Valley-polarized exciton currents in a van der Waals heterostructure

Abstract

Valleytronics is an appealing alternative to conventional charge-based electronics that aims at encoding data in the valley degree of freedom, that is, the information as to which extreme of the conduction or valence band carriers are occupying. The ability to create and control valley currents in solid-state devices could therefore enable new paradigms for information processing. Transition metal dichalcogenides (TMDCs) are a promising platform for valleytronics due to the presence of two inequivalent valleys with spin–valley locking1 and a direct bandgap2,3, which allows optical initialization and readout of the valley state4,5. Recent progress on the control of interlayer excitons in these materials6,7,8 could offer an effective way to realize optoelectronic devices based on the valley degree of freedom. Here, we show the generation and transport over mesoscopic distances of valley-polarized excitons in a device based on a type-II TMDC heterostructure. Engineering of the interlayer coupling results in enhanced diffusion of valley-polarized excitons, which can be controlled and switched electrically. Furthermore, using electrostatic traps, we can increase the exciton concentration by an order of magnitude, reaching densities in the order of 1012 cm−2, opening the route to achieving a coherent quantum state of valley-polarized excitons via Bose–Einstein condensation.

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Fig. 1: Device characterization.
Fig. 2: Exciton diffusion.
Fig. 3: Valley-polarized excitonic switch.
Fig. 4: Electrostatic control of exciton concentration.

Data availability

The data that support the findings of this study are available from the corresponding author on reasonable request.

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Acknowledgements

We are grateful to J.F. Gonzalez Marin for useful discussions. We acknowledge the help of Z. Benes (EPFL Center of MicroNanoTechnology (CMI)) with electron-beam lithography. D.U., A.C., A.A. and A.K. acknowledge support by the Swiss National Science Foundation (grant no. 153298), H2020 European Research Council (ERC, grant no. 682332) and Marie Curie-Sklodowska-Curie Actions (COFUND grant no. 665667). A.K. acknowledges funding from the European Union’s Horizon H2020 Future and Emerging Technologies under grant no. 696656 (Graphene Flagship). K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan and JSPS KAKENHI grants nos. JP15K21722 and JP25106006.

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Contributions

A.K. initiated and supervised the project. A.C. fabricated the devices. D.U. performed optical measurements with assistance from A.C. A.C. and D.U. analysed the data. Z.S., A.C. and D.U. performed SHG measurements. K.W. and T.T. grew the hBN crystals. A.C., D.U., A.A. and A.K. wrote the manuscript, with input from all authors.

Corresponding author

Correspondence to Andras Kis.

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The authors declare no competing interests.

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Peer review information Nature Nanotechnology thanks Min-Kyu Joo and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary Information

Supplementary Figs. 1–11 and refs. 1–5.

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Unuchek, D., Ciarrocchi, A., Avsar, A. et al. Valley-polarized exciton currents in a van der Waals heterostructure. Nat. Nanotechnol. 14, 1104–1109 (2019). https://doi.org/10.1038/s41565-019-0559-y

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