Intrinsic circular polarization in centrosymmetric stacks of transition-metal dichalcogenide compounds.

Abstract

The circular polarization (CP) that the photoluminescence inherits from the excitation source in n monolayers of transition-metal dichalcogenides (MX_{2})_{n} has been previously explained as a special feature of odd values of n, where the inversion symmetry is absent. This "valley polarization" effect results from the fact that, in the absence of inversion symmetry, charge carriers in different band valleys could be selectively excited by different circular polarized light. Although several experiments observed CP in centrosymmetric MX_{2} systems, e.g., for bilayer MX_{2}, they were dismissed as being due to some extrinsic sample irregularities. Here we show that also for n=even, where inversion symmetry is present and valley polarization physics is strictly absent, such intrinsic selectivity in CP is to be expected on the basis of fundamental spin-orbit physics. First-principles calculations of CP predict significant polarization for n=2 bilayers: from 69% in MoS_{2} to 93% in WS_{2}. This realization could broaden the range of materials to be considered as CP sources.