Intermediate energy structure of cuprates using Resonant Inelastic X-ray Scattering

We present a comprehensive study of the charge-transfer excitations in the 1-8 eV range using the burgeoning technique of resonant inelastic X-ray scattering (RIXS). Surprisingly, we find that the charge-transfer gap, distinct at around 2.25 eV in Mott insulating La$_{2}$CuO$_{4}$, is also discernible in the high-T$_{c}$ superconductor HgBa$_{2}$CuO$_{4+\delta }$. In addition, we are able to identify many distinct, weakly dispersive features above the charge-transfer gap of La$_{2}$CuO$_{4}$ [1-3] and the model high-Tc superconductor HgBa$_{2}$CuO$_{4+\delta }$ [1]. Detailed extension of this work in La$_{2}$CuO$_{4}$ reveals previously unresolved systematics in the vicinity of the charge-transfer gap, and a distinct dependence on scattering geometry of both the charge-transfer gap and the high-energy excitations. We interpret this scattering-geometry dependence as arising from the intrinsic symmetry selectivity of the RIXS/Raman process, and suggest that similar experiments can give definitive identification of excitation symmetry. [1] L. Lu et al., Phys. Rev. Lett. 95, 217003 (2005). [2] L. Lu et al. (to appear Phys. Rev. B 74; cond-mat/0607311) [3] J. N. Hancock et al. (in preparation).