Scaling of Particle Trajectories on a Lattice II: The Critical Region

Abstract

The scaling behavior of the closed trajectories of a moving particle generated by randomly placed rotators or mirrors on a square or triangular lattice in the critical region are investigated. We study numerically two scaling functions: $f(x)$ related to the trajectory length distribution $n_S$ and $h(x)$ related to the trajectory size $R_S$ (gyration radius) as introduced by Stauffer for the percolation problem, where $S$ is the length of a closed trajectory. The scaling function $f(x)$ is in most cases found to be symmetric double Gaussians with the same characteristic size exponent $\sigma =0.43\approx 3/7$ as was found at criticality. In contrast to previous assumptions of an exponential dependence of $n_S$ on $S$, the Gaussian functions lead to a stretched exponential dependence of $n_S$ on $S$, $n_S\sim e^{-S^{6/7}}$. However, for the rotator model on the partially occupied square lattice, an alternative scaling function near criticality is found, leading to a new exponent ${\sigma }^{\prime }=1.6\pm 0.3$ and a super exponential dependence of $n_S$ on $S$. The appearance of the same exponent $\sigma =3/7$ describing the behavior at and near the critical point is discussed. Our numerical simulations show that $h(x)$ is essentially a constant, which depends on the type of lattice and on the concentration of the scatterers.

22 pages LaTex and 18 postscript figures