Investigation of a d-electron Kondo insulator iron antimonid

FeSb2 is a narrow band semiconductor, whose magnetic susceptibility strongly resembles that of FeSi, an archetype example of a Kondo insulator. The unusual spectral weight transfer in the optical conductivity measurement and ab initio LDA+U calculations imply that FeSb2 is an unconventional semiconductor with strong electron correlations. In order to study the anisotropy and probe the electronic structure of FeSb2, single crystals are grown by high temperature flux method and carriers are introduced into FeSb 2 by substituting Fe and Sb with Co/Cr and Te respectively. We investigated the magnetic and electrical properties of FeSb2 and the doping effects. Hall measurement on FeSb2 reveals that multiple band carriers with very large mobility difference are involved in the electrical transport. This can be explained by the orbital selective hybridization of the Fe - d orbitals with Sb - p electrons in FeSb2 involving only one electronic subsystem. Band structure calculation suggests that FeSb 2 is close to a magnetic instability. Indeed, and various magnetic orders are experimentally observed in doped FeSb2. For example, the activated Pauli paramagnetism of FeSb2 evolves into a weak ferromagnetism in (Fe1--xCox)Sb 2 and a complex canted antiferromagnetic structure in (Fe1-- xCrx)Sb 2 and Fe(Sb1--xTe x)2. Metallic states are readily induced by doping FeSb2 as a result of band filling. Positive colossal magnetoresistance mainly attributed to quasi 1D weak localization is observed in Co doped FeSb 2. With Te substitution, FeSb2 evolves from a strongly correlated semiconductor into a moderate heavy fermion metal. Most of the Kondo insulator materials are cubic, with the exception of CeRhSb and CeNiSn, and 4f intermetallics. An important question is that if the Kondo insulator scenario can be applied to 3d materials, e.g. FeSi and FeSb2. FeSb2 can be obtained in single crystal form with high quality and readily doped with carriers, thus it represents a model system with only 3d elements to study this physics. Our results indicate that the underling physics of FeSb2 can be well described by the Kondo insulator picture and FeSb2 resembles FeSi in many ways as a prototypical example of a nonrare-earth containing Kondo insulator.