Showing papers on "Bipolaron published in 2008"
TL;DR: In this paper, the bipolaron model for large OMAR at room temperature is extended to an analytically solvable two-site scheme, which reproduces some of the key features of OMAR, including the possibility to have both positive and negative magnetoresistance, as well as its universal line shapes.
Abstract: The recently proposed bipolaron model for large “organic magnetoresistance” (OMAR) at room temperature is extended to an analytically solvable two-site scheme. It is shown that even this extremely simplified approach reproduces some of the key features of OMAR, viz., the possibility to have both positive and negative magnetoresistance, as well as its universal line shapes. Specific behavior and limiting cases are discussed. Extensions of the model, to guide future experiments and numerical Monte Carlo studies, are suggested.
68 citations
TL;DR: A series of oligothiophene dications doped with Cl3− ions were studied using density functional theory (DFT) at the B3LYP/6−31G(d) level.
Abstract: A series of oligothiophene dications doped with Cl3− ions were studied using density functional theory (DFT) at the B3LYP/6−31G(d) level. The balance between the bipolaron and polaron pair states w...
58 citations
TL;DR: In this paper, the authors conducted in-situ EPR spectroelectrochemical studies of poly(3,4-ethylenedioxythiophene) (PEDOT) to gain new insights into the doping processes taking place in this conjugated polymer.
52 citations
TL;DR: In this paper, the authors discuss the interpretation of electrochemical and spectroelectrochemical measurement of charging and transport of electronic species in conducting polymers, as a function of the polymer potential.
46 citations
TL;DR: In this paper, the effect of counterions on the properties and structure of conducting polymers was studied by using a series of Cl 3 -doped polythiophenes (PTs) as a case example.
Abstract: The effect of counterions on the properties and structure of conducting polymers was studied by using a series of Cl 3 - doped polythiophenes (PTs) as a case example. Hybrid density functional theory (DFT) with periodic boundary conditions (PBC) at the B3LYP/6-31G(d) level has been used. This is the first theoretical study of infinitely long doped PT using DFT with counterions explicitly taken into account. The balance between charge carrier states was addressed by studying the singlet and triplet state unit cells of differently doped PTs. The relative isomer energies, density of states diagrams, bond length alternation, and charge distribution patterns were analyzed. Interestingly, the position of the counterion is flexible over the polymer chain and the electronic structure of the polymer and, consequently, optical properties are sensitive to the position of the counterion. A bipolaron electronic configuration is preferred at high dopant concentrations (one dopant per six or less thiophene rings) while a polaron pairs configuration is preferred at low dopant concentrations (one dopant per ten or more thiophene rings) which is in line with many experimental observations.
38 citations
TL;DR: Theoretical UV spectra for cations and dications of short oligomers (six to eight rings) match experimental spectra of polypyrrole at low and at high doping levels, respectively, and the error in the theoretical calculations is about 0.4 eV.
Abstract: Changes in absorption spectra during doping of oligopyrroles were investigated with time-dependent density functional theory on optimized structures of neutral, singly, and doubly charged pyrrole oligomers with up to 24 rings. In the absence of counterions, defects are delocalized. Counterions induce localization. For dications two polarons on the same chain are preferred over a bipolaron. Intragap absorptions arise in charged species, no matter whether defects are localized or delocalized. Cations and dications give rise to two sub-band transitions. The cation peaks have lower energies than those of dications. The first excitations of cations have lower oscillator strengths than the second; for dications the second peak is weaker than the first. For very long oligomers, the second sub-band absorption vanishes and a third one appears at higher energy. The behavior of pyrrole oligomers is analogous to that of thiophene oligomers. Theoretical UV spectra for cations and dications of short oligomers (six to eight rings) match experimental spectra of polypyrrole at low and at high doping levels, respectively. The error in the theoretical calculations is about 0.4 eV, slightly larger than for thiophene oligomers at the same level of theory.
37 citations
TL;DR: A new mechanism for the formation of doped polyaniline is presented, which allows for the rationalization of the experimentally observed equilibrium between polaron and bipolaron defects in emeraldine salt.
Abstract: A new mechanism for the formation of doped polyaniline is presented. Besides providing suitable structural and spectroscopic parameters, the new mechanism allows for the rationalization of the experimentally observed equilibrium between polaron and bipolaron defects in emeraldine salt. The magnetic behavior and the “metallic island” model for conduction in doped polyaniline are also theoretically supported by the new proposal.
27 citations
TL;DR: In this paper, the magnetic properties of doped pellets of poly(3-methylthiophene) showing room temperature ferromagnetic behavior have been discussed in a previous article.
Abstract: The magnetic properties of doped pellets of poly(3-methylthiophene) showing room temperature ferromagnetic behaviour have been discussed in a previous article. The magnetic behaviour was attributed to a weak ferromagnetic phase, due to the superexchange interaction of polarons via the dopant anions. The Dzialoshinsky–Morya interaction among canted spins was proposed to explain the ferromagnetism. In this article the main conclusions of that work concerning the magnetic behaviour are revised. The basic assumption now is that the magnetic moments are spin 1/2 polarons that can interact antiferromagnetically and/or ferromagnetically. In the small crystalline regions of the polymer, which are identified with the polymer portion that remains ferromagnetic at room temperature, the interaction gives rise to S = 0a nd 1 bipolarons and the S = 1 triplet state is lower in energy. In the disordered region, disorder will prevent the complete S = 1 and 0 coupling and bands of polarons ferromagnetically and antiferromagnetically coupled will appear. Using this approach, all the magnetization data can be qualitatively explained, as well as the electron spin resonance data.
25 citations
TL;DR: BaTi1−xNbxO3 films, spanning the entire range of x, have been deposited on MgAl2O4 substrates and X-ray diffraction measurements indicate single phase films for all values of x as discussed by the authors, contrary to a previously reported niobium solubility limit.
Abstract: BaTi1−xNbxO3 films, spanning the entire range of x, have been deposited on MgAl2O4 substrates. X-ray diffraction measurements indicate single phase films for all values of x, contrary to a previously reported niobium solubility limit. Films show extreme sensitivity to high temperature oxygen exposure, which destroys conductivity and severely disrupts crystallinity. Under optimum growth conditions increasing x gives rise to a Ti4+ to Ti3+ transformation in the oxidation state accompanied by increased conductivity with a semiconductor-metal transition near x=0.2. Temperature dependent magnetic measurements show an anomalous rise in the spin moment, a feature supportive of the small singlet bipolaron model.
20 citations
TL;DR: In this article, the authors performed charge-induced absorption and electroluminescence spectroscopy in a polyfluorene organic magnetoresistive device and measured the singlet exciton, triplet exciton and polaron densities in a live device under an applied magnetic field.
Abstract: We perform charge-induced absorption and electroluminescence spectroscopy in a polyfluorene organic magnetoresistive device. Our experiments allow us to measure the singlet exciton, triplet exciton, and polaron densities in a live device under an applied magnetic field and to test the predictions of three different models that were proposed to explain organic magnetoresistance. These models are based on different spin-dependent interactions, namely, exciton formation, triplet-exciton polaron quenching, and bipolaron formation. We show that the singlet exciton, triplet exciton, and polaron densities and conductivity all increase with increasing magnetic field. Our data appear to be inconsistent with the exciton formation and triplet-exciton polaron quenching models.
20 citations
TL;DR: In this article, a polaron-polaron interaction model is proposed to study the spin injection and transport in an organic semiconductor (OSC) device, and the evolutions of spin polarons and spinless bipolarons are calculated from the drift-diffusion equations, in which both the polaron−bipolaron transition and the spin-flipping of a spin polaron are included.
TL;DR: It was established that there exists a noticeable interaction between pi-orbitals of individual macrocycles in tubular aggregates as follows from decreasing of the band gap with a number of repeating units in aggregates and the polaron delocalization toward tube axes in oxidized species.
Abstract: The geometries of neutral, mono-, and dioxidized tubular aggregates of cyclo[8]thiophenes containing up to 5 repeating units were fully optimized at the MPWB1K/3-21G* level of theory. Calculated interplane distances between macrocycles were found to be close to 3.1 A for neutral and charged aggregates. The binding energies between macrocycles in neutral intermediates were in the range of 40-45 kcal/mol, increasing for monocations and dropping strongly for dicationic species due to electrostatic repulsion between polarons. It was established that there exists a noticeable interaction between pi-orbitals of individual macrocycles in tubular aggregates as follows from decreasing of the band gap with a number of repeating units in aggregates and the polaron delocalization toward tube axes in oxidized species. A polaron pair is the most stable dicationic state for all studied molecules according to the calculations. A singlet polaron pair is more stable than a triplet one. The energy difference between singlet and triplet states is growing smaller with the size of the system, becoming zero for the pentamer corresponding to a completely dissociated bipolaron.
TL;DR: Scheinert et al. as discussed by the authors showed that trap recharging or mobile ions, or formation/dissociation of bipolarons in the accumulation layer can cause hysteresis in field effect transistors.
Abstract: Hysteresis effects do occur usually in polymer field-effect devices. It has been suggested that trap recharging or mobile ions, or formation/dissociation of bipolarons (BP) in the accumulation layer can cause this effect. Here, at first a literature survey on the hysteresis in field-effect transistors is given. Then examples of measured hysteresis in field-effect transistors and in metal-insulator-semiconductor (MIS). capacitors are presented. The typical peculiarity is the dependency of the flat band voltage on the sweep direction of the gate voltage. Our recent numerical simulations on the trap recharging mechanism in MIS capacitors are continued and extended to transistors. Energetically distributed traps can lead to hysteresis. But the form of the hysteresis deviates from the observed one and extreme parameter values are needed. Thus it is more likely that trap recharging can modily a hysteresis caused by another mechanism. For discussing the bipolaron mechanism the equilibrium between polarons and doubly charged states of the polymer chains was analyzed anew. With a rate constant for the bipolaron formation determined recently by Salleo and Street relaxation times for formation/dissociation are determined. They indicate that these processes can cause the hysteresis. A further possible mechanism influencing the hysteresis is connected with complexes between polarons, bipolarons and mobile counter ions. The estimated rate constants indicate processes on the time scale of the measurements. Numerical simulations of these processes as well as of the role of mobile ions and their reactions are required for a full description of the hysteresis. Dynamic capacitance-voltage curves of an organic MIS capacitor measured at 1 Hz for different gate-bulk voltage (V GB ) sweep directions at different temperatures. The organic semi-conductor is a 48 run thick poly(3-octylthiophene) (P3OT) layer, the silicon dioxide insulator is 50 nm thick. The curves for the two sweep directions are shifted against each other. Thus, one has for the two sweep directions different interface charges causing the different flat band voltages. [Data from S. Scheinert, Habilitationsschrift, Technische Universitat Ilmenau (2006)].
TL;DR: In this article, the electronic structure of doped-oligoaniline with various dopants is investigated by means of DFT method, and the doping mechanism is proposed based on the calculation results.
Abstract: The electronic structure of doped-oligoaniline with various dopants is investigated by means of DFT method. After doping by hydrochloric acid (HCl) and camphorsulfonic acid (HCSA), the alternation of bond-lengths is decreased and the co-planarity of adjacent aromatic rings is increased. The π-conjugating effect is increased in the electronic nature of Ph-N system because the electrons can be delocalized along the backbone of oligoaniline where the hydrogen bonds as a bridge transfer the electrons. The electronic structure of polaron and bipolaron conformation and their relative stability is discussed, indicating that the preferable conformation is dependant on various dopants. The calculation results reveal that there is a relatively stronger interaction between the organic dopant of HCSA and N atoms of PANI, and more charge transfer between PANI and HCSA is a reason for the fact that the conductivity of HCSA-doped PANI is higher than that of HCl-doped PANI. The doping mechanism is proposed based on the calculation results. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008
TL;DR: The long-range transport of polaron/bipolaron under high electric field at low temperature is the field-facilitated sequential tunneling through spatial-disordered potential barriers via multiple intrastrand and interstrand pathways.
Abstract: We investigate the pathways of polaron and bipolaron transports in DNA double strands with an extended Su-Schrieffer-Heeger model involving the effects of solvent polarization. We find that the long-range transport of polaron/bipolaron under high electric field at low temperature is the field-facilitated sequential tunneling through spatial-disordered potential barriers via multiple intrastrand and interstrand pathways. Although the interstrand pathways may be very active and effective in some DNA sequences, the intrastrand ones always dominate the charge transfer when the excess charge moves close to the final acceptor.
TL;DR: In this article, it was shown that spin polarons can be created preferentially in energy when electrons are doped under a reasonable magnetic field considering the influence of electron-electron interactions and electron-phonon coupling.
Abstract: Spin degeneracy in conjugated polymers will be destroyed in the presence of magnetic field because of the existence of Zeeman splitting. The characteristic of carriers in polythiophene is studied in the framework of a tight binding model. It is found that spin polarons can be created preferentially in energy when electrons are doped under a reasonable magnetic field considering the influence of electron–electron interactions and electron–phonon coupling, which is favorable for spin polarized transport in organic semiconductors and is of great importance in the field of organic spintronics.
TL;DR: In this paper, the exact polaron and bipolaron Green's functions for a model with a highly inhomogeneous electron-boson coupling were derived, where the polaron ground-state energy and quasiparticle weight are similar to those of the Holstein polaron with equal effective coupling.
Abstract: We derive the exact polaron and bipolaron Green's functions for a model with a highly inhomogeneous electron-boson coupling , where . While the polaron ground-state energy and quasiparticle weight are similar to those of the Holstein polaron with equal effective coupling, the polaron dispersion is very different. Unlike that of a Holstein polaron, which is monotonically increasing with the polaron momentum, the polaron dispersion in this case is folded inside the Brillouin zone. For strong coupling, the polaron and bipolaron effective masses increase linearly with the effective coupling and exhibit no isotope effect, as opposed to an exponential increase and a strong isotope effect, in the Holstein model. As a result, this model exhibits strongly bound yet light bipolarons at strong couplings. Generalizations are discussed.
TL;DR: In this article, it was shown that a bipolaron can open a channel of electroluminescence, which does not involve a triplet exciton, and can enhance the efficiency of the system.
TL;DR: In this article, a quasi-0D quantum dot with a spherical parabolic confinement potential was investigated by applying the Feynman variational principle and the bipolaron coupling energy and self-action potential energy were found to increase with an increase in the Frohlich electron-phonon-coupling constant.
TL;DR: The relation between the bipolaron entanglement and the correlation functions demonstrates that the largeEntanglement corresponds to the large magnitude of lattice deformations induced by electrons.
Abstract: We revisit the two-site Hubbard-Holstein model by using extended phonon coherent states. The nontrivial singlet bipolaron is studied exactly in the whole coupling regime. The ground-state (GS) energy and the double occupancy probability are calculated. The linear entropy is exploited successfully to quantify bipartite entanglement between electrons and their environment phonons, displaying a maximum entanglement of the singlet-bipolaron in strong coupling regime. A dramatic drop in the crossover regime is observed in the GS fidelity and its susceptibility. The bipolaron properties is also characterized classically by correlation functions. It is found that the crossover from a two-site to single-site bipolaron is more abrupt and shifts to a larger electron-phonon coupling strength as electron-electron Coulomb repulsion increases.
TL;DR: In this article, the effects of interchain interaction on the polaron-bipolaron transition on conjugated polymer chains were investigated using a Su-Schrieffer-Heeger model modified to include interchain interactions, an external electric field and electron-electron interaction via extended Hubbard terms.
Abstract: We investigate the effects of interchain interaction on the polaron–bipolaron transition on conjugated polymer. We use a Su-Schrieffer-Heeger model modified to include interchain interaction, an external electric field and electron–electron interaction via extended Hubbard terms. We study the dynamics within the time-dependent unrestricted Hartree-Fock approximation. We find that adding a hole in interacting conducting polymer chains bearing a single positively charged polaron leads to the direct transition of polaron to bipolaron state. The transition which is produced is single-polaron to bipolaron transition whose excitation spectrum explains the experimental data. The competing mechanism of two polarons merging to form a bipolaron is also observed under special circumstances. 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.
TL;DR: In this paper, the conditions of the virial theorem imposed on the dependence of the total energy of a system consisting of two polarons on the distance between the centers of the polarization wells are analyzed for various trial two-electron functions in the limit of strong electron-phonon coupling.
Abstract: The conditions of the virial theorem imposed on the dependence of the total energy of a system consisting of two polarons on the distance between the centers of the polarization wells are analyzed for various trial two-electron functions in the limit of strong electron-phonon coupling. It is shown that the inclusion of the direct dependence of the wave function of the system on the interelectron distance (correlation effects) does not break the conditions of the virial theorem. This theorem holds for a single-center and a two-center configuration corresponding to a subsidiary minimum, which disappears when the electron correlations are included.
TL;DR: The major conclusion of this work concerns the effects of the interchain interactions in bipolaron systems, which are usually neglected in theoretical works and is predicted that as a consequence of the latter in the bulk system a bipolaron generated on a single polymer chain will soon split into two polarons on adjacent chains.
Abstract: Long neutral and doubly charged oligodiacetylenes (with a maximum of 30 repeat units) as well as some of their aggregates have been studied in a semiempirical framework. By analyzing the plots of the one-electron ground-state density matrices, we have shown that the 12-units long oligomer is the borderline between two different behaviors of the electronic structure of the isolated charged systems. For oligomers shorter than this limit the bipolaronic structure is the dominant one, while for larger oligomers the polaron-pair structure is preferred. The major conclusion of this work concerns the effects of the interchain interactions in bipolaron systems, which are usually neglected in theoretical works. It is predicted that as a consequence of the latter in the bulk system a bipolaron generated on a single polymer chain will soon split into two polarons on adjacent chains.
TL;DR: In this paper, the polaron to bipolaron transition in disordered media under the influence of a broadband ac electric field is suggested: ac conductivity vs. frequency measurements induce spatio-temporal fluctuations of the density of polarons yielding polaron-to-bipolaron transformation.
Abstract: The polaron to bipolaron transition in disordered media under the influence of a broadband ac electric field is suggested: ac conductivity vs. frequency measurements in disordered media with inhomogeneous disorder induce spatio-temporal fluctuations of the density of polarons yielding polaron to bipolaron transformation. The external field results in the transition, alternatively to an increasing doping level. The assertion is confirmed by performing successive complex impedance measurements in disordered media. A systematic increase of the real part of the ac conductivity in the frequency domain, followed by mutual reduction of the magnetic inductivity of conducting polypyrrole, are explained.
TL;DR: In this article, the authors reported the temperature and frequency dependence of a.c. conductivity in glassy Se 70 Te 30− x Zn x (x = 0, 2, 4 and 6) alloys in the temperature range 300-500 K and frequency range 1-kHz.
Abstract: The present work reports the temperature and frequency dependence of a.c. conductivity in glassy Se 70 Te 30− x Zn x ( x = 0, 2, 4 and 6) alloys in the temperature range 300–500 K and frequency range 1 kHz. An agreement between experimental and theoretical results suggests that the a.c. conductivity behaviour of the present samples can be successfully explained by correlated barrier hopping (CBH) model. The density of defect states has been determined using this model for all the glassy alloys. The results show that bipolaron hopping dominates over single-polaron hopping in this glassy system. This is explained in terms of lower values of the maximum barrier height for single-polaron hopping.
TL;DR: In this article, the transport properties of sintered samples NdBa 2− x La x Cu 3 O 7− δ with 0.30 have been studied in the normal state by X-ray diffraction, the resistivity, and the thermoelectric power measurements.
Abstract: The transport properties of sintered samples NdBa 2− x La x Cu 3 O 7− δ with 0 ⩽ x ⩽ 0.30 have been studied in the normal state by X-ray diffraction, the resistivity, and the thermoelectric power measurements. La doping results in a larger normal state resistivity, the thermoelectric power, a lower critical temperature T c , a larger pseoudogap temperature T g , and a change from a metal-like to a semiconductor-like behavior at low temperature and high doping concentration. The strong increase in the room temperature resistivity for x ⩾ 0.05 suggested that hole filling picture is not only mechanism to decrease hole concentration but there is also another mechanism such as mobile hole localization. The results of the resistivity as a function of temperature and doping concentration was analyzed within bipolaron model while the thermoelectric power was analyzed within a phenomenological narrow band model. A good agreement between models and data were obtained. An increased tendency for localization is found within both models. Therefore, the results of both models suggested that both hole localization and hole filling mechanisms decrease the hole concentration.
TL;DR: In this paper, the effects of removing one more electron from conjugated polymers with polarons were studied using a Su-Schrieffer-Heeger model modified to include an external electric field and electron-electron interactions via a Parr-Pariser-Pople mean field Hamiltonian.
Abstract: We describe the effects of removing one more electron from conjugated polymers with polarons. We use a Su-Schrieffer-Heeger model modified to include an external electric field and electron–electron interactions via a Parr-Pariser-Pople mean field Hamiltonian. Within the Unrestricted Hartree–Fock approximation, we study the dynamics performing numerical simulations. We have found that removing one electron from a conducting polymer bearing a single polaron leads to the creation of a structure that contains a bipolaron coupled to a breather. The bipolaron energy levels in the gap oscillate with higher amplitude due to the bipolaron–breather coupling. The bipolaron–breather pair can be decoupled through the action of an external electric field. Lattice oscillation modes are created due to the bipolaron motion and the energy levels associated to the bipolaron still oscillate, nevertheless, with smaller amplitude. The effects of the external electric field on the bipolaron–breather dynamics are studied by changing the electric field intensity.
TL;DR: In this article, the polaron to bipolaron transition in disordered media under the influence of a broadband ac electric field is suggested: ac conductivity versus frequency measurements induce spatiotemporal fluctuations of the density of polarons yielding polaron-to-bipolaron transformation.
Abstract: The polaron to bipolaron transition in disordered media under the influence of a broadband ac electric field is suggested: ac conductivity versus frequency measurements in disordered media with inhomogeneous disorder induce spatiotemporal fluctuations of the density of polarons yielding polaron to bipolaron transformation. The external field results in the transition, alternatively to an increasing doping level. The assertion is confirmed by performing successive complex impedance measurements in disordered media. A systematic increase in the real part of the ac conductivity in the frequency domain, followed by mutual reduction in the magnetic inductivity of conducting polypyrrole, are explained.
TL;DR: In this article, the authors used the single-path integral to calculate the impedance function of the polaron and bipolaron in quantum confinement with the presence of the external magnetic fields.
Abstract: We use the single-path-integral to calculate the impedance function of the polaron and bipolaron in quantum confinement with the presence of the external fields. The expectation values of the classical equation of motion is considered in order to obtain the impedance function. The mobility of the polaron and bipolaron in quantum confinement is also calculated in the direction parallel and perpendicular to the magnetic field. Without trapping, we also calculate the effective mass of the bipolaron in the magnetic field.
TL;DR: In this article, the authors review the bipolaron theory of unconventional proximity effects, symmetry and checkerboard modulations of the order parameter and quantum magneto-oscillations discovered recently in cuprates, and point to the possibility that high-Tc cuprate superconductors may not derive from the Bose-Einstein condensation (BEC) of real-space pairs.
Abstract: There is compelling evidence for a strong electron-phonon interaction (EPI) in cuprate superconductors from the isotope effects on the supercarrier mass, high resolution angle resolved photoemission spectroscopies (ARPES), a number of optical and neutron-scattering measurements in accordance with our prediction of high-temperature superconductivity in polaronic liquids. A number of observations point to the possibility that high-Tc cuprate superconductors may not be conventional Bardeen-Cooper-Schrieffer (BCS) superconductors, but rather derive from the Bose-Einstein condensation (BEC) of real-space pairs, which are mobile small bipolarons. Here I review the bipolaron theory of unconventional proximity effects, the symmetry and checkerboard modulations of the order parameter and quantum magneto-oscillations discovered recently in cuprates.