VIBRATING REED STUDIES OF VORTEX PINNING IN HIGH-TEMPERATURE SUPERCONDUCTORS

The vibrating reed method has been used to study the dissipation due t o vortex motion and the elastic coupling between vortices and pinning sites in single crystals of highly anisotropic superconductors. The re sults were obtained on small crystals of Bi2Sr2CaCu2O8+x (T(c) = 90.5 K), Y(6%Gd)Ba2Cu3O6.83 (T(c) = 67 K) and the organic superconductor ka ppa-(BEDT-TT)2Cu(NCS)2 (T(c) = 9.5 K) attached in different arrangemen ts to a dielectric reed vibrating in a static magnetic field. The angl e theta between the superconducting planes and the field direction cou ld be varied from parallel to perpendicular orientation. For all these anisotropic quasi-2D materials, we observe two dissipation peaks as a function of temperature when 0-degrees < theta < 90-degrees for all v alues of field. We show that the ''depinning lines'' defined by the pe ak position correspond to two flux diffusion modes within the model of thermally activated flux diffusion. They can be rescaled by B sin the ta to coincide with two constant resistivity lines.