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Collective Excitations of Quantum Anomalous Hall Ferromagnets in Twisted Bilayer Graphene

TitleCollective Excitations of Quantum Anomalous Hall Ferromagnets in Twisted Bilayer Graphene
Publication TypeJournal Article
Year of Publication2020
AuthorsF. Wu, and S. Das Sarma
JournalPhys. Rev. Lett.
Volume124
Pagination046403
Date PublishedJAN 30
Type of ArticleArticle
ISSN0031-9007
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.

DOI10.1103/PhysRevLett.124.046403