The electronic structure and its relationship to superconductivity in NiH

Abstract

With the aim of studying the characteristics of the electronic states at the Fermi energy, the authors have investigated the electronic structure of NiH by means of an ab initio augmented plane wave calculation. The d band holes of the pure Ni metal are filled in the hydride, in agreement with magnetisation measurements and as suggested by a comparative study of the K absorption edges of Ni and NiH. This feature is qualitatively similar to that found for PdH; however, the Fermi level in NiH is much closer to the top of the d bands than in PdH. The bandstructure results are used in conjunction with recent experimental estimates of the Debye and Einstein temperatures for the acoustic and optic modes respectively, to calculate the electron-phonon coupling parameter lambda using the model of Gaspari and Gyorffy (1972). A large decrease of the electronic contribution to lambda is found from PdH to NiH. This contributes, together with the hardening of the optic modes evidenced by electrical resistivity measurements, to a drastic lowering of the superconducting transition temperature, T_c and should lead to the non-occurrence of superconductivity in NiH above 1.5K. A site and angular momentum density-of-states analysis of the occupied energy bands is also presented which hopefully will stimulate photoemission and X-ray emission experiments.