Showing papers on "Bipolaron published in 1992"

Two-band model for halogen-bridged mixed-valence transition-metal complexes. I. Ground state and excitation spectrum.

Abstract: We consider a 3/4-filled, two-band discrete tight-binding Peierls-Hubbard model for an isolated chain of a halogen-bridged, mixed-valence, transition-metal linear-chain complex (HMMC or {ital MX} chain). We have employed the adiabatic approximation in which the quantum fluctuations associated with phonons are implicitly treated as an external field for the electrons, and treat electron-electron effects in the Hartree-Fock approximation. We investigate ground states as functions of the model parameters and doping-induced and photoinduced excitations---kinks, polarons, bipolarons, and excitons. Results for several experimental observables, including the lattice distortion, the excess charge and spin densities of defects, and the optical absorption, are compiled. For the ground state, we find that the bond-order-wave (BOW) portion of the one-band phase diagram is eliminated from the two-band phase diagram, in agreement with the lack of real materials in the pure BOW phase. The extent of electron-hole asymmetry and of spatial localization or delocalization of defects is explored. Two separate solitons or polarons are compared with corresponding bipolarons. We demonstrate explicitly the need to employ the two-band model for a realistic modeling of the {ital MX} systems, focusing on three specific systems: (a) highly distorted, valence-localized (strongly charge-disproportionated) PtCl, (b) moderately distorted PtBr, and (c) weakly distorted, valence-delocalizedmore » (weak charge-density wave) PtI. The compilation of results reported here constitutes a reference resource against which the rapidly expanding experimental data can be compared.« less

Synthesis, characterization, and properties of poly(N-alkylanilines)

Abstract: A series of soluble N-substituted poly(alkylanilines) have been chemically and electrochemically synthesized with elkyl groups ranging from methyl to n-dodecyl. The syntheses were made in 1.0 M HClO 4 using (NH 4 ) 2 S 2 O 8 as oxidant or a constant-potential technique. The electrical and physical properties of the poly(N-alkylanilines) were found to depend on the length of the substituents. These polymers also show multiple and reversible optical transitions that can be ascribed to the formation of polaron and bipolaron states

Polarons in C60.

Abstract: A simple Su-Schrieffer-Heeger-type model of ${\mathrm{C}}_{60}$ and doped ${\mathrm{C}}_{60}$ is presented The model is solved numerically in the adiabatic approximation In the half-filled case the cluster dimerizes, in agreement with experiment and more complicated calculations When an electron is added to half-filling, a polaron is present in the ground state This polaron possesses an unusual geometry; it is a loop circling the ${\mathrm{C}}_{60}$ cluster For two added electrons a bipolaron forms with a similar geometry It is suggested that this bipolaron may be observable in optical experiments

Many-body effects in the normal-state polaron system.

Abstract: Dielectric response function of small polarons (SP's) is studied. The Debye radius is small, which reduces a short-range Coulomb repulsion to the magnitude of the order of the small-polaron (SP) bandwidth. Polaron-polaron attraction is enhanced by screening. A critical temperature of the bipolaron formation is found. The dielectric response becomes dynamic in a very low-frequency region. A multiphonon diagram technique is developed to obtain vibration excitations, that are a mixture of phonons with polaronic plasmons

Strong-coupling analysis of large bipolarons in two and three dimensions.

Abstract: In the limit of strong electron-phonon coupling, we use either a Pekar-type or an oscillator wave function for the center-of-mass coordinate and either a Coulomb or an oscillator wave function for the relative coordinate, and are able to reproduce all the results from the literature for the large-bipolaron binding energy. Lower bounds are constructed for the critical ratio {eta}{sub {ital c}} of dielectric constants below which bipolarons can exist. It is found that, in the strong-coupling limit, the stability region for bipolaron formation is much larger in two dimensions (2D) than in 3D. We introduce a model that combines the averaging of the relative coordinate over the asymptotically best wave function with a path-integral treatment of the center-of-mass motion. The stability region for bipolaron formation is increased compared with the full path-integral treatment at large values of the coupling constant {alpha}. The critical values are {alpha}{sub {ital c}}{approx}9.3 in 3D and {alpha}{sub {ital c}}{approx}4.5 in 2D. Phase diagrams for the presented models are also obtained in both 2D and 3D.

Polarons in high-temperature superconductors

Abstract: The review deals with the main results of a consistent study of electron-lattice interaction in crystals in the strong-coupling limit. Small polaron and bipolaron formation is shown to provide a number of new physical phenomena both in the normal and superconducting states of the system. Two mechanisms of superconductivity are discussed in detail. The first one arises from the Cooper pairing of small polarons in momentum space (polaron superconductivity). The second one is due to polaron pairing in real space leading to on-site or intersite bipolaron formation with superconductivity analogous to the superfluidity of 4He (bipolaron superconductivity). Highly non-adiabatic motion of (bi)polarons in the narrow band results in fundamental differences of their superconducting state properties with respect to the predictions of the BCS theory and its well known strong-coupling generalization. A number of basic properties of high-temperature metallic oxides being analyzed in terms of (bi)polaron theory of superconductivity is shown to reveal a satisfactory agreement of its findings with the available experimental data.

Simultaneous EPR and electrochemical measurements on polyaniline in ambient temperature molten salts

Abstract: Simultaneous electrochemical measurements and EPR are carried out on polyaniline in ambient temperature molten salts. Maximum EPR response was where half the total observed charge had been passed in both cyclic voltametry and potential step experiments. The number of spins observed and electrons removed were 1:1 to about 25% of full oxidation. Two unresolved one-electron steps are used to explain the experiments with neutral, polaron, and bipolaron thermodynamic equilibrium. Reduced and oxidized form equilibrium constants change with the film`s ionic environment and conductivity. Doping stage dominance and EPR decay response of the polaron are also determined. 32 refs., 10 figs.

Large-bipolaron transport and cuprate superconductors.

Abstract: The in-plane electronic transport of large bipolarons that move within parallel conducting sheets is considered The Coulomb interactions between the heavy bipolarons in this planar system produce a distinctive spectrum for collective excitations Transport of the quasiparticles associated with the collective excitations is treated via the conventional Boltzmann transport theory However, the scattering of the heavy quasiparticles by acoustic phonons is taken to be analogous to the distinctive scattering of individual large polarons In particular, as with strong-coupling (adiabatic) large polarons, the scattering time is taken to be proportional to the quasiparticle effective mass The resistivity, the Hall effect, and the thermoelectric power are determined with this scheme With a constant density of carriers, the resistivity is proportional to the temperature The Hall number, the number of carriers deduced from the reciprocal of the Hall constant, exceeds the true carrier density by a ratio that rises linearly with temperature In addition, the thermoelectric power falls in magnitude and ultimately changes sign as the temperature is raised These unusual transport properties, as well as the ir conductivity, are consistent with the observed behaviors of the high-${\mathit{T}}_{\mathit{c}}$ cuprate superconductors

Photoinduced defects in the bromide-bridged platinum linear chain

Abstract: Photoinduced defects in the bromide-bridged platinum linear chain (Pt(en){sub 2})(Pt(en){sub 2}Br{sub 2})(ClO{sub 4}){sub 4} (PtBr, where en=ethylenediamine) yield Raman, absorption, and electron paramagnetic resonance (EPR) signals that have been examined as a function of temperature in order to determine the kinetics of the decay of these metastable defects in an intermediate-strength low-dimensional charge-density-wave (CDW) solid Both diamagnetic and paramagnetic defects are photogenerated with the initial excitonic transient primarily decaying to form electron bipolarons and hole polarons The photogenerated Raman signal is indicative of the former and the EPR signal of the latter The type and stability of these photoinduced defects are dependent on the sample history, with PtBr crystals grown in the orthorhombic phase (above 30 {degree}C) leading to greater variety and stability of defects as compared with those photoinduced in crystals grown in the monoclinic phase (below 10 {degree}C) The temperature dependence of the photogenerated spectroscopic signals reveals complicated decay routes, with the eventual recombination of valence defects mediated by lattice pinning potentials of various strengths, interconversion of defect states (including the possible melting'' of electron bipolarons into electron polarons) and a strengthening of the CDW as the crystals are warmed toward room temperature The observation of bromide hyperfinemore » splitting in the EPR data and the reversible intensity changes in the Raman defect signals indicates that these defects are spatially quite disperse, with ten or greater platinum sites revealed in the EPR signal, and demonstrates that the weakening of the CDW in PtBr relative to the strong CDW solid PtCl has a significant effect on the ground- and defect-state properties« less

Electrochemistry of conductive polymers. 11. Spectroelectrochemical studies of poly(3-methylthiophene) oxidation

Abstract: The oxidation of electrochemically grown poly(3-methylthiophene) and its other spectroscopic properties have been studied by in-situ spectroelectrochemical techniques as well as in-situ conductivity measurements, and the results are reported. An absorptivity of 1.1×10 5 cm -1 is reported for the absorption band at 490 nm for a neutral, reduced polymer film propylene carbonate. The oxidation of the neutral polymer to the cation radical, or polaron, and its further oxidation to the dication, or bipolaron, are shown to take place at about 0.45 and 0.80 V, respectively, and be controlled by the diffusion of counterious

Random-phase-approximation approach to phonon spectra in doped and undoped halogen-bridged transition-metal compounds

Abstract: We have developed a method based on a real-space random-phase approximation (RPA) to study the phonon spectra of doped and undoped halogen-bridged transition-metal-chain complexes. The method is very convenient in cases of spatially inhomogeneous mean-field structures. Thus, we are able to find the usual extended phonon modes as well as local modes associated with polaron, bipolaron, kink, exciton, or impurity structures in intrinsically or extrinsically doped systems. The corresponding infrared absorption spectra and Raman spectra (as a function of the excitation energy) are calculated. We demonstrate the method for a homogeneous charge-density-wave system, and for electron and hole polarons. We find that each type of defect has specific signatures in both infrared and Raman spectra, which can help to identify different types of defects experimentally.

Stability of large bipolarons

Abstract: The problem of the stability of bipolarons is discussed under the assumption of the Frohlich electron-phonon interaction. A variational wavefunction for the bipolaron is proposed in a general form, which yields the translationally invariant wavefunction as a special case. A minimization of the expectation value of the Hamiltonian leads to the best estimates if the trial wavefunction is localized. In a broad range of material parameters, the upper bounds obtained for the ground-state energy of the bipolaron are better than those obtained with the help of methods fully exploiting the translational symmetry. A similar property has been observed for the free polarons for which the lowest upper bounds on the ground-state energy in the strong-coupling limit are obtained with the help of localized trial wavefunctions. This suggests that the stability of large bipolarons is connected with their self-trapping. Because of this tendency to localization the authors do not expect that the large bipolarons contribute to the bipolaronic mechanism of superconductivity in three-dimensional crystals.

Extended stability region for large bipolarons through interaction with multiple phonon branches

Abstract: The large (bi)polaron is investigated for the case where the electron interacts with multiple LO-phonon branches. Explicit expressions for the groundstate energy and the effective mass are obtained within the Feynman polaron model approximation and they are applied to the material SrTiO3. The results of an effective LO-phonon branch approximation are compared with the results in which all LO-phonon branches are explicitly included. We show how the stability region for large bipolaron formation is enlarged when the electrons interact with multiple LO-phonon branches. The possible relevance of this result for the high-Tc superconductors is pointed out.

Optical and magnetic properties of electrochemically doped p- and n-type poly(3-hexylthiophene)

Abstract: The electrochemical, optical and magnetic properties of poly(3-hexylthiophene) (PAT-6) in comparison with those of polythiophene (PT) during electrochemical p- and n-type doping have been investigated by cyclic voltammetry, optical absorption spectroscopy and electron spin resonance measurements The results are discussed in terms of polaron and/or bipolaron models and lead to a better understanding of the electronic levels Specifically, as the top of the valence band of PAT-6 is located at a higher energy state than that of PT by about 03 eV, the p-type dopant in PAT-6 is relatively stable compared with that in PT However, because the bottom of the conduction band of PAT-6 was evaluated to be located at a higher energy state by about 03 eV than that of PT, the n-type doping in PAT-6 is more difficult than in PT

Preparation and oxidative doping studies of α,ω-dithienyl polyenes stabilized by long chain alkylthio substituents

Abstract: Polyenes containing mesomerically interactive substituent groups can be oxidatively doped with SbCl5 in solution to yield stabilized bipolaron-like charge states. However, in both α,ω-diphenyl and α,ω-dithienyl polyene series, lack of solubility has precluded studies of bipolaron formation for polyenes containing more than six double bonds in the polyene segment. In the present study, the use of long-chain alkylthio substituents to stabilize the bipolaronic charge states has allowed the synthesis of dithienyl polyenes containing up to ten double bonds, and observation of bipolaron formation in the octaene. Models for copolymers containing these bipolaronic repeat units have also been synthesized, and oxidatively doped.

MCSCF study of polaron- and bipolaron-like defects in small all-trans conjugated polyenes

Abstract: A Multiconfigurational Self-Consistent Field study of neutral, singly and doubly ionized C4H6, C6H8, C8H10, and C10H12 as model systems for polaron and bipolaron defects in polyacetylene has been carried out. In all computations a double zeta quality basis set was used. The post-Hartree-Fock computations were carried out in the complete active π subspace and the fully optimized structures are reported. Comparison with SCF results reveals that the inclusion of electron correlation attenuates the double bond/single bond alternancy in conjugated polyenes as well as the charge waves associated with the cations.

Equivalence of 3D bipolarons in a strong magnetic field to 1D bipolarons

Abstract: We show that in the strong magnetic-field limit the 3D-bipolaron problem reduces to the one of a 1D bipolaron with a renormalized electron-phonon coupling constant. This results in a much smaller critical value for bipolaron formation.

Electronic conduction in Pb-modified amorphous semiconductors Pb20GexSe80-x exhibiting a p-n transition

Abstract: A.c. conductivity and dielectric relaxation studies of Pb-modified amorphous semiconductors Pb20GexSe80-x, exhibiting a p-n transition at x = 21, are reported as functions of frequency in the range 100Hz-10kHz and as functions of temperature in the range 180–450 K. The a.c. conductivity σa.c(ω) is found to be proportional to ω5. The temperature dependence of σa.c (ω) and of the exponent s for all the compositions examined have been interpreted using the correlated-barrier-hopping model. Analysis of the results reveals that electronic conduction in GeSe3·5 takes place via bipolaron hopping in the whole temperature range of study. The addition of Pb impurities induces new types of defect (C1 1 and Pb2+) which contribute predominantly to single-polaron hopping at higher temperatures. However, the conduction at lower temperatures is of bipolaron type in compositions with or without Pb. Strong temperature and frequency dependences of the loss tangent tan δ at higher temperatures in the compositions co...

Comparison of bipolaron-like charge state generation in the oxidative doping of α,ω-dithienyl and α,ω-diphenyl polyenes stabilized by methoxy and methylthio substituents

Abstract: Several α,ω-dithienyl and diphenyl polyenes substituted with mesomerically interactive methylthio substituents have been synthesized by Wittig condensation. These polyenes can be oxidatively doped with SbCl5 in solution to yield stable bipolaron-like charge states. The methylthio-substituted bipolaron spectra are considerably more red-shifted in the diphenyl polyene series than the methoxy-substituted counterparts, or the methylthiothienyl series. The potential of these states for enhanced non-linear optical response is presented.

Plasmon Effects on Fröhlich Bipolaron Binding Energies

Abstract: We have studied the possibility of formation of bipolarons in the presence of plasmons taking into account short-range electron screening. We have found that the coexistence of phonons and plasmons strongly enhances the binding energy. Its dependence on the electron density has been studied and the existence of a maximum has been shown. Finally the possible connection between these results and high-Tc superconductors is briefly commented.

N-Type doping properties and electronic states of poly(2,5-thienylene vinylene)

Abstract: Evidence for electrochemical n-type doping of poly(2,5-thienylene vinylene) (PTV) is obtained from in situ measurements of a cyclic voltammogram, optical absorption spectrum and electron spin resonance. Bipolaron levels in the gap states appear upon n-type doping and the electronic band scheme of PTV is discussed.

Electron-correlation effects in one-dimensional large-bipolaron formation

Abstract: We study the effects of electron correlation on the ground state of a one-dimensional large singlet bipolaron. The electron-lattice interaction is taken to be the short-range interaction of Holstein's molecular-crystal model. We represent the Coulomb repulsion with the Hubbard short-range repulsion. This adoption of the Hubbard model is equivalent to replacing a strict one-dimensional system that has a short-range logarithmic divergence of the Coulomb repulsion energy between overlapping charges, with a quasi-one-dimensional system of finite width. Two types of electronic correlation are considered. With ``in-out'' correlation, we permit one of the two self-trapped carriers of the bipolaron to have a larger radius than the other. With ``left-right'' correlation, we permit the centroids of the self-trapped carriers to be displaced from one another. For both types of correlation, variational calculations are performed to determine the magnitudes of the correlation effects in the ground state. There are three parameters in the model: the electronic bandwidth parameter t, the electron-lattice coupling strength ${\mathit{E}}_{\mathit{b}}$, and the Hubbard repulsion, ${\mathit{V}}_{\mathit{c}}$. The electron-lattice interaction provides an indirect intercarrier attraction that fosters the coalescence of the two carriers. In opposition, the carriers' Coulomb repulsion and the kinetic energy required for carrier confinement foster the carriers spreading. With bipolaron formation the intercarrier attraction dominates the Coulomb repulsion.

Bipolaron theory approach to the microwave surface resistance of high-temperature superconductors

Abstract: The temperature dependence of the high-temperature superconductor surface resistance is calculated on the basis of the bipolaron Bose condensation model. The correlation between the theoretical results and experimental data is discussed. Several constituents of the residual resistance are described.

Conductivity, chemical equilibria and small polaron in oxide superconductor : YBa2Cu3O7-δ

Abstract: An investigation of the structures of the thermal variations of the conductivities, σ(T,δ)'s, and an examination of the chemical equilibria in YBa2Cu3O7−δ indicate the existence of a small polaron in the normal state. The overall structures of σ (T),δ)'s can be analyzed in terms of delocalized and localized (thermally activated) carriers with narrow band tunneling (Fermi energy of order of 10−4 eV) and hopping (activation energy of the order of 10−2eV), respectively. The rapidly increasing σ(T) observed in some cases at low decreasing temperatures corresponds to the heretofore unrecognized narrow band tunneling in the small polaron theory: as a consequence of the chemical equilibria which must be recognized in any mechanism of conductivity in the normal state, thermodynamic equilibria can be associated with small polarons in which charge is transported through the time rate of change in the dipole moments. A precondition for the onset of the superconducting state is the co-existence of the two mechanisms and some relation between their two parameters such as the one derived by Allender, Bray and Bardeen (Phys. Rev. B 7 (1973) 1020). The superconducting state appears to be a bipolaronic state.

Trapped bipolaron due to interimpurity interactions in doped conjugated polymers: Two-impurity problem and dependence on impurity-potential forms.

Abstract: Effects of interimpurity interaction in conjugated polymers are numerically investigated. The Su-Schrieffer-Heeger Hamiltonian is studied with two acceptor-type long-ranged impurities that have anisotropic dielectric constants and a screening factor. Direct interimpurity repulsion is taken into account. Impurity clustering occurs and a trapped bipolaron is formed when the range of the impurity potential is long enough. Optical absorption has only one midgap absorption peak due to the strong impurity potential. Impurities are isolated from one another when the range becomes shorter.

Electronic properties of polyarylene-vinylene conducting polymers

Abstract: The electrochemical, optical and magnetic properties in polyarylene-vinylene conducting polymers, such as poly-p-phenylene vinylene (PPV), poly-1,4-naphthalene vinylene (PNV), and poly-2,5-thienylene vinylene (PTV), during electrochemical p- and n-type doping have been investigated by cyclic voltammetry, optical absorption spectroscopy, and ESR measurements. These results are discussed in terms of polaron and/or bipolaron models and lead to a better understanding of the electronic levels. The evolution of localized states upon doping is dependent on molecular structure and successful n-type doping in PTV and PNV is suggested. Furthermore, new optical recording techniques utilizing the precursor films of PPV and its derivatives have also been prepared. >