Magnetism and Superconductivity in Crystalline Few-Layer Graphene

Bernal bilayer and rhombohedral trilayer graphene are two crystalline graphene allotropes. Their energy band structures share similar aspects -- both feature van Hove singularities at the band edges that can be enhanced by an electric field. Tunning the Fermi surface close to the van Hove singularities tends to induce instability and therefore novel electronic phenomena. Combining quantum capacitance measurement and electronic transport measurement, we observed a series of gate-tuned phase transitions associated with spontaneous spin- and valley-symmetry breaking in both the bilayer and trilayer systems. More surprisingly, superconductivity was observed in rhombohedral trilayer graphene near some of the phase boundaries and in Bernal bilayer graphene when an in-plane magnetic field is applied. Although the pairing mechanism remains puzzling, we believe our observation introduces significant new constraints to the theory of superconductivity in graphene systems.