Infrared Investigations of the Neutral-Ionic Phase Transition in TTF-CA and Its Dynamics

Control of electronic polarization via charge ordering and electron transfer: electronic ferroelectrics and electronic pyroelectrics

Ferroelectric, pyroelectric, and piezoelectric compounds whose electric polarization properties can be controlled by external stimuli such as electric field, temperature, and pressure have various applications, including ferroelectric memory materials, sensors, and thermal energy-conversion devices. Numerous polarization switching compounds, particularly molecular ferroelectrics and pyroelectrics, have been developed. In these materials, the polarization switching usually proceeds via ion displacement and reorientation of polar molecules, which are responsible for the change in ionic polarization and orientational polarization, respectively. Recently, the development of electronic ferroelectrics, in which the mechanism of polarization change is charge ordering and electron transfer, has attracted great attention. In this article, representative examples of electronic ferroelectrics are summarized, including (TMTTF)2X (TMTTF = tetramethyl-tetrathiafulvalene, X = anion), α-(BEDT-TTF)2I3 (BEDT-TTF = bis(ethylenedithio)-tetrathiafulvalene), TTF-CA (TTF = tetrathiafulvalene, CA = p-chloranil), and [(n-C3H7)4N][FeIIIFeII(dto)3] (dto = 1,2-dithiooxalate = C2O2S2). Furthermore, polarization switching materials using directional electron transfer in nonferroelectrics, the so-called electronic pyroelectrics, such as [(Cr(SS-cth))(Co(RR-cth))(μ-dhbq)](PF6)3 (dhbq = deprotonated 2,5-dihydroxy-1,4-benzoquinone, cth = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraaza-cyclotetradecane), are introduced. Future prospects are also discussed, particularly the development of new properties in polarization switching through the manipulation of electronic polarization in electronic ferroelectrics and electronic pyroelectrics by taking advantage of the inherent properties of electrons.

Interlayer Sliding Phonon Drives Phase Transition in the Ph-BTBT-10 Organic Semiconductor

In the field of organic electronics, the semiconductor 7-decyl-2-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10) has become a benchmark due to its high charge mobility and chemical stability in thin film devices. Its phase diagram is characterized by a crystal phase with a bilayer structure that at high temperature transforms into a Smectic E liquid crystal with monolayer structure. As the charge transport properties appear to depend on the phase present in the thin film, the transition has been the subject of structural and computational studies. Here such a process has been investigated by polarized low frequency Raman spectroscopy, selectively probing the intermolecular dynamics of the two phases. The spectroscopic observations demonstrate the key role played by a displacive component of the transition, with the interpenetration of the crystal bilayers driven by lattice phonon mode softening followed by the intralayer rearrangement of the molecule rigid cores into the herringbone motif of the liquid crystal. The mechanism can be related to the effectiveness of thermal annealing to restore the crystal phase in films.

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.

Strong Terahertz Radiation via Rapid Polarization Reduction in Photoinduced Ionic-To-Neutral Transition in Tetrathiafulvalene-p-Chloranil

Terahertz lights are usually generated through the optical rectification process within a femtosecond laser pulse in noncentrosymmetric materials. Here, we report a new generation mechanism of terahertz lights based upon a photoinduced phase transition, in which an electronic structure is rapidly changed by a photoirradiation. When a ferroelectric organic molecular compound, tetrathiafulvalene-p-chloranil, is excited by a femtosecond laser pulse, the ionic-to-neutral transition is driven and simultaneously a strong terahertz radiation is produced. By analyzing the terahertz electric-field waveforms and their dependence on the polarization direction of the incident laser pulse, we demonstrate that the terahertz radiation originates from the ultrafast decrease of the spontaneous polarization in the photoinduced ionic-to-neutral transition. The efficiency of the observed terahertz radiation via the photoinduced phase transition mechanism is found to be much higher than that via the optical rectification in the same material and in a typical terahertz emitter, ZnTe.

Light-Induced Current Oscillations in the Charge-Ordered State of (TMTTF)2SbF6

Below T CO = 157 K the quasi-one-dimensional charge-transfer salt (TMTTF) 2 SbF 6 undergoes a pronounced phase transition to a charge-ordered ground state. We have explored the non-linear and photoconductive behavior as a function of applied voltage, laser pulse energy and temperature. Besides a decay of the photoconductive signal in a double exponential fashion in the millisecond range, we discover current oscillations in the kHz range induced by the application of short laser pulses. While the resonance frequencies do not depend on voltage or laser intensity and vary only slightly with temperature, the amplitude changes linearly with the laser intensity and voltage. We suggest that photo-induced fluctuations of the charge-ordered state alter the current flow of the single particles and hence, the photocurrent. The findings are discussed and compared to comparable phenomena in other low-dimensional electron systems.

Shift current photovoltaic effect in a ferroelectric charge-transfer complex

Shift current is a steady-state photocurrent generated in non-centrosymmetric single crystals and has been considered to be one of the major origins of the bulk photovoltaic effect. The mechanism of this effect is the transfer of photogenerated charges by the shift of the wave functions, and its amplitude is closely related to the polarization of the electronic origin. Here, we report the photovoltaic effect in an organic molecular crystal tetrathiafulvalene-p-chloranil with a large ferroelectric polarization mostly induced by the intermolecular charge transfer. We observe a fairly large zero-bias photocurrent with visible-light irradiation and switching of the current direction by the reversal of the polarization. Furthermore, we reveal that the travel distance of photocarriers exceeds 200 μm. These results unveil distinct features of the shift current and the potential application of ferroelectric organic molecular compounds for novel optoelectric devices.The bulk photovoltaics refers to an effect whereby electrons move directionally in non-centrosymmetric crystals upon light radiation. Here, Nakamura et al. observe this effect in a ferroelectric organic charge-transfer complex, which shows large diffusion distance of photogenerated electrons over 200 µm.