Finite Temperature Retarded and Advanced Fields
  By employing retarded and advanced propagators, Aurenche and Becherrawy showed how to rewrite the real-time thermal Feynman rules so that the temperature dependence is removed from the free propagators and transferred to the vertices. The present paper introduces retarded and advanced field operators and incorporates all temperature dependence into the interaction term of the Hamiltonian. Perturbative expansions of the Green functions in the Hamiltonian formulation give the correct results order by order in perturbation theory.   The spectrum of the temperature-dependent Hamiltonian contains the thermal quasiparticles that produce poles in the propagators. 
H. Arthur Weldon