On the Finite Temperature Effective Potential in Scalar QED with N   Flavors
  The effective potential of scalar quantum electrodynamics with N flavors of complex scalar fields is studied, by performing a self consistent 1/N expansion up to next to leading order in $1/N$. Starting from the broken phase at zero temperature, the theory exhibits a phase transition to the symmetric phase at some finite temperature $T_c$. We work in general covariant gauges and demonstrate the gauge invariance of both, the critical temperature $T_c$ and the minimization condition at any finite temperature T. Furthermore, the only minimum of the potential is at zero scalar vacuum expectation value for any temperature $T \geq T_c$ and varies continuously to nonzero values for temperatures below $T_c$, implying the existence of a second order phase transition. 
M. Carena and C.E.M. Wagner