Kondo insulators are strongly correlated electron systems in which the interaction of localized electrons (mostly from unfilled 4f-shells of rare earth elements) with conduction electrons leads to the opening of a narrow energy gap at the Fermi level. The microscopic mechanism for this process is far from being well understood.

Our recent investigations have focussed on several systems:

• CeRu₄Sn₆: The overall behaviour of the physical properties of this cage compound ressemble those of typical Kondo insulators. In addition, non-Fermi-liquid-like features are observed at low temperatures. In collaboration with Rand Africaans University Johannesburg and MPI CPfS Dresden.
• CeNiSn (image): The opening of the energy gap is accompanied by a huge maximum in the thermopower below 10 K. In collaboration with Hiroshima University and MPI CPfS Dresden.
• U₂Ru₂Sn: The first tetragonal U-based Kondo insulator. In collaboration with Nagoya University, with Rand Africaans University Johannesburg and MPI CPfS Dresden.
• (Fe,Mn)Si: FeSi is a narrow gap semiconductor with strong hybridisation effects. Substitution of Fe by Mn keeps the B₂₀ crystal structure unchanged but drives the system towards a metallic state. Right at the insulator to metal transition, transport coefficients are promising with respect to thermoelectric applications. In addition, non-Fermi liquid features are expected here, similar to those originated by pressure in the Kondo insulator SmB₆. In co-operation with University of Pavia and Slovak Academy of Sciences, Kosice.

Contact: E. Bauer, S. Bühler-Paschen

Contact - Theory: K. Held