Transition Metal Dichalcogenide Excitons in Periodic Electrostatic Potentials: Center-of-Mass Models

TL;DR

This study models how periodic electrostatic potentials influence exciton properties in 2D transition-metal dichalcogenides, revealing valley splitting and dispersion effects that could enable exciton Bose condensation.

Contribution

It introduces a center-of-mass model to analyze electrostatic effects on excitons in TMDs, highlighting valley splitting and dispersion control.

Findings

Electrostatic potentials induce up to 10 meV valley splitting.

Valley splitting and dispersion are sensitive to potential symmetry.

Linear dispersion near the lowest exciton band may enable Bose condensation.

Abstract

Two-dimensional (2D) van-der-Waals materials are a promising platform for exciton state engineering. In this paper, we study the properties of excitons in 2D group VI transition-metal dichalcogenide (TMD) semiconductors that are modified by a periodic electrostatic potential through the quadratic Stark effect. Using a model that retains only center-of-mass and valley degrees-of-freedom, we find that electrostatic potentials can drive optical valley splitting up to 10meVs and induce valley selective exciton dispersion. We explain why both properties are sensitive to the rotational symmetry of the electrostatic trapping potential using a combination of numerical results and analytical approximations. An important consequence of valley-splitting is that the lowest exciton band is non-degenerate and has a linear dispersion around $\gamma$ that is expected to suppress thermal excitations, allowing true Bose condensation and superfluidity of excitons in two space dimensions.