Abstract:
A novel ultrasonic technique (sampled cw method) is used to probe into the
viscosity of vortex systems in untwinned and twinned YBCO single crystals.
Motion of the flux lines in the superconducting sample is induced via coupling
with the moving ionic lattice in the bulk of the sample. Hence, the observed
dissipation has its origin in the (induced) motion of the vortices with respect
to the ionic background of conduction charge carriers. In the experiments which
were performed, pronounced field-dependent attenuation changes were observed
which are indicative of transitions from a soft vortex system at low fields to a
rigid vortex system at high fields. [Research supported by Office of Naval
Research and C. Hucho was supported by Deutsche Forschungsgemeinschaft.] [sup