Nonequilibrium effects in ballistic point contacts Ta−CuTa-Cu and 2H−NbSe2−Cu2H-NbS{{e}_{2}}-Cu. Two-gap superconductivity in 2H−NbSe22H-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: 16 pages, 9 figure

On the Static Dielectric Permittivity for Coulomb System in the Long Wavelength Limit

On the basis of the exact relations the general formula for the static dielectric permittivity for Coulomb system is found in the region of small wave vectors. The obtained formula describes the dielectric function of the Coulomb system in both limits: in the "metallic" state and in the "dielectric" one. On this basis the determinations of the "apparent" dielectric and the "apparent" radius of screening are introduced. In the random phase approximation (RPA) the exact relations for the function dielectric function of the electron gas in the long-wavelength region of the wave vectors are found for an arbitrary degeneration of the particles.Comment: 5 pages, no figure