Showing papers on "Bipolaron published in 2013"

Multiple reduction of 2,5-bis(borolyl)thiophene: isolation of a negative bipolaron by comproportionation.

Abstract: The 2,5-bis(borolyl)thiophene 2, a conjugated acceptor-π-acceptor system, can be reduced to the monoradical anion [2](.-) , the dianion [2](2-) , and the tetraanion [2](4-) . The dianion [2](2-) was also prepared by a comproportionation reaction and features an absorption maximum in the near-IR region (λmax =800 nm), which is characteristic of a bipolaron with a quinoidal structure.

Effects of temperature and electric field induced phase transitions on the dynamics of polarons and bipolarons

Abstract: The stability of charge carriers in conjugated polymers is investigated in terms of a nonadiabatic evolution method by using an extended version of the Su–Schrieffer–Heeger (SSH) model that includes the effects of an external electric field and temperature. On the basis of this physical picture, different patterns of applied electric field and temperature dependence of polaron and bipolaron kinematics as well as the transitions between different regimes are found. Phase transitions from subsonic to supersonic velocities are also discussed in terms of the system conditions. We were able to describe at which thermal regime each quasi-particle loses its stability and also to determine under which circumstances do the electric field and temperature rise or dampen its motion. The results indicate that thermal effects on polaron and bipolaron stability may provide guidance for improving the charge carrier conduction in organic optoelectronic devices.

Bipolarons and multipolarons consisting of impurity atoms in a Bose-Einstein condensate

Abstract: The variational Feynman formalism for the polaron, extended to an all-coupling treatment of bipolarons, is applied for two impurity atoms in a Bose-Einstein condensate. This shows that if the polaronic coupling strength is large enough, the impurities will form a bound state (the bipolaron). As a function of the mutual repulsion between the impurities, two types of bipolaron are distinguished: a tightly bound bipolaron at weak repulsion and a dumbbell bipolaron at strong repulsion. Apart from the binding energy, the evolution of the bipolaron radius and its effective mass are also examined as a function of the strength of the repulsive interaction between the impurities and of the polaronic coupling strength. We then apply the strong-coupling formalism to multiple-impurity atoms in a condensate, which leads to the prediction of multipolaron formation in the strong-coupling regime. The results of the two formalisms are compared for two impurities in a condensate, which results in a general qualitative agreement and a quantitative agreement at strong coupling. Typically, the system of impurity atoms in a Bose-Einstein condensate is expected to exhibit the polaronic weak-coupling regime. However, the polaronic coupling strength is, in principle, tunable with a Feshbach resonance.

Excess dielectron in an ionic liquid as a dynamic bipolaron.

Abstract: We report an ab initio molecular dynamics simulation study on the accommodation of a dielectron in a pyridinium ionic liquid in both the singlet and triplet state. In contrast to water and liquid ammonia, a dielectron does not prefer to reside in cavity-shaped structures in the ionic liquid. Instead, it prefers to be distributed over more cations, with long-lived diffuse and short-lived localized distributions, and with a triplet ground state and a low-lying, open-shell singlet excited state. The two electrons evolve nonsynchronously in both states via a diffuse-versus-localized interconversion mechanism that features a dynamic bipolaron with a modest mobility, slightly lower than a hydrated electron. This work presents the first detailed study on the structures and dynamics of a dielectron in ionic liquids.

Translation invariant theory of polaron (bipolaron) and the problem of quantizing near the classical solution

Abstract: A physical interpretation of translation-invariant polarons and bipolarons is presented, some results of their existence are discussed. Consideration is given to the problem of quantization in the vicinity of the classical solution in the quantum field theory. The lowest variational estimate is obtained for the bipolaron energy E(η) with E(0) = -0.440636α2, where α is a constant of electron-phonon coupling, η is a parameter of ion binding.

Dynamical study of impurity effects on bipolaron-bipolaron and bipolaron-polaron scattering in conjugated polymers.

Abstract: Combining the one-dimensional tight-binding Su–Schrieffer–Heeger (SSH) model and the extended Hubbard model (EHM), the scattering of two oppositely charged bipolarons and a bipolaron–polaron pair is investigated under the influence of impurity effects using a nonadiabatic evolution method. These novel results for bipolarons show that the oppositely charged quasi-particles scatter into a mixed state composed of bipolarons and excitons. The excitation yield depends sensitively on the strength of the applied electric field. In the presence of an impurity, the critical electric field regime for formation of a state composed by bipolarons and excitons is increased. Additionally, we were able to obtain critical values of electric fields that played the role of drastically modifying the system dynamics. These facts suggest that the scattering between bipolarons and a bipolaron–polaron pair in the presence of impurities is crucial for the understanding of electroluminescence in optoelectronics devices, such as po...

Symmetry of Bipolaron Bound States for Small Coulomb Repulsion

Abstract: We consider the bipolaron in the Pekar–Tomasevich approximation and address the question whether the ground state is spherically symmetric or not. Numerical analysis has, so far, not completely settled the question. Our contribution is to prove rigorously that the ground state remains spherical for small values of the electron-electron Coulomb repulsion.

Possibility of a (bi)polaron high-temperature superconductivity in Poly A/ Poly T DNA duplexes

Abstract: Dynamical equations for a polaron and bipolaron in a DNA duplex are obtained on the basis of Holstein-Hubbard Hamiltonian. It is shown that in a PolyA/PolyT duplex especially stable is a bipolaron state in which holes are localized on different chains of the duplex. With the use of the polaron and bipolaron free energy, the temperature of bipolaron decay is found to be T_d \approx 350 K which can serve as an approximate estimate of the superconducting transition temperature. The way of constructing superconducting nanowires on the basis of DNA is suggested.

On-site and inter-site electron―phonon interaction in 2D Hubbard model

Abstract: An exact diagonalization study of Hubbard model having on-site electron–phonon interaction and inter-site phonon mediated interaction shows that on-site electron–phonon interaction generates an effective attraction between electrons which eventually forms on-site bipolaron. On the other hand, phonon mediated interaction between electrons at nearest-neighbor sites is suppressed by on-site Coulomb repulsion initially, but at certain larger strength this interaction strengthen the on-site electron–phonon interaction. There is also formation of neighboring-site bipolaron. Results show that interplay between electron–electron interaction and electron–phonon interaction is very much relevant.

Translation invariant theory of polaron (bipolaron) and the problem of quantizing near the classical solution

Abstract: A physical interpretation of translation-invariant polarons and bipolarons is presented, some results of their existence are discussed. Consideration is given to the problem of quantization in the vicinity of the classical solution in the quantum field theory. The lowest variational estimate is obtained for the bipolaron energy E(\eta) with E(0)=-0,440636\alpha^2, where \alpha is a constant of electron-phonon coupling, \eta is a parameter of ion binding.

Strong-coupling theory of high temperature superconductivity

Abstract: The observation of high-temperature superconductivity in complex layered cuprates by Bednorz and Muller must now rate as one of the greatest experimental discoveries of the last century. Identifying and understanding the microscopic origin of high-temperature superconductivity stands as one of the greatest theoretical challenges of this century. These lectures describe an approach, based on the extension of the BCS theory to the strong-coupling regime with small polarons and bipolarons. We discuss the canonical Migdal-Eliashberg theory of strongly coupled electrons and phonons (or any bosons) and its breakdown in the strong-coupling regime. Then, starting from the dynamic properties of a single polaron, we show how the multi-polaron problem is reduced to a weakly interacting charged Bose gas. Finally we discuss a few recent applications of the bipolaron theory to cuprates, in particular the essential interaction in oxides, the parameter-free expression for the superconducting critical temperature, upper c...

Local Lattice Distortion in Superconducting Cuprates Studied by XAS

Abstract: In-plane local distortion in cuprates characterized by strong doping and temperature dependence is a signature of polaron (bipolaron). The nature of dynamic lattice response in relation to the onset of superconductivity was probed by polarized x-ray absorption spectroscopy (XAS), for pure and magnetic impurity-doped MxLa1.85Sr0.15Cu1-xO4 (M=Mn, Ni, Co, x<0.05) single crystals grown by a TSFZ method. The results confirm that the dynamical anti Jahn-Teller (AJT) distorted domains maximize at Tdmax~Tconset, which is described by the disappearance of the Cu-O(1) bond alternation upon the completion of phase coherence (onset of superconductivity). In contrast to the Ni and Co dopings which quickly suppress superconductivity by magnetic pair breaking, the Mn doping shows sustainable superconductivity i.e., a constant Tdmax and Tconset upto x=0.03 with the presence of dynamical AJT distortion. The results suggest that the dynamical AJT type distortion with either Q2 or pseudo JT symmetry extending over the basal plane could be a prerequisite for high temperature superconductivity.

Reduction and transition in thermal activation for charge transport through electrolyte-gated polythiophene thin films with high hole densities

Abstract: We investigate the activation energy for electrical charge transport through a polythiophene thin film. The results are particularly obtained under high hole density (5.9 × 1020 holes/cm3) through electrolyte-gating and are compared with previous results attained from the same polymer film but with low induced carrier densities. Comparative analyses show that introducing high hole densities into the polymer results in (i) suppression of the transport activation energy and (ii) reduction of the temperature that yields transition in the activation energy. These results are explained based on the formation of bipolaron states, which are generated at such high hole densities.

The normal state properties of nano-sized CoFe{sub 2}O{sub 4} added Bi-based superconductors in bipolaron model

Abstract: The effect of nano-sized CoFe2O4 particles (10 nm in diameter) addition on the structure and the normal state transport properties of polycrystalline Bi-based superconductors were systematically studied The additional amount, x wt%, of CoFe2O4 in this case varied from 00 to 1 wt% of the total mass of the sample Phase analysis by X-ray diffraction (XRD) and Electrical resistance as a function of temperature, ρ(T) were carried out Nano-sized particles addition modifies the electrical behavior of the normal state with increasing the CoFe2O4 concentration The bipolaron model can explain properly the normal state resistivity of the samples

A Study on the Sign Inversion Behavior of Organic Magnetoresistance

Abstract: We study the sign inversion behavior of organic magnetoresistance (OMR) in organic thin-film devices, to elucidate the mechanisms governing the well-known OMR phenomenon. From the combination of a percolation theory with a magnetic field modulated bipolaron mechanism, we derive a model that accounts for OMR sign inversion (OMRSI) behavior. It exhibits how an applied magnetic field acts together with other factors (temperature, bias, and film thickness) on the device current. Under the framework of space-charge-limited current, we reproduce two kinds of OMRSI behavior. In the end, we discuss the influence of hyperfine field on OMRSI lines.

Bipolaron by inter-site electron-phonon interaction

Abstract: An exact diagonalization calculation of the Hubbard model with inter-site electron-phonon interaction on a 2D square cluster shows that inter-site electron-phonon interaction effectively creates on-site (S0) bipolaron. There is also formation of neighboring site (S1) bipolaron. Entropy calculation shows that system goes into more ordered state with the formation of these self-trapped bipolaron.

Influence of bipolaron density on the transport properties of thermalized organic conductors

Abstract: We investigate the influence of the number of bipolarons and temperature on the transport mechanism of doped conducting polymer under the action of an electric field. To do so, a version of the one-dimensional Su-Schrieffer-Heeger model, modified to include thermal effects, impurities, and an external electric field, is used. It is observed that the interaction between the bipolarons is of fundamental importance to the correct description of the system dynamics. For higher densities, the overall systems mobility tends to increase. This increase is found to arise from the two different, yet correlated, processes of carrier untrapping and velocity gain. Temperature is also observed to play a major role in increasing the mobility of the charge carriers. Hence, for the first time in the literature, it is possible to estimate bipolaron responses to these stimuli, in agreement with experimental evidence. © 2013 Wiley Periodicals, Inc.

Study of the optical properties of Zinc doped Polypyrrole

Abstract: Polypyrrole is a well known conducting polymer whose electrical properties have been studied deeply. Amongst the various ways of polymerizing Pyrrole, chemical route was chosen for the present study. Ferric chloride was chosen as an oxidizing agent. Zinc was found to be an appropriate dopant. Zinc belongs to group IIB with atomic number 30 having electronic configuration [Ar] 3d10 4s2. An expected substitution of Zn with Nitrogen of Pyrrole would leave a single electron with Zn after bond formation, creating a probability of polaron and bipolaron band formations. Characteristic peak around 450nm is observed in all the samples confirming the formation of polypyrrole. Band gap of samples are found in semiconducting range. Apart from this, some of the important optical constants such as absorption coefficient, extinction coefficient, refractive index and optical conductivity have also been evaluated for all the samples and have been studied as a function of energy. The work presents the optical properties of this conducting polymer for making it a suitable candidate for optoelectronic devises.

Distribution function of charge carriers in electron-phonon systems with spontaneously broken translational symmetry and 'vertical dispersion' in ARPES spectra of cuprates

Abstract: We show that spontaneous breaking of the translational symmetry (SBTS) of strongly interacting electron and phonon fields acknowledged in lightly doped cuprates to interpret broad bands in their ARPES and optical conductivity spectra influences the carrier distribution function as charge carrier momentum is no longer certain. We develop appropriate distribution and use it to calculate the carrier concentration in different states in essentially doped systems with SBTS. This allows studying doping and temperature evolution of ARPES spectra of such systems. We reveal that at increasing both temperature or doping polarons concede dominance to bipolarons that results in a shift of the broad band maximum to higher binding energy as it occurs in ARPES spectra of cuprates. At further doping delocalized carriers appear and coexist with bipolarons. We show that intimate property of systems with SBTS is partition of the momentum space between autolocalized and delocalized carriers caused by Pauli exclusion rule. We demonstrate that in ARPES spectra such partition manifests itself as `vertical dispersion` or waterfalls universally observed in all the cuprates at essential doping and in undoped parent compounds. Thus, studying the carriers distribution in systems where electron-phonon interaction breaks the translation symmetry leads simultaneously to both types of waterfalls observed in significantly doped and undoped cuprates. The article also represents the methods developed to calculate the band in ARPES spectrum caused by bipolaron photodissociation and the bipolaron binding energy and radius using coherent states for the phonon field characterization.

Comments to a series of works by V.K. Mukhomorov on the theory of a continuum polaron and two-center bipolaron (axially symmetrical quasimolecular dimer)

Abstract: Some critical remarks are made regarding a series of works by V.K. Mukhomorov dealing with polarons and oscillatory and rotational spectrum of a large-radius bipolaron near a subsidiary minimum corresponding to its two-center configuration. It is shown that V.K. Mukhomorov's conclusion that by varying the bipolaron functional one should look for a constrained rather than absolute minimum is erroneous. Consideration of interelectronic correlations corresponding to a direct dependence of the wave function of the studied system on the distance between the electrons does not break the virial theorem. In the strong coupling limit the virial theorem holds true for both one-center and two-center bipolaron states, the latter representing a subsidiary minimum which arises due to a choice of an insufficiently versatile variation function.