Nematic spin density wave in magic-angle twisted bilayer graphene

We study theoretically many-body properties of magic-angle twisted bilayer graphene for different doping levels. At vanishing interactions, the low-energy spectrum of the system studied consists of four almost-flat almost-degenerate bands. Electron-electron repulsion lifts this degeneracy. To account for such an interaction effect, a numerical mean-field theory is used. Assuming that the ground state has spin-density-wave-like order, we introduce a multicomponent order parameter. Structure of this order parameter depends on the doping level. We show that doping away from the charge neutrality point reduces the rotational symmetry of the ordered state, indicating the appearance of an electron nematic state. Manifestations of the nematicity can be observed in the spatial distribution of the spin magnetization within a moirè cell, as well as in the single-electron band structure. The nematicity is strongest at half-filling (two extra electron or holes per supercell). We argue that nematic symmetry breaking is a robust feature of the system ground state, stable against model parameters variations. [1]

[1] A.O. Sboychakov, A.V. Rozhkov, A.L. Rakhmanov, Franco Nori, arXiv:2005.07749 (2020).