Yukawa Institute for Theoretical Physics, Kyoto University, Kyoto 606-8502, Japan, Yukawa Institute for Theoretical Physics, Kyoto University, Kyoto 606-8502^* Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan^**
○takami Tohyama^* Kenji Tsutsui^** Sadamichi Maekawa^**

Cu K-edge Resonant inelastic x-ray scattering (RIXS) in two-dimensional insulating cuprates reveals characteristic momentum dependence of a charge-transfer process from the occupied Zhang-Rice singlet band to the unoccupied upper Hubbard band, i.e., Mott-gap excitation. Using the numerically exact diagonalization technique for finite-size Hubbard cluster, we have proposed that the Mott-gap excitations are seen in the Cu K-edge RIXS even for doped cuprates but with different doping dependence between hole- and electron-doping [1]. Recently we examined theoretically the Cu K-edge RIXS on electron-doped Nd_2-xCe_xCuO₄ and compared with the experimental data obtained in SPring-8. The low-energy spectrum is assigned as the intra-band excitation and is qualitatively similar to the dynamical charge correlation function [2]. These results indicate that the RIXS is a powerful technique to investigate the charge dynamics in the strongly correlated electron systems. We also examine the momentum dependence of Cu L-edge RIXS in insulating cuprates [3]. Using the two-band Hubbard model, we find that charge excitations characteristic of the Cu L-edge RIXS process appear in the spectrum.
[1] T. Tohyama, K. Tsutsui, S. Maekawa, J. Phys. Chem. Solids 66, 2139 (2005).
[2] K. Ishii, K. Tsutsui, et al, Phys. Rev. Lett. 94, 207003 (2005).
[3] K. Tsutsui, H. Yamamoto, T. Tohyama, S. Maekawa, J. Phys. Chem. Solids 67, 274 (2006).