Dilution studies of the Kondo insulators and alkaline earth and rare earth hexaborides

Experimental results are presented on a doping study of strongly correlated electron materials. These include the Kondo insulators FeSi and Ce3Bi 4Pt3 and the hexaboride compounds XB6, where X is a rare earth or alkaline earth metal.; Both FeSi and Ce3Bi4Pt3 have nonmagnetic and insulating ground states. The transport and spin gaps of these materials are extracted from activated fits to the electrical resistivity and magnetic susceptibility, respectively. The behavior of the gaps are analyzed as a function of doping. FeSi has been doped at the Fe site with Co, Mn, and Ru. Both Co and Mn doping lead to moment formation, increase in carriers, a large reduction in the low-temperature resistivity, and a suppression of the transport gap. At the Si site, FeSi has been doped with P and Ge. Phosphorus doping results in moment formation for low concentrations, an increase in carrier concentration, and a suppression of the transport gap. The isovalent substitution of Si with Ge produces ferromagnetism where Tc increases with increasing Ge concentration. The transport gap is also suppressed as a function of doping, and thermoelectric power measurements resemble those of other doped Kondo insulators. Similar results are observed for doped compounds of Ce3Bi4Pt3.; A doping study is made of the semimetallic feromagnet EuB6 by replacing Eu with three nonmagnetic substitutions: La, Ca, and Ba. A semiquantitative treatment of the RKKY interaction predicts the antiferromagnetic ordering observed in the doping studies of EuB6 and reconciles the findings of high-pressure measurements which appear to be at odds with the chemical substitutions.; Finally, a study was made to track the development of the Kondo-insulating ground state from a nonmagnetic insulator. At very low doping levels (<1%), metallic behavior is observed in the electrical resistivity, and magnetic measurements show an anomalously large, nearly temperature-independent susceptibility and weak ferromagnetism. Similar results are observed for Ce-doped SrB 6 and CaB6, suggesting a long-range interaction between the f-moments in these materials. However, doping with nonmagnetic La produces the same results. The data are consistent with a partial spin polarization of a nearly free conduction electron gas.