Charge dynamics in the colossal magnetoresistance pyrochlore ${\mathrm{Tl}}_2{\mathrm{Mn}}_2{\mathrm{O}}_7$

Optical conductivity data $\left[\sigma \right(\omega \left)\right]$ 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 $\mathrm{}\left(B\right).$ Upon cooling and upon applying B near the Curie temperature, where the CMR manifests itself, $\sigma \left(\omega \right)$ shows a clear transition from an insulatorlike to a metallic electronic structure as evidenced by the emergence of a pronounced Drude-like component below $\sim 0.2\hspace{0.5em}\mathrm{eV}.$ Analyses on the $\sigma \left(\omega \right)$ 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 $\mathrm{}\left(M\right).$ In particular, the effective carrier density obtained from $\sigma \left(\omega \right)$ 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.