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.
PHYSICAL REVIEW LETTERS
95, November, 217003-1-217003-5 (2005)
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.