Phenomenological Landau Theory of Charge Density Wave Phase Transitions in Layered Compounds

Abstract

The Landau theory is well known as a general and convenient framework for studying the second-order phase transition in various systems [1]: magnetic or structural phase transitions, superconductivity, superfluid 3He, liquid crystal phases, etc. It is based on the symmetry of the system in the normal state without the long-range order and transformation properties of the order parameter with respect to the symmetry operation. Because of this general feature, it is particularly suited to investigate possible ordered phases in complicated phase transitions, for which it is not easy to develop a microscopic theory. The charge density wave (CDW) phase transition in layered transition-metal dichalcogenides is one such field [2]. In fact, as described in the article above by Motizuki and Suzuki, the mechanism of the CDW formation in this system is fairly intricate. In addition, available experiments show that instead of a single phase transition, a series of successive transitions is usually observed from a normal phase to a commensurate (C) phase via one or two incommensurate (I) phases when the temperature is lowered. Here the I (C) state is such that the period of the CDW relative to the underlying lattice constant is irrational (rational). At present a theoretical study with a realistic and microscopic model is not available on the problem of successive phase transitions. On the other hand, the phenomenological Landau theory has been very fruitful to our understanding of successive CDW phase transitions.