Nonequilibrium effects in ballistic point contacts $Ta-Cu$ and $2H-NbS{{e}_{2}}-Cu$. Two-gap superconductivity in $2H-NbS{{e}_{2}}$

The $Ta-Cu$ and $NbS{e}_{2}-Cu$ heterocontacts have been studied. For $Ta-Cu$ contacts the theoretical estimation of the value of $\delta$-functional barrier at the boundary arising due to the mismatch of Fermi parameters of the contacting metals was carried out and a good agreement between the calculation and experiment was obtained. An expression for the estimation of the diameter of the heterocontact on either side of the boundary is obtained. The magnitude of the jump-like decrease in the excess current (and the superconducting gap) due to the phase transition of the superconductor region near the point contact into a spatially inhomogeneous state when the critical concentration of nonequilibrium quasiparticles is reached has been determined. Obtained dependence of the additive differential resistance on the offsets at the contact arising after the phase transition, due to the excess charge of quasiparticles and the associated reverse current (or additive voltage). In $2H-NbS{{e}_{2}}$ there is a two-zone superconductivity character with $\sim$8 times different energy gap values. Under the influence of current injection of nonequilibrium quasiparticles there is a sequential phase transition of the layers adjacent to the point contact into a spatially inhomogeneous state with a suppressed gap, which is accompanied by a step change in the slope of the $I-V$ curve with a discrete increase in the differential resistance.
Comment: 18 pages, 9 figures