ABSTRACT
The band structure of a prototypical dilute magnetic semiconductor (DMS), Ga1-xMnxAs, is studied across the phase diagram via infrared and optical spectroscopy. We prove that the Fermi energy (EF) resides in a Mn-induced impurity band (IB). Specifically the changes in the frequency dependent optical conductivity [sigma1(omega)] with carrier density are only consistent with EF lying in an IB. Furthermore, the large effective mass (m*) of the carriers inferred from our analysis of sigma1(omega) supports this conclusion. Our findings demonstrate that the metal to insulator transition in this DMS is qualitatively different from other III-V semiconductors doped with nonmagnetic impurities. We also provide insights into the anomalous transport properties of Ga1-xMnxAs.
ABSTRACT
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.
