Dynamics of phase separation in two-dimensional tricritical systems

Abstract

We present a Monte Carlo study of the tricritical spinodal decomposition of a two-dimensional model obeying Kawasaki dynamics which describes both an Ising metamagnet as well as a simple model of chemisorption. Quenches to three different points in the unstable domain are studied. In all cases the structure functions for both the nonconserved order parameter and conserved secondary order parameter exhibit dynamical instabilities as manifest by peaks which increase with increasing time. Each structure function satisfies a simple scaling behavior with respect to characteristic lengths whose time dependence can be approximated by simple powerlaw behavior in certain domains of time. In addition we find that the tricritical spinodal decomposition exhibits an unusual asymmetry with respect to the quench value of the conserved variable (the magnetization) as compared to the case of simple binary alloys. Namely, the peak in the magnetic structure factor (at fixed time) increases with increasing magnetization as one moves from one side of the unstable region toward the other side of the coexistence curve. Eventually, however, as one continues to increase the magnetization this peak value ceases to increase, but rather decreases dramatically in a very narrow transition region. This sharp transition would seem to be a fundamental dynamical distinction between tricritical and critical instabilities.