Finite Size Analysis of the U(1) Background Field Effective Action
  We apply the finite size scaling analysis to the derivative of the density of the effective action for the lattice U(1) pure gauge theory in an external constant magnetic field. We found the presence of a continuous phase transition. Moreover, our extimate of of the critical parameters gives values consistent with those extracted from the analysis of the specific heat. 
Paolo Cea, Leonardo Cosmai and Antonio D. Polosa