Long-Range Superconducting Proximity Effect in Nickel Nanowires

TL;DR

This study demonstrates that inserting a ferromagnetic CuO buffer layer between superconducting Nb contacts and Ni nanowires significantly extends the superconducting proximity effect from a few nanometers to tens of nanometers, likely due to triplet supercurrents.

Contribution

The paper reveals that a ferromagnetic CuO buffer layer enables long-range superconducting proximity effects in Ni nanowires, a novel approach to enhancing supercurrent transmission.

Findings

Proximity effect extends from 2 nm to tens of nanometers with CuO layer.

CuO layer exhibits ferromagnetism confirmed by magnetization measurements.

Long-range effect likely due to conversion to triplet supercurrent.

Abstract

When a ferromagnet is placed in contact with a superconductor, owing to incompatible spin order, the Cooper pairs from the superconductor cannot survive more than one or two nanometers inside the ferromagnet. This is confirmed in the measurements of ferromagnetic nickel (Ni) nanowires contacted by superconducting niobium (Nb) leads. However, when a 3 nm thick copper oxide (CuO) buffer layer made by exposing an evaporated or a sputtered 3 nm Cu film to air, is inserted between the Nb electrodes and the Ni wire, the spatial extent of the superconducting proximity range is dramatically increased from 2 to a few tens of nanometers. Scanning transmission electron microscope study confirms the formation of a 3 nm thick CuO layer when an evaporated Cu film is exposed to air. Magnetization measurements of such a 3 nm CuO film on a SiO2/Si substrate and on Nb/SiO2/Si show clear evidence of ferromagnetism. One way to understand the long-range proximity effect in the Ni nanowire is that the CuO buffer layer with ferromagnetism facilitates the conversion of singlet superconductivity in Nb into triplet supercurrent along the Ni nanowires.