DOUBLED SHOT-NOISE IN DISORDERED NORMAL-METAL SUPERCONDUCTOR JUNCTIONS

The low-frequency shot-noise power of a normal-metal-superconductor ju nction is studied for an arbitrary normal region. Through a scattering approach, a formula is derived that expresses the shot-noise power in terms of the transmission eigenvalues of the normal region. The noise power divided by the current is enhanced by a factor 2 with respect t o its normal-state value, due to Cooper-pair transport in the supercon ductor. For a disordered normal region, it is still smaller than the P oisson noise, as a consequence of noiseless open scattering channels.