MODEL CALCULATION OF DIELECTRIC FUNCTION IN TRIGONAL SE AND TE WITH LOCAL-FIELD CORRECTIONS INCLUDED

The diagonal as well as the off-diagonal elements of the dielectric matrix epsilon G,G' (q, omega =0) are explicitly evaluated in trigonal Se and Te. A simple tight-binding model Hamiltonian has been used to describe the electron states, while the wavefunctions have been drawn from sp3 hybrid orbitals, modified according to the chemical bond. The dielectric matrix has been inverted and the local-field corrections to the dielectric function have been computed. These are found to be very large for q<qf and, at least in the case of selenium, their introduction seems essential to reproduce the experimental dielectric tensor. This conclusion is discussed after the very recent, and disputed, experimental results.