Meissner Shielding Currents and Magnetic Flux Penetration in Thin-Film Superconductors

We have observed the magnetic flux penetration at 4.2 K into superconducting. lead films of about 4 pm thickness using a high-resolution magnetooptical technique. The specimens had the shape of small squares or long strips. In increasing magnetic field it was found that magnetic flux starts to penetrate into the bulk of the specimens in the form of flux tubes containing about 60 flux quanta. For the samples with the strip geometry the magnetic field H*, at which flux penetration into the bulk sets in, was investigated in the presence of an external electrical current I along the strip superimposed on the Meissner shielding currents. The field H* was found to decrease linearly with increasing transport current I. The magnetic field near a long superconducting cylinder in the presence of both a transverse applied magnetic field and an applied electrical current parallel to the cylinder axis has been calculated. The calculation is valid for a cylinder of elliptical cross section, which approximates a long,flat strip. The experimental observations for the slope OW RI are in good agreement with the calculations.