Anomalous Long-Range Proximity Effect Observed in Single-Crystal Superconducting Nanowires*

An anomalous proximity effect has been observed in single-crystal Pb, Sn, and Zn nanowires, each in contact with a pair of macroscopic electrodes. With electrodes having a higher critical temperature Tc, superconductivity is induced at the Tc of the electrodes in Sn and Zn nanowires as long as 60 $\mu$m, which is more than 10 times the expected length defined by current theories. This effect is further confirmed by the field dependence of the resistive transitions and I-V characteristics. It is found to depend sensitively on the residual-resistance-ratio of the nanowires.