Abstract
Heisenberg interactions are ubiquitous in magnetic materials and play a central role in modelling and designing quantum magnets. Bond-directional interactions1,2,3 offer a novel alternative to Heisenberg exchange and provide the building blocks of the Kitaev model4, which has a quantum spin liquid as its exact ground state. Honeycomb iridates, A2IrO3 (A = Na, Li), offer potential realizations of the Kitaev magnetic exchange coupling, and their reported magnetic behaviour may be interpreted within the Kitaev framework. However, the extent of their relevance to the Kitaev model remains unclear, as evidence for bond-directional interactions has so far been indirect. Here we present direct evidence for dominant bond-directional interactions in antiferromagnetic Na2IrO3 and show that they lead to strong magnetic frustration. Diffuse magnetic X-ray scattering reveals broken spin-rotational symmetry even above the Néel temperature, with the three spin components exhibiting short-range correlations along distinct crystallographic directions. This spin- and real-space entanglement directly uncovers the bond-directional nature of these interactions, thus providing a direct connection between honeycomb iridates and Kitaev physics.
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Acknowledgements
Work in the Materials Science Division of Argonne National Laboratory (sample preparation, characterization, and contributions to data analysis) was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Division. Use of the Advanced Photon Source, an Office of Science User Facility operated for the US Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the US DOE under Contract No. DE-AC02-06CH11357. K.M. acknowledges support from UGC-CSIR, India. Y.S. acknowledges DST, India for support through Ramanujan Grant #SR/S2/RJN-76/2010 and through DST grant #SB/S2/CMP-001/2013. J.C. was supported by ERDF under project CEITEC (CZ.1.05/1.1.00/02.0068) and EC 7th Framework Programme (286154/SYLICA).
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B.J.K. conceived the project. S.H.C., J-W.K., J.K. and B.J.K. performed the experiment with support from Y.C., T.G., A.A-Z., M.M.S. and M.K. H.Z. and K.M. grew the single crystals; C.C.S., C.D.M. and K.M. characterized the samples under the supervision of J.F.M. and Y.S. S.H.C., J-W.K. and B.J.K. analysed the data. J.C. performed the numerical calculations. J.C., G.J. and G.K. developed the theoretical model. All authors discussed the results. B.J.K. led the manuscript preparation with contributions from all authors.
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Hwan Chun, S., Kim, JW., Kim, J. et al. Direct evidence for dominant bond-directional interactions in a honeycomb lattice iridate Na2IrO3. Nature Phys 11, 462–466 (2015). https://doi.org/10.1038/nphys3322
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DOI: https://doi.org/10.1038/nphys3322
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