The infrared reflectivity of single crystals of Bi2Mm+1ComOy (M=Ca, Sr, Ba; m=1,2), Bi2Sr3Fe2O9.2, and Bi2Sr2MnO6.25 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 Bi2Mm+1ComOy 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 Bi2M3Co2Oy, as observed in the high-Tc cuprates in which the carrier density is changed by doping. In addition, as observed in the high-Tc cuprates, the reflectivity-frequency profile below the apparent plasma edge is less curved than predicted by the Drude model. For the Bi2Mm+1ComOy system, the intensity of the mid-ir absorption approximately scales with that of the free-carrierlike absorption. The compounds Bi2Sr3Fe2O9.2 and Bi2Sr2MnO6.25 appear more insulating in reflectivity measurements than does Bi2Mm+1ComOy. However, the intensity of the mid-ir absorption in these crystals is slightly larger. Our results suggest that the existence of a mid-ir absorption band is not a sufficient condition for the occurrence of high-Tc superconductivity.
All Science Journal Classification (ASJC) codes
- Condensed Matter Physics