Infrared-absorption and photoinduced-absorption spectroscopy of semiconductingYBa2Cu3O6+xA(A=16 and 18; 0≤x≤0.3)

Abstract

We report the infrared (ir) absorption and photoinduced absorption (PA) spectra of semiconducting ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${}_{}{}^{\mathit{A}}{}_{}{}^{}\mathrm{O}_{6+\mathit{x}}^{}{}_{}{}^{}$ for A=16 and 18 and for 0≤x≤0.3. We observe ten ir absorption modes for x=0.3 and report the first observation for all 11 allowed ir modes for x=0.0. The ir modes for x=0.0 and x=0.3 are nearly the same. Shifts of the ir modes with substitution of ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ with ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ are in accord with the expected role of oxygen in the various vibrational modes. The PA spectra of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathit{x}}$ for pumping with 2.71-eV light across the band gap reveals the presence of a photoinduced electronic absorption band that varies with increasing oxygen content from a broad and nearly featureless peak at 1300 ${\mathrm{cm}}^{\mathrm{-}1}$ for x=0.0, to photoinduced transitions at 750, 950, 1100, and 1300 ${\mathrm{cm}}^{\mathrm{-}1}$ for x=0.13 and 0.20, and to a stronger PA peak centered at 950 ${\mathrm{cm}}^{\mathrm{-}1}$ with a shoulder at 750 ${\mathrm{cm}}^{\mathrm{-}1}$ for x=0.3. There are corresponding changes in the PA ir spectra with increasing x, with, in particular, bleaching of ir modes associated with the tetragonal x=0 lattice and enhancement of ir modes associated with the orthorhombic x=1.0 lattice. Based on our results and those from the following paper (Leng et al.), we propose a model for the formation of positively and negatively charged defects, ${\mathit{D}}^{+}$ and ${\mathit{D}}^{\mathrm{-}}$. We suggest that while the ${\mathit{D}}^{+}$ defects reside on the ${\mathrm{CuO}}_2$ planes, the ${\mathit{D}}^{\mathrm{-}}$ defects consist of electrons initially in the conduction band for x=0.0. With increasing oxygen content, the electrons are trapped in CuO chains of increasing length and form localized ${\mathit{D}}^{\mathrm{-}}$ defects.