A ug 1 99 3 Correlated mean field Ansatz for the Kondo necklace

We study the ground state phase diagram of the pseudospin model introduced by Doniach to describe the essential physics of Kondo lattices. We use variational trial states which augment the usual mean field solution by incorporating various intersite correlations. A composite spin correlation describing the antiparallel alignment of fluctuating triplets is found to be particularly favourable for large Kondo couplings. With this trial state, the magnetic–to–Kondo transition is suppressed and the strong coupling ground state is ordered with strongly reduced moments. The relevance of the findings is discussed.