Anomalous dispersion of LO phonons in La{sub 1.85}Sr{sub 0.15}CuO{sub 4}

Abstract

The dispersion of the highest energy LO phonon branch in La. 1.85 Sr 0.15 CuO 4 in the (100) direction has been reinvestigated by high resolution inelastic neutron 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 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 softening 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.