Evidence for Macroscopic Quantum Tunneling in One-Dimensional Superconductors

Measurements of the superconducting-to-normal transition in very-small-diameter In wires are presented. The results are discussed in terms of the currently accepted theory, in which dissipation below $T_c$ is attributed to thermally activated motion of the Ginzburg-Landau order parameter over the free-energy barriers which separate metastable states. This theory is consistent with our results for temperatures within about 0.2 K of $T_c$, but at lower temperatures it fails qualitatively. We suggest that the low-temperature behavior is dominated by quantum-mechanical tunneling through the free-energy barrier, and show that our results are consistent with this picture.