Fig. 3: Cu L-edge RIXS spectra recorded at selected incident photon energies in comparison with multiplet calculations.

a RIXS spectra measured with the incident photon energies tuned to the resonance of T_d Cu²⁺ compared with calculated RIXS. The solid black lines plot the calculated spectra of T_d Cu²⁺ with the Jahn-Teller distortion; the dashed gray lines are for those in a spin-orbital entangled state. The calculated RIXS intensity of T_d Cu²⁺ in the J_eff = 1/2 state vanishes. See the main text for the discussion on the origin of peaks A and B. b Measured RIXS of O_h Cu²⁺ in comparision with calculations. The solid black lines plot the calculated spectra of O_h Cu²⁺ without a distortion. The incident photon energies E_in indicated in (a) and (b) correspond to the red dashed lines shown in Fig. 2. Both experimental and calculated RIXS spectra of Cu L₂ edge are magnified by a factor of four.