Competition between Electronic Cooling and Andreev Dissipation in a Superconducting Micro-Cooler

We discuss very low temperature experiments on superconducting micro-coolers made of a double Normal metal-Insulator-Superconductor junction. We investigate with a high resolution the differential conductance of the micro-cooler as well as of additional probe junctions. There is an explicit crossover between the single quasi-particle current and the phase-coherent Andreev current. We establish a thermal model by considering the thermal contribution due to the Andreev current. The related increase of the electron temperature is discussed, including the influence of several parameters like the phase-coherence length or the tunnel junction transparency.