Ultrasound studies of the high-TcsuperconductorLa1.85Sr0.15CuO4

Abstract

Sound-velocity, attenuation and thermal-expansion measurements are reported on ${\mathrm{La}}_{1.85}$ ${\mathrm{Sr}}_{0.15}$ ${\mathrm{CuO}}_4$. From room temperature to just above $T_c$ the sound velocity is found to soften significantly as the temperature is reduced. At $T_c$ there is a drop of ∼150 ppm in accord with the thermodynamics of the superconducting transition, and below $T_c$, as the gap opens, there is a large increase in the sound velocity. The magnitudes of these effects are anomalously large.