Charge-orbital-lattice coupling in a quasi-one-dimensional cuprate revealed through energy shifts in the dd-excitation profile

One-dimensional edge-sharing copper oxides provide a unique opportunity to study the effects of electron-lattice (e-l) interactions without complication from magnetic degrees of freedom which have a much lower energy scale in these compounds. Building on the characterization of e-l coupling in these materials from the elastic line profile found in resonant inelastic x-ray scattering (RIXS) at the O K-edge, new analysis of dd-excitation peak positions in Cu L-edge RIXS reveals abrupt shifts as one tunes the incident photon energy through the resonance. The observations point toward an orbital-specific coupling of the high-energy excited states of the system to the low-energy degrees of freedom. A Franck-Condon treatment of e-l coupling, consistent with other measurements, reproduces these shifts and highlights charge-orbital-lattice renormalization in the high energy d-manifold with obvious repercussions for other copper oxides.