Reentrant Flat Bands with Nontrivial Topology and Electronic Correlations in Artificial Rhombohedral Trilayer Graphene.

Rhombohedral trilayer graphene provides a platform for exploring exotic quantum phases arising from the interplay of electron correlations and band topology. The development of strategies to engineer rhombohedral trilayer graphene with flat bands is important for both fundamental studies of correlated topological states and potential device applications. Here, we demonstrate that a slight rotation of monolayer graphene relative to a bilayer offers an effective approach to achieving this goal. In minimally twisted monolayer-bilayer graphene (mtMBG), significant structural relaxation gives rise to stable, large-scale ABA and ABC domain arrays. When the twist angle falls below a critical threshold, the ABC regions exhibit electronic properties akin to those of rhombohedral trilayer graphene, featuring robust and spatially uniform flat bands with tunable layer polarization. At partial filling, the conduction flat band progressively splits into four sub-bands, driven by electron-electron interactions and indicative of quantum phase transitions between correlated states. Moreover, spectroscopic signatures of electronic polarization and topological edge states at the domain walls separating the ABA and ABC regions are observed. These results establish mtMBG as a versatile platform for investigating emergent flat-band phenomena and related electronic states.