The next to leading order effective potential in the 2+1 dimensional   Nambu-Jona-Lasinio model at finite temperature
  The finite temperature effective potential in the 2+1 dimensional Nambu-Jona-Lasinio model is constructed up to the next to leading order in the large $N$ expansion, where $N$ is the number of flavors in the model. The distinctive feature of the analysis is an inclusion of an additional scalar field, which allows us to circumvent the well known, and otherwise unavoidable problem with the imaginary contribution to the effective potential. In accordance with the Mermin-Wagner-Coleman theorem, applied to the dimensionally reduced subsystem of the zero Matsubara modes of the composite boson fields, the finite temperature effective potential reveals a global minimum at the zero of the composite order parameter. This allows us to conclude that the continuous global symmetry of the NJL model is not broken for any arbitrarily small, finite temperature. 
E. P. Esposito, I. A. Shovkovy and L. C. R. Wijewardhana