Flavor Ferromagnetism and Superconductivity in Graphene Multilayers

Band states in graphene multilayers have a 4-fold spin x valley flavor degeneracy. Recent experiments have demonstrated that many-electron ground states of graphene multilayers that break flavor degeneracy without breaking translational symmetry are common when correlations are strong in the absence of a magnetic field, just as they are in the quantum Hall regime. I will discuss properties of these flavor ferromagnets, commenting on the Fermi surface reconstructions [1] and soft collective modes [2] that they produce, on the influence of dynamic screening of the long-range Coulomb interaction on the pattern of broken symmetries [3], and on the relationship to superconductivity [4].<br/> <br/>[1] Xie M, MacDonald AH. Weak-Field Hall Resistivity and Spin-Valley Flavor Symmetry Breaking in Magic-Angle Twisted Bilayer Graphene. Physical Review Letters. 2021 Nov 2;127(19):196401.<br/>[2] Kumar A, Xie M, MacDonald AH. Lattice collective modes from a continuum model of magic-angle twisted bilayer graphene. Physical Review B. 2021 Jul 8;104(3):035119.<br/>[3] Zhu Jihang, Torre Iacoppo, Polini Marco, MacDonald AH. GW correlations and flavor ferromagnetism in magic-angle twisted bilayer graphene. (in preparation).<br/>[4] Huang C, Wei N, Qin W, MacDonald AH. Pseudospin Paramagnons and the Superconducting Dome in Magic Angle Twisted Bilayer Graphene. arXiv preprint arXiv:2110.13351. 2021 Oct 26.