Showing papers on "Bipolaron published in 2004"
TL;DR: In this article, the morphology of electrosynthesized poly(3-methylthiophene) (P3MT) thin films was analyzed by atomic force microscopy and different film morphologies were found for different electrodeposition technique and solution temperature.
111 citations
PARC1
TL;DR: In this paper, the bias-stress effect is associated with bipolaron formation, based on the observation that the stressing rate is proportional to the square of the hole concentration, and the thermal dissociation rate is also deduced from the data.
Abstract: Studies of the kinetics of the bias-stress effect are reported for regioregular poly(thiophene) thin-film transistors. We associate the bias-stress effect with bipolaron formation, based on the observation that the stressing rate is proportional to the square of the hole concentration. The bipolaron formation rate is measured as a function of temperature, and the thermal dissociation rate is also deduced from the data. We discuss the hole capture process and suggest that tunneling through the Coulomb barrier dominates. The temperature dependence of bipolaron formation and recovery kinetics leads to a maximum in their formation at about $220\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and we also find other changes in the electrical properties near this temperature. A separate bias-stress effect is observed to operate at much longer time scales and might be associated with a different population of bipolarons.
97 citations
TL;DR: In this paper, the authors present a diagrammatic Monte Carlo study of the properties of the Hubbard-Holstein bipolaron on a two-dimensional square lattice, showing that the bipolaron system suffers a sharp transition from a state formed by two weakly bound light polarons to a heavy, strongly bound on-site bipolaron.
Abstract: We present a diagrammatic Monte Carlo study of the properties of the Hubbard-Holstein bipolaron on a two-dimensional square lattice. With a small Coulomb repulsion $U$ and with increasing electron-phonon interaction, and when reaching a value about two times smaller than the one corresponding to the transition of light polaron to heavy polaron, the system suffers a sharp transition from a state formed by two weakly bound light polarons to a heavy, strongly bound on-site bipolaron. Aside from this rather conventional bipolaron a new bipolaron state is found for large $U$ at intermediate and large electron-phonon coupling, corresponding to two polarons bound on nearest-neighbor sites. We discuss both the properties of the different bipolaron states and the transition from one state to another. We present a phase diagram in parameter space defined by the electron-phonon coupling and $U$. Our numerical method does not use any artificial approximation and can be easily modified to other bipolaron models with longer range electron-phonon and∕or electron-electron interaction.
77 citations
TL;DR: A new model that assumes a Gaussian distribution of energies related to molecular geometry fluctuations is suggested, and this model shows excellent agreement with the results of electrochemical oxidation of polypyrrole in quasiequilibrium conditions.
Abstract: We discuss the interpretation of usually broad oxidation peaks observed in electronically conducting polymers, in terms of the statistical distributions functions of polarons and bipolarons. The analysis is based on examining the chemical capacitance, that relates the change of concentration to a modification of the chemical potential of a given species, for different statistical models. We first review the standard models for single energy species that provide a nernstian dependence, and the limitations of these models are discussed. A new model that assumes a Gaussian distribution of energies related to molecular geometry fluctuations is suggested, and this model shows excellent agreement with the results of electrochemical oxidation of polypyrrole in quasiequilibrium conditions. From a fit of the data, it is found that the density of conjugated chain segments in polypyrrole, Ns≈1021 cm−3, shows a Gaussian distribution of half width σ≈170 meV, tentatively attributed to bipolaron formation energies.
58 citations
TL;DR: A quinoid molecule is proposed as a reliable structural and electronic model for dication species in doped oligothiophenes or for bipolaron charged defects in doping polythiophene.
Abstract: The UV/Vis, infrared absorption, and Raman scattering spectra of 3',4'-dibutyl-5,5"-bis(dicyanomethylene)-5,5"-dihydro-2,2':5',2"-terthiophene have been analyzed with the aid of density functional theory calculations. The compound exhibits a quinoid structure in its ground electronic state and presents an intramolecular charge transfer from the terthiophene moiety to the C(CN)2 groups. The molecular system therefore consists of an electron-deficient terthiophene backbone end-capped with electron-rich C(CN)2 groups. The molecule is characterized by a strong absorption in the red, due to the HOMO-->LUMO pi-pi* electronic transition of the terthiophene backbone that shifts hypsochromically on passing from the solid state to solution and with the polarity of the solvent. The analysis of the vibrational spectra confirms the structural conclusions and supports the existence of an intramolecular charge transfer. Vibrational spectra in several solvents and as a function of temperature have also been studied. Significant frequency upshifts of the vibrations involved in the pi-electron-conjugated pathway have been noticed upon solution in polar solvents and with the lowering of the temperature. Finally, we propose a quinoid molecule as a reliable structural and electronic model for dication species in doped oligothiophenes or for bipolaron charged defects in doped polythiophene.
45 citations
TL;DR: In this article, the potential-determining ions were used to establish an interfacial Galvani potential difference between the potentials of different thiophene oligomers in 1,2-dichloroethane.
Abstract: Heterogeneous electron transfer (HET) from an aqueous redox couple, Ce(III):Ce(IV) to 2,2‘:5‘,2‘ ‘-terthiophene (3T) in 1,2-dichloroethane was investigated using potential-determining ions to establish an interfacial Galvani potential difference. The reaction was found to be potential dependent, and different thiophene oligomers species were identified spectroelectrochemically depending on the interfacial potential and the concentration of 3T present in the organic phase. At low concentrations, sexithiophene and its bipolaron were found to be the main products. At higher concentrations an insoluble black polymer was precipitated at the interface and sexithiophene radical cations and radical cation dimers were found to be present in solution.
24 citations
TL;DR: In this article, a dynamical simulation in polymers shows photoinduced carrier fission where a positive carrier (polaron) is split into two carriers by photoexcitation One carrier is a negative polaron and the other is a positive bipolaron.
Abstract: Polarons and bipolarons are carriers in conducting polymers A dynamical simulation in polymers shows photoinduced carrier fission where a positive carrier (polaron) is split into two carriers by photoexcitation One carrier is a negative polaron and the other is a positive bipolaron This fission is an ultrafast process of photoinduced charge generation without the aid of an electric field with a relaxation time of $100\phantom{\rule{03em}{0ex}}\mathrm{fs}$
21 citations
TL;DR: In this article, the filling level dependence of the physical properties of MnCl 2 through the polystyrene (PS) matrix was explored and the assigned conjugated double bonds deduced from infrared (IR) analysis suggested the presence of polarons and/or bipolarons in the polymeric matrix.
Abstract: The filling level (FL) dependence of the physical properties of MnCl 2 through the polystyrene (PS) matrix was explored. The assigned conjugated double bonds deduced from infrared (IR) analysis suggested the presence of polarons and/or bipolarons in the polymeric matrix. The FL dependence of certain IR absorption peaks was correlated with the obtained physical parameters characterizing the other properties. The optical studies revealed the existence of two optical windows, the first one (325–365 nm) in the UV region and the second (365–578 nm) in the UV and VIS regions. X-ray diffraction spectra of two halos have been noticed indicating the amorphous feature of PS. The temperature dependence of the DC electrical conduction was interpreted using a model based on the phonon-assisted charge carrier hopping. The electron spin resonance spectra revealed an unresolved complicated signal for pure PS due to hyperfine interactions while for high FLs a Lorentzian like shape has been observed suggesting the Mn 2+ –Mn 2+ exchange interaction.
14 citations
TL;DR: In this article, a set of unitary transformations were used to decouple electrons and phonons in the strong-coupling limit of superconducting cuprates, and the Hall Lorenz number of the system was calculated to show that the Wiedemann-Franz law breaks down due to interference of the polaron and bipolaron contributions to heat flow.
Abstract: With the superconducting cuprates in mind, a set of unitary transformations was used to decouple electrons and phonons in the strong-coupling limit. While phonons remain almost unrenormalised, electrons are transformed into itinerent singlet and triplet bipolarons and thermally excited polarons. The triplet/singlet exchange energy and the binding energy of the bipolarons are thought to account for the spin and charge pseudogaps in the cuprates, respectively. We calculated the Hall Lorenz number of the system to show that the Wiedemann-Franz law breaks down due to the interference of the polaron and bipolaron contributions to heat flow. The model provides a quantitative fit to magnetotransport data in the cuprates. Furthermore we are able to extract the phonon component of the thermal conductivity with the use of experimental data and the model. Our results further validate the use of a charged Bose gas model to describe normal and superconducting properties of unconventional superconductors.
9 citations
TL;DR: In this paper, the bipolaron extension of the BCS theory to the strong-coupling regime could be a solution to the problem of cuprates, where the symmetry and space modulations of the order parameter are explained in the framework of bipolaron theory.
Abstract: The seminal work by Bardeen, Cooper, and Schrieffer taken further by Eliashberg to the intermediate coupling solved the problem of conventional superconductors about half a century ago. The Frohlich and Jahn–Teller electron–phonon interactions were identified as an essential piece of physics in all novel superconductors. The BCS theory provides a qualitatively correct description of some of them like magnesium diborade and doped fullerenes (if the polaron formation is taken into account). However, cuprates remain a problem. Here I show that the bipolaron extension of the BCS theory to the strong-coupling regime could be a solution. Low-energy physics in this regime is that of small “superlight” bipolarons, which are real-space mobile bosonic pairs dressed by phonons. The symmetry and space modulations of the order parameter are explained in the framework of the bipolaron theory. A d-wave Bose–Einstein condensate of bipolarons reveals itself as a checkerboard modulation of the hole density and of the gap below Tc.
8 citations
TL;DR: In this paper, the effect of various filling levels (FLs) of mixtures of two transition compounds (X)AgNO3 (10−X) FeCl3, on structural, electrical and magnetic properties of polystyrene (PS) films was investigated.
TL;DR: In this article, the stability region of two-dimensional magnetobipolarons in a parabolic quantum dot subject to a uniform magnetic field was investigated and the ground-state energy of the two dimensions was calculated.
Abstract: Using a variational procedure based on Lee-Low-Pines and Huybrechts canonical transformations, we study the stability region of two-dimensional bipolarons confined in a parabolic quantum dot, subject to a uniform magnetic field. In the framework of our approach, we calculate the ground-state energy for two-dimensional magnetobipolarons, together with the free polaron ground-state energy, by performing a self consistent calculation. We then obtain the binding energy for two-dimensional magnetobipolarons, in the usual way, to explore the properties of bipolaron formation in two-dimensional quantum dot structures. The stability region is found to be very sensitive to the confinement length of the parabolic potential and to the magnetic field strength, as well as to the material parameters $\alpha $
and $\eta $
. The stability region is also found to be remarkably enhanced by increasing the degree of spatial confinement and magnetic field. Our results are both in qualitative and quantitative agreement with those found in the literature.
TL;DR: The authors used Landau-Pekar variational method to investigate a strong-coupling singlet optical bipolaron in different configuration of quantum confinement, showing that confinement favors bipolaron formation in a quantum well.
Abstract: The authors used Landau-Pekar variational method to investigate a strong-coupling singlet optical bipolaron in different configuration of quantum confinement. Numerical and analytical results showed that when configuration changes from quantum dot and wire to well, confinement shows different effect on the formation of a bipolaron. In contrast to a bipolaron in a quantum dot or wire, the binding energy of a bipolaron in a quantum well increases with increasing confinement, indicating that confinement favors bipolaron formation in a quantum well.
TL;DR: The range of existence of a translation invariant continuum three-dimensional bipolaron in space of two variables: of electron-phonon coupling constant and the dielectric parameters of polar medium is investigated in this article.
Abstract: The range of existence of a translation invariant continuum three-dimensional bipolaron in space of two variables: of electron–phonon coupling constant and the dielectric parameters of polar medium is investigated The critical values of the electron–phonon coupling constant are found The possibility of existence of metastable bipolaron formations is scored On the basis of definition of bipolaron existence the requirement of existence of a bound condition in a bipolaron potential well is necessary It is shown, that use of a rigider requirement, than positivity of dissociation energy, namely presence of even one bound state narrows the range of existence of a bipolaron The criteria for a bound state to arise are found by solving an integral equation, which corresponds to a Schrodinger equation describing internal states of a bipolaron
TL;DR: In this paper, the possibility of spontaneous formation of periodical structure of polarons in the case of adiabatically strong coupling is considered, based on the assumption that two-electron bound formation of the bipolaron type exists in the system.
Abstract: The possibility of the spontaneous formation of periodical structure of polarons in the case of adiabatically strong coupling is considered. For metalammonia systems, the critical temperature and polaron concentration at which the spatially uniform temperature and polaron distributions may become spatially periodic are found. The theory is based on the assumption that two-electron bound formation of the bipolaron type exist in the system . A dispersion relation for the excitation of the uniformly distributed polaron system is derived, and conditions for sustained excitation are found. Tests for instability correlate with conditions where the periodical structures appear.
TL;DR: In this article, a Feynman path-integral variational approach combining with the average for the relative motion was applied to study the stability of bipolaron in a quantum dot.
Abstract: We apply a Feynman path-integral variational approach combining with the average for the relative motion to study the stability of bipolaron in a quantum dot. The binding energy is calculated in different parameters. We find that an optimum quantum potential favors the formation of bipolaron. Compared with other methods in literature, the present approach is better than Laudau–Pekar one in all coupling regime and full path-integral one in the strong coupling regime.
TL;DR: In this article, a single vortex in the charged Bose-gas (CBG) has a charged core and its profile different from the vortex in neutral and BCS superfluids, and the unusual resistive upper critical field, H c 2 (T ), of many cuprates and a few other unconventional superconductors is described as the BoseEinstein condensation field of preformed bosons-bipolarons.
Abstract: A single vortex in the charged Bose-gas (CBG) has a charged core and its profile different from the vortex in neutral and BCS superfluids Lower and upper critical fields of CBG are discussed The unusual resistive upper critical field, H c2 ( T ), of many cuprates and a few other unconventional superconductors is described as the Bose–Einstein condensation field of preformed bosons–bipolarons Its nonlinear temperature dependence follows from the scaling arguments Exceeding the Pauli paramagnetic limit is explained Controversy in the determination of H c2 ( T ) of cuprates from kinetic and thermodynamic measurements is addressed in the framework of the bipolaron theory
TL;DR: In this article, an electron paramagnetic resonance study of electron polarons in BaTiO3 and hole polarons (HP) was performed and their properties were determined by Jahn-Teller effects.
Abstract: An electron paramagnetic resonance study of electron polarons in BaTiO3 and hole polarons in BaTiO3 and KTaO3 is presented. Their properties are strongly determined by Jahn-Teller effects. There are three manifestations of electron polarons, including bipolarons. Hole polarons can exhibit static as well as dynamic Jahn-Teller coupling.
TL;DR: In this paper, superconductivity was analyzed in a body-centered orthorhombic crystal structure of A 3 C 60, where A=K, Rb, Cs and a model is proposed.
Abstract: Superconductivity is analyzed in a body-centered orthorhombic crystal structure of A 3 C 60 , where A=K, Rb, Cs and a model is proposed. The model is based on a characteristic chain of A elements that can be identified in the crystal structure and it is found that a bipolaron mechanism through these atoms, not the C 60 , is responsible for the occurrence of superconductivity. The later merely acts as the supporting skeleton. A qualitative discussion on the consequences of the model is also provided.
TL;DR: In this article, the infrared and Raman spectra of biphenyle, the aromatic A-phase dimer and a quinoidal B-phase polyparaphenylene at Density Functional Theory (DFT) level using a Becke-3 exchange and a Lee-Yang-Parr correlation functional (B3LYP).
Abstract: We have calculated infrared and Raman spectra of biphenyle, the aromatic A-phase dimer and of a quinoidal B-phase dimer of polyparaphenylene at Density Functional Theory (DFT) level using a Becke-3 exchange and a Lee-Yang-Parr correlation functional (B3LYP). We found in both types of spectra features typical for the two phases, which offer the possibility to detect the presence of a quinoidal chain segment in conducting samples of the polymer. Together with more sophisticated future calculations on longer oligomers and on higher level of calculation, it should be possible to gauge intensity ratios in the spectra theoretically so that bipolarons could be found and their width be estimated spectroscopically. This is due to the fact that usually only within a bipolaron the B-phase is fully formed, which is not the case in a singly charged polaron. Calculations on neutral model molecules, while bipolarons are doubly charged, can be used for the purpose because in bipolarons the two like charges are usually quite localized in the parts of the bipolaron where the phase is changed from A to B on one side of the bipolaron or from B to A on the other side. Further, we found very interesting features useful for bipolaron detection, which are connected with mostly intra-ring motions of the atoms and thus the use of dimers, as model seems to be justified. For example, the Raman spectra of the B-phase show lines ten times more intense than the aromatic phase, which are due to ring breathing and ring deformations. To obtain quantitative informations about the atomic motions involved in each line in the spectra, in addition the Potential Energy Distribution of each normal mode among symmetry coordinates was calculated.
TL;DR: In this paper, a photo-induced splitting of a bipolaron into two polarons was shown to convert a charge carrier (bipolaron) into two spin carriers (polarons).
Abstract: In conducting polymers, a polaron carries charge with spin and a bipolaron carries charge without spin. Hence, the polaron is a spin carrier but the bipolaron is not. Through a dynamical simulation, our study of photoinduced phenomena shows that, by absorbing a photon, a bipolaron is split into two polarons. This photoinduced splitting converts a charge carrier (bipolaron) into two spin carriers (polarons). (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
TL;DR: In this article, the bipolaron theory numerically explains high Tc values without any fitting parameters and describes other key features of the cuprates and provides a new insite into the theory of transport through molecular nanowires and quantum dots (MQD).
Abstract: There is a growing understanding that transport properties of complex oxides and individual molecules are dominated by polaron physics In superconducting oxides the long-range Frohlich and short-range Jahn-Teller electron-phonon in-teractions bind carriers into real space pairs — small bipolarons with surprisingly low mass but sufficient binding energy, while the long-range Coulomb repul-sion keeps bipolarons apart preventing their clustering The bipolaron theory numerically explains high Tc values without any fitting parameters and describes other key features of the cuprates The same approach provides a new insite into the theory of transport through molecular nanowires and quantum dots (MQD) Attractive polaron-polaron correlations lead to a “switching” phenomenon in the current-voltage characteristics of MQD The degenerate MQD with strong electron-vibron coupling has two stable current states (a volatile memory), which might be useful in molecular electronics
01 Jan 2004
TL;DR: In this paper, a simple model was used to simulate the dynamics of polaron-, bipolaron- and Drude-type electrons, and the relaxation times for polarons and bipolarons were 210 fs and 3 ps, respectively.
Abstract: We investigated the ultrafast dynamics in a Na-NaBr melt at 1073 K by fs pump probe absorption spectroscopy. A simple model was used to simulate the dynamics of polaron-, bipolaron- and Drude-type electrons. The relaxation times for polarons and bipolarons are 210 fs and 3 ps, respectively. The existence of an isosbestic point at ∼ 1.35 eV indicates an inter-conversion between bipolarons and Drude-type electrons.
TL;DR: In this article, a quantum dynamical manifold theory (QDMT) was used to obtain the Dirac quasiparticle pair states (cooperons and potential cooperons or Dirac polaron pairs) in a polarizable medium appropriate for high-temperature superconductors.
Abstract: The formation of Dirac pair states is shown to be crucial to the formation of the superconductive state The properties of these Dirac bipolaron states is studied using a quantum dynamical extension of Cartan's moving frame approach to differential geometry The overall geometric method, called quantum dynamical manifold theory (QDMT), is used to obtain the Dirac quasiparticle pair states (cooperons and potential cooperons or Dirac polaron pairs) in a polarizable medium appropriate for high-temperature superconductors (HTSCs) The matrix solutions of the equations describing the quantum dynamical manifold have a (flavor) symmetry also occurring in other closed-shell systems By independently coupling each Dirac electron to a polarizable background in a semiclassic approximation, and solving the resulting quantum dynamical manifold equations (QDMEs), there are found new, nonperturbative, analytic solutions to the polaron pair (bipolaron) problem This enables us to show that these cooperon states are closed shells with su(2) Lie algebraic symmetry These are used to describe the cooperon system above and below the superconduction critical temperature It is shown how these Dirac polaron pairs can be correlated for electron–electron interactions via virtual exciton exchange and screened Coulomb repulsion and then used in a homogeneous Bethe–Salpeter equation approach to study the onset of superconductivity of polaronic superconductors involving pairs of Dirac polarons The relation of these states to the formation of a Bose–Einstein condensation (BEC) is discussed To numerically investigate the question of the existence of high-temperature superconductivity mediated by the screened Coulomb and boson exchange of virtual excitons, a wave vector- and frequency-dependent effective interaction, Veff(k, ω), between electron or hole polaron pairs in the electronic polaron model is used in lieu of a full pairing interaction, thereby simplifying the analysis of interacting pair states We include the temperature dependence of the dielectric function, e−1(k, ω, T), for many-body systems of these quasiparticles An effective mass method is employed to further simplify the computations Analysis of the results shows that the attractive part of Veff for the electronic polaron model persists to high temperature Further, polarons not participating in the superconductive state produce a measurable Fermi surface even in the presence of the bipolaron BEC © 2004 Wiley Periodicals, Inc Int J Quantum Chem, 2004