Refined DFT recipe and renormalisation of band-edge parameters for electrons in monolayer MoS$_2$ informed by the measured spin-orbit splitting

TL;DR

This paper refines DFT calculations to accurately predict the conduction band-edge spin-orbit splitting in monolayer MoS$_2$, aligning theoretical results with experimental measurements and elucidating the orbital contributions involved.

Contribution

The study introduces a refined DFT+U+V approach with optimized parameters to better match experimental SOS values in monolayer MoS$_2$, improving understanding of its electronic structure.

Findings

Measured SOS exceeds DFT predictions by an order of magnitude.

Refined DFT+U+V approach achieves close agreement with experimental SOS.

Orbital contributions from sulfur p and molybdenum d orbitals are crucial for accurate predictions.

Abstract

Conduction band-edge spin-orbit splitting (SOS) in monolayer transition metal dichalcogenides determines a competition between bright and dark excitons and sets conditions for spintronics applications of these semiconductors. Here, we report the SOS measurement for electrons in monolayer MoS$_2$, found from the threshold density, $n_*$, for the upper spin-orbit-split band population, which exceeds by an order of magnitude the values expected from the conventional density functional theory (DFT). Theoretically, half of the observed value can be attributed to the exchange enhancement of SOS in a finite-density electron gas, but explaining the rest requires refining the DFT approach. As the conduction band SOS in MoS$_2$ is set by a delicate balance between the contribution of sulphur $p_x$ and $p_y$ orbitals and $d_{z^2}-d_{xz}$ and $d_{z^2}-d_{yz}$ mixing in molybdenum, we use a DFT+U+V framework for fine-tuning the orbital composition of the relevant band-edge states. An optimised choice of Hubbard U/V parameters produces close agreement with the experimentally observed conduction band SOS in MoS$_2$, simultaneously resulting in the valence-band SOS and the quasi-particle band gap which are closer to their values established in the earlier-published experiments.