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arXiv · 0911.0619

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

James C. T. Lee·Shi Yuan·Siddhartha Lal·Young Il Joe·Yu Gan·Serban Smadici·Ken Finkelstein·Yejun Feng·Andrivo Rusydi·Paul M. Goldbart·S. Lance Cooper·Peter Abbamonte
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Abstract

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.

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James C. T. Lee, Shi Yuan, Siddhartha Lal, Young Il Joe, Yu Gan, Serban Smadici, Ken Finkelstein, Yejun Feng, Andrivo Rusydi, Paul M. Goldbart, S. Lance Cooper, Peter Abbamonte. 2009-11-03. Two-step stabilization of orbital order and the dynamical frustration of spin in the model charge-transfer insulator KCuF3. https://doi.org/10.1038/nphys2117

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