PHONON RENORMALIZATION AT SMALL Q-VALUES IN THE HIGH-TEMPERATURE PHASE OF CSCUCL3

The hexagonal perovskite CsCuCl3 exhibits a structural phase transitio n from a dynamically disordered high-temperature phase to an ordered l ow-temperature phase due to the cooperative Jahn-Teller effect. The la ttice dynamics of the high-temperature phase has been studied by inela stic neutron scattering experiments. The investigations concentrated o n small wave vectors q, where for the first time renormalized phonons at q = 0.02-0.05 Angstrom(-1) could be observed. The measurements conf irm the predictions of a theoretical approach based on the coupling be tween dynamic reorientation processes and acoustic lattice waves (pseu do-spin phonon coupling).