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

Abstract

We study the collective spin and orbital excitations in the canonical orbital system KCuF3. Using the Cu L3-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 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 local Jahn-Teller effect in KCuF3, which predominantly drives orbital ordering.