F. Zuo, S. Khizroev, V.N. Kopylov, N.N. Kolesnikov.
Pinning of Josephson vortices in single crystals of Tl₂Ba₂CuO_6+d superconductors.

  Abstract:
 Magnetic hysteresis measurements are performed on single crystals of  Tl₂Ba₂CuO_6+d    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.