Abstract
The recently discovered monolayer transition metal dichalcogenides (TMDCs) provide a fertile playground to explore new coupled spin–valley physics1,2,3. Although robust spin and valley degrees of freedom are inferred from polarized photoluminescence (PL) experiments4,5,6,7,8, PL timescales are necessarily constrained by short-lived (3–100 ps) electron–hole recombination9,10. Direct probes of spin/valley polarization dynamics of resident carriers in electron (or hole)-doped TMDCs, which may persist long after recombination ceases, are at an early stage11,12,13. Here we directly measure the coupled spin–valley dynamics in electron-doped MoS2 and WS2 monolayers using optical Kerr spectroscopy, and reveal very long electron spin lifetimes, exceeding 3 ns at 5 K (two to three orders of magnitude longer than typical exciton recombination times). In contrast with conventional III–V or II–VI semiconductors, spin relaxation accelerates rapidly in small transverse magnetic fields. Supported by a model of coupled spin–valley dynamics, these results indicate a novel mechanism of itinerant electron spin dephasing in the rapidly fluctuating internal spin–orbit field in TMDCs, driven by fast inter-valley scattering. Additionally, a long-lived spin coherence is observed at lower energies, commensurate with localized states. These studies provide insight into the physics underpinning spin and valley dynamics of resident electrons in atomically thin TMDCs.
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Acknowledgements
We gratefully acknowledge D. L. Smith and H. Dery for helpful discussions, and W. D. Rice for laser expertise. This work was supported by the Los Alamos LDRD programme. These optical studies were performed at the National High Magnetic Field Laboratory, which is supported by NSF DMR-1157490 and the State of Florida. We also acknowledge the support from AFOSR (grant FA9550-14-1-0268) and the Welch Foundation (grant C1716).
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L.Y. and S.A.C. conceived and built the experiments. W.C., J.Y., J.Z. and J.L. grew the samples. L.Y. performed the optical measurements. N.A.S. provided theoretical insight. L.Y., N.A.S. and S.A.C. wrote the paper in consultation with all authors.
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Yang, L., Sinitsyn, N., Chen, W. et al. Long-lived nanosecond spin relaxation and spin coherence of electrons in monolayer MoS2 and WS2. Nature Phys 11, 830–834 (2015). https://doi.org/10.1038/nphys3419
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DOI: https://doi.org/10.1038/nphys3419
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