Abstract
Breaking the space-time symmetries in materials can markedly influence their electronic and optical properties. In 3R-stacked transition metal dichalcogenides, the explicitly broken inversion symmetry enables valley-contrasting Berry curvature and quantization of electronic angular momentum, providing an unprecedented platform for valleytronics. Here, we study the valley coherence of 3R WS2 large single-crystal with thicknesses ranging from monolayer to octalayer at room temperature. Our measurements demonstrate that both A and B excitons possess robust and thickness-independent valley coherence. The valley coherence of direct A (B) excitons can reach 0.742 (0.653) with excitation conditions on resonance with it. Such giant and thickness-independent valley coherence of large single-crystal 3R WS2 at room temperature would provide a firm foundation for quantum manipulation of the valley degree of freedom and practical application of valleytronics.
Highlights
Valley degree of freedom known as valley pseudospin, the local degenerate energy extrema in momentum space, can open up new ways to encode and process binary information: valleytronics [1,2,3]
Highly selective circular polarization was observed in different 2Hstacked bilayer transition metal dichalcogenides (TMDCs) [18,19,20,21], such circular dichroism stems from the hidden spin polarization and cannot signify valley polarization [17, 22, 23], which can be clearly seen in the diagram of the electronic structure (Figure 1(a))
We demonstrate the giant valley coherence in 3R-stacked WS2 large single-crystal at room temperature
Summary
Valley degree of freedom known as valley pseudospin, the local degenerate energy extrema in momentum space, can open up new ways to encode and process binary information: valleytronics [1,2,3]. The extremely low carrier mobility and poor valley response at room temperature are significant roadblocks to the valleytronic applications with monolayer TMDCs [6, 12, 13]. In the 2H-stacked bilayer TMDCs, the lower layer is a π in-plane rotation of the upper layer, which leads to the inversion symmetry restore. Highly selective circular polarization was observed in different 2Hstacked bilayer TMDCs [18,19,20,21], such circular dichroism stems from the hidden spin polarization and cannot signify valley polarization [17, 22, 23], which can be clearly seen in the diagram of the electronic structure (Figure 1(a)). Few-layer TMDCs with 2H symmetry are not a good playground for valleytronic applications
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