We study the collective particle-hole modes of the correlated insulating state in magic-angle twisted bilayer graphene at -3/4 filling within the time-dependent Hartree-Fock approximation, using a continuum model. We show that the fully spin and valley polarized Hartree-Fock ground state is stable to collective mode excitations and find seven independent, relatively flat, low-energy modes within the flat-band subspace, which lie well below the charge gaps throughout the moire Brillouin zone. This indicates the presence of an effective SU(8) spin-valley-band degree of freedom per moire unit cell at low energies.

Ajesh Kumar

University of Texas