Factorization of finite temperature graphs in thermal QED
  We extend our previous analysis of gauge and Dirac fields in the presence of a chemical potential. We consider an alternate thermal operator which relates in a simple way the Feynman graphs in QED at finite temperature and charge density to those at zero temperature but non-zero chemical potential. Several interesting features of such a factorization are discussed in the context of the thermal photon and fermion self-energies. 
Fernando T. Brandt, Ashok Das, Olivier Espinosa, Josif Frenkel,   Silvana Perez