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Transition-metal dichalcogenide (TMD) monolayers are direct-gap semiconductors with peculiar spin-valley coupling. Combining two different TMDs can lead to a type-II band alignment and formation of interlayer excitons (ILE). In MoSe2-WSe2 heterobilayers, these are only optically bright if the layers are aligned in parallel orientation, or with an interlayer twist of 60 degrees. Depending on alignment, ILE transitions are either valley-conserving or between valleys. This allows us to engineer the ILE emission energy as well as its g factor, changing its magnitude and even its sign. Additionally, applied magnetic fields induce a valley polarization of the ILE, and its buildup can directly be observed in helicity- and time-resolved photoluminescence (PL), with peculiar features due to the dependence of ILE optical selection rules on interlayer registry [1]. Stacking TMD monolayers with parallel orientation leads to ferroelectric fields at the interfaces of adjacent layers, whose orientation can be switched by in-plane sliding, potentially giving rise to a novel type of nonvolatile data storage. While artificial stacking of TMDs always results in finite interlayer twist, leading to small, moiré-like ferroelectric domains, naturally grown parallel-stacked TMD crystals can have mesoscopic domains. Remarkably, the ferroelectric order in such crystals can be determined using optical spectroscopy, as it leads to a stacking-dependent energetic splitting of intralayer exciton transitions [2]. This allows us to map ferroelectric domains and track their switching in externally applied electric fields. [1] J. Holler, TK et al., Phys. Rev. B 105, 085303 (2022). [2] S. Deb, TK et al., Nature Comms. 15, 7595 (2024).

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Tobias Korn, Uni Rostkock
Seminar room Ph2
Contact: Paul van Loosdrecht