Phase Competition, Solitons, and Domain Walls in Neutral-Ionic Transition Systems

Phase competition and excitations in the one-dimensional neutral-ionic transition systems are theoretically studied comprehensively. From the semiclassical treatment of the bosonized Hamiltonian, we examine the competition among the neutral (N), ferroelectric-ionic (I-ferro), and paraelectric-ionic (I-para) states. The phase transitions between them can become first-order when the fluctuation-induced higher-order commensurability potential is considered. In particular, the description of the first-order phase boundary between N and Iferro enables us to analyze N-I-ferro domain walls. Soliton excitations in the three phases are described explicitly and their formation energies are evaluated across the phase boundaries. The characters of the soliton and domain-wall excitations are classified in terms of the topological charge and spin. The relevance to the experimental observations in the molecular neutral-ionic transition systems is discussed. We ascribe the pressure-induced crossover in tetrathiafulvalene-p-chloranil (TTF-CA) at a high-temperature region to that from the N to the I-para state, and discuss its consequence.