Abstract
The orbital degree of freedom is integral to many exotic phenomenain condensed matter, including colossal magnetoresistance and unconventional superconductivity. The standard model of orbital physics is the Kugel–Khomskii model1, which first explained the symmetry of orbital and magnetic order in KCuF3 and has since been applied to virtually all orbitally active materials2. Here we present Raman and X-ray scattering measurements showing that KCuF3 exhibits a previously unidentified structural phase transition at T=50 K, involving rotations of the CuF6 octahedra. These rotations are quasi-ordered and exhibit glassy hysteresis, but serve to stabilize Néel spin order at T=39 K. We propose an explanation for these effects by supplementing the Kugel–Khomskii model with a direct, orbital exchange term that is driven by a combination of electron–electron interactions and ligand distortions3. The effect of this term is to create a near degeneracy that dynamically frustrates the spin subsystem but is lifted at low temperature by subdominant, orbital–lattice interactions. Our results suggest that direct orbital exchange may be crucial for the physics of many orbitally active materials, including manganites, ruthenates and the iron pnictides.
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Acknowledgements
We gratefully acknowledge discussions with D. I. Khomskii, M. V. Mostovoy, A. J. Millis, A. K. Sood and H. R. Krishnamurthy. This work was supported by the US Department of Energy through grant DE-FG02-07ER46453, with soft-X-ray studies supported by DE-FG02-06ER46285.The Advanced Photon Source was supported by DE-AC02-06CH11357 and the NSLS by DE-AC02-98CH10886. CHESS and ChemMatCARS are supported by National Science Foundation grants CHE-0822838 and DMR-0225180, respectively. S.L. gratefully acknowledges financial support from the Department of Science and Technology, Government of India, through a Ramanujam Fellowship.
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S.Y. and J.C.T.L. grew the crystals. S.Y. carried out Raman experiments. J.C.T.L., Y.I.J., S.S., Y.F., Y.G., A.R. and K.F. carried out the X-ray experiments. S.L., P.M.G. and P.A. developed the model. P.A. and S.L.C. wrote the paper.
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Lee, J., Yuan, S., Lal, S. et al. Two-stage orbital order and dynamical spin frustration in KCuF3. Nature Phys 8, 63–66 (2012). https://doi.org/10.1038/nphys2117
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DOI: https://doi.org/10.1038/nphys2117
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