Magnetorotons in moir\'e fractional Chern insulators

We perform a comprehensive study of the intraband neutral excitations of the fractional Chern insulators (FCIs) in the moir\'e flatband systems, particularly the twisted transition metal dichalocogenide homobilayers. Utilizing the exact diagonalization, our work provides a detailed description of the magnetorotons in FCIs. We elucidate the nature of the excitations at the long-wavelength limit by recognizing the chiral angular momentum-2 features and find the modes become chiral-mixing when the FCI is less ideal. Curiously, we unravel nonchiral long-wavelength excitations when the FCI transits into the charge density wave (CDW). We conclude with a discussion on potential experimental work for probing the collective neutral excitations in moir\'e flatband systems.