Excitons in atomically thin transition metal dichalcogenides

TL;DR

This paper reviews recent advances in understanding excitons in monolayer transition metal dichalcogenides, highlighting their unique optical, electronic, and valleytronic properties driven by strong Coulomb interactions and spin-valley coupling.

Contribution

It provides a comprehensive overview of excitonic phenomena in TMD monolayers, emphasizing the effects of Coulomb interactions, light-matter coupling, and external fields on their properties.

Findings

Strong Coulomb interactions dominate optical properties.

Valley polarization can be tuned with electric and magnetic fields.

Excitonic effects are central to TMD optoelectronic behavior.

Abstract

Atomically thin materials such as graphene and monolayer transition metal dichalcogenides (TMDs) exhibit remarkable physical properties resulting from their reduced dimensionality and crystal symmetry. The family of semiconducting transition metal dichalcogenides is an especially promising platform for fundamental studies of two-dimensional (2D) systems, with potential applications in optoelectronics and valleytronics due to their direct band gap in the monolayer limit and highly efficient light-matter coupling. A crystal lattice with broken inversion symmetry combined with strong spin-orbit interactions leads to a unique combination of the spin and valley degrees of freedom. In addition, the 2D character of the monolayers and weak dielectric screening from the environment yield a significant enhancement of the Coulomb interaction. The resulting formation of bound electron-hole pairs, or excitons, dominates the optical and spin properties of the material. Here we review recent progress in our understanding of the excitonic properties in monolayer TMDs and lay out future challenges. We focus on the consequences of the strong direct and exchange Coulomb interaction, discuss exciton-light interaction and effects of other carriers and excitons on electron-hole pairs in TMDs. Finally, the impact on valley polarization is described and the tuning of the energies and polarization observed in applied electric and magnetic fields is summarized.