Current-voltage measurements as a probe of the activation barriers for flux creep in thin films of YBa 2Cu 3O 7−δ

Abstract

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 YBa 2Cu 3O 7−δ thin films at temperatures close to 77.4 K and fields up to 8 T. Using Taylor's expansion the logarithmic derivative ( j/ E) d E/d j and the second derivative d 2 E/d j 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) ∝ (1− j/ j c), a logarithmic U( j) ∝ 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) ∝ ( j c/ j) μ , as expected for a glassy vortex phase. Although the exponent μ is field dependent, our results suggest a Bose glass phase at low fields.