Anomalous Transport Phenomena in Eu-Chalcogenide Alloys

TLDR

Eu‑chalcogenide alloys exhibit anomalous optical, magnetic, and transport phenomena. These anomalies stem from strong d–f and s–f exchange interactions that form magnetic excitons and giant spin molecules, producing bound states, enhanced optical responses, and temperature‑dependent transitions between hopping and metallic‑impurity conduction.

Abstract

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.