Incipient Berezinskii-Kosterlitz-Thouless transition in two-dimensional coplanar Josephson junctions

TL;DR

This paper reports the observation of an incipient Berezinskii-Kosterlitz-Thouless transition in two-dimensional graphene-based Josephson junctions, revealing unique hysteretic supercurrent behavior linked to low-dimensional vortex physics.

Contribution

It demonstrates the realization of a quasi-ideal 2D Josephson junction with graphene and provides evidence of BKT transition effects in such hybrid devices, including hysteresis phenomena.

Findings

Evidence of Josephson coherence below 400 nm graphene gaps

Observation of hysteretic collapse and revival of supercurrent

Partial explanation of hysteresis using a modified Bean Critical State model

Abstract

Superconducting hybrid junctions are revealing a variety of novel effects. Some of them are due to the special layout of these devices, which often use a coplanar configuration with relatively large barrier channels and the possibility of hosting Pearl vortices. A Josephson junction with a quasi ideal two-dimensional barrier has been realized by growing graphene on SiC with Al electrodes. Chemical Vapor Deposition offers centimeter size monolayer areas where it is possible to realize a comparative analysis of different devices with nominally the same barrier. In samples with a graphene gap below 400 nm, we have found evidence of Josephson coherence in presence of an incipient Berezinskii-Kosterlitz-Thouless transition. When the magnetic field is cycled, a remarkable hysteretic collapse and revival of the Josephson supercurrent occurs. Similar hysteresis are found in granular systems and are usually justified within the Bean Critical State model (CSM). We show that the CSM, with appropriate account for the low dimensional geometry, can partly explain the odd features measured in these junctions.