Current noise in biased superconducting weak links

¹ Department of Applied Physics, Chalmers University of Technology and Göteborg University - S-412 96, Göteborg, Sweden
² B. Verkin Institute for Low Temperature Physics and Engineering, 47 Lenin ave., 310164 Kharkov, Ukraine
³ Department of Physics, University of Oslo P.O. Box 1048 Blindern, N 0316 Oslo 3, Norway A. F. Ioffe Physico-Technical Institue - 194021 St. Petersburg, Russia

Received: 8 September 1995
Accepted: 12 February 1996

Abstract

Non-equilibrium current noise of a short quasi–one-dimensional constriction between two superconductors is considered. We derive a general expression for the frequency-dependent current-current correlation function valid for arbitrary temperatures, transparencies, and bias voltages. This formalism is then applied to a single current-carrying quantum mode with perfect transparency, and at zero frequency and temperature. Contrary to a transparent channel separating two normal conductors, a weak link between two superconductors exhibits a finite level of noise. The source of noise is the fractional Andreev scattering of quasi-particles with energies |E| greater than the half-width of the superconducting gap. For high bias voltages, , the zero-frequency limit of the noise spectrum, S(0), as well as the excess current , are twice as large than for a normal-superconductor junction, .