Anomalous Transport Phenomena in Eu-Chalcogenide Alloys

Various anomalous optical, magnetic, and transport phenomena have been found in Eu-chalcogenide alloys. Anomalous optical properties are explained by the magnetic exciton in which a hole in the $4f$ conduction band and an electron (for optically active magnetic exciton) in the $5d$ conduction band make a bound state. Strong $d\ensuremath{-}f$ exchange interaction between the magnetic-exciton electron of $5d$ character and the $4f$ electrons in the surrounding ${\mathrm{Eu}}^{2+}$ ions is the origin of the anomaly. Anomalous magnetic properties in dilute ${\mathrm{R}}^{3+}$ alloys are explained by the properties of the giant spin molecule which comes from the strong $s\ensuremath{-}f$ exchange interaction between the impurity electron and the $4f$ electrons at the central ${R}^{3+}$ and the surrounding ${\mathrm{Eu}}^{2+}$. The anomalous transition from the hopping type to the metallic-impurity band conduction in dilute ${\mathrm{R}}^{3+}$ alloys is also explained by the temperature dependence of the activation energy due to the $s\ensuremath{-}f$ exchange interaction in the magnetic-impurity state.