FIELD-DEPENDENCE OF THE CURRENT-VOLTAGE CHARACTERISTICS OF THIN-FILM YBACUO AT LOW MAGNETIC-FIELDS

Current-voltage isotherms for YBa2Cu3O6+x thin-film superconductors fo r several magnetic fields in the low-field regime have been obtained. A crossover from ohmic behavior at low currents to exponentially decre asing behavior occurs for all fields down to ambient The relation betw een the crossover temperature and field is predicted by the vortex gla ss theory. The isotherms within about 1 K of the crossover can be coll apsed onto two curves for each field, according to the prescription of the vortex glass theory; however some parameters are field dependent. In particular, the parameter z increases with decreasing field, and s hows a correlation with the number of vortices in the film.