Heterostructures of atomically thin van der Waals bonded monolayers have opened a unique platform to engineer Coulomb correlations, shaping excitonic1,2,3, Mott insulating4 or superconducting phases5,6. In transition metal dichalcogenide heterostructures7, electrons and holes residing in different monolayers can bind into spatially indirect excitons1,3,8,9,10,11 with a strong potential for optoelectronics11,12, valleytronics1,3,13, Bose condensation14, superfluidity14,15 and moiré-induced nanodot lattices16. Yet these ideas require a microscopic understanding of the formation, dissociation and thermalization dynamics of correlations including ultrafast phase transitions. Here we introduce a direct ultrafast access to Coulomb correlations between monolayers, where phase-locked mid-infrared pulses allow us to measure the binding energy of interlayer excitons in WSe2/WS2 hetero-bilayers by revealing a novel 1s–2p resonance, explained by a fully quantum mechanical model. Furthermore, we trace, with subcycle time resolution, the transformation of an exciton gas photogenerated in the WSe2 layer directly into interlayer excitons. Depending on the stacking angle, intra- and interlayer species coexist on picosecond scales and the 1s–2p resonance becomes renormalized. Our work provides a direct measurement of the binding energy of interlayer excitons and opens the possibility to trace and control correlations in novel artificial materials.
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The data sets generated during and/or analysed during the current study are available from the corresponding author upon reasonable request.
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The authors thank M. Furthmeier for technical assistance. This work was supported by the Deutsche Forschungsgemeinschaft (DFG) through research training group GRK 1570, Collaborative Research Center SFB 1277 (projects A05 and B03) and project grant KO3612/3-1. The Chalmers group acknowledges funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 696656 (Graphene Flagship) and the Swedish Research Council (VR).
The authors declare no competing interests.
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Merkl, P., Mooshammer, F., Steinleitner, P. et al. Ultrafast transition between exciton phases in van der Waals heterostructures. Nat. Mater. 18, 691–696 (2019). https://doi.org/10.1038/s41563-019-0337-0
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