Positive and negative Hanbury-Brown and Twiss correlations in normal metal-superconducting devices

In the light of the recent analogs of the Hanbury-Brown and Twiss experimen ts [1] in mesoscopic beam splitters, negative current noise correlations ar e recalled to be the consequence of an exclusion principle. Here, positive (bosonic) correlations are shown to exist in a fermionic system, composed o f a superconductor connected to two normal reservoirs. In the Andreev regim e, the correlations can either be positive or negative depending on the ref lection coefficient of the beam splitter. For biases beyond the gap, the tr ansmission of quasiparticles favors fermionic correlations. The presence of disorder enhances positive noise correlations. Potential experimental appl ications are discussed.