Observation of phonon softening at the superconducting transition in Bi2Sr2CaCu2O8.

Abstract

Resonant neutron-absorption spectroscopy has been used to determine the temperature dependence of the average kinetic energy {l angle}{ital E}{r angle} of the Cu in Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} for the directions along and perpendicular to the {ital c} axis. {l angle}{ital E}{r angle} is directly related to the Cu phonon density of states and shows essentially normal behavior as a function of temperature for the {ital c}-axis direction. However, a sudden drop in {l angle}{ital E}{r angle} is observed at the superconducting transition for the motions in the Cu-O planes.