, a postdoctoral fellow in Oleg Shpyrko’s lab at the University of California, San Diego. “We’ve now shown that there is a way to enhance charge density waves in crystals of chromium using laser light, and this method could potentially also be used to tweak the properties of other materials.”

    This could mean, for example, that scientists might be able to switch a material from a normal conductor to a superconductor with a single flash of light. Singer and his colleagues reported their results on July 25 in Physical Review Letters.

    The research team used the chemical element chromium as a simple model system to study charge density waves, which form when the crystal is cooled to about minus 280 degrees Fahrenheit. They stimulated the chilled crystal with pulses of optical laser light and then used LCLS X-ray pulses to observe how this stimulation changed the amplitude, or height, of the charge density waves.

    “We found that the amplitude increased by up to 30 percent immediately after the laser pulse,” Singer says. “The amplitude then oscillated, becoming smaller and larger over a period of 450 femtoseconds, and it kept going when we kept hitting the sample with laser pulses. LCLS provides unique opportunities to study such process because it allows us to take ultrafast movies of the related structural changes in the lattice.”

    Based on their results, the researchers suggested a mechanism for the amplitude enhancement: The light pulse interrupts the electron-phonon interactions in the material, causing the lattice to vibrate. Shortly after the pulse, these interactions form again, which boosts the amplitude of the vibrations, like a pendulum that swings farther out when it receives an extra push.

    A Bright Future for Studies of the Electron-Phonon Dance

    Studies like these have a high priority in solid-state physics and materials science because they could pave the way for new materials and provide new ways to control material properties.

    With its 120 ultrabright X-ray pulses per second, LCLS reveals the electron-phonon dance with unprecedented detail. More breakthroughs in the field are on the horizon with LCLS-II – a next-generation X-ray laser under construction at SLAC that will fire up to a million X-ray flashes per second and will be 10,000 times brighter than LCLS.

    “LCLS-II will drastically increase our chances of capturing these processes,” Dunne says. “Since it will also reveal subtle electron-phonon signals with much higher resolution, we’ll be able to study these interactions in much greater detail than we can now.”

    Other research institutions involved in the studies were University College Cork, Ireland; Imperial College London, UK; Duke University; Oak Ridge National Laboratory; RIKEN Spring-8 Center, Japan; University of Tokyo, Japan; University of Michigan; and University of Kiel, Germany. Funding sources included DOE Office of Science; Science Foundation Ireland; Volkswagen Foundation, Germany; and Federal Ministry of Education and Research, Germany. Preliminary X-ray studies on lead telluride were performed at SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL), a DOE Office of Science User Facility, and at the Spring-8 Angstrom Compact Free-electron Laser (SACLA), Japan.

    SLAC/SSRL

    SACLA Free-Electron Laser Riken Japan


    his movie introduces LCLS-II, a future light source at SLAC. It will generate over 8,000 times more light pulses per second than today’s most powerful X-ray laser, LCLS, and produce an almost continuous X-ray beam that on average will be 10,000 times brighter. (SLAC National Accelerator Laboratory)

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    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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  • richardmitnick 7:27 am on May 10, 2016 Permalink | Reply
    Tags: , , RIKEN SACLA, ,   

    From BNL: “Ultra-fast X-ray Lasers Illuminate Elusive Atomic Spins” 

    Brookhaven Lab

    May 9, 2016
    Justin Eure
    (631) 344-2347
    jeure@bnl.gov

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

    New x-ray technique reveals never-before-seen, trillionth-of-a-second magnetic fluctuations that transform the electronic and magnetic properties of materials.

    1
    Brookhaven Lab physicists Pavol Juhas, John Hill, Mark Dean, Yue Cao, and Vivek Thampy, all of the Condensed Matter Physics and Materials Science Department, except Hill, who is director of NSLS-II.

    A quick flash of light can make ordinary materials extraordinary, potentially inducing qualities such as the perfect efficiency of superconductivity even at room temperature. But these subatomic transformations are infamously fleeting—they vanish in just trillionths of a second.

    Now, an international team of scientists has used synchronized infrared and x-ray laser pulses to simultaneously manipulate and reveal the ultra-fast magnetic properties of this promising quantum landscape. The rapid, light-driven switching between magnetic states, explored here with unprecedented precision, could one day revolutionize the reading and writing of data in computers and other digital devices.

    The study, published* May 9, 2016, in the journal Nature Materials, was led by scientists at the U.S. Department of Energy’s Brookhaven National Laboratory and included researchers from the U.S., China, Germany, Japan, Spain, and the UK.

    “We developed a way to reveal light-induced femtosecond magnetic dynamics in as yet unseen detail,” said Mark Dean, a physicist at Brookhaven Lab and lead author on the study. “This brings us closer to perfecting a recipe for manipulating these materials on ultra-fast time scales.”

    This novel x-ray technique, called time-resolved resonant inelastic scattering, revealed the subtle spin correlations, which travel as waves through the material and define its magnetic properties. Crucially, they behaved differently between two- and three-dimensional spaces when sparked by an infrared laser pulse.

    “Within a two-dimensional atomic plane, the novel state lasted just a few picoseconds,” said Brookhaven physicist and study coauthor Yue Cao. “But three-dimensional correlations also cross between planes, and these took hundreds of picoseconds to vanish—on this scale, that difference is tremendous. It is enormously exciting to help pioneer a new technique and see it succeed.”

    The bulk of the experimental work relied on the powerful and precise x-ray lasers available at SLAC National Accelerator Laboratory’s Linac Coherent Light Source, a DOE Office of Science User Facility, and the SACLA facility in Japan.

    SLAC/LCLS
    SLAC/LCLS

    SACLA Free-Electron Laser Riken Japan
    SACLA Free-Electron Laser Riken Japan

    Doping with light

    To introduce novel magnetic and electronic qualities, scientists often use a technique called chemical doping to augment the atomic configuration of a material. Electrons can be meticulously added or removed, but the process is permanent.

    “We wanted to access similar states transiently, so we used photo-doping,” Dean said. “A laser pulse supplies the needed photons, which changes the electron and spin configuration in the sample—the same spins thought to be responsible for phenomena like superconductivity. Moments later, the material returns to its native state.”

    In this work, the scientists used a strontium-iridium-oxygen compound (Sr2IrO4), selected for its strong magnetic interactions. Manipulating spin in the material was relatively easy—the real challenge was catching it in motion.

    Bright, fast flashes

    The collaboration turned to two powerful photon sources: the LCLS and SACLA, both uniquely capable of illuminating a quantum spin wave mid-stride. Both facilities can produce x-ray pulses with extremely short duration and high brightness.

    “Knowing that these facilities could produce fast and accurate enough laser pulses inspired this entire collaboration,” said study coauthor John Hill, the director of Brookhaven Lab’s National Synchrotron Light Source II, another DOE Office of Science User Facility.

    BNL NSLS-II Interior
    BNL NSLS-II

    For the experiment, an initial infrared laser pulse struck the layered Sr2IrO4 compound, destroying the native magnetic state. For a brief moment, the electrons inside the material formed spin waves that rippled through the material and radically changed its electronic and magnetic properties. Trillionths of a second later, an x-ray pulse followed and bounced off those emergent waves. By measuring the change in both momentum and the angles of diffraction, the scientists could deduce the transient electronic and magnetic qualities.

    This specific process of bouncing and tracking x-rays, called resonant inelastic x-ray scattering (RIXS), was also pioneered by members of this collaboration to explore similar phenomena in condensed matter systems. The new research builds on that to include time-resolved data points.

    “Beyond the remarkable capabilities of LCLS and SACLA to supply ultra-short femtosecond x-ray pulses, the challenge we were facing was how to detect the response of the spins,” said study coauthor Xuerong Liu from the Institute of Physics in Beijing. “That is, we needed a specialized x-ray detection system or ‘camera.'”

    The scientists developed a highly specialized RIXS spectrometer, which used millimeter-sized silicon crystals to measure the exact energy of the rebounding x-rays.

    The data revealed a clear difference in the propagation and timescale of the magnetic phenomena, with the inter-layer correlations taking hundreds of times longer to recover than those within each layer.

    “The findings match theoretical expectations, which is encouraging, but more importantly they demonstrate the strength and precision of this technique,” said collaborator Michael Först of the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg, Germany. “We can now dive deeper into the mechanism and think of strategies to fine-tune the control of magnetic properties with light.”

    Next, the scientists plan to explore optical pulses at even longer wavelengths, which will shift atoms within the material without directly exciting the electrons and spins. That work may help reveal the native magnetic coupling within the material, which in turn will clarify how to best break that coupling and toggle between different electronic and magnetic states.

    The research was funded in part by the DOE’s Office Science (BES), which supported experimentation at LCLS.

    *Science paper:
    Ultrafast energy- and momentum-resolved dynamics of magnetic correlations in the photo-doped Mott insulator Sr2IrO4

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition
    BNL Campus

    One of ten national laboratories overseen and primarily funded by the Office of Science of the U.S. Department of Energy (DOE), Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. The Laboratory’s almost 3,000 scientists, engineers, and support staff are joined each year by more than 5,000 visiting researchers from around the world.Brookhaven is operated and managed for DOE’s Office of Science by Brookhaven Science Associates, a limited-liability company founded by Stony Brook University, the largest academic user of Laboratory facilities, and Battelle, a nonprofit, applied science and technology organization.
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