Nonequilibrium in normal-conductor/superconductor microconstrictions

By high-current injection from a point contact into bulk superconducting Ta a nonequilibrium state is produced that may be described as a normal bubble within the superconductor half-space. In driving the diameter of this bubble by the external bias we observe two different types of oscillatory behavior of the conductance. At zero magnetic field the quasiparticle current across the droplet is modulated due to geometrical resonances by multiple Andreev and normal reflections. At magnetic fields somewhat below the critical field, the superconducting surrounding of the normal region forms a symmetric dc SQUID. The magnetic flux through the normal region thus modulates a small supercurrent contribution. Equal numerical values of the bubble diameter are obtained by quantitative evaluation of both effects.