Controllable superconducting weak links

We report the experimental characteristics of controllable superconducting weak links (CLINKs), in which the necessary localized weak region is produced by creating a nonequilibrium excess quasiparticle population which suppresses the superconducting order parameter. Direct quasiparticle injection, photo-injection, and phonon injection have been used. The characteristics (e.g., critical current) of these CLINKs can be electrically or optically tuned continuously at any temperature below Tc. The ac Josephson effect has been observed in each type. We have developed a rudimentary theory which appears to account for some of the experimental observations. The theory is based on the phenomenological Rothwarf-Taylor equations for a nonequilibrium superconductor, modified to include quasiparticle and phonon diffusion spatial dependence of the densities, together with the modified heating or T* model of Parker and existing theoretical models of weak links.