We consider the modification of the Cahn-Hilliard equation when a time delay process through a memory function is taken into account. We then study the process of spinodal decomposition in fast phase transitions associated with a conserved order parameter. The introduced memory effect plays an important role to obtain a finite group velocity. Then, we discuss the constraint for the parameters to satisfy causality. The memory effect is seen to affect the dynamics of phase transition at short times and have the effect of delaying, in a significant way, the process of rapid growth of the order parameter that follows a quench into the spinodal region.Keywords: Non-equilibrium field dynamics; Memory effects; Relativistic heavy-ion collisions
I. BACKGROUNDDiffusion is a typical relaxation process and appears in various fields of physics: thermal diffusion processes, spin diffusion processes, Brownian motions and so on. It is empirically known that the dynamics of these processes is approximately given by the diffusion equation. Although the diffusion equation has broad applicability, there exist the applicability limitations. First, the diffusion equation does not obey causality [1]. Let us consider the following telegraph equation,Then, the propagation speed is given by v = D/τ. Thus, the propagation speed of the diffusion equation is infinite because the telegraph equation is reduced to the diffusion equation in the limit of vanishing τ. Second, the diffusion equation does not satisfy exact relations, for instance, the Kramers-Kronig relation and the f-sum rule [2,3]. By solving the Heisenberg equation of motion, the exact Laplace-Fourier transform of the time-evolution of a conserved number density is given bywhere δ means the fluctuations from the equilibrium value and C(k) represents the Fourier transform of the correlation function of the number density. Because of the Kramers-Kronig relation and the f-sum rule, the term proportional to 1/z 2 disappears and the coefficient of the second term is proportional to the equilibrium expectation value of the total number n(0) eq . These are not satisfied if the coarse-grained dynamics of the number density is assumed to be given by the diffusion equation [2,3].It is known that the problem of causality and the sum rules can be solved by using the telegraph equation instead of the diffusion equation. Interestingly enough, it has been shown recently that the coarse-grained equation derived by employing systematic coarse-grainings from the Heisenberg equation of motion is not the diffusion equation but the telegraph equation [3,4].The discussion so far is applicable to conserved quantities because the diffusion equation is a coarse-grained equation of conserved quantities. On the other hand, the corresponding equation for a non-conserved quantity is the timedependent Ginzburg-Landau (TDGL) equation in the sense that the TDGL equation is a overdamping equation. The microscopic calculation [7] again shows that the relaxation phenomenon is accompanied by oscillation and cannot...