Transition metal dichalcogenides are useful in studying the manifestations of spin-valley physics under external stimulus. A study published in the New Journal of Physics investigated the effect of strain on orbital angular momenta, Berry curvatures and effective g-factors using the ab initio method.
Research: First-Principles Insights into the Spin-Valley Physics of Strained Monolayers of Transition Metal Dichalcogenides. Image credit: Andrei Keno/Shutterstock.com
The results revealed an unexpected decrease in the conduction band spin expectation upon compressive strain in K-valleys, increasing the dark exciton dipole strength by more than an order of magnitude. Furthermore, the g-factors of the direct exciton under tension reveal that as the tensile stress increases, the absolute value of the g-factors increases.
A one percent variation in voltage changes the bright exciton g-factors by approximately 0.3 and 0.2 for tungsten (W) and molybdenum (Mo), and the dark exciton g-factors by approximately 0.5 and 0.3 for W and Mo, respectively. Conducting magneto-optical experiments helped visualize these predictions in the strained sample at low temperatures. The calculations suggest that the voltage effect is a possible cause of the g-factor fluctuations.
Moreover, comparing different transition metal dichalcogenides revealed the direct correlation between spin-orbit coupling (SOC) and spin-valley. Under applied voltage, the sensitivity of the spin-valley characteristics increases with SOC. Thus, monolayer tungsten selenide (WSe2) was a suitable material to study the role of strain on spin-valley physics due to its high SOC.
Dichalcogenides of transition metals
Transition metal dichalcogenides are van der Waals materials that enable fundamental and applied physics research in electronics, optoelectronics, spintronics, opto-spintronics and valetronics. Single-layer transition metal dichalcogenides with a hexagonal crystal structure and an optical band gap are direct semiconductors with electrons and holes localized at the first K-points of the Brillouin zone and exciton signatures in the optical spectra.
The lack of inversion symmetry of the crystal lattice and the presence of heavy metal elements mark a strong SOC physics in K-valleys by spin polarization in the out-of-plane direction. Thus, spin-valley locking of holes and electrons allows selective excitation of excitonic quasiparticles disappearing from the K or -K valley.
To this end, magneto-optical spectroscopy helps to probe the spin-valley physics of holes, electrons, and excitons in monolayer transition metal dichalcogenides. Zeeman splitting of the valley is observed due to the lifting of the degeneracy of the K and −K valleys under an external magnetic field.
Although exciton spectra measure the g-factor of the exciton while simultaneously accounting for the hole and electron contributions. Additional emission peaks are needed to assess the individual contributions of the valence and conduction bands in the transition metal dichalcogenides.
In addition to spin-valley physics, transition metal dichalcogenides are suitable materials for straintronics. Applying a controllable voltage across them can tune the optical emission energy of the exciton by several hundreds of millielectronvolts. In addition, the voltage suppresses nonradiative exciton recombination, keeping the photoluminescence quantum yield close to unity.
Spin-valley physics of transition metal dichalcogenides with strain
The present study investigates transition metal dichalcogenides with hexagonal crystal structures for their spin-valley physics under applied voltage. Previously, multiple phonon-mediated emission peaks were used to unravel the valence and bandgap g-factors, whose measurements were consistent with first-principles calculations. Here, first-principles calculations helped to estimate the Bloch contribution to the bandgap g-factors.
In the current work, first-principles calculations helped to estimate the spin and orbital angular moments, effective g-factors, and Berry curvatures of molybdenum (MoS2), molybdenum selenide (MoSe2), molybdenum telluride (MoTe2), tungsten sulfide (WS2), and tungsten selenide (WSe2).
The K-valley under compressive strain showed an unexpected spin-mixing regime for the conduction band with spin-down electrons. Direct excitons originating from the low-energy K-valley bands (dark excitons) reveal two trends in the Zeeman effect.
There is an increasing trend in the absolute value of the g-factor for positive strain value. On the other hand, there is a tendency to decrease the absolute value of the factor for the value of negative deformation. Among the various trends exhibited by transition metal dichalcogenides, the larger SOC effect makes WSe2 a suitable material to study the strain effect on spin-valley physics.
While the previous literature lacked the combination of magneto-optics and strained transition metal dichalcogenides. In this work, magneto-optics was used to investigate g-factor characteristics in strained transition metal dichalcogenides, where coupling of the dipole matrix elements to g-factor trends revealed that the dipole strength of dark excitons is modified based on spin -mixing.
Conclusion
In conclusion, transition metal dichalcogenides were investigated to investigate their spin-valley physics under biaxial strain. Several transition metal dichalcogenides with hexagonal crystal structures were used to analyze orbital angular moments, spin-mixing, g-factors, and Berry curvatures. The results revealed compressive stress-dependent spin characteristics in K-valleys.
Furthermore, analysis of the symmetry of the energy bands and the SOC Hamiltonian reveals that the mechanism behind the decrease in spin value (Sz) in the K-valley is based on spin-flip coupling between the spin-down conduction band and the spin-up conduction band .
The present study established the effect of voltage on the spin-valley properties of monolayer transition metal dichalcogenides. In addition to insight into these systems where many effects compete with strain, the study helps to investigate proximity effects and interfacial excitons in transition metal dichalcogenides and their heterostructures.
reference
Junior, PEF, Zollner, K., Woźniak, T., Kurpas, M., Gmitra, M., Fabian, J. (2022). First-Principles Insights into the Spin Valley Physics of Strained Monolayers of Transition Metal Dichalcogenides. New Journal of Physics. https://iopscience.iop.org/article/10.1088/1367-2630/ac7e21
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