OBSERVATION OF ANDREEV REFLECTION ENHANCED SHOT-NOISE

We have experimentally investigated the quasiparticle shot noise in Nb N/MgO/NbN superconductor-insulator-superconductor tunnel junctions. Th e observed shot noise is significantly larger than theoretically expec ted. We attribute this to the occurrence of multiple Andreev reflectio n processes in pinholes present in the MgO barrier. This mechanism cau ses the current to flow in large charge quanta (Andreev clusters), wit h a voltage dependent average value of m approximate to 1 + 2 Delta/eV times the electron charge. Because of this charge enhancement effect, the shot noise is increased by the factor m.