High-energy phonon dispersion in La1.85Sr0.15CuO4

We have measured two phonon-dispersion curves above 50 meV in a superconducting (Tc=33 K) single crystal of La1.85Sr0.15CuO4. In the normal phase (T=45 K), we show that the Λ1 mode compatible with the Raman A1g (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 the tetragonal (0, 0, ξ) direction. For the Λ1 mode compatible with the infra-red A2u (480 cm-1) mode, we observe a significant dispersion of the phonon energy which drops monotonically from 60 to 56 meV. The global features of these dispersion curves are similar to those previously reported in studies of both pure and 10% Sr doped La2CuO4. However significant differences in the shape of the dispersion of the highest Λ1 branch is shown, as well as a significant softening with increasing carrier concentration. (orig.)