Charge-transfer excitations in the model superconductor HgBa2CuO4+δ

We report a Cu K-edge resonant inelastic x-ray scattering (RIXS) study of charge-transfer excitations in the 2-8eV range in the structurally simple compound HgBa2CuO4+δ at optimal doping (Tc=96.5K). The spectra exhibit a significant dependence on the incident photon energy which we carefully utilize to resolve a multiplet of weakly dispersive (<0.5eV) electron-hole excitations, including a mode at 2eV. The observation of this 2eV excitation suggests the existence of a remnant charge-transfer gap deep in the superconducting phase. Quite generally, our results, which include additional data for the Mott insulator La2CuO4, demonstrate the importance of exploring the incident photon-energy dependence of the RIXS cross section.