Kondo atoms, double exchange molecules, and a novel large S expansion for the ordered Kondo lattice

Kondo lattice models are widely used to describe systems where local magnet ic moments couple to itinerant electrons. We construct a novel large S expa nsion scheme for an ordered Kondo lattice, starting from the eigenstates of the Kondo coupling on a single site, and paying particular attention to th e special case of two sites sharing a single electron, which we solve exact ly. The expansion scheme which we introduce is shown to reproduce all featu res of this exact solution, to O(1/S-2), and to offer a better physical sta rting point than previous approaches for calculating the properties of syst ems on a lattice in the 'double exchange' limit of strong Hund's rule coupl ing.