Valley-contrasting optics of interlayer excitons in Mo- and W-based bulk transition metal dichalcogenides

Abstract

Recently, spatially indirect (“interlayer”) excitons have been discovered in bulk 2H-MoTe₂. They are theoretically predicted to exist in other Mo-based transition metal dichalcogenides (TMDCs) and are expected to be present in W-based TMDCs as well. We investigate interlayer excitons (X_IL) in bulk 2H-MoSe₂ and 2H-WSe₂ using valley-resolved magneto-reflectance spectroscopy under high magnetic fields of up to 29 T combined with ab initio GW-BSE calculations. In the experiments, we observe interlayer excitons in MoSe₂, while their signature is surprisingly absent in WSe₂. In the calculations, we find that interlayer excitons exist in both Mo- and W-based TMDCs. However, their energetic positions and their oscillator strengths are remarkably different. In Mo-based compounds, the interlayer exciton resonance X_IL is clearly separated from the intralayer exciton X^1s_A and has a high amplitude. In contrast, in W-based compounds, X_IL is close in energy to the intralayer A exciton X^1s_A and possesses a small oscillator strength, which explains its absence in the experimental data of WSe₂. Our combined experimental and theoretical observations demonstrate that interlayer excitons can gain substantial oscillator strength by mixing with intralayer states and hence pave the way for exploring interlayer exciton physics in Mo-based bulk transition metal dichalcogenides.