Topological Exciton Bands in Moiré Heterojunctions (1610.03855v2)

Abstract: Moir\'e patterns are common in Van der Waals heterostructures and can be used to apply periodic potentials to elementary excitations. We show that the optical absorption spectrum of transition metal dichalcogenide bilayers is profoundly altered by long period moir\'e patterns that introduce twist-angle dependent satellite excitonic peaks. Topological exciton bands with non-zero Chern numbers that support chiral excitonic edge states can be engineered by combining three ingredients: i) the valley Berry phase induced by electron-hole exchange interactions, ii) the moir\'e potential, and iii) the valley Zeeman field.

• The paper demonstrates that moiré patterns in TMD bilayers induce topological exciton bands with nonzero Chern numbers.
• The paper employs a plane-wave expansion method to reveal twist-angle dependent satellite excitonic peaks in the optical absorption spectrum.
• The paper highlights the potential of engineered moiré heterojunctions for chiral excitonic edge states and advanced optoelectronic applications.

Topological Exciton Bands in Moiré Heterojunctions

The paper conducted in this paper examines the phenomenon of topological exciton bands in moiré heterostructures, specifically focusing on transition metal dichalcogenide (TMD) bilayers such as MoS$_2$ and WS$_2$. The research capitalizes on the emerging understanding of moiré patterns in van der Waals heterostructures, which demonstrate significant potential in altering the electronic properties of these materials.

Research Overview

The authors delve into the optical absorption spectrum of TMD bilayers, demonstrating how it is substantially affected by moiré patterns that vary with the twist angle. These patterns introduce twist-angle dependent satellite excitonic peaks, producing topological exciton bands harboring non-zero Chern numbers. This work integrates three main factors: (i) the valley Berry phase arising from electron-hole exchange interactions, (ii) the moiré potential, and (iii) the valley Zeeman field to engineer these excitonic phenomena.

Theoretical Insights and Numerical Results

• Impact of Moiré Patterns: Moiré patterns manifest in these heterostructures due to small twist angles and lattice mismatches, leading to significant electronic property modulation. The effect is akin to observed phenomena in other graphene-based heterostructures.
• Exciton Modulation: The moiré-induced periodic potential modifies exciton properties, mixing momentum states separated by moiré reciprocal lattice vectors, thus revealing satellite optical absorption peaks. The shifts in these peaks with varying twist angles provide measurable evidence of modified exciton energy-momentum dispersions.
• Topological Band Structure: By computing the exciton bands using a plane-wave expansion, the authors demonstrate the formation of bands characterized by quantized Chern numbers. The establishment of these topological bands, through combined Berry phase effects and the moiré potential, underscores their ability to host chiral excitonic edge states fostering unidirectional transport channels.
• Numerical Calculations: Several twist angles were analyzed, with exciton moiré bands calculated for $\theta = 1^{\circ}$. The results highlight strong twist angle dependencies and the creation of topological bands in substantial parameter spaces, indicating practical feasibility in these systems.

Practical and Theoretical Implications

This exploration carries significant implications for the design of optoelectronic devices based on TMD heterostructures. The advent of tunable excitonic properties via moiré engineering presages advances in quantum information applications, where the manipulation of excitonic edge states could prove advantageous. Furthermore, the potential to routinely achieve topological exciton bands through practical engineering avenues could unlock new paths for researching collective excitations and their applications.

Future Developments

The comprehensive analysis and modeling of moiré excitonic systems can be extended to various TMD compositions and configurations, examining further twist angles and external field modulations. Additionally, exploring heterostructure stability and the impact of environmental factors on moiré patterns will be crucial to advancing device applications. The topological nature of excitonic states suggests possibilities for innovative materials with unique transport properties, prompting ongoing investigations into other low-dimensional materials.

This paper effectively demonstrates that moiré pattern design in TMD bilayers presents a potent methodology for tailoring electronic and optical properties, opening up vistas for fundamental and applied research in solid-state physics and materials science.