Irreversibility line of the HgBa2CaCu2O6+δ high-temperature superconductors

Reversible and irreversible magnetizations (M) have been measured on a polycrystalline sample of HgBa2CaCu2O6+δ (Hg 1:2:1:2) before and after oxygenation. For the preoxygenated sample, the superconducting transition temperature (Tc) is ∼114 K with a transition width of ∼2 K. The irreversibility line (Hirr) is approximated by a power law Hirr∼(1-Tirr/Tc)n with n∼5/2 between 0.1 and 5.5 T. For the oxygenated sample, Tc is ∼120 K with a width ∼4 K and the n for Hirr is almost the same. The values of n, which are ∼3/2 for Y 1:2:3:, ∼5/2 for Hg 1:2:1:2, and ∼11/2 for the Bi and Tl families, seem to correlate well with the distance between adjacent CuO2 blocks. A scaling law M[1-(T/T*)2]F(H), is observed for reversible M up to a few degrees below Tc with a T*∼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.