Optical Absorption by Small Polarons inp- andn-Type Lanthanum Cobaltite

Experimental data for the absorption coefficient of $p$- and $n$-type LaCo${\mathrm{O}}_3$ are presented for wavelengths between 3 and 14 μ for the temperatures $T_1=290\mathrm{}°$K, $T_2=200\mathrm{}°$K, $T_3=77\mathrm{}°K$, $T_4=4.2\mathrm{}°$K, and four different dopant concentrations. At high temperatures, the absorption coefficient has a minimum at 11 μ. The frequency dependence and the absolute magnitude of the optical absorption cannot be explained by a Drude-Zener-type carrier absorption. At $T_4=4.2\mathrm{}°$K considerable structure is seen in the absorption. This structure cannot be ascribed to localized modes due to the dopant atoms. The data at $T_2$ and $T_3$ show the gradual change of the absorption from high- to low-temperature behavior. The data at $T_1$ and $T_4$ can be consistently explained using the high- and low-temperature versions of the small-polaron theory. The agreement between theory and experiment is quantitative for $T_1$ and for $T_4$ in the region between 3 and 8 μ. For higher wave-lengths the agreement is only qualitative because of the incomplete knowledge of the lattice properties which is needed in order to explain the two-phonon structures in the polaron absorption.