Effective action for the Kondo lattice model. New approach for S=1/2

In the partition function of the Kondo lattice (KL), spin matrices are exac tly replaced by bilinear combinations of Fermi operators with the purely im aginary chemical potential lambda = - i pi T/2 (Popov representation). This new representation of spin operators allows one to introduce new Green's F unctions (GF) with Matsubara frequencies omega(n) = 2 pi T(n + 1/4) for S = 1/2. A simple temperature diagram technique is constructed with the path i ntegral method. This technique is standard and does not contain the complic ated combinatoric rules characteristic of most of the known variants of the diagram techniques for spin systems. The effective action for the almost a ntiferromagnetic KL problem is derived. (C) 1999 Elsevier Science B.V. All rights reserved.