Charge-Transfer Excitations in the Model Superconductor   HgBa$_2$CuO$_{\bf 4+\delta}$
  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 HgBa$_2$CuO$_{4+\delta}$ at optimal doping ($T_{\rm 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 charge-transfer pseudogap deep in the superconducting phase. Quite generally, our data demonstrate the importance of exploring the incident photon energy dependence of the RIXS cross section. 
L. Lu, X. Zhao, G. Chabot-Couture, J. N. Hancock, N. Kaneko, O. P.   Vajk, G. Yu, S. Grenier, Y. J. Kim, D. Casa, T. Gog, M....