Two-stage orbital order and dynamical spin frustration in KCuF sub(3)

• Published in: Nature physics 2012-01, Vol.8 (1), p.63-66
• Main Authors: Lee, James CT, Yuan, Shi, Lal, Siddhartha, Joe, Young Il, Gan, Yu, Smadici, Serban, Finkelstein, Ken, Feng, Yejun, Rusydi, Andrivo, Goldbart, Paul M, Lance Cooper, S, Abbamonte, Peter
• Format: Article
• Language: English
• Subjects: Condensed matter; Degrees of freedom; Frustration; Glassy; Iron; Manganites; Orbitals; Scattering
• ISSN: 1745-2473
• DOI: 10.1038/nphys2117

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 sub(3) and has since been applied to virtually all orbitally active materials. Here we present Raman and X-ray scattering measurements showing that KCuF sub(3) exhibits a previously unidentified structural phase transition at T=50K, involving rotations of the CuF sub(6) octahedra. These rotations are quasi-ordered and exhibit glassy hysteresis, but serve to stabilize Neel 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.