Dependence of the Critical Currents in Superconducting Films on Applied Magnetic Field and Temperature

Measurements of critical currents $i_c$ were performed on planar and cylindrical films of tin of 650 to 2000 Å thickness. Both pulsed currents (80-nsec duration) and dc currents with stabilizing network were used. Close agreement was obtained for the dc data in the temperature dependence and approximate agreement in the magnitude of the critical current when compared with Bardeen's calculation as long as the stabilizing network was effective. The pulse measurements deviate from theory and from the dc data for temperatures $\frac{T}{T_c}<0.9\mathrm{}$. Oscilloscope pictures of the transitions indicate that this deviation towards lower currents is not caused by usual Joule heating. This and other similar work with pulsed currents and fields would indicate that an entirely new mechanism is responsible for the discrepancy in fast-pulsed critical currents. Critical-current measurements were also taken with the superposition of external magnetic fields ${\mathrm{H}}_e$. The whole range from the critical current in zero field to the critical field as measured with a current of 100 μA was covered. When the data are plotted as $log{i}_c$ versus $H_e$, they form a series of similar curves as the temperature is lowered. If adjusted for a match at ${\mathrm{H}}_e=0\mathrm{}$, these curves closely agree with the theoretical predictions of Ginzburg.