Two-step stabilization of orbital order and the dynamical frustration of spin in the model charge-transfer insulator KCuF3

• Published in: arXiv.org 2009-11
• Main Authors: Lee, James C T, Shi, Yuan, Lal, Siddhartha, Young Il Joe, Gan, Yu, Serban Smadici, Finkelstein, Ken, Feng, Yejun, Rusydi, Andrivo, Goldbart, Paul M, Cooper, S Lance, Abbamonte, Peter
• Format: Article
• Language: English
• Subjects: Charge transfer; Critical point; Degrees of freedom; Frustration; Liquid phases; Neel temperature; Orbital stability; Pressure; Spin liquid; Variation; X-ray scattering
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.0911.0619

We report a combined experimental and theoretical study of KCuF3, which offers - because of this material's relatively simple lattice structure and valence configuration (d9, i.e., one hole in the d-shell) - a particularly clear view of the essential role of the orbital degree of freedom in governing the dynamical coupling between the spin and lattice degrees of freedom. We present Raman and x-ray scattering evidence that the phase behaviour of KCuF3 is dominated above the Neel temperature (T_N = 40 K) by coupled orbital/lattice fluctuations that are likely associated with rotations of the CuF6 octahedra, and we show that these orbital fluctuations are interrupted by a static structural distortion that occurs just above T_N. A detailed model of the orbital and magnetic phases of KCuF3 reveals that these orbital fluctuations - and the related frustration of in-plane spin-order-are associated with the presence of nearly degenerate low-energy spin-orbital states that are highly susceptible to thermal fluctuations over a wide range of temperatures. A striking implication of these results is that the ground state of KCuF3 at ambient pressure lies near a quantum critical point associated with an orbital/spin liquid phase that is obscured by emergent Neel ordering of the spins; this exotic liquid phase might be accessible via pressure studies.