A theory for the low-voltage impedance of a superconductor-insulator-normal metal tunnel junction is developed that includes the effects of charge imbalance and of quasiparticle fluctuations. A novel, inelastic, charge-imbalance relaxation process is identified that is associated with the junction itself. This new process leads to the surprising result that the charge-imbalance component of the dc resistance of a junction becomes independent of the electron-phonon scattering rate as the insulator resistance decreases.

• Received 12 December 1983

DOI:https://doi.org/10.1103/PhysRevLett.52.1029