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Far out-of-equilibrium spin populations trigger giant spin injection into atomically thin MoS2

Abstract

Injecting spins from ferromagnetic metals into semiconductors efficiently is a crucial step towards the seamless integration of charge- and spin-information processing in a single device1,2. However, efficient spin injection into semiconductors has remained an elusive challenge even after almost three decades of major scientific effort3,4,5, due to, for example, the extremely low injection efficiencies originating from impedance mismatch1,2,5,6, or technological challenges originating from stability and the costs of the approaches7,8,9,10,11,12. We show here that, by utilizing the strongly out-of-equilibrium nature of subpicosecond spin-current pulses, we can obtain a massive spin transfer even across a bare ferromagnet/semiconductor interface. We demonstrate this by producing ultrashort spin-polarized current pulses in Co and injecting them into monolayer MoS2, a two-dimensional semiconductor. The MoS2 layer acts both as the receiver of the spin injection and as a selective converter of the spin current into a charge current, whose terahertz emission is then measured. Strikingly, we measure a giant spin current, orders of magnitude larger than typical injected spin-current densities using currently available techniques. Our result demonstrates that technologically relevant spin currents do not require the very strong excitations typically associated with femtosecond lasers. Rather, they can be driven by ultralow-intensity laser pulses, finally enabling ultrashort spin-current pulses to be a technologically viable information carrier for terahertz spintronics.

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Fig. 1: Ultrafast spin-injection process.
Fig. 2: Characterization of MoS2 and Co/MoS2 samples.
Fig. 3: THz emission of Co/MoS2 under different experimental conditions.
Fig. 4: Strongly out-of-equilibrium distribution of spin injection.

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 acknowledge funding from the A*STAR PHAROS Programme on Topological Insulators (SERC Grant No. 152 74 00026) and 2D Materials (SERC Grant No. 152 70 00012 and 152 70 00016), and Singapore Ministry of Education AcRF Tier 1 (MOE2018-T1-001-97) and Tier 2 (MOE2015-T2-2-065, MOE2016-T2-1-054) grants. J.C.W.S. acknowledges the support of the Singapore National Research Foundation under fellowship award NRF-NRFF2016-05. M.B. gratefully acknowledges Nanyang Technological University, NAP-SUG and the Austrian Science Fund (FWF) through Lise Meitner position M1925-N28 for the funding of this research. The work was supported in part by the Center for Integrated Nanotechnologies, a US DOE BES user facility. We acknowledge B. Tang from the National University of Singapore and D. Seng from the Institute of Materials Research and Engineering, A*STAR, for Raman and X-ray photoelectron spectroscopy data.

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Contributions

E.E.M.C. and H.Y. conceived the experiments. W.Y., Y.W. and M.C. fabricated the heterostructures. L.C. and X.W. carried out the THz measurements and data analysis with the help and guidance of E.E.M.C. and H.Y. M.B., J.C.W.S. and J.X.Z. provided theoretical inputs. W.Y. and S.W. performed and analysed the X-ray photoelectron spectroscopy and Raman measurements. L.C., X.W., J.C.W.S, M.B. and E.E.M.C wrote the manuscript together. All authors discussed the results and commented on the manuscript.

Corresponding authors

Correspondence to Justin C. W. Song or Marco Battiato or Hyunsoo Yang or Elbert E. M. Chia.

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Supplementary Chapters 1–8, Supplementary Figures 1–7 and Supplementary References 1–12

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Cheng, L., Wang, X., Yang, W. et al. Far out-of-equilibrium spin populations trigger giant spin injection into atomically thin MoS2. Nat. Phys. 15, 347–351 (2019). https://doi.org/10.1038/s41567-018-0406-3

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