Lifetime of phonons in semiconductors under pressure

We have measured the pressure dependence of the width of the first-order Raman lines in Si and Ge at low temperature. The width increases linearly with hydrostatic pressure, which implies a decrease of the lifetime of the long-wavelength optical phonons. The results are compared with recent first-principles calculations of anharmonic decay into two phonons of lower energy, based on third-order density-functional perturbation theory. Provided the calculations are slightly adjusted so that the relevant frequencies agree exactly with the measured ones, the agreement for the linewidths between theory and experiment is excellent.