Spatial extent of the excited exciton states in WS$_2$ monolayers from diamagnetic shifts

TL;DR

This study measures the sizes of excitons in WS2 monolayers using magneto-optical spectroscopy, confirming their Wannier-Mott nature and supporting hydrogen-like models for these quasiparticles.

Contribution

The paper provides experimental diamagnetic shift measurements to determine exciton radii in WS2 monolayers, validating theoretical models and clarifying exciton properties.

Findings

Exciton radii are 2 nm for ground state and 5-8 nm for excited state.

Ground and excited states have similar g factors around -4.

Results support the Wannier-Mott exciton model in monolayer WS2.

Abstract

We experimentally study the radii of excitons in hBN-encapsulated WS2 monolayers by means of magneto-optical reflectance spectroscopy at cryogenic temperatures in magnetic fields up to 29 T. We observe field-induced energy shifts of the exciton ground and excited states due to valley Zeeman and diamagnetic effects. We find the g factor of the first excited state of $-4.2(+/-0.1) to be essentially equal to that of the ground state of -4.35(+/-0.1). From diamagnetic shifts we determine the root mean square radii of the excitons. The radius of the first excited state is found to be 5-8 nm and that of the ground state around 2 nm. Our results further confirm the Wannier-Mott nature of the exciton quasiparticles in monolayer semiconductors and the assignment of the optical resonances in absorption-type measurements. They also provide additional support for the applicability of the effective mass hydrogenlike models in these systems.