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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Spin-Layer- and Spin-Valley-Locking Due to Symmetry in Differently-Stacked Tungsten Disulfide Bilayers
摘要： The coupling of the spin and valley degrees of freedom and valley-selective optical selection rules in transition-metal dichalcogenides (TMDCs) monolayers (ML) has been a major research topic in recent years. In contrast, valleytronic properties of TMDC bilayers have not been in the focus so much by now. In our contribution, we study the effect of the relative layer alignment in TMDC homo-bilayer samples on their polarization-dependent optical properties. To this end, CVD-grown WS2 bilayer samples have been prepared that during synthesis favour either the inversion symmetric AA’ or AB stacking without inversion symmetry. For the optical studies, the bilayer samples were transferred either onto a bare SiO2 or a few-layer h-BN buffer. To verify the difference in symmetry for these bilayer configurations, second-harmonic-generation (SHG) raster-scans have been performed, confirming inversion symmetry for the AA’ configuration. Subsequently, a detailed analysis of reflection contrast and photoluminescence (PL) spectra under different polarization conditions has been performed. In photoluminescence, we find a circular and linear dichroism for both stacking configurations that is more pronounced for the AB stacked sample. Furthermore, a lifting of degeneracy and a small shift (~10 meV) of the excitons for different stacks relative to each other is found. Amended theoretical calculations based on our recently developed approach combining density-functional theory (DFT), the gap equations (GE) and the Dirac-Bloch equations (DBE), show that the spin–valley locking is maintained in the AB-stacked WS2 bilayer, whereas the inversion symmetric AA’ bilayer shows a spin–layer-locking instead. Differences in the optical selection rules together with the substrate dependent renormalizations lead to the observed dichroism. Furthermore, the observed lifting of the degeneracy between different stacking configurations is confirmed by theory.
关键词： spin-valley locking,optical properties,transition-metal dichalcogenides,bilayer stacking,spin-layer-locking
更新于2025-09-12 10:27:22
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Room Temperature Valley Polarization and Coherence in Transition Metal Dichalcogenide-Graphene van der Waals Heterostructures
摘要： Van der Waals heterostructures made of graphene and transition metal dichalcogenides (TMD) are an emerging platform for opto-electronic, -spintronic and -valleytronic devices that could benefit from (i) strong light-matter interactions and spin-valley locking in TMDs and (ii) exceptional electron and spin transport in graphene. The operation of such devices requires significant valley polarization and valley coherence, ideally up to room temperature. Here, using a comprehensive Mueller polarimetry analysis, we report artifact-free room temperature degrees of valley polarization up to 40 % and, remarkably, of valley coherence up to 20 % in monolayer tungsten disulfide (WS2)/graphene heterostructures. At a temperature of 20 K, we measure a record degree of valley coherence of 60 %, a value that exceeds the degree of valley polarization (50 %) and indicates that our samples are minimally affected by pure dephasing processes. Valley contrasts have been particularly elusive in molybdenum diselenide (MoSe2), even at cryogenic temperatures. Upon interfacing monolayer MoSe2 with graphene, the room temperature degrees of valley polarization and coherence are as high as 14 % and 20 %, respectively. Our results are discussed in light of recent reports of highly efficient interlayer exciton and carrier transfer in TMD/graphene heterostructures and hold promise for room temperature chiral light-matter interactions and opto-valleytronic devices.
关键词： Mueller polarimetry,spin-valley locking,Transition metal dichalcogenides,graphene,chiral optics,opto-valleytronics,excitons
更新于2025-09-10 09:29:36