Giant Tc shift in HgBa2CaCu2O6+δ and TlBa2CaCu2O7-δ superconductors due to Hg-Tl exchange

The irreversibility lines of epitaxial thin films of HgBa2CaCu2O6+delta and TlBa2CaCu2O7-delta, whether deduced from field-induced magnetoresistive broadening or onset of nonhysteretic magnetization, were found to coincide when plotted against reduced temperature, showing the same exponential-decay temperature behavior for T/T-c < 0.8 and power-law behavior for T/T-c > 0.8 (where T-c is zero-field, zero-resistivity critical temperature). These results indicate that replacing Tl with Hg has no noticeable effect on the anisotropy and suggests that T-c is not determined by anisotropy. [S0163-1829(99)04601-9].