Showing papers on "Bipolaron published in 2001"
TL;DR: In this article, the authors numerically and analytically calculate the properties of the bipolaron in an extended Hubbard-Holstein model, which has a longer-range electron-phonon coupling like the Fr\"ohlich model.
Abstract: We numerically and analytically calculate the properties of the bipolaron in an extended Hubbard-Holstein model, which has a longer-range electron-phonon coupling like the Fr\"ohlich model. In the strong-coupling regime, the effective mass of the bipolaron in the extended model is much smaller than the Holstein bipolaron mass. In contrast to the Holstein bipolaron, the bipolaron in the extended model has a lower binding energy and remains bound with substantial binding energy even in the large-$U$ limit. In comparison with the Holstein model where only a singlet bipolaron is bound, in the extended Holstein model a triplet bipolaron can also form a bound state. We discuss the possibility of phase separation in the case of finite electron doping.
86 citations
TL;DR: The Seebeck coefficients of boron carbides are consistent with large contributions from softening bipolarons, singlet bipolarons whose stabilization is significantly affected by their softening of local vibrations as discussed by the authors.
Abstract: The most conspicuous feature of boron carbides' electronic transport properties is their having both high carrier densities and large Seebeck coefficients. The magnitudes and temperature dependencies of the Seebeck coefficients are consistent with large contributions from softening bipolarons: singlet bipolarons whose stabilization is significantly affected by their softening of local vibrations. Boron carbides' high carrier densities, small activation energies for hopping ({approx} 0.16 eV), and anomalously large Seebeck coefficients combine with their low, glass-like thermal conductivities to make them unexpectedly efficient high-temperature thermoelectrics.
66 citations
TL;DR: In this article, the use of polythiophene films as supporting matrix for Pt particles was reported, and the polymer film hosts were prepared using a strong acid aqueous solution, impregnated with Pt particles, and then tested for electrocatalytic properties for the oxygen reduction reaction (ORR).
Abstract: This work reports the use of polythiophene films as supporting matrix for Pt particles. The polymer film hosts were prepared using a strong acid aqueous solution, impregnated with Pt particles, and then tested for electrocatalytic properties for the oxygen reduction reaction (ORR). Cyclic voltammetry shows that the redox behavior of polythiophene in 12.5 M H 2 SO 4 presents a general feature equivalent to that of several other conducting polymers, especially polypyrrole. Features of the UV-visible spectra obtained during the redox process are consistent with the polaron/bipolaron model. Scanning electron microscopy measurements show that a very homogeneous film layer is obtained with the Pt deposited as very fine particles. Polarization measurements have shown that the ORR occurs through a mechanism involving four electrons with Tafel slopes of ca. 0.12 V dec -1 for potentials below 0.85 V vs. RHE (reference hydrogen electrode), in agreement with previous results for other types of dispersed Pt electrocatalysts.
43 citations
TL;DR: This work is the first deconvolution of conjugated polymer spectroelectrochemistry, which assigns as neutral polymer, polaron species, bipolaron species, and a species formed by further bipolaron reaction.
Abstract: Iterative target transformation factor analysis (ITTFA) was used to determine the spectra of the individual species generated during the oxidative p-doping of films of poly(para-phenyleneethynylene) (PPE). UV-visible spectra of PPE films on transparent electrodes were obtained in-situ during an anodic sweep. ITTFA identified 4 species present during the oxidation, which we assign as neutral polymer, polaron species, bipolaron species, and a species formed by further bipolaron reaction. The region of electrochemical stability for each of these species was identified and their potential-dependent profiles were obtained. This work is the first deconvolution of conjugated polymer spectroelectrochemistry.
19 citations
TL;DR: In this article, structural and electronic transitions in poly(phenylenesulfide phenyleneamine) (PPSA) upon electrochemical doping have been investigated, and the results indicate that polarons are the predominant charge defects at low doping levels, yielding an electronically and mechanically stable material.
Abstract: Structural and electronic transitions in poly(phenylenesulfide phenyleneamine) (PPSA) upon electrochemical doping have been investigated. The results indicate that polarons are the predominant charge defects at low doping levels, yielding an electronically and mechanically stable material. The electrochemical doping at high potentials induces a transition of the polarons to a bipolaron state. Due to the heterogeneous nature of the polymer chain and different oxidation potentials associated with aniline and phenylene sulfide units, the formation of polaron and bipolaron is distinguished as two separated steps. The dopant perchlorate ion exists in the polymer matrix, not only as charge neutralizer but also as a ligand that is simultaneously Coulombically bound to the positively charged S or N sites on one polymer chain and hydrogen bonded to the N−H group on the neighboring chain. The formation of such perchlorate anion centered Coulombic/hydrogen-bonded complexes has a major impact on the electrochemical a...
17 citations
TL;DR: In this paper, an accurate numerical consideration of 1D spinless fermion model with next-nearest neighbor (NNN) interactions is carried out for the electron concentrations 4/7.
Abstract: An accurate numerical consideration of 1D spinless fermion model with next-nearest neighbour (NNN) interactions is carried out for the electron concentrations 4/7. It is shown that depending on the parameters of the model it can be either Luttinger liquid or bipolaron liquid. In the former case competing interactions can result in a smooth behavior of one-electron distribution function at the Fermi surface with a divergence in the second derivative with respect to the quasimomentum (and not in the first one, as usual). In this connection, uncommon photoemission spectra for some 1D conductors can be explained qualitatively.
15 citations
TL;DR: In this article, the structure and stability of quaterthiophene radical cations were studied using density functional calculations, and it was shown that embedding a dimer in a dielectric medium results in stabilizing the dimer, as observed experimentally.
9 citations
TL;DR: In this paper, the authors provided an analytic theory by means of a response function to prove that a polymeric molecule with a bipolaron has a negative static polarizability and explained the physical origin of this photoinduced phenomenon in detail.
Abstract: Numerical simulation has shown that a polymeric molecule can possess a photoinduced phenomenon---photoinduced polarization reversion---in which the electric dipole of the polymeric molecule is reversed by absorbing one photon. This paper provides an analytic theory by means of a response function to prove that a polymeric molecule with a bipolaron has a negative static polarizability and explains the physical origin of this photoinduced phenomenon in detail. This paper also presents a dynamical calculation for the photoinduced polarization reversion, from which the relaxation time for the dipole reversion can be determined.
8 citations
TL;DR: In this paper, the influence of next-neighbor electron hopping on polaron and bipolaron formation in a square Hubbard-Holstein model was investigated within a variational approach.
Abstract: The influence of next-nearest-neighbor electron hopping ${t}^{\ensuremath{'}}$ on the polaron and bipolaron formation in a square Hubbard-Holstein model is investigated within a variational approach. The results for electron-phonon and electron-electron correlation functions show that a negative value of ${t}^{\ensuremath{'}}$ induces a strong anisotropy in the lattice distortions favoring the formation of nearest-neighbor intersite bipolaron. The role of ${t}^{\ensuremath{'}},$ electron-phonon and electron-electron interactions is briefly discussed in view of the formation of charged striped domains.
8 citations
TL;DR: The electrochemical and electronic properties, as well as the redox induced ion and charge transport mechanism in poly[1,4-bis(pyrrol-2-yl)phenylene] (PBPyP) have been studied by electrochemical, optical and electron spin resonance (ESR) spectroscopies, along with electrogravimetric techniques.
8 citations
TL;DR: In this article, the advantages of the in situ ESR/UV-Vis-NIR spectroelectrochemical measurements are presented in the study of different polymeric structures like polypyrrole, polythiophene, polyaniline and oligothiophenes.
TL;DR: In this article, the ground-singlet state of an optical bipolaron in the region of intermediate coupling strengths is analyzed and the spatial structure of a bipolaron with intermediate coupling strength is shown to be that of an axially symmetric quasi-molecular dimer.
Abstract: The ground-singlet state of an optical bipolaron in the region of intermediate coupling strengths is analysed It is shown that the spatial structure of a bipolaron with intermediate coupling strength is that of an axially symmetric quasi-molecular dimer The bipolaron energies of coupling are determined for different coupling constants and dielectric parameters of a medium The electronic terms of the ground-dimer state are represented as a function of the distance between quasi-particles The intermediate region of coupling constants is shown to give rise to an additional specific term in the total energy, which depends on the dielectric properties of a medium and makes the bipolaron energy of coupling higher (at e*/e∞>106) or lower (at e*/e∞<106) than the value in the limit of adiabatic and strong coupling
TL;DR: In this article, a review is made of D-wave pairing in real space between electrons on a Cuprate lattice due to any effective interaction with a short range repulsive part and a longer range attractive tail such as might arise from Bipolaronic coupling.
Abstract: The question as to whether Fermion pairs interacting with a short range Coulomb repulsion and longer range attractive tail can form a Condensate is a key question for High Temperature Superconductivity [1–30] If such a form of interaction can support a bound state it is commonly suggested that Bose-Einstein condensation of such pairs may occur, as in widely discussed Bipolaron models of High Temperature Cuprate superconductivity where the paired Fermions are holes On the other hand it is frequently stated that strongly overlapping real-space pairing with a strong short range Coulomb repulsion is a priori implausible [4] In this paper a review is made of our work on D-wave pairing [7–19] in real-space between electrons on a Cuprate lattice due to any effective interaction with a short range repulsive part and a longer range attractive tail such as might arise from Bipolaronic coupling
TL;DR: In this article, a variational procedure is developed to study the properties of the bipolaron in the Frohlich model including explicitly the electron band structure and taking into account the long-range repulsive electron-electron interaction.
Abstract: A variational procedure is developed to study the properties of the bipolaron in the Frohlich model including explicitly the electron band structure and taking into account the long-range repulsive electron-electron interaction. Adopting two different ansatze for the longitudinal optical phonon distribution function, the large-bipolaron and small-bipolaron limits are obtained. The evolution of the bipolaron ground state as a function of the electron-phonon coupling constant and of the bare-electronic bandwidth is discussed and the bipolaron phase diagram is presented. A clear crossover from the large-bipolaron to the small-bipolaron regime takes place.
TL;DR: In this paper, the response of a large bipolaron to an external electromagnetic field is derived in the framework of the memory-function approach, where the bipolaron optical absorption spectrum consists of a series of relatively narrow peaks.
Abstract: The absorption of large bipolarons is investigated using the path-integral method. The response of a bipolaron to an external electromagnetic field is derived in the framework of the memory-function approach. The bipolaron optical absorption spectrum consists of a series of relatively narrow peaks. The peculiarities of the bipolaron optical absorption as a function of the frequency of the electromagnetic field may be attributed to the transitions involving relaxed excited states and scattering states of a bipolaron.
TL;DR: In this paper, the Pariser-Parr-Pople model for poly (phenylene vinylene) (PPV) was utilized to expose the subtle electronic and lattice structures of PPV.
Abstract: We utilize the Pariser–Parr–Pople model for poly (phenylene vinylene) (PPV) and take into account the realistic lattice structures of PPV considering the electron–electron and the electron–phonon interactions so that we can expose the subtle electronic and the lattice structures of PPV We find that the lattice deformation is mainly in the vinylene group, and we also discuss the spin density waves of polaron and the charge density waves of polaron and bipolaron The spin density wave is modulated by charge density wave The competition between polaron and bipolaron are studied We find that when the electron–phonon interaction λ 021, v favors the bipolaron
TL;DR: In this paper, a series of I2-doped poly[2-buthoxy-5methoxy-1,4-phenylenevinylene] (PBMPV) conducting polymers were studied.
TL;DR: In this article, the authors explore the properties of the bipolaron in a simplified one-dimensional (1D) Frohlich-Hubbard model in comparison with results in the Holstein-Holstein model, where the former has a longer range electron-phonon interaction.
Abstract: We explore the properties of the bipolaron in a simplified one-dimensional (1D) Frohlich–Hubbard model in comparison with results in the Holstein–Hubbard model, where the former has a longer range electron–phonon interaction. We solve the model with an exact diagonalization method on an infinite 1D lattice. In the strong coupling regime, the effective mass of the Frohlich bipolaron is much smaller than the Holstein bipolaron mass. In contrast to the Holstein model where only a singlet bipolaron is bound, in the Frohlich model, a triplet bipolaron can also form a bound state.
TL;DR: In this article, the authors used the Su, Schrieffer and Heeger (SSH) Hamiltonian extended to include on-site and nearest-neighbor Coulomb repulsion terms, and the damping was introduced by adding a phenomenological friction term in the lattice equation of motion.
Abstract: In order to estimate the mobility of an acoustic bipolaron in a typical non-degenerated polymer such as polydiacetylene (PDA), which can be regarded as a one-dimensional electron-lattice system, the dynamical behavior of an acoustic bipolaron in an external electric field is numerically investigated in the presence of a phenomenological damping of the lattice motion. In these simulations, we use the Su, Schrieffer and Heeger (SSH) Hamiltonian extended to include on-site and nearest-neighbor Coulomb repulsion terms, and the damping is introduced by adding a phenomenological friction term in the lattice equation of motion. The repulsive interactions are treated within the unrestricted time-dependent Hartree–Fock approximation. It is confirmed that the mobility of a bipolaron increases with the increase of the repulsion strength and that the relevant quantity determining the mobility of a bipolaron is its width as far as the phenomenological friction is kept fixed. The mobility of a bipolaron is compared wit...
TL;DR: In this article, a transport theory for high temperature superconductors is proposed and supported with extensive calculations, and the binding energies of Cooper-pairs both in a weak Fermion system and for a spinless boson are calculated.
Abstract: A new transport theory for high temperature superconductors is propos#ed and supported with extensive calculations. The Cooper-pairs while remaining on a pseudo Fermi surface behave like weak Fermions. When the Cooper-pairs become free they behave as spinless bosons. The polarons and bipolarons are highly localized quantum states with virtually no gap existing in between them and are considered as bosons with nondegenerate spining. The binding energies of Cooper-pairs both in a weak Fermion system and for a spinless boson are calculated. A kind of a semimetallic softening transition is responsible for high temperature superconductivity and is temperature independent. There is no electron–phonon coupling for high temperature superconductors. High temperature ceramic superconductors before the onset of superconductivity are found to be Mott–Hubbard dielectric insulating materials.
TL;DR: In this paper, a theoretical analysis of polarons and bipolarons in poly(OSO) is presented, where both syn and doubly oxidized oligomers, whose all rings are linked at syn or anti-forms, are used as model.
TL;DR: In this article, the authors suggest theoretical interpretation to a long-debated discussion on a nature of the intrinsic "green" luminescence observed in many ABO3 perovskites, using the Intermediate Neglect of the Differential Overlap combined with the Large Unit Cell periodic model.
Abstract: We suggest theoretical interpretation to a long-debated discussion on a nature of the intrinsic “green” luminescence observed in many ABO3 perovskites. For this purpose we performed quantum chemical calculations using the Intermediate Neglect of the Differential Overlap combined with the Large Unit Cell periodic model. Triplet exciton which is very likely responsible for the “green” luminescence is shown to be in a good approximation a pair of nearest Jahn-Teller electron and hole polarons (a bipolaron).
TL;DR: In this paper, the authors examined the existing theory of bipolaron electrical conductivity by performing a systematical study on the formation energies of the defects in boron carbide using a state-of-the-art ab-initio electronic structure method.
Abstract: The electronic structure of a crystalline boron carbide has an energy forbidden gap of ∼ 3 eV and is hence a good insulator. But, on the other hand, the electrical conductivity of boron carbide is measurable. It is therefore believed that the defects formation in boron carbide is responsible for its electrical conductivity and a theory of hopping conduction of bipolaron through localized defects were developed, accordingly. Although the bipolaron electrical conductivity model does not rely on any specific type of defect, the bipolaron formation in boron carbide is believed to be a defective CBB intraicosahedral chain in connection with an B 11 C icosahedron. The current study examined the existing theory of bipolaron electrical conductivity by performing a systematical study on the formation energies of the defects in boron carbide using a state-of-the-art ab-initio electronic structure method. The studied defects cover a) stoichiometric variations of carbon concentration, b) missing boron atoms, and c) distribution of carbon atoms in the materials. It is found that the ground state of a fully carbonated boron carbide consists of B 11 C icosahedra connected by CBC intraicosahedral chains, i.e. consistent with the reported structural model of B 4 C . When carbon concentration is reduced, however, the population of CBC chain is found to be intact, while the population of B 11 C icosahedron is reduced by the replacements of B 12 icosahedron. This observation is fundamentally different from the existing model of boron-rich boron carbide. The localized states associated with missing boron atoms are identified and the electrical conductivity through these localized defects states is studied.
TL;DR: In this article, the out-of-plane resistance of BSCCO-2212 single crystals with T c 0 =91-93 K was measured in magnetic fields up to 50 T over a wide temperature range.
Abstract: The out-of-plane resistance of BSCCO-2212 single crystals with T c0 =91–93 K has been measured in magnetic fields up to 50 T over a wide temperature range. The results are characterized by a positive linear magnetoresistance in the superconducting state and a negative linear magnetoresistance in the normal state. We show that the zero-field normal state c -axis resistance, the negative linear normal state magnetoresistance and the divergent upper critical field, H c2 , may be understood within the framework of the bipolaron theory of superconductivity.
TL;DR: In this article, a model Hamiltonian based on tight-binding approximation is presented, and the critical temperature of superconductivity T c is not related to the reduced mass of the positive and negative ions.
Abstract: A model Hamiltonian has been presented based on the tight-binding approximation. The critical temperature of superconductivity T c is not related to the reduced mass of the positive and negative ions. Therefore, there is no isotopic effect according to this model.
TL;DR: By exact diagonalization of a small cluster, this article showed that an interplay of electron-phonon and onsite electron-electron interactions results in "intersite" or "onsite" two-electrons (bipolaronic) solutions of the Holstein-Hubbard model, depending on the strengths of the interactions.
Abstract: By exact diagonalization of a small cluster, we show that an interplay of electron–phonon and onsite electron–electron interactions results in “intersite” or “onsite” two-electron (bipolaronic) solutions of the Holstein–Hubbard model, depending on the strengths of the interactions. On this basis, we argue that the decrease in the superconducting transition temperature of Bi-2212 compounds, following the enhancement of the electron–phonon interaction recently reported by Devereaux et al. [10] might be a consequence of the above mechanism, which leads to a transition from itinerant (intersite) to bound immobile (onsite) bipolarons, thus effectively reducing the number of superconducting carriers.
TL;DR: In this article, the dynamics of polarons and bipolarons were numerically studied using the Su-Schrieffer-Heeger model combined with the Pariser-Parr-Pople model extended to include an external electric field and a Brazovskii-Kirova (BK) symmetry breaking interaction.
01 Jan 2001
TL;DR: In this article, the authors attributed the softening bipolaron properties to the singlets being formed amongst four-fold degenerate icosahedral orbitals, a type of bipolaron that softens the symmetry-breaking vibrational modes to which they are coupled.
Abstract: Holes in boron carbides pair to form singlet bipolarons on twelve-atom icosahedral structures. Transport occurs by carriers hopping between icosahedra. However, the optical conductivity and the Seebeck coefficients differ qualitatively from expectations for small bipolarons. These novel features are attributed to the singlets being formed amongst four-fold degenerate icosahedral orbitals. In particular, carriers in highly degenerate states can form a novel type of bipolaron, a “softening bipolaron.” Distinctive features of softening bipolarons result from singlets’ softening the symmetry-breaking vibrational modes to which they are coupled.
TL;DR: In this article, a non-rigid bipolaron model modified with the correction of the pseudogap effect was used to study the temperature dependence of the Hall coefficient in the normal state of several typical high-Tc superconductors (e.g. YBa2Cu3O7-δ).
Abstract: Using a non-rigid bipolaron model modified with the correction of the pseudogap effect, we have studied the temperature (T) dependence of the Hall coefficient (RH) in the normal state of several typical high-Tc superconductors (e.g. YBa2Cu3O7-δ). The results are in agreement with experiments. It is suggested that the positive maximum in the RH-T diagram corresponds to a point near the bottom of the pseudogap. We have also found that for these high-Tc superconductors, W-T can be scaled into a single universal function form, where W is the bandwidth of the non-rigid bipolaron system.
TL;DR: In this paper, two distinct energy scales, the d-wave superconducting order parameter and charge segregation in the form of stripes in cuprates, are unified in the framework of the bipolaron theory.
Abstract: The Frohlich electron-phonon interaction in cuprates and other charge-transfer oxides is shown to be much stronger than any magnetic interaction. The polaron shift due to the Frohlich interaction of about 1 eV suggests that carriers are smali (bi)polarons at all temperatures and dopings, in agreement with the oxygen isotope effect on the carrier mass, optical conductivity and other experimental observations. Two distinct energy scales, the d-wave superconducting order parameter and charge segregation in the form of stripes in cuprates, are unified in the framework of the bipolaron theory as a result of the formation of mobile bipolarons in the normal state and their Bose-Einstein condensation. Within the theory both the d-wave superconducting order parameter and the striped charge distribution result from the bipolaron (centre-of-mass) energy band dispersion rather than from any particular interaction.