Spin-Layer- and Spin-Valley-Locking Due to Symmetry in Differently-Stacked Tungsten Disulfide Bilayers

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 [1]. 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 WS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> 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 SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (cf. fig. 1 a,b) or a few-layer h-BN buffer (fig. 1 c,d). To verify the difference in symmetry for these bilayer configurations, second-harmonic-generation (SHG) raster-scans have been performed (cf. fig. 1 e-h), 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.