Raman linewidths of phonons in Si, Ge, and SiC under pressure

We have investigated the pressure dependence of the lifetimes of zone-cente r optical phonons in Si, Ge, and 3C-SiC. The mechanism leading to a finite lifetime is the anharmonic decay into phonons of lower energy. Experimental ly, the lifetime is determined from the full width at half maximum of Raman lines measured at low temperature. The experimental results for the linewi dths as well as phonon frequencies are compared to results of ab-initio cal culations which are based. on third-order density functional perturbation t heory.