External field dependence of magnetization and long range order in quantum field theory

Field theoretical models of self-interacting bosons are considered in the framework of the Euclidean formulation of quantum field theory. The methods of classical statistical mechanics are used. It is proved that, for a large class of states, the magnetisation is equal to the derivative of the free energy, and that the long range order vanishes for all values of the external field strength for which the free energy is differentiable. (P.D.)