Character of charge transfer excitons in Sr2CuO2Cl2 -: art. no. 180512

High resolution electron energy loss spectroscopy (EELS) measurements of the insulating, layered cuprate compound Sr2CuO2Cl2 have been performed with a transferred momentum near k=0 and in a wide energy range up to 20 eV. To interpret the data a semiquantitative embedded cluster approach is applied to describe the electron-hole excitations in insulating cuprates. The theory includes the complete set of Cu 3d and O 2p orbitals and predicts a rather wide (2/13 eV) spectral range of the main dipole-allowed one- and two-center charge transfer (CT) transitions whose positions are in good quantitative agreement with the experimental spectra. Both, theory and the experimental data show that the CT gap is determined by a superposition of nearly degenerate one-center, localized CT excitations and two-center CT excitons with sizable dispersion.