Speakers
Description
Van der Waals heterostructures made by stacking dissimilar materials or by introducing a twist angle
between layers can exhibit many exotic properties that are not present from their bulk counterparts.
Indeed, recent experiments on transition metal dichalcogenide (TMD) bilayers suggest that this
structural engineering can lead to periodic confinement of excitons and exciton hybridization. These
systems, a solid-state analogue of an optical lattice for ultracold atoms and molecules, are therefore a
particularly promising platform for a multitude of applications, such as the creation of quantum dot
arrays and the realization of excitonic topological insulators. However, in order to harness their full
potential, it is critical to first understand how the local exciton properties are modified by the crystal
structure. To date, such studies have remained a significant challenge due to the small size in
reconstructed domains in typical heterostructures (on the order of 1-10 nanometers and thus much
smaller than the optical diffraction limit).
In the present work, we overcome this limitation and explore the impacts of the local crystal structure
on TMD excitons by fabricating near zero twist angle MoSe2/MoSe2 bilayers. This enables us to create
large, twinned rhombohedral AB and BA domains with broken mirror and inversion symmetry. By
performing far-field, spatially resolved, spectroscopic measurements of the individual domains, we
demonstrate that both momentum-indirect interlayer excitons (XI,1) and momentum-direct interlayer
excitons (XI,2) have domain-specific dipole orientations that are perpendicular to the basal plane.
Furthermore, we show that it is possible to electrostatically control both the preferred dipole
orientation of XI,1 and hybridization between XI,2 and the intralayer excitons (X0). Importantly, we
support our experimental observations with first-principle density functional theory calculations, which
are in quantitative agreement. Our results shed light on the effect of crystal symmetry on TMD optical
properties and form the foundation for engineering exciton properties via domain engineering in van
der Waals heterostructures. Therefore, they open up new avenues for designing periodic domain
structures to explore quantum emitter arrays, topological exciton insulators and strongly correlated
exciton lattices for Hubbard model physics.
| TItle | Excitons in twisted MoSe2/MoSe2 bilayers: the effect of broken mirror symmetry |
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