IRREVERSIBILITY LINE OF THE HGBA2CACU2O6+DELTA HIGH-TEMPERATURE SUPERCONDUCTORS

Reversible and irreversible magnetizations (M) have been measured on a polycrystalline sample of HgBa2CaCu2O6+delta (Hg 1:2:1:2) before and after oxygenation. For the preoxygenated sample, the superconducting transition temperature (T(c)) is approximately 114 K with a transition width of approximately 2 K. The irreversibility line (H(irr)) is approximated by a power law H(irr) approximately (1 - T(irr)/T(c))n with n approximately 5/2 between 0.1 and 5.5 T. For the oxygenated sample, T(c) is approximately 120 K with a width approximately 4 K and the n for H(irr) is almost the same. The values of n, which are approximately 3/2 for Y 1:2:3:, approximately 5/2 for Hg 1:2:1:2, and approximately 11/2 for the Bi and Tl families, seem to correlate well with the distance between adjacent CuO2 blocks. A scaling law M is-proportional-to [1 - (T/T*)2]F(H), is observed for reversible M up to a few degrees below T(c) with a T* approximately 112 K before oxygenation. This suggests that Hg 1:2:1:2 is a quasi-two-dimensional system, where the effects of thermal fluctuations have been previously demonstrated.