Abundant electric-field tunable symmetry-broken states in twisted monolayer-bilayer graphene

Electron-electron correlations can lift the high degeneracies in strong correlated systems, resulting in various symmetry-broken states. Twisted monolayer-bilayer graphene (tMBG) is an especially rich system due to its low crystalline symmetry. Here, we report abundant electric-field tunable symmetry-broken states in tMBG. The ground state at half filling of the conduction flat band is spin- and valley-polarization dominated under positive and negative electric field, respectively, consistent with our theoretical calculations. In addition, we find a symmetry-broken Chern insulator emanating from 1.5 electrons per moiré unit with C = 3 emerges at high magnetic field in a negative electric field range. The C = 3 suggests that one and a half flavor-polarized Chern 2 bands within the same valley are filled, consistent with the valley-polarization-dominated half-filling state under negative electric field, while the fractional filling stems from a density-wave state held by enlarged unit cells containing two moiré units.