Emergent excitons in super-twisted spiral transition metal dichalcogenide*

Twisted transition metal dichalcogenide heterostructures have created a flatland for studying strongly correlated electrons by the formation of tunable moiré bands. Compared to the initial demonstration of magic-angle twisted bilayer graphene, the twisted semiconductor layers not only enable flat bands over a wide continuous range of twist angles but also provide bounded electron-hole pairs, excitons, enabling emergent optical phenomena. Here we unveil a new type of exciton created by bulk-like super-twisting of WS2 layers, which is caused by the third-dimensional symmetry breaking and is much stronger than the moiré excitons.

The hundred-nanometer thick super-twisted WS2 are directly grown by chemical vapor deposition utilizing screw dislocations and non-Euclidean substrate geometry. The high crystal quality is proven by real-space imaging of waveguide modes and their large quality factors by scattering-type scanning nearfield microscopy. The super-twisted spirals bring about a new collection of symmetries observed by second-harmonic generation mapping. Most importantly, by measuring temperature-dependent photoluminescence, emergent long-wavelength excitonic peaks are uncovered under 160 K with comparable intensity with A excitons. The position and strength of these peaks are dependent on super-twist angles and absent on non-twisted spirals. The formation of these excitons is inherited from the drastically modified band structure by 3D super-twisting, supported by tight-binding calculations.