Berry Curvature of Low-Energy Excitons in Rhombohedral Graphene

TL;DR

This paper introduces a refined low-energy model for rhombohedral graphene, revealing electrically tunable exciton properties and Berry curvature effects, positioning it as a platform for exploring exciton topology in moiré materials.

Contribution

A new low-energy two-band model for rhombohedral graphene accurately captures band structure and reveals tunable exciton properties and Berry curvature effects.

Findings

Exciton Wannier centers are displaced and can be electrically tuned.

Excitonic Berry curvature affects semiclassical transport.

Potential for detecting excitonic topological modes.

Abstract

We investigate low energy excitons in rhombohedral pentalayer graphene encapsulated by hexagonal boron nitride (hBN/R5G/hBN), focusing on the regime at the experimental twist angle $\theta = 0.77^\circ$ and with an applied electric field. We introduce a new low-energy two-band model of rhombohedral graphene that captures the band structure more accurately than previous models while keeping the number of parameters low. Using this model, we show that the centres of the exciton Wannier functions are displaced from the moir\'e unit cell origin by a quantised amount - they are instead localised at $C_3$-symmetric points on the boundary. We also find that the exciton shift is electrically tunable: by varying the electric field strength, the exciton Wannier centre can be exchanged between inequivalent corners of the moir\'e unit cell. Our results suggest the possibility of detecting excitonic corner or edge modes, as well as novel excitonic crystal defect responses in hBN/R5G/hBN. Lastly, we find that the excitons in hBN/R5G/hBN inherit excitonic Berry curvature from the underlying electronic bands, enriching their semiclassical transport properties. Our results position rhombohedral graphene as a compelling tunable platform for probing exciton topology in moir\'e materials.