Ultrafast pseudospin quantum beats in multilayer WSe 2 and MoSe 2 .

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, which behaves conceptually like the electron spin. Here, we present investigations of excitonic transitions in mono- and multilayer WSe 2 and MoSe 2 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 contrast, we observe pronounced temporal oscillations in multilayer samples for B ∥  > 0. Our first-principles calculations 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.