Showing papers on "Bipolaron published in 1988"
TL;DR: In this paper, band-structure calculations for different lattice conformations of doped polyacetylene and polythiophene were presented for different polaron lattice conformation, and the results of these calculations were compared with results of the Takayama, Lin-Liu, and Maki (TLM) model.
Abstract: Band-structure calculations are presented for different lattice conformations of doped polyacetylene and polythiophene. At intermediate doping levels, the intraband transition energies are found to be in good agreement with experimental optical-absorption data for a soliton lattice conformation in polyacetylene and a bipolaron lattice conformation in polythiophene. At high doping levels, where both polymers exhibit a metalliclike behavior, we find the best agreement with observed optical-absorption and magnetic data to occur for a polaron lattice conformation. Important qualitative differences are found between the polaron lattice conformations obtained in this work and those reported previously. The polaron lattice band structures for polyacetylene and polythiophene are calculated to be very similar to one another, which is also consistent with the experimental trends. The evolutions of the \ensuremath{\pi}-${\ensuremath{\pi}}^{\mathrm{*}}$ and the \ensuremath{\pi}-to-soliton band energy gaps for polyacetylene are studied and compared with results of the Takayama, Lin-Liu, and Maki (TLM) model. We find that, in comparison with optical data probing these transitions, the valence effective Hamiltonian results are superior to the results of the TLM model. We stress that our results do not constitute a proof of the existence of polaron lattice conformation at high doping levels but indicate that the presence of such a conformation is in agreement with a large number of experimental (ir, optical conductivity, magnetic, electron-energy-loss spectroscopy) data.
123 citations
TL;DR: In this article, a one-to-one correspondence between the photoinduced and doping-induced spectral changes is observed, typical of conducting polymers, demonstrating that charges are predominantly stored in bipolarons in P3HT.
Abstract: Comprehensive (0.05--3-eV) spectroscopic measurements are reported for poly(3-hexylthienylene), P3HT, during photoexcitation (i.e., photoinduced absorption) and after doping with ${\mathrm{NOPF}}_{6}$. A one-to-one correspondence between the photoinduced and doping-induced spectral changes is observed, typical of conducting polymers. The results demonstrate that charges are predominantly stored in bipolarons in P3HT. We find two subgap electronic absorptions and a series of infrared-active vibrational (IRAV) modes. The four localized IRAV modes (1088, 1161, 1200, 1354 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) associated with bipolaron distortions of the P3HT chains and made infrared active through coupling to the uniform translation of the bipolaron were observed in both the doping-induced and photoinduced absorption spectra, as in polythiophene and poly(3-methylthienylene). An additional absorption (1396 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) was identified as arising from an ir-active localized mode associated with the nonuniform translation (shape oscillation) of the bipolaron. Comparison of the energies of the photoinduced and doping-induced electronic transitions yields an estimate of the change in Coulomb energy of the bipolaron on photoexcitation, ${U}_{B}$\ensuremath{\approxeq}0.25 eV; this relatively small value of ${U}_{B}$ is consistent with bipolaron formation in P3HT.
99 citations
TL;DR: In this article, the same authors deduced key structural features of acceptor-doped polyaniline (polyemeraldine salt) from the crystal structures for the ClO 4 − and the BF 4 − dication salts of H(C 6 H 4 ǫ NH 6 H 6 H 5, radical cation salts of shorter oligomers and neutral H 5 C 6 C 6 NC 6H 4 NNH 6 H5, and showed that radical cations are
80 citations
TL;DR: In this paper, an interatomic potential model for La2CuO4 has been derived, which correctly reproduce the structure of the orthorhombic phase, and yield calculated phonons dispersion curves showing a soft mode.
Abstract: Inter-atomic potential models have been derived for La2CuO4. The models correctly reproduce the structure of the orthorhombic phase, and yield calculated phonons dispersion curves showing a soft mode. Defect simulations show a large energy barrier to the Cu2+ disproportionation reaction, but find that doping of the material and oxidation of the doped material are energetically favoured. The calculations show that the latter takes place with formation of Cu3+ ions. Bipolaron formation is also investigated.
73 citations
01 Nov 1988
TL;DR: The cis/trans isomerization of cis-PA upon cathodic reduction can be detected voltammetrically as discussed by the authors, showing that the trans-form is thermodynamically more favorable in the charged than in the neutral state.
Abstract: The cis/trans isomerization of cis-PA upon cathodic reduction can be detected voltammetrically. The analysis of the redox data reveals that the trans-form is thermodynamically more favourable in the charged than in the neutral state. Cyclic voltammetry also reflects geometric relaxation processes which occur in conjugated materials such as PPy or PTh during charging and discharging. The results are in good agreement with quantummechanical predictions based on the bipolaron model. Additional spectroelectrochemical studies unambiguously show that the charging process in the whole accessible potential range is faradaic.
72 citations
TL;DR: An explanation of some of the anomalous properties of La-Ba-Cu-O, La-Sr-Cu -O, and Y- Ba- Cu-O superconductors on the basis of the small bipolaron theory of Alexandrov and co-workers is proposed.
Abstract: An explanation of some of the anomalous properties of La-Ba-Cu-O, La-Sr-Cu-O, and Y-Ba-Cu-O superconductors on the basis of the small bipolaron theory of Alexandrov and co-workers is proposed.
62 citations
TL;DR: In this article, it was shown that the superconducting properties are due to bose condensation of pairs of spinless small-polarons, bound together at high temperatures into (exchange-) bipolarons by the associated gain in antiferromagnetic exchange energy.
59 citations
TL;DR: In this article, the authors investigated the electronic excitation in aniline tetramers at various levels of oxidation and protonation and showed that for certain geometrical conformations there is excellent agreement between their results and recently published experimental data.
Abstract: We investigate the electronic excitation in aniline tetramers at various levels of oxidation and protonation. We show that for certain geometrical conformations there is excellent agreement between our results and recently published experimental data. The neutral forms of the tetramer are shown to have excitation spectra almost identical to polymer equivalents. For the doubly protonated emeraldine tetramer we propose a bipolaron type of defect state, unlike in its polymer equivalent for which a polaronic metal model has been proposed. The electronic transitions associated with the bipolaron are shown to involve only one bipolaron level in contrast to the common picture of two bipolaron levels.
42 citations
TL;DR: In this paper, the migration of photo-induced excitons from quenching of photoluminescence during in situ electrochemical doping in conducting polythiophene and poly(3-methyl thiophene) films was studied.
Abstract: Migration of photoinduced excitons has been studied from the quenching of photo-luminescence during in situ electrochemical doping in conducting polythiophene and poly(3-methylthiophene) films. An analysis based on one-dimensional model gives an average migration distance of the excitons about 35 thiophene rings during their lifetimes. The results indicate that an aggregation of dopants follows the uniform doping, being consistent with the model of polaron and bipolaron formations.
22 citations
TL;DR: In this article, the electronic structures of polyiminovinylene (PIV) and polyaniline (PAn) were studied theoretically using the one-dimensional tight-binding self consistent field-crystal orbital (SCF-CO) method.
21 citations
TL;DR: A simple scaling model of magnetic-polaron and magnetic-bipolaron formation in antiferromagnetic systems is presented and the dependence of the formation criteria on the dimensionality of the electronic system is specifically addressed.
Abstract: We present a simple scaling model of magnetic-polaron and magnetic-bipolaron formation in antiferromagnetic systems. Within this picture, we determine the criteria for an excess electron forming a magnetic polaron and for a pair of excess electrons forming a (triplet) magnetic bipolaron. The dependence of the formation criteria on the dimensionality of the electronic system is specifically addressed. In addition, we consider the effect of the electron-lattice interaction on the size and energetic stability of magnetic polarons and bipolarons as a function of the system's dimensionality. Beyond the formation of isolated magnetic polarons and/or magnetic bipolarons, we consider how a system composed of magnetic polarons within an antiferromagnetic system is driven ferromagnetic as the density of magnetic polarons is increased. Finally, we discuss the large paramagnetic susceptibility associated with the formation of magnetic polarons. The magnetic susceptibility is composed of a Curie-like temperature-dependent component as well as a temperature-independent component. The temperature-dependent term arises from the reorienting of the magnetic polarons under the influence of a magnetic field. The temperature-independent contribution arises from the expansion and contraction of magnetic polarons which are, respectively, aligned parallel or antiparallel with the applied magnetic field.
TL;DR: In this article, a theoretical calculation of the optical absorption due to polarons and bipolarons in doped polythiophene is presented, and the results are used to describe the absorption spectra at thermal equilibrium.
TL;DR: The influence of the Coulomb repulsion associated with the double occupation of the gap levels that occurs in bipolarons is investigated within a phenomenological model and its contribution is not strong enough to destabilize bipolarons that remain the most stable excitation.
Abstract: We study the effect of Coulomb interactions on the formation of polarons and bipolarons in polythiophene. First, the interaction of an ionized dopant is considered and these defects are found to be always energetically favored upon doping, bipolarons being the lowest-lying charged excitations. The influence of the Coulomb repulsion associated with the double occupation of the gap levels that occurs in bipolarons is then investigated within a phenomenological model. This interaction leads to a modification of the different equilibrium parameters and reduces appreciably the difference between the creation energy of a pair of polarons and that of a bipolaron. However, this contribution is not strong enough to destabilize bipolarons that remain the most stable excitation. Implications of these results for the existence of polarons are discussed depending on whether the excitations are photogenerated or created by doping.
TL;DR: In this paper, the lattice energy in semiconducting Ti 4 O 7, below the metal-to-semiconductor transition temperature, in which bipolarons are formed, is calculated, using the polarizable point ion shell model developed by Dienes et al.
TL;DR: An effective strong-coupling (or weak-hybridization) Hamiltonian is derived for the low-lying states of the bipolaron (negative-U) centers and magnetic impurities which includes a repulsive on- site Coulomb interaction as well as a coupling of the electronic on-site energy to the displacement amplitude of a local phonon mode.
Abstract: We have studied a model of bipolaron (negative-U) centers and magnetic impurities which includes a repulsive on-site Coulomb interaction as well as a coupling of the electronic on-site energy to the displacement amplitude of a local phonon mode. By means of a generalized Schrieffer-Wolff transformation, we derive an effective strong-coupling (or weak-hybridization) Hamiltonian for the low-lying states of the system. In the case of a bipolaron center (where the phonon-mediated on-site attraction dominates over the Coulomb repulsion), the effective Hamiltonian describes conduction-electron scattering and electron-pair tunneling processes between the localized impurity orbital and the conduction band of the metallic host. In the limit of very strong electron-phonon coupling, the pair tunneling matrix elements become exponentially small, whereas conduction electron scattering is suppressed only algebraically. The impurity orbital occupation number is then almost ``frozen in'' and the bipolaron center becomes inefficient in enhancing the superconducting pairing correlations of the host. In the case of a magnetic impurity (with a predominant repulsive Coulomb interaction), the effective Hamiltonian reduces to the form of an ordinary Kondo spin-exchange model. The spin-exchange coupling constants are not affected by the presence of the electron-phonon interaction.
TL;DR: In this article, an alternative type of small polaron associated with an off-center configurational distortion can arise from the additional coupling of the electron to some appropriate odd vibrational mode.
Abstract: An alternative type of small polaron associated with an off-center configurational distortion can arise from the additional coupling of the electron to some appropriate odd vibrational mode. The off-center polaron is inherently polarizable in an electric field. Consequently, neighboring entities can couple through the universal Van der Waals force to form bipolarons. Implications for the high-T c superconductivity are discussed, bipolarons in superconducting cuprates being envisaged as asymmetric Cu O central-cage linear chains.
TL;DR: In this paper, the authors reported an even further example of the experimental data of the Ca-Sr-Bi-Cu-O system amazingly similar to those of the Ba-Pb-BiO system.
Abstract: Quite recently, we reported a preliminary part of the experimental results of “ the Novel Spectral Photoconductivity of the Ba-Pb-Bi-O System potentially correlative with Superconductivity ”. Here, we report an even further example of the preliminary part of the experimental data of the Ca-Sr-Bi-Cu-O system amazingly similar to those of the Ba-Pb-Bi-O system rather than those of the La-Cu-O, Y-Cu-O or Y-Ba-Cu-O systems. These results suggest the undisputed importance of “the exciton-mediated bipolaron mechanis” mainly in the Bi 2 O 3 -like part in the Ca-Sr-Bi-Cu-O system.
TL;DR: In this article, it has been shown that poly(3-alkylthiophene) in solution can be doped just as in the case of polythiophen films, resulting in the suppression of the interband absorption and the appearance of absorption peaks in the infrared region originating in the transitions between polaron states and from the valence band to the low-lying polaron state.
Abstract: Poly(3-alkylthiophene) in solution can be doped just as in the case of polythiophene films, resulting in the suppression of the interband absorption and the appearance of absorption peaks in the infrared region originating in the transitions between polaron states and from the valence band to the low-lying polaron state. Enhancement of the spin density by doping in an ESR measurement was also observed, which supports the polaron formation but not bipolaron formation. From these results, it has been clearly demonstrated that even a single polymer chain has the characteristics of conducting polymers.
TL;DR: In this article, the polaron lattice band structures for polyacetylene, polythiophene, and polyemeraldine were analyzed and compared with the observed optical absorption and magnetic data for the three polymers which exhibit a metallic-like behavior in their highly doped state.
Abstract: Band structure calculations are presented for the polaron lattice conformation in the highly doped regime of polyacetylene, polythiophene, and polyemeraldine. We have also performed calculations for the soliton lattice in polyacetylene and for the bipolaron lattice in polythiophene and polyemeraldine. The polaron lattice band structures are found to be in good agreement with the observed optical absorption and magnetic data for the three polymers which all exhibit a metallic-like behavior in their highly doped state. The polaron lattice band structures for polyacetylene and polythiophene are calculated to be very similar, which is consistent with the experimental trends. The polyemeraldine salt bipolaron and polaron band structures present a single defect band deep in the gap, instead of the two defect bands usually found in other conjugated systems. This peculiarity is related to the lack of electron-hole symmetry in polyaniline and leads to a modified picture for the electronic transitions appe...
TL;DR: In this paper, the authors describe an etude du poly (phenylene-1,4 vinylene [diphenyl-1-2] et composes oligomeres et polymeres voisins.
Abstract: Etude du poly (phenylene-1,4 vinylene [diphenyl-1,2]) et de composes oligomeres et polymeres voisins. Entre 400 et 900 nm. Observation de 3 bandes d'absorption, attribuees respectivement a une espece chargee photogeneree, a un bipolaron et a un etat neutre excite
TL;DR: Schlieslich et al. as mentioned in this paper derived the polaron lattice solution of the Brazovskii-Kirova model in the continuum approximation, and discussed the stability of the bipolaron lattice phase of this solution as well as the nature of the transition to the dimerized C-phase.
Abstract: The polaron lattice solution of the Brazovskii-Kirova model is derived in the continuum approximation. The stability of the bipolaron lattice phase of this solution as well as the nature of the transition to the dimerized C-phase are discussed. The optical conductivity is calculated for different excess electron densities and the optical sum rule is verified in each case. Finally a comparison with experimental results for polypyrrole, polythiophene, and its alkyl-derivatives is made.
Im Rahmen der Kontinuum-Approximation des Brazovskij-Kirova-Modells wird die Polarongitter-Losung abgeleitet. Die Stabilitat der Bipolarongitter-Phase sowie die Natur des Ubergangs in die dimerisierte C-Phase werden diskutiert. Die optische Leitfahigkeit wird fur verschiedene Dotierungsgrade (Uberschuselektronendichten) berechnet und die optische Summenregel jeweils nachgepruft. Schlieslich erfolgt ein Vergleich mit experimentellen Resultaten an Polypyrrol, Polythiophen und seinen alkylierten Derivaten.
Journal Article•
TL;DR: Spectrometrie RPE du complexe forme par l'oligo(dibromo-4,4' diphenyl-co-diethynyl-1,4 benzene) avec l'iode as discussed by the authors.
Abstract: Spectrometrie RPE du complexe forme par l'oligo(dibromo-4,4' diphenyl-co-diethynyl-1,4 benzene) avec l'iode
TL;DR: In this article, the mid-IR spectra of iodine doped polyazines is reported as a function of doping level and two new bands develop while the other peaks in the vibrational spectrum remain unchanged.
TL;DR: In this article, the bipolaron-bipolaron interaction is found to be repulsive and varies with bipolaron density as (1/${\rm{\ensuremath{\rho}}}_{\mathit{b}}$)exp(-2/${\mathrm{\denuremath{b})exp(2/${denoremath{\xi}}{p}}$), where p is the characteristic length of bipolaron.
Abstract: Using a lattice sum over single bipolaron potentials displaced by periodicity d, we have analytically obtained a solution for a bipolaron lattice for a tetrahedrally bonded homopolymer within the continuum model of Rice and Phillpot. This solution is used to derive the band structure, which consists of two bipolaron bands symmetrically located about the middle of the band gap in addition to the conduction and valence bands. The electronic density of states, chemical potential \ensuremath{\mu}, and the energy of formation of a bipolaron lattice are also calculated as a function of the bipolaron density ${\ensuremath{\rho}}_{b}$. The bipolaron chemical potential lies between the conduction-band edge and the upper edge of the upper bipolaron band, indicating that the bipolaron lattice is energetically the most favorable charge configuration at low ${\ensuremath{\rho}}_{b}$. In the strict weak-coupling limit (infinite momentum cutoff \ensuremath{\Lambda}) the bipolaron-bipolaron interaction is found to be repulsive and varies with bipolaron density as (1/${\mathrm{\ensuremath{\rho}}}_{\mathit{b}}$)exp(-2/${\mathrm{\ensuremath{\rho}}}_{\mathit{b}}$${\ensuremath{\xi}}_{\mathit{p}}$), ${\ensuremath{\xi}}_{\mathit{p}}$ being the bipolaron characteristic length. Thus, the bipolaron lattice is stable only in the range 0${\ensuremath{\rho}}_{b}$2/${\ensuremath{\xi}}_{p}$. This suggests the possibility of a phase separation of a doped homopolymer into conducting bipolaron droplets at ${\ensuremath{\rho}}_{b}$=2/${\ensuremath{\xi}}_{p}$, while the rest of the system is insulating. Our results apply to polysilylenes, polygermylenes, and their derivatives, as well as to a wide class of carbon-based polymers.
TL;DR: In this article, a model to describe the free carrier-bipolaron interacting system is proposed, and a characteristic temperature T ∗ is obtained, below which the system enters a coherent state.
Abstract: In this paper a model to describe the free carrier-bipolaron interacting system is proposed. Effective hopping of the bipolaron is studied in the slave-boson approach, and a characteristic temperature T ∗ is obtained, below which the system enters a coherent state. The density of states in the normal state and the superconductivity of the system are discussed in a quasiparticle picture. The results show that the mixing between the free carrier and the bipolaron results in an enhancement of the effective mass of the quasiparticle and meanwhile the renormalized coupling interaction, arising from the negative correlation energy in the bipolaron region, enhances the effective superconducting coupling interaction. Under the most favourable conditions, the superconducting transition temperature T c ∼ ω c , where ω c is the Debye frequency related with local electron-phonon coupling. In general we have T ∗ > T c ⪢ T c0 ( T c0 is the superconducting transition temperature of a usual superconductor). Therefore the system will firstly enter a coherent state before becoming a high- T c superconductor.
TL;DR: A possible connection between bipolaron pairing in one dimension and the occurrence of superconductivity in the layered copper-oxide perovskites is pointed out in this article, which suggests that the superconductivities in the lanthanum compounds may be associated with an anisotropic pairing mechanism that involves holes in narrow, non-bonding, oxygen p-bands.
TL;DR: In this paper, the Pseudo-Jahn-Teller effect was used to model the mixing of two different-parity electronic states of the truncated Coulomb potential by an odd parity vibration of two neighbors.
Abstract: An alternative type of small polaron associated with an off-center configurational distortion can arise from the additional coupling of the electron to some appropriate odd vibrational mode. The model is exemplified by quantum-mechanical calculations of the vibronic mixing of two different-parity electronic states of the truncated Coulomb potential by an odd-parity vibration of two-neighboring ions: the Pseudo-Jahn-Teller effect. These calculations manifest the feasibility of the model and suggest that off-center polarons may occur in perovskites. Each off-center polaron is inherently polarizable in an electric field. Consequently, neighboring entities can couple through the universal Van der Waals force to form bipolarons. The off-center vibronic polarizability is temperature-dependent and so is the binding which decreases gradually as the temperature is raised. As a result, these Van der Waals bipolarons may cover the range from small-radius bipolarons, exhibiting a specific type of superconductivity, to large-radius species, which are similar to Cooper pairs. The model compares favorably with many specific features of the high-T
c
superconductivity.
TL;DR: In this article, an explanation of some anomalous properties of LBSO, LSCO and YBCO superconductors on the basis of the bipolaron theory of superconductivity proposed by Alexandrov and Ranninger /AR, 1981/ is proposed.
Abstract: The results of synthesis, measurements of the magnetic susceptibility, the upper critical field, X-ray photoelectron spectroscopy, and the intrinsic Josephson tunneling in Sr-La-Cu-O, and Y-La-Cu-O are presented. An explanation of some of the anomalous properties of LBSO, LSCO and YBCO superconductors on the basis of the bipolaron theory of superconductivity proposed by Alexandrov and Ranninger /AR, 1981/ is proposed.