Anisotropy of the Irreversible Magnetic Behaviour of YBaCuO and TlBaCaCuO Single Crystals–A Comparative a.c.-Susceptibility Study

Abstract

Applying an a.c.-susceptibility technique, the lower critical fields Hc1(T) as well as the irreversibility lines Hc*(T/Tc) ∼ (1 - T/Tc)α have been determined for YBaCuO and Tl2Ba2CaCu2O8 single crystals. Pronounced anisotropies are observed for the above quantities in both systems indicating the strong influence of the Cu-O planes. As a main result, the exponent α describing the temperature behaviour of the irreversibility lines is found to exhibit a clear dependence on the field orientation: if the field is oriented perpendicular to the Cu-O planes, one observes α = 4/3 for both HTSC systems. For fields parallel to the Cu-O planes α = 3/2 for YBaCuO and α = 1 for Tl2Ba2CaCu2O8.