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Quantum spin Hall effect from multi-scale band inversion in twisted bilayer Bi$_2$(Te$_{1-x}$Se$_x$)$_3$

Moiré materials have become one of the most active fields in material science in recent years due to their high tunability, and their unique properties emerge from the Moiré-scale structure modulation. Here, we propose twisted bilayer Bi$_2$(Te$_{1-x}$Se$_x$)$_3$ as a new Moiré material where the Moiré-scale modulation induces a topological phase transition. We show, in twisted bilayer Bi$_2$(Te$_{1-x}$Se$_x$)$_3$, a topological insulator domain and a normal insulator domain coexist in the Moiré lattice structure, and edge states on the domain boundary make nearly flat bands that dominate the material properties. The edge states further contribute to a Moiré-scale band inversion, resulting in Moiré-scale topological states. There are corresponding Moiré-scale edge states and they are so to speak "edge state from edge state", which is a unique feature of twisted bilayer Bi$_2$(Te$_{1-x}$Se$_x$)$_3$. Our result not only proposes novel quantum phases in twisted bilayer Bi$_2$Te$_3$-family, but also suggests the twisting of stacking sensitive topological materials paves an avenue in the search for novel quantum materials and devices.