Fermi Velocity Dependent Critical Current in Ballistic Bilayer Graphene Josephson Junctions

TL;DR

This study investigates how the Fermi velocity influences the critical current in ballistic bilayer graphene Josephson junctions, revealing a gate-dependent Fermi velocity and its impact on device tunability.

Contribution

It introduces a method to determine Fermi velocity from critical current measurements and demonstrates its dependence on gate voltage in bilayer graphene junctions.

Findings

Critical current follows an exponential temperature dependence.

Fermi velocity increases with gate voltage.

Carrier density affects the energy scale in bilayer graphene.

Abstract

We perform transport measurements on proximitized, ballistic, bilayer graphene Josephson junctions (BGJJs) in the intermediate-to-long junction regime ($L>\xi$). We measure the device's differential resistance as a function of bias current and gate voltage for a range of different temperatures. The extracted critical current $I_{C}$ follows an exponential trend with temperature: $ \exp(-k_{B} T/ \delta E)$. Here $\delta E = \hbar \nu_F /2\pi L $: an expected trend for intermediate-to-long junctions. From $\delta E$, we determine the Fermi velocity of the bilayer graphene, which is found to increase with gate voltage. Simultaneously, we show the carrier density dependence of $\delta E$, which is attributed to the quadratic dispersion of bilayer graphene. This is in contrast to single layer graphene Josephson junctions, where $\delta E$ and the Fermi velocity are independent of the carrier density. The carrier density dependence in BGJJs allows for additional tuning parameters in graphene-based Josephson Junction devices.