title: First-principles theory of anharmonicity and the inverse isotope effect in superconducting palladium-hydride compounds.
abstract: Palladium hydrides display the largest isotope effect anomaly known in the 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 mainly responsible for the isotope anomaly.
authors.affiliation: Université Pierre et Marie Curie, CNRS, IMPMC, 4 Place Jussieu, 75252 Paris, France and IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain.
authors.forename: Ion
authors.initials: I
authors.lastname: Errea
authors.forename: Matteo
authors.initials: M
authors.lastname: Calandra
authors.forename: Francesco
authors.initials: F
authors.lastname: Mauri