OPTICAL-PROPERTIES OF AN EFFECTIVE ONE-BAND HUBBARD-MODEL FOR THE CUPRATES

We study the Cu and O spectral density of states and the optical condu ctivity of CuO2 planes using an effective generalized one-band Hubbard model derived from the extended three-band Hubbard model. We solve ex actly a square cluster of 10 unit cells and average the results over a ll possible boundary conditions, which leads to smooth functions of fr equency. Upon doping, the Fermi energy jumps to Zhang-Rice states whic h are connected to the rest of the valence band (in contrast to an iso lated band in the middle of the gap). The transfer of spectral weight depends on the parameters of the original three-band model not only th rough the one-band effective parameters but also through the relevant matrix elements. We discuss the evolution of the gap upon doping. The optical conductivity of the doped system shows a mid-infrared peak due to intraband transitions, a pseudogap, and a high-frequency part rela ted to interband transitions. Its shape and integrated weight up to a given frequency (including the Drude weight) agree qualitatively with experiments in the cuprates for low to moderate doping levels, but sig nificant deviations exist for doping x>0.3.