Spectral functions of 2D systems with coupling of electrons to   collective or localized spin degrees of freedom
  The spectral properties of itinerant 2D systems with (nearly) ferromagnetic ground state are studied within the spin-fermion and the classical s-d exchange models. While the former model describes the effect of collective magnetic excitations on the electronic properties, the latter one considers the effect of local moments. We use the equation of motion approach combined with the 1/M and 1/z expansions (M is the number of spin components, z is the coordination number) to investigate spectral functions. In both the models the spectrum splitting occurs at low temperatures T in the renormalized classical regime due to strong magnetic fluctuations. For the interaction J between electronic and magnetic degrees of freedom comparable to the bandwidth (the intermediate-coupling regime) the 1/z expansion predicts full splitting of electronic spectrum below some temperature T_s. At the same time, the spectrum remains only partly split in the 1/M expansion with the quasiparticle structure which becomes fully coherent at low T. 
A. A. Katanin and V. Yu. Irkhin