Anisotropy of the lower critical field and irreversibility lines of c-axis-aligned HgCa2Ba2Cu3O8+δ powders

We present anisotropy of the lower critical field and irreversibility lines observed in c-axis-aligned powders of the HgCa2Ba2Cu3O8+δ compound with Tc = 131 ± 0.5 K. Temperature dependence of lower critical field, Hc1(T), exhibits an anomalous behavior without the usual low temperature saturation, and the temperature extrapolated value is Hc1(T = 0 K) = 2550 ± 200 G and 1350 ± 100 G for both H|c and Hτc, respectively. The anisotropic ratio Hirr(τc)Hirr(|c) below 2 is observed in all temperature region. The simple 3D-like power law Hirr = a · (1 − TTc)n is observed only in the low field region (≤ 2000G) with n = 1.40 ± 0.1 for H τc and n = 1.63 ± 0.1 for H |c. In the higher field region up to 5 Tesla, the temperature dependence of Hr(T) lines changes into a 2D-like exponential function Hirr = a · exp(−TTo) due to the breakdown of the interlayer and/or intralayer coupling of the conduction channel which consists of three Cu-O planes, with To = 16.0 ± 1.3 K for H τc and To = 16.6 ± 1.3 K for H |c.