Gap-anisotropic model for the narrow-gap Kondo insulators

A theory is presented which accounts for the dynamical generation of a hybr idization gap with nodes in the Kondo insulating materials CeNiSn and CeRhS b. We show that Hund's interactions acting on virtual 4f(2) configurations of the cerium ion can act to dynamically select the shape of the cerium ion by generating a Weiss field which couples to the shape of the ion. In low symmetry crystals where the external crystal fields are negligible, this pr ocess selects a nodal Kondo semimetal state as the lowest energy configurat ion.