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Optical initialization of a single spin-valley in charged WSe2 quantum dots

Abstract

Control and manipulation of single charges and their internal degrees of freedom, such as spin, may enable applications in quantum information technology, spintronics and quantum sensing1,2. Recently, atomically thin semiconductors with a direct bandgap such as group VI-B transition-metal dichalcogenide monolayers have emerged as a platform for valleytronics—the study of the valley degree of freedom of charge carriers to store and control information. They offer optical, magnetic and electrical control of the valley index, which, with the spin, is locked into a robust spin-valley index3,4. However, because recombination lifetimes of photogenerated excitations in transition-metal dichalcogenides are of the order of a few picoseconds, optically generated valley excitons possess similar lifetimes. On the other hand, the valley polarization of free holes has a lifetime of microseconds5,6,7,8,9. Whereas progress has been made in optical control of the valley index in ensembles of charge carriers10,11,12, valley control of individual charges, which is crucial for valleytronics, remains unexplored. Here we provide unambiguous evidence for localized holes with a net spin in optically active WSe2 quantum dots13,14,15,16,17 and we initialize their spin-valley state with the helicity of the excitation laser under small magnetic fields. Under such conditions, we estimate a lower bound of the valley lifetime of a single charge in a quantum dot from the recombination time to be of the order of nanoseconds. Remarkably, neutral quantum dots do not exhibit such spin-valley initialization, which illustrates the role of the excess charge in prolonging the valley lifetime. Our work extends the field of two-dimensional valleytronics to the level of single spin- valleys, with implications for quantum information and sensing applications.

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Fig. 1: Singly charged QDs and charge control in a monolayer WSe2 field effect transistor.
Fig. 2: Charged and neutral QDs in monolayer WSe2.
Fig. 3: Zeeman splitting and circular polarization of positively charged QDs in monolayer WSe2.
Fig. 4: Optical initialization of a positively charged QD under magnetic field.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

We thank A. Imamoğlu and M. Kroner for many discussions. We also acknowledge technical help from T. Neal and E. Liu. A.S. acknowledges support from Emory University startup funds and the NSF through the EFRI programme (grant number EFMA-1741691). Q.X. gratefully acknowledges strong support from Singapore National Research Foundation via an NRF-ANR joint grant (numbef NRF2017-NRF-ANR002 2D-Chiral) and the Singapore Ministry of Education via an AcRF Tier2 grant (number MOE2017-T2-1-040) and Tier1 grants (RG 113/16 and RG 194/17).

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X.L. and A.S. conceived and designed the experiments. X.L., X.C., S.D., Q.Y. and W.L. performed the experiments. X.L. and X.W. prepared the samples. X.L., X.C., S.D., Q.X. and A.S. analysed the data. Q.X. contributed materials. X.L. and A.S. co-wrote the paper.

Corresponding authors

Correspondence to Xin Lu or Ajit Srivastava.

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The authors declare no competing interests.

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Journal peer review information: Nature Nanotechnology thanks Ziliang Ye and other anonymous reviewer(s) for their contribution to the peer review of this work.

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Supplementary Figures 1–16, Supplementary Tables 1,2

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Lu, X., Chen, X., Dubey, S. et al. Optical initialization of a single spin-valley in charged WSe2 quantum dots. Nat. Nanotechnol. 14, 426–431 (2019). https://doi.org/10.1038/s41565-019-0394-1

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