电子关联对聚乙炔中激子类型的影响

在SSH哈密顿基础上引进电子关联,对反式聚乙炔链中光致激子的产生和演化过程实施分子动力学模拟。弱关联强度U取值0～1.250 eV,V =U/2取值0～0.625 eV .计算结果表明,关联强度的大小影响链中元激发类型,U＜0.555eV时产生的元激发为孤子-反孤子对,U ＞0.555eV 时产生的元激发为正负荷电极化子对。为进一步讨论该类型一维系统中不同类型激子的产生、输运、衰减等动态过程,关联强度U的选择提供参考。     

Long-lived electronic-deformation excitations in a polyacetylene chain — the photoexcitation spectrum

It is observed that the photoexcitation efficiency of neutral long-lived states in a trans-polyacetylene chain increases with increasing photon energy of the excitation. The observed effect is given an interpretation according to which the energy of a photon above the optical absorption edge is added to the excitation of the vibrational subsystem, increasing sharply the probability that the chain relaxes into a long-lived deformed neutral state. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 10, 765–770 (25 May 1998)     

Photoconductivity in polyacetylene

The results of steady state photoconductivity experiments on cis- and trans-(CH)_x are interpreted in terms of charged solitons, photogenerated either directly (threshold hv = 4Δ/π) or indirectly through coupling of the lattice to electron-hole pair excitations (hv ? 2Δ). The transient photocurrent, after laser pulse excitation, decays as a power law, I(t) ? t^?0.6, suggesting dispersive transport of the photogenerated carriers.     

Hot luminescence in polyacetylene

We propose that the unusual frequency dependent lineshapes observed in Raman scattering experiments on polyacetylene are due to hot luminescence in very long polyene chains. Employing a generalized equation of motion technique we find that a photoexcited polyene is unstable with respect to a variety of symmetry lowering distortions which vary as a function of the frequency of the exciting radiation. Radiative decay during this relaxation is observably strong in the inelastically scattered photon spectrum and would explain the dominant observed lineshape variations.     

Lithium intercalation in polyacetylene

The coulometric titration curve for lithium intercalation in polyacetylene has been determined using poly(ethylene oxide) based solid electrolyte. The plateaus observed suggest the existence of stages, comparable to graphite. This interpretation seems also supported by the cyclic voltammetry scans on polyacetylene. Lithium intercalation appears as a very slow process with

estimated at ? 10^-13cm².s^-1.     

Infrared photoluminescence in polyacetylene

More extensive photoluminescence measurements of polyacetylene reveal a broad new emission band between 1.2 and 1.6 eV in samples with various isomeric contents. The peak energy and the intensity of this low energy luminescence decrease as the cis fraction decreases, but the band is still present in fully converted trans samples. These characteristics suggest that the infrared emission either is due to perturbed fragments of cis polyacetylene or is unquenched ¹Ag luminescence from segments of trans (CH) x.     

Conduction mechanisms in polyacetylene nanofibres

The current–voltage (I–V) characteristics of individual nanofibres of lightly-doped polyacetylene show very strong nonlinearities. At low temperatures the I–V characteristics are consistent with Zener-type tunnelling, and independent of temperature and magnetic field. We propose that this behaviour arises from tunnelling of a segment of the conjugated bond system in the presence of an electric field, in analogy to the soliton-pair creation mechanism proposed by Maki for conduction in charge-density-wave (CDW) materials. A comparison is made with analogous tunnelling conduction mechanisms reported in CDW and spin-density-wave systems at low temperatures. At higher temperatures the I–V characteristics deviate from Zener-type behaviour and are temperature dependent, so other conduction mechanisms are important.     

Polarization-dependent EXAFS in bromine-doped polyacetylene

The local dopant structure was investigated in stretch-oriented (CHBr_y)_x at 77 K by polarization dependent EXAFS measurements at the Br-K edge. Definite nn Br-C and Br-Br distances are derived and nnn distances are tentatively assigned. The data clearly show that a large portion of Br intercalates in form of linear Br^-₃ (or longer Br^-_2n+1) molecules with the molecular axis parallel to the (CH)_x chains. Two models for the local dopant structure are proposed and discussed on the basis of other experimental results.     

Asymmetrically doped polyacetylene

Doped one-dimensional (1D) conjugated polymers, such as polyacetylene, have a conductivity of some metals, like copper. We show that when this polymer is asymmetrically doped, unexpected properties are revealed, when compared to the behavior of the symmetrically standard doped systems (SDS). Depending on the level of imbalance between the chemical potentials of the two involved fermionic species, the polymer can be converted into a 1D partially or fully polarized organic conductor.     

Insulator-to-metal transition in doped polyacetylene

A field-theoretic model of the insulator-to-metal transition in doped polyacetylene is presented. The likelihood of an inhomogeneous distribution of charge in the (CH)_x fibrils is stressed.     

Optical conductivity of solitons in polyacetylene

The soliton lattice solution of the continuum model for polyacetylene is used to derive the frequency dependent conductivity of charged solitons. The limit of low soliton density is explicitly evaluated and shown to be free of singularities. The conductivity sum rule is proven for the model Hamiltonian, and is shown to be satisfied by the explicit solution. Implications for polyacetylene and for TTF-TCNQ are discussed.     

Defect states in polyacetylene and polydiacetylene

Possible intrinsic conformational defect states (e.g. mobile paramagnetic centres) in polyacetylene and polydiacetylene chains are compared and contrasted. Experimental evidence and implications in the more complicated polydiacetylene cases are reviewed, and theoretical repercussions are suggested (e.g. for exciton and polaron calculations).     

Electronic oscillations in paired polyacetylene chains

An interacting pair of polyacetylene chains are initially modeled as a couple of undimerized polymers described by a Hamiltonian based on the tight-binding model representing the electronic behavior along the linear chain, plus a Dirac’s potential double well representing the interaction between the chains. A theoretical field formalism is employed, and we find that the system exhibits a gap in its energy band due to the presence of a mass-matrix term in the Dirac’s Lagrangian that describes the system. The Peierls instability is introduced in the chains by coupling a scalar field to the fermions of the theory via spontaneous symmetry breaking, to obtain a kink-like soliton, which separates two vacuum regions, i.e., two spacial configurations (enantiomers) of the each molecule. Since that mass-matrix and the pseudo-spin operator do not commute in the same quantum representation, we demonstrate that there is a particle oscillation phenomenon with a periodicity equivalent to the Bloch oscillations.     

Trapping of phase kinks in polyacetylene

Molecular orbital calculations with full geometry optimization using a semi-empirical tight-binding Hamiltonian for finite molecular models of phase kinks (solitons) in polyacetylene, (CH)_X′ were done. This bond alternation defect extends over many atoms from which only ～30 atoms are energetically important for stabilizing it. The neutral (spin-carying) phase kink is attracted by an energy on the meV scale to carbon atoms joining a non-hydrogen side-group, in contrast to charged phase kinks, where this attraction is estimated to be 100 to 1000 times larger.