Chiral Lagrangians at finite temperature and the Polyakov Loop
  Heat kernel expansions at finite temperature of massless QCD and chiral quark models generate effective actions relevant for both low and high temperature QCD. The key relevance of the Polyakov Loop to maintain the large and non-perturbative gauge invariance at finite temperature is stressed. 
E. Megias, E. Ruiz Arriola, L.L. Salcedo