AndreaBianchi

Kondo insulators are the insulating equivalent of the heavy fermions; here however the electronic correlations lead to the opening of a gap in the bandstructure. Again, as with the heavy fermions, this is largely driven by magnetic fluctuations, which lead to a non-magnetic ground state. FeSi was one of the first compounds to be discussed in this context.

Bandstructure of a Kondo insulator

FeSi crystallizes in the so-called B20 structure which lacks inversion symmetry. Pure FeSi is a socalled Kondo insulator, as at higher temperatures the magnetic moment shows a thermally activated behavior, whereas at low temperatures the system is a non-magnetic insulator. This observation can be explained if one assumes that the Kondo effect leads to the fully screened magnetic Fe moment and the opening of a gap in the electric excitation spectrum. In this picture, the excitation of charge carrier into the conduction electron is always accompanied with the generation of a magnetic moment. FeGe has the same structure but it is a metal which orders ferromagnetically at room temperature. As the compound misses the inversion symmetry, the magnetic moment orders with a long wavelength spiral. By chemically substituting germanium with silicon in FeSi1−xGex, we were able to suppress the magnetic order (1). At a critical concentration xc of 25 %, the magnetic moment disappears in a first order phase transition and a gap is opened in the electronic excitation spectrum.

1. S. Yeo, S. Nakatsuji, A. D. Bianchi, P. Schlottmann, Z. Fisk, L. Balicas, P. A Stampe, and R. J. Kennedy, “First-order transition from a Kondo insulator to a ferromagnetic metal in single crystalline FeSi1−xGex”, Phys. Rev. Lett. 91, 046401 (2003).