ELLIPSOMETRIC STUDY OF THE CHARGE-TRANSFER EXCITATION IN SINGLE-CRYSTAL LA2CUO4

The dielectric response functions parallel and perpendicular to the c axis of single-crystal La2CuO4 are determined at room temperature from 0.7 to 4.9 eV by analysis of ellipsometric data. Strong anisotropy is observed; for E parallel-to c the optical properties are nearly featu reless in this energy range, while for E perpendicular-to c there is a sharp peak in Im(E) near 2 eV associated with the O(2p) to Cu(3d) cha rge-transfer excitation. Additional features are observed on the high- energy side of this peak, and the origin of these is discussed in the light of recent theoretical predictions.