Observation of magnetism and superconductivity in crystalline graphene multilayers

Bernal bilayer graphene and rhombohedral trilayer graphene are two crystalline allotropes of layered carbon materials. They inherited the simple energy band dispersion and gate tunability of monolayer graphene. In addition, the existence of Van Hove singularities and flat band edges opens up the opportunity for them to host correlation-driven electronic phases. In this talk, I will report our observation of magnetism and superconductivity in these two materials. We found that both of them host gate-tuned spin- and valley-ordered magnetic phases. The interplay of Coulomb interaction, inter-valley scattering, and subtle features of the single-particle energy bands makes the phase diagram rich and complex. More surprisingly, several superconducting phases were observed near some of the phase boundaries. They show distinct responses to external perturbations such as magnetic field and change of temperature, indicating different configurations of the Cooper pairs. Apart from gate-tuned phase transitions, we found that the proximity-induced spin-orbit coupling can alter the phase diagram and stabilize superconducting phases with significantly higher critical temperature. Our observation of the novel phases together with the homogeneity of the crystalline materials make the Bernal bilayer graphene and rhombohedral trilayer graphene ideal prototype materials to study the manybody physics and mechanisms of superconductivity.