Superconductor-Insulator Transition in Long MoGe Nanowires

Abstract

The properties of one-dimensional superconducting wires depend on physical processes with different characteristic lengths. To identify the process dominant in the critical regime we have studied the transport properties of very narrow (9-20 nm) MoGe wires fabricated by advanced electron-beam lithography in a wide range of lengths, 1-25 μm. We observed that the wires undergo a superconductor-insulator transition (SIT) that is controlled by cross sectional area of a wire and possibly also by the width-to-thickness ratio. The mean-field critical temperature decreases exponentially with the inverse of the wire cross section. We observed that a qualitatively similar superconductor-insulator transition can be induced by an external magnetic field. Our results are not consistent with any currently known theory of the SIT. Some long superconducting MoGe nanowires can be identified as localized superconductors; namely, in these wires the one-electron localization length is much smaller than the length of a wire.

Zero-bias anomaly in homogeneously disordered MoGe nanowires undergoing a superconductor-insulator transition

“…Therefore, the formation of a structural discontinuity at the nanowire-electrode interface, which could serve as a tunneling junction, is unlikely. This assumption is supported by the fact that we didn't detect any extra interface resistance in slightly thicker superconducting MoGe nanowires fabricated by the same method 7 . The picture, however, can be more complicated since recent theories suggest that a tunneling junction, in fact, is not needed for observation of a Coulomb blockade; any coherent scatterer can serve as an "effective" tunneling junction 46,47 .…”

“…Exactly as in the case of MoGe films, the ZBA in nanowires changes sign from negative to positive at the critical point of the SIT 7 . On the superconducting side, the non-linear behavior is likely related to the critical superconducting current in nanowires; the origin of the ZBA on the insulating side is not understood and is a focus of the present paper.…”

“…Rather unexpectedly, recent experiments indicate that a direct SIT can also occur in long one-dimensional superconducting nanowires 7,8 . It was found that this transition can be driven by a magnetic field, nanowire cross sectional area, and possibly by aspect ratio of a nanowire cross section.…”

“…These nanowires were fabricated using negative electron beam lithography with hydrogen silsesquioxane (HSQ) electron-beam lithography resist; the details of the fabrication are given in the Supplementary Materials to Ref. [7]. In our fabrication procedure, a nanowire is smoothly connected to rectangular film electrodes, which are made from the same original MoGe film.…”

“…Voltage and current leads are connected to the opposite sides of the electrodes; we refer to this connection as the quasi-4-probe geometry. In our precious work we fabricated several nanowires in true-4-probe geometry, where nanowires served as the voltage probes 7 . No difference between two methods was found.…”