Superconductivity and Pronounced Electron-Phonon Coupling in Rock-Salt Al_1−xO_1−x and Ti_1−xO_1−x

The highest ambient-pressure Tc among binary compounds is 40 K (MgB2). Higher Tc is achieved in high-pressure hydrides or multielement cuprates. Alternatively, are explored superconducting properties of binary, metastable sub-oxides, that may emerge under extremely low oxygen partial pressure. The emphasis is on the rock-salt structure, which is known to promote superconductivity, and exploring AlO, ScO, TiO, and NbO. Dynamic lattice stability is achieved by introducing metal and oxygen vacancies in the fashion of Nb_1−xO_1−x-type structure (x = ¼). The electron-phonon (e-ph) coupling is remarkably large in Al_1−xO_1−x and Ti_1−xO_1−x (λ ≈ 2 at x = ¼), with Tc ≈ 35 K according to the Allen–Dynes equation. Significantly, the coupling strength is comparable to that in high-pressure hydrides, yet, in contrast to hydrides and MgB2, the coupling is largely driven by low frequency phonons. Sc_1−xO_1−x and Nb_1−xO_1−x show significantly smaller λ and Tc. Further, hydrogen intercalation to boost λ and Tc is investigated. Only Ti_1−x(O_1−xHx) and Nb_1−x(O_1−xHx) are dynamically stable upon intercalation, where H, respectively, decreases and increases Tc. The effect of H doping on electronic structure and Tc is discussed. Altogether, the study suggests that metal sub-oxides are promising compounds to achieve strong e-ph coupling at ambient pressure.