Molecular Pairing in Twisted Bilayer Graphene Superconductivity

Molecular Pairing in Twisted Bilayer Graphene Superconductivity

August 7, 2024 | Yi-Jie Wang, Geng-Dong Zhou, Shi-Yu Peng, Biao Lian, Zhi-Da Song
This paper presents a theory explaining how superconductivity emerges in magic-angle twisted bilayer graphene (MATBG) despite strong Coulomb repulsion. The authors propose that phonon-mediated interactions, specifically $ J_A = 1-4 $ meV, overcome the Coulomb repulsion $ U = 30-60 $ meV to enable superconductivity. They compare this mechanism to that in the $ A_3C_{60} $ family of molecular superconductors, where optical phonons dynamically lift the degeneracy of moiré orbitals, similar to the dynamical Jahn-Teller effect. The resulting $ J_A $ acts like an anti-Hund's coupling and is less suppressed by Kondo screening near a Mott insulator. Additionally, an intra-orbital Hund's coupling $ J_H $ is considered, which arises from on-site repulsion of carbon atoms. The authors show that the renormalized local interaction between quasi-particles has a pairing (negative) channel in a doped correlated insulator at $ \nu = \pm(2+\delta\nu) $, despite the bare interaction being positive definite. This is proven non-perturbatively using exact asymptotic behaviors of the vertex function imposed by Ward identities. The paper also predicts an optimal $ U $ for superconductivity and finds that the ground state has a nematic d-wave singlet pairing, which can lead to a p-wave-like nodal structure due to Berry's phase on Fermi surfaces. The study highlights the role of phonon interactions in overcoming Coulomb repulsion and enables superconductivity in MATBG.This paper presents a theory explaining how superconductivity emerges in magic-angle twisted bilayer graphene (MATBG) despite strong Coulomb repulsion. The authors propose that phonon-mediated interactions, specifically $ J_A = 1-4 $ meV, overcome the Coulomb repulsion $ U = 30-60 $ meV to enable superconductivity. They compare this mechanism to that in the $ A_3C_{60} $ family of molecular superconductors, where optical phonons dynamically lift the degeneracy of moiré orbitals, similar to the dynamical Jahn-Teller effect. The resulting $ J_A $ acts like an anti-Hund's coupling and is less suppressed by Kondo screening near a Mott insulator. Additionally, an intra-orbital Hund's coupling $ J_H $ is considered, which arises from on-site repulsion of carbon atoms. The authors show that the renormalized local interaction between quasi-particles has a pairing (negative) channel in a doped correlated insulator at $ \nu = \pm(2+\delta\nu) $, despite the bare interaction being positive definite. This is proven non-perturbatively using exact asymptotic behaviors of the vertex function imposed by Ward identities. The paper also predicts an optimal $ U $ for superconductivity and finds that the ground state has a nematic d-wave singlet pairing, which can lead to a p-wave-like nodal structure due to Berry's phase on Fermi surfaces. The study highlights the role of phonon interactions in overcoming Coulomb repulsion and enables superconductivity in MATBG.
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