LOGARITHMIC CURRENT-DENSITY DEPENDENCE OF THE ACTIVATION BARRIER IN SUPERCONDUCTING HGBA2CACU2O6+X

We report relaxation measurements of the nonequilibrium magnetic moment m(j)(t) with B parallel to to the c axis in a grain aligned sample of the HgBa2CaCu2O6+x high-temperature superconductor. The nonlinear curent density dependence of the activation barrier U(j) is extracted from the m(j)(t) data, and it is found that U(j) is well described by U(j)= U(c)1n(j(c)/j). We also find the field dependence of the parameters U-c and j(c). The creep rate d In(m(j))/d in(t) is found to be a monotonically increasing function of T, reaching a finite value at low T, in the quantum creep regime.