Inducing superconductivity in bilayer graphene by alleviation of the Stoner blockade

Abstract

External magnetic fields conventionally suppress superconductivity, both by orbital and paramagnetic effects. A recent experiment has shown that in a Bernal stacked bilayer graphene system, the opposite occurs -- a finite critical magnetic field is necessary to observe superconducting features occurring in the vicinity of a magnetic phase transition. We propose an extraordinary electronic-correlation-driven mechanism by which this anomalous superconductivity manifests. Specifically, the electrons tend to avoid band occupations near high density of states regions due to their mutual repulsion. Considering the nature of spontaneous symmetry breaking involved, we dub this avoidance Stoner blockade. We show how a magnetic field softens this blockade, allowing weak superconductivity to take place, consistent with experimental findings. Our principle prediction is that a small reduction of the Coulomb repulsion would result in sizable superconductivity gains, both in achieving higher critical temperatures and expanding the superconducting regime. Within the theory we present, magnetic field and spin-orbit coupling of the Ising type have a similar effect on the Bernal stacked bilayer graphene system, elucidating the emergence of superconductivity when the system is proximitized to a $\rm WSe_2$ substrate. We further demonstrate in this paper the sensitivity of superconductivity to disorder in the proposed scenario. We find that a disorder that does not violate Anderson's theorem may still induce a reduction of $T_c$ through its effect on the density of states, establishing the delicate nature of the Bernal bilayer graphene superconductor.