Gigahertz ultrasound in single crystal superconducting YBa/sub 2/Cu/sub 3/O/sub 7/

Abstract

Anisotropic sound propagation at 1 GHz in single-crystal YBa/sub 2/Cu/sub 3/O/sub 7/ is described. Longitudinal phonons have been studied from 0.01 K to 300 K, with emphasis on the temperature region near the superconducting transition T/sub c/ at 88 K. A discontinuous sound velocity is observed at T/sub c/ for propagation parallel and perpendicular to the c axis. Strain derivatives of T/sub c/ are evaluated using mean field theory. Below 1 K the temperature-dependent sound velocity has positive slope, which indicates tunneling of defects.<>