FeSi exhibits a Curie-like susceptibility accompanied by a moderate electronic conductivity at high temperature. At low temperature the magnetic susceptibility goes to zero, and a small charge gap appears. We discuss phenomenological and theoretical aspects of the relationship between FeSi and various rare-earth correlated systems often referred to as Kondo insulators.

We report the reflectivity and optical conductivity of the Kondo insulator Ce3Bi4Pt3. For temperatures less than 100 K, depletion of the conductivity below about 300 cm-1 signifies the development of a charge gap. The temperature dependence of the disappearance of the spectral weight scales with the quenching of the Ce 4f moments. © 1994.

We report the first infrared and optical measurements of the absolute conductivity of FeSi. Above 200 K the ac conductivity of FeSi resembles that of a dirty metal, while at low temperature most of the conductivity below 60 meV is depleted, consistent with an energy gap of that magnitude. Thus in contrast to what is expected from a picture based on simple thermal activation, the gap disappears at a temperature which is low relative to its size. Moreover, the spectral weight lost below the gap does not reappear just above the gap, but is instead distributed over an energy range of order eV. © 1993 The American Physical Society.

We combine the results of terahertz time-domain spectroscopy with far-infrared transmission and reflectivity to obtain the conductivity of SrRuO_3 over an unprecedented continuous range in frequency, allowing us to characterize the approach to zero frequency as a function of temperature. We show that the conductivity follows a simple phenomenological form, with an analytic structure fundamentally different from that predicted by the standard theory of metals.

The Kondo insulating materials present a particularly simple limiting case of the strongly correlated electron lattice problem, one occupied f-state interacting with a single half-filled conduction band. Experiment shows that the solution to this problem has some remarkably simple aspects. Optical conductivity data display the strong coupling nature of this physics. © 1995.