The phase transition in superconductors II. Phase propagation above the critical field

Abstract Detailed measurements have been made of the rate at which the superconducting phase collapses radially in cylindrical rods of tin, when they are suddenly subjected to a magnetic field greater than the critical. This is probably the simplest example of phase propagation in superconductors. The results in most respects confirm the theory of Pippard (1950 a) and Lifshitz (1950), according to which the propagation is controlled by an electromagnetic damping associated with the setting up of eddy currents. This theory explains in detail the way in which the rate of propagation depends on specimen radius and conductivity, and on field strength; its only failure is at the higher temperatures, where the magnitude of the rate of propagation tends to be slightly less than the theory predicts. Other factors besides eddy currents which might be retarding the transition are latent heat, the interphase surface energy, and a finite relaxation time governing the destruction of superconductivity by a magnetic field; but none of these proves altogether adequate to account for the discrepancy mentioned. The experiments provide evidence that the relaxation time is less than 2 x 10-7 s in tin.