ANOMALOUS MAGNETIZATION IN SINGLE-CRYSTAL TL2BA2CUO6 - EVIDENCE OF DIMENSIONAL CROSSOVER

Magnetization as a function of field parallel to the c axis shows anom alous secondary peak for a single-crystal Tl(2)Ba(2)Cu(O)6. The peak d isappears above a crossover temperature T-cr, which is correlated with a crossover in the irreversibility line H-rev(T). For T>T-cr, H-rev(T )proportional to(1-T/T-c)(n) with n=3/2; for T<T-cr, H-rev(T) increase s with 1/T exponentially, an indication of melting of two-dimensional (2D) vortices. Combined with magnetic-relaxation measurements at T<T-c r, we suggest the anomalous magnetization in this system is due to dim ensional crossover from 3D to 2D vortex structure.