CONSISTENT ANHARMONIC-SHELL-MODEL CALCULATION FOR THE RAMAN MODE IN SILICON

We show that a previous failure to describe the frequency shift Delta and linewidth Gamma of the optical zone-center mode in silicon with an anharmonic shell model arises from the neglect of the electronic pola rizability effects in the perturbative treatment of the anharmonic int eractions. We consider cubic anharmonic shell-shell interactions betwe en nearest-neighbor atoms in a shell model used in the past for silico n. We apply an anharmonic perturbation theory which properly takes int o account the adiabatic constraint for the shell's polarization. We ob tain Delta and Gamma as functions of temperature in good agreement wit h experimental data.