The role of the perovskite lattice oxygen in the oxygen evolution reaction (OER) is systematically studied in the PrBaCo2O5+δ family. The reduced number of physical/chemical variables combined with in‐depth characterizations such as neutron dif‐fraction, O K‐edge X‐ray absorption spectroscopy (XAS), electron energy loss spectroscopy (EELS), magnetization and scanning transmission electron microscopy (STEM) studies, helps investigating the complex correlation between OER activity and a single perovskite property, such as the oxygen content. Larger amount of oxygen vacancies appears to facilitate the OER, possibly contributing to the mechanism involving the oxidation of lattice oxygen, i.e., the lattice oxygen evolution reaction (LOER). Furthermore, not only the number of vacancies but also their local arrangement in the perovskite lattice influences the OER activity, with a clear drop for the more stable, ordered stoichiometry.
Many complex electronic systems exhibit so-called pseudogaps, which are poorly-understood suppression of low-energy spectral intensity in the absence of an obvious gap-inducing symmetry. Here we investigate the superconductor Ba 1−x K x BiO 3 near optimal doping, where unconventional transport behavior and evidence of pseudogap(s) have been observed above the superconducting transition temperature T c , and near an insulating phase with longrange lattice distortions. Angle-resolved photoemission spectroscopy (ARPES) reveals a dispersive band with vanishing quasiparticle weight and "tails" of deep-energy intensity that strongly decay approaching the Fermi level. Upon cooling below a transition temperature T p > T c , which correlates with a change in the slope of the resistivity vs. temperature, a partial transfer of spectral weight near E F into the deep-binding energy tails is found to result from metal-insulator phase separation. Combined with simulations and Raman scattering, our results signal that insulating islands of ordered bipolarons precipitate out of a disordered polaronic liquid and provide evidence that this process is regulated by a crossover in the electronic mean free path.Pseudogaps represent a departure from the expectations of standard band theory and the Fermi liquid theory of electronic excitations, which together serve as a successful starting point for understanding many condensed matter systems. They could potentially originate from any ways in which the conventional theories might break down, e.g., due to disorder, fluctuations, strong interactions, and/or strong correlations. But it is also conceivable that some observed pseudogaps might be less mysterious than they first seem, in the sense that they are rooted a "hidden" order
Single crystals of FeTe0.65Se0.35, with the onset of critical temperature (Tc) at 14 K, were hydrogenated for 10–90 hours at various temperatures, ranging from 20 to 250 oC. It is shown that tetragonal matrix becomes unstable and crystal symmetry is reduced for the crystals hydrogenated already at 200 oC despite that molecular impurities do not change matrix symmetry, unless the material is not destroyed under hydrogenation at 250 oC. Bulk Tc, takenat the middle of the transition, equal to about 12–13 K for the as-grown FeTe0.65Se0.35, increases by 1–2 K. The critical current density determined in magnetic field range of 0–70 kOe increases 4–30 times as a result of hydrogenation at 200 oC for 10 h. Electron paramagnetic resonance studies confirmed higher value of the bulk Tc for hydrogenated crystals. Thermal diffusion of hydrogen leads to substantial structural changes, causes degeneration of crystal quality, and significantly affects superconducting properties. A strong correlation was observed between the structural changes and changes in the parameters of the superconducting state for the hydrogenated crystals.
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