OVERTONE PHONON STATES AND THE SHARP 2-PHONON RAMAN PEAK IN DIAMOND

Phonon dispersions near the Brillouin zone center and the two-phonon overtone states at the high frequency region in diamond are studied with a tight-binding potential model previously developed. We found that the uppermost phonon branch has its minimum frequency at the zone center leading to a peak in the two-phonon overtone density-of-states at frequency bigger than twice of the one-phonon Raman frequency. The frequency shift and line shape of the two-phonon peak are compared with the experimental data.