Charge-transfer excitations in the model superconductor HgBa2CuO4+δ -: art. no. 217003

We report a Cu K-edge resonant inelastic x-ray scattering (RIXS) study of charge-transfer excitations in the 2-8 eV range in the structurally simple compound HgBa2CuO4+delta at optimal doping (T-c=96.5 K). The spectra exhibit a significant dependence on the incident photon energy which we carefully utilize to resolve a multiplet of weakly dispersive (< 0.5 eV) electron-hole excitations, including a mode at 2 eV. The observation of this 2 eV 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.