Spatially inhomogeneous discrete states of a superconductor under injection of nonequilibrium quasi-particles from the point contact with a normal metal

The current-voltage characteristics (IV) of S-c-N point contacts of superconductors with a small coherence length ξ0 display a stepwise structure with discrete values of differential resistance. The structure is determined by the transition of the superconductor contact region to a spatially inhomogeneous state due to injection of nonequilibrium particles which penetrate the superconductor at a depth lE. The relaxation of disbalance of populations of electron- and hole-like branches in the quasi-particle spectrum results in a position shift of the IV peculiarities towards high energies with increase in magnetic field and temperature. This effect was observed for superconductors with different relations between contact diameter d and ξ0, lE in the series Ta (d<<ξ0, lE) →NbSe2, Nb3Sn (d > or approx. ξ0, lE)→YBa1.25Sr0.75Cu3O7-δ (d>> ξ0, lE). A jump-like motion of the boundary between regions with suppressed and equilibrium values of the energy gap near the contact appears to cause oscillations in the IV derivatives obtained for the point contacts between high temperature superconductor single crystals and normal metal. A resistance periodicity of the stepwise structure in the I-V characteristics permits the depth of electric field penetration, lE, into NbSe2 and YBaSrCuO to be estimated