Dissipation in a one-dimensional superconductor: Evidence for macroscopic quantum tunneling

The results of a study of the superconducting transition in very-small-diameter In wires are presented. Several different types of measurements have been performed. First, we have studied the resistive transition in the limit of low applied currents, i.e., I≪${\mathit{I}}_{\mathit{c}}$, where ${\mathit{I}}_{\mathit{c}}$ is the critical current. Second, the transition to the dissipative state as a function of I has been examined. The results are discussed initially in terms of the thermal-activation model, according to which the dissipation is due to thermally activated motion of the Ginzburg-Landau order parameter over the free-energy barrier which separates metastable states. For certain ranges of sample size, temperature, and current, the thermal-activation theory is consistent with our results. However, for a wide range of these parameters we find that this theory fails, as the observed transition rates are much larger, and have a qualitatively different temperature dependence, than those predicted by the thermal-activation model. We suggest that in addition to thermal activation, quantum-mechanical tunneling of the order parameter through the free-energy barrier may also take place. We show that our results are consistent with this picture.