In-plane orbital magnetization as a probe for symmetry breaking in strained twisted bilayer graphene

Abstract

Three symmetries prevent a twisted bilayer of graphene from developing an in-plane spontaneous magnetization in the absence of a magnetic field - time reversal symmetry, $C_2$ symmetry to $\pi$ rotation and $C_3$ symmetry to $2\pi /3$ rotation. In contrast, there are experimental and theoretical indications that, at certain electron densities, time reversal and $C_2$ symmetries are broken spontaneously, while the $C_3$ symmetry is often broken due to strain. We show that in-plane orbital magnetization is a very sensitive probe to the simultaneous breaking of these three symmetries, exhibiting surprisingly large values (of the order of one Bohr magneton per moiré unit cell) for valley polarized states at rather small values of heterostrain. We attribute these large values to the large magnitude of the characteristic magnetization of individual Bloch states, which we find to reflect the fast Dirac velocity of single layer graphene, rather than the slow velocity of the twisted bilayer. These large values are forced to mutually cancel in valley-symmetric states, but the cancellation does not occur in valley-polarized states. Our analysis is carried out both for non-interacting electrons and within a simplified Hartree-Fock framework.

arXiv comment17 pages, 9 figures

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Cited by in corpus (7)

• Symmetries as the guiding principle for flattening bands of Dirac fermions
• Valley-polarized nematic order in twisted moiré systems: In-plane orbital magnetism and non-Fermi liquid to Fermi liquid crossover
• Chirality probe of twisted bilayer graphene in the linear transport regime
• Neutral magic-angle bilayer graphene: Condon instability and chiral resonances
• Effects of Boundary on Orbital Magnetization for a Bilayer System with Different Chern Numbers
• Mean-field Modelling of Moiré Materials: A User's Guide with Selected Applications to Twisted Bilayer Graphene
• Magic-angle twisted bilayer graphene under orthogonal and in-plane magnetic fields