Irreversibility in the Superconducting Transition of Lead

The nature and origins of the hysteresis in the magnetic transition of pure superconducting lead have been studied in some detail. The effect can be introduced in nonhysteretic specimens by plastic deformation at liquid helium temperatures. The hysteresis becomes apparent somewhat below $T_c$ and its width increases monotonically with decreasing temperature. Similar effects can be caused by dilute additions of Ca to Pb. Strain induced hysteresis anneals out near 300°K but temperatures approaching the melting point appear necessary to remove impurity induced effects. Isothermal resistive measurements show a small fraction of the superconducting phase persisting to fields several hundred gauss above $H_c$. The residual superconductivity at high fields is increased by plastic deformation, and in general seems closely related to the hysteresis effect. The observations suggest the existence of a connected network which pervades the entire specimen volume and consists of very small filaments having a critical field exceeding the reversible critical field of bulk lead. The filaments are believed to be associated with defects in the crystalline lattice.