This chapter studies the time evolution of classical statistical systems in the critical domain. Typical quantities of interest are relaxation rates towards equilibrium, time-dependent correlation functions, and transport coefficients. The main motivation for such a study is that, in systems in which the dynamics is local (on short time scales a modification of a dynamic variable has an influence only locally in space), when the correlation length becomes large, a large time scale emerges which characterizes the rate of time evolution. This phenomenon called critical slowing down leads to universal behaviour and scaling laws for time-dependent quantities.
Universal critical properties can manifest themselves not only in spatial but also in temporal directions. It has been found that critical point with emergent symmetry exhibits intriguing spatial critical properties characterized by two divergent length scales, attracting long-term investigations. However, how the temporal critical properties are affected by emergent symmetry is largely unknown. Here, we study the nonequilibrium critical dynamics in the three-dimensional (
3D) clock model, whose critical point has emergent
U(1) symmetry. We find that in contrast to the magnetization
M, whose relaxation process is described by the usual dynamic exponent
z of the
3D XY universality class, the angular order parameter ${\ensuremath{\phi}}_{q}$ shows a remarkable two-stage evolution characterized by different dynamic critical exponents. While in the short-time stage the relaxation dynamics is governed by
z, in the long-time stage the dynamics is controlled by a new dynamic exponent ${z}^{\ensuremath{'}}$. Further scaling analyses confirm that ${z}^{\ensuremath{'}}$ is an indispensable dynamic critical exponent. Our results may be detected in the hexagonal
RMnO3 (
R=rare earth) materials experimentally.