Nonequilibrium supercurrent transport in controllable superconductor-normal-metal-superconductor junctions

We study the supercurrent through a superconductor (Nb)-normal-metal (Ti-Au bilayer)-superconductor junction as a function of the electron distribution in the normal region. The normal region of the junction is coupled to the center of a mesoscopic wire (control channel) over which a control voltage V-c can be applied. The effective electron-electron relaxation in the wire determines the shape of the electron distribution and therefore the critical current of the SNS junction. For a short (1 mum) control channel we observe the transition to a pi state of the junction whereas for a junction with a long (9 mum) control channel we observe a monotonic decrease of the critical current. The data are in good quantitative agreement with theory at higher energies, but at low energies important geometry dependent deviations arise. The inferred electron-electron interaction time in the normal metal tau (e-e) equals 10 ps.