Theory of optical conductivity for dilute GaMnAs

We construct a semi-microscopic theory, to describe the optical conductivity of GaMnAs in the dilute limit, x = 1%. We construct an effective Hamiltonian that captures inside-impurity band optical transitions as well as transitions between the valence band and the impurity band. All parameters of the Hamiltonian are computed from microscopic variational calculations. We find a metal-insulator transition within the impurity band in the concentration range, x = 0.2 -0.3 for uncompensated and x = 1-3% for compensated samples, in good agreement with the experiments. We find an optical mass m_opt = m_e, which is almost independent of the impurity concentration excepting in the vicinity of the metal-insulator transition, where it reaches values as large as m_opt = 10 m_e. We also reproduce a mid-infrared peak at \hbar \omega = 200 meV, which redshifts upon doping, in quantitative agreement with the experiments.