Logarithmic current density dependence of the activation barrier in superconducting HgBa2CaCu2O6+x

We report relaxation measurements of the nonequilibrium magnetic moment mj(t) with B 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 mj(t) data, and it is found that U(j) is well described by U(j)=Ucln(jc/j). We also find the field dependence of the parameters Uc and jc. The creep rate d ln(mj)/d ln(t) is found to be a monotonically increasing function of T, reaching a finite value at low T, in the quantum creep regime.