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

TL;DR

This study combines experimental and theoretical approaches to understand how orbital fluctuations and structural distortions influence spin frustration and phase behavior in the charge-transfer insulator KCuF3, revealing proximity to a quantum critical orbital/spin liquid phase.

Contribution

It provides a detailed model showing how orbital fluctuations and structural distortions govern the magnetic phases and spin frustration in KCuF3, highlighting its proximity to a quantum critical point.

Findings

Orbital fluctuations dominate above T_N, linked to CuF6 octahedra rotations.

A static structural distortion interrupts orbital fluctuations near T_N.

KCuF3 is near a quantum critical point with an orbital/spin liquid phase.

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.