Enhancing superconducting transition temperature of lanthanum superhydride by increasing hydrogen vacancy concentration

Hydrogen deficiency has been observed in experimental samples of various clathrate superhydride superconductors, while its impact on superconductivity is often neglected. Previous path-integral simulations suggest that hydrogen vacancies could diffuse due to thermal and quantum fluctuations, potentially altering superconducting properties from those in stoichiometric systems, while also hindering the application of conventional harmonic theories.

In this study, we investigate the superconductivity of lanthanum superhydride with hydrogen deficiency (LaH10-δ) from first principles, using the non-perturbative stochastic path-integral approach (SPIA), which is developed by some of us and collaborators to study superconductivity mediated by various forms of ion motion. We find that the diffusion of hydrogen vacancies modifies ion vibrations, electronic structure and electron-phonon coupling. These changes result in a nonmonotonic dependence of superconducting transition temperature (Tc) on the vacancy concentration (δ). By comparing the experimental and theoretical equations of state, we suggest that δ varies across samples under different pressures. This explains the positive pressure dependence of Tc in experiments below 150 GPa. Remarkably, within this pressure range, we find that Tc could be further raised by increasing δ.