Intermediate state structure in superconducting films of Indium

We have studied the intermediate state structure in type I superconducting strips of In with a high resolution magneto-optical method. The strips were about 5 μm thick and 160 μm wide. Both the magnetic structure due to an external magnetic field applied perpendicular to the film and the current induced intermediate state due to an electrical transport current in zero external field were investigated. The latter experiments were carried out with average current densities of 105 to 106 A/cm2. From our results and Landau's non-branching model we obtain the wall energy parameter Δ(1.7°K) = 0.76 μm. The current induced intermediate state consists of channels of the normal phase growing abruptly from both edges to the center of the strip. The magnetic field in these channels has opposite orientation for the two sides of the strip. The first channel was observed when the magnetic field of the transport current was about three times the critical field at the sample edge. With increasing current the number of normal channels increases, whereas the width and length of the individual channel remain nearly constant. The abrupt growth of each normal channel from the edge to the center of the strip appears to be analogous to the kink instability in magneto-hydrodynamics. Simultaneously with the current induced magnetic structure resistive voltage appears in the specimen.