Infrared reflectivity of single-crystal Bi2Mm+1ComOy (M=Ca,Sr,Ba; m=1,2), Bi2Sr3Fe2O9.2, and Bi2Sr2MnO6.25, isomorphic to Bi-Cu-based high-Tc oxides

Abstract

The infrared reflectivity of single crystals of ${\mathrm{Bi}}_2$ ${\mathit{M}}_{\mathit{m}+1\mathrm{}}$ ${\mathrm{Co}}_{\mathit{m}}$ ${\mathrm{O}}_{\mathit{y}}$ (M=Ca, Sr, Ba; m=1,2), ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_3$ ${\mathrm{Fe}}_2$ ${\mathrm{O}}_{9.2}$, and ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{MnO}}_{6.25}$ was measured at room temperature between 0.08 and 1.4 eV. A broad absorption band is observed in the mid-ir range (near 0.5 eV) in all the compounds studied, and an absorption band near 0.3 eV is observed in the insulating system. In terms of a conventional Drude-Lorentz model, the measured reflectivity and the frequency-dependent conductivity between 0.2 and 1 eV can be fitted with three broad Lorentzians and a Drude term. For the ${\mathrm{Bi}}_2$ ${\mathit{M}}_{\mathit{m}+1\mathrm{}}$ ${\mathrm{Co}}_{\mathit{m}}$ ${\mathrm{O}}_{\mathit{y}}$ system, the reflectivity increases and assumes a more metallic profile as the number of Co-O layers per unit cell increases, or as the ionic radius of M increases. The apparent plasma edge of this system is about 0.3 eV, and remains unshifted for all ${\mathrm{Bi}}_2$ ${\mathit{M}}_3$ ${\mathrm{Co}}_2$ ${\mathrm{O}}_{\mathit{y}}$, as observed in the high-${\mathit{T}}_{\mathit{c}}$ cuprates in which the carrier density is changed by doping. In addition, as observed in the high-${\mathit{T}}_{\mathit{c}}$ cuprates, the reflectivity-frequency profile below the apparent plasma edge is less curved than predicted by the Drude model. For the ${\mathrm{Bi}}_2$ ${\mathit{M}}_{\mathit{m}+1\mathrm{}}$ ${\mathrm{Co}}_{\mathit{m}}$ ${\mathrm{O}}_{\mathit{y}}$ system, the intensity of the mid-ir absorption approximately scales with that of the free-carrierlike absorption.