Soliton In Trans-polyacetylene Research Articles

Stability of moving solitons in trans -polyacetylene in an electric field

In this work we study the dynamics and stability of charged solitons in trans-polyacetylene (tPA), and revisit the issue of the stability of these non-linear excitations under the effect of an external electric field applied parallel to the polymer. Using the formalism of the Su-Schrieffer-Heeger (SSH) model, we solve the coupled dynamical equations for electrons and classical nuclei at the mean-field level and in the regime of low external electric field $E$, where the dynamics of the moving soliton is adiabatic. Analyzing observable quantities in real space and frequency space, we identify the microscopic mechanisms triggering the dynamical instabilities of the soliton. In addition, we put forward the definition of a proper quantitative measure of its stability, an issue which to the best of our knowledge has remained an open question. Besides its intrinsic interest from the fundamental point of view, our work might be relevant for the design of novel organic electronic devices based on soliton-mediated transport.

Chaotic Soliton Dynamics in Photoexcited trans-Polyacetylene

We study the photogeneration of topological solitons in trans-polyacetylene and their time evolution using ab initio excited-state dynamics. The system is excited to the optically allowed 1(1)Bu state, and the atoms are then propagated classically using quantum mechanical forces computed using hybrid time-dependent density functional theory (TD-DFT). A soliton/antisoliton pair nucleates spontaneously and creates two independent solitons moving at constant velocity, similar to simulations based on uncorrelated lattice models like the Su-Schrieffer-Heeger (SSH) Hamiltonian [Su, W. P.; Schrieffer, J. R.; Heeger, A. J. Phys. Rev. Lett. 1979, 42, 1698]. At T = 0, the solitons coalesce into bound pairs with a two-soliton functional form, whereas chaotic dynamics, in the form of 2-bounce resonances, is observed at soliton/antisoliton collisions at T ≠ 0. This behavior is related to the onset of a strong correlation regime at short intersoliton distance, which is not accounted for by SSH simulations.

Ultrafast real-time vibronic coupling dynamics of a breather soliton in trans-polyacetylene with a few-optical-cycle-pulse laser

The dynamics preceding the spatial separation of a charged soliton pair after photoexcitation in trans-polyacetylene was successfully investigated by using ultrafast spectroscopy with a 6.2fs pulse laser. It was directly verified that after photoexcitation, the electron–hole pair relaxes with a breather mode (i.e. multi-quanta vibronic states), as theory predicts, with an electron–hole pair lifetime of 33–50fs. By applying spectrogram analysis to the time trace of the absorbance change, the ultrafast amplitude and frequency modulations of CC and CC stretching modes, induced by breathers and lasting no longer than 100fs, can be observed simultaneously for the first time. The frequency shifts of both modes were in good agreement with a simulation based on the Su–Schrieffer–Heeger model. It was found that the intensities of transition dipoles changed due to breathers, whereas transition energies were dominantly modulated by CC stretching modes as recent theoretical work predicted.

Atomic Vibration and Sliding Speed of Charged Solitons in Trans-Polyacetylene

To clarify the soliton behavior in heavily doped trans-polyacetylene, a sliding simulation of the charged soliton lattice is carried out. The mechanism to determine the sliding speed is studied. The speed is determined by a coupling between the passing solitons and the C-C stretching vibration. The period of the passing solitons is synchronized with one time the period of the C-C stretching vibration with higher soliton density, i.e., more than 6.7% in this simulation, where the energy of the carbon vibration is effectively used for the soliton lattice translation. And if the soliton density is lower, where the next soliton cannot catch up with the carbon vibration, the period is synchronized with more than one time and integer times the period of the C-C stretching vibration.

Spin solitons in organic charge-transfer salts

Organic charge-transfer (CT) salts that crystallize in face-to-face stacks of π-electron donors (D) and acceptors (A) are one-dimensional electronic systems with strong coupling to lattice and molecular vibrations. Peierls–Hubbard models of CT salts resemble π-electron theory of conjugated polymers, although transfer integrals t along mixed DA stacks are considerably smaller. Strong D and A yield Peierls systems with dimerized ground states of ion radicals, D + and A −. Topological spin solitons separate regions of opposite dimerization, similarly to solitons in trans-polyacetylene, but are thermally accessible in ionic CT salts due to small t. Salts with still smaller t have Peierls transitions at T P < 300 K to undimerized stacks. A Peierls–Hubbard model, H CT, describes both types of salts and estimates of t are consistent with T P < 300 K in some salts and spin solitons in others with higher T P. Electron paramagnetic resonance (epr) spectra of single crystals provide direct evidence for spin solitons and one-dimensional electronic states. Spin solitons and H CT resolve longstanding conflicts between vibrational and magnetic data that indicate dimerized stacks in several prototypical CT salts, while structural data point to undimerized stacks with large thermal ellipsoids. The low energy scale, availability of single crystals and diversity of CT salts offer opportunities for detailed modeling.

Spin soliton in aπ-conjugated ladder polydiacetylene

The electronic structure and the spin dynamics of a spin soliton observed in a polydiacetylene have been investigated using multifrequency pulsed and continuous-wave (cw) electron paramagnetic resonance (EPR), electron spin echo (ESE) spectroscopy, electron-nuclear double resonance (ENDOR), and electron-nuclear-nuclear triple resonance (TRIPLE). The undoped ladder polymer in which two polydiacetylene wires are linked by bridges consisting of two triple bonds exhibits a strong EPR signal. In contrast, no EPR signal is observed in the ladder-structured polydiacetylene, where the bridges consist of single bonds. The observed bridge effect on the probability of formation of paramagnetic centers is interpreted in terms of a quasidegenerate ground state due to the π-conjugated ladder structure that creates a π-bond kink corresponding to a spin soliton. High-frequency and high-field EPR reveals a small anisotropy of the g tensor (principal values g 1 =2.0037, g 2 =2.0028, and g 3 =2.0023) for the spin soliton. By nutation spectroscopy, the spin multiplicity was confirmed to be S=½. The hyperfine pattern attributed to the β protons was detected by ENDOR and TRIPLE methods, providing evidence for both the positive and negative spin densities of the soliton wave function. These experimental results led to the conclusion that the unpaired electron of the spin soliton was substantially delocalized over the two polydiacetylenes through the π-conjugation. At temperatures (T) below 50 K, the width of the EPR spectrum narrows by about 45% with increasing temperature and the width remains nearly constant for T>50 K. The pronounced change in the line width for T<50 K suggests the presence of a mobile spin soliton in the ladder-structured polydiacetylene. A temperature dependence of the electron spin system's phase memory time T M for T<50 K was also observed by ESE spectroscopy. The decay of the ESE exhibits a nonexponential behavior, with T M increasing monotonically with rising temperature These findings represent clear experimental evidence for motional narrowing phenomena. Based on a one-dimensional random-walk model for the unpaired electron spin along the ladder polymer, the soliton's diffusion rate constant was estimated from an analysis of the ESE dephasing process. An Arrhenius plot of the diffusion rate was asymptotic to 8×10 7 s - 1 at very low temperature, which is two orders of magnitude slower than that of the soliton in trans-polyacetylene.

Infrared intensity and local vibrations of charged solitons

We present a simple microscopic model for infrared active vibrations (IRAV's) of charged solitons in trans-polyacetylene (PA), by combining the Su-Schrieffer-Heeger model for $\ensuremath{\pi}$ electrons and a force field based on pristine PA. The model rationalizes the one-to-one correspondence between Raman modes of pristine PA samples and IRAV's of doped or photoexcited samples and provides a microscopic basis for the phenomenological amplitude mode formalism. The softening of the IRAV modes relative to Raman frequencies and their huge intensities are clearly due to their coupling to $\ensuremath{\pi}$ electrons, and specifically to the large $\ensuremath{\pi}$-electron fluctuations induced by small nuclear displacements along a single local coordinate. The model accounts for effective mass and infrared spectra of photoexcited samples, and qualitatively, for dopant-induced spectra. Our microscopic picture also clarifies two long-standing puzzles. A soliton defect extending over $\ensuremath{\sim}14$ sites generates many local modes, but only special ones are coupled strongly to $\ensuremath{\pi}$ electrons. At the same time, local vibrations are sensitive to chain lengths much longer than the defect due to the long electronic coherence length.

EPR lineshape analysis of one-dimensional soliton diffusion in trans-polyacetylene

The one-dimensional diffusion processes of solitons in trans-polyacetylene are studied by timedomain analysis of EPR measurements. The temperature dependence of the diffusion rate constant appears to obey a simple power law. Monte Carlo simulations of the diffusion processes indicate that impurity traps can cause a significant slow down of the overall diffusion rate.

EFFECTS OF HUBBARD INTERACTION AND SITE-TYPE IMPURITY POTENTIAL ON ELECTRONIC STATES IN TRANS-POLYACETYLENE

In this paper, the SSH mode] has been extended to include the site-type impurity potential and the Hubbard interaction. The energy spectra and the electronic states of soliton in transpolyacetylene have been studied by the self-consistent iterative approach. Our numerical calculations have shown that the important differences in the effects of impurity potential and Hubbard interaction on electronic states of soliton. Namely, the site-type impurity potential is to shift the midgap states of the charged solitons in the opposite direction with Hubbafd interaction. Our conclusion are coincident with experimental results

Ring-torsional solitons in polyaniline.

Topological defects involving distortions of the phenyl ring torsion angles in the leucoemeraldine base form of polyaniline are studied. A ring-torsional order parameter is introduced; two solitonlike defects of this order parameter are found, one of which has been discussed previously. These solitons are anticipated to exhibit a conventional spin-charge relationship, unlike the bond-alternation solitons in trans-polyacetylene. The creation energies of these solitons are estimated, and possible experimental signatures and relevance of neutral and charged solitons are discussed.

Ab initio calculations of trans-polyacetylene clusters including sodium counterions

The geometrical and electronic properties of a negatively charged soliton in trans-polyacetylene are calculated at the ab initio Hartree-Fock level. The optimized geometry of a system including a sodium counterion is compared with that of a negatively charged soliton with no counterion present. It is shown that in the presence of the dopant, the characteristics width of the soliton is reduced by a factor of 2.5. For higher doping levels, a single counterion does not represent the dopant potential accurately. Instead, the situation of high doping concentration is represented by a cluster of three polymer chains, carrying one solition each and surrounding a channel of three sodium ions. In our calculations, only one of these chains is treated quantum mechanically. Theother two chains are represented by an array of point charges located at the carbon sites of these chains. When the electrostatic interaction with these point charges is taken into account, as well as the full interaction with the sodium ions, the soliton width is increased as compared to the case of the soliton interacting with a single counterion. Due to the attractive potential of soliton-counterion complexes on neighboring chains, additional geometrical distortions in the soliton geometry occur. This effect must be considered when discussing the transition into a metallic state for highly doped trans-polyacetylene.

Influence of local strains of chemical bonds on the movement of solitons in trans-polyacetylene

Local binding or compression of chemical bonds in trans-polyacetylene chain are considered with respect to their influence on movement of a soliton along the chain. Compression of double and single bond by an equal length (bending by an angle) results in different ground state energies of the system. Therefore while passing along the chain with a strained double or single bond the soliton can be reflected, decelerated or accelerated. Being placed between strained double bonds, the soliton appears to be in potential well.

Temperature-dependent microsecond decay dynamics of photogenerated charged solitons in trans-polyacetylene

The decay behavior of photogenerated charged solitons in trans-polyacetylene has been observed with a newly constructed microsecond time-resolved infrared spectrometer in the temperature range 77–240 K. A unimolecular decay ascribable to geminate recombination is dominant at 77 K, while bimolecular decay processes become more significant at higher temperatures. This characteristic temperature dependence of the decay behavior is discussed in relation to known temperature-dependent photoconductivity of trans-polyacetylene and its microscopic mechanism.

The structure of the soliton in trans-polyacetylene: A pulse endor investigation

Detailed ENDOR, TRIPLE resonance and ENDOR Multiple-Quantum spectroscopy on trans-polyacetylene below 20 K reveal a soliton halfwidth at half maximum of w=12 CH-units and a negative-to-positive spin density ratio of ug=0.42.

Displacement vectors of localized vibrational modes around a charged soliton in Trans-polyacetylene

The localized vibrational modes of finite polyacetylene with a charged soliton are calculated taking full degrees of freedom into consideration. The displacement vectors of each localized vibrational mode are obtained and shown in this report. The vectors are discussed in connection with mode mixing. The localized modes are successfully classified into branch modes of four modes: translational, amplitude oscillation, third localized and staggered. Displacement patterns of localized modes indicate that the coupling between the two types of C-C stretching branch modes is weak, but there is a strong coupling between one of the C-C stretching modes and the C-H bending mode. Calculated frequencies and infrared intensities are found to be in good agreement with the infrared spectrum. Furthermore, it has been found that a force constant matrix obtained from a short cluster model is not suitable for calculation in the case of a long polyacetylene chain.

Dynamic effects of the impurity potential and electron interactions on a soliton in trans-polyacetylene.

We calculate the potential energy of a charged soliton on a chain of polyacetylene in the presence of an (interstitial) impurity atom and Hubbard on-site interactions. Using this potential, we find the lowest two bound states of the soliton which determine the ``Goldstone-mode'' contribution to infrared absorption. The wave functions are then used to incorporate soliton dynamics into the calculation of optical absorption by the chain. Also included is a study of interaction effects on neutral solitons in pure polyacetylene.

Dynamics of photogenerated solitons in trans-polyacetylene

Time-resolved absorption using pole wavelengths of 2.5-5.5 mu m has been used to study photoexcitations in the quasi-one-dimensional semiconductor, trans-polyacetylene. Present apparatus has 0.5-ps resolution, which is sufficient to observe the decay of the photogenerated charged soliton pairs which form after intrachain excitation of electron-hole pairs. The tunability of the apparatus permits the unambiguous identification of solitons and observation of spectral relaxation during their formation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Calculation of localized vibrational modes around a soliton in trans-polyacetylene using the molecular orbital method

Vibrational frequencies of polyacetylene with a charged soliton are calculated for finite chain lengths using the molecular orbital method. A translational mode, an amplitude oscillation mode, a third localized mode and a staggered mode are obtained as localized modes. It is found that the frequency of the translational mode is inversely proportional to the square of the chain length, and approaches zero when the length becomes infinite. This can be explained if the soliton is regarded as a quantum particle in a box. A type of electron-hole symmetry holds, even with the Coulomb effect, between occupied orbitals of the cation and unoccupied orbitals of the anion and between unoccupied orbitals of the former and occupied orbitals of the latter.

Picosecond dynamics of photogenerated charged solitons in trans-polyacetylene.

Measurements of the dynamics of the photoinduced 0.45-eV midgap absorption associated with photogenerated charged solitons in trans-polyacetylene are reported. We have studied the intensity, polarization, and temperature dependences of the dynamics of this band with 5-ps resolution. Two types of charged solitons are clearly identified. First, there are directly photogenerated Su-Schrieffer soliton pairs which form following intrachain absorption and geminately recombine on subpicosecond time scales. At later times (\ensuremath{\gtrsim}20 ps), there are charged solitons formed by an interchain absorption mechanism where polarons are formed and diffuse to convert residual neutral solitons to charged solitons.

Chain length dependence of localized vibrational modes around a soliton in trans-polyacetylene

Vibrational frequencies of finite polyacetylene with a charged soliton are calculated using the molecular orbital method. It is found that the frequency of the translational mode has a remarkable length effect and is inversely proportional to the square of chain length. The dependence is explained by regarding the soliton as a quantum particle in a one dimensional box.