Weak-coupling theory of magic-angle twisted bilayer graphene

Strong correlations occur in magic-angle twisted bilayer graphene (MATBG) when the octet of flat moiré minibands centered on charge neutrality (CN) is partially occupied. The octet consists of a single valence band and a single conduction band for each of four degenerate spin-valley flavors. Motivated by the importance of Hartree electrostatic interactions in determining the filling-factor-dependent band structure, we use a time-dependent Hartree approximation to gain insight into electronic correlations. We find that the electronic compressibility is dominated by Hartree interactions, that paramagnetic states are stable over a range of density near CN, and that the dependence of energy on flavor polarization is strongly overestimated by mean-field theory.