Theoretical studies on phonon density of states in nonstoichiometric palladium hydrides

We investigate the phonon density of states in nonstoichiometric palladium hydrides PdH _x , in order to clarify the contribution of lattice vibrations to superconductivity. We employ the Burger-MacLachlan model, which considers a one-dimensional triply-connected diatomic chains with randomly distributed vacancies, focusing on the <111> direction in the rock-salt lattice of PdH _x . The phonon density of states is calculated by the Forced Oscillator Method suitable for random systems. We find that phonon modes at high frequencies change from localized to collective modes with increasing x and the crossover stoichiometry is consistent with that of the appearance of superconductivity. These results support the optical phonon mechanism for superconductivity in PdH _x . (paper)