The dielectric functions of single-crystal Bi&M3Co209+q {M=Ca, Sr, and Ba) were obtained by optical-reflectivity measurements.The Bi-based Co oxides have a characteristic Bi202 layer with two sheets of Co02 plane per primitive cell, and can be a reference material for Bi&Sr2CaCu208+z. The obtained spectra are transformed to give the optical conductivity, and the optical properties of the Co02 plane are discussed. Comparison of the electronic states of the Co and Cu oxides reveals two characteristics of high-T, 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 Cu02 plane is con6ned in the low-energy region and is separated from other excitations such as those in the Bi202 layer.
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