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, 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 ∥ &gt; 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. 

/pdf/ultrafast-pseudospin-quantum-beats-in-multilayer-wse2-and-3tc7mln5.pdf

Ultrafast pseudospin quantum beats in multilayer WSe2 and MoSe2

The optical properties of two-dimensional materials can be effectively tuned by strain induced from a deformable substrate. In the present work we combine first-principles calculations based on density functional theory and the effective Bethe-Salpeter equation with high-pressure optical measurements in order to thoroughly describe the effect of strain and dielectric environment onto the electronic band structure and optical properties of a few-layered transition-metal dichalcogenide. Our results show that WS2 remains fully adhered to the substrate at least up to a 0.6% in-plane compressive strain for a wide range of substrate materials. We provide a useful model to describe effect of strain on the optical properties on general strain conditions. Within this model, exceptionally large compressive uniaxial and biaxial in-plane gauge factors were obtained, which confirm transition metal dichalcogenides as very promising candidates for flexible functionalities. Finally, we discuss the pressure evolution of an optical transition closely-lying to the A exciton for bulk WS2 as well as the direct-to-indirect transition of the monolayer upon compression.

Strong substrate effects in multilayered WS2 revealed by high-pressure optical measurements

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Strong substrate effects in multilayered WS2 revealed by high-pressure optical measurements