Positive current cross-correlations in a highly transparent normal-superconducting beam splitter due to synchronized Andreev and inverse Andreev reflections

Abstract

Predictions are established for linear differential current-current cross-correlations dSa,b/dV in a symmetrically biased three-terminal normal metal-superconductor-normal metal (NSN) device. Highly transparent contacts turn out to be especially interesting because they feature positive dSa,b/dV. At high transparency, processes based on crossed Andreev reflection (CAR) contribute only negligibly to the current and to dSa,b/dV. Under these circumstances, current-current cross-correlations can be plausibly interpreted as a coherent coupling between the two NS interfaces in the form of synchronized Andreev and inverse Andreev reflections (AR-AR¯), corresponding to the process where a pair of electronlike quasiparticles and a pair of holelike quasiparticles arrive from the normal electrodes and annihilate in the superconductor. Hence, positive dSa,b/dV does not automatically imply CAR. For tunnel contacts, dSa,b/dV is positive because of CAR. In between these two extremities, at intermediate transparencies, dSa,b/dV is negative because both processes which cause positive correlations occur only with small amplitude. We use scattering theory to obtain analytic expressions for current and noise and microscopic calculation using a tight-binding model in order to obtain a clear interpretation of the physical processes.

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