Resistance anomaly and excess voltage in inhomogeneous superconducting aluminum thin films

We report measurements of the resistance and the current-voltage characteristics of aluminum thin films which have two regions of different superconducting transition temperatures. Local suppression of the transition temperature is achieved by the use of a CF4 reactive ion-etching technique. A small gradient in the transition temperature is induced around the lithographically defined etched-unetched interface. As the temperature is increased from the superconducting state, we observe voltage ~and hence resistance! increases above the normal-state value when measured with superconducting voltage probes located within ;20 mm on either side of the interface. The excess voltage persists over a range of bias currents above IC and eventually disappears, approaching the normal-state value at high bias currents. In an experiment where there are multiple voltage probes arranged along the film, the excess voltage is eliminated in steps with increasing bias current, resulting in a series of negative differential resistance peaks. These unusual phenomena are explained by a nonequilibrium charge imbalance model which requires the spatial dependences of the quasiparticle and pair electrochemical potentials to be different near normal-superconducting interfaces and phase-slip centers. We also report the observation of an asymmetry in the current-voltage characteristics which cannot be understood in terms of the nonequilibrium model. @S0163-1829~97!03814-9#