Temperature Dependence of the Chiral Condensate from an Interacting Pion   Gas
  By exploiting the fact that the chiral condensate is related to the derivative of the free-energy density with respect to the bare quark mass we calculate the temperature dependence of the condensate ratio $\ave{\bar qq}_T/\ave{\bar qq}_0$ from an iteracting pion gas. When using the Weinberg Lagrangian at the Hartree level we find a depression of the condensate with temperature. For $T < 100$ MeV the results are in good agreement with chiral perturbation theory in the three loop approximation. Near $T_c$, however, there are marked differences due to non-perturbative nature of our approach. 
G. Bunatian and J. Wambach