Measurements of the transverse sound attenuation and velocity in a ceramic ${\mathrm{Bi}}_{1.6}$${\mathrm{Pb}}_{0.4}$${\mathrm{Sr}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{y}}$ superconductor (${\mathit{T}}_{\mathit{c}}$=110 K) were performed with and without a magnetic field. Pronounced peaks in the field-induced attenuation and an increase of the velocity at low temperatures were observed. These effects can be attributed to the transition of a rigidly pinned flux-line lattice into a depinned state. A quantitative explanation is presented within the framework of thermally assisted flux flow and without any adjustable parameter.

• Received 11 June 1990

DOI:https://doi.org/10.1103/PhysRevLett.65.3052