Dynamics and Spin-Valley Locking Effects in Monolayer Transition Metal Dichalcogenides

Abstract

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, MoSe, WS, WSe) 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 of ∼140 ps in WSe. Finally, we calculate excited-carrier generation profiles which indicate that direct transitions dominate across optical energies, even for WSe 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 toward interesting quantum properties on-demand in transition metal dichalcogenides that could be leveraged via driving spin, valley, and phonon degrees of freedom.

Enhanced electron–phonon scattering in Janus MoSSe

“…Consequently, the short MFPs of MoSSe here are attributed to the out-of-plane dipole moments. Our results also indicate that the MFPs of hot holes in MoSSe and MoS 2 are larger that hot electrons, which means hot holes transport more easily than hot electrons, agrees well with the results of the mobility analysis in [28,[43][44][45][46]. Furthermore, the MFPs along AM direction are much shorter than along ZZ direction, meaning the highly anisotropic propagation of hot carriers in MoSSe and MoS 2 .…”

“…From the mode-resolved scattering rate (figure S2), we expect that below the energy E Qc1 , LO1 phonon mode dominates the scattering in MoSSe while LA and LO2 modes are the main source of the scattering. Here we focus only on the e-ph scattering while the electron-electron scattering is not included, since it is dominant in the total scattering with the energy range far away from the band edges [28].…”

“…The carrier's lifetime and the hot carrier cooling process in materials are dependent strongly on the electronphonon (e-ph) interaction [17][18][19][20][21][22][23][24][25]. The carrier relaxation dynamics of several TMDs are investigated experimentally and theoretically [26][27][28][29][30], which mainly focus on the spin dependent e-ph interaction. Although synthesized successfully, Janus MoSSe structures have been studied mostly on the electronic structures [15,16].…”

Theory of field-modulated spin valley orbital pseudospin physics

“…Such pair of states define a unique "spin-valley-orbital pseudospin", extremely noise-resilient due to strong SOI-induced interlocking among sz, τ, and lz against individual index fluctuations. [29] Experimentally [57][58][59] and theoretically, [60] the valley lifetime of holes is reported to be enhanced over that of electrons by 10-100 times reaching O(10 µs) at 5-10 K. Such advantage fosters quite an exciting promise for pseudospin-based studies, applications at low temperatures such as quantum computing, and also roomtemperature devices such as pseudospin filters and FETs, and has motivated researchers from a wide range of disciplines.…”

“… involving secondary coupling parameters have been ignored. Two points are noted below based on Eqn (60)…”