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 scattering. In contrast to what has been recently reported by McQueeney et al. [Phys. Rev. Lett. 82, 628 (1999)], we find that the dispersion is conti nuous throughout the Brillouin zone. Our results are in conflict with the i dea of a short-range unit-cell doubling in the CuO2 plane existing on a tim e scale which is long compared to that of the LO frequencies. On the other hand, the strong softening and broadening of LO bond-stretching phonon:mode s :observed upon doping might be taken as a precursor phenomenon to stripe charge ordering which has been observed in nonsuperconducting cuprates. [S0 163-1829(99)51946-2].