Abstract
Magnetic order is usually associated with well-defined magnon excitations. Exotic magnetic fluctuations with fractional, topological or multipolar character have been proposed for unconventional forms of magnetic matter such as spin liquids1. As a result, considerable effort has been expended to search for, and uncover, low-spin materials with suppressed dipolar order at low temperatures2,3. However, long-range order of magnetic dipoles is much more common. Here we report neutron-scattering experiments and quantitative theoretical modelling of a spin-1 system—the uniaxial triangular magnet FeI2 (ref. 4)—where a dispersive band of mixed dipolar–quadrupolar fluctuations with large spectral weight emerges just above a dipolar ordered ground state. This excitation arises from anisotropic exchange interactions that hybridize overlapping modes carrying fundamentally different quantum numbers. A generalization of spin–wave theory to local SU(3) degrees of freedom5 accounts for all details of the low-energy dynamical response of FeI2, without going beyond quadratic order. Our work highlights that quantum excitations without classical counterparts can be realized, even in the presence of fully developed magnetic order.
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The data that support the plots within this paper and other findings of this study are available from the corresponding authors upon request.
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The source codes used in this study are available from the corresponding authors upon request.
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Acknowledgements
We thank C. Broholm, I. Kimchi, S. Nagler, O. Starykh and A. Tennant for valuable discussions. The work of X.B., Z.D. and M.M. at Georgia Tech was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division under award DE-SC-0018660. The work of S.-S.Z. and C.D.B. at the University of Tennessee was supported by the Lincoln Chair of Excellence in Physics and the work by H. Zhang was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. The work of Q.H. and H. Zhou at the University of Tennessee was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division under award DE-SC-0020254. This research used resources at the High Flux Isotope Reactor and Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory.
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X.B. and M.M. conceived the project, which was supervised by C.D.B. and M.M. Z.D. and X.B. grew the samples with the assistance of Q.H. and H. Zhou using the floating zone furnace. Z.D. aligned the sample for measurements. X.B., Z.D., M.B.S., A.I.K., F.Y. and M.M. performed the neutron-scattering measurements. X.B. analysed the data and performed fits. S.-S.Z., H. Zhang and C.D.B. carried out the GSWT calculations and assisted with the theoretical interpretation. S.-S.Z. made the GSWT code used to fit the inelastic spectra. X.B. and M.M. wrote the manuscript with input from all authors.
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Bai, X., Zhang, SS., Dun, Z. et al. Hybridized quadrupolar excitations in the spin-anisotropic frustrated magnet FeI2. Nat. Phys. 17, 467–472 (2021). https://doi.org/10.1038/s41567-020-01110-1
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DOI: https://doi.org/10.1038/s41567-020-01110-1
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