Twisted Type-II Rashba Homobilayers: A New Platform for Tunable Topological Moiré Flat Bands

The recent discovery of topological flat bands in twisted transition metal dichalcogenide homobilayers and multilayer graphene has sparked significant research interest. This paper proposes a new platform for realizing tunable topological moiré flat bands: twisted type-II Rashba homobilayers. By maintaining centrosymmetry, the interplay between Rashba spin-orbit coupling and interlayer interactions will create a gapped Dirac cone, accompanied by non-zero Berry curvature. Using twisted BiTeI bilayers as an example, we predict the emergence of flat bands with a remarkably narrow bandwidth (below 20 meV). Notably, as the twist angle increases, the system undergoes a transition from a valley Hall insulator to a quantum spin Hall insulator. This transition arises from the competition between twisting-driven effective spin-orbit coupling and sublattice onsite energies in type-II Rashba moiré structures. The high tunability of Rashba materials in terms of spin-orbit coupling strength, interlayer interaction, and twisting angle expands the range of materials suitable for realizing and manipulating correlated topological properties.