Current-voltage measurements as a probe of the activation barriers for flux creep in thin films of YBa2Cu3O7-delta

In order to investigate the nature of the energy barrier U(j) for flux creep below the irreversibility line, accurate current-voltage (j-E) curves have been measured on YBa2Cu3O7-delta thin films at temperatures close to 77.4 K and fields up to 8 T. Using Taylor's expansion the logarithmic derivative (j/E) dE/dj and the second derivative d(2)E/dj(2) can be obtained reliably from the j-E data. Below the irreversibility line the logarithmic derivative increases with decreasing j, displaying an upward curvature in a low-field regime. An evaluation of the results in terms of various models proposed for U(j) is presented. The Anderson-Kim picture U(j) alpha (1-j/j(c)), a logarithmic U(j) alpha ln(j) and various distribution functions p(U) for the activation energy could not be reconciled with the j dependence of the logarithmic derivative. The experimental data are, however, consistent with U(j) alpha (j(c)/j)(mu), as expected for a glassy vortex phase. Although the exponent mu is field dependent, our results suggest a Bose glass phase at low fields.