2ND-ORDER RAMAN-SPECTRA OF DIAMOND FROM AB-INITIO PHONON CALCULATIONS

The second-order Raman spectra of diamond and silicon have been calcul ated using ab initio phonons and phenomenological polarizability coeff icients. The sharp peak in the spectrum of diamond near the two-phonon cutoff is explained by a maximum in the vibrational density of states ; this maximum originates from the uppermost phonon branch whose frequ encies are calculated to have a minimum at the Brillouin-zone center. This frequency minimum as well as the sharp Raman peak are unique to d iamond and do not occur for the other group-IV semiconductors. In our calculation based on harmonic ab initio lattice dynamics neither two-p honon bound states nor polarizability matrix element effects are neede d to explain the peak, and we feel that the longstanding controversy a bout its origin has been resolved.