Signatures of evanescent transport in ballistic suspended graphene superconductor junctions

TL;DR

This study demonstrates evanescent transport in ballistic suspended graphene Josephson junctions near the Dirac point, confirming theoretical predictions of pseudo-diffusive behavior in ultra-clean graphene devices.

Contribution

First experimental observation of evanescent transport in ballistic suspended graphene Josephson junctions near the Dirac point, validating longstanding theoretical models.

Findings

Transition from ballistic to evanescent transport below 10^10 cm^-2 carrier density

Pronounced sub-harmonic gap structures near the Dirac point

Sharp decrease in normalized excess current at low carrier densities

Abstract

In Dirac materials, the low energy excitations behave like ultra-relativistic massless particles with linear energy dispersion. A particularly intriguing phenomenon arises with the intrinsic charge transport behavior at the Dirac point where the charge density approaches zero. In graphene, a 2-D Dirac fermion system, it was predicted that charge transport near the Dirac point is carried by evanescent modes, resulting in unconventional pseudo-diffusive charge transport even in the absence of disorder. In the past decade, experimental observation of this phenomenon remained challenging due to the presence of strong disorder in graphene devices which limits the accessibility of the low carrier density regime close enough to the Dirac point. Here we report transport measurements on ballistic suspended graphene-Niobium Josephson weak links that demonstrate a transition from ballistic to pseudo-diffusive like evanescent transport below a carrier density of $(10^{10} cm^{-2})$.Approaching the Dirac point, the sub-harmonic gap structures due to multiple Andreev reflections display a strong Fermi energy-dependence and become increasingly pronounced, while the normalized excess current through the superconductor-graphene interface decreases sharply. Our observations are in qualitative agreement with the longstanding theoretical prediction for the emergence of evanescent transport mediated pseudo-diffusive transport in graphene.