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Enhancement and decrease of critical current due to suppression of superconductivity by a magnetic field

We present experimental results on the transport properties of a superconducting aluminum loop connected to reservoirs. As function of the applied magnetic field, an unexpected behavior is found—a steep enhancement for temperatures close to the critical temperature and a sharp drop at lower temperatures of the critical current at some value of the magnetic field Ba T. These effects are a consequence of a sudden suppression of superconductivity at Ba =B a in the banks to which the loop is attached by the current leads. As a result, the normal metal–superconductor boundaries appear at the ends of the current leads and the quasiparticle distribution function fE in the superconductor deviates strongly from its equilibrium value. Calculations based on the Usadel equations show that the critical current should be enhanced at high temperature due to the penetration of the normal current from the normal metal–superconductor boundaries, whereas at low temperatures, the suppression of the order parameter by the nonequilibrium fE dominates and the critical current decreases. The latter provides a direct experimental verification of the recently proposed mechanism of the destruction of superconductivity by an applied voltage R. S. Keizer, M. G. Flokstra, J. Aarts, and T. M. Klapwijk, Phys. Rev. Lett. 96, 147002 2006.