The dynamics of a hole in a CuO_4 plaquette: electron energy-loss   spectroscopy of Li_2CuO_2
  We have measured the energy and momentum dependent loss function of Li_2CuO_2 single crystals by means of electron energy-loss spectroscopy in transmission. Using the same values for the model parameters, the low-energy features of the spectrum as well as published Cu 2p_(3/2) x-ray photoemission data of Li_2CuO_2 are well described by a cluster model that consists of a single CuO_4 plaquette only. This demonstrates that charge excitations in Li_2CuO_2 are strongly localized. 
S. Atzkern, M. Knupfer, M. S. Golden, J. Fink, C. Waidacher, J.   Richter, K. W. Becker, N. Motoyama, H. Eisaki, and S. Uchida