Logarithmic current density dependence of the activation barrier in superconductingHgBa2CaCu2O6+x

Abstract

We report relaxation measurements of the nonequilibrium magnetic moment ${\mathit{m}}_{\mathit{j}}$(t) with B\ensuremath{\parallel} to the c axis in a grain aligned sample of the ${\mathrm{HgBa}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{6+\mathit{x}}$ high-temperature superconductor. The nonlinear curent density dependence of the activation barrier U(j) is extracted from the ${\mathit{m}}_{\mathit{j}}$(t) data, and it is found that U(j) is well described by U(j)=${\mathit{U}}_{\mathit{c}}$ln(${\mathit{j}}_{\mathit{c}}$/j). We also find the field dependence of the parameters ${\mathit{U}}_{\mathit{c}}$ and ${\mathit{j}}_{\mathit{c}}$. The creep rate d ln(${\mathit{m}}_{\mathit{j}}$)/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.