The metal-insulator transition in disordered tungsten bronzes. Results of an Anderson-Mott-Hubbard model

Abstract

To understand the electronic properties — in particular the metal—insulator transition (MIT) — of cubic tungsten bronzes Na x WO 3 and Na x Ta y W 1 − y O 3, a microscopic model incorporating electron interactions and correlated disorder i presented and treated at the mean-field level of unrestricted Hartree-Fock. The conduction band is found to exhibit a pseudogap at the Fermi level for sufficiently large interaction strengths, in agreement with experiment. The pseudogap has a profound effect on localization properties of Fermi-level states and the position of the MIT. The lower band-edge states are found to be essentially two-dimensional, with a progressive crossover to three-dimensional character with increasing energy. An exception to this behaviour occurs in the pseudogap, where the states are virtually two-dimensional.