Liquid-Crystal-Amorphous Phase Transitions in Optical Memory Devices

Abstract

Phase transitions including ones to the amorphous state are studied based on the phase field model and the heat diffusion equation. Two order parameters are introduced to describe the three different phases. There is a Lyapunov function with three local minima corresponding to the liquid, crystal and amorphous phases. The simplest interaction term is assumed, and the energy barriers between the states control the transition rate. Although the energy level of amorphous phase is higher than the crystal phase one, the amorphous phase cannot change into the crystal without the thermal noise due to the energy barrier. The key point of the present model is that phase transitions depend on not only the temperature but also the cooling rate. Rapid cooling leads to the amorphous phase below the melting temperature, whereas slow cooling leads to the crystal phase. With this simple model, the phase transitions are investigated by simulations. There is a kink solution connecting the crystal phase with the liquid or amorphous phase; it travels with a speed that is dependent on the temperature at the interface. The stabilities of different phases are compared when they are in collision. Although the kink solution is stable, the planar solution in two dimensions is destabilized through lateral inhibition. This result is theoretically supported by the stability analysis of the interface. A crystallization from a super-cooled liquid induced by noise is observed in the case of slow cooling. The crystallization occurs without a fixed seed and suggests that the present system is a good candidate for the present phase transitions.