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

Superconductor (SC)-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. The superconductivity onset temperature at low magnetic fields 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 SC and metal. The finite size scaling analysis shows that this transition belongs to the (2+1)D-XY universality class without disorder.