Magnetic relaxation and the activation energies of grain-aligned HgBa2Can-1CunOy (n=1, 2, 3) superconductors

The magnetic relaxations of grain-aligned HgBa2Can-1CunOy (n = 1, 2, 3) superconductors were investigated in the temperature range of 5 to 70 K and in a magnetic field of 1 T. These results led to the functional expression U-eff(J, T) = U(i)G(T)(J(i)/J)(mu), where U-i is the scale of the activation energy and G(T) = (1 - (T/T-c)(2))(m) for n = 1, 2, and 3, respectively. The observed power-law relationship of U-eff(J, T) clearly demonstrates two of three regions as predicted by the theory of collective flux creep; namely mu = 7/9 and 3/2 for J << J(c) and J < J(c), respectively.