Evidence for Impurity Bands in La-Doped EuS

Abstract

A study has been made of the effect of La doping on the electrical conductivity of EuS. The doping has been observed to cause considerable decreases of the resistivity at all temperatures between 4.2° and 300°K. In the paramagnetic state, the electrical data are well represented by a straight line of the form ρ/T=ρ0 exp (ΔE/kT),where ΔE is the activation energy for conduction. The resistivity reaches a maximum value at a temperature which is closely related to the Curie temperature (Tc). A decrease of resistivity of approximately three orders of magnitude in each case is observed at the magnetic transition. Resistivities below Tc display a metallic-type temperature dependence. The resistivity anomaly may be attributed to a change of the conduction mechanism from an activated hopping process to that involving impurity bands. It is shown that for temperatures below Tc the existing ideas concerning impurity banding are valid of one properly takes into account the effects of the magnetization of the host lattice. Above Tc the excess electrons are in localized states and the conduction process resembles that of Li-doped NiO. A full account of this work will be published in Physics.