Showing papers on "Bipolaron published in 2006"

Fermi-level pinning at conjugated polymer interfaces

Abstract: Photoelectron spectroscopy has been used to map out energy level alignment of conjugated polymers at various organic-organic and hybrid interfaces. Specifically, we have investigated the hole-injection interface between the substrate and light-emitting polymer. Two different alignment regimes have been observed: (i) Vacuum-level alignment, which corresponds to the lack of vacuum-level offsets (Schottky–Mott limit) and (ii) Fermi-level pinning, where the substrate Fermi level and the positive polaronic level of the polymer align. The observation is rationalized in terms of spontaneous charge transfer whenever the substrate Fermi level exceeds the positive polaron/bipolaron formation energy per particle. The charge transfer leads to the formation of an interfacial dipole, as large as 2.1 eV.

Theoretical study of long oligothiophene dications: bipolaron vs polaron pair vs triplet state.

Abstract: A series of oligothiophene dications (from the sexithiophene dication to the 50-mer oligothiophene dication, nT2+, n = 6-50) were studied. Density functional theory (DFT) at the B3LYP/6-31G(d) level and, in some cases, also at BLYP/6-31Gd, was applied to study the singlet and triplet states of the whole series. We found that the singlet state is the ground state for all oligothiophene dications up to the 20-mer, and that the singlet and triplet states are degenerate for longer oligomers. Thus, the triplet state is never a pure ground state for these dications. We found that, for short oligothiophenes dication (e.g., 6T2+), the bipolaron state is the more important state, with only a small contribution made by the polaron pair state. For medium size oligothiophene dications (e.g., 14T2+), both the bipolaron state and the polaron-pair state contribute to the electronic structure. Finally, in long oligothiophene dications, such as 30T2+ and 50T2+, the contribution from the polaron pair state becomes dominant, and these molecules can be considered as consisting of two independent cation radicals or a polaron pair. Results from isodesmic reactions show that the stability of oligothiophene cation radicals over dications is inversely proportional to chain length. Small oligothiophene dications (n = 6-12) were studied at the CASSCF(m,m)/6-31G(d) (m = 4, 6, and 10) level. The major conclusions of this paper regarding the relative energy of the singlet state versus the triplet state and regarding the relative stability of the bipolaron versus the polaron pair were also supported by CASSCF calculations.

Dynamical evolution of polaron to bipolaron in conjugated polymers

Abstract: We investigate the insertion of holes in conjugated polymers bearing polarons. We use a Su-Schrieffer-Heeger model modified to include electron-electron interactions via an extended Hubbard Hamiltonian, a Brazovskii-Kirova symmetry breaking term, and an external electric field. We study the dynamics performing numerical calculations within the time-dependent unrestricted Hartree-Fock approximation. We find that adding a hole in a conducting polymer bearing a single positively charged polaron leads to the direct transition of polaron to bipolaron state. The transition produced is a single-polaron to biplaron transition whose excitation spectrum explains the experimental data. The competing mechanism of two polarons merging to form a bipolaron is not observed. We also find that depending on how fast the hole is inserted, a structure that contains a bipolaron coupled to a breather is created. The bipolaron-breather pair can be decoupled under the action of an external electric field. We determine the value of the critical electric field to untrap the bipolaron from the breather.

The "crab" bipolaron as a possible route to room temperature superconductivity

Abstract: We study a lattice bipolaron on a staggered triangular ladder and triangular and hexagonal lattices with both long-range electron-phonon interaction and strong Coulomb repulsion using a novel continuous-time quantum Monte-Carlo (CTQMC) algorithm extended to the Coulomb-Frohlich model with two particles. The algorithm is preceded by an exact integration over phonon degrees of freedom, and as such is extremely efficient. The bipolaron effective mass and bipolaron radius are computed. Lattice bipolarons on such lattices have a novel crablike motion, and are small but very light in a wide range of parameters, which leads to a high Bose-Einstein condensation temperature. We discuss the relevance of our results with current experiments on cuprate high-temperature superconductors and propose a route to room temperature superconductivity.

Bipolarons or Polaron Pairs in Conducting Polymers: Equilibrium and Kinetics* , **

Abstract: Equilibrium and kinetics are compared for conjugated polymers, supposing the charged states are either polarons (Ps) and bipolarons (BPs) or polarons and polaron pairs (PPs). For low concentrations equilibrium and kinetics are virtually indistinguishable. Both are essentially different for high concentrations due to the fact that the extension of the bipolaron is almost the same as that of one polaron whereas the polaron pair has the extension of two polarons. As a result, the polaron concentration, i.e. the spin concentration, shows for a system with bipolarons a maximum and decreases for continuing increase of the potential. This dependence corresponds to results of ESR spectroelectrochemistry. In contrast, for the system with PPs both P and PP concentrations saturate at high potentials. Thus, the PP model is only reasonable if an additional subsequent second oxidation step is possible. The kinetics of the formation and dissociation of PPs or BPs, is essentially the same for concentrations that can be achieved in accumulation layers in organic MOS devices. The estimated relaxation times point to the formation and dissociation of either BPs or PPs as processes that can either cause or influence hysteresis effects in polymeric MOS capacitors, which can be described by an interface charge, depending on the sweep direction in the capacitance-voltage measurements.

Electrochemical, Magnetic, and Electrical Properties of α,ω-Capped Sexithiophene Films. 2. Conduction in Sexithiophenes with α,ω-Aryl-Extended Conjugation

Abstract: Thin films of tetrahexylsexithiophenes, fluorenonyl- and fluorenyl-protected at the terminal α,ω-positions, were investigated by cyclic voltammetry, electrochemical quartz crystal microbalance analysis, in situ ESR, and in situ conductivity. Reversible oxidation is composed of three separate steps, two one-electron processes and a further multielectron process. ESR indicates strong magnetic dimerization for the one-electron-oxidized species. Conductivity is redox type at the cation−dication (polaron−bipolaron) state and metal-like at doping levels higher than the bipolaron with a 20-fold increase at full oxidation. Hexyl-substituted α,ω-capped octathiophene, decathiophene, and dodecathiophene films have been similarly investigated to compare the effects of fluorenonyl and fluorenyl extension of sexithiophene with those given by additional thiophene rings. The conductivity, which increases progressively (by 3 orders of magnitude) with the oligothiophene chain length, is metal-like. In dodecathiophene the d...

Theoretical investigation of static characterization on nonlinear elementary excitations in trans-polyacetylene.

Abstract: Semiempirical quantum chemical studies on neutral and positively charged H(CH) n H homologues have been performed for systems with n up to 101, where different kinds of nonlinear excitation are found with increasing chain length. The Pariser-Parr-Pople (PPP) model has been employed and solved with the density matrix renormalization group (DMRG) method. The geometrical and electronic distortions induced by defects are obtained and compared with previous theoretical work, indicating that an adequate account of the electron correlation is essential for describing such systems. The structural distortion of a charged soliton (half-width calculated as L = 13) is shown to be more extended than that of a neutral soliton (L = 6); the geometrical distortion is even more extended in a polaron. In bipolarons, our calculations show that the coupling of the soliton-antisoliton pair might be longer ranged than expected. The phase transition from a bipolaron to a separated soliton-antisoliton pair occurs when n is close to 100.

Raman study of order-disorder transition of bipolarons in Ti4O7

Abstract: We have studied the phase transition of bipolarons in Ti4O7 by means of Raman spectroscopy. It is found that on warming across the phase transition, the Raman spectrum shows distinct changes, i.e., broadening of the linewidths, decrease in the phonon frequencies, and appearance of the fine structure. These features indicate the existence of structural disorder above the transition temperature, and hence are consisitent with a model that the phase transition is an order-disorder transition of bipolarons. The mechanism of the line broadening is discussed in relation to the bipolaron motion in the disordered phase. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Stability of bipolarons, electron-electron correlations, the variational principle, and the virial theorem

Abstract: For bipolaron states, a relation is established between the variational principle and the virial theorem optimizing the electronic wave functions. It is shown that a bipolaron one-center state is unstable under any conditions that do not violate basic physical principles. It is established that a stage-by-stage increase in the flexibility of the electronic wave function due to the electron-electron correlations does not stabilize a one-center bipolaron. It is argued that the results of calculations based on the one-center bipolaron model are incorrect.

Changes in electrochemical and optical properties of oligoalkylthiophene film induced by bipolaron formation.

Abstract: An unusual electrochemical response is found for oligoalkylthiophene (13T) films in acetonitrile. A pristine 13T film exhibits a stable redox wave in cyclic voltammogram when the anodic potential limit is below 0.6 V, whereas the redox peaks shift clearly to the negative side by ca. 0.1 V once the 13T film experiences a potential beyond 0.7 V. After this electrochemical stimulus is imposed on a pristine 13T film, the absorption and emission spectra change their features. These findings, along with the measurements with varying thicknesses of 13T films, suggest that a pristine 13T film is a mixture of two phases to be oxidized at separate potentials and that one phase can be switched to the other by the potential stimulation. A plausible model proposed for explaining these observations assumes that a key parameter characteristic of the two phases is a conformation of 13T molecules in the solid state and a change in conformation, leading to the shift of the redox potentials and the change of optical properties, is induced by electrochemical generation of bipolarons on the half units of the oligomers in the pristine 13T film.

Stability of Formation of Large Bipolaron: Nonrelativistic Quantum Field Theory

Abstract: We are concerned with the stability of formation of large bipolaron in a 3-dimensional (3D) crystal. This problem is considered in the framework of nonrelativistic quantum field theory. Thus, the Hamiltonian formalism, as Froehlich introduced, is employed to describe the bipolaron. We approach the problem by characterizing some sufficient or necessary conditions for the bipolaron being stable. This paper gives a full detail of the author's talks at ESI, RIMS, and St. Petersburg State Univ. in 2005.

Theory of magnetoresistance based on variable-range hopping in the presence of Hubbard interaction and spin-dynamics

Abstract: We develop a theory of magnetoresistance based on variable-range hopping. An exponentially large, low-field and necessarily positive magnetoresistance effect is predicted in the presence of Hubbard interaction and spin-dynamics under certain conditions. The theory was developed with the recently discovered organic magnetoresistance in mind. To account for the experimental observation that the organic magnetoresistance effect can also be negative, we tentatively amend the theory with a mechanism of bipolaron formation.

Polaron Stability Under Collision With Different Defects in Conjugated Polymers

Abstract: Polarons are charge carriers generated by doping or photo-excitation in conjugated polymers. The dynamics of polaron-soliton and polaron- bipolaron collisions in conjugated polymers is numerically studied using the Su-Schrieffer-Heeger model extended to include an external electric field, Coulomb interactions, and impurities interactions. The time-dependent unrestricted Hartree-Fock approximation is considered. The stability of polarons determines whether the charge transport by these structural defects are modified under these collisions. An energetic analysis determines whether the energy levels associated with solitons, polarons, and bipolarons change with collisions. We show that these structural defects have great stability in collisions. The bipolaron mobility does not change under polaron- bipolaron collisions. In contrast, we find that there is a great exchange of linear momentum in the collisions between solitons and polarons.

Electron Paramagnetic Resonance and High Temperature Superconductivity

Abstract: This lecture is dedicated to the discovery of the electron paramagnetic resonance (EPR) by professor E.K. Zavoisky here in the Kazan University 60 years ago. Today, I want to discuss the contribution made by the EPR method to the field of cuprate superconductivity. I shall start with the Orbach relaxation of Jahn–Teller (JT) ions, especially Ni3+, Pt3+, and Cu2+ [1]. This work was performed 40 years ago and relates to the problem of vibronic character of the ground state of the JT ions. This concept gave later an impulse to search for high −Tc superconductivity in cuprates. The next example will be the three-spin polaron detected by EPR in LSCO. Then I shall discuss the Jahn–Teller bipolaron concept derived from EPR, EXAFS, and neutron scattering. The bipolarons observed in lightly doped LSCO can be a good candidate for the elementary quasiparticle in superconducting cuprates. I would like to mention further the electronic phase separation detected on the microscopic level by the EPR measurements.

Ground state and polaron and bipolaron excited states in polydiacetylene

Abstract: The electronic properties of ground state and charged excited states of nondegenerate polydiacetylene were investigated by means of a tight-binding model. The parameters of the model were obtained by comparison of the experimental and other theoretical results. It was found that there is a stable dimerized structure of polydiacetylene in ground state and the doping induces the nonlinear excitations, such as polarons and bipolarons. In order to compare the stability of polaron and bipolaron, the creation energy and binding energy were separately defined. By neglecting the electron-electron Coulomb interaction, a bipolaron is more stable than two independent polarons.

The influence of magnetic field on the stability region of the bipolaron in high-Tc superconductors with the Van Hove scenario

Abstract: The properties of polarons and bipolarons are studied by the variational method taking into account the density of state for Van Hove singularity (V.H.S) in two-dimension (2D) in the presence of a perpendicular magnetic field. It should be noticed that the bipolaron stability region is relatively sensitive to the application of a magnetic field. In 2D, for ωc = 0.8 the value of αc = 1 is compared to αc = 1.6 for ωc = 0. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The bipolaron in the strong coupling limit

Abstract: The bipolaron are two electrons coupled to the elastic deformations of an ionic crystal. We study this system in the Frohlich approximation. If the Coulomb repulsion dominates, the lowest energy states are two well separated polarons. Otherwise the electrons form a bound pair. We prove the validity of the Pekar-Tomasevich energy functional in the strong coupling limit, yielding estimates on the coupling parameters for which the binding energy is strictly positive. Under the condition of a strictly positive binding energy we prove the existence of a ground state at fixed total momentum $P$, provided $P$ is not too large.

Bipolaron phase diagram of the 1D adiabatic Holstein-Hubbard model in the strong coupling limit

Abstract: We derive the bipolaron phase diagram of the one-dimensional adiabatic Holstein–Hubbard model in the strong coupling (small hopping) limit. We show the existence of a threshold value for the Coulomb interaction beyond which the ground state of the system consists of two free polarons (infinitely far apart from each other). This result, obtained by means of an exact perturbative expansion, cannot be reproduced by the usual exponential ansatz for the wave function.

Electronic Excitations and Metal-Insulator Transition in Poly(3-hexylthiophene) Organic Field-Effect Transistors

Abstract: We carry out a comprehensive theoretical and experimental study of charge injection in poly(3-hexylthiophene) (P3HT) to determine the most likely scenario for metal-insulator transition in this system. Wecalculate the optical-absorption frequencies corresponding to a polaron and a bipolaron lattice in P3HT. We also analyze the electronic excitations for three possible scenarios under which a first- or a second-order metal-insulator transition can occur in doped P3HT. These theoretical scenarios are compared with data from infrared absorption spectroscopy on P3HT thin-film field-effect transistors (FETs). Our measurements and theoretical predictions suggest that charge-induced localized states in P3HT FETs are bipolarons and that the highest doping level achieved in our experiments approaches that required for a first-order metal-insulator transition.

Lang-Firsov approaches to polaron physics: From variational methods to unbiased quantum Monte Carlo simulations

Abstract: We review variational and quantum Monte Carlo approaches based on (extended) Lang-Firsov transformations of the Hamiltonian. Derivations for one, two and many electrons are given, and results for the Holstein polaron, the Holstein-Hubbard bipolaron, and the spinless Holstein model at finite carrier densities are presented.