Low-temperature critical field of YBCO

We have measured the upper critical field HC2(T) of YBa2Cu3O7 (YBCO) thin films in magnetic fields up to 140 T for H applied parallel to the c-axis of the film. We prepared a 1700 A film of YBCO on a LaAlO3 substrate using the BaF2 process. Such a film has the c-axis perpendicular to the substrate. A meander pattern with large tabs for electrical contact was photolithographically prepared on this film, and the sample was cut into small squares less than 10 mm on their sides. We prepared evaporated niobium films to match the YBCO films and used them to cancel out the induced emf from the area of the pattern by connecting the two films in series-opposition. We measured R(T) of the films for linearity and transition width both in the laboratory and outdoors in the actual experiment during cooling before the field was applied. The critical field in the zero-temperature limit is 138 T, and the temperature dependence does not fit any simple model. 9 refs., 1 fig