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Physics Seminar

Resonantly hybridized excitons in moire superlattices in van der Waals heterostructures

Speaker: Prof. Alexander Tartakovskii (University of Sheffield)
Date: Wednesday 6 March 2019
Time: 15:00
Venue: Queens Buildings N/3.28

Recent years have seen significant effort in exploration of monolayer semiconductors such as transition metal dichalcogenides (TMDs) MoS2, WS2, MoSe2, WSe2 etc. Of particular interest is a possibility to combine few-atomic-layer crystals to create artificial heterostructures with tailored electronic and optical properties. This route opens possibilities inaccessible for traditional semiconductors, where the strict lattice matching requirement limits possible combinations of materials in a heterostructure. In contrast, atomically-thin layers of two-dimensional materials can be assembled in vertical stacks held together by relatively weak van der Waals forces, allowing for coupling between monolayer crystals with incommensurate lattices and arbitrary mutual rotation. The lattice constant difference and the mutual rotation angle present new degrees of freedom for the design of novel meta-materials. A profound consequence of using these new degrees of freedom is the emergence of an overarching periodicity in the local atomic registry of the constituent crystal structures, known as a moire superlattice. Its presence in graphene/hexagonal boron nitride structures led to observation of the Hofstadter butterfly spectra, and recently culminated in the discovery of the intriguing superconductor-insulator transition at magic twist angles. In my talk, I will show that in semiconducting heterostructures built of incommensurate MoSe2 and WS2 monolayers, excitonic bands can hybridize, which results in the resonant enhancement of the moire superlattice effects. MoSe2 and WS2 are specifically chosen for the near degeneracy of their conduction band edges to promote the hybridization of intra- and interlayer excitons. For MoSe2/WS2 heterostructures with almost aligned pairs of monolayer crystals, the resonant mixing of the electron states leads to amplified effects of the heterostructure's geometrical moire pattern on the hX dispersion. Further in my talk, I will also discuss the tuning of the band-structure in van der Waals heterostructures containing alloys such as Mo_xW_{1-x}Se2, where novel hybridisation effects are also observed.