Unraveling the orbital physics in a canonical orbital system KCuF${}_3$

Abstract

We explore the existence of the collective orbital excitations, orbitons, in the canonical orbital system KCuF${}_3$. Using the Cu $L_3$-edge resonant inelastic X-ray scattering we show that the non-dispersive high-energy peaks result from the Cu${}^{2+}$ $dd$ orbital excitations. These high-energy modes show good agreement with the {\it ab-initio} quantum chemistry calculation based on a single cluster, indicating that the $dd$ excitations are highly localized. At the same time, the low-energy excitations present clear dispersion. They match extremely well with the two-spinon continuum following the comparison with Mueller Ansatz calculations. The localized $dd$ excitations and the observation of the strongly dispersive magnetic excitations suggest that orbiton dispersion is below the resolution detection limit. Our results can reconcile with the strong {\it local} Jahn-Teller effect in KCuF${}_3$, which predominantly drives orbital ordering.

arXiv comment6 pages, 4 figures

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• Collective nature of orbital excitations in layered cuprates in the absence of apical oxygens
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• Orbital-slective behaviour in cubanite CuFe2S3
• Electronic excitations and spin interactions in chromium trihalides from embedded many-body wavefunctions
• Observation of spin-conserving two-spinon continuum in the $S$=1/2 antiferromagnetic chain system Sr${}_2$CuO${}_3$ using Cu $K$-edge resonant inelastic x-ray scattering
• Spectroscopy of the frustrated quantum antiferromagnet Cs${}_2$CuCl${}_4$
• Witnessing Spin-Orbital Entanglement using Resonant Inelastic X-Ray Scattering