Kondo Semiconductor CeNiSn

Abstract

Since the discovery of semiconducting behavior of SmB6 in 1969,1 the formation of a small energy gap in 4f-electron systems has been the subject of intensive studies. The presence of energy gap of several 10 K was found in valence-fluctuating (VF) compounds gold SmS, TmSe and YbB12 with cubic structures.2–4 A simple picture of these systems is that the hybridization of the 4f electron states and the conduction band leads to the small gap at the Fermi level.5 However, the detailed mechanism of the gap formation remains unsettled. Recently, CeNiSn has been found to be the first example of a cerium compound showing the behavior of a small-gapped semiconductor.6 From the activation-type resistivity, the gap energy Eg was estimated to be 6 K. This compound crystallizes in an orthorhombic structure (Pn21a),7 which is closely related to the e-TiNiSi type structure. Subsequently, similar gap formation has been found in Ce3Pt3Bi4 and CeRhSb,8,9 where the values of Eg are 70 and 8 K, respectively. The latter crystallizes in the same type of structure as CeNiSn, whereas the former in the cubic Y3Au3Sb4-type structure. These findings have renewed the interest in the problem of the insulating ground state of the Kondo lattice.10,11