MESOSCOPIC FLUCTUATIONS OF THE DC JOSEPHSON CURRENT IN A DIRTY SNS JUNCTION

We present theoretical studies on mesoscopic fluctuations of the dc Jo sephson current in superconductor (S)-dirty normal conductor (N)-super conductor junctions. The analytical expression for the Josephson-curre nt fluctuation delta J(2) is obtained by using the thermal Green's fun ction method, which is shown to he valid when the separation between t wo superconductors is much lager than the coherence length of the dirt y superconductor. The effects of temperature and inelastic scattering on delta J(2) are examined. The results show that the characteristic t emperature (size) for the reduction of delta J(2) is of the order of t he Thouless correlation energy E(c) (the phase choherence length). Fro m the current correlation function, the energy correlation range is es timated to be at about E(c). In order to prepare a specific reference for the system under our consideration, we numerically examine the cri tical-current fluctuations delta J(c)(2) in the long-junction limit in a model system. Numerical results show that the magnitude of delta J( c)(2) and energy correlation range agree with our analytical result. T he effect of the barrier potential at the NS interfaces is numerically investigated. We find that the experimental result by Takayanagi et a l. can be explained quantitatively within the frame work of our theory .