Optical investigation of room-temperature chain ordering in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

We have investigated the near-infrared (0.5–1.5 eV) optical response of quenched ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ samples with intermediate oxygen content as a function of the time τ the samples spent at room temperature. In a previous experiment, we determined the dielectric function in the spectral region between 3.8 and 4.3 eV under similar conditions using spectroscopic ellipsometry. We find an increase of the concentration of Cu(I${)}^{1+}$ ions with τ, indicating an oxygen-reordering process in the course of which Cu-O dumbells [linearly coordinated O(IV)-Cu(I)-O(IV) complexes] are generated. The reflectance measurements in the infrared regime between 0.5 and 1.5 eV indicate that simultaneously additional carriers are generated in the ${\mathrm{CuO}}_2$ planes. Both findings are in accordance with previous explanations of the rise of ${\mathit{T}}_{\mathit{c}}$ with τ.