Neutral-ionic Phase Transition Research Articles

Giant infrared intensity of the Peierls mode at the neutral-ionic phase transition.

We present exact diagonalization results on a modified Peierls-Hubbard model for the neutral-ionic phase transition. The ground state potential energy surface and the infrared intensity of the Peierls mode point to a strong, nonlinear electron-phonon coupling, with effects that are dominated by the proximity to the electronic instability rather than by electronic correlations. The huge infrared intensity of the Peierls mode at the ferroelectric transition is related to the temperature dependence of the dielectric constant of mixed-stack organic crystals.

Lattice and magnetic instabilities near the neutral-ionic phase transition of the one-dimensional extended Hubbard model with alternating potentials in the thermodynamic limit

The effects of dimerization causing the alternation of transfer integrals on the neutral-ionic phase transition are studied in the one-dimensional extended Hubbard model with alternating potentials at half filling. The finite-temperature density-matrix renormalization-group method is used to treat the thermodynamic limit. In the ionic phase, the free-energy gain is proportional to ${\ensuremath{\delta}}^{4/3}$ with $\ensuremath{\delta}$ being the degree of dimerization even near the phase boundary at low temperatures. In the neutral phase, the free-energy gain is quadratic for small transfer integral far from the phase boundary, but it is nonlinearly enhanced near the boundary. Large transfer integrals make the gain faster than ${\ensuremath{\delta}}^{2},$ so that they facilitate dimerization. The dimerization in the neutral phase increases the ionicity and lowers the spin excitation energies. These lattice effects are in contrast to the effects of a staggered magnetic field. The relevance of these results to recently observed dimerization in the neutral phase of ClMePD-DMeDCNQI is discussed.

Quantum and thermal charge-transfer fluctuations for neutral-ionic phase transitions in the one-dimensional extended Hubbard model with alternating potentials

Effects of quantum and thermal fluctuations on transitions between the ionic phase and the neutral phase are studied by applying the finite-temperature density-matrix renormalization-group method to the one-dimensional extended Hubbard model with alternating potentials at half filling. Charge-transfer fluctuations lower the energy of the neutral phase more than that of the ionic phase, which is in contrast to the spin fluctuations favoring the ionic phase. As a consequence, with increasing intermolecular overlap between the neighboring donor and acceptor sites, we expect a spin-fluctuation-induced transition from the neutral phase to the ionic phase, and a charge-transfer-fluctuation-induced transition from the ionic phase to the neutral phase, near the phase boundary. Relevance to a recently observed phase transition in a new class of mixed-stack charge-transfer complexes is discussed.

Ultrafast Dynamics of Photo-Induced Cooperative Phenomena: Neutral-Ionic Phase Transition in Charge Transfer Complex.

Primary dynamics of photo-induced ionic (I)-neutral (N) phase transition in a charge transfer complex tetrathiafulvalene-p-chloranil (TTF-CA) was investigated by femtosecond reflection spectroscopy with varying excitation densities. In I-to-N transition, charge transfer excitation in the I phase produces N donor-acceptor (D0A0) strings within 200 fs immediately after the weak photo-excitation. These initial N states decay with a life time of 300 ps at 4 K, whereas those cause multiplication leading to macroscopic I-to-N conversion within 20 ps at 77 K just below the N-I transition temperature TNI. Such a multiplication process is not observed for the N-to-I transition at 90 K which is a reverse process of I-to-N transition. Ionic (D+A−) string with a lifetime of several picosecond was observed even for strong excitation in N-to-I transition. Vibrational coherence due to the collective motion of the D+A− dimerization as well as of N-I domain boundary in several tenth ps and sub-ps time regions is observed.

Variation Mechanisms of Ground-State and Optical-Excitation Properties in Quasi-One-Dimensional Two-Band Electron Systems

We study i) the ground-state and optical-excitation properties of halogen-bridged binuclear metal complexes, which are known as MMX chain compounds, by the strong-coupling expansion for a one-dimensional two-orbital extended Peierls-Hubbard model, and ii) the dynamics of domain walls between the neutral and ionic phases in mixed-stack charge-transfer complexes, by solving the time-dependent Schrödinger equation for a one-dimensional extended Peierls-Hubbard model with alternating energy levels. We find in i) that competition between electron-electron and electron-lattice interactions and competition between short- and long-range interactions are both important for the electronic phase variation, and in ii) that charge and lattice dynamics immediately after photoexcitations is complex and not explained simply within the domino picture. MMX chain charge polarization charge density wave mixed-stack charge-transfer complex neutral-ionic phase transition photoinduced phase transition

Far-Infrared Optical Response of Neutral-Ionic Phase Transition in an Organic Charge-Transfer Complex

Infrared optical spectra have been investigated for a charge-transfer complex of tetrathiafulvalene and 2-bromo-3,5,6-trichloro- $p$-benzoquinone, which undergoes a neutral-ionic ( $N\ensuremath{-}I$) transition at ${T}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}68\mathrm{K}$. With the decrease of temperature toward ${T}_{c}$, the far-infrared reflectivity spectrum is gradually transformed into a Drude-like high reflectance band edging around $100{\mathrm{cm}}^{\ensuremath{-}1}$, and the optical conductivity shows an anomalous peak around $10{\mathrm{cm}}^{\ensuremath{-}1}$. The far-infrared anomaly is related to the gigantic dielectric response just above ${T}_{c}$, which are both discussed in terms of dynamics of $N\ensuremath{-}I$ domain walls with fractional charge.

Neutral-ionic phase transition of (BEDT-TTF)(ClMeTCNQ) under pressure

Neutral-ionic phase transition of (BEDT-TTF)(ClMeTCNQ) under pressure

A new type of neutral–ionic interface in mixed stack organic charge transfer crystals: Temperature induced ionicity change in ClMePD–DMeDCNQI

Raman and polarized infrared spectra of the mixed stack charge transfer crystal 2-chloro-5methyl-p-phenylendiamine- -2,5-dimethyl-dicyanoquinonediimine (ClMePD-DMeDCNQI) are reported as a function of temperature. A detailed spectral interpretation allows us to gain new insight into the temperature induced neutral-ionic transition in this compound. In particular, the crossing of the neutral-ionic borderline appears to be quite different from that of the few known temperature induced neutral-ionic phase transitions. First of all, the ionicity change is continuous. Furthermore, the onset of stack dimerization precedes, rather than accompanies, the neutral-ionic crossing. The (second order) phase transition is then driven by the dimerization, but the extent of dimerization is in turn affected by the ionicity change.

Electronic phase transition of BEDT-TTF based mixed-stack charge-transfer complexes

The BEDT-TTF based mixed-stack charge-transfer (CT) complexes have been investigated for a series of isostructural compounds. The ionic complex of (BEDO-TTF)(Cl 2TCNQ) is a magnetic insulator with showing Curie-Weiss behavior in magnetic susceptibility, in sharp contrast to the conventional mixed-stack CT complexes. The (BEDT-TTF)(ClMeTCNQ) compound undergoes neutral-ionic (NI) phase transition at high pressure.

Phase control of neutral-ionic states by chemical doping

The effect of molecular substitution on the neutral-ionic (NI) phase transition has been investigated for TTF- p-chloranil (QCl 4) by the measurements of dielectric constant. Due to the ferroelectric nature in the transformed ionic state, the dielectric constant shows a large anomaly at the NI transition temperature, which was remarkably but continuously lowered down to zero temperature by the molecular substitution. X-ray structural analysis revealed the large “chemical pressure” effect for TSF doped crystals.

Effect of chemical doping on the ferroelectric neutral-ionic phase transition in tetrathiafulvalene-p-chloranil(TTF−QCl4)

Effects of molecular substitution on the neutral-ionic (NI) phase transition have been investigated for tetrathiafulvalene (TTF)-p-chloranil $({\mathrm{QCl}}_{4})$ complex doped with tetraselenafulvalene (TSF) or trichloro-p-benzoquinone $({\mathrm{QCl}}_{3})$ molecules. Except for the fully substituted compound ${\mathrm{T}\mathrm{T}\mathrm{F}\ensuremath{-}\mathrm{Q}\mathrm{C}\mathrm{l}}_{3}$ of minor structural modifications, x-ray-diffraction measurements confirm the isostructurality over a wide compositional range as well as smooth change in the lattice constants by molecular replacement. With increasing TSF concentration, the peak in the dielectric constant corresponding to the ferroelectric NI transition smoothly shifts toward zero temperature due to the lattice expansion. Around the critical concentration are found some characteristics of quantum ferro(para)electricity. Although the increment of ${\mathrm{QCl}}_{3}$ concentration also lowers the peak temperature of the dielectric constant, the dielectric anomalies reveal glasslike frequency dependence. We attributed the glasslike behavior to the strong ``pinning-impurity'' effect of ${\mathrm{QCl}}_{3}$ which yields binary-phase separation into ferroelectric (ionic) and paraelectric (neutral) regions.

Ultrafast Dynamics of Reversibly Photoinduced Neutral-Ionic Transition in Tetrathiafulvalene-Chloranil Single Crystals

We report that bi-directional neutral-ionic (N-I) phase transition in the single crystal of tetrathiafulvalene-chloranil (TTF-CA) can be triggered by pulsed laser excitation with 100 femto-second width. The excitation intensity dependence of the dynamics of N-I domain-wall indicates that cooperative electron-lattice and/or electron-electron interactions play a key role in the driving mechanism of N-I photoinduced phase transition.

Model of photo-induced neutral-ionic phase transition in organic charge-transfer salts

One-dimensional donor-acceptor mixed chains are modeled by a periodic DADA tetramer. Electron coupling to intramolecular vibrations are taken into account explicitly. Generalized adiabatic potentials are calculated for the cases of regular and dimerized stacks which are characteristic, respectively, of quasi-neutral (N) and quasi-ionic (I) phases of a tetrathiafulvalene-chloranil compound. A sharp difference in life-times of photo-induced I-states in the N-phase and N-states in the I-phase is discussed within the periodic DADA tetramer model.

Langmuir-Blodgett films of molecular conductors based on alkylTCNQ derivatives

The charge transfer properties of alkylTCNQ derivatives in the LB films of molecular conductors were examined. TMTTF, OMTTF and BEDOTTF were adopted as donors to form charge transfer complexes with C 10 TCNQ or C 14TCNQ. The degree of charge transfer of the alkylTCNQ in the LB films could be controlled from nearly neutral to fully ionic state by adopting appropriate donors for the counterpart. OMTTF-C 10TCNQ formed a neutral complex which is located near the neutral-ionic boundary and is expected to be a candidate to realise neutral-ionic phase transition in the LB films.

Charge Transfer in Donor-Acceptor Crystalline Complexes

The pressure-induced neutral-ionic transition of TTF-DMDCNQI complex is analyzed by using a simple dimer model with electron-intramolecular vibration coupling. The coupling is shown to be responsible for a sharp neutral-ionic phase transition and characteristic hysteresis in the degree of ionicity dependence upon pressure in this complex. Ground state charge distribution is calculated as a function of small-polaron binding energies and transfer integral between donor and acceptor sites.

Tetramethylbenzidine-TCNQ: Studies of the neutral-ionic transition by time-resolved non-degenerate four-wave mixing

Y. Iwasa et al. reported recently experimental evidence for the occurrence of a temperature-induced neutral-ionic transition (TINT) in a new mixed stack charge transfer (CT) compound consisting of 3,3′,5,5′ tetramethyl benzidine (TMB) as the donor and TCNQ as the acceptor (TC = 205 ± 5K) We have used femtosecond laser pulses and the technique of time resolved non degenerate four-wave mixing to measure the third order susceptibility χ(3) of thin films of TMB-TCNQ, and the corresponding temporal response as a function of temperature above and below the neutral-ionic transition. The change of the decay time of different contributions to χ(3) with T will be discussed in connection with the physical process related to this neutral-ionic phase transition. We point out the importance of the electron-phonon interactions in this phase transition.

Interacting electrons and non-adiabatic holstein phonons: The numerical treatment of the neutral-ionic phase transition

Abstract We present exact results on the N-I phase transitions obtained by non-adiabatic valence bond diagonalization on finite size clusters.

Mixed-stack charge-transfer films prepared by Langmuir-Blodgett technique and donor doping

The formation of mixed-stack charge-transfer (CT) films was studied. Spectroscopic analysis suggested that donor doping to a 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (C18TCNQ) film prepared by the Langmuir-Blodgett technique resulted in the formation of the mixed-stack CT film when 3,3′,5,5′-tetramethylbenzidine (TMB) was used as a donor, while degradation of the CT complex was observed when N,N,N′,N′-tetramethyl-p-phenylenediamine was used. Although X-ray diffraction measurements suggested that the order of the layered structure and crystallinity of the film were decreased by the doping, it was proved that the molecular columns of TMB and C18TCNQ were preferentially oriented along a substrate from polarized electronic spectra and scanning electron microscope observation. Therefore, non-linear electrical conduction as observed in mixed-stack CT crystals was expected. In the current density-electric field characteristic for the TMB-C18TCNQ film, the non-linearity was weaker than that observed in a TMB-TCNQ crystal. Furthermore, a temperature-induced neutral-ionic (N-I) phase transition was not observed from 4.2 K to room temperature in a spectroscopic measurement.

Ground state optical properties of charge transfer crystals close to the neutral-ionic interface: Tetrathiafulvalene-2,5-dichloro-p-benzoquinone

The crystal structure and the optical data of the (1:1) charge-transfer (CT) complex of tetrathiafulvalene (TTF) with 2,5-dichloro-p-benzoquinone (2,5Cl2BQ) are reported, yielding a full characterization of the ground state properties. The complex crystallizes in the triclinic space group P1̄, Z=1. TTF and 2,5Cl2BQ form mixed regular stacks along the b axis. The degree of ionicity ρ is ∼0.18, and the CT integral ∼0.22 eV. A detailed analysis of the optical data yields estimates of the strength of electron–molecular vibration coupling and of intersite electron–electron interactions. The latter has also been computed in the atom-in-molecule fractional charge approximation. The TTF-2,5Cl2BQ structure and microscopic parameters at ambient conditions are found to be rather similar to those of the extensively studied complex of TTF with tetrachloro-p-benzoquinone (CA). The different behavior of the two complexes under pressure (both exhibit a neutral–ionic phase transition, but ρ changes discontinuously for TTF–CA and continuously for TTF-2,5Cl2BQ) is ascribed to the competitive role of the Madelung energy and of the CT integral.

Lattice-Dynamical Aspects of Neutral-Ionic Transition in TTF-Chloranil Charge-Transfer Crystal

Abstract The paper presents results of numerical calculations of the statics (crystal structure minimisation) and lattice dynamics for the neutral (higher temperature) phase of the TTF-Chloranil crystal. The results, in particular the low frequency phonons are discussed in relation to neutral-ionic phase transition, which takes place in the system at around 80 K.