Two-stage orbital order and dynamical spin frustration in KCuF₃

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 model, which first explained the symmetry of orbital and magnetic order in KCuF 3 and has since been applied to virtually all orbitally active materials. Here we present Raman and X-ray scattering measurements showing that KCuF₃ exhibits a previously unidentified structural phase transition at T=50K, involving rotations of the CuF₆ octahedra. These rotations are quasi-ordered and exhibit glassy hysteresis, but serve to stabilize Néel spin order at T=39K. 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 distortions. 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.