On the true quasiparticle structure of the conduction band of a ferromagnetic semiconductor

In order to clarify controversial discussions concerning the electronic excitation spectrum of ferromagnetic semiconductors such as EuO and EuS exact T=0 results for the spectral density and the quasiparticle density of states are presented and discussed within the framework of the sf model. The physics of the sf model turns out to be strikingly different in the strong-coupling region g/W>or approximately=0.5 (atomic-like behaviour, g is the sf exchange constant and W is the Bloch bandwidth), in the intermediate-coupling region 0.1>or approximately=g/W. It has been overlooked in the past that typical magnetic semiconductors (e.g. europium chalcogenides) are spread over all three regions. Mean-field treatments are reasonable in a certain sense for weakly coupled systems such as EuO but are, however, completely inappropriate for more strongly coupled systems such as EuS. Due to the sf exchange the conduction band consists of several quasiparticle sub-bands, overlapping in the weak-coupling but with real gaps between them in the intermediate- and strong-coupling regions.