ULTRASONIC STUDIES OF FLUX-LINE PINNING IN SUPERCONDUCTING BI2SR2CACU2O8+Y

Measurements have been made of ultrasonic shear wave attenuation and s ound velocity in highly textured polycrystalline samples of Bi2Sr2CaCu 2O8+y in magnetic fields up to 9 T and at temperatures down to 4.2 K. Magnetic field dependent attenuation peaks and velocity enhancements w ere in good agreement with theoretical predictions based on the assump tions of the thermally assisted flux-flow model of the mixed state. Fl ux-line pinning activation energy obtained from the data showed a weak anisotropy, similar to 160 K for field parallel and similar to 240 K for field perpendicular to the c-axis. The flux-line lattice was found to be fully pinned at lowest temperatures. Experiments were also perf ormed in low magnetic fields (up to 9 mT), but showed no evidence of r eported ultrasonic effects that could be attributed to weak supercondu ctivity in grain boundaries.