Study: First-principles insights into the spin-valve physics of strained transition dichloride monolayers. Image credit: Andrey Keno/Shutterstock.com The results revealed an unexpected decrease in the expected conduction band spin value under compressive strain in the K valleys, increasing the dark exciton dipole strength by more than one order of magnitude. Furthermore, the direct exciton g-factors under tension revealed that with increasing tensile stress, the absolute value of the g-factors increased. One percent variation in strain modified the bright exciton g-factors by about 0.3 and 0.2 for tungsten (W) and molybdenum (Mo), and for the dark exciton g-factors, they were about 0.5 and 0.3 for W and Mo, respectively. Conducting magneto-optical experiments helped to visualize these predictions in the stretched sample at low temperatures. Calculations suggested that the distortion effect was a possible cause of the g-factor variations. Moreover, the comparison of different transition metal dichalcogenides revealed the direct correlation between spin-orbit coupling (SOC) and spin-valley. Under applied strain, the sensitivity of 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 investigations in fundamental and applied physics in electronics, optoelectronics, spintronics, opto-spintronics and valetronics. Monolayer transition metal dichalcogenides with hexagonal crystal structure and 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 signal strong physical SOC in the K valleys through spin polarization in the out-of-plane direction. Thus, spin-valley locking of holes and electrons allows selective excitation of quasi-exciton particles that fade from the K or -K valley. To this end, magneto-optical spectroscopy helps to probe the natural spin valley of holes, electrons, and excitons in monolayer transition metal dichalcogenides. Zeeman valley splitting is observed due to degeneracy of K and −K valleys that rise under an external magnetic field. Although exciton spectra measure the exciton g-factor, simultaneously accounting for hole and electron contributions. Additional emission peaks are required to assess the individual contributions of the valence and conduction bands in transition metal dichalcogenides. In addition to spin-valley physics, transition metal dichalcogenides are suitable materials for strassitrons. Applying controlled pressure to them can tune the optical emission energy of the exciton by several hundred millielectrovolts. In addition, the strain suppresses nonradiative exciton recombination, keeping the photoluminescence quantum yield close to unity.
Spin-Valley Physics of Strained Transition Metal Dichalcogenides
The present study investigated transition metal dichalcogenides with hexagonal crystal structures for their spin-valley physics under applied strain. Previously, multiple phonon-mediated emission peaks were used to reveal valence and g-band factors whose measurements were in agreement with first-principles calculations. Here, first-principles calculations helped to evaluate the Bloch contribution to the g-factors of the complex. 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), sulfide tungsten (WS2) and tung 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 downward trend in the absolute value of the factor was observed for the negative strain value. Among the various trends exhibited by transition metal dichalcogenides, the larger SOC effect made WSe2 a suitable material for studying the effect of strain on spin valley physics. While the combination of magneto-optics and strained transition metal dichalcogenides was not present in the previous literature. In this work, magneto-optics was used to probe g-factor features in strained transition metal dichalcogenides, where linking the matrix dipole elements to g-factor stresses revealed that the dipole strength of dark excitons was modified based on mixing with spin
conclusion
In conclusion, transition metal dichalcogenides were investigated to study their spin-valley physics under biaxial strain. Various transition metal dichalcogenides with hexagonal crystal structures were used to analyze orbital angular momentum, spin mixing, g-factors and Berry curvatures. The results revealed compression-dependent mixing characteristics in the K valleys. Additionally, symmetry analysis of the energy bands and SOC Hamiltonian revealed that the mechanism behind the decrease in the spin value (Sz) in the K-valley was based on the spin-flip coupling between the spin-down conduction band and the spin-up conduction band . The present study determined the effect of strain on the spin-valley properties of monolayer transition metal dichalcogenides. In addition to insights into these systems, where multiple effects compete with distortion, the study helps to investigate proximity effects and interlayer excitations in transition metal dichalcogenides and their heterostructures.
Report
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 in his capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork, owner and operator of this website. This disclaimer forms part of the Terms and Conditions of use of this website.
