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A novel ferroelectric material of phenol derivatives, DHBA, provides a special platform regarding the mechanisms of polarization reversal: proton-transfer and flip-flop mechanisms.

The odd-even effects are renowned as a mysterious phenomenon in broad fields of science but have never been established as an effective approach for materials engineering. We demonstrate that the parity of alkyl carbon number n can cause alternating emergence of polar/antipolar organic semiconductor crystals. This is achieved by the development of a series of polar rod-like molecules, composed of a linkage between extended π-core (head) and alkyl chains (tail), exhibiting both high layered crystallinity and well-balanced end-to-end affinity. The molecules are unidirectionally aligned to form two-dimensional array, and the eventual polar monomolecular layers present two distinct types of interlayer stacking depending on the parity of n: alternating head-to-head and tail-to-tail (antipolar) alignment in odd-n crystals, and uniform head-to-tail (polar) alignment in even-n crystals. The latter allows to obtain polar semiconductor films that considerably improve interfacial carrier transport characteristics. The findings are key for creating polarity-controlled optoelectronic materials and devices.

The competition between polar and antipolar states causes the ferroelectric/antiferroelectric hybrid-like dielectric behaviors, whereas all the antipolar degenerate ground states cause distinct situations, which have been overlooked to date.

A noncontact optical imaging technique called “ferroelectrics field modulation imaging (FFMI)” is developed to visualize ferroelectric domains and domain walls. The difference image sensing techniq...

The construction and control of 2D layered packing motifs with π-extended fused-ring molecules is of crucial importance for developing organic electronic materials and devices. Herein, we demonstrate that, when adequately substituted, two kinds of layered packing motifs are obtainable for benzo[1,2-b:4,5-b′]bis[b]benzothiophene (BBBT), which itself does not show layered crystallinity. We synthesized BBBT derivatives substituted with a combination of alkyl chains and a phenyl ring in a symmetric/asymmetric manner, 2,8-didecyl-BBBT (diC10-BBBT) and 2-decyl-8-phenyl-BBBT (Ph-BBBT-C10). We found that diC10-BBBT forms a layered π-stack (LπS) structure mainly composed of slipped parallel stacks, while Ph-BBBT-C10 forms a typical layered herringbone (LHB) packing structure chiefly composed of T-shaped contacts. This feature is associated with the non-layered packing motif in BBBT: typical π-stack and herringbone structures, both of whose polymorphs show a large slip along the molecular long axis. Calculations of intermolecular interaction energies between neighbouring molecules in the crystals reveal that the interchain interactions suppress the long-axis slip, leading to the formation of the LπS and the LHB, respectively. Both diC10-BBBT and Ph-BBBT-C10 form uniform (ultra)thin films originating from the layered crystallinity, and exhibit good transistor characteristics with a hole mobility of about 1 cm2 V−1 s−1. We discuss how the substituent modifications are useful as crystal engineering to explore the potential of π-extended molecules for electronic applications.

We apply van der Waals density-functional theory (vdW-DFT) calculations to predict crystal structure parameters and spontaneous polarization values for seven hydrogen-bonded single-component organic ferroelectrics. The results show good agreement with experimental results, implying that an important step for the computational materials design of organic ferroelectrics has been achieved. This approach also enables the simulation of electromechanical responses. Calculations using the vdW-DFT method are performed for croconic acid (CRCA), 2-phenylmalondialdehyde (PhMDA), and 5,6-dichloro-2-methylbenzimidazole (DC-MBI) under uniaxial stresses or electric fields. Direct piezoelectric ${d}_{33}$ constants are evaluated from the polarization change as a function of stress, whereas converse piezoelectric ${d}_{33}$ constants are evaluated from the change in lattice parameter as a function of electric field. The obtained values show acceptable agreement with the experimental values if possible objective factors are considered. The stress-induced or electric-field-induced variation of polarization is analyzed considering two types of contributions. One is from proton transfer as a classical point charge motion and the other residual part corresponds to the redistribution of $\ensuremath{\pi}$ electrons.

Bilayer-type layered herringbone (LHB) packing is of great importance because of its remarkably high layered crystallinity and its ability to form extremely smooth ultrathin films which are useful ...

Supramolecular cocrystals of anilic acids with 2,2′-bipyridines exhibit successive phase transitions as well as unusual isotope effects. Ferroelectricity driven by a cooperative proton transfer along the supramolecular chains is accompanied by huge permittivity (a maximum of 13 000) at the Curie point, as well as a large spontaneous polarization (maximum 5 μC cm−2) and a low coercive field ranging from 0.5 to 10 kV cm−1. Deuterium substitutions over the hydrogen bonds smoothly raise the Curie point and simultaneously reduce other phase-transition temperatures by a few tens of degrees. The coexistence of opposite isotope effects reduces the temperature interval of the intermediate paraelectric phase from 84 to 10 K for the 5,5′-dimethyl-2,2′-bipyridinium bromanilate salt. The bipyridine molecules exhibit interplanar twisting, which represents the order parameter relevant to the high-temperature phase transitions. The normal and inverse temperature shifts are ascribed to the direct and indirect effects, respectively, of the lengthened hydrogen bonds, which adjusts the molecular conformation of the flexible bipyridine unit so as to minimally modify their adjacent intermolecular interactions.

The imidazole ring is chemically stable and forms an excellent building block for compounds that show externally switchable polarization in their crystalline states. We examined the structural prop...

Dielectrics that undergo electric-field-induced phase changes are promising for use as high-power electrical energy storage materials and transducers. We demonstrate the stepwise on/off switching of large polarization in a series of dielectrics by flipping their antipolar or canted electric dipoles via proton transfer and inducing simultaneous geometric changes in their π-conjugation system. Among antiferroelectric organic molecular crystals, the largest-magnitude polarization jump was obtained as 18 μC cm−2 through revisited measurements of squaric acid (SQA) crystals with improved dielectric strength. The second-best polarization jump of 15.1 μC cm−2 was achieved with a newly discovered antiferroelectric, furan-3,4-dicarboxylic acid. The field-induced dielectric phase changes show rich variations in their mechanisms. The quadruple polarization hysteresis loop observed for a 3-(4-chlorophenyl)propiolic acid crystal was caused by a two-step phase transition with moderate polarization jumps. The ferroelectric 2-phenylmalondialdehyde single crystal having canted dipoles behaved as an amphoteric dielectric, exhibiting a single or double polarization hysteresis loop depending on the direction of the external field. The magnitude of a series of observed polarizations was consistently reproduced within the simplest sublattice model by the density functional theory calculations of dipole moments flipping over a hydrogen-bonded chain or sheet (sublattice) irrespective of compounds. This finding guarantees a tool that will deepen our understanding of the microscopic phase-change mechanisms and accelerate the materials design and exploration for improving energy-storage performance. The excellent energy-storage performance of SQA was demonstrated by both a high recoverable energy-storage density Wr of 3.3 J cm−3 and a nearly ideal efficiency (90%). Because of the low crystal density, the corresponding energy density per mass (1.75 J g−1) exceeded those derived from the highest Wr values (∼8–11 J cm−3) reported for several bulk antiferroelectric ceramics , without modification to relaxor forms., Electric-field induced phase changes, which are promising for use in high-power electrical energy storage, can be realized in a series of organic dielectrics by flipping the antipolar or canted electric dipoles via proton transfer.

We investigated microscopic lattice states in the donor-acceptor ionic Mott insulator, TTF-BA, by $^{79}$Br-NQR spectroscopy to explore cross-correlated fluctuations between spin, charge and lattice. A ferroelectric transition with lattice dimerization is captured by a NQR line splitting with the critical exponent $\beta$ of 0.40, as expected in the 3D Ising universality class, and a peak formation in the spin-lattice relaxation rate $T_1^{-1}$ at the transition temperature, $T_\mathrm{c}$, of 53 K. Notably, $T_1^{-1}$ does not obey the conventional $T^2$ law expected for the Raman process of phonons even far above $T_\mathrm{c}$, indicating the emergence of extraordinary lattice fluctuations. They are very probably associated with polar fluctuations in the paraelectric and paramagnetic phase of TTF-BA and explain the previous observation of the anomalously suppressed paramagnetic spin susceptibility, which was conjectured to be due to the local spin-singlet pairing prior to the nonmagnetic ferroelectric order [K. Sunami $et$ $al$., Phys. Rev. Res. 2, 043333 (2020)]., Comment: 4 pages, 4 figures

We report the dielectric, magnetic, and ultrasonic properties of a one-dimensional organic salt $\mathrm{TTF}\text{\ensuremath{-}}{\mathrm{QBr}}_{3}\mathrm{I}$. These indicate that $\mathrm{TTF}\text{\ensuremath{-}}{\mathrm{QBr}}_{3}\mathrm{I}$ shows a ferroelectric spin-Peierls (FSP) state in a quantum critical regime. In the FSP state, coupling of charge, spin, and lattice leads to emergent excitation of spin solitons as topological defects. Amazingly, the solitons are highly mobile even at low temperatures, although they are normally stationary because of pinning. Our results suggest that strong quantum fluctuations enhanced near a quantum critical point enable soliton motion governed by athermal relaxation. This indicates the realization of quantum topological transport at ambient pressure.

For the application of a ferroelectric thin film to optical and electric devices, it is important to align its ferroelectric polarization parallel or perpendicular to the film surface. In this context, an effective method to characterize the polarization directions in the whole area of devices is needed. Here, we report an alternative method to determine a polarization vector in three dimensions using an emission of terahertz waves. The target material is an organic hydrogen-bonded molecular ferroelectric 2-methylbenzimidazole (MBI). By the irradiation of a femtosecond laser pulse on a bulk single-crystal thin film of MBI, we observe the emission of the terahertz waves originating from the polarization modulation of infrared-active phonons via the impulsive stimulated Raman-scattering mechanism. By measuring the terahertz electric field parallel to the ferroelectric polarization with rotating the single-crystal film around two independent axes, we succeed in determining the polarization vector in three dimensions, the direction of which is tilted by 45\ifmmode^\circ\else\textdegree\fi{} normal to the film surface and substrate. This method can be a powerful tool to characterize directions of ferroelectric polarizations in any kinds of thin-film samples.

Observations of ferroelectric domains and domain walls are essential for understanding ferroelectric switching characteristics. Here, we demonstrate that the use of optical birefringence, coupled with a first-order electro-optic effect (Pockels effect), allows the visualization of ferroelectric domains in transparent ferroelectric materials. We utilize a field-modulation imaging technique, with the use of a complementary metal-oxide-semiconductor (CMOS) area image sensor to detect slight differences in parallel- and crossed-Nicols polarized optical microscopy images between forward- and reverse-electric-field applications. The ferroelectric domains with antiparallel polarizations are clearly discriminated from each other by the sign of the ellipticity modulation of outgoing elliptically polarized light. The observed images are consistent with piezoresponse force microscopy images of transparent single-crystal films of 2-methylbenzimidazole (MBI). The modulation signal at both polarization configurations exhibit clear sign changes in the wavelength dispersion, owing to the anisotropic nature of the MBI films. The unique optical-probe nature also reveals the existence of two types of domain walls with different three-dimensional orientations.

The construction and control of 2D layered molecular packing motifs with functionally substituted π-electron cores are crucial for developing organic electronic materials and devices. We investigated a regioisomeric structure–property relationship in high-performance and solution-processable layered organic semiconductors based on mono-octyl-substituted benzothieno[3,2-b]naphtho[2,3-b]thiophene (mono-C8-BTNT). We demonstrated that an isomorphous bilayer-type layered herringbone packing motif is obtainable in a series of four positional isomers of mono-C8-BTNTs whose π-electron core is substituted by an octyl chain at one of the four most peripheral positions with roughly keeping the rod-like molecular shape. These regioisomeric compounds exhibited systematic variations in the solvent solubility and liquid-crystalline phase transitions at elevated temperatures. The analysis of intermolecular interaction energies in the crystals based on dispersion-corrected DFT calculations revealed that the crystals of 2- and 8-mono-C8-BTNTs are more stable than those of 3- and 9-mono-C8-BTNTs owing to the higher ordering of alkyl chain layers in the crystals. Such differences of the stability in their crystal formation are closely correlated with TFT performances, where the single-crystal devices of the 2- and 8-mono-C8-BTNTs substituted at the most peripheral positions exhibit high-performance TFT characteristics with a mobility of approximately 10 cm2 V−1 s−1., An isomorphous bilayer-type layered herringbone crystal packing is reported for a series of four positional isomers of mono-C8-BTNTs, where the single-crystal devices with the isomers exhibit high-performance TFT characteristics.

An applied electric field and/or temperature changes can interconvert the low- and high-dipole states of molecules in a metaelectric organic crystal., Metaelectric transition, i.e. an abrupt increase in polarization with an electric field is just a phase change phenomenon in dielectrics and attracts increasing interest for practical applications such as electrical energy storage and highly deformable transducers. Here we demonstrate that both field-induced metaelectric transitions and temperature-induced phase transitions occur successively on a crystal of highly polarizable bis-(1H-benzimidazol-2-yl)-methane (BI2C) molecules. In each molecule, two switchable polar subunits are covalently linked with each other. By changing the NH hydrogen location, the low- and high-dipole states of each molecule can be interconverted, turning on and off the polarization of hydrogen-bonded molecular ribbons. In the low-temperature phase III, the tetragonal crystal lattice comprises orthogonally crossed arrays of polar ribbons made up of a ladder-like hydrogen-bond network of fully polarized molecules. The single-step metaelectric transition from this phase III corresponds to the forced alignment of antiparallel dipoles typical of antiferroelectrics. By the transition to the intermediate-temperature phase II, the polarity is turned off for half of the ribbons so that the nonpolar and polar ribbons are orthogonal to each other. Considering also the ferroelastic-like crystal twinning, the doubled steps of metaelectric transitions observed in the phase II can be explained by the additional switching at different critical fields, by which the nonpolar ribbons undergo “metadielectric” molecular transformation restoring the strong polarization. This mechanism inevitably brings about exotic phase change phenomena transforming the multi-domain state of a homogeneous phase into an inhomogeneous (phase mixture) state.

Herein, we report the rational design of practical small-molecule organic semiconductors based on a π-electron skeleton of benzothieno[3,2-b]naphtho[2,3-b]thiophene (BTNT) whose layered herringbone (LHB) packing is intentionally modulated by the designated asymmetric substitutions with the phenyl group and normal alkyl chains. The thermal stability of the hybrid BTNT core is high enough, as it lies between those of dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) and benzothieno[3,2-b]benzothiophene (BTBT), although the solvent solubility for the substituted BTNT at ordinary 2,8-substituting positions by the alkyl chain and phenyl group remains extremely low. We show in the BTBT and BTNT derivatives that the tuning of the substituting position works to slightly bend the rodlike organic semiconductor molecules and thus to decrease the cohesive energy of the crystals with retention of the bilayer-type herringbone (b-LHB) packing for the asymmetric rodlike molecules. This modification eventually leads t...

The field-induced antipolar-polar structural transition in an organic antiferroelectric 2-trifluoromethylnaphthimidazole crystal is investigated by performing synchrotron X-ray diffraction. The polarities of all of the hydrogen-bonded chains become parallel with each other in the presence of an external electric field. The switching is accompanied by a giant electrostriction, which provides 1 order of magnitude larger strain than the piezoelectric strain of the organic ferroelectrics: croconic acid and poly(vinylidene fluoride); however, it is comparable to those of typical commercial piezoelectric ceramics. The crystal structure analysis with electric field shows that the origin of the observed giant electrostriction can be attributed to the shear strain that emerges from the polarity switching of the hydrogen-bonded chains. The antipolar-polar structural transition in antiferroelectrics could be employed for the development of high-performance electrostrictive organic materials.

The replacement of lead-containing piezoceramics is increasingly in demand for environmentally benign applications in various electromechanical devices. However, small organic molecules have attracted little interest in relation to their piezoelectric properties. We examined the direct and the converse piezoelectric effects at room temperature of several prototropic organic ferroelectrics having large spontaneous polarizations. The magnitude of the piezoelectric coefficient d33 showed a positive correlation with the spontaneous polarization. Whereas the maximum d33 (∼15 pC N−1 for croconic acid) is comparable to the highest level ever observed among nonferroelectric small molecules, it is about half that of ferroelectric polymers and one or two orders of magnitude smaller than those of the commercially available piezoceramics. The modest piezoelectricity of prototropic ferroelectrics, despite their strong polarization, is opposite in behavior to that of ferroelectric Rochelle salt or triglycine sulfate. Plausibly, its origin can be ascribed to thermally robust ferroelectricity and a microscopic mechanism of ferroelectric switching that minimizes molecular deformations and reorientations.

Highly efficient switching of strong polarization in squaric acid crystal arises from 90° rotation of sheet polarization in a pseudo-tetragonal lattice., The electric-field-induced phase transition from antipolar to polar structures is at the heart of antiferroelectricity. We demonstrate direct evidence of antiferroelectricity by applying a strong electric field to two antipolar crystals of squaric acid (SQA) and 5,5′-dimethyl-2,2′-bipyridinium chloranilate. The field-induced polarization of SQA is quite large and reasonably explained by the theoretically calculated polarization on the hydrogen-bonded sheet sublattice. The pseudo-tetragonal lattice of SQA permits unique switching topologies that produce two different ferroelectric phases of low and high polarizations. By tilting the applied field direction, the electrical switching mechanism can be attributed to a 90° rotation of the sheet polarization. From the viewpoint of applications, the strong polarization, high switching field, and quite slim hysteresis observed in the polarization versus electric field curve for SQA are advantageous for excellent-efficiency energy storage devices.

Ferroelectrics based on proton tautomerism are promising in low-field and above-room-temperature operations. Here seven organic ferroelectric crystals are examined to search for efficient switching of strong spontaneous polarization on proton tautomerism. Solution-grown crystals exhibit strong pinning of ferroelectric domain walls, but excellent switching performance is awakened by depinning domain walls under thermal annealing and/or repetitive bipolar pulses with a high voltage. Compared with ferroelectric polymers such as polyvinylidefluoride, the optimized polarizations are comparable or stronger in magnitude whereas the coercive fields are two orders of magnitude weaker. The polarization of croconic acid, in particular, breaks its own record for organic systems in increasing from 21 to 30 μC cm−2 and now exceeds those of some commercial ferroelectric materials such as SrBi2Ta2O9 and BaTiO3. Optimization reduces the discrepancy of the spontaneous polarization with the results of the first-principles calculations to less than 15%. The cooperative roles of proton transfer and π-bond switching are discussed by employing the point-charge model and hydrogen-bond geometry., Ferroelectrics based on proton tautomerism are promising in low-field and above-roomtemperature operations. Here the authors establish a procedure to optimize spontaneous polarizations, finding that the polarization in croconic acid breaks its own record for organic systems.

Resistivity and NMR measurements demonstrate an electrical conduction mechanism by topological domain walls and solitons., The concept of topology has been widely applied in condensed matter physics, leading to the identification of peculiar electronic states on three-dimensional (3D) surfaces or 2D lines separating topologically distinctive regions. In the systems explored so far, the topological boundaries are built-in walls; thus, their motional degrees of freedom, which potentially bring about new paradigms, have been experimentally inaccessible. Here, working with a quasi-1D organic material with a charge-transfer instability, we show that mobile neutral-ionic (dielectric-ferroelectric) domain boundaries with topological charges carry strongly 1D-confined and anomalously large electrical conduction with an energy gap much smaller than the one-particle excitation gap. This consequence is further supported by nuclear magnetic resonance detection of spin solitons, which are required for steady current of topological charges. The present observation of topological charge transport may open a new channel for broad charge transport–related phenomena such as thermoelectric effects.

We have investigated the microscopic charge/lattice/spin states in the neutral-ionic transition material $\mathrm{DMTTF}\ensuremath{-}{\mathrm{QCl}}_{4}$ under pressure by $^{35}\mathrm{Cl}\ensuremath{-}\mathrm{NQR}$ and $^{1}\mathrm{H}\ensuremath{-}\mathrm{NMR}$ measurements to reveal the mechanism of the phase transition and the nature of the pressure-temperature phase diagram. At ambient pressure, the $^{35}\mathrm{Cl}\ensuremath{-}\mathrm{NQR}$ frequency indicates low ionicity and shows neither discontinuous jump nor precursory decrease around the transition, suggesting that the spin degrees of freedom are not involved in a dimerization transition. These experimental features are consistent with the theoretically proposed picture that the electronic energy gain is acquired by modulations in transfer integrals (so-called Peierls mechanism) with the lattice dimerization. As pressure is applied, the ionicity is increased and the system is driven into an ionic state with the degree of charge transfer exceeding 0.5 above $\ensuremath{\sim}9\phantom{\rule{0.28em}{0ex}}\mathrm{kbar}$ at room temperature, where the $^{1}\mathrm{H}\ensuremath{-}\mathrm{NMR}$ captures paramagnetic spin fluctuations indicative of the emergence of a nondimeric ionic phase. Furthermore, $^{35}\mathrm{Cl}\ensuremath{-}\mathrm{NQR}$ and x-ray-diffraction measurements found a distinct antiferroelectric state at high pressures above 11 kbar. Based on these results, we propose a pressure-temperature phase diagram.

Domain structure accounts for the robust ferroelectricity of some organic compounds, but the limits of instrumentation have prevented detailed domain characterization. Using a state-of-the-art image sensor, the authors develop a sensitive, rapid visualization technique based on the first-order electro-optic effect, enabling mm-scale visualization of ferroelectric domain structures with sub-$\ensuremath{\mu}$m spatial resolution. The ability to probe depth allows unambiguous discrimination of a domain wall's charge state, and thus its topological nature and motion, to promote applications of organic ferroelectrics in high-performance, low-cost devices.

Photoexcitation effects on ferroelectricity were investigated in a hydrogen-bonded supramolecular cocrystal by using nonlinear femtosecond spectroscopy. Irradiation of a terahertz pulse that caused...

Cross-correlation between magnetism and dielectric is expected to offer novel emergent phenomena. Here, magnetic excitations in the organic donor-acceptor spin-chain system, TTF-BA, with a ferroelectric ground state is investigated by $^1$H-NMR spectroscopy. A nonmagnetic transition with a ferroelectric order is marked by sharp drops in NMR shift and nuclear spin relaxation rate $T_1^{-1}$ at 53 K. Remarkably, the analyses of the NMR shift and $T_1^{-1}$ dictate that the paramagnetic spin susceptibility in TTF-BA is substantially suppressed from that expected for the 1D Heisenberg spins. We propose that the spin-lattice coupling and the ferroelectric instability cooperate to promote precursory polar singlet formation in the ionic spin system with a nonmagnetic ferroelectric instability., Comment: 6 pages, 4 figures

Here we discuss requirements for high performance and solution processable organic semiconductors, by presenting a systematic investigation of 7-alkyl-2-phenyl[1]benzothieno[3,2-b][1]benzothiophenes (Ph-BTBT-C n ’s). We found that the solubility and thermal properties of Ph-BTBT-C n ’s depend systematically on the substituted alkyl-chain length n. The observed features are well understood in terms of the change of molecular packing motif with n: The compounds with n ≤ 4 do not form independent alkyl chain layers, whereas those with n ≥ 5 form isolated alkyl chain layers. The latter compounds afford a series of isomorphous bilayer-type crystal structures that form two-dimensional carrier transport layers within the crystals. We also show that the Ph-BTBT-C10 afford high performance single-crystalline field-effect transistors the mobility of which reaches as high as 15.9 cm2/Vs. These results demonstrate a crucial role of the substituted alkyl chain length for obtaining high performance organic semiconductors and field-effect transistors.

The development of organic ferroelectrics and antiferroelectrics with significantly improved performances has recently increased. Their attractive features are environmentally benign nature and tec...

Rapid polarization control by an electric field in ferroelectrics is important to realize high-frequency modulation of light, which has potential applications in optical communications. To achieve this, a key strategy is to use an electronic part of ferroelectric polarization. A hydrogen-bonded molecular ferroelectric, croconic acid, is a good candidate, since π-electron polarization within each molecule is theoretically predicted to play a significant role in the ferroelectric-state formation, as well as the proton displacements. Here, we show that a sub-picosecond polarization modulation is possible in croconic acid using a terahertz pulse. The terahertz-pulse-pump second-harmonic-generation-probe and optical-reflectivity-probe spectroscopy reveal that the amplitude of polarization modulation reaches 10% via the electric-field-induced modifications of π-electron wavefunctions. Moreover, the measurement of electric-field-induced changes in the infrared molecular vibrational spectrum elucidates that the contribution of proton displacements to the polarization modulation is negligibly small. These results demonstrate the electronic nature of polarization in hydrogen-bonded molecular ferroelectrics. The ultrafast polarization control via π-electron systems observed in croconic acid is expected to be possible in many other hydrogen-bonded molecular ferroelectrics and utilized for future high-speed optical-modulation devices.

NMR captures mobile spin solitons deconfined from a charge-lattice–coupled nonmagnetic ferroelectric order., Topological defects have been explored in different fields ranging from condensed matter physics and particle physics to cosmology. In condensed matter, strong coupling between charge, spin, and lattice degrees of freedom brings about emergent excitations with topological characteristics at low energies. One-dimensional (1D) systems with degenerate dimerization patterns are typical stages for the generation of topological defects, dubbed “solitons”; for instance, charged solitons are responsible for high electrical conductivity in doped trans-polyacetylene. Here, we provide evidence based on a nuclear magnetic resonance (NMR) study for mobile spin solitons deconfined from a strongly charge-lattice–coupled spin-singlet ferroelectric order in a quasi-1D organic charge-transfer complex. The NMR spectral shift and relaxation rate associated with static and dynamic spin susceptibilities indicate that the ferroelectric order is violated by dilute solitonic spin excitations, which were further demonstrated to move diffusively by the frequency dependence of the relaxation rate. The traveling solitons revealed here may promise the emergence of anomalous electrical and thermal transport.

Antiferroelectric-to-ferroelectric transitions in squaric acid are simulated by computationally applying a static electric field. Depending on the direction of the electric field, two different metastable ferroelectric (and piezoelectric) phases have been found. One of them corresponds to the experimentally confirmed phase, whereas the other is an optimally polarized phase. The structural details of these phases have been determined as a function of the electric field. The spontaneous polarization values of the phases are 14.5 and 20.5 $\ensuremath{\mu}\mathrm{C}$/${\mathrm{cm}}^{2}$, respectively, and are relatively high among those of the existing organic ferroelectrics.

We observe a terahertz radiation from a hydrogen-bonded organic molecular ferroelectric 5,6-dichloro-2-methylbenzimidazole excited by a femtosecond laser pulse at room temperature. The emitted terahertz wave consists of three oscillatory components, the frequencies of which agree with those of Raman- and infrared-active phonon modes. This suggests that the terahertz radiation is attributed to polarization modulations by infrared-active phonons excited via the impulsive stimulated Raman scattering processes. By taking into account the Raman polarizability tensor and dipole-moment for each phonon, we succeeded in reproducing not only the spectrum of the terahertz radiation, but also its time characteristic. The analysis method is discussed in detail. Our result provides a new way for the light-induced terahertz radiation in organic ferroelectrics., 39 pages including Supplemental Material

The fabrication of single-crystalline thin-film arrays for an organic ferroelectric small molecule is achieved by a simple solution process without additional thermal annealing. Based on a cooperative proton tautomerism through a hydrogen-bonding network, films show the polarity switching with an operating voltage of less than 5 V at room temperature. This approach provides a low-cost and eco-friendly fabrication of ferroelectric devices.

We report quasi-three-dimensional imaging of the ferroelectric domains in a hydrogen-bonded supramolecular ferroelectric 1:1 salt of 5,5′-dimethyl-2,2′-bipyridine and deuterated iodanilic acid by mapping out terahertz waves radiated from the crystal upon femtosecond laser irradiation. Polarization dependence of the effective depth radiating the terahertz waves due to the absorption anisotropy for terahertz waves allows us to distinguish domains in the inside and surface regions of the crystals. In an as-grown crystal, a large domain covering almost all the area is discerned in the inside region, while multidomains are discerned in the surface regions. We observed polarization switching via domain boundary motion in the direction perpendicular to the direction of hydrogen-bonded chains in the surface regions under an external electric field. Obtained images suggest that an uncharged 180° domain wall (DW) parallel to the stacking plane of hydrogen-bonded chains covers the whole area at coercive fields. We a...

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.

Croconic acid crystals show proton displacive-type ferroelectricity with a large spontaneous polarization reaching $20\text{ }\text{ }\ensuremath{\mu}\mathrm{C}/{\mathrm{cm}}^{2}$, which originates from the strong coupling of proton and $\ensuremath{\pi}$-electron degrees of freedom. Such a coupling makes us expect a large polarization change by photoirradiations. Optical-pump second-harmonic-generation-probe experiments reveal that a photoexcited croconic-acid crystal loses the ferroelectricity substantially with a maximum quantum efficiency of more than 30 molecules per one absorbed photon. Based on density functional calculations, we theoretically discuss possible pathways toward the formation of a one-dimensional domain with polarization inversion and its recovery process to the ground state by referring to the dynamics of experimentally obtained polarization changes.

Three polymorphic forms of 6,6′-dimethyl-2,2′-bipyridinium chloranilate crystals were characterized to understand the origin of polarization properties and the thermal stability of ferroelectricity. According to the temperature-dependent permittivity, differential scanning calorimetry, and X-ray diffraction, structural phase transitions were found in all polymorphs. Notably, the ferroelectric α-form crystal, which has the longest hydrogen bond (2.95 A) among the organic acid/base-type supramolecular ferroelectrics, transformed from a polar structure (space group, P21) into an anti-polar structure (space group, P21/c) at 378 K. The non-ferroelectric β- and γ-form crystals also exhibited structural rearrangements around hydrogen bonds. The hydrogen-bonded geometry and ferroelectric properties were compared with other supramolecular ferroelectrics. A positive relationship between the phase-transition temperature (TC) and hydrogen-bond length ( ) was observed, and was attributed to the potential barrier height for proton off-centering or order/disorder phenomena. The optimized spontaneous polarization (Ps) agreed well with the results of the first-principles calculations, and could be amplified by separating the two equilibrium positions of protons with increasing . These data consistently demonstrated that stretching is a promising way to enhance the polarization performance and thermal stability of hydrogen-bonded organic ferroelectrics.