Positional Order and Diffusion Processes in Particle Systems

Nonequilibrium behaviors of positional order are discussed based on diffusion processes in particle systems. With the cumulant expansion method up to the second order, we obtain a relation between the positional order parameter $\mathrm{\Psi }$ and the mean square displacement $M$ to be $\Psi \sim \exp(- {\bf K}^2 M /2d)$withareciprocalvector${\bf K}$andthedimensionofthesystem$d$. On the basis of the relation, the behavior of positional order is predicted to be $\mathrm{\Psi }\sim \exp (-{\mathbf{K}}^{2}Dt)$ when the system involves normal diffusion with a diffusion constant $D$. We also find that a diffusion process with swapping positions of particles contributes to higher orders of the cumulants. The swapping diffusion allows particle to diffuse without destroying the positional order while the normal diffusion destroys it.