On the theory of tunnel injection into a superconductor

Nonequilibrium states in the superconductor of an S–I–N (superconductor–insulator–normal metal) tunnel junction are considered, such a junction being formed by a superconducting film and a bulky normal electrode. It is shown that at the temperature T = 0 the distribution function of electron excitations at eV–Δ ≪ Δ almost does not depend on the energy and its value does not exceed 1/2 (V denoting the voltage across the junction and Δ denoting the energy gap of the superconductor material). Such properties as ultrasound attenuation along with generation of 2Δ phonons and the high-frequency conductivity in these nonequilibrium states are examined. It is shown, furthermore, that these properties differ appreciably from those in the equilibrium state. The effect of departure from equilibrium due to tunnel injection from the N metal into the S film on the quasiparticle tunnel current flowing through the S– I–S’ junction of an N–I–S–I–S’ structure is subsequently analyzed.