Positive cross correlations of noise in superconducting hybrid structures:: Roles of interfaces and interactions

Shot noise cross-correlations in normal metal-superconductor-normal metal structures are discussed at arbitrary interface transparencies using both the scattering approach of Blonder, Tinkham, and Klapwik and a microscopic Green's function approach. Surprisingly, negative crossed conductance in such setups [R. Melin and D. Feinberg, Phys. Rev. B 70, 174509 (2004)] does not preclude the possibility of positive noise cross-correlations for almost transparent contacts. We conclude with a phenomenological discussion of interactions in the one-dimensional leads connected to the superconductor, which induce sign changes in the noise cross-correlations.