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arXiv · 1908.05417

Collective Excitations of Quantum Anomalous Hall Ferromagnets in Twisted Bilayer Graphene

Fengcheng Wu·Sankar Das Sarma
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Abstract

We present a microscopic theory for collective excitations of quantum anomalous Hall ferromagnets (QAHF) in twisted bilayer graphene. We calculate the spin magnon and valley magnon spectra by solving Bethe-Salpeter equations, and verify the stability of QAHF. We extract the spin stiffness from the gapless spin wave dispersion, and estimate the energy cost of a skyrmion-antiskyrmion pair, which is found to be comparable in energy with the Hartree-Fock gap. The valley wave mode is gapped, implying that the valley polarized state is more favorable compared to the valley coherent state. Using a nonlinear sigma model, we estimate the valley ordering temperature, which is considerably reduced from the mean-field transition temperature due to thermal excitations of valley waves.

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Fengcheng Wu, Sankar Das Sarma. 2019-11-19. Collective Excitations of Quantum Anomalous Hall Ferromagnets in Twisted Bilayer Graphene. https://doi.org/10.1103/physrevlett.124.046403

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