Spontaneous symmetry breaking and linear effective potentials

The convexity of a scalar effective potential is a well known property, and, in the situation of spontaneous symmetry breaking, leads to the so-called Maxwell construction, characterised by a flat effective potential between the minima of the bare potential. Simple derivations are given here, which show how linear effective potentials arise from non-trivial saddles points which dominate the partition function, for a self-interacting scalar field and for a Yukawa model.