Infrared probe of itinerant ferromagnetism in Ga(1-x)Mn(x)As

The doping and temperature dependence of the complex conductivity is determined for the ferromagnetic semiconductor Ga(1-x)Mn(x)As. A broad resonance develops with Mn doping at an energy scale of approximately 200 meV, well within the GaAs band gap. Possible origins of this feature are explored in the context of a Mn induced impurity band and intervalence band transitions. From a sum rule analysis of the conductivity data the effective mass of the itinerant charge carriers is found to be at least a factor of 3 greater than what is expected for hole doped GaAs. In the ferromagnetic state a significant decrease in the effective mass is observed, demonstrating the role played by the heavy carriers in inducing ferromagnetism in this system.