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 temperature s close to 77.4 K and fields up to 8 T. Using Taylor's expansion the l ogarithmic 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 irreversibilit y line the logarithmic derivative increases with decreasing j, display ing an upward curvature in a low-field regime. An evaluation of the re sults in terms of various models proposed for U(j) is presented. The A nderson-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 deriv ative. The experimental data are, however, consistent with U(j) alpha (j(c)/j)(mu), as expected for a glassy vortex phase. Although the expo nent mu is field dependent, our results suggest a Bose glass phase at low fields.