Charged Solitons In Polyacetylene Research Articles

Excitation spectrum for ultrafast photogeneration of charged solitons in polyacetylene

Ultrafast photoinduced absorption by IRAV modes is used to detect charged solitons in oriented polyacetylene. We find that soliton pairs are photogenerated within our time resolution of ∼250 fs with -100% quantum efficiency (Φ ch ). The excitation spectrum of Φ ch shows an onset at 1.0 eV, with a weak photon energy dependence up to 4.7 eV. These results agree with the ultrafast soliton formation predicted by Su and Schrieffer and with the SSH threshold of 2E g /π for soliton pair production.

Excitation spectrum for ultrafast photogeneration of charged solitons in polyacetylene

Ultrafast photoinduced absorption by infrared-active vibrational modes is used to detect charged solitons in oriented trans-polyacetylene. Soliton pairs are photogenerated within ;250 fs with quantum efficiencies (fch) approaching unity. The excitation spectrum of fch shows an onset at ;1.0 eV with a weak photon energy dependence up to 4.7 eV. The results are consistent with the ultrafast soliton formation predicted by Su and Schrieffer and with the Su-Scrieffer-Heeger threshold of 2Eg /p for soliton pair production. The recombination dynamics of charged solitons is very fast ~initial decay,1 ps) with a modest dependence on the pump photon energy.

Collisions of solitons in polyacetylene

The dynamics of collisions of structural defects in polyacetylene are studied numerically using the Su-Schrieffer-Heeger model extended to include the influence of an external electric field. The electric field is used to accelerate charged solitons in polyacetylene and it is introduced in terms of a time-dependent vector potential. Collisions between solitons with different charges in a soliton network are considered. It is found that although two isolated solitons on a chain may exchange their moments in a collision, several solitons work like a barrier attenuating the transfer moment. A charge oscillation is observed between the colliding solitons and partial charge transfers. It is also found that if the colliding solitons hold different charges (charged soliton-charged soliton or neutral soliton-charged soliton), the pair is more stable, surviving the collision. Neutral solitons are unstable against collisions and make a dynamic bound state. The energies involved are also discussed.

Electric Conduction Due to Charged Solitons in Polyacetylene*

In order to estimate the mobility of a charged soliton in polyacetylene, the dynamics of a soliton in an electric field is numerically investigated in the presence of a phenomenological damping of the lattice motion. Except for the damping term in the equation of motion for the lattice displacements, the dynamics is assumed to be determined by Su, Schrieffer and Heeger's model. It is shown that a Drude-type treatment of the soliton motion is applicable. The mobility is found to be µ=88.9 cm 2 /Volt·sec, and the conductivity is estimated to be σ=7.54×10 3 Ω -1 cm -1 on the assumption that the concentration of the solitons is one per 100 carbon atoms. The present result is compatible with experimental data in the non-magnetic regime.

Motion of Charged Soliton in Polyacetylene Due to Electric Field. IV. Damping of Soliton Velocity

We study velocity damping of a moving soliton in polyacetylene by using numerical simulations within Su, Schrieffer and Heeger's model. The system is assumed to be at absolute zero of temperature and involves no impurity. In order to get an initial velocity the soliton is accelerated by applying an external electric field. The relaxation time of the soliton velocity has a minimum value when the initial velocity is around the sound velocity of this system. This is a result of the interaction between the moving soliton and the acoustic phonons emitted by the soliton itself. We find that the damping of the soliton velocity is caused by the energy transfer from the moving soliton to the acoustic phonons. The transfer is enhanced when the soliton is surrounded by the acoustic phonons.

A quantum chemical study of interchain hopping model of negatively charged solitons in polyacetylene

Phonon-assisted interchain hopping of negatively charged solitons in polyacetylene has been studied using a local chemical reaction model CH + CH4 CH4 + CH. Quantum chemical characteristics of the electron transfer process have been analyzed in terms of the dynamic electron density and the mutual polarization moment. The CH stretching vibrational motion of CH4, which is a local model of the sp3 defect, has been found to play a significant role for the electron transfer. The excitation of the corresponding vibrational mode of the sp3 defect would promote the interchain hopping of the charged soliton. The electron transfer process has also been studied in terms of the “regional” density functional theory. It has been shown that the driving force of the electron transfer is represented by the regional chemical potentials.

Motion of Charged Soliton in PolyacetyleneDue to Electric Field.III. Energetics

In numerical simulations on a moving charged soliton in polyacetylene, the behavior of the total, electric, lattice potential and lattice Kinetic energies are analyzed. In order to induce physically natural motion, the soliton is accelerated by a uniform electronic field as in the previous papers of this series. During the acceleration the electronic energy shows an almost monotone decrease while the lattice potential energy increases in an almost monotone way. These two energies have also an oscillating component which is related to the previously reported oscillation of the soliton width. On the contrary the total and the lattice kinetic energies show no oscillation. The relation between the total energy e tot and the soliton velocity υ is well fitted to a functional form e tot (υ)=e tot (0)-( M s υ m 2 /2) ln (1-υ 2 /υ m 2 ) where υ m is the saturation velocity of about three times the sound velocity and M s the soliton effective mass of four to five times the bare electron mass.

Motion of charged soliton in polyacetylene due to electric field

The behaviour of a moving soliton in polyacetylene is numerically studied from the first principle by using Su-Schrieffer-Heeger's (SSH) model. The motion is introduced by applying an electric field to a system with a static charged soliton. The time dependences of the electronic wave functions and the lattice displacements are determined selfconsistently. The electric field is given in terms of a time dependent vector potential, and therefore the periodic boundary condition can be maintained.

Motion of Charged Soliton in Polyacetylene Due to Electric Field. II. Behavior of Width

By numerical simulations treating the motion of a charged soliton in polyacetylene, we analyze the width of a moving soliton. The motion of the soliton is induced by applying an electric field as in the previous work. The soliton width is estimated by two methods; one is from the excess charge density profile and the other from the distortion in the lattice displacement. Both results indicate that the width decreases as the soliton velocity increases. Quantitatively, however, the amount of decrease is different in the two cases; the maximum change of the width is about 10% in the former case whereas it is 20% in the latter. Furthermore, the width obtained from the lattice distortion is found to oscillate as a function of time, consistently with the excitation of the amplitude mode around a soliton.

Motion of Charged Soliton in Polyacetylene Due to Electric Field

Real time dynamics of a moving charged soliton in polyacetylene is studied numerically by using Su-Schrieffer-Heeger's model. An electric field is introduced to the system through a time dependent vector potential which can be included into the Hamiltonian through the Peierls substitution of the phase factor to the transfer integral. Several interesting properties of the moving soliton are obtained, e.g. , the Brownian-like motion of the soliton due to the soliton-phonon interaction, the saturation of the soliton velocity, the saturation velocity being independent of the applied electric field strength, while the time needed to attain the saturation velocity is, roughly speaking, linearly dependent on the logarithm of the applied field.

Gap states of charged solitons in polyacetylene.

By considering the electron interactions in polyacetylene, it is found that there exist two gap states in charged solitons of trans-polyacetylene: One is deep level, and the other is shallow level. The deep one shifts 0.23 eV down (for a positive soliton) or up (for a negative soliton) from the center of the gap, while the shallow one is 0.06 eV under the bottom edge of the conduction band (positive soliton) or above the top edge of the valence band (negative soliton). These results agree with the absorption spectra of trans-polyacetylene. Other shallow states outside the energy bands are also predicted.

Mechanism for photogeneration of metastable charged solitons in polyacetylene.

We present a detailed investigation of the optical excitations, both electronic and vibrational, from photogenerated charged solitons in cis-rich and all-trans-polyacetylene prepared by the Shirakawa method. We find that the saturation concentration of photogenerated solitons on the trans sequences in cis-rich material (30--40 % trans) is substantially higher than in all-trans material, and that the lifetimes in this cis-rich material are considerably longer than in all-trans, in excess of 10 msec at low temperatures. For both the cis-rich and the all-trans materials, we find that photoinduced bleaching occurs only above the band edge, as expected if the photogenerated soliton states are formed from band states. These experimental results provide strong support for the stabilization of the photogenerated charges as like-charged pairs of solitons on the same chain rather than as charged solitons formed by trapping of free carriers (polarons) on neutral soliton defects.

Vibrational Excitations of Charged Solitons in Polyacetylene

Theoretical studies of the vibrational excitations of charged solitons in polyacetylene are reported. Two strongly infrared active modes, derived from bulk optical phonons, are associated with these defects and we suggest the presence of a third, as yet unobserved, infrared-active mode in the acoustic phonon regime.