State memory and reentrance in a paramagnetically limited superconductor

We report observations of a new quasistatic nonequilibrium phenomenon in th e density of electronic states (DOS) of a superconductor. Tunneling measure ments of the DOS of ultrathin Al films, at the spin paramagnetically limite d parallel magnetic field transition, reveal a strongly hysteretic DOS spec trum. We show that the hysteresis can be characterized as a quasistatic sta re memory effect in which the state of a film (normal or superconducting) i s determined by its state prior to entering the hysteretic region. We also show that state memory can lead to the onset of superconductivity with incr easing temperature, i.e., reentrance.