Superconductivity Under Magnetic Fields in Nanobridges of Lead

Abstract

We study the properties of superconducting bridges of Pb of nanoscopic dimensions fabricated by an STM under magnetic fields. The bridges behave as weak links of variable dimensions whose form and smallest contact radius can be varied in-situ. We find that these structures have upper critical fields several times the bulk critical field of Pb, forming at large fields an N - s - junction - s - N system whose superconducting part s is of mesoscopic dimensions. We could find no evidence for the nucleation of vortices in these structures, in agreement with the estimation for the geometry which we can get from R-z curves and the model of Ref... Therefore, their phenomenology cannot be explained by the usual type II superconducting behavior. The physical properties of the system are then expected to be dominated by the pair breaking effect of the magnetic field. For instance, in the case of a bridge whose smallest contact radius is of the order of a single atom, we can follow the subharmonic gap structure as a function of the pair breaking parameter. By contrast, in bridges formed by two long cone like structures with a smallest contact of the order of 20Åor larger a new phenomenology appears. We observe two well defined conduction regimes as a function of the field and under strong current flow. In an intermediate field regime, strong peaks appear in the dV/dI curves which disappear at large fields, and at the highest fields, an new anomaly in dV/dI appears. This behavior can be understood in terms of nonequilibrium superconductivity by considering the magnetic field dependence of the quasiparticle charge imbalance length ΛQ _* through the pair breaking effect. At intermediate fields, our curves can be interpreted as an indication for the nucleation of phase slip centers within the bridge (of dimensions orders of magnitude smaller than usual phase slip wires), and at high magnetic fields the observed anomaly should be related to the contact resistance of the N -s interface formed by the destruction of superconductivity in the bulk. The different magnetic field dependence of ΛQ _* and of the length of the superconducting part L _s makes the appearence of two regimes possible.