Tunneling as a Probe of Nonequilibrium Superconducting States

The $I(V)$ characteristic of a superconductor-insulator-superconductor tunneling junction in which both of the superconductors are driven out of equilibrium is investigated. We show that the steady-state quasiparticle distribution function is related to the $I(V)$ characteristic by a linear integral equation. Modifications of the $I(V)$ characteristic due to nonequilibrium distributions produced by phonon and microwave irradiations are discussed.