Raman study of the anomalous TO phonon structure in GaP with controlled isotopic composition

The transverse optical (TO) phonon in GaP has been measured in crystals wit h varying Ga isotopic composition by means of Raman scattering. In high res olution experiments at low temperature, we find a striking double peak stru cture in the line shape of natural GaP which varies with the Ga isotope abu ndance in a systematic manner. We calculate the spectra assuming anharmonic interaction of the TO phonon with a band of acoustic modes given by a mode l two-phonon density of states. This approach explains the measured Raman p rofiles and the isotope effects.