Ideal Topological Flat Bands in Two-dimensional Moir\'e Heterostructures with Type-II Band Alignment

TL;DR

This paper proposes a design principle for creating topological flat bands with ideal quantum geometry in 2D moiré heterostructures with type-II band alignment, enabling tunable and potentially exotic quantum phases.

Contribution

It introduces a moiré Chern-band model and a topological heavy fermion model to realize and control ideal topological flat bands in type-II heterostructures, independent of twist angle.

Findings

Topological flat bands can be realized with strong moiré potential exceeding atomic band gap.

Band flatness and quantum geometry are tunable via external gate voltages.

Design strategy is insensitive to twist angle and applicable to various materials.

Abstract

Topological flat bands play an essential role in inducing exotic interacting physics, ranging from fractional Chern insulators to superconductivity, in moir\'e materials. In this work, we propose a design principle for realizing topological flat bands with "ideal quantum geometry", namely the trace of Fubini-Study metric equals to the Berry curvature, in a class of two-dimensional moir\'e heterostructures with type-II band alignment. We first introduce a moir\'e Chern-band model to describe this system and show that topological flat bands can be realized in this model when the moir\'e superlattice potential is stronger than the type-II atomic band gap of the heterostructure. Next, we map this model into a topological heavy fermion model that consists of a localized orbital for "f-electron" and a conducting band for "c-electron". We find that both the flatness and quantum geometry of the flat band in the topological heavy fermion model depend on the energy gap between c-electron and f-electron bands at $\Gamma$ which is experimentally controllable via external gate voltages. This tunability will allow us to realize an ideal topological flat band with zero band-width and ideal quantum geometry. Our design strategy of topological flat bands is insensitive of twist angle. We also discuss possible material candidates for moir\'e heterostructures with type-II band alignment.