Evidence for two types of low-energy charge transfer excitations in Sr2CuO3 -: art. no. 037001

A comparative analysis of electron energy-loss spectroscopy (EELS) spectra for the 1D insulating cuprate Sr2CuO3 with transferred momentum (q) over right arrow parallel to and perpendicular to to the chain axis allows one to elucidate the structure of the charge transfer gap in in-chain response. It is determined by the superposition of two types of excitations with different magnitudes of dispersion. The low-energy response with (q) over right arrow perpendicular to to the chain direction, but yet within the plane of CuO4 plaquettes, exhibits also a dispersionless peak near 2 eV. The theoretical simulation of the EELS data using exact diagonalizations of an appropriate extended Hubbard Hamiltonian for relevant clusters requires the explicit consideration of low-lying oxygen 2ppi states within the CuO4 plaquette plane beyond the standard pdsigma extended Hubbard model widely used for cuprates with corner-shared CuO4 plaquettes.