Low-Energy Charge Excitations in an Undoped Cuprate: Description Beyond the Standard pdσ-Model?

Abstract

Within a joint experimental and theoretical study the density-density response of the prototypical linear chain compound Sr 2 CuO 3 has been investigated by means of polarization and momentum dependent electron energy loss spectroscopy (EELS) and exact diagonalizations including also the continued fraction technique for the determination of spectral densities such as the optical conductivity and the loss function, respectively. It has been shown that the lowest charge excitations of this 1D charge transfer insulator cannot be described properly within the usual pdσ-model for cuprates which is based on the Cu 3dx2-y2 and the corresponding O 2pσ states, only. The lowest energy charge excitations are dominated by O 2pπ final states within the CuO 4 plaquette plane. A detailed analysis of the EELS-data allows a classification into two types of charge transfer excitations with predominant π and σ character and various sizes of internal localization and dispersion.