Influence of collective modes on the order of the phase transition
Volkov B.A., Kopaev Yu.V.
It is shown that the contribution made to the system energy by the zero-point oscillations of the order parameter near the phase-transition point determines essentially the order of the phase transition. For example, in the case of a structure phase transition, such an oscillation is a soft phonon mode, with which the lattice instability is connected and which vanishes only at the transition point. This behavior of the branch of the collective excitations leads to a first-order phase transition.