Infrared Studies of a La_{0.67}Ca_{0.33}MnO_3 Single Crystal: Optical Magnetoconductivity in a Half-Metallic Ferromagnet

The infrared reflectivity of a $\mathrm{L}{\mathrm{a}}_{0.67}\mathrm{C}{\mathrm{a}}_{0.33}\mathrm{M}\mathrm{n}{\mathrm{O}}_3$ single crystal is studied over a broad range of temperatures (78-340 K), magnetic fields (0-16 T), and wavenumbers (20-9000 cm${}^{-1}$). The optical conductivity gradually changes from a Drude-like behavior to a broad peak feature near 5000 cm${}^{-1}$ in the ferromagnetic state below the Curie temperature $T_C=307K$. Various features of the optical conductivity bear striking resemblance to recent theoretical predictions based on the interplay between the double exchange interaction and the Jahn-Teller electron-phonon coupling. A large optical magnetoconductivity is observed near $T_C$.