Nonequilibrium effects in ballistic point contacts Ta-Cu and 2H-NbSe2-Cu. Two-gap superconductivity in 2H-NbSe2

Abstract

Ta-Cu and NbSe2-Cu heterocontacts have been studied. For Ta-Cu contacts, a theoretical estimation of the delta-functional barrier at the boundary, arising due to the mismatch of the Fermi parameters of the contacting metals, has been carried out, and good agreement between the calculation and experiment has been obtained. An expression for estimating the diameter of the heterocontact on both sides of the boundary is obtained. The value of the jump-like reduction of the excess current (and superconducting gap) associated with the phase transition of the superconducting region near the point contact to a spatially inhomogeneous state when the critical concentration of nonequilibrium quasiparticles is reached is determined. The dependence of the additive differential resistance on the displacement at the contact arising after the phase transition due to the excess charge of quasiparticles and the associated reverse current (or additive voltage) is obtained. In 2H-NbSe2, a two-zone superconductivity character with an 8-fold difference in the energy gap values takes place. Under the action of current injection of nonequilibrium quasiparticles, there is a sequential phase transition of layers adjacent to the point contact into a spatially inhomogeneous state with a suppressed gap, which is accompanied by a stepwise change in the slope of the I-V curve with a discrete increase in the differential resistance.