Macroscopic-ranged proximity effect in graphite

Bruno Cury Camargo,Yakov Kopelevich,Ramon Ferreira De Jesus,Oleksandr Onufriienko,Marek Kuzmiak,Piotr Gierłowski,Pavol Szabó

doi:10.1088/1361-648x/ac24c5

Abstract

We report proximity-induced superconducting features over macroscopic lengths in highly oriented pyrolytic graphite. The phenomenon is triggered when electrical currents are injected in the material through superconducting electrodes, few millimeters apart from each other. Such a large range is anomalous, as proximity-induced features in normal conductors hardly surpass few micrometers. The results can be explained as due to the presence of pre-existing superconductivity in graphite on small, localized regions.

Highlights

When superconducting (S) and normal (N) materials are brought together, Cooper pairs from the S region drift into N
Very recently, percolative (R = 0) superconductivity has been measured in twisted bi-18 and multi-layer[19] graphenes. It is conceivable the superconducting proximity effect (PE) in graphite can have a far longer reach than ξN ≈ 1 μm, possibly reaching macroscopic scales
Voltage electrodes of samples contacted in the 6probe Hall bar configuration were spread across the whole height of the sample, for the same reasons

Summary

INTRODUCTION

When superconducting (S) and normal (N) materials are brought together, Cooper pairs from the S region drift into N. Usually confined to regions tenths or hundredths of nanometers near the S-N interface, in some cases, the PE can occur over several thousand times the length ξN This is observed in selected S-N-S systems, and is related mostly to the properties of the N material. These values lead to large estimated normal coherence lengths at low temperatures ξN (T = 4 K) = vF /2πkBT ≈ 2 μm It presents various indications of SC, reported across the literature. Very recently, percolative (R = 0) superconductivity has been measured in twisted bi-18 and multi-layer[19] graphenes Given these signatures, it is conceivable the superconducting PE in graphite can have a far longer reach than ξN ≈ 1 μm, possibly reaching macroscopic scales. Results revealed signatures of a long-range PE in our devices, persisting above 700 μm (200 − 300 ξN ) from the superconducting electrodes - an unusually long distance in bulk systems

Sample preparation

Control samples and the superconducting alloy

Transport measurements

Surface characterization

Current distribution

Discussion