Theory of Relaxation Phenomena near the Second-Order Phase-Transition Point

One may expect that the observables connected with nonequilibrium processes will show abrupt changes if a substance undergoes a second-order phase transition, since the equilibrium thermodynamic quantities appear to do so. In order to study these phenomena in a connected way the assumption is made that the long-range order parameter and the short-range order parameter can be treated as fluxes and forces in the sense of Onsager's theory of irreversible thermodynamics. Actual calculations are performed for two cases: an order—disorder system with short- and long-range order and a system with two modes of long-range order (antiferromagnet). The absorption of sound is calculated and its behavior near the critical temperature is analyzed. The function is continuous with a discontinuity in the slope provided the phenomenological constants are smooth functions of the temperature.