Infrared conductivity of metallic (III,Mn)V ferromagnets

We present a theory of the infrared conductivity and absorption coefficients of metallic (III,Mn)V ferromagnetic semiconductors. We find that the conductivity is dominated by intervalence-band transitions that produce peaks at (h) over bar omegasimilar to220 meV and obscure the broadened Drude peak. We demonstrate that transverse f-sum rule measurements can be used to extract accurate values for the free-carrier density, bypassing the severe characterization difficulties that have till now been created by the large anomalous Hall effect in these materials.