DIAGONALIZATION OF PROPAGATORS IN THERMO-FIELD DYNAMICS FOR RELATIVISTIC QUANTUM-FIELDS

Two-point functions for interacting quantum fields in statistical syst ems can be diagonalized by matrix transformations. It is shown, that w ithin the framework of time-dependent Thermo Field Dynamics this diago nalization can be understood as a thermal Bogoliubov transformation to non-interacting statistical quasi-particles. The condition for their unperturbed propagation relates these states to the thermodynamic prop erties of the system: It requires global equilibrium for stationary si tuations, or specifies the time evolution according to a kinetic equat ion.