Superconductivity enhanced conductance fluctuations in few layer graphene nanoribbons

TL;DR

This study demonstrates that superconducting contacts significantly enhance conductance fluctuations in few-layer graphene nanoribbons, especially below the superconducting gap, due to phase-coherent Andreev reflection effects.

Contribution

It provides the first detailed measurement of conductance fluctuation enhancement in graphene nanoribbons caused by superconducting proximity effects.

Findings

Conductance fluctuations increase at lower temperatures and voltages.

Enhanced fluctuations are observed below the superconducting gap due to Andreev reflection.

Maximum conductance variance reaches 0.58 e^2/h at 230 mK.

Abstract

We investigate the mesoscopic disorder induced rms conductance variance $\delta G$ in a few layer graphene nanoribbon (FGNR) contacted by two superconducting (S) Ti/Al contacts. By sweeping the back-gate voltage, we observe pronounced conductance fluctuations superimposed on a linear background of the two terminal conductance G. The linear gate-voltage induced response can be modeled by a set of inter-layer and intra-layer capacitances. $\delta G$ depends on temperature T and source-drain voltage $V_{sd}$. $\delta G$ increases with decreasing T and $|V_{sd}|$. When lowering $|V_{sd}|$, a pronounced cross-over at a voltage corresponding to the superconducting energy gap $\Delta$ is observed. For $|V_{sd}|\ltequiv \Delta$ the fluctuations are markedly enhanced. Expressed in the conductance variance $G_{GS}$ of one graphene-superconducutor (G-S) interface, values of 0.58 e^2/h are obtained at the base temperature of 230 mK. The conductance variance in the sub-gap region are larger by up to a factor of 1.4-1.8 compared to the normal state. The observed strong enhancement is due to phase coherent charge transfer caused by Andreev reflection at the nanoribbon-superconductor interface.