Magnetic polaron in ferro- and antiferromagnetic semiconductors

The temperature-dependent quasiparticle spectrum of a single conduction electron exchange coupled to a ferro- or antiferromagnetically ordered localized-spin system (e.g., EuO, EuTe) is calculated by a moment-conserving Green function technique. In the weak coupling regime the exchange interaction leads to an almost rigid shift of the Bloch dispersion. The induced spin splitting of the conduction band states is proportional to the magnetization [S-Z] of the localized-spin system. As soon as the coupling constant exceeds a critical value an additional splitting of the quasiparticle dispersion for each spin projection sets in due to different elementary excitations. One is based on a repeated emission and reabsorption of a magnon by the conduction electron resulting in an effective attraction between magnon and electron. This gives rise to a polaronlike quasiparticle (''magnetic polaron''). Another excitation is due to a direct magnon emission or absorption by the electron thereby flipping its own spin (''scattering states''). For the exactly calculable special case of a ferromagnetically saturated spin system (T=0 K), the magnetic polaron appears only in the down arrow spectrum and turns out to be a stable quasiparticle. For finite temperatures it gets a finite Lifetime. In antiferromagnetic systems each quasiparticle band exhibits an additional ''Slater splitting'' due to the reduced magnetic Brillouin zone. The predicted strong correlation effects in the excitation spectrum require unconventional interpretations of respective inverse photoemission experiments.