ANISOTROPY OF THE LOWER CRITICAL-FIELD AND IRREVERSIBILITY LINES OF C-AXIS-ALIGNED HGCA2BA2CU3O8+DELTA POWDERS

We present anisotropy of the lower critical field and irreversibility lines observed in c-axis-aligned powders of the HgCa2Ba2Cu3O8+delta co mpound with T-c = 131+/-0.5 K. Temperature dependence of lower critica l field, H-c1(T), exhibits an anomalous behavior without the usual low temperature saturation, and the temperature extrapolated value is H-c 1(T = 0 K) = 2550+/-200 G and 1350+/-100 G for both H parallel to c an d H perpendicular to c, respectively. The anisotropic ratio H-irr (per pendicular to c)/H-irr(parallel to c) below 2 is observed in all tempe rature region. The simple 3D-like power law H-irr = a . (1 - T/T-c)(n) is observed only in the low field region (less than or equal to 2000G ) with n = 1.40 +/- 0.1 for H perpendicular to c and n = 1.63 +/- 0.1 for H parallel to c. In the higher field region up to 5 Tesla, the tem perature dependence of H-r(T) lines changes into a 2D-like exponential function H-irr = a . exp(-T/T-o) due to the breakdown of the interlay er and/or intralayer coupling of the conduction channel which consists of three Cu-O planes, with T-o = 16.0 +/- 1.3 K for H perpendicular t o c and T-o = 16.6 +/- 1.3 K for H parallel to c.