Solid state Pomeranchuk effect in unstable Kondo lattice systems

In this paper we study the first order transitions which occur in Kondo lattice systems accompanied by volume instabilities. In general, in non-magnetic Kondo lattice materials there is a smooth crossover along a coherence line between a Fermi liquid and a local moment regime. We investigate here the conditions that for certain materials this crossover changes into an abrupt transition at which local moment and Fermi liquid phases coexist. The system YbInCu4 is such a material and we point out the similarities of its thermodynamic behavior with that of 3He including the existence of a solid state Pomeranchuk effect. Finally, the possibility of constructing a cooling machine based on unstable Kondo lattice systems is discussed.