PINNING OF JOSEPHSON VORTICES IN SINGLE-CRYSTALS OF TL2BA2CUO6+DELTA SUPERCONDUCTORS

Magnetic hysteresis measurements are performed on single crystals of Tl2Ba2CuO6+delta superconductors with field parallel to the ab-plane. Our results show anomalously the Bean type critical state behavior with constant critical current in this orientation. The penetration field H-p(T) depends on temperature T according to H-p(T) = H-p(0)[1 - (T/T-c)(2)], in sharp contrast to H-p(T) with field in the c-axis direction. For samples with high transition temperatures, the magnetization undergoes abrupt changes as a function of field at low temperatures. We propose that pinnings on the Josephson vortices in this direction are due to structure defects in these crystals. A local variation in the tunneling distance will introduce a finite pinning on Josephson vortices moving along the ab-plane. The abrupt change in magnetization may be due to field induced decoupling of the Josephson interaction between the layers.