From the Kondo-Hubbard model to the spin brush model

In the regime of large Hubbard coupling and at half-filling, the Kondo-Hubb ard model is mapped onto a new effective model. This is a model of spins in teracting antiferromagnetically on each site of a brush (spin brush model). A phase diagram is established. Two phases are found, an antiferromagnetic phase and a phase of local singlet with a band of spin-triplet excitations . Using a recent I;ind of coherent state, the long wavelength fluctuations are described by the nonlinear sigma model. The quantum critical transition is found and is shifted in comparison to the classical one. (C) 1999 Elsev ier Science B.V. All rights reserved.