ANGULAR-DEPENDENCE OF ULTRASONIC-ATTENUATION AT H-CL IN YBA2CU3O7 SINGLE-CRYSTAL

The resistive behavior of type-II superconductors in the superconducti ng state is largely influenced by the materials' ability to pin magnet ic flux lines. While an externally applied magnetic field leads to an ordered flux-lattice in defect-free superconductors, spatial variation s in the superconducting order parameter tend to deform the vortex sys tem. This has immediate implications for the temperature- and field-de pendence of the critical current density. Simple estimates of the bala nce between the intervortex interaction and the vortex-pinsite interac tion allow for qualitative predictions of the temperature- and field-d ependent resistive behavior of type-II superconductors. A low frequenc y ultrasonic technique was developed, which was used to study the visc ous properties of the vortex system.