Tunable Topological Transitions Probed by the Quantum Hall Effect in Twisted Double Bilayer Graphene

The moiré system provides a tunable platform for investigating exotic quantum phases. Particularly, the displacement field D is crucial for tuning the electronic structures and topological properties of twisted double bilayer graphene (TDBG). Here, we present a series of D-tunable topological transitions by the evolution of quantum Hall phases (QHPs) in the valence bands of TDBG. As D increases, we observe the alternating emergence of two distinct quantum Hall regions originating from full-filling and half-filling, which we attribute to the D-induced Lifshitz transition. Moreover, we delve into the remote valence bands of TDBG and observe a transition in the sequence of Landau levels upon the application of D, shifting from 8N + 4 to 8N. This observation, combined with theoretical calculations, unveils an alteration in the Berry phase. Our findings highlight the TDBG as an exemplary platform for understanding the origin of the topological transitions in the graphene-based moiré systems.