Friday, May 31, 2013

1305.7123 (Ion Errea et al.)

Inverse Isotope Effect in Palladium Hydrides Explained by the
Self-Consistent Harmonic Approximation

Ion Errea, Matteo Calandra, Francesco Mauri
Palladium hydrides display the largest isotope effect anomaly known in literature. Replacement of hydrogen in PdH with the heavier deuterium or tritium leads to higher superconducting temperatures, a behavior inconsistent with harmonic theory. Solving the self-consistent harmonic approximation by a stochastic approach, we show that the phonon spectra of palladium hydrides are strongly renormalized by anharmonic effects, far beyond the perturbative regime. We obtain the anharmonic free energy, the thermal expansion and the superconducting properties of palladium hydrides fully ab initio. We show that superconductivity is phonon mediated and we explain the inverse isotope effect, obtaining a -0.38 value for the isotope coefficient in good agreement with experiments. Our work demonstrates that hydrogen anharmonicity is the main responsible for the isotope anomaly in palladium hydrides and provides a non-perturbative framework to investigate strongly anharmonic systems.
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