Date of Award

1993

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics and Astronomy

First Advisor

Joseph Callaway

Abstract

We present a self-consistent cluster embedding method to study the electronic structure of solids by ab initio, local spin density functional calculations. A theoretical discussion from the viewpoint of total energy is given, and a definition of the total energy for an embedded cluster is introduced. We have constructed a general LCGO cluster-embedding program package, and developed a procedure to simulate the spin disordered states with local antiferromagnetic (AF) order. The electronic structures of NiO and CoO are studied by this method with a high quality basis set. The two materials show similar antiferromagnetic insulating ground states with both localized and band properties: A small energy gap separates the well localized unoccupied and occupied 3d orbitals. Each 3d orbital is attached to a particular cation. Two diffuse oxygen 2p bands are below the 3d levels; empty oxygen 3s bands are above the 3d levels. Calculations show that the excited 3d electrons are also well localized. We propose a new explanation of the insulating nature for transition-metal monoxides which can explain both NiO and CoO consistently: The overlap of excited 3d electrons is too small to form a metallic band, but the overlap is sufficient for the hole to migrate through the crystal. In this sense, both NiO and CoO are charge transfer insulators with gaps of about 4 and 5 ev (mostly from oxygen ion to cation), respectively. The spin magnetic moments of both ions and the Neel temperatures of NiO and CoO are calculated directly. The theoretical simulations of the paramagnetic phases for both materials show that the electronic structure in the local AF pairs is independent of long-range spin order. Our theoretical results lead to a natural interpretation of almost all experimental data.

Pages

261

Share

COinS