Anisotropic irreversibility lines due to thermal fluctuation for quasi-two-dimensional high-${\mathit{T}}_{\mathit{c}}$ superconductors (HTS’s) were observed in c-axis-aligned powders of the ${\mathrm{HgBa}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{8+\mathrm{\delta }}$ compound with ${\mathit{T}}_{\mathit{c}}$=131±0.5 K. The anisotropic ratio ${\mathit{H}}_{\mathrm{irr}}$(⊥c)/${\mathit{H}}_{\mathrm{irr}}$(∥c) below 2 was observed in all temperature regions. The simple three-dimensional-like power law ${\mathit{H}}_{\mathrm{irr}}$=a(1-T/${\mathit{T}}_{\mathit{c}}$${)}^{\mathit{n}}$ was observed only in the low-field region (≤2000 G) with n=1.40±0.1 for H⊥c and n=1.63±0.1 for H∥c. In the higher field region up to 5 T, the temperature dependence of ${\mathit{H}}_{\mathrm{irr}}$(T) lines changes into a two-dimensional-like exponential function ${\mathit{H}}_{\mathrm{irr}}$=a exp(-T/${\mathit{T}}_0$) due to the breakdown of the interlayer coupling of the conduction channel, which consists of three Cu-O planes, with ${\mathit{T}}_0$=16.0±1.3 K for H⊥c and ${\mathit{T}}_0$=16.6±1.1 K for H∥c. The exponent n is independent of the pinning strength or anisotropy related to interlayer coupling between ${\mathrm{CuO}}_2$ planes. The characteristic crossover field giving deviation from the power function depends strongly on the anisotropic nature of materials; e.g., ≥5 T for Y HTS’s, ∼0.2 T for Hg HTS’s, ∼0.02 T for Bi HTS’s, and ∼0.01 T for Tl HTS’s; this means that flux dynamics depend strongly on coupling strength between ${\mathrm{CuO}}_2$ blocks in the HTS.

• Received 28 March 1994

DOI:https://doi.org/10.1103/PhysRevB.50.16644