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Exciton-coupled coherent magnons in a 2D semiconductor

Abstract

The recent discoveries of two-dimensional (2D) magnets1,2,3,4,5,6 and their stacking into van der Waals structures7,8,9,10,11 have expanded the horizon of 2D phenomena. One exciting application is to exploit coherent magnons12 as energy-efficient information carriers in spintronics and magnonics13,14 or as interconnects in hybrid quantum systems15,16,17. A particular opportunity arises when a 2D magnet is also a semiconductor, as reported recently for CrSBr (refs. 18,19,20) and NiPS3 (refs. 21,22,23) that feature both tightly bound excitons with a large oscillator strength and potentially long-lived coherent magnons owing to the bandgap and spatial confinement. Although magnons and excitons are energetically mismatched by orders of magnitude, their coupling can lead to efficient optical access to spin information. Here we report strong magnon–exciton coupling in the 2D A-type antiferromagnetic semiconductor CrSBr. Coherent magnons launched by above-gap excitation modulate the exciton energies. Time-resolved exciton sensing reveals magnons that can coherently travel beyond seven micrometres, with a coherence time of above five nanoseconds. We observe these exciton-coupled coherent magnons in both even and odd numbers of layers, with and without compensated magnetization, down to the bilayer limit. Given the versatility of van der Waals heterostructures, these coherent 2D magnons may be a basis for optically accessible spintronics, magnonics and quantum interconnects.

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Fig. 1: Coupling of excitons to coherent magnons.
Fig. 2: Magnetic-field-dependent magnon frequencies and calculated dispersions.
Fig. 3: Exciton sensing of propagating coherent AFM spin waves in the 2D plane.
Fig. 4: Magnon dispersions.
Fig. 5: Detection of coherent spin waves from excitonic transitions in CrSBr down to the 2D limit.

Data availability

The data that support the plots within this paper are available from the corresponding author upon reasonable request. Source data are provided with this paper.

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Acknowledgements

The spectroscopic and imaging work was supported by the Materials Science and Engineering Research Center (MRSEC) through NSF grant DMR-2011738, with partial support for experimental apparatus by the Vannevar Bush Faculty Fellowship through the Office of Naval Research grant number N00014-18-1-2080. The synthesis of the CrSBr crystals was supported as part of Programmable Quantum Materials, an Energy Frontier Research Center funded by the US Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under award DE-SC0019443. The magnetic resonance spectroscopy work was supported by the Air Force Office of Scientific Research under grant FA9550-19-1-0307. The magnetic-field-dependent experiment in Fig. 2c was supported by the Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division (DE-SC0012509). The vibrating sample magnetometry was purchased with financial support from the NSF through a supplement to award DMR-1751949. This research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. This research was supported by an appointment to the Intelligence Community Postdoctoral Research Fellowship Program at University of Washington administered by Oak Ridge Institute for Science and Education (ORISE) through an interagency agreement between the US Department of Energy and the Office of the Director of National Intelligence (ODNI). We are grateful to K. Lee, T. Handa and Y. Dai for providing help and discussions.

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Contributions

X.Z. and Y.J.B. conceived this work. Y.J.B. carried out all optical measurements at fixed magnetic fields with assistance from J.W., Y.B. and M.D. Bulk crystals were synthesized and characterized by D.G.C. and M.E.Z. under the supervision of X.R. and C.R.D. The magnetic resonance measurements were carried out by Y.J.B., J.X. and H.R. under the supervision of A.D.K. The magnetic-field-dependent optical measurements were performed by G.M.D. and J.C. under the supervision of X.X. Theoretical analysis was performed by Y.J.B. and A.S. X.Z. supervised the project. The manuscript was prepared by Y.J.B. and X.Z. in consultation with all other authors. All authors read and commented on the manuscript.

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Correspondence to Xiaoyang Zhu.

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Nature thanks Jun-Bo Han and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary information

Supplementary Information

This file contains Supplementary Sections 1–15, including Supplementary Figs. 1–23, Tables 1–5 and References. The sections include supplementary data, including temperature-dependent, pump-power-dependent and zero-field transient reflectance spectra, calculations of exciton energy modulation in bulk and a few-layer samples, temperature-dependent antiferromagnetic resonance spectra and analysis using Landau–Lifshitz equations 1–18 and linear spin-wave theory, coherent magnon propagation data and group velocity analysis, magnon–phonon coupling in the frequency domain, magnon coherence time analysis, optical and atomic force microscopy images of samples, optical set-up including transient reflectance and magneto-optical Kerr effect, and anisotropy field calculations from vibrating sample magnetometry at variable temperatures.

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Bae, Y.J., Wang, J., Scheie, A. et al. Exciton-coupled coherent magnons in a 2D semiconductor. Nature 609, 282–286 (2022). https://doi.org/10.1038/s41586-022-05024-1

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