Many-electron multiplet effects in the optical spectra of NiO, CoO and MnO

Abstract

A new theoretical approach to the multiplet structure of localized states in solids is applied to the Mott insulators NiO, CoO, and MnO. This method circumvents the major approximations underlying the conventional Tanabe-Sugano approach to the multiplet problem in that differential hybridization for e and t 2 states is incorporated directly, the Racah parameter A is allowed to depend on the one-electron configurations, and the crystal-field parameter incorporates both bare-ion effects (as traditionally done) and average multiplet corrections. Analysis of the observed optical spectra of NiO, CoO, and MnO in light of this approach produces (i) an excellent fit to the spectra and new assignments for some of the transitions, (ii) the values of the mean-field parameters for these materials, (iii) a direct measure for the amount by which an ideal, spin-restricted mean-field electronic structure theory would fail to reproduce the excitation energies, and (iv) analysis of the optical transitions above the interband onset.