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Register a new userSpin Hall effect in spin-valley coupled monolayer transition-metal dichalcogenides
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We study both the intrinsic and extrinsic spin Hall effect in spin-valley coupled monolayers of transition metal dichalcogenides. We find that whereas the skew-scattering contribution is suppressed by the large band gap, the side-jump contribution is comparable to the intrinsic one with opposite sign in the presence of scalar and magnetic scattering. Intervalley scattering tends to suppress the side-jump contribution due to the loss of coherence. By tuning the ratio of intra- to intervalley scattering, the spin Hall conductivity shows a sign change in hole-doped samples. Multiband effect in other doping regime is considered, and it is found that the sign change exists in the heavily hole-doped regime, but not in the electron-doped regime.
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We study valley-dependent spin transport theoretically in monolayer transition-metal dichalcogenides in which a variety of spin and valley physics are expected because of spin-valley coupling. The results show that the spins are valley-selectively excited with appropriate carrier doping and valley polarized spin current (VPSC) is generated. The VPSC leads to the spin-current Hall effect, transverse spin accumulation originating from the Berry curvature in momentum space. The results indicate that spin excitations with spin-valley coupling lead to both valley and spin transport, which is promising for future low-consumption nanodevice applications.
In this work, we predict the emergence of the valley Edelstein Effect (VEE), which is an electric-field-induced spin polarization effect, in gated monolayer transition metal dichalcogenides (MTMDs). We found an unconventional valley-dependent response in which the spin-polarization is parallel to the applied electric field with opposite spin-polarization generated by opposite valleys. This is in sharp contrast to the conventional Edelstein effect in which the induced spin-polarization is perpendicular to the applied electric field. We identify the origin of VEE as combined effects of conventional Edelstein effect and valley-dependent Berry curvatures induced by coexisting Rashba and Ising SOCs in gated MTMDs. Experimental schemes to detect the VEE are also considered.
Transition metal dichalcogenides have been the primary materials of interest in the field of valleytronics for their potential in information storage, yet the limiting factor has been achieving long valley decoherence times. We explore the dynamics of four monolayer TMDCs (MoS$_2$, MoSe$_2$, WS$_2$, WSe$_2$) using ab initio calculations to describe electron-electron and electron-phonon interactions. By comparing calculations which both omit and include relativistic effects, we isolate the impact of spin-resolved spin-orbit coupling on transport properties. In our work, we find that spin-orbit coupling increases carrier lifetimes at the valence band edge by an order of magnitude due to spin-valley locking, with a proportional increase in the hole mobility at room temperature. At temperatures of 50~K, we find intervalley scattering times on the order of 100 ps, with a maximum value ~140 ps in WSe$_2$. Finally, we calculate excited-carrier generation profiles which indicate that direct transitions dominate across optical energies, even for WSe$_2$ which has an indirect band gap. Our results highlight the intriguing interplay between spin and valley degrees of freedom critical for valleytronic applications. Further, our work points towards interesting quantum properties on-demand in transition metal dichalcogenides that could be leveraged via driving spin, valley and phonon degrees of freedom.
In transition-metal dichalcogenides, electrons in the K-valleys can experience both Ising and Rashba spin-orbit couplings. In this work, we show that the coexistence of Ising and Rashba spin-orbit couplings leads to a special type of valley Hall effect, which we call spin-orbit coupling induced valley Hall effect. Importantly, near the conduction band edge, the valley-dependent Berry curvatures generated by spin-orbit couplings are highly tunable by external gates and dominate over the intrinsic Berry curvatures originating from orbital degrees of freedom under accessible experimental conditions. We show that the spin-orbit coupling induced valley Hall effect is manifested in the gate dependence of the valley Hall conductivity, which can be detected by Kerr effect experiments.
Orbital Hall effect (OHE) is the phenomenon of transverse flow of orbital moment in presence of an applied electric field. Solids with broken inversion symmetry are expected to exhibit a strong OHE due to the presence of an intrinsic orbital moment at individual momentum points in the Brillouin zone, which in presence of an applied electric field, flows in different directions causing a net orbital Hall current. Here we provide a comprehensive understanding of the effect and its tunability in the monolayer 2D transition metal dichalcogenides (TMDCs). Both metallic and insulating TMDCs are investigated from full density-functional calculations, effective $d$-band tight-binding models, as well as a minimal four-band model for the valley points that captures the key physics of the system. For the tuning of the OHE, we examine the role of hole doping as well as the change in the band parameters, which, e. g., can be controlled by strain. We demonstrate that the OHE is a more fundamental effect than the spin Hall effect (SHE), with the momentum-space orbital moments inducing a spin moment in the presence of the spin-orbit coupling, leading to the SHE. The physics of the OHE, described here, is relevant for 2D materials with broken inversion symmetry in general, even beyond the TMDCs, providing a broad platform for future research.
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