  We consider the electronic properties of ferromagnetic bulk GaMnAs at zero
temperature using two realistic tight-binding models, one due to Tang and
Flatte and one due to Masek. In particular, we study the density of states, the
Fermi energy, the inverse participation ratio, and the optical conductivity
with varying impurity concentration x=0.01-0.15. The results are very sensitive
to the assumptions made for the on-site and hopping matrix elements of the Mn
impurities. For low concentrations, x<0.02, Masek's model shows only small
deviations from the case of p-doped GaAs with increased number of holes while
within Tang and Flatte's model an impurity-band forms. For higher
concentrations x, Masek's model shows minor quantitative changes in the
properties we studied while the results of the Tang and Flatte model exhibit
qualitative changes including strong localization of eigenstates with energies
close to the band edge. These differences between the two approaches are in
particular visible in the optical conductivity, where Masek's model shows a
Drude peak at zero frequency while no such peak is observed in Tang and
Flatte's model. Interestingly, although the two models differ qualitatively the
calculated effective optical masses of both models are similar within the range
of 0.4-1.0 of the free electron mass.
