MAGNETIC POLARON IN FERROMAGNETIC AND ANTIFERROMAGNETIC SEMICONDUCTORS

The temperature-dependent quasiparticle spectrum of a single conductio n electron exchange coupled to a ferro- or antiferromagnetically order ed localized-spin system (e.g., EuO, EuTe) is calculated by a moment-c onserving Green function technique. In the weak coupling regime the ex change interaction leads to an almost rigid shift of the Bloch dispers ion. The induced spin splitting of the conduction band states is propo rtional to the magnetization [S-Z] of the localized-spin system. As so on as the coupling constant exceeds a critical value an additional spl itting of the quasiparticle dispersion for each spin projection sets i n due to different elementary excitations. One is based on a repeated emission and reabsorption of a magnon by the conduction electron resul ting in an effective attraction between magnon and electron. This give s rise to a polaronlike quasiparticle (''magnetic polaron''). Another excitation is due to a direct magnon emission or absorption by the ele ctron thereby flipping its own spin (''scattering states''). For the e xactly calculable special case of a ferromagnetically saturated spin s ystem (T=0 K), the magnetic polaron appears only in the down arrow spe ctrum and turns out to be a stable quasiparticle. For finite temperatu res it gets a finite Lifetime. In antiferromagnetic systems each quasi particle band exhibits an additional ''Slater splitting'' due to the r educed magnetic Brillouin zone. The predicted strong correlation effec ts in the excitation spectrum require unconventional interpretations o f respective inverse photoemission experiments.