First-Principles Theory of Anharmonicity and the Inverse Isotope Effect in Superconducting Palladium-Hydride Compounds

TL;DR

This paper presents a first-principles theoretical study of anharmonic effects in palladium hydrides, explaining the inverse isotope effect and accurately predicting superconducting properties beyond harmonic approximations.

Contribution

The study introduces a stochastic self-consistent harmonic approximation to fully account for anharmonicity in superconducting palladium hydrides, revealing its crucial role in isotope effects.

Findings

Anharmonic phonon spectra are significantly renormalized.

Harmonic approximation overestimates critical temperatures.

Hydrogen anharmonicity explains the inverse isotope effect.

Abstract

Palladium hydrides display the largest isotope effect anomaly known in literature. Replacement of hydrogen with the heavier isotopes leads to higher superconducting temperatures, a behavior inconsistent with harmonic theory. Solving the self-consistent harmonic approximation by a stochastic approach, we obtain the anharmonic free energy, the thermal expansion and the superconducting properties fully ab initio. We find that the phonon spectra are strongly renormalized by anharmonicity far beyond the perturbative regime. Superconductivity is phonon mediated, but the harmonic approximation largely overestimates the superconducting critical temperatures. We explain the inverse isotope effect, obtaining a -0.38 value for the isotope coefficient in good agreement with experiments, hydrogen anharmonicity being the main responsible for the isotope anomaly.