Contribution of hydrogen vibration to superconducting pairing at ambient pressure: A first principles study on hole-doped perovskite oxyhydrides

The recent technique of chemical substitution of oxide (O^2-) to hydride (H^-) ions in the perovskite oxides ABO₃ has paved a way to explore new physics involving special features of hydrogen. Along this line, we have recently proposed an unsynthesized group of compounds ATiO₂H (A: Alkaline metal), where H-1s orbitals comprise an appreciable fraction of the highest valence band [J. Chem. Phys. 147，034507 (2017)]. The hole doping into this band could realize the superconductivity boosted by the Cooper pairing via the hydrogen vibration, which has been thought to occur only at megabar pressure. To examine this possibility, we investigate the superconducting properties of hole-doped ATiO₂H by density functional theory for superconductors. Both the uniform doping and direct substitution (Ti-->Sc) schemes yield superconducting state with Tc~1K. In the latter case Tc is lowered by subtle modification of the electronic states such as deformation of the Fermi surface and charge density wave; nevertheless, by fine-tuning of the doping ratio, superconducting phase contributed by hydrogen 1s orbital can be realized. Our results draw a new prospective path to the metallic and superconducting hydrogen at accessibly low pressures.