Electronic Quasiparticle Spectrum of Ferromagnetic Semiconductors

We discuss the electronic quasiparticle spectrum of a ferromagnetic semiconductor, within the framework of the s-f model. Some exactly calculable limi ting cases (ferromagnetic saturation, atomic limit, centre of gravity of the conduction band, ....) as well as approximate theories for the general case are reviewed in order to understand the influence of temperature and finite band filling on the conduction band structure. The simple picture that below Tc the conduction band of a ferromagnetic semiconductor is split into two spinpolarized.subbands is obviously wrong. The splitting turns out to be essentially more complex, where the spectral weights (' reduced' densities of states) of the various subbands strongly depend on temperature T and band occupation n. Even at T=o the electron spin polarization is complete only for the special case of the exactly half-filled band (n=l) . For fixed T and n < 1 the polarization increases with increasing band filling n.