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Highlights

  • Researchers have overcome the extreme challenge of directly observing the dynamics of how light excites electrons and generates electricity in solar cell and photovoltaic technologies.  The formation and dissociation of bound electron and hole pairs, known as excitons, were studied using a combination of broadband terahertz pulses (a trillion cycles per second) and selective laser pumping to reveal the light-induced excitation dynamics and charge transport mechanism within perovskites.  Perovskites are a class of materials that show promise as industrial solar energy materials...

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  • Phase diagram of Ca(Fe1-xCox)2As2 in the free (black) and strained (red) state.

    By applying strain to iron-arsenide based superconductors, researchers were able to study the interplay between magnetic states, the tetragonal phase, the orthorhombic phase, and the onset of superconductivity of these materials. The Ca(Fe1-xCox)2As2 series is exceptionally pressure sensitive and crystals of these materials expand differently in each direction as temperature is applied. By rigidly adhering one side of the crystals to silica and taking advantage of the differences in their thermal expansion, in situ measurements of the...

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  • Schematic showing the probable location of the carbon dangling bonds via EPR and electronic measurements.

    Unattached, dangling carbon bonds have been shown experimentally to be the main culprit in degradation of polymer solar cells.  This type of solar cell has attracted intense attention due to their potential for use in flexible large-area, low-cost photovoltaic panels; degradation is the primary obstacle to commercialization.  Theoretical studies suggested photo-induced carbon-hydrogen bond rearrangement and breaking can lead to carbon dangling bonds and now this has been shown experimentally for the first time.  Researchers studied polymer:fullerene solar cells using the...

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  • Salt-induced 2D hexagonal superlattices of polyethylene-glycol functionalize gold nanoparticles at the vapor-liquid interface.

    Researchers have developed a method to self-assemble close to perfect structures of gold nanoparticles.  The gold nanoparticles were grafted with polymer chains into 2D supercrystals by controlling salt concentration.  The materials were characterized using high-resolution synchrotron surface X-ray scattering methods, including grazing incidence small angle X-ray scattering and X-ray reflectivity, at the Advanced Photon Source, a U.S. Department of Energy, Office of Science User Facility.  At low salt concentrations, no ordered structures exist.  As salt concentration...

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  • Loci of the measured momentum dependence of the superconductor gap of the Fermi surface sheets.  The dashed line shows the expected variation of the gap based on a spin fluctuation model.   The significant deviation from predictions demonstrates the deficiency of current models.

    Measurements of the superconducting gap in a new member of the family of iron-based superconductors revealed substantial deviation from predictions of the well-established theory. In superconductors, electrons form Cooper pairs that behave like single particles. The force binding the electrons in pairs, known as the superconducting gap, often depends on the momentum of the electrons and its measurement shows fingerprints of the mechanism that causes superconductivity. Researchers used a technique known as laser angle-resolved photoemission spectroscopy combined with electronic structure...

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  • Cover art showing V. Smetana and A.-V. Mudring's work in Angewandte Chemie

    A double salt of cesium platinum and cesium hydride has been made for the first time. Simple salts, like sodium chloride, contain one positively charged element and one negatively charged element. This double salt contains two anions (Pt2- and H-) and one cation (Cs+). Einstein’s Theory of Relativity helps explain how this is possible. The average radial velocity of electrons closest to the nucleus is larger for heavier elements, which leads to a higher effective mass that then leads to a smaller than expected atomic radius, and changes in chemical...

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  • Constant energy intensity contour 10 meV above EF in the momentum space. White, gray and green are locations of the bands. points. Theoretically predicted locations for the Weyl points (green) and the experimentally determined points (red). Fermi arcs are seen as white strikes of intensity connecting “experimental” Weyl points.

    A new type of semimetal has been proven to exist in a crystal made of molybdenum and tellurium atoms.  In this recently postulated state, the electron and hole bands normally separated by a gap touch at a few discrete points, called Weyl points. The orientation of electron spin at those points in momentum space resembles magnetic field lines of magnetic monopoles. Weyl points are connected by “Fermi arcs” at the surface of the sample, which allow their identification.  Fermi arcs are disconnected, ungapped areas in density of states of these materials.  The proof of the...

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  • Suppression of the superconducting transition by controlled disorder for different compositions.

    Studying electronic excitations by intentionally creating point-like defects in the crystal lattice has helped distinguish between competing electron pairing states and led to a better understanding of the origins of superconductivity in barium-potassium-iron-arsenide.  By adjusting two independent "knobs", the ratio of barium and potassium and the scattering by defects introduced by 2.5 MeV electron irradiation, the response of two key independent parameters, superconducting transition temperature, Tc, and low-temperature magnetic susceptibility is compared to theoretical...

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  • Polarized-light image of an FeSe single crystal at 7 K reveals orthorhombic domains oriented along the tetragonal [100] direction (parallel to the sample sides). For detwinning, the sample is cut along the [110] tetragonal direction. Lower frames show the selected region at different temperatures across the nematic/structural transition at 90 K. 

    For the first time, the directionally dependent electrical resistivity in iron-selenium, an iron-based superconductor, could be extrapolated to the zero-stress limit, and studied over a broad range of temperatures without interference from long-range magnetic order. In contrast to other iron-based superconductors, iron-selenium does not develop long-range magnetic order below the structural (nematic) transition at Ts ≈90 K. This allows for the disentanglement of the contributions to the directionally dependent resistivity due to magnetic and nematic order. The results suggest...

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  • Evolution of the in-plane lattice parameters at various pressures determined from the splitting of the tetragonal (HH0) Bragg peaks.

    Recent experiments resolve an important open question concerning the interplay between magnetism and structure, which is ubiquitous in iron-based superconductors.  Studies of the iron-selenium compound using x-ray diffraction and time-domain Mössbauer spectroscopy under applied pressure at the Department of Energy’s Advanced Photon Source confirm that structural nematicity—long-range, orientational order—and magnetic order in FeSe are indeed strongly and cooperatively coupled.  At pressures between 1.0 GPa (about 10,000x that of ambient pressure) and 1.7 GPa, separate structural...

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