AI Article Synopsis
- The study explores collective excitations in quantum anomalous Hall ferromagnets within twisted bilayer graphene by calculating spin and valley magnon spectra using Bethe-Salpeter equations.
- It assesses the stability of quantum anomalous Hall ferromagnets, finding that the energy cost for skyrmion-antiskyrmion pairs is similar to the Hartree-Fock gap, indicating a balance between different energy states.
- The valley wave mode is found to be gapped, making valley polarization more favorable, and the valley ordering temperature is determined to be lower than mean-field predictions due to thermal effects of valley waves.
Article 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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| http://dx.doi.org/10.1103/PhysRevLett.124.046403 | DOI Listing |
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