State Memory and Reentrance in a Paramagnetically Limited Superconductor

We report observations of a new quasistatic nonequilibrium phenomenon in the density of electronic states (DOS) of a superconductor. Tunneling measurements of the DOS of ultrathin Al films, at the spin paramagnetically limited parallel magnetic field transition, reveal a strongly hysteretic DOS spectrum. We show that the hysteresis can be characterized as a quasistatic state memory effect in which the state of a film (normal or superconducting) is determined by its state prior to entering the hysteretic region. We also show that state memory can lead to the onset of superconductivity with increasing temperature, i.e., reentrance.