Direct observation of spin-orbit-induced 3d hybridization via resonant inelastic extreme ultraviolet scattering on an edge-sharing cuprate

Using high resolution resonant inelastic x-ray scattering measurements, we have observed that the orbital excitations of the quasi-1D spin chain compound CuGeO3 has nontrivial and noticeable orbital mixing effects from 3d valence spin-orbit coupling. In particular, the SOC leads to a significant correction of dz2 state, which has a direct interplay with the low energy physics of cuprates. Guided by atomic multiplet based modeling, our results strongly support a 3d spin-orbit mixing scenario and explore in detail the nature of these excitations.