Charge-Transfer Excitations in the Model Superconductor HgBa[subscript 2] CuO[subscript 4 + delta]

---

L. Lu, G. Chabot-Couture, X. Zhao, J.N. Hancock, N. Kaneko, O.P. Vajk, G. Yu, S. Grenier, Y.J. Kim, D. Casa, T. Gog, M. Greven

Abstract:
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.