Electron Correlations Study toward Understanding Twisted Bilayer Graphene: Two-Orbital Hubbard Model and Exact Numerical Calculations

We investigated the honeycomb lattice using a two-orbital (α and β) Hubbard model and many-body exact calculations. The spin gap (Δs), charge gap (Δc), intrasite (γiiαβ=⟨S→iα·S→iβ⟩), intersite (γijαβ=⟨S→iα·S→jβ⟩), spin–spin correlations, and orbital magnetic moments μiα are analyzed for different band-filling (n), which denotes the number of electrons per site. Special attention was set in one-quarter-filled (n = 1) and half-filled (n = 2) bands. The results are presented by varying Coulomb repulsion (U), Hund’s coupling (JH), external magnetic field (B), and orbital energy splitting (Δϵ). For the one-quarter-filled band, an intrasite magnetic disorder (γiiαβ≈0) begins to form for large values of U/t, which is interpreted as a possible signature of a resonating valence bond state. Full orbital polarization (γiiαβ=0.25) with intersite antiferromagnetic (AFM) order (γijαβ=−0.25) was observed in the half-filled band for U/t ≫ 1, indicating that the two-orbital Hubbard model can be mapped to an AFM Heisenberg model with S = 1. The increase of JH is conductive to insulating and metallic phases for half- and one-quarter-filled bands, respectively. For a fractional n, the honeycomb lattice can exhibit spin-glass-like behavior, predominating as a disordered magnetic system characterized by random values of γijαβ. The magnetic field induced a reduction in Δc, which became zero at the critical external magnetic field. Twisted bilayer graphene (TBG) exhibits approximately flat electronic bands at magic angles and interesting correlated phenomena, such as unconventional superconductivity and correlated insulating states around n = 1 and n = 2, respectively. The dominance of electron–electron interactions in TBG makes the Hubbard model, in various versions (extended or multi-orbital), offer a practical, structure-scale account for investigating many-body effects in large moiré unit cells, where first-principles density-functional theory calculations are computationally challenging.