The 2014-2015 prediction, discovery, and confirmation of record high temperature superconductivity above 200K in compressed H3S, followed by the 2018 extension to superconductivity in the 250-260K range in lanthanum hydride, marks a new era in the longstanding quest for room temperature superconductivity: quest achieved, at the cost of supplying 1.5-2 megabars pressure. Predictions of numerous high temperature superconducting metal hydrides XHn (X=metal atom) have appeared, but are providing limited understanding of why some transition temperatures Tc are high while others are low. We make use of the small mass ratio MH /MX to obtain an atomic decomposition of the coupling strength to reveal that although the X atom provides coupling strength via λX as commonly calculated, it is irrelevant for Tc because the resulting lowering of frequency moments compensates (and sometimes over-compensates) for the increase in λ. It is important for analysis and for understanding that the X atom contribution is neglected, because Tc depends more transparently on λH . Five XHn compounds, predicted in harmonic approximation to have Tc in the 150-300K range, are analyzed consistently for their relevant properties, revealing some aspects that confront conventional wisdom. A phonon frequency -critical temperature (ω2-Tc) phase diagram is obtained that reveals a common structural phase instability boundary limiting Tc at the low pressure range of each compound. The hydrogen scattering matrix elements are obtained and found to differ strongly over the hydrides. A quantity directly proportional to Tc in these hydrides is identified, indicating that (in common notation) NH (0)I 2 H /ωH = ηH /ωH is the parameter combination to be maximized in hydrides.

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SrTiO3 (STO) is the substrate of choice to grow oxide thin-films and oxide heterojunctions, which can form quasi-two-dimensional electronic phases that exhibit a wealth of phenomena, and, thus, a workhorse in the emerging field of metal-oxide electronics. Hence, it is of great importance to know the exact character of the STO surface itself under various oxygen environments. Using density functional theory within the spin generalized gradient approximation we have investigated the structural, electronic and magnetic properties of the oxygen-deficient STO surface. We find that the surface oxygen vacancies order in periodic arrays giving rise to surface magnetic moments and a quasi two-dimensional electron gas in the occupied Ti 3-d orbitals. The surface confinement, the oxygen-vacancy ordering, and the octahedra distortions give rise to spin-polarized t2g dispersive sub-bands; their energy split near the Brillouin zone center acts as an effective Zeeman term, which, when we turn on a Rashba interaction, produces bands with momentum-spin correlations similar to those recently discovered on oxygen deficient STO surface.

There are reasons to believe that the ground state of the magnetic rare earth pyrochlore Yb2Ti2O7 is on the boundary between competing ground states. We have carried out ab initio density functional calculations to determine the most stable chemical formula as a function of the oxygen chemical potential and the likely location of the oxygen atoms in the unit cell of the "stuffed" system. We find that it is energetically favorable in the "stuffed" crystal (with an Yb replacement on a Ti site) to contain oxygen vacancies which dope the Yb 4f orbitals and qualitatively change the electronic properties of the system. In addition, with the inclusion of the contribution of spin-orbit-coupling (SOC) on top of the GGA+U approach, we investigated the electronic structure and the magnetic moments of the most stable "stuffed" system. In our determined "stuffed" structure the valence bands as compared to those of the pure system are pushed down and a change in hybridization between the O 2p orbitals and the metal ion states is found. Our first-principle findings should form a foundation for effective models describing the low-temperature properties of this material whose true ground state remains controversial.