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

TL;DR

This paper provides a comprehensive theoretical analysis of phase competition, solitons, and domain walls in one-dimensional neutral-ionic transition systems, elucidating phase boundaries, excitations, and their experimental relevance.

Contribution

It introduces a semiclassical bosonized Hamiltonian approach to analyze phase transitions, domain walls, and soliton excitations in neutral-ionic systems, including the first-order phase boundary description.

Findings

Identification of phase boundaries among N, I_ferro, and I_para states.

Explicit description and energy evaluation of soliton excitations.

Classification of excitations by topological charge and spin.

Abstract

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$_\mathrm{ferro}$) and paraelectric-ionic (I$_\mathrm{para}$) 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 I$_\mathrm{ferro}$ enables us to analyze N-I$_\mathrm{ferro}$ 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$_\mathrm{para}$ state, and discuss its consequence.