Critical Behavior of Damping Rate for Plasmon with Finite Momentum in   \phi^4 Theory
  Applying thermal renormalization group (TRG) equations to $\phi^4$ theory with spontaneous breaking symmetry, we investigate the critical behavior of the damping rate for the plasmons with finite momentum at the symmetry-restoring phase transition. From the TRG equation the IR cutoff provided by the external momentum leads to that the momentum-dependent coupling constant stops running in the critical region. As the result, the critical slowing down phenomenon reflecting the inherently IR effect doesn't take place at the critical point for the plasmon with finite external momentum. 
Hanzhong Zhang, Luan Cheng and Enke Wang