Andreev reflection off a fluctuating superconductor in the absence of equilibrium

Abstract

Andreev reflection between a normal metal and a superconductor whose order parameter exhibits quantum phase fluctuations is examined. The approach chosen is nonperturbative in the tunneling Hamiltonian and enables us to probe the whole range of voltage biases up to the gap amplitude. Results are illustrated using the one-dimensional Josephson junction array model previously introduced in the linear response regime. Phase fluctuations are shown to affect the differential conductance and are compared to the result of Blonder, Tinkham, and Klapwijk for a rigid BCS superconductor. The noise spectrum of the Andreev current is also obtained and its second derivative with respect to frequency is proposed as a direct tool to analyze the phase fluctuations.