Observation of a second Dirac point in a graphene/superconductor bilayer

Abstract

Two-dimensional (2D) materials have attracted vast research interest since the breakthrough discovery of graphene. One major benefit of such systems is the ability to tune the Fermi level through the charge neutrality point between electron and hole doping. Here we show that single layer graphene coupled to the low-density superconductor indium oxide (InO) exhibits two charge neutrality points, each of them representing electronic regions in which the carrier density can be tuned from hole to electron dominated. This is not seen in clean graphene or in a bilayer where the carrier density is extremely low. We suggest that the second charge neutrality point results from regions in the graphene layer just below superconducting islands in InO, where pairing is induced via the proximity effect; gating of this hybrid system therefore allows the tuning from hole to electron superconductivity through an ultralow carrier density regime. We propose this as a “superconducting Dirac point (SDP)” where intravalley scattering is greatly enhanced. Our results suggest that the electronic states around SDP behave like those in a strongly coupled superconductor.