Decay of high-frequency phonons in amorphous silicon

We present measurements concerning the dynamics of high-frequency phonons ({}{ω}{≥}150 cm^-1) in amorphous silicon at 2 K. From pulsed anti-Stokes Raman experiments, estimates of the population decay time are obtained for high-frequency phonons, generated by fast carrier relaxation. The fracton model explains the counterintuitive sense of the decay-time dependence on frequency in terms of the decrease of the localization length of the vibrational states towards higher frequencies, and the concomitant suppression of the anharmonic decay rate. This model fits our data with a very realistic third-order elastic constant. {} 1996 The American Physical Society.