Effect of spin-orbit interaction on the excitonic effects in single-layer, double-layer, and bulk MoS2

TL;DR

This study uses advanced ab-initio methods to analyze how spin-orbit interaction influences excitonic effects in single-layer, double-layer, and bulk MoS2, revealing similarities and differences in their optical absorption spectra.

Contribution

It provides a comprehensive, spin-orbit-inclusive analysis of excitonic effects across different MoS2 forms using GW and Bethe-Salpeter calculations.

Findings

Excitonic peaks are split by spin-orbit interaction in monolayer MoS2.

Double-layer and bulk MoS2 spectra are mainly affected by inter-layer interactions.

Single and double-layer MoS2 show strongly bound excitons around 3 eV.

Abstract

We present converged ab-initio calculations of the optical absorption spectra of single-layer, bi-layer, and bulk MoS$_2$. Both the quasiparticle-energy calculations (on the level of the GW approximation) and the calculation of the absorption spectra (on the level of the Bethe-Salpeter equation) explicitly include spin-orbit coupling, using the full spinorial Kohn-Sham wave-functions as input. Without excitonic effects, the absorption spectra would have the form of a step-function, corresponding to the joint-density of states of a parabolic band-dispersion in 2D. This profile is deformed by a pronounced bound excitonic peak below the continuum onset. The peak is split by spin-orbit interaction in the case of single-layer and (mostly) by inter-layer interaction in the case of double-layer and bulk MoS$_2$. The resulting absorption spectra are thus very similar in the three cases but the interpretation of the spectra is different. Differences in the spectra can be seen around 3 eV where the spectra of single and double-layer are dominated by a strongly bound exciton.