The polar metal is a material that hosts both polar distortion and metallicity. Such a material is expected to show exotic magneto-electric phenomena if superconducts. Here, we theoretically explore ferroelectric and superconducting properties in a series of perovskite-type oxyhydrides ATiO2H (A=K, Rb, Cs) under hole-doping conditions using the first-principles calculations based on the density functional theory. Our simulation shows that these compounds host spontaneous polarization and superconductivity at optimal doping concentration. The unusual coexistence of superconductivity and large polarization (~100 μC/cm 2 ) originates from weak coupling of the polar distortion and superconducting states, the reason of which is separation of the displaced atoms and spacially confined metallic carriers.Besides, the superconductivity is enhanced by the unique electronic properties near the valence band maximum: quartic band dispersion with a sizable contribution of hydrogen 1s states. Our study thus feature the oxyhydrides as possible model polar superconducting systems, which may be utilizable for future magneto-electric devices.

No abstract

No abstract

Kohn anomaly is a non-smooth phonon softening induced by electron-phonon coupling in low-dimensional metals. Some measurements claimed that Kohn anomalies are present in topological materials due to the Dirac fermions in the bulk or on the surface. However, first-principles calculations have not reproduced the Kohn anomalies, especially, on the surface of topological insulators. It is still unclear about the origin of the controversy for the existence of the Kohn anomaly whether it is a numerical shortcoming or misinterpretation in measurement. In this study, we investigate the surface Kohn anomaly in two topological insulators Bi2Se3 and SnSe using the state-of-the-art Wannier interpolation schemes. We find that Bi2Se3 exhibits the Kohn anomaly but only in the bulk-like phonon modes by structural confinement along the c-axis. Interestingly, SnSe exhibits the surface Kohn anomaly in support of the experimental report on Pb0.7Sn0.3Se. We show that double Dirac cones in SnSe surface states are responsible for the Kohn anomaly, which is even enhanced if the subsurface states are partially occupied.