ABSORPTION AND PHOTOINDUCED-ABSORPTION SPECTROSCOPY IN SEMICONDUCTING YBA2CU3O6+X

Semiconducting YBa2Cu3O6+x thin films with x = 0.0 and 0.3 have been studied extensively via direct-absorption (0.2-5.5 eV) and photoinduced-absorption (0.5-3.3 eV) techniques. Optical absorptions at approximately 0.5, approximately 1.7, approximately 3.0, approximately 4.3, and approximately 4.8 eV were observed in direct-absorption studies for x = 0.0. With addition of oxygen to form the x = 0.3 composition, these absorptions evolved to approximately 0.5, approximately 1.7, approximately3.0, approximately 4.3, and approximately 5.2 eV. The photoinduced-absorption spectra showed several common features: Two broad photoinduced-absorption bands peaked at 1.1 and 1.4 eV and two strong photoinduced-bleaching peaks at 1.9 and 2.6 eV. The photoinduced-absorption peaks are assigned to optical transitions from O p bands to a photoinduced charged-defect state. This self-localized positively charged defect state may be direct evidence of the presence of an oxygen 2p hole polaronic state in semiconducting YBa2Cu3O6+x upon photoexcitation. Within this model, the double peaks of photoinduced absorption reflect the excitation spectrum of the Zhang-Rice singlet predicted by Chen and Schuttler for the strong-coupling-limit t-J model. Possible alternative origins of the photoinduced-absorption peaks are pointed out. A photoinduced absorption peaked at 0.13 eV and varying with oxygen content is suggested to be associated with excitation within the CuO chains in the preceding paper (Ye et al.). The photoinduced-absorption spectra together with the pump-laser power and modulation frequency dependence of each of the photoinduced features suggest that the 1.9- and 2.6-eV photoinduced-bleaching peaks arise from in-plane O p-sigma-band to Cu dx2-y2and from in-plane O p-pi-x,y to Cu dx2-y2 and/or out-of-plane O p-pi-z to Cu d3z2-r2 band transitions, respectively.