Soliton In Trans-polyacetylene Research Articles

Brownian Motion of a Soliton in Trans-Polyacetylene. II. Friction Mechanism

Second mechanism for diffusive motion of a soliton in trans-polyacetylene is investigated in Takayama Lin-Liu and Maki's model. It is induced by momentum transfer with thermally excited phonons, which dissipates by friction. By applying Mori's formula to the equation of motion of the solitons, the friction and diffusion constant are calculated, using classical statistics. The friction depends linearly on temperature in the low temperature region. According to the Einstein relation, the diffusion constant becomes temperature independent. Relation between the present mechanism and random walk mechanism, previously studied, is also clarified. When the frequency is not zero (or in the finite time regime), the two mechanisms are working and the dynamical diffusion constant, D (ω), becomes proportional to T 2 . Possible importance of acoustic phonons is pointed out in polyacetylene.

μSR studies on polyacetylenes

Positive muons were injected into both cis- and trans-polyacetylenes. From the various μSR measurements, the μ+ was found to form a radical state with an unpaired electron localized near μ+ in cis-polyacetylene, while in trans-polyacetylene it was found to be in a diamagnetic state. Combined with the observed H−1/2 dependence of the μ+ relaxation rate on the applied longitudinal field H in trans-polyacetylene, it was explained by the formation of soliton in trans-polyacetylene. Also the radical state was found to become unstable in iodine-doped polyacetylenes. An explanation is given with respect to the soliton model.

Soliton diffusion in trans-polyacetylene.

We have calculated acoustic-phonon scattering and the resulting intrachain diffusion constant ${\mathrm{D}}_{\mathrm{?}}$ for neutral solitons in trans-polyacetylene. The result, with no arbitrary parameters, agrees with ${\mathrm{D}}_{\mathrm{?}}$ determined by magnetic resonance experiments from \ensuremath{\sim}150 to 300 K. The more rapid decrease with decreasing temperature seen below 150 K can be obtained with the introduction of a barrier-to-soliton motion of 0.01 eV. Such barriers may result from remanent cis linkages in the Shirakawa trans-polyacetylene used for the experiments.

Soliton Diffusion at Low Temperatures

Friction of a soliton is studied in the one-dimensional φ4 system. It is generated by momentum transfer between the soliton and phonons. Quantum statistics is used for the phonon distribution It has the temperature dependence of the activation type, since the number of thermally excited phonons reduces drastically. The lowest frequency of the extended phonon modes plays the role of the activation energy. The result is compared with the soliton in trans-polyacetylene.

Infrared activity of phonons around a soliton in trans-polyacetylene

Abstract The dynamical conductivity due to phonons around a charged soliton in trans -polyacetylene is calculated, using three kinds of models: the Takayama-Lin-Liu-Maki (TML) model, the Su-Schrieffer-Heeger (SSH) model, and the multi-phonon-band model. In the TLM model, two of the three localized modes, i.e. the Goldstone mode and the third localized mode, are found to be infrared (ir) active. These modes are found also in the SSH model to contribute to the optical absorption, while the staggered localized mode yields no contribution to the conductivity. In the last model, three weak ir modes, which have their origins in the third localized mode, are found in addition to the already known three intense ones. The vibrational frequencies and relative optical intensities agree well with a recent photo-induced absorption experiment. The isotope effect on the absorption spectrum is also discussed.

Charged-soliton dynamics in trans-polyacetylene.

Piscosecond time-resolved absorption studies of the midgap absorption associated with photogenerated charged solitons in trans-polyacetylene are reported. Both direct photogeneration of charged-soliton pairs and neutral- to charged-soliton conversion are observed. The former are created by intrachain absorption and decay on a subpicosecond time scale. The latter are formed following interchain photoexcitation and are the dominant population of charged solitons at times greater than 20 ps.

Brownian Motion of a Soliton in trans-Polyacetylene. I. Random Walk Mechanism

Diffusive motion of a soliton in trans-polyacetylene is studied theoretically within the adiabatic approximation of the Takayama Lin-Liu and Maki model. The collective coordinate method is applied to obtain an equation of motion of the soliton. It bears a remarkable similarity to that in the φ 4 model. Shift of the soliton location is induced by collision with classical phonon, which is excited thermally. It leads to a random walk of the soliton. The dynamical diffusion constant D (ω) and the friction Γ(ω) are calculated in the low temperature expansion. It is shown that the real part of D (ω) is proportional to T 2 , when the frequency is not zero and Γ(0) is larger than Γ(ω). Its magnitude is hundred times as large as that of the φ 4 kink.

Polyacetylene: Coulomb interactions and localized collective excitations of solitons

Abstract A charged (neutral) soliton in trans-polyacetylene is associated with a single localized charged density excitation (spin density excitation) and with several localized spin (charge) density excitations. The number of the latter depends on the strength of Coulomb e- -e- interactions. Examination of these modes in experiments should give information about the importance of the mentioned interelectron interactions.

Photogenerated charged solitons intrans-polyacetylene

It is suggested that the apparent pinning of photogenerated solitons in trans-polyacetylene is due to solitonic exciton formation and that recombination occurs via the photodecay process ${S}^{+}{S}^{\ensuremath{-}}\ensuremath{\rightarrow}{S}^{0}{S}^{0}+h\ensuremath{\nu}$.