First-principles insights into the spin-valley physics of strained transition metal dichalcogenides monolayers

Abstract

Transition metal dichalcogenides (TMDCs) are ideal candidates to explore the manifestation of spin-valley physics under external stimuli. In this study, we investigate the influence of strain on the spin and orbital angular momenta, effective g -factors, and Berry curvatures of several monolayer TMDCs (Mo and W based) using a full ab initio approach. At the K -valleys, we find a surprising decrease of the conduction band spin expectation value for compressive strain, consequently increasing the dipole strength of the dark exciton by more than one order of magnitude (for $\sim 1\%-2\%$ strain variation). We also predict the behavior of direct excitons g -factors under strain: tensile (compressive) strain increases (decreases) the absolute value of g -factors. Strain variations of ∼1% modify the bright (A and B) excitons g -factors by ∼0.3 (0.2) for W (Mo) based compounds and the dark exciton g -factors by ∼0.5 (0.3) for W (Mo) compounds. Our predictions could be directly visualized in magneto-optical experiments in strained samples at low temperature. Additionally, our calculations strongly suggest that strain effects are one of the possible causes of g -factor fluctuations observed experimentally. By comparing the different TMDC compounds, we reveal the role of spin–orbit coupling (SOC): the stronger the SOC, the more sensitive are the spin-valley features under applied strain. Consequently, monolayer WSe _2 is a formidable candidate to explore the role of strain on the spin-valley physics. We complete our analysis by considering the side valleys, Γ and Q points, and by investigating the influence of strain in the Berry curvature. In the broader context of valley- and strain-tronics, our study provides fundamental microscopic insights into the role of strain in the spin-valley physics of TMDCs, which are relevant to interpret experimental data in monolayer TMDCs as well as TMDC-based van der Waals heterostructures.

Keywords

• spin-valley
• valley Zeeman
• spin-mixing
• TMDC
• valleytronics
• straintronics