Theory of Coupled Phase Transitions: Phase Separation and Variation of Order Parameter

TL;DR

This paper presents a simplified Ginzburg-Landau model showing how coupling between first- and second-order phase transitions can cause phase separation and alter the behavior of the order parameter, revealing new critical phenomena.

Contribution

It introduces a theoretical framework demonstrating how coupling induces phase separation and modifies critical exponents in phase transitions.

Findings

Coupling causes second-order transitions to exhibit phase separation similar to first-order transitions.

The order parameter's variation is dominated by the proportion of the ordered phase during FOPT.

Critical exponents can increase from 1/2 to 1 or higher due to coupling effects.

Abstract

A simplified Ginzburg-Landau theory is presented to study generally a coupling of a first-order phase transition (FOPT) to a second-order phase transition (SOPT). We show analytically that, due to the coupling between the two phase transitions, the SOPT may exhibit a FOPT-like phase separation in which an ordered phase is separated from a disordered one. This phase separation results in a distinct behavior in the variation of the order parameter of the SOPT, namely, it is primarily the proportion of the ordered phase that contributes to the total order of the whole system during the FOPT. This growth mode may turn a mean-field critical exponent from 1/2 to 1 or even bigger.