Quasiparticle density of states and edge shifts of doped ferromagnetic semiconductors

We use a new spectral-density approach to find the temperature behavior of the quasiparticle density of states for the conduction band of a ferromagnetic semiconductor. Because of the exchange interaction between conduction electrons and localized spin system ($4f$ moments in the case of Eu chalcogenides), the conduction band splits into several quasiparticle subbands. The famous red shift of the optical absorption edge is almost quantitatively explained as a bandwidth effect of the lower ↑ subband. The striking band-filling dependence of the red shift, first observed by Schoenes and Wachter for ${\mathrm{Eu}}_{1-x}{\mathrm{Gd}}_x\mathrm{O}$, finds a natural interpretation by the existence and temperature dependence of quasiparticles in the conduction band of a ferromagnetic semiconductor EuO.