Fluctuation of vortices in single-crystal Tl₂Ba₂CuO₆ superconductors

We report magnetization studies on single crystals of Tl₂Ba₂CuO₆ superconductors. For magnetic fields parallel to the c-axis configuration, fluctuations of vortices are observed in this layered compound. The characteristic temperature T^* where M is independent of applied field is about 84.2 K. Low-field magnetization measurements show H_c1=0 at T_s=86.5 K while the mean-field transition temperature T_c0=88.8 K. The experimental results strongly suggest the Tl compound is Josephson-coupled between superconducting layers, and the spontaneous excitations are thus the wandering vortex lines rather than pancakes.