FINITE LIFETIMES FOR INTERMEDIATE PHONON LINES IN ANHARMONIC PERTURBATION-THEORY

The spectral functions for a linear chain show strong singularities wh en calculated with the usual perturbation-theory expressions. Results from molecular-dynamics simulations show no such signularities. We mod ify the perturbation-theory calculation to include lifetimes, and henc e widths, for the intermediate phonons in the cubic term. The widths a nd shifts of the intermediate lines an assumed, for simplicity, to be proportional to their frequencies, and are calculated self-consistentl y, The agreement with the molecular-dynamics results is substantially improved.