Spatial Separation of Carrier Spin by the Valley Hall Effect in Monolayer WSe₂ Transistors

Abstract

We investigate the valley Hall effect (VHE) in monolayer WSe2 field-effect transistors using optical Kerr rotation measurements at 20 K. While studies of the VHE have so far focused on n-doped MoS2, we observe the VHE in WSe2 in both the n- and p-doping regimes. Hole doping enables access to the large spin-splitting of the valence band of this material. The Kerr rotation measurements probe the spatial distribution of the valley carrier imbalance induced by the VHE. Under current flow, we observe distinct spin-valley polarization along the edges of the transistor channel. From analysis of the magnitude of the Kerr rotation, we infer a spin-valley density of 44 spins/μm, integrated over the edge region in the p-doped regime. Assuming a spin diffusion length less than 0.1 μm, this corresponds to a spin-valley polarization of the holes exceeding 1%.

Light-field and spin-orbit-driven currents in van der Waals materials

“…However, due to the locking of spin and valley in monolayer TMDs, this charge neutral Hall current results in the accumulation of an opposite spin polarization at opposite edges ( Figure 3C), which is also called a spin Hall effect. Such a spin-valley accumulation has been confirmed by magneto-optical Kerr microscopy both for electrons and holes in gate-tuned monolayer WSe 2 [19]. The spin Hall effects is not unique to strictly spin-valley coupled systems, but it is a general consequence of spinorbit interaction [148].…”

“…In the monolayer limit, these prototypical TMDs are directgap semiconductors, whose optical properties are dominated by many-body exciton physics, even at room temperature [15,16]. Due to their strong spin-orbit coupling, monolayer TMDs inherently intertwine angular momentum, out-of-plane spin, and crystal momentum degrees of freedom, such that under polarized optical excitation directed spin and charge currents can emerge [17][18][19][20][21]. Directly after a pulsed photoexcitation, the presence of a large density of (photogenerated) charge carriers can alter the Coulomb screening in monolayer TMDs [22][23][24][25][26][27][28][29], such that both the quasi-particle band gap and the excitonic binding energies are renormalized on femtosecond time scales.…”

Control of light–valley interactions in 2D transition metal dichalcogenides with nanophotonic structures

“…19,27 Such information could be further modulated using an out-of-plane magnetic field (the valley Zeeman effect) 34,54 or electrical gating. 55 The valley pseudospin can be read out by measuring the valley-dependent Hall voltage 28,29 or observing the circular polarization of the PL emission. 19,27 Fig.…”

“…valley polarization, in TMDCs can be introduced by spin injection 26 or optical excitation with circularly polarized light. 19,27 Besides the encoding methods, the valley information can be read out through the valley Hall effect 28,29 or circularly polarized photoluminescence (PL) measurements. 19,27,30 However, the valley contrasting signals of single layer TMDCs are severely weakened by phononassisted intervalley scattering 31,32 as the temperature increases and vanish at room temperature.…”

Experimental observation of coupled valley and spin Hall effect in p‐doped WSe2 devices

“…Several optical measurements such as pump-probe using circularly polarized light [19][20][21][22] and Kerr rotation microscopy 23 have shown ultra-long valley life time in TMD and their heterostructures and associate that to long spin life time based on the spin-locking theory 10,11 . However, in these measurements, the spin and valley degrees of freedom are convoluted, thus do not directly probe the spin polarization.…”