Anomalous dispersion of LO phonons in La1.85Sr0.15CuO4

The dispersion of the highest energy LO phonon branch in La1.85Sr0.15CuO4 i n the (100) direction has been reinvestigated by high resolution inelastic neutron scattering. In contrast to what has been recently reported by McQue eney et al. (Phys. Rev. Lett. 82, 628 (1999)), we find that the dispersion is continuous throughout the Brillouin zone. These results are in conflict with the idea of charge stripe order existing on a time scale which is long compared to that of the LO frequencies. On. the other hand, the strong sof tening and boradening of LO phonons observed on doping might be taken as a precursor phenomenon to a-stripe order phase with a periodicity of about 4 lattice constants.