CRITICAL CURRENTS AND IRREVERSIBILITY LINES IN HGBA2(CA1-XSRX)(2)CU3O8+DELTA AND HG0.5TL0.5BA2(CA1-XSRX)(2)CU3O8+DELTA

We have investigated the temperature dependence and applied magnetic-field dependences of critical current densities J(c)(H, T) and the irreversibility lines H-irr(T) obtained from the magnetization measurements in HgBa2(Ca1-xSrx)(2)Cu3O8+delta and Hg0.5Tl0.5Ba2(Ca1-xSrx)(2)Cu3O8+delta J(c)(H,T) and H-irr(T) of Hg0.5Tl0.5Ba2(Ca1-xSrx)(2)Cu3O8+delta are found to be higher than those of HgBa2(Ca1-xSrx)(2)Cu3O8+delta, indicating that the T1 substitution at Hg sites provides additional flux pinning. J(c)( H,T) has been analyzed using the flux-creep model. J(c)(H,T) in magnetic fields above 1 T is well described by the flux-creep model. However, the thermal activation energy estimated by fitting the data to the flux-creep model shows different behaviors above and below about 65 K for both HgBa2(Ca1-xSrx)(2)Cu3O8+delta and Hg0.5Tl0.5Ba2(Ca1-xSrx)(2)Cu3O8+delta Also, in H-irr(T) a crossover from a power-law temperature dependence above 65 K to a more rapid dependence below 65 K is observed in both samples. Although the origin of such a crossover in H-irr(T) and J(c)(H,T) is not clear, the two phenomena are considered to be closely related.