Impurity Band Conduction in a High Temperature Ferromagnetic   Semiconductor
  The band structure of a prototypical dilute ferromagnetic semiconductor, Ga$_{1-x}$Mn$_{x}$As, is studied across the phase diagram via optical spectroscopy. We prove that the Fermi energy ($E_{F}$) resides in a Mn induced impurity band (IB). This conclusion is based upon careful analysis of the frequency and temperature dependence of the optical conductivity ($\sigma_{1}(\omega,T)$). From our analysis of $\sigma_{1}(\omega,T)$ we infer a large effective mass ($m^*$) of the carriers, supporting the view that conduction occurs in an IB. Our results also provide useful insights into the transport properties of Mn-doped GaAs. 
K.S. Burch, D.B. Shrekenhamer, E.J. Singley, J. Stephens, B.L. Sheu,   R.K. Kawakami, P. Schiffer, N. Samarth, D.D. Awschalo...