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Abstract
The valley Hall effect (VHE) in two-dimensional (2D) van der Waals (vdW) crystals is a promising approach to study the valley pseudospin. Most experiments so far have used bound electron-hole pairs (excitons) through local photoexcitation. However, the valley depolarization of such excitons is fast, so that several challenges remain to be resolved. We address this issue by exploiting a unipolar VHE using a heterobilayer made of monolayer MoS2/WTe2 to exhibit a long valley-polarized lifetime due to the absence of electron-hole exchange interaction. The unipolar VHE is manifested by reduced photoluminescence at the MoS2 A exciton energy. Furthermore, we provide quantitative information on the time-dependent valley Hall dynamics by performing the spatially-resolved ultrafast Kerr-rotation microscopy; we find that the valley-polarized electrons persist for more than 4 nanoseconds and the valley Hall mobility exceeds 4.49 × 103 cm2/Vs, which is orders of magnitude larger than previous reports.
Highlights
The valley Hall effect (VHE) in two-dimensional (2D) van der Waals crystals is a promising approach to study the valley pseudospin
We show that the intrinsic contribution of our unipolar VHE is deeply rooted in Berry curvature of the occupied conduction electrons, whereby we obtained a good agreement on the gate-dependent depolarization time between the theoretical estimation and the ultrafast Kerr-rotation data
We tuned the Fermi level to be located within the MoS2 bandgap (Δ) such that the optical excitation by the ultra-short pump pulse takes place only in WTe2, not in MoS2, i.e., ρ < ε1 < Δ; since the short-pulse pump excitation is spatially separated from the valley Hall transport region, we call it a remote pump
Summary
The valley Hall effect (VHE) in two-dimensional (2D) van der Waals (vdW) crystals is a promising approach to study the valley pseudospin. We show that the intrinsic contribution of our unipolar VHE is deeply rooted in Berry curvature of the occupied conduction electrons, whereby we obtained a good agreement on the gate-dependent depolarization time between the theoretical estimation and the ultrafast Kerr-rotation data.
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