Infrared reflectivity of La1.89Ca1.11Cu2O6±δ single crystals at various O2 annealing pressures

The ab-plane reflectivity of La1.89Ca1.11Cu2O6±δ single crystals, whose properties change from semiconductor to superconductor under high-oxygen-pressure annealing, is measured between 250 and 20 000 cm-1 at room temperature. Although the annealing increases the oxygen content very slightly, the spectra change in the same way as seen in other cuprates with carrier doping: a charge-transfer peak disappears and low-energy excitation increases. We suggest the possibility that holes in the apex oxygen redistribute to the CuO2 planes. Mid-infrared absorption is observed in all samples. Relation between the mid-infrared absorption and the influence of La and Ca disorder on the CuO2 planes is also discussed.