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

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.

💡 Research Summary

The paper presents a comprehensive experimental and theoretical investigation of the conduction‑band spin‑orbit splitting (SOS) in monolayer MoS₂, a key parameter governing bright‑dark exciton competition and spintronic functionality in transition‑metal dichalcogenides. Using a dual‑gated, hBN‑encapsulated MoS₂ device, the authors measured four‑terminal resistance under magnetic field and extracted Shubnikov‑de Haas oscillations (SdHO). At low carrier densities the oscillations are described by a single spin‑split band with an effective mass mₗ ≈ 0.65 mₑ, significantly larger than the DFT‑predicted mₗ₀ ≈ 0.43 mₑ. By Fourier analysis of the SdHO as a function of total density, they identified a clear threshold density n* = 4.2 × 10¹² cm⁻² at which the upper spin‑split conduction band begins to populate. This threshold is an order of magnitude higher than what would be expected from conventional DFT values (Δ₀ ≈ 3 meV, mₗ₀ ≈ 0.43 mₑ), indicating a serious discrepancy.

To resolve the mismatch, the authors first consider many‑body exchange enhancement of the bare SOS. They compute the screened exchange self‑energy Σ(k,ω) using the Keldysh potential for 2D dielectric screening (ε ≈ 4.6) and a static Lindhard polarisation bubble Π(q,0). By expanding Π in the Wigner‑Seitz radius rₛ up to second order (Π ≈ mₗ₀/πħ²