Band structure and optical properties of graphite and of the layer compounds GaS and GaSe

Abstract

The band structure of graphite and of the layer compounds GaS and GaSe is computed by using the tight-binding approach in a semi-empirical way. The band structure is related to the basic properties of these compounds and some features of the optical excitation spectrum are explained. We show that the reason why graphite is a semi-metal and GaS and GaSe are semiconductors can be understood in the two-dimensional approximation and is due to the existence of the inversion symmetry in the former case. The absorption edge in GaS corresponds to indirect transitions between the statesΓ1+ andP1+, while in GaSe it corresponds to direct transitions between the statesΓ1+ andΓ3+. Sharp peaks ine2(0,ω) are attributed to saddle point singularities in the joint density of states. The effect on the optical properties produced by a change in the polarization of light is discussed.