Decay of metastable and unstable states: Mechanisms, concepts and open problems

The decay of metastable states is initiated by a nucleation process in which a free energy barrier is overcome, while in unstable states fluctuations can grow unaffected by such a barrier. The first part of this review considers the significance of the spinodal line separating the metastable and unstable regime. A Ginzburg criterion for the validity of the spinodal line, as well as for the mean-field theory of nucleation and the linear theory of spinodal decomposition will be discussed. Evidence from simulations and experiments will be mentioned.