Anomalous Dispersion of LO Phonons in La{sub 1.85 }Sr{sub 0 .15}CuO{sub 4} at Low Temperatures

Inelastic neutron scattering measurements of La{sub 1.85} Sr{sub 0.15} CuO{sub 4} show that a discontinuity in dispersion develops in the middle of the highest energy LO phonon branch at low temperatures. The result suggests dynamic short range unit cell doubling in the CuO{sub 2} plane along the direction of the Cu-O bond, such as charge ordering on every other row of oxygen. Such charge ordering is related to, but is different from, the stripe charge ordering observed for nonsuperconducting cuprates. {copyright} {ital 1999} {ital The American Physical Society}