Transition metal dichalcogenides are useful for investigating the manifestations of spin valley physics under external stimulation. A study published in the New Journal of Physics explored the effect of strain on orbital angular momentum, Berry curvatures and effective g-factors using the ab initio method.
Study: First-Principles Insights into the Spin Valley Physics of Strained Transition Metal Dichalcogenide Monolayers. Image credit: Andrey Keno/Shutterstock.com
The results revealed an unexpected decrease in the spin expectation value of the conduction band under compressive strain in the K valleys, increasing the dipole strength of the dark exciton by more than an order of magnitude. Furthermore, the direct exciton g-factors under strain revealed that with an increase in tensile stress, the absolute value of the g-factors increased.
One percent variation in voltage changed the bright exciton g-factors by about 0.3 and 0.2 for tungsten (W) and molybdenum (Mo), and for the exciton g-factors dark, was about 0.5 and 0.3 for W and Mo, respectively. Conducting magneto-optical experiments helped to visualize these predictions in the strained sample at low temperatures. Calculations suggested that the strain effect was a possible cause of the g-factor fluctuations.
Furthermore, comparison of different transition metal dichalcogenides revealed the direct correlation between spin-orbit coupling (SOC) and spin valley. Under the applied voltage, the sensitivity of the spin valley features increased with SOC. Thus, monolayer tungsten selenide (WSe2) was a suitable material to investigate the role of strain in spin valley physics due to its high SOC.
Transition metal dichalcogenides
Transition metal dichalcogenides are van der Waals materials that enable fundamental and applied physics research in electronics, optoelectronics, spintronics, optospintronics and valleytronics. Monolayer transition metal dichalcogenides with hexagonal crystal structure and optical band are direct semiconductors with electrons and holes located at the first K points of the Brillouin zone and exciton signatures in the optical spectrum.
The lack of inversion symmetry of the crystal lattice and the presence of heavy metal elements mark a strong SOC physics in the K valleys through spin polarization in the out-of-plane direction. Thus, hole and electron spin valley blocking allows selective excitation of exciton quasi-particles that decay from the K or -K valley.
To this end, magneto-optical spectroscopy helps to explore the spin valley physics of holes, electrons and excitons in monolayer transition metal dichalcogenides. A splitting of the Zeeman valley is observed due to the lifting of the degeneracy of the K and -K valleys under an external magnetic field.
Although excitonic spectra measure the g-factor of the exciton, simultaneously accounting for hole and electron contributions. Additional emission peaks are required to estimate the individual contributions of the valence and conduction bands in transition metal dichalcogenides.
In addition to thorn valley physics, transition metal dichalcogenides are suitable materials for strattronics. Applying a controllable voltage to it can tune the optical emission energy of the exciton by several hundred millielectronvolts. Furthermore, the strain suppresses non-radiative exciton recombination, preserving the photoluminescence quantum yield close to unity.
Spin-Valley Physics of Deformed Transition Metal Dichalcogenides
The present study investigated transition metal dichalcogenides with hexagonal crystal structures for their spin valley physics under applied stress. Previously, multiple phonon-mediated emission peaks were used to unravel the valence and g-band factors whose measurements agreed with first-principles calculations. Here, first-principles calculations helped to assess the Bloch contribution to the band g-factors.
In the current work, first-principles calculations helped to evaluate the spin and orbital angular momentum, 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 stress showed an unexpected spin-mixing regime for the conduction band with spin-down electrons. Direct excitons originating from the low energy bands of the K valley (dark excitons) revealed two trends in the Zeeman effect.
An increasing trend in the absolute value of the g-factor was observed for the positive strain value. On the other hand, a decreasing trend was observed in the absolute value of the factor for the negative strain value. Among the various trends exhibited by transition metal dichalcogenides, the largest SOC effect made 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 the g-factor characteristics in strained transition metal dichalcogenides, where connecting the dipole matrix elements to the g-factor trends revealed that the dipole strength of dark excitons was modified as a function of spin mixing.
Conclusion
To conclude, transition metal dichalcogenides were explored to study their spin valley physics under biaxial strain. Various transition metal dichalcogenides with hexagonal crystal structures were used to analyze orbital angular moments, spin mixing, gi factors and Berry curvatures. The results revealed compressive strain-dependent spin mixing features in the K valleys.
Furthermore, symmetry analysis of the energy bands and the SOC Hamiltonian revealed that the mechanism behind the reduction of the spin (Sz) value in the K valley was based on the spin-flip coupling between the spin-down driving band and spin-up driving band.
The present study established the impact of strain on the spin valley properties of monolayer transition metal dichalcogenides. In addition to the information on these systems in which the many effects competed with the strain, the study helps to investigate proximity effects and interlayer 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 Transition Metal Dichalcogenide Monolayers. New Journal of Physics. https://iopscience.iop.org/article/10.1088/1367-2630/ac7e21
Disclaimer: The views expressed here are those of the author expressed privately and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and Conditions of Use for this website.