PULSE INDUCED SUPERCONDUCTING TO NORMAL TRANSITIONS IN THIN FILMS

Abstract

The superconducting to normal transition in thin films of Al, Sn, In, and Pb has been studied by applying square voltage pulses to a coaxial line terminated by the film and observing the reflected pulse shape. Interest has centered on the nearly isothermal transitions which could be induced below the lambda temperature of liquid helium. The results have been analyzed in terms of the nucleation and growth of normal domains. The use of very rapidly rising pulses has allowed an estimation of the nucleation time of a normal domain as not more than 3 × 10−10 second. The number of nucleation sites was a sensitive function of the current density in the film. The subsequent growth of normal domains proceeded with a velocity characteristic of the material, of the order of the velocity of transverse sound waves in the bulk metal. Roughly the same velocity was deduced for normal domain growth both parallel and perpendicular to the direction of the current in the film. Growth velocities smaller than these have not been observed, but the "pinning" of domain boundaries by defects in the film has been encountered.