Giant and Helical Exciton Dipole from Berry Curvature in Flat Chern Bands

TL;DR

This paper demonstrates that excitons in flat Chern bands exhibit large, tunable electric dipoles with helical textures influenced by Berry curvature, enabling control over many-body interactions in 2D materials.

Contribution

It reveals the emergence of giant, helical exciton dipoles in flat Chern bands and shows how electric fields can switch exciton types and dipole textures, advancing topological exciton engineering.

Findings

Excitons have dipole moments around 100 Debye comparable to moiré lattice parameters.

In-plane helical dipole textures arise from Berry curvature in twisted TMDs.

Out-of-plane displacement fields induce a Frenkel-Wannier exciton transition and reverse dipole helicity.

Abstract

We show that excitons forming between moir\'e flat Chern bands possess a substantial electric dipole moment comparable to the moir\'e lattice parameter times the elementary charge ($\sim10^2$ Debye). At a hole filling factor of one in twisted MoTe$_2$, the dipole moment of the lowest-energy exciton branch develops in-plane helical texture in momentum space from the intrinsic Berry curvature of electron and hole. By solving the Bethe-Salpeter equations, we demonstrate that an out-of-plane displacement field induces a Frenkel-to-Wannier exciton transition, accompanied by a reversal of the dipole texture helicity. The resulting attractive exciton dipole-dipole interactions lead to quadrupolar biexcitons that can be probed via two-photon spectroscopy. Our findings establish band topology as a tunable knob to engineer exciton dipole moments and pave the way to manipulate many-body interactions in the terahertz regime.