Showing papers on "Bipolaron published in 2011"
TL;DR: In this paper, the stochastic Liouville equation is applied to the field of organic magnetoresistance to perform detailed microscopic calculations on the different proposed models, and the influence of a magnetic field on the current in bipolaron, electron-hole pair, and triplet models is calculated.
Abstract: The stochastic Liouville equation is applied to the field of organic magnetoresistance to perform detailed microscopic calculations on the different proposed models. By adapting this equation, the influence of a magnetic field on the current in bipolaron, electron-hole pair, and triplet models is calculated. The simplicity and wide applicability of the stochastic Liouville equation makes it a powerful tool for interpreting experimental results on magnetoresistance measurements in organic semiconductors. New insights are gained on the influence of hopping rates and disorder on the magnetoresistance.
106 citations
TL;DR: In this paper, the molecular structure of a cyclic oligothiophene, C10T, has been determined by single-crystal X-ray structure analysis, and the exclusive synconformation of all thiophene units as confirmed in the solid state and the ring strain in this macrocycle result in its unusual and optoelectronic properties.
Abstract: The molecular structure of a cyclic oligothiophene, C10T, has been determined by single-crystal X-ray structure analysis. The exclusive syn-conformation of all thiophene units as confirmed in the solid state and the ring strain in this macrocycle result in its unusual and optoelectronic properties. This does not only apply to neutral C10T but also to its oxidized states, as demonstrated by absorption and ESR spectroscopy, supporting the formation of a polaron-pair structure upon oxidation of C10T to C10T2(·+) as has been discussed for linear oligothiophenes. To the best of our knowledge, C10T2(·+) represents an unambiguous example comprising a two-polaron structure (polaron-pair) of a thiophene-based conjugated macrocycle.
55 citations
TL;DR: In this paper, the bipolaron theory is used to explain high-temperature superconductivity in real-space tightly bound pairs, but with a purely electronic mechanism of pairing rather than with an electron?phonon interaction (EPI).
Abstract: Soon after the discovery of the first high-temperature superconductor by Georg Bednorz and Alex M?ller in 1986, the late Sir Nevill Mott in answering his own question 'Is there an explanation?' (1987 Nature 327 185) expressed the view that the Bose?Einstein condensation (BEC) of small bipolarons, predicted by us in 1981, could be the one. Several authors then contemplated BEC of real-space tightly bound pairs, but with a purely electronic mechanism of pairing rather than with an electron?phonon interaction (EPI). However, a number of other researchers criticized the bipolaron (or any real-space pairing) scenario as incompatible with some angle-resolved photoemission spectra, with experimentally determined effective masses of carriers and unconventional symmetry of the superconducting order parameter in cuprates. Since then, the controversial issue of whether EPI is crucial for high-temperature superconductivity or is weak and inessential has been one of the most challenging problems of contemporary condensed matter physics. Here I outline some developments in the bipolaron theory suggesting that the true origin of high-temperature superconductivity is found in a proper combination of strong electron?electron correlations with a significant finite-range (Fr?hlich) EPI, and that the theory is fully compatible with key experiments.
24 citations
TL;DR: In this article, the true origin of high-temperature superconductivity is found in a proper combination of strong electron-electron correlations with a significant finite-range EPI, and that the theory is fully compatible with the key experiments.
Abstract: Soon after the discovery of the first high temperature superconductor by Georg Bednorz and Alex Mueller in 1986 the late Sir Nevill Mott answering his own question "Is there an explanation?" [Nature v 327 (1987) 185] expressed a view that the Bose-Einstein condensation (BEC) of small bipolarons, predicted by us in 1981, could be the one. Several authors then contemplated BEC of real space tightly bound pairs, but with a purely electronic mechanism of pairing rather than with the electron-phonon interaction (EPI). However, a number of other researchers criticized the bipolaron (or any real-space pairing) scenario as incompatible with some angle-resolved photoemission spectra (ARPES), with experimentally determined effective masses of carriers and unconventional symmetry of the superconducting order parameter in cuprates. Since then the controversial issue of whether the electron-phonon interaction (EPI) is crucial for high-temperature superconductivity or weak and inessential has been one of the most challenging problems of contemporary condensed matter physics. Here I outline some developments in the bipolaron theory suggesting that the true origin of high-temperature superconductivity is found in a proper combination of strong electron-electron correlations with a significant finite-range (Froehlich) EPI, and that the theory is fully compatible with the key experiments.
23 citations
TL;DR: The model modified to include electron-electron interactions, the Brazovskii-Kirova symmetry breaking term and an external electric field is used to investigate the scattering process between a negative and a positive bipolaron in a system composed of two coupled polymer chains.
Abstract: By using the Su-Schrieffer-Heeger model modified to include electron-electron interactions, the Brazovskii-Kirova symmetry breaking term and an external electric field, we investigate the scattering process between a negative and a positive bipolaron in a system composed of two coupled polymer chains. Our results show that the Coulomb interactions do not favor the bipolaron recombination. In the region of weak Coulomb interactions, the two bipolarons recombine into a localized excited state, while in the region of strong Coulomb interactions they can not recombine. Our calculations show that there are mainly four channels for the bipolaron recombination reaction: (1) forming a biexciton, (2) forming an excited negative polaron and a free hole, (3) forming an excited positive polaron and a free electron, (4) forming an exciton, a free electron, and a free hole. The yields for the four channels are also calculated.
18 citations
TL;DR: In this paper, the effect of different dopants and polarity of the solvents on the electronic structure and properties of the emeraldine salts is evaluated and interpreted from the standpoint of available experimental data.
Abstract: Quantum mechanical simulations of a variety of inorganic and organic emeraldine salts (doping agents HCl, HBr, H2SO4, MSA, BSA, and CSA) in bipolaron and polaron form with account of solvents of different polarity (chloroform, m-cresol, and water) are reported for the first time. The models are based on tetramers, and the calculations are performed with the DFT method. The polarizable continuum model is used for the treatment of solute-solvent interaction. The effect of the different dopants and polarity of the solvents on the electronic structure and properties of the salts is evaluated and interpreted from the standpoint of the available experimental data. © 2010 Wiley Periodicals, Inc.
18 citations
TL;DR: In this article, the authors investigated the magnetic field effects on electron mobility in tri-8-hydroxyquinoline-aluminum based light-emitting devices by the transient electroluminescence (EL) method upon application of various offset voltages (Voffset).
Abstract: We investigated the magnetic field effects (MFEs) on electron mobility in tri-(8-hydroxyquinoline)-aluminum based light-emitting devices by the transient electroluminescence (EL) method upon application of various offset voltages (Voffset). It is found that the rising edges of the EL overlap closely but the falling edges are separated by the magnetic field both when Voffset = 0 V and Voffset > Vturnon. The results suggest the bipolaron model and the triplet-polaron interaction model related to the carriers’ mobility are not the dominant mechanisms for explaining the MFEs under our experimental conditions, and the external magnetic field is confirmed to affect the carriers’ recombination process.
14 citations
TL;DR: In this paper, the mganetic field effects (MFEs) on electron mobility in tri-(8-hydroxyquinoline)-aluminum based light-emitting devices by transient-electroluminescence method upon application of various offset voltages.
Abstract: We investigated the mganetic field effects (MFEs) on electron mobility in tri-(8-hydroxyquinoline)-aluminum based light-emitting devices by transient-electroluminescence method upon application of various offset voltages. It is found the rising edges of EL pulses are well overlapped and the falling edges of EL pulses are separated for the magnetic field on and off when Voffset=0 V and Voffset>Vturnon of the devices. The results suggest the bipolaron model and the triplet-polaron interaction model related to the carriers mobilities are not the dominant mechanisms to explain the MFEs under our experimental conditions, and the magnetic field affects the carriers recombination process is confirmed.
14 citations
TL;DR: In this article, the effect of Zn substitution on superconducting and normal state properties of new Y-based Y3Ba5Cu8−xZnxO18−δ superconductor was reported.
Abstract: We report the effect of Zn Substitution on superconducting and normal state properties of new Y-based Y3Ba5Cu8−xZnxO18−δ superconductor. The resistivity of the samples rather shows linear temperature dependence. Two cases of bipolaron model, in presence and absence of thermally excited individual polarons, are used to analyze normal state resistivity. The effect of Zn substitution on carrier localization, transition temperature, boson–boson relaxation time, and extended bosons are discussed.
12 citations
TL;DR: In this paper, temperature dependence of the properties of a strong coupling bipolaron in a quantum dot (QD) is studied based on the Lee-Low-Pines-Huybrechts variational method and quantum statistical theory.
Abstract: Temperature dependence of the properties of strong-coupling bipolaron in a quantum dot (QD) is studied based on the Lee–Low–Pines–Huybrechts variational method and quantum statistical theory. Results of the numerical calculation show that the vibration frequency as well as the absolute value of the induced potential and the effective potential all increase with increasing coupling strength and temperature, respectively, and they also increase with decreasing relative distance of electrons. The bipolarons are closer and more stable when the temperature is higher and coupling strength is larger. The influence of radius of QD and dielectric constant ratio on the effective potential is little.
11 citations
TL;DR: In this article, a unified parameter-free explanation of the observed oxygen-isotope effects on the critical temperature, the magnetic-field penetration depth, and on the normal-state pseudogap for underdoped cuprate superconductors within the framework of the bipolaron theory compatible with the strong Coulomb and Froehlich interactions was provided.
Abstract: Developing a theory of high-temperature superconductivity in copper oxides is one of the outstanding problems in physics. Twenty-five years after its discovery, no consensus on the microscopic theory has been reached despite tremendous theoretical and experimental efforts. Attempts to understand this problem are hindered by the subtle interplay among a few mechanisms and the presence of several nearly degenerate and competing phases in these systems. Here we provide unified parameter-free explanation of the observed oxygen-isotope effects on the critical temperature, the magnetic-field penetration depth, and on the normal-state pseudogap for underdoped cuprate superconductors within the framework of the bipolaron theory compatible with the strong Coulomb and Froehlich interactions, and with many other independent observations in these highly polarizable doped insulators. Remarkably, we also quantitatively explain measured critical temperatures and magnitudes of the magnetic-field penetration depth. The present work thus represents an ultimate proof of the bipolaron theory of high-temperature superconductivity, which takes into account essential Coulomb and electron-phonon interactions.
TL;DR: In this paper, the true origin of high-temperature superconductivity is found in a proper combination of strong electron-electron correlations with a significant finite-range (Frohlich) EPI so that low energy quasi-particles are small mobile polarons and bipolarons in cuprate superconductors.
Abstract: It has been now over 20 years since the discovery of the first high temperature superconductor by Georg Bednorz and Alex Muller and yet, despite intensive effort, no universally accepted theory exists about the origin of superconductivity in cuprates. A controversial issue on weather the electron–phonon interaction (EPI) is crucial for high-temperature superconductivity or weak and inessential has been one of the most challenging problems of contemporary condensed matter physics. Here, it is suggested that the true origin of high-temperature superconductivity is found in a proper combination of strong electron–electron correlations with a significant finite-range (Frohlich) EPI so that low energy quasi-particles are small mobile polarons and bipolarons in cuprate superconductors. Our recent development of the bipolaron theory of tunnelling accounting for two energy scales, their temperature and doping dependencies, asymmetry and inhomogeneity of tunnelling spectra of cuprate superconductors is briefly discussed.
TL;DR: In this paper, it was shown that polarizability effects and Jahn-Teller centers are of special importance, since these enable dynamical charge transfer, interband interac- tions, polaron and bipolaron formation.
Abstract: The discovery of high temperature supercon- ductivity in layered systems has evoked a heated discus- sion about the microscopic electron (hole) pairing mechan- ism. In particular, the proximity of these systems to magnetic phases has been taken as evidence for a purely electronic mechanism. However, structural anomalies and unconventional isotope effects suggest that the lattice plays a crucial role and must be incorporated in modeling the mechanism. Here we show that polarizability effects and Jahn-Teller centers are of special importance, since these enable dynamical charge transfer, interband interac- tions, polaron and bipolaron formation. In this respect the layered superconductors approach ferroelectrics, where dy- namical covalency is known to trigger the structural in- stability.
TL;DR: In this paper, the authors analyzed the tunneling oscillations of polarons about its equilibrium positions and the interaction between polarons in quasi-two-dimensional dielectric layers.
Abstract: It is established, that action of intense laser field results in occurrence of two equivalent positions of a large polaron localization. This paper analyzes the tunneling oscillations of polarons about its equilibrium positions and the interaction between polarons in quasi two-dimensional dielectric layers. It is shown that oscillations of polarons in intense laser field lead to effective interpolaron attraction in coordinate space. This effect promotes Bose-Einstein condensation of bipolarons and to emerge of high-temperature superconductivity.
TL;DR: In this paper, it was shown that a bipolaron superconductivity with the transition temperature T c ǫ = 6 − − 20 K can take place in DNA, and that transition temperature depends on the parameter values of the model.
TL;DR: In this article, a GA-driven density matrix method for calculating the equilibrium geometry and charge storage configurations of hole-bipolaron-doped polythiophene (PT) oligomers is proposed.
Abstract: We propose a genetic algorithm (GA)-driven density matrix method for calculating the equilibrium geometry and charge storage configurations of hole(bipolaron)-doped Polythiophene (PT) oligomers. A modified version of the Su–Schrieffer–Heeger Hamiltonian is used to describe the PT chain. A population of geometry strings are used to generate the corresponding PT-Hamiltonians which act as generators of corresponding unitary transformations which transform a single trial one-electron density matrix into a population of density matrices for different geometries. As the geometry strings evolve under the action of GA operators, the density matrices also evolve on the fitness landscape. Once the fitness reaches maximum, the optimum geometry string, the corresponding Hamiltonian, and the density matrix lead to equilibrium geometry, energy, charge distribution, band gap, Fermi energy, etc. The bipolaronic defect–induced conducting state is predicted to have a symmetric lattice-like admixture of “aromatic” and “quin...
TL;DR: In this article, a two-site model for large magnetic field effect on current of nonmagnetic organic semiconductor, based on bipolaron formation in the presence of both hyperfine field and an applied magnetic field, was presented.
TL;DR: In this paper, the 2D lattice gas model with competing short and long range interactions is applied for calculation of the incoherent charge transport in the classical strongly correlated charge segregated polaronic state.
Abstract: The 2D lattice gas model with competing short and long range interactions is appliedused for calculation of the incoherent charge transport in the classical strongly-correlated charge segregated polaronic state. We show, by means of Monte-Carlo simulations, that at high temperature the transport is dominated by hopping of the dissociated correlated polarons, where with thetheir mobility is inversely proportional to the temperature. At the temperatures below the clustering transition temperature the bipolaron transport becomes dominant. The energy barrier for the bipolaron hopping is determined by the Coulomb effects and is found to be lower than the barrier for the single-polaron hopping. This leads to drastically different temperature dependencies of mobilities for polarons and bipolarons at low temperatures.
01 Jan 2011
TL;DR: In this article, a positive spin-1/2 ELDMR was observed in 2wt% Rubrene doped Tris(8-hydroxyquinolinato)aluminium (Alq3) devices.
Abstract: Optically Detected Magnetic Resonance (ODMR) techniques were used to investigate the dynamics of excitons and charge carriers in π-conjugated organic semiconductors. Degradation behavior of the negative spin-1/2 electroluminescence-detected magnetic resonance (ELDMR) was observed in Alq3 devices. The increase in the resonance amplitude implies an increasing bipolaron formation during degradation, which might be the result of growth of charge traps in the device. The same behavior of the negative spin-1/2 ELDMR was observed in 2wt% Rubrene doped Tris(8-hydroxyquinolinato)aluminium (Alq3) devices. However, with increasing injection current, a positive spin-1/2 ELDMR, together with positive spin 1 triplet powder patterns at {delta}m{sub S}={+-}1 and {delta}m{sub S}={+-}2, emerges. Due to the similarities in the frequency dependences of single and double modulated ELDMR and the photoluminescence-detected magnetic resonance (PLDMR) results in poly[2-methoxy-5-(2 -ethyl-hexyloxy)-1,4-phenyl ene vinylene] (MEH-PPV) films, the mechanism for this positive spin-1/2 ELDMR was assigned to enhanced triplet-polaron quenching under resonance conditions. The ELDMR in rubrene doped Alq3 devices provides a path to investigate charge distribution in the device under operational conditions. Combining the results of several devices with different carrier blocking properties and the results from transient EL, it was concluded trions not only exist near buffer layer but also exist in the electronmore » transport layer. This TPQ model can also be used to explain the positive spin-1/2 PLDMR in poly(3-hexylthiophene) (P3HT) films at low temperature and in MEH-PPV films at various temperatures up to room temperature. Through quantitative analysis, TE-polaron quenching (TPQ) model is shown having the ability to explain most behaviors of the positive spin-1/2 resonance. Photocurrent detected magnetic resonance (PCDMR) studies on MEH-PPV devices revealed a novel transient resonance signal. The signal may originate from the higher concentration of deep traps near cathode. A quantitative analysis based on this assumption was carried out and found to be consistent with the experimental results.« less
TL;DR: In this article, the electrical transport properties of Li x Cr 0.11 V 2 O 5.16 powders were investigated in a broad frequency range of 10−10 10 10 ǫ at temperature varying between 300 and 400 K. The transport properties are consistent with small polaron and bipolaron conduction models.
TL;DR: In this paper, the role of holes that drift beyond the recombination zone and approach the cathode in small molecular organic light-emitting diodes (OLEDs) with specific materials and structures is discussed.
Abstract: Recent electroluminescence (EL) detected magnetic resonance and transient EL studies reveal the presence and role of holes that drift beyond the recombination zone and approach the cathode in small molecular organic light-emitting diodes (OLEDs) with specific materials and structures. In particular, these studies suggest that these holes are responsible for trion (i.e., a bipolaron stabilized by a counterpolaron on an adjacent molecule) formation in the electron transport layer, and may contribute to EL spikes observed at the end of a bias pulse. The significance of these holes to overall OLED performance is discussed.
TL;DR: The mechanisms of chemical dedoping of conductive polypyrrole with the initial conductivity values differing by an order of magnitude have been studied using EPR spectroscopy.
Abstract: The mechanisms of chemical dedoping of conductive polypyrrole with the initial conductivity values differing by an order of magnitude have been studied using EPR spectroscopy. An analysis of the temperature dependences of the electrical resistance of the initial highly conductive samples and dedoped samples has revealed a transition from three-dimensional conduction in the initial samples to one-dimensional conduction in the dedoped samples. This transition is caused by the transformation of transverse bipolarons into longitudinal bipolarons and ipolarons. It has been established that transverse and longitudinal quasiparticles are stabilized by counterions involving iron ions in different spin states: the spin of iron in the counterion of a transverse bipolaron is 2 and iron is inactive in the EPR spectrum, whereas in the counterion of longitudinal quasiparticles, the spin of iron is 5/2 and iron becomes paramagnetic.
01 Jan 2011
TL;DR: In this article, a closed analytical expression for charge carrier velocity dependence on electric field has been derived and analyzed in detail, with special attention given to: dynamical behavior of electrons in rigid chains, band structure of regular polynucleotide chains, dynamics of polaron states formation in Holstein chain, polaron motion in an electric field, role of dispersion, Bloch oscillations and breather states, the bipolaron mechanism of a superconducting state in DNA based on Holstein-Habbard model.
Abstract: General properties of electron motion in molecular chains are considered. The charge motion is described in terms of quantum mechanics, whereas vibrational degrees-offreedom are treated both classically and quantum mechanically. A typical charge transfer/transport pattern can physically be viewed as a polaron and/or soliton. A closed analytical expression for charge carrier velocity dependence on electric field has been derived and analyzed in detail. Special attention is given to: dynamical behavior of electrons in rigid chains, band structure of regular polynucleotide chains, dynamics of polaron states formation in Holstein chain, polaron motion in an electric field, the role of dispersion, Bloch oscillations and breather states, the bipolaron mechanism of a superconducting state in DNA based on Holstein-Habbard model.
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TL;DR: In this paper, the authors explain the observed specific heat anomaly (and hence entropy change) in the colossal magnetoresistive manganite Sm0.55Sr0.45MnO3 by introducing phase separation into the current carrier density collapse theory via the notion of the ferromagnetic volume fraction.
Abstract: We explain the observed specific heat anomaly (and hence entropy change) in the colossal magnetoresistive manganite Sm0.55Sr0.45MnO3, by introducing phase separation into the current carrier density collapse theory via the notion of the ferromagnetic volume fraction. Within the same framework, we have also been able to explain the observed electrical resistivity of Sm0.55Sr0.45MnO3 by using appropriate expressions governing the scattering mechanisms far away from the transition. Fitting specific heat and resistivity results has allowed us to separate the hopping activation energy of polaronic carriers and the bipolaron binding energy contributions to the exponential behaviour of resistivity in the paramagnetic phase.
TL;DR: In this paper, a possible mechanism of polaron to bipolaron transition is investigated based on the well known SSH model, and it is found that when the relative velocity of collision is not very large, the collision leads to a direct transition of two polarons to a bipolaron.
Abstract: A possible mechanism of polaron to bipolaron transition is investigated based on the well known SSH model. We have studied the dynamical process of collision between two positively charged polarons with same spin in the presence of external electric fields. It is found that when the relative velocity of collision is not very large, the collision leads to a direct transition of two polarons to a bipolaron.
TL;DR: In this article, the poly(3-methylthiophene) (P3MT) was synthesized electrochemically on transparent conductive substrates of F-doped tin oxide (FTO), using as supporting electrolyte Et 4 NBF 4 or LiClO 4 in acetonitrile.
Abstract: In this work was synthesized electrochemically the poly(3-methylthiophene) (P3MT) on transparent conductive substrates of F-doped tin oxide (FTO), using as supporting electrolyte Et 4 NBF 4 or LiClO 4 in acetonitrile. To dedope the material, the samples were reduced electrochemically and chemically (in NH 4 OH solution). The films were characterized by electron paramagnetic resonance spectroscopy (EPR) and infrared spectroscopy (FTIR) and their optical properties examined by ultraviolet-visible absorption spectroscopy (UV-Vis) and photoluminescence (PL). Scanning electron microscopy (SEM) images of the first synthesized samples were performed to verify the degree of homogeneity of them. The FTIR spectra show the characteristic features of P3MT and, together with the results of EPR, allowed to evaluate the charge carriers and to identify benzoid and quinoid structures in the formation of chains. The UV-Vis absorption spectra helped to obtain the average gap energy of P3MT presenting the ?-?* transition band and a bipolaron band. In the PL spectra, with variation of excitation power, studies were performed through the fit with Gaussian functions, given the strong probability of the existence of two distinct contributions in the formation of the emission spectra. These two contributions were assigned to the emission of mixed chains (Gaussian centered at higher energy) and emission of benzoid chains (Gaussian of lower energy) in the formed polymeric material.
TL;DR: In this paper, the motion of a bipolaron under an external electric field is theoretically investigated in a conjugated polymer with magnetic impurities, and it is shown that the bipolaron can pass through the magnetic impurity, or break down into two polarons with different spins, depending on the width of the impurity region.
Abstract: Both polarons and bipolarons are composite particles with internal structures in nondegenerate conjugated polymers. A bipolaron is a spinless species with charge ±2|e|, while a polaron is a spin-bearing one (spin 1/2, charge ±|e|). Serving as charge carriers, they play an important role in the transport properties of polymer-based optoelectronic devices. By using a nonadiabatic dynamic method, the motion of a bipolaron under an external electric field is theoretically investigated in a conjugated polymer with magnetic impurities. Our results show that a bipolaron can pass through the magnetic impurities, or break down into two polarons with different spins, or be trapped by the magnetic impurities, depending on the width of the impurity region. When the width of the impurity region is comparable to the polaron width, the bipolaron is transformed into two polarons easily. As a polaron and a bipolaron possess different spin characteristics, the decomposition of bipolarons induced by the magnetic impurities ...