Ultrafast pseudospin quantum beats in multilayer WSe$_2$ and MoSe$_2$

Abstract

Layered van-der-Waals materials with hexagonal symmetry offer an extra degree of freedom to their electrons, the so called valley index or valley pseudospin. This quantity behaves conceptually like the electron spin and the term valleytronics has been coined. In this context, the group of semiconducting transition-metal dichalcogenides (TMDC) are particularly appealing, due to large spin-orbit interactions and a direct bandgap at the K points of the hexagonal Brillouin zone. In this work, we present investigations of excitonic transitions in mono-and multilayer WSe2 and MoSe2 materials by time-resolved Faraday ellipticity (TRFE) with in-plane magnetic fields, B , of up to 9 T. In monolayer samples, the measured TRFE time traces are almost independent of B , which confirms a close to zero in-plane exciton g factor g , consistent with first-principles calculations. In stark contrast, we observe pronounced temporal oscillations in multilayer samples for B > 0.Remarkably, the extracted in-plane g are very close to reported out-of-plane exciton g factors of the materials, namely |g 1s | = 3.1 ± 0.2 and 2.5 ± 0.2 for the 1s A excitons in WSe2 and MoSe2 multilayers, respectively. Our first-principles calculations nicely confirm the presence of a non-zero g for the multilayer samples. We propose that the oscillatory TRFE signal in the multilayer samples is caused by pseudospin quantum beats of excitons, which is a manifestation of spin-and pseudospin layer locking in the multilayer samples. Our results demonstrate ultrafast pseudospin rotations in the GHz-to THz frequency range, which pave the way towards ultrafast pseudospin manipulation in multilayer TMDC samples.

First-principles insights into the spin-valley physics of strained transition metal dichalcogenides monolayers

“…The measured values are in great agreement with state-of-the-art first-principles calculations [38][39][40][41] that properly evaluate the Bloch contribution to the band g-factors, finally establishing a connection to the vast existing knowledge in the area of III-V materials [42][43][44][45][46][47][48][49][50][51][52]. These new theoretical developments on g-factor calculations allow us to grasp the fundamental microscopic physics that drives the g-factors in more complex TMDC-based systems, such as van der Waals heterostructures [38,40,41], doping effects [53] and defect states [54] in monolayers, and bulk materials [55].…”