NONEQUILIBRIUM FLUCTUATIONS OF CURRENT IN SUPERCONDUCTOR-NORMAL METALTUNNEL STRUCTURES

General expressions for the current and the spectral density of curren t fluctuations in the low frequency limit for the normal metal-superco nductor contact with an arbitrary barrier in the-presence of a constan t external potential difference have been obtained in terms of the sca ttering problem formalism. It is demonstrated that the dynamic and flu ctuation characteristics of the system can behave quite differently, a s dictated by the tunnelling character (nonresonant in a structureless barrier; resonant in a barrier containing localized states). For a pa rticular model of resonance tunneling, the expression is obtained For the current allowing for existence of negative differential resistance in a certain range of the scattering parameters as well as a formula for the noise. of the current whose spectral density contains more con tributions than nonresonance tunneling that are due to interference el ectron-hole process of scattering from a localized state.