Bias-resolved measurements of charge imbalance in superconductors at ultra-low temperatures

In this study we explore charge imbalance in mesoscopic normal-metal/superconductor multiterminal hybrid structures at ultra-low temperatures (T <<Tc). The investigated samples, structured by e-beam lithography and shadow evaporation, consist of a superconducting aluminum bar with several copper fingers forming tunnel contacts at different distances from each other. We measured in detail the local and non-local conductance of these structures as a function of the applied bias voltage V, the applied magnetic field B, the temperature T and the contact distance d. From these data the relaxation length λQ* was derived. The bias-resolved measurements show a transition from dominant elastic scattering close to the gap to inelastic scattering at higher bias. We measured a strong suppression of charge imbalance with magnetic field, which can be directly linked to the pair breaking parameter. In contrast practically no temperature dependence of the charge imbalance signal was observed below 0.5 K. These results are relevant for the investigation of other non-local effects such as crossed Andreev reflexion and spin diffusion.