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

We report relaxation measurements of the nonequilibrium magnetic momen t m(j)(t) with B parallel to to the c axis in a grain aligned sample o f the HgBa2CaCu2O6+x high-temperature superconductor. The nonlinear cu rent density dependence of the activation barrier U(j) is extracted fr om 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 parameter s U-c and j(c). The creep rate d In(m(j))/d in(t) is found to be a mon otonically increasing function of T, reaching a finite value at low T, in the quantum creep regime.