Superconductor to metal quantum phase transition with magnetic field in Josephson coupled lead islands on Graphene

TL;DR

This study investigates the superconductor-to-metal quantum phase transition in a Josephson junction array on graphene, revealing a gate-tunable transition belonging to the (2+1)D-XY universality class.

Contribution

It provides experimental evidence of a gate-controlled quantum phase transition in a graphene-based Josephson array, modeled by the Werthamer-Helfand-Hohenberg theory and finite size scaling analysis.

Findings

Superconductivity onset temperature fits WHH theory.

Magnetoresistance shows thermally activated flux flow.

Quantum phase transition is gate-dependent and belongs to (2+1)D-XY class.

Abstract

Superconductor-to-metal transition with magnetic field and gate-voltage is studied in a Josephson junction array comprising of randomly distributed lead islands on exfoliated single-layer graphene with a back-gate. The low magnetic-field superconductivity onset temperature is fitted to the Werthamer-Helfand-Hohenberg theory to model the temperature dependence of the upper critical field. The magnetoresistance in the intermediate temperature and field regime is described using thermally activated flux flow dictated by field dependent activation barrier. The barrier also depends on the gate voltage which dictates the inter-island Josephson coupling and disorder. The magnetoresistance near the upper critical field at low temperatures shows signatures of a gate dependent continuous quantum phase transition between superconductor and metal. The finite size scaling analysis shows that this transition belongs to the $(2+1)$D-XY universality class without disorder.