Showing papers on "Bipolaron published in 1986"
TL;DR: In this paper, a quantitative study of polaron and bipolaron formation in polypyrrole upon electrochemical doping is presented, where the steady-state spin concentration has been measured as a function of the electrical potential in an in situ ESR-electrochemical experiment and the charge injection has been determined by coulometry and by optics.
134 citations
TL;DR: The infinite polyyne chain, ( CXC )/sub infinity/, or polyyne, is interpreted as a Peierls distorted one-dimensional metal with strong intrinsic electron-phonon coupling at the half-filled band level, which leads to a rich variety of kink-soliton and polaron states and to the interesting result that polyyne is an approximate physical realization of an N-italic = 4 Gross-Neveu model relativistic field theory.
Abstract: The infinite polyyne chain, ( C?C ${)}_{\mathrm{\ensuremath{\infty}}}$, or polyyne, is interpreted as a Peierls distorted one-dimensional metal with strong intrinsic electron-phonon coupling at the half-filled band level. Because there are two degenerate \ensuremath{\pi}-electron bands in polyyne the effective internal degeneracy (N) of the electrons is 4, rather than 2 as in the model of polyacetylene. This leads to a rich variety of kink-soliton and polaron states and, in the limit of a continuum description, to the interesting result that polyyne is an approximate physical realization of an N=4 Gross-Neveu model relativistic field theory. The low-lying electronic excitations of the polymer are kink-solitons with charges 0, \ifmmode\pm\else\textpm\fi{}e, and \ifmmode\pm\else\textpm\fi{}2e. The lowest-lying ionization states are a polaron, a bipolaron, and a tripolaron, with charges \ifmmode\pm\else\textpm\fi{}e, \ifmmode\pm\else\textpm\fi{}2e, and \ifmmode\pm\else\textpm\fi{}3e, respectively. Photoexcitation of the polymer leads to a neutral polaron consisting of an electron and a hole bound by lattice distortion (``polarexciton''). Both the soliton and the polarons involve the appearance of localized intragap levels that are not present in the ground state. Moreover, a localized excited state exists for the polaron, while photoexcitation of the bipolaron and the tripolaron result in soliton-antisoliton pair production. Photoinduced absorption measurements on long finite polyynes in solution should be able to confirm the photogeneration of the polarexciton. Formation of the polaron states can be expected if doping with strong electron withdrawing species is possible. An ?dd?membered polyyne is predicted to contain a soliton in its ground-state configuration, the doubly ionized closed-shell chain being particularly stable. Soliton- or polaron-bearing acetylenic chains might be of relevance in astrophysics. The ground-state acoustic and optical branches of the phonon spectrum of the discrete chain are also calculated and an expression derived for the reduction of the speed of sound due to the electron-phonon coupling.
88 citations
TL;DR: In this article, electron spin resonance measurements carried out on thin films of polythiophene in situ during electrochemical doping with ClO4 were used to demonstrate the spinless character of the doping induced charge states at dilute to intermediate doping concentrations.
80 citations
TL;DR: In this article, the photoinduced infrared absorption of polythiophene was measured and spectral features below the noise level of previous doping and photogeneration experiments provided evidence for weakly infrared active bipolaron shape oscillations predicted by calculations based on the continuum model for conjugated polymers.
76 citations
TL;DR: In this paper, the effect of electrical conductivity and ESR on the semiconductor-to-metal transition in polythiophene films was investigated. But the authors focused on the relatively narrow dopant concentration region of 2×10 3 < y <5×10 -3 and showed that the ESR signal also changes significantly, showing a maximum spin density and a minimum linewidth.
Abstract: Dependences of electrical conductivity and ESR of iodine-doped polythiophene films, (C 4 H 2 SI y ) x , on iodine concentrations which are precisely determined by a neutron activation technique (from a dilute level of y ∼4×10 -4 to a deep level of y ∼4×10 -1 ) are reported. A drastic increase of conductivity from 10 -8 to 10 -2 S/cm is observed at the fairly narrow dopant concentration region of 2×10 3 < y <5×10 -3 , accompanied by a remarkable decrease in the activation energy of conductivity. At this transitional region, the ESR signal also changes significantly, showing a maximum spin density and a minimum linewidth. These results are discussed to understand the semiconductor-to-metal transition in iodine-doped polythiophene, taking polaron and bipolaron models into account.
39 citations
TL;DR: In this paper, the electronic properties of polypyrrole and polythiophene involving α-β linkages have been studied using the one-dimensional tight-binding SCF-CO (self consistent field-crystal orbital) method.
27 citations
TL;DR: In this paper, an in situ study of the absorption spectra of poly(p-phenylene) films during the electrochemical doping with AsF 6 has been carried out.
Abstract: Poly( p -phenylene) films with a conductivity of ca . 100 S/cm are synthesized by a new method of electrical oxidation of benzene using the composite electrolytes of CuCl 2 /LiAsF 6 . An in situ study of the absorption spectra of the poly( p -phenylene) films during the electrochemical doping with AsF 6 has been carried out. From the absorption spectrum of undoped state, we find a strong absorption peak at ca . 3.4 eV which is associated with the transition from ground-state to excited-state. The remarkable changes of the absorption spectra below the photon energy of 3.0 eV are observed, which are related to the polaron and/or bipolaron states of this polymer. The electrochemical n -type doping is also demonstrated.
27 citations
TL;DR: In this article, a complete derivation of the statistics of a polaron-bipolaron system as a function of their relative lengths and energies is presented, and it turns out that the state of the chain at full charge is governed by the length ratio rather than by the energy ratio.
Abstract: We present a complete derivation of the statistics of a polaron-bipolaron system as a function of their relative lengths and energies. It turns out that the state of the chain at full charge is governed by the length ratio rather than by the energy ratio. The results are used to interpret the spin-charge relation observed in polypyrrole.
24 citations
TL;DR: It is shown that the creating of such a dipolaron can be energetically favorable in the case of either doping or photoexcitation.
Abstract: We study a nondegenerate conjugated polymer by means of a modified Su-Schrieffer-Heeger-type model Hamiltonian. Analytical solutions are presented for the ground state and dynamic equations are solved numerically. We present solutions representing a bipolaron oscillating between two stages (a kink-antikink pair and a bound-polaron pair). We show that the creating of such a dipolaron can be energetically favorable in the case of either doping or photoexcitation. Our results compare well with the experimental photoluminescence spectrum of cis-(CH${)}_{\mathrm{x}}$.
23 citations
TL;DR: In this paper, Monte Carlo computer simulation techniques are used to study the formation of bipolarons on a lattice and the transition between the three possible states, extended, twopolaron, and bipolaron is studied.
Abstract: Monte Carlo computer simulation techniques are used to study the formation of bipolarons on a lattice. The transition between the three possible states, extended, twopolaron, and bipolaron is studied. The phase diagram as a function of the strengths of the electron-phonon coupling and repulsive interaction is determined.
20 citations
TL;DR: In this paper, the spin-lattice relaxation rate (T 1 −1 ) was measured as a function of NMR frequency over a wide range (4-300MHz). And the singular dependence upon frequency characteristic of the low-dimensional (one and two-dimensional) diffusive motion of electrons was found.
TL;DR: In this article, the reflection spectra of polypyrrole perchlorate were measured in the range 20 − 35 000 cm−1 and attributed to the sliding mode (Goldstone mode) of the charged bipolaron, and its implication in the conduction mechanism.
TL;DR: In this paper, the authors extended the coupled electron-phonon system in conjugated polymers to include the lattice kinetic energy and the coupling of several structural degrees of freedom.
Abstract: The continuum model of the coupled electron-phonon system in conjugated polymers is extended to include the lattice kinetic energy and the coupling of several structural degrees of freedom. The model is used to study defect vibrational spectra for a variety of defects that may occur, e.g., solitons, polarons, and bipolarons, in conjugated polymer systems. In agreement with previous theoretical studies of polyacetylene, a class of infrared-active modes common to all mobile bond-alternation defects is obtained. In addition the study reveals a new class of bound vibrations localized at the defect center of a polaron or a bipolaron. These additional infrared-active modes can be used spectroscopically to identify defect types in conjugated polymers such as polyacetylene or polythiophene. Examples considered include doping and photoinduced defects (solitons and polarons) in both (CH${)}_{\mathrm{x}}$ and (CD${)}_{\mathrm{x}}$ as well as photoinduced defects (bipolarons) in polythiophene.
TL;DR: In this article, conditions de transition supraconductrice par condensation bipolaronique and en particulier des effets de la pression are discussed in terms of transition transition.
Abstract: Discussion des conditions de transition supraconductrice par condensation bipolaronique et en particulier des effets de la pression
15 Apr 1986
TL;DR: In this article, the authors used quantum chemical calculational techniques to consider the relation between charge state and distortion for a system consisting of a B12 icosahedron and 12 tieoff atoms.
Abstract: It has been proposed that electrical transport in boron carbides is accomplished by hopping of singlet electron pairs between bipolaronic states localized on the icosahedra. Formation of a bipolaron will be accompanied by a distortion of the associated icosahedron. We have used quantum‐chemical calculational techniques to consider the relation between charge state and distortion for a system consisting of a B12 icosahedron and 12 tieoff atoms. It is found that localization of two extra electrons on an icosahedron results in a contraction of the icosahedral cage and a reduction of the total energy of the cluster by ∼1.3 eV compared to that of the neutral cluster. The reduction in energy corresponds to the formation energy of a bipolaron in our model. Extensions of the model planned include calculations of polaron vs. bipolaron formation energies.
TL;DR: In this paper, a bipolaron microscopic model is proposed to theoretically interpret the resonance Raman scattering spectra in cis-polyacetylene on the basis of lattice relaxation theory, and the coupling constants of electronic motion to different phonon modes are determined from the observed infrared and Raman frequencies.
Journal Article•
TL;DR: In this article, the degenerate perturbation theory is used to calculate the bipolaronic energy bands in various one-, two-, and three-dimensional crystal structures, from which the stability of the bipolaron is investigated.
Abstract: The degenerate perturbation theory is used to calculate the bipolaronic energy bands in various one-, two-, and three-dimensional crystal structures. The dissociation energy of bipolaron is calculated, from which the stability of the bipolaron is investigated. Higher stability is found in lower-dimensional systems. The triplet bipolaron state is almost as stable as the singlet bipolaron state, and so plays a very important role in determining the thermodynamic properties of bipolaronic systems.
Entartete Storungstheorie wird benutzt, um die Bipolaronenenergiebander in verschiedenen ein-,zwei- und dreidimensionalen Kristallstrukturenzzu berechnen. Die Dissoziationsenergie des Bipolarons wird berechnet und damit die Stabilitat des Bipolarons untersucht. Es wird hohere Stabilitat in Systemen mit niedrigerer Dimension gefunden. Der Bipolaron-Triplett-Zustand ist fast so stabil wie der Bipolaron-Singulett-Zustand und spielt somit eine sehr wesentliche Rolle bei der Bestimmung der thermodynamischen Eigenschaften des Bipolaronensystems.
Journal Article•
18 Aug 1986
TL;DR: In this article, the electronic properties of boron-rich icosahedral clusters were studied as a means of understanding the electronic structure of the borides such as Boron carbide.
Abstract: The electronic properties of boron-rich icosahedral clusters were studied as a means of understanding the electronic structure of the icosahedral borides such as boron carbide. A lower bound was estimated on bipolaron formation energies in B12 and B11C icosahedra, and the associated distortions. While the magnitude of the distortion associated with bipolaron formation is similar in both cases, the calculated formation energies differ greatly, formation being much more favorable on B11C icosahedra. The stable positions of a divalent atom relative to an icosahedral borane was also investigated, with the result that a stable energy minimum was found when the atom is at the center of the borane, internal to the B12 cage. If incorporation of dopant atoms into B12 cages in icosahedral boride solids is feasible, novel materials might result. In addition, the normal modes of a B12H12 cluster, of the C2B10 cage in para-carborane, and of a B12 icosahedron of reduced (D sub 3d) symmetry, such as is found in the icosahedral borides, were calculated. The nature of these vibrational modes will be important in determining, for instance, the character of the electron-lattice coupling in the borides, and in analyzing the lattice contribution to the thermal conductivity.
TL;DR: Using a variational method of the Pekar type, this article examined the formation of surface bipolarons by coupling of two electrons to ripplons, the symmetry property of the system being taken into account.
TL;DR: In this paper, a systematical study of electrical conductivity and EPR of iodine-doped anthracene polymers (C14H6Ix)n is carried out within a wide range of iodine concentrations (0.07 ≦ x ≦ 0.85).
Abstract: A systematical study of electrical conductivity and EPR of iodine-doped anthracene polymers (C14H6Ix)n is carried out within a wide range of iodine concentrations (0.07 ≦ x ≦ 0.85). The room temperature electrical conductivity has a broad maximum near x = 0.5. No metallic-type conduction is observed. The intensity of EPR signal for all of the investigated samples depends on temperature according to the Curie law and has a minimum near x = 0.19. A clear decoupling of paramagnetism with iodine concentration and electrical conductivity is detected. An idealized model of the structure of (C14H6Ix)n including polarons and bipolarons is proposed. On the base of the model a mechanism of generation and compensation of spin and charge during the doping process is discussed and an interpretation of experimental data is given.
[Russian Text Ignored]
TL;DR: In this article, the phase diagram of one-and two-dimensional N-site N-electron (N>>1) systems with a site-diagonal electron-phonon(e-p) coupling is studied in the context of polaron theory, so as to clarify the competition between the superconductihg(SC) state and the crystalline state of bipolarons, in such a strong coupling region that even the standard “strong coupling behavior for superconductivity breaks down.
Abstract: The phase diagram of one- and two-dimensional N-site N-electron (N>>1) systems with a site-diagonal electron-phonon(e-p) coupling are studied in the context of polaron theory, so as to clarify the competition between the superconductihg(SC) state and the crystalline state of bipolarons, in such a strong coupling region that even the standard “strong coupling thoery” for superconductivity breaks down. Phonon clouds moving with electrons as well as the frozen phonon are taken into account by a variational method combined with the mean-field theory. It covers the whole region of the three basic parameters characterizing the system: the inter-site electron transfer T, the e-p coupling S and the phonon energy ω. In both one- and two-dimensional cases, the polaron is found to shrink its radius from a large polaron to a small one as T/ω (retardation effect) decreases. In the one-dimensional case, however, the bipolaronic crystal with a frozen phonon is always stable than the SC state, because of the perfect nesting of the Fermi surface. In the two-dimensional case with a round Fermi surface, on the other hand, the SC region appears in the phase diagram and it expands as T/ω decreases. It is found, for the first time, that the energy gap of the SC state for given T and S becomes maximum in the intermediate region ω~T, indicating the importance of the polaron effect. The collective excitation within the energy gap of the SC state is also studied by RPA, and found to change its nature continuously from the BCS-type pair breaking excitation to the superfluid-type one as S/T increases.