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 〈E〉 of the Cu in ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$ for the directions along and perpendicular to the c axis. 〈E〉 is directly related to the Cu phonon density of states and shows essentially normal behavior as a function of temperature for the c-axis direction. However, a sudden drop in 〈E〉 is observed at the superconducting transition for the motions in the Cu-O planes.