An infrared probe of the insulator-to-metal transition in GaMnAs and   GaBeAs
  We report infrared studies of the insulator-to-metal transition (IMT) in GaAs doped with either magnetic (Mn) or non-magnetic acceptors (Be). We observe a resonance with a natural assignment to impurity states in the insulating regime of Ga$_{1-x}$Mn$_x$As, which persists across the IMT to the highest doping (16%). Beyond the IMT boundary, behavior combining insulating and metallic trends also persists to the highest Mn doping. Be doped samples however, display conventional metallicity just above the critical IMT concentration, with features indicative of transport within the host valence band. 
B. C. Chapler, R. C. Myers, S. Mack, A. Frenzel, B. C. Pursley, K. S.   Burch, E. J. Singley, A. M. Dattelbaum, N. Samarth, ...
cond-mat.mtrl-sci