DOPING DEPENDENCE OF THE ELLIPSOMETRIC SPECTRA OF ND2-XCEXCUO4-DELTA SINGLE-CRYSTALS

We study the evolution with doping of the optical spectra of the Cu-O planes in Ln(2-x)Ce(x)CuO(4-delta) (Ln=lanthanide, mostly Nd) with 0 l ess than or equal to x less than or equal to 0.21. We analyse in detai l the spectral features that appear between 0.7 and 5.2 eV in room-tem perature ellipsometric measurements. The intensity and energy position of the observed spectral features show systematic dependences with th e composition of the samples. Already light doping causes relevant cha nges in the line shape of the charge-transfer excitation, located at 1 .55 eV, due to the interaction of localized and mobile carriers, confi rming the excitonic character of this transition. The additional shoul der that appears at 2 eV has band character. The doping-induced featur e near 4.3 eV is assigned to an intraionic Cu transition. Its smooth s hift with x gives the progressive Fermi level rise with doping. The sy stematics of all the results supports the picture of a doped insulator . (C) 1998 Elsevier Science B.V.