Showing papers on "Bipolaron published in 1996"
TL;DR: Electroabsorption measurements of built-in electric fields and internal photoemission measurements of Schottky barriers are reported to determine the charge transfer and single-particle energy gaps of the conjugated polymer poly[2-methoxy, 5-(2{prime}-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV).
Abstract: We report electroabsorption measurements of built-in electric fields and internal photoemission measurements of Schottky barriers to determine the charge transfer and single-particle energy gaps of the conjugated polymer poly[2-methoxy, 5-(2\ensuremath{'}-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV). For MEH-PPV, with an exciton absorption peak of 2.25 eV, or results yield a single-particle energy gap of 2.45 eV and a charge transfer energy gap of at least 2.35 eV. Therefore the exciton binding energy is 0.2 eV and the bipolaron binding energy is less than 0.1 eV.
410 citations
TL;DR: In this article, the transition between polaron, bipolaron, and metallic states of poly(3,4-ethylenedioxythiophene) (PEDOT) was detected using in situ UV−VIS−NIR spectroscopy.
Abstract: Different oxidation levels, regarded as polaron, bipolaron, and metallic states, are usually found in conjugated heterocyclic polymers. We found that poly(3,4-ethylenedioxythiophene) (PEDOT) also has these different oxidation levels, using in situ UV−VIS−NIR spectroscopy. The transitions between the different oxidation levels were, however, never clearly observed in cyclic voltammetry (CV). Instead it usually shows a broad oxidation peak and two reduction peaks. The CV of PEDOT at macroelectrodes shows a pair of redox peaks separated by 0.9 V at low scan rates, indicating two irreversible electron transfer steps. Using an ultramicroelectrode, we found these two pairs of redox peaks in PEDOT, as well as for poly(3-methylthiophene) (PMeT). These two peaks cannot be explained by the existence of two forms of the materials, whether due to two different conjugation lengths, crystalline or noncrystalline phases, or created by conformational changes. We deduced that there should be a third redox peak. With fast ...
174 citations
TL;DR: It is demonstrated that the formation of hole polarons is favored by strong Coulomb correlations, and it is found that hole binding is enhanced as a dynamical effect of the electron-phonon interaction.
Abstract: The Holstein Hubbard and Holstein t-J models are studied for a wide range of phonon frequencies, electron-electron, and electron-phonon interaction strengths on finite lattices with up to ten sites by means of direct Lanczos diagonalization Previously the necessary truncation of the phononic Hilbert space caused serious limitations to either very small systems (four or even two sites) or to weak electron-phonon coupling, in particular in the adiabatic regime Using parallel computers we were able to investigate the transition from ``large'' to ``small'' polarons in detail By resolving the low-lying eigenstates of the Hamiltonian and by calculating the spectral function, we can identify a polaron band in the strong-coupling case, whose dispersion deviates from the free-particle dispersion at low and intermediate phonon frequencies For two electrons (holes) we establish the existence of bipolaronic states and discuss the formation of a bipolaron band For the two-dimensional Holstein t-J model, we demonstrate that the formation of hole polarons is favored by strong Coulomb correlations Analyzing hole-hole correlation functions, we find that hole binding is enhanced as a dynamical effect of the electron-phonon interaction \textcopyright{} 1996 The American Physical Society
116 citations
TL;DR: In this paper, a hole bipolaron band structure with two flat anisotropic bands is derived for oxide superconductors and strong anisotropy leads to one-dimensional localization in a random field which explains the metal-like value of the Hall effect and the semiconductor-like doping dependence of resistivity of overdoped oxides.
Abstract: Hole bipolaron band structure with two flat anisotropic bands is derived for oxide superconductors. Strong anisotropy leads to one-dimensional localization in a random field which explains the metal-like value of the Hall effect and the semiconductorlike doping dependence of resistivity of overdoped oxides. Doping dependence of ${T}_{c}$ and ${\ensuremath{\lambda}}_{H}(0)$ as well as the low-temperature dependence of resistivity, of the Hall effect, ${H}_{c2}(T)$ and robust features of angle-resolved photoemission spectroscopy of several high-${T}_{c}$ copper oxides are explained.
107 citations
TL;DR: In this paper, the electronic absorption and Raman spectra of as-polymerized BF 4 − -doped polythiophene have been analyzed on the basis of the spectra, viewed as models of a positive polaron and a positive bipolaron, respectively.
84 citations
TL;DR: In this paper, the bending beam method was used to detect volume changes during electrochemical transformations of poly(3,4-ethylenedioxythiophene) and found that the volume changes cannot be explained as a simple consequence of ion transport but must be due to the structural change of the polymer between different states.
Abstract: The electrochemical properties of poly(3,4-ethylenedioxythiophene) are studied using the bending beam method to detect volume changes during electrochemical transformations of the material Thin films of poly(3,4-ethylenedioxythiophene) immersed in different supporting electrolytes first contract very rapidly and then expand on doping, while upon undoping they contract directly, or first expand and then contract, to their original positions It is clearly observed that the oxidation or reduction of the polymer contains two steps, one due to a redox potential close to −05 V vs Ag/AgCl, and another potential around 0 V We find that the volume changes cannot be understood as a simple consequence of ion transport but must be due to the structural change of the polymer between the different states A hypothetical picture is that during the transition from the neutral to the polaron state, the polymer is slightly charged and thus contracted; on further doping to the bipolaron and to the metallic state, the co
79 citations
TL;DR: In this paper, the authors show that the phase separation is due to the competition between the single and double occupancies of the Mn 3d orbitals by the doped electrons, and that the doubly occupying electrons form on-site triplets which are mobile at high temperatures and form a bipolaron lattice below ∼ 250 K.
71 citations
TL;DR: In this paper, the stability of a strong-coupling singlet optical bipolaron was studied for the first time in two-and three-dimensional parabolic quantum dots using the Landau-Pekar variational method.
Abstract: The stability of a strong-coupling singlet optical bipolaron is studied for the first time in two- and three-dimensional parabolic quantum dots using the Landau - Pekar variational method. It is shown that the confining potential of the quantum dot reduces the stability of the bipolaron.
53 citations
TL;DR: A model based on nondegenerate continuum model of Brazovskii and Kirova for electronic properties of polymers is described, finding that the bipolaron lattice is unstable at high density when intrinsic gap parameter exceeds a critical fraction of total energy gap.
Abstract: We describe a model for metal-polymer interfaces based on the nondegenerate continuum model of Brazovskii and Kirova for the electronic properties of polymers. The correct analytic equations for a bipolaron lattice in this model are stated and the electronic properties of the bulk polymer, i.e., the energy-level structure, the energy density, and the chemical potential as a function of electron density are obtained numerically. We find that the bipolaron lattice is unstable at high densities when the intrinsic gap parameter exceeds a critical fraction of the total energy gap. The electronic properties of the bulk polymer are used for modeling the metal-polymer interface. The charge density near a metal-polymer interface is found from the electrostatic potential and an analytic expression for the bipolaron chemical potential assuming that the contact is in equilibrium with the polymer layer. Poisson's equation is integrated to determine the electrostatic potential. We find that a large charge density is transferred into the polymer layer if the Fermi level of the metal contact is higher than the negative bipolaron formation energy per particle or lower than the positive bipolaron formation energy per particle. The transferred charge lies very close to the metal-polymer interface as a bipolaron lattice with charge density progressively decreasing away from the interface.The transferred charge gives rise to a region of rapid ``band bending,'' pins the Fermi level, and establishes the effective Schottky energy barrier. Upon increasing the metal Fermi level above the bipolaron formation energy per particle, the effective Schottky barrier saturates at the energy difference between the polaron formation energy and the bipolaron formation energy per particle. The model results are useful in interpreting recent measurements of internal photoemission, device electroabsorption, and capacitance-voltage characteristics in polymer light-emitting diodes. \textcopyright{} 1996 The American Physical Society.
44 citations
TL;DR: In this article, the structure and stability of dications of oligo-thiophenes up to the decamer are calculated by semi-empirical calculations which explicitly take electron-electron interactions into account.
38 citations
TL;DR: The Holstein-Hubbard model is used to study high-temperature hopping transport when it is energetically favorable for carriers to pair as singlet small bipolarons, and the semiclassical rates for one-electron transfers involving small polarons are found to be much greater than those that only involve smallipolarons.
Abstract: The Holstein-Hubbard model is used to study high-temperature (T\ensuremath{\gtrsim}${\mathit{T}}_{\mathrm{phonon}}$/3) hopping transport when it is energetically favorable for carriers to pair as singlet small bipolarons. The semiclassical rates for one-electron transfers involving small polarons are found to be much greater than those that only involve small bipolarons. In particular, the most rapid one-electron-transfer processes have a small-polaron hopping to a vacant site and one of a small bipolaron's two carriers jumping onto the site of an adjacent small polaron. As a result, even when most carriers form small bipolarons rather than small polarons, one-electron transfers involving small polarons always dominate the dc conductivity. The energy to thermally generate small polarons from small bipolarons thus contributes to the conductivity's activation energy. This pair-breaking energy also manifests itself in a thermally activated contribution to the system's paramagnetic susceptibility. In addition, the carriers' Seebeck coefficient garners a contribution with a temperature dependence that is characterized by the pair-breaking energy. \textcopyright{} 1996 The American Physical Society.
TL;DR: In this paper, a superposition of Drude-type and hopping-type dynamical conductivity is assumed for the FIR spectra of boron-rich solids.
Abstract: The FIR spectra of boron-rich solids measured down to exhibit strongly increasing reflectivities towards lower frequencies. These spectra are well described when a superposition of Drude-type and hopping-type dynamical conductivity is assumed. The corresponding fits were simultaneously made for the real and the imaginary part of the dielectric function to achieve unambiguous parameters. The results obtained can be described consistently, for example, with DC conductivity measurements, and moreover in the case of -rhombohedral boron, with the known energy band scheme. The bipolaron hopping model of the electronic transport in boron carbide is proved to be inappropriate.
TL;DR: In this paper, the ground state of the acoustical polaron and bipolaron in two dimensions was studied using the Feynman pathintegral formalism and the phase diagram for the self-trapping of polarons was obtained.
Abstract: Using the Feynman path-integral formalism we study the ground state of the acoustical polaron and bipolaron in two dimensions. The phase diagram for the self-trapping of polarons is obtained. For a Debye cutoff of ${\mathit{k}}_{0}$130 the self-trapping transition is continuous while for ${\mathit{k}}_{0}$g130 it is discontinuous and the polaron mass exhibits a discontinuity as function of the electron-phonon coupling constant (\ensuremath{\alpha}). A phase diagram for the polaron-bipolaron transition is obtained and we found that (1) the bipolaron transition is a first-order transition and (2) it is also possible between two polarons which are in the quasifree state, i.e., not self-trapped. At the transition point the effective mass changes substantially. Analytic results for the energy and the phase diagram in the weak- and strong-coupling constant regime are also obtained. \textcopyright{} 1996 The American Physical Society.
TL;DR: In this article, the formation of the Ca/poly( p -phenylene vinylene) (PPV) interface using X-ray photoelectron emission spectroscopy (XPS) and near edge Xray absorption fine structure (NEXAFS) was studied.
TL;DR: ERC measurements at temperatures well above those of previous works unearth thermally generated spins in boron carbides, and it is suggested that these thermally induced spins arise from thermal dissociation of bor on carbides singlet small bipolarons.
Abstract: Using ESR, the spin susceptibilities of a crystal and ceramics of boron carbides have been measured between 4 and 1200 K. A thermally generated contribution to the spin susceptibility, which may be discerned emerging above 100 K, becomes unmistakable above 300 K. The $g$ values of these spins differ from those of the temperature-independent background spins that dominate low-temperature ESR measurements. Thus, ESR measurements at temperatures well above those of previous works unearth thermally generated spins in boron carbides. We suggest that these thermally induced spins arise from thermal dissociation of boron carbides' singlet small bipolarons. The energy with which a small bipolaron is bound with respect to dissociating into two small polarons is estimated to be about 0.2 eV.
TL;DR: In this article, the reversible hydroxide-anion doping of polypyrrole (PPy) has been investigated by FT-IR, cyclic voltammetry, in situ conductivity and ESR techniques.
TL;DR: In this article, the authors conclude that there is no evidence that photogeneration results in a significant number of bipolarons in PPV and its derivatives, apart from samples with particular impurities or defects that foster bipolaron creation.
TL;DR: In this article, the dynamical properties of an acoustic bipolaron in non-degenerate conjugated polymers such as polydiacetylene (PDA), which can be regarded as one-dimensional electron-lattice systems, are studied through numerical simulations.
Abstract: The dynamical properties of an acoustic bipolaron in non-degenerate conjugated polymers such as polydiacetylene (PDA), which can be regarded as one-dimensional electron-lattice systems, are studied through numerical simulations. In these simulations, use has been made of Su-Schrieffer-Heeger's model extended to involve on-site as well as nearest-neighbor electron-electron Coulomb repulsion terms. The electron-electron interactions are treated within the unrestricted Hartree-Fock approximation. It is confirmed that the velocity of the acoustic bipolaron accelerated by a constant electric field saturates at the sound velocity of the system, and that the width of the bipolaron shrinks to zero as its velocity approaches the saturation velocity (= sound velocity). The effective mass of a bipolaron is estimated to be several hundred times as heavy as the free electron mass, in the case of PDA. These properties are compared with those of a polaron.
TL;DR: This model predicts an insulating behavior of the ordered bipolaron lattice of PT and predicts that the superlattice structure of doped PT's will be semiconductors at room temperature but with a high resistivity.
Abstract: We study here theoretically the evolution of the electronic structure of polythiophene (PT) due to bipolaron doping after modifying the \ensuremath{\sigma}-bond compressibility model. Since the electron-lattice coupling in this model alters both the site energies and the hopping integrals, effects due to the changes in the site energies on the formation of polarons and bipolarons in PT are incorporated. The inductive effect of sulfur is also considered. The ground state geometry of the neutral PT and the experimentally observed bipolaronic optical transitions are reproduced. We predict an insulating behavior of the ordered bipolaron lattice of PT. The bipolaron cluster model also fails to develop a near degeneracy of the highest occupied and the lowest unoccupied molecular orbitals at and above 20 mol % defect concentration. We attribute this to the altering of the site energies upon doping. Our model further predicts that the superlattice structure of doped PT's will be semiconductors at room temperature but with a high resistivity. \textcopyright{} 1996 The American Physical Society.
TL;DR: In this paper, the authors measured the ultraviolet photoelectron spectra for two π-conjugated polymers, poly(pyridine-2,5-diyl) (PPy) and poly(2,2′-bipyridine -5,5′-dyl), which exhibit n-type electrically conducting properties.
TL;DR: In this paper, a dielectric relaxation study of well-characterized Bi modified amorphous thin films of Ge20Te80-x Bi x (x=0, 0.19, 2.93 and 7.35) prepared by flash evaporation, exhibiting an absence of pn transitions in the frequency range 500 Hz•10 kHz and the temperature span 180 K•450 K, is reported.
Abstract: Ac conductivity and dielectric relaxation studies of well‐characterized Bi‐modified amorphous thin films of Ge20Te80‐x Bi x (x=0, 0.19, 2.93 and 7.35) prepared by flash evaporation, exhibiting an absence of p‐n transitions in the frequency range 500 Hz‐10 kHz and the temperature span 180 K‐450 K, are reported. The ac conductivity, σ ac(ω), is found to be proportional to ωs. The temperature dependences of σ ac(ω), and the exponent s are interpreted using the correlated‐barrier hopping model. It is revealed that electronic conduction takes place via bipolaron/single polaron hopping processes at low/high temperatures in all the compositions. At a lower Bi concentration (x=0.19), Bi‐induced defects that take part in single polaron hopping are produced. Further increases in the Bi concentration (x=2.93 and 7.35) cause little change in the density of such defects. The density of defect states taking part in bipolaron conduction is not significantly affected by addition of Bi atoms to the alloy. This fe...
TL;DR: In this article, a condition for the bipolaron stability has been reformulated to take into account the fact that a bipolaron is a part of the charge carrier system, and possible consequences of this condition were considered in the special case when a single bipolaron was placed into the polaron gas.
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TL;DR: In this article, the electronic structures of undoped and doped cyclic polythiophene (PT) were studied using modified σ-bond compressibility model and they showed that the form having two singly occupied degenerate orbitals will be stable in a sufficiently large cyclic PT.
Abstract: Electronic structures of undoped and doped cyclic polythiophene (PT) are studied using modified $\sigma$-bond compressibility model. Cyclic PT doped with odd number of bipolarons creates an aromatic polyene backbone containing (4$n$+2) $\pi$-electrons and the system is driven towards the quinoid form. Consequently, we find an insulator-metal transition for dopant concentration $\geq$ 14 mol $\%$ and a $\sim$ 0.8 eV redshift in Fermi energy at 30 mol$\%$. For even number of bipolarons, we propose here that the form having two singly occupied degenerate orbitals will be stable in a sufficiently large cyclic PT.
TL;DR: In this paper, the ground state energy of an acoustical bipolaron in 3D was studied using the Feynman path integral method and the corresponding phase diagram was calculated as function of the cut-off, the electron - phonon coupling strength, and the repulsion (U) between the electrons.
Abstract: The ground-state energy of an acoustical bipolaron in three dimensions (3D) is studied within the Feynman path integral method. The acoustical polaron exhibits a transition from the quasi-free state to the self-trapped state. Depending on the value of the Debye cut-off in phonon wavevector space this transition can be continuous or discontinuous. If two electrons are present a second transition becomes possible: the transition to the bipolaron state in which the two electrons are self-trapped around the same position. The corresponding phase diagram is calculated as function of the cut-off , the electron - phonon coupling strength , and the repulsion (U) between the electrons. We found that: (i) the bipolaron transition is a first-order transition; (ii) it is also possible between two polarons which are in the quasi-free state, i.e. are not self-trapped; and (iii) the effective particle mass increases substantially at the transition point.
TL;DR: In this paper, the evolution of the electronic structure of cyclic polythiophene (PT) upon bipolaron doping is studied to explore the possibility of uniform charge density ground state in the metallic regime.
Abstract: The evolution of the electronic structure of cyclic polythiophene (PT) upon bipolaron doping is studied to explore the possibility of uniform charge density ground state in the metallic regime. The ground state geometry of the neutral PT, the structure of a single bipolaron, and the experimentally observed optical transitions due to it are reproduced. Since the cyclic PT doped with an odd number of bipolarons (the O configuration) creates an aromatic polyene backbone containing (4n+2)π electrons, it is driven toward the quinoid form. Consequently, we find an insulator‐metal transition for dopant concentration ≥14 mol % and an ∼0.8 eV redshift in Fermi energy with respect to the neutral system at 30 mol % which agree very well with experimental findings. For an even number of bipolarons, there are two possible configurations, namely (i) the (1,1) or the T configuration and (ii) the (2,0)/(0,2) or the S configuration. The T configuration and the O configuration behave similarly and merge to a single asympto...
TL;DR: In this article, it was shown that the magnetic bipolaron can be formed if the distance between two polarons is less than some critical value, i.e., the critical radius is larger, the lower the temperature and the larger the polaron radius.
TL;DR: In this paper, it was shown that Frohlich bipolaron can exist in both two and three dimensions in the strong coupling region, the binding being stronger in the lower dimension.
Abstract: Within the framework of Rayleigh-Schrodinger perturbation theory the bound state of two electrons in polar crystals is investigated in various dimensional systems. It is found that Frohlich bipolaron can exist in both two and three dimensions in the strong-coupling region, the binding being stronger in the lower dimension. The stability criterion for the formation of the Frohlich bipolaron is estimated to yield better results compared to the values quoted in the literature.
TL;DR: The Ising, small-biopolaron (ISB) theory is a strong coupling theory of cuprate superconductivity which is based in the negative-U, Hubbard Hamiltonian as mentioned in this paper.
Abstract: The Ising, small-biopolaron (ISB) theory is a strong-coupling theory of cuprate superconductivity which is based in the negative-U, Hubbard Hamiltonian. Its ground state is composed of (small) biopolarons and (small-biopolaron) holes with a vibronically induced, biopolaron-hole exchange interaction, J{sub BH}, between them. The energy gap, {Delta}(0), is taken to be equal to the dissociation energy of a small biopolaran and which, since it is defined spectroscopically, is not an order parameter. The application of the Ising meanfield theory to the highly degenerate ground-state yields a second-order phase change with kT{sub c}/2=J{sub BH} and a real order parameter, {Omega}(T), which is valid over the entire temperature range from zero to T{sub c}. Near T{sub c}, the Ising free energy functional takes the same form as does the Landau. In the presence of an electromagnetic field, the Ising functional is a generalization of the Ginzburg-Landau functional which employs a complex order parameter and which is invariant under the electromagnetic gauge transformation. The breaking of the gauge invariance yields the London theory of superconductivity. 6 refs., 9 figs.
TL;DR: In this article, the 183 W NMR chemical shifts of polyoxotung states reduced by two electrons regarded as a bipolaron have been reconsidered and a quantitative estimation of the chemical shifts resulting from the delocalization process is given.