Semi-fermionic approach for quantum spin systems

We present a general derivation of semi-fermionic representation for generators of SU(N) group as a bilinear combination of Fermi operators. The constraints are fulfilled by means of imaginary Lagrange multipliers. The important case of SU(2) group is discussed. We demonstrate how the idea of semi-fermionic representation might be extended to the groups possessing dynamic symmetries. As an example, SO(4) group is considered. We illustrate the application of semi-fermionic representations for various problems of strongly correlated physics.