Correlations between critical current density and penetration depth in ion irradiated YBa/sub 2/Cu/sub 3/O/sub 7/ thin films

Point defects have been introduced into YBa/sub 2/Cu/sub 3/O/sub 7/ through low energy helium ion irradiation in order to probe the origin of dissipation in a current-carrying superconductor. Resistivity, infrared reflectance and x-ray diffraction measurements indicate that the films are not chemically altered and that the induced point defects act as scattering centres. Measured electric field-current density characteristics are found to be well described by a model based on quantum current fluctuations. This description is used to extract the change in the superconducting carrier density with ion damage which agrees well with direct measurements of the same quantity by infrared reflectance. The implications of the relation between dissipation and the superconducting carrier density, or alternatively the magnetic penetration depth, are discussed.