High electric field study of the superconducting transition of Y Ba 2Cu3O7-x

The superconducting transitions of YBa2Cu3O 7-x (YBCO) films of different thicknesses in high current densities and high electric fields are investigated. A pulsed-current technique allows for true four-probe measurements with a pulse duration as short as 50 ns, resulting in a significant reduction of self-heating. Electric fields up to 900 V cm-1 can be applied without sample damage. The data are compared to a theoretical model that describes the superconducting transition by the Hartree approximation of the Lawrence-Doniach model for a layered superconductor, together with a temperature correction in order to account for the self-heating. Although this model fits the electric-field dependence of the resistivity of a 330 nm thick YBCO film well, it fails for a 36 nm ultra-thin film, where self-heating is much smaller. This observation is a direct indication of intrinsic non-ohmic behaviour, resulting from the suppression of superconducting order parameter fluctuations in high electric fields.