|Title||Collective Excitations of Quantum Anomalous Hall Ferromagnets in Twisted Bilayer Graphene|
|Publication Type||Journal Article|
|Year of Publication||2020|
|Authors||F. Wu, and S. Das Sarma|
|Journal||Phys. Rev. Lett.|
|Date Published||JAN 30|
|Type of Article||Article|
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.