Local-field effects in the electric field gradient of dilute transition-metal alloys.

Abstract

A formalism is developed for impurity screening in a transition-metal (TM) dilute alloy in the dielectric screening approach. The full dielectric tensor is inverted using two schemes: (1) the linear combination of atomic orbitals approximation, and (2) the mixed-band-structure scheme, where s electrons are represented in the free-electron approximation and the d electrons in a local representation. In both schemes the exact expressions are obtained for the excess impurity scattering potential \ensuremath{\Delta}V(r) and the impurity-induced charge perturbation \ensuremath{\Delta}n(r). In the mixed-band-structure scheme, \ensuremath{\Delta}V(r) and \ensuremath{\Delta}n(r) are separable into isotropic and anisotropic contributions. The latter is the manifestation of local-field (LF) effects. The numerical results for \ensuremath{\Delta}V(r) and \ensuremath{\Delta}n(r) are obtained for vanadium (V) alloys with TM impurities using a noninteracting-model band structure. The results are consistent with the other theoretical results and experimental predictions. The LF effects in \ensuremath{\Delta}V(r) and \ensuremath{\Delta}n(r) are found to be significant. The electric field gradient (EFG) is calculated for V alloys including LF effects. The calculations of the EFG suggest that both valence and size effects are equally important in explaining the experimental results.