Classical microscopic theory of mixed states

A classical microscopic theory for mixed states has recently been formulated in order to deal with the theoretical description of nuclear phenomena observed in highly excited states. In this theory, which is re-examined in detail, the degrees of freedom of real fermions are doubled by fictitious fermions, in the context of the formalism provided by thermofield dynamics. The relevant role played in the theory by the requirement of static stability of the state of equilibrium is discussed. The significance of the constraint which the theory imposes on the density matrix of the fictitious fermions is analysed, a rigorous justification being provided for the followed procedure. The concept of canonical collective variables for mixed states is analysed and illustrated through the example of the Lipkin model. (orig.)