Th. Koslowski

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 NaxWO3 and NaxTayW1 − yO3, 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.

Electron localization in amorphous solids: numerical studies for the distorted diamond lattice

The authors present a numerical study of localization properties on models for amorphous solids in three dimensions. Tight-binding Hamiltonians that are based on the diamond structure are used to describe the electronic structure of amorphous modifications of carbon, silicon and germanium. The localization behaviour of electronic states is calculated by studying the sensitivity of eigenvalues to a change in boundary conditions. A wide spectrum of localization effects can be observed, including electron localization both at the band edges and weak localization within the bands. The origin of electron localization is discussed using a population analysis of eigenfunctions.

Electronic structure of metal-molten salt solutions: Electron localization and the metal-non-metal transition

Abstract A theoretical and numerical description is presented of the metal-non-metal transition in a simple model of metal-molten salt solutions. Theoretical results are obtained applying a recent proposal by Yurdabak et al. to describe the metal-non-metal transition as a balance between strongly localized electrons (F-centers) and nearly free electrons, with screening of the Coulombic interaction in the ionic melt. The validity of the Thomas-Fermi screened mean spherical approximation is supported by Monte Carlo simulations. Applied to Kc[KCl]1 − c, the model exhibits a crossover from a defect-rich phase to a metallic phase solely governed by free electrons. This crossover is strongly influenced by packing effects. For a variety of models (screened/unscreened, homogeneous/inhomogeneous charge distribution), the free energy is computed as a function of composition and compared to experiments.

Theoretical approaches to the electronic structure of disordered solids

Abstract A short review is presented on a theoretical numerical approach to the calculation of the electronic structure of disordered and amorphous solids. A chemically specific approach is thereby advocated. A new procedure to fit in a tight-binding method the band structure of complex solids is introduced. The electronic structure of different defect models of a-Si and a-Si:H is described. Then the interplay of disorder and electron–electron interactions in the disordered cubic tungsten bronzes, NaxWO3, is addressed which leads to a pseudogap at the Fermi level. This work results in an understanding of the metal–insulator transition for finite doping degree.

Anderson localization in tungsten bronzes A numerical study

Abstract We report numericai results for electron localization induced by disorder in the tungsten bronzes NaxWO3, NaxTayW1−y O3 and NaxWO3-y Fy The bronzes are described by a nearest-neighbour tight-binding Hamiltonian. Anderson-type disorder is introduced into the transition-metal atoms. The localization properties of eigenfunctions are deduced from the scaling behaviour of the correlation length. The calculated density of states is used to compare the numerical results with photoelectron spectra. A metal-non-metal transition is indicated at a critical concentration of electrons in the conduction band of xc  0.3, which is independent of the degree of F or Ta substitution.