It is of fundamental importance to establish whether there is a limit to how thin a superconducting wire can be, while retaining its superconducting character—and if there is a limit, to determine what sets it. This issue may also be of practical importance in defining the limit to miniaturization of superconducting electronic circuits. At high temperatures, the resistance of linear superconductors is caused by excitations called thermally activated phase slips1,2,3,4. Quantum tunnelling of phase slips is another possible source of resistance that is still being debated5,6,7,8. It has been theoretically predicted8 that such quantum phase slips can destroy superconductivity in very narrow wires. Here we report resistance measurements on ultrathin (≲10 nm) nanowires produced by coating carbon nanotubes with a superconducting Mo–Ge alloy. We find that nanowires can be superconducting or insulating depending on the ratio of their normal-state resistance (RN) to the quantum resistance for Cooper pairs (Rq). If RN < Rq, quantum tunnelling of phase slips is prohibited by strong damping, and so the wires stay superconducting. In contrast, we observe an insulating state for RN > Rq, which we explain in terms of proliferation of quantum phase slips and a corresponding localization of Cooper pairs.
We thank E. Demler, Y. Oreg and D. S. Fisher for discussions about their QPS theory, M.Bockrath, B. I. Halperin, L. Levitov, J. E. Mooij, C. van der Wal, R. M. Westervelt and A.D. Zaikin for discussions about other aspects of the work, and S. Shepard for help with fabrication. This work was supported in part by the NSF and ONR.