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-f exchange interaction between the magnetic-exciton electron of 5d character and the 4f electrons in the surrounding Eu^2+ ions is the origin of the anomaly. Anomalous magnetic properties in dilute R^3+ alloys are explained by the properties of the giant spin molecule which comes from the strong s-f exchange interaction between the impurity electron and the 4f electrons at the central R^3+ and the surrounding Eu^2+. The anomalous transition from the hopping type to the metallic-impurity band conduction in dilute R^3+ alloys is also explained by the temperature dependence of the activation energy due to the s-f exchange interaction in the magnetic-impurity state.