Syed Ismat Ullah Shah

ABSORPTION AND PHOTOINDUCED ABSORPTION STUDIES OF La2CuO4 and YBa2Cu3O6+x: ROLE OF DEFECT STATES

ABSTRACT Linear and photoinduced optical absorption studies of YBa 2 Cu 3 O 6+x are described and compared with those of La 2 CuO 4 . Absorption studies of thin films of YBa 2 Cu 3 O 6+x show a series of optical absorptions at 0.5, 1.6, 3.0, 4.3, and 4.8 eV for x=0. As × increases, the energies of each of the peaks are unchanged except for the highest-energy peak, which shifts considerably with increasing oxygen concetration. Photoinduced optical absorptions studies of YBa 2 Cu 3 O 6+x reveal similar results for x=0 and x=0.3. These spectra differ from those of La 2 CuO 4 in that the crossover from photoinduced absorption to photoinduced bleaching is shifted down in energy from 2.0 eV to 1.5 eV. There remain two photoinduced electronic absorptions, though their energies and character differ substantially from those observed in La 2 CuO 4 . For x=0.0 samples, the high energy photoinduced absorption is very broad, with a weak maximum at ˜ 1.2 eV. The low energy peak is much sharper, with a maximum at ˜0.13 eV. The infrared spectrum of x=0.0 samples is similar to that previously reported. Photoinduced infrared studies demonstrate the important role of electron-phonon coupling. The phonon modes shift as expected with substitution of 18 O for 16 O. A new photoinduced mode at 315 cm −1 is observed for YBa 2 Cu 3 16 O 6.0 . This mode is associated with the breaking of the tetragonal symmetry to orthorhombic or lower symmetry upon photoexcitation of charge carriers. It is likely associated with the formation of O 1− sites upon photoexcited charge transfer.