Gigahertz Ultrasound in Single Crystal Superconducting YBa sub 2 Cu sub 3 O sub 7

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 0.01K to 300K with emphasis on the temperature region near the superconducting transition T sub c at 88K. 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 1K the temperature-dependent sound velocity has positive slope which indicates tunneling of defects.