Dynamics of a hole in a CuO 4 plaquette: Electron energy-loss spectroscopy of Li 2 CuO 2

We have measured the energy- and momentum-dependent loss function of ${\mathrm{Li}}_{2}{\mathrm{CuO}}_{2}$ 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 ${\mathrm{Li}}_{2}{\mathrm{CuO}}_{2}$ are well described by a cluster model that consists of a single ${\mathrm{CuO}}_{4}$ plaquette only. This demonstrates that charge excitations in ${\mathrm{Li}}_{2}{\mathrm{CuO}}_{2}$ are strongly localized.