Heisenberg and interaction representations in thermo field dynamics

In this paper the so-called α degree of freedom appearing in thermal quantum field theory is discussed, using thermo field dynamics (TFD). This paper is confined to stationary thermal situations, both nonequilibrium and equilibrium. The main result is that when the stationary number distribution differs from the equilibrium one the use of time ordered and antitime ordered formalisms picks up α = 1 and α = 0, in contrast to the general belief that the Feynman diagram method is usable for any other a as well. This situation in TFD will be compared with the other approaches. The reason why the Feynman diagram method becomes available for any a in the case of the equilibrium distributions is also studied.