The low-temperature phase of the Heisenberg antiferromagnet in a fermionicrepresentation

Thermal properties of the ordered phase of the spin-1/2 isotropic Heisenber g Antiferromagnet on a d-dimensional hypercubical lattice are studied withi n the fermionic representation when the constraint of a single occupancy co ndition is taken into account by the method suggested by Popov and Fedotov. Using a saddle point approximation in the path integral approach we discus s not only the leading order but also the fluctuations around the saddle po int at one-loop level. The Influence of taking into account the single occu pancy condition is discussed at all steps.