Ultrasonic studies of thermally assisted flux flow and intrinsic pinning in high-Tc phase Bi-Pb-Sr-Ca-Cu-O ceramics

The temperature dependence of the ultrasonic attenuation coefficient α ( T, H ) and sound velocity V s ( T, H ) were measured under magnetic fields for high- T c phase Bi-Pb-Sr-Ca-Cu-O ceramic samples aligned with the c -axis. In the case H · k , where k is the direction of sound propagation, a broad peak was observed in the α( T , 10T)-α( T , 0 T) versus T curve at T ∼ 65 K accompanied with a broad step in the V s ( T , 10 T) - V s ( T , 0 T) versus T curve. On the other hand, the peak and the step were also observed in the case of H | k , were the temperature position shifted down to T ∼50 K, and the width was narrower than in H · k . The behavior was explained in terms of thermally assisted flux flow in which flux lines are pinned by intrinsic pinning centers.