Topological Excitations in Neutral-Ionic Transition Systems

The existence and physical properties of topological excitations in ferroelectrics, especially mobile topological boundaries in one dimension, are of profound interest. Notably, topological excitations emerging in association with the neutral-ionic (NI) phase transition are theoretically suggested to carry fractional charges and cause anomalous charge transport. In recent years, we experimentally demonstrated mobile topological excitations in a quasi-one-dimensional (1D) ferroelectric, tetrathiafulvalene-p-chloranil [TTF-CA; TTF (C6H4S4) and CA (C6Cl4O2)], which shows the NI transition, using NMR, NQR, and electrical resistivity measurements. Thermally activated topological excitations carry charges and spins in the NI crossover region and in the ionic phase with a dimer liquid. Moreover, free solitons show a binding transition upon a space-inversion symmetry-breaking ferroelectric order. In this article, we review the recent progress in the study of mobile topological excitations emerging in TTF-CA, along with earlier reports that intensively studied these phenomena, aiming to provide the foundations of the physics of electrical conductivity and magnetism carried by topological excitations in the 1D ferroelectric.