The upper critical field and the normal-state resistivity of YBa2Cu3O7-δ thin films in megagauss fields

We measured the DC and AC magnetoresistance of YBa2Cu3O7-delta thin films at temperatures between 4.2 K and 100 K in magnetic fields parallel to the c-axis of up to 130 T. By minimizing the sample size and the area of the measurement circuit perpendicular to the magnetic field, a very good signal-to-noise ratio was obtained using a high-frequency digital lock-in technique for the AC measurements. Suppressing the superconductivity down to 4.2 K using magnetic fields up to 130 T, we found that the normal-state resistivity remains metallic down to 4.2 K. We determined the upper critical fields (H-c2) down to 4.2 K; H-c2 tends to saturate at low temperature.