Inhomogeneous superconducting state in hydrogen storage alloy PdDx
Disorder affects the coherent state of a superconductor in characteristic ways. In conventional superconductors with s-wave pairing symmetry, magnetic defects act as strong pair breakers, whereas nonmagnetic defects have negligible effect. In this review, we will show that nonmagnetic defects promote the superconductivity mediated by optical phonon in palladium-hydride system. Temperature dependence of the critical current is well fitted by the Ambegaokar-Baratoff relation, suggesting the existence of Josephson junction between the superconducting(SC) grains. Appearance of the coherent SC grains through the Josephson coupling may be associated with the interaction between deuterium and lattice defects such as vacancies and dislocations. In addition, we theoretically investigated whether or not optical phonon modes formed by the collective motion of a large number of atoms exist in nonstoichiometric PdHx, possessing random or inhomogeneous lattices. We found that individual excitations compete with collective excitations in controlling the system’s randomness, and that optical phonons develop from localized phonon modes as the stoichiometry x increases.