Spatially inhomogeneous discrete states of a superconductor upon injection of nonequilibrium quasiparticles from a point contact with a normal metal

The current-voltage characteristics (IVC) of $S-c-N$ point contacts of superconductors with a small coherence length ${\xi }_0$ reveal steps with discrete values of the differential resistance. This peculiarity is associated with a transition of the contact region of the superconductor to a spatially inhomogeneous state under the influence of the current injection of nonequilibrium quasiparticles penetrating the superconductor to a depth $l_E$ . The role of the relaxation of the disbalance between the occupancies of electron- and hole-like branches of the quasiparticle spectrum is manifested in the displacement of the position of the singularities on the IVC towards higher energies upon an increase in the magnetic field and/or temperature. This effect was observed in superconductors with different ratios of the contact diameter $d$ and ${\xi }_0$ or $l_E$ in the series $Ta
(d\ll {{\xi}{0}},\ {{l}{E}})\to NbS{{e}{2}}$,$N{{b}{3}}Sn\ (d\gtrsim {{\xi}{0}},\ {{l}{E}})\to YB{{a}{1.25}}S{{r}{0.75}}C{{u}{3}}{{O}{7-\delta}}(d\gg {{\xi}{0}},
{{l}{E}})$. Apparently, a jumplike displacement of the boundary between regions with suppressed and equilibrium values of the energy gap near the contact is responsible for the oscillations observed on the IVC of point contacts between single crystals of high- temperature superconductors (HTS) and a normal metal. The resistance-periodic step-like structure of the IVC allows us to estimate the penetration depth $l_E$ of the electric field in $NbS{e}_2$ and $YBaSrCuO$.