HIGH-ENERGY PHONON-DISPERSION IN LA1.85SR0.15CUO4

We have measured two phonon-dispersion curves above 50 meV in a superc onducting (T-c = 33 K) single crystal of La1.85Sr0.15CuO4. In the norm al phase (T = 45 K), we show that the Lambda(1) mode compatible with t he Raman A(1g) (425 cm(-1)) mode is weakly dispersive, falling from 53 .7 meV at the zone center to about 51 meV at the zone boundary along t he tetragonal (0, 0, xi) direction. For the Lambda(1) mode compatible with the infra-red A(2u) (480 cm(-1)) mode, we observe a significant d ispersion of the phonon energy which drops monotonically from 60 to 56 meV. The global features of these dispersion curves are similar to th ose previously reported in studies of both pure and 10% Sr doped La2Cu O4. However significant differences in the shape of the dispersion of the highest Lambda(1) branch is shown, as well as a significant soften ing with increasing carrier concentration.