Vortex bending and avalanche in a type-II superconductor observed with contactless ultrasonics

Severe vortex bending as large as 90 degrees and giant avalanche were observed over a large area of a disk-shaped type-ii superconductor in the mixed state using a contactless ultrasonic technique. This phenomenon is greatly different from a conventional critical state model in which the vortices are assumed to stand perpendicular to the disk plane. Equations describing the contactless ultrasonics are derived using a dynamic equation of vortex motion that takes account of crystal displacements arising from ultrasonic wave propagation. Using the equations and the ultrasonic data, the variations of the bending angle and the vortex densities are obtained as a function of applied magnetic field. [S0163-1829(98)01526-4].