RESUMO
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 WSe2 and MoSe2 materials by time-resolved Faraday ellipticity (TRFE) with in-plane magnetic fields, Bâ¥, of up to 9 T. In monolayer samples, the measured TRFE time traces are almost independent of Bâ¥, which confirms a close to zero in-plane exciton g factor gâ¥, 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.