ab-plane optical properties ofYBa2Cu3O7−δBrysingle crystals

Room-temperature infrared- and optical-spectroscopic measurements are reported on brominated ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.2}$ single-crystal samples. Incorporation of bromine in the lattice was observed in both Auger electron and infrared-spectroscopy experiments but the exact lattice site for bromine dopant atoms remains unknown. Five of the six in-plane infrared-active ${\mathit{E}}_{\mathit{u}}$ phonon modes were observed in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.2}$ ${\mathrm{Br}}_{\mathit{y}}$ samples and phonon frequency softening was observed for the 591-${\mathrm{cm}}^{\mathrm{-}1}$ phonon [Cu(2)-O(2) in-plane stretch] mode. The frequency-dependent optical conductivity σ(ω) was obtained from the Kramers-Kronig transformation of the near-normal-incidence reflectance R(ω) measurements, and the resulting σ(ω) showed the presence of a mid-infrared band (centered at ∼2500 ${\mathrm{cm}}^{\mathrm{-}1}$) in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.2}$ ${\mathrm{Br}}_{\mathit{y}}$.