OPTICAL REFLECTIVITY OF SINGLE-CRYSTAL BI2M3CO2O9+DELTA (M = CA, SR, AND BA) FROM THE INFRARED TO THE VACUUM-ULTRAVIOLET REGION

The dielectric functions of single-crystal Bi2M3Co2O9+delta (M=Ca, Sr, and Ba) were obtained by optical-reflectivity measurements. The Bi-based Co oxides have a characteristic Bi2O2 layer with two sheets of CoO2 plane per primitive cell, and can be a reference material for Bi2Sr2CaCu2O8+delta. The obtained spectra are transformed to give the optical conductivity, and the optical properties of the CoO2 plane are discussed. Comparison of the electronic states of the Co and Cu oxides reveals two characteristics of high-T(c) cuprates. One is that the peak of the midinfrared absorption seen in optical conductivity decreases in energy toward 0 eV upon doping. The other is that the doping-dependent excitation associated with the CuO2 plane is confined in the low-energy region and is separated from other excitations such as those in the Bi2O2 layer.