Novel Electronic Structure of Nitrogen-Doped Lutetium Hydrides

First-principles density functional theory (DFT) calculations of Lu-H-N compounds reveal low-energy configurations of Fm$\overline{3}$m Lu$_{8}$H$_{23-x}$N structures that exhibit novel electronic properties such as flat bands, sharply peaked densities of states (van Hove singularities, vHs), and intersecting Dirac cones near the Fermi energy (E$_F$). These N-doped LuH$_3$-based structures also exhibit an interconnected metallic hydrogen network, which is a common feature of high-T$_c$ hydride superconductors. Electronic property systematics give estimates of T$_c$ for optimally ordered structures that are well above the critical temperatures predicted for structures considered previously. The vHs and flat bands near E$_F$ are enhanced in DFT+U calculations, implying strong correlation physics should also be considered for first-principles studies of these materials. These results provide a basis for understanding the novel electronic properties observed for nitrogen-doped lutetium hydride.Comment: To be submitted. 4 main figures, 15 SI Figures, 1 table. 5 pages (13 pgs SI

High-Pressure Reentrant Ferroelectricity in PbTiO3_3 Revisited

We study ferroelectricity in the classic perovskite ferroelectric PbTiO$_3$ to very high pressures with density functional theory (DFT) and experimental diamond anvil techniques. We use second harmonic generation (SHG) spectroscopy to detect lack of inversion symmetry, if present. Consistent with early understanding and experiments, we find that ferroelectricity disappears at moderate pressures. However, the DFT calculations show that the disappearance arises from the overtaking of zone boundaries instabilities, and not to the squeezing out of the off-centering ferroelectric displacements with pressure, as previously thought. Our computations also predict a dramatic double reentrance of ferroelectricity at higher pressures, not yet seen in experiments

Quantum and Classical Orientational Ordering in Solid Hydrogen

We present a unified view of orientational ordering in phases I, II, and III of solid hydrogen. Phases II and III are orientationally ordered, while the ordering objects in phase II are angular momenta of rotating molecules, and in phase III the molecules themselves. This concept provides quantitative explanation of the vibron softening, libron and roton spectra, and increase of the IR vibron oscillator strength in phase III. The temperature dependence of the effective charge parallels the frequency shifts of the IR and Raman vibrons. All three quantities are linear in the order parameter.Comment: Replaced with the final text, accepted for publication in PRL. 1 Fig. added. Misc. text revision

Superconductivity in the Chalcogens up to Multimegabar Pressures

Highly sensitive magnetic susceptibility techniques were used to measure the superconducting transition temperatures in S up to 231($\pm$5) GPa. S transforms to a superconductor with T$_c$ of 10 K and has a discontinuity in T_c dependence at 160 GPa corresponding to bco to beta-Po phase transition. Above this pressure T_c in S has a maximum reaching about 17.3(+/-0.5) K at 200 GPa and then slowly decreases with pressure to 15 K at 230 GPa. This trend in the pressure dependence parallels the behavior of the heavier members Se and Te. Superconductivity in Se was also observed from 15 to 25 GPa with T_c changing from 4 to 6 K and above 150 GPa with T_c of 8 K. Similiarities in the T_c dependences for S, Se, and Te, and the implications for oxygen are discussed.Comment: 4 pages, 10 figure