  Resonant inelastic x-ray scattering (RIXS) tuned for the Cu L edge is a
possible tool to detect charge excitations in cuprate superconductors. We
theoretically investigate the possibility for observing a collective charge
excitation by the RIXS. The RIXS process via the intermediate state inevitably
makes the spectral weight of charge excitation stronger in electron doping than
in hole doping. Electron-hole asymmetry also appears in the dynamical charge
structure factor, showing a new enhanced small-momentum low-energy mode in
electron doping. These facts indicate a possibility of detecting the new charge
mode by RIXS in electron-doped systems.
