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Highlights

  • 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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  • Using 207Pb solid-state NMR, UV-Vis, and XRD, researchers showed that semi-crystalline phases and non-stoichiometric impurities permeate samples made in solution. Solid-phase synthesis avoids the former, but not the latter.

    How you make perovskite materials matters. Lead (Pb) halide (e.g. chloride) perovskites are promising semiconducting materials for photovoltaic solar cells, because of their low cost and their high efficiency for converting sunlight into electricity.  Semi-crystalline phases and compositional imperfections were found to permeate these materials when made by solution phase synthesis, even after heat treatment. In contrast, a new solvent-free, solid phase synthesis avoids the formation of semi-crystalline phases, and enables a better understanding of the origin and nature of those...

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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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  • Predicted reaction energy path for AlH3 formation on Ti-doped Al(111)

    A defect-mediated direct synthesis of alane (AlH3) from its elements was predicted and verified experimentally. Alane has potential use as a hydrogen storage material with its capacity of 10.1 weight % H2 and relative ease of releasing hydrogen below 100 °C. Alane has not found broad practical use for over 70 years due to difficulties of direct synthesis from its elements. Using density functional theory researchers showed defect-mediated formation of alane monomers on crystalline faces of aluminum, namely Al(111), in a two-step process: (1) dissociative adsorption...

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  • Schematic of the Pt atom inside the MOF structure, and its corresponding wideline spectrum acquired using DNP NMR

    Researchers have uncovered the atomic-scale geometry of platinum ions with unprecedented precision within metal organic frameworks (MOFs)—a growing class of porous materials that consist of organic ligands and inorganic components.  Due to the immense structural diversity and large surface areas of MOFs, often exceeding the size of a professional football field per gram of material, many uses of MOFs have been discovered in purification, separation, capture, and storage of gases, especially hydrogen, carbon dioxide and methane, as well as applications as catalysts and sensors.  ...

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  • The anisotropic upper critical field as a function of temperature for a stoichiometric single crystal.

    A member of the complex family of iron-based superconductors has been newly synthesized, shown to be highly ordered, and exhibits nearly optimal properties.  Well-ordered, single crystals of CaKFe4As4 superconductors have allowed researchers to remove one of the veils of complexity for iron-based superconductors—disorder.  Previously, our understanding of the higher transition temperatures, and promising high-magnetic field properties of iron-based superconductors has been associated with compounds with significant chemical disorder, making it hard to...

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  • Schematic representation of how the adatoms (small spheres) are guided by the electric field. The non-intercalated domain shown in the middle of the figure has higher electrostatic potential. An electric field is generated at the boundary and, in this case, the middle region attracts the adatoms.

    Researchers have shown how the motion of individual atoms on surfaces of graphene—a one atom thick layer of carbon—can be controlled. The adatom diffusion rate and hopping direction can be tuned by lowering the diffusion barrier using an effective electric field. This was shown using in situ scanning tunneling microscopy at low temperatures and the mechanism was elucidated using first-principles calculations. The electric field is locally tuned by inserting metal atoms below graphene. The metal atoms can be added to form regions of higher charge compared to areas without intercalated...

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  • The image in the center shows a sketch of topological dispersion in PtSn4. The data on the left demonstrate the presence of Dirac cones near edges of the central image. The data on the right show double Dirac dispersion that forms Dirac arc nodes visible near the center of the sketch.

    Electrons in a newly discovered metal, PtSn4, nearly reach speed of light in ways not seen in other materials. Like navigating the seas, electrons are shipped through metals and their movement is governed by the features of their own liquid, the Fermi sea. Like the sea, which can be calm to stormy, with currents and whirlpools that challenge navigation, electrons are shipped through metals governed by the varying topology of the electronic sea, which govern the electron’s speed. To date, only isolated points (Dirac points) with relatively small numbers of conduction electrons have been...

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  • Image of the Fermi surface (left) and band dispersion (right) along a red line cut. At 130 K a gapped branch appears that is due to the surface CDW

    Discovery of an unconventional charge density wave (CDW) in purple bronze, a molybdenum oxide, points to a possible new pathway to high temperature superconductivity. A CDW is a state of matter where electrons bunch together periodically, like a standing wave of light or water. CDWs and superconductivity are frenemies, since they share a common origin and often coexist, yet compete for dominance. Conventional CDWs and superconductivity arise from interactions of electrons with lattice vibrations (phonons), while electron-electron interactions are the likely origin of unconventional, high...

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