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| Abstract |
The infrared reflectivity of single crystals of Bi_2M_m+1Co_mO_y (M=Ca, Sr, Ba; m=1,2), Bi_2Sr_3Fe_2O_9.2, and Bi_2Sr_2MnO_6.25 was measured at room temperature between 0.08 and 1.4 eV. A broad absorp...tion 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 Bi_2M_m+1Co_mO_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 Bi_2M_3Co_2O_y, as observed in the high-T_c cuprates in which the carrier density is changed by doping. In addition, as observed in the high-T_c cuprates, the reflectivity-frequency profile below the apparent plasma edge is less curved than predicted by the Drude model. For the Bi_2M_m+1Co_mO_y system, the intensity of the mid-ir absorption approximately scales with that of the free-carrierlike absorption.
The compounds Bi_2Sr_3Fe_2O_9.2 and Bi_2Sr_2Mn_O6.25 appear more insulating in reflectivity measurements than does Bi_2M_m+1Co_mO_y. 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-T_c superconductivity.show more
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