Rydberg series of intralayer K-excitons in WSe$_2$ multilayers

TL;DR

This study investigates the Rydberg series of intralayer K-excitons in WSe$_2$ multilayers using magneto-reflectance and theoretical modeling, revealing multiple excitonic series depending on the number of layers.

Contribution

It provides the first detailed analysis of excitonic Rydberg series in multilayer WSe$_2$, showing how the number of series varies with layer thickness.

Findings

Multiple excitonic Rydberg series are observed in multilayers.

The number of Rydberg series increases with the number of layers.

Excitonic resonances originate from different series depending on layer count.

Abstract

Semiconducting transition metal dichalcogenides of group VI are well-known for their prominent excitonic effects and the transition from an indirect to a direct band gap when reduced to monolayers. While considerable efforts have elucidated the Rydberg series of excitons in monolayers, understanding their properties in multilayers remains incomplete. In these structures, despite an indirect band gap, momentum-direct excitons largely shape the optical response. In this work, we combine magneto-reflectance experiments with theoretical modeling based on the $\mathbf {k\cdot p}$ approach to investigate the origin of excitonic resonances in WSe$_2$ bi-, tri-, and quadlayers. For all investigated thicknesses, we observe a series of excitonic resonances in the reflectance spectra, initiated by a ground state with an amplitude comparable to the ground state of the 1$s$ exciton in the monolayer. Higher energy states exhibit a decrease in intensity with increasing energy, as expected for the excited states of the Rydberg series, although a significant increase in the diamagnetic shift is missing in tri- and quadlayers. By comparing the experimental observations with theoretical predictions, we discover that the excitonic resonances observed in trilayers originate from two Rydberg series, while quadlayers exhibit four such series, and bilayers host a single Rydberg series similar to that found in monolayers.