Charge dynamics in the colossal magnetoresistance pyrochlore Tl 2 Mn 2 O 7

Optical conductivity data $[\ensuremath{\sigma}(\ensuremath{\omega})]$ of the colossal magnetoresistance (CMR) pyrochlore ${\mathrm{Tl}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ are presented as functions of temperature (T) and external magnetic field $(B).$ Upon cooling and upon applying B near the Curie temperature, where the CMR manifests itself, $\ensuremath{\sigma}(\ensuremath{\omega})$ shows a clear transition from an insulatorlike to a metallic electronic structure as evidenced by the emergence of a pronounced Drude-like component below $\ensuremath{\sim}0.2 \mathrm{eV}.$ Analyses on the $\ensuremath{\sigma}(\ensuremath{\omega})$ spectra show that both T- and B-induced evolutions of the electronic structure are very similar to each other, and that they are universally related to the development of macroscopic magnetization $(M).$ In particular, the effective carrier density obtained from $\ensuremath{\sigma}(\ensuremath{\omega})$ scales with ${M}^{2}$ over wide ranges of T and B. The contributions to the CMR from the carrier effective mass and scattering time are also evaluated from the data.