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We report a combined experimental and theoretical study of KCuF3, whichoffers - because of this material's relatively simple lattice structure andvalence configuration (d9, i.e., one hole in the d-shell) - a particularlyclear view of the essential role of the orbital degree of freedom in governingthe dynamical coupling between the spin and lattice degrees of freedom. Wepresent Raman and x-ray scattering evidence that the phase behaviour of KCuF3is dominated above the Neel temperature (T_N = 40 K) by coupled orbital/latticefluctuations that are likely associated with rotations of the CuF6 octahedra,and we show that these orbital fluctuations are interrupted by a staticstructural distortion that occurs just above T_N. A detailed model of theorbital and magnetic phases of KCuF3 reveals that these orbital fluctuations -and the related frustration of in-plane spin-order-are associated with thepresence of nearly degenerate low-energy spin-orbital states that are highlysusceptible to thermal fluctuations over a wide range of temperatures. Astriking implication of these results is that the ground state of KCuF3 atambient pressure lies near a quantum critical point associated with anorbital/spin liquid phase that is obscured by emergent Neel ordering of thespins; this exotic liquid phase might be accessible via pressure studies.

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