Showing papers on "Bipolaron published in 2009"

Advances in Polaron Physics

Abstract: Continuum Polaron.- Lattice Polaron.- Bipolaron.- Multipolaron Problem.- Polarons and Bipolarons in Advanced Materials.- Current Status of Polarons and Open Problems.

Electron small polarons and bipolarons in LiNbO3

Abstract: An overview of the properties of electron small polarons and bipolarons is given, which can occur in the congruently melting composition of LiNbO3 (LN). Such polarons influence the performance of this important optical material decisively. Since coupling to the lattice strongly quenches the tunnelling of free small polarons in general, they are easily localized at one site even by weak irregularities of a crystal. The mechanism of their optical absorptions is thus shared with those of small polarons localized by binding to selected defects. It is shown that the optical properties of free electrons in LN as well as those bound to NbLi antisite defects can be attributed consistently to small polarons. This is extended to electron pairs forming bipolarons bound to NbLi?NbNb nearest neighbours in the LN ground state. On the basis of an elementary phenomenological approach, relying on familiar concepts of defect physics, the peak energies, lineshapes, widths of the related optical absorption bands as well as the defect binding energies induced by lattice distortion are analysed. A criterion universally identifying small polaron absorption bands in oxide materials is pointed out. For the bipolarons, the dissociation energy, 0.27?eV, derived from a corresponding study of the mass action behaviour, is shown to be consistent with the data on isolated polarons. Based on experience with simple O? hole small polaron systems, a mechanism is proposed which explains why the observed small polaron optical absorptions are higher above the peak energies of the bands than those predicted by the conventional theory. The parameters characterizing the optical absorptions are seen to be fully consistent with those determining the electrical conductivity, i.e.?the bipolaron dissociation energy and the positions of the defect levels as well as the activation energy of mobility. A reinterpretation of previous thermopower data of reduced LN on the basis of the bipolaron model confirms that the mobility of the free polarons is activated by 0.27?eV. On the basis of the level scheme of the bipolarons as well as the bound and free polarons the temperature dependence of the electronic conductivity is explained. The polaron/bipolaron concept also allows us to account for the concentrations of the various polaron species under the combined influence of illumination and heating. The decay of free and bound polarons dissociated from bipolarons by intense short laser pulses of 532?nm light is put in the present context. A critical review of alternative models, being proposed to explain the mentioned absorption features, is given. These proposals include: single free polarons in the (diamagnetic) LN ground state, oxygen vacancies in their various conceivable charge states, quadpolarons, etc. It is shown why these models cannot explain the experimental findings consistently.

Present knowledge of electronic properties and charge transport of icosahedral boron-rich solids

Abstract: B12 icosahedra or related structure elements determine the different modifications of elementary boron and numerous boron-rich compounds from α-rhombohedral boron with 12 to YB66 type with about 1584 atoms per unit cell. Typical are well-defined high density intrinsic defects: Jahn-Teller distorted icosahedra, vacancies, incomplete occupancies, statistical occupancies and antisite defects. The correlation between intrinsic point defects and electron deficiencies solves the discrepancy between theoretically predicted metal and experimentally proved semiconducting character. The electron deficiencies generate split-off valence states, which are decisive for the electronic transport, a superposition of band-type and hopping-type conduction. Their share depends on actual conditions like temperature or pre-excitation. The theoretical model of bipolaron hopping is incompatible with numerous experiments. Technical application of the typically p-type icosahedral boron-rich solids requires suitable n-type counterparts; doping and other possibilities are discussed.

Bipolaron in the t-J model coupled to longitudinal and transverse quantum lattice vibrations.

Abstract: We explore the influence of two different polarizations of quantum oxygen vibrations on the spacial symmetry of the bound magnetic bipolaron in the context of the t-J model by using exact diagonalization within a limited functional space. Linear as well as quadratic electron-phonon coupling to transverse polarization stabilize d-wave symmetry. The existence of a magnetic background is essential for the formation of a d-wave bipolaron state. With increasing linear electron-phonon coupling to longitudinal polarization the symmetry of a d-wave bipolaron state changes to a p wave. Bipolaron develops a large anisotropic effective mass.

Electrochemical Properties of Thin Films of Polythiophene Polymerized on Basal Plane Platinum Electrodes in Nonaqueous Media

Abstract: In this paper the electrochemical properties of polythiophene thin films synthesized on single-crystal platinum electrodes are studied. It was found that the electrochemical properties, ion transport kinetics, and morphology of the polythiophene films depend on the surface orientation of the single-crystal platinum electrode used for their electropolymerization. Different oxidation levels, regarded as neutral, polaron, bipolaron, and metallic states, are usually found in conjugated heterocyclic polymers. However, the transitions between the different oxidation levels were never clearly observed in cyclic voltammetry. Instead the voltammograms usually show broad oxidation and reduction peaks with some shoulders. With the use of single-crystal platinum electrodes, it was found that polythiophene has a well-defined redox process at low potential, not observed before, possibly related to the conversion from the neutral state to polarons. On the other hand, two well-defined consecutive steps were found during the ion exchange reaction of thin films of polymer, both characterized by nucleation kinetics. This is the first report of two consecutive nucleation processes during the ion exchange process of a conducting polymer. The results presented here could further illuminate the mechanism in which the electron is transported in organic semiconductor materials.

Scattering processes between bipolaron and exciton in conjugated polymers

Abstract: The scattering processes between a bipolaron and an exciton in a conjugated polymer chain are investigated by numerical simulations based on the model extended with an external electric field of Su et al. It is found that there are two channels for these scattering processes. In one channel, the exciton is annihilated and the bipolaron dissociates into a polaron and an excited polaron. In the other channel, the bipolaron and the exciton are converted into each other. Furthermore, the probabilities for these two channels are calculated, and we found that they depend sensitively on the strength of the electric field. Our results show that the interactions of bipolaron-exciton open a channel to enhance the quantum efficiency of electroluminescence due to radiative decay of the excited polaron.

Periodic density functional theory studies of Li-doped polythiophene: Dependence of electronic and structural properties on dopant concentration

Abstract: We report periodic B3LYP6-31G(**) density functional theory calculations on Li-doped polythiophene at various dopant concentrations using (SC(4)H(2))(m)Li(2) unit cells for m=2, 6, and 10. Uniform doping by Li atoms and by pairs of Li atoms on adjacent thiophene rings are considered with the primary aim of comparing polaron versus bipolaron properties. Properties examined include geometries, charge distributions, polaron/bipolaron formation energies, dopant binding energies, band structures, and densities of states.

Organic single-crystal surface-induced polymerization of conducting polypyrroles.

Abstract: Polypyrrole hexagonal microplates (PHMs) (50-100 microm long, 10-20 microm wide, and 0.8-1.2 microm thick) with a quasicrystalline structure and high electrical conductivity (up to 400 S/cm) are simply fabricated using single crystals of 4-sulfobenzoic acid monopotassium salt (KSBA) in aqueous medium. Moreover, the fabrication process described here differs strikingly from traditional methods, such as template-free, soft template, and hard template methods. Synthetic time-resolved polypyrrole (PPy) morphology dynamics reveals that the fabrication process of PHMs composed of PPy nanostructures combines a shape-copying process for forming a PPy preform that imitates the shape of a KSBA single crystal and the self-assembly process of PPys on the preform. The PHMs exhibit the improved pi-stacking and bipolaron structure. The strong pi-stacks among PPy rings of bipolaron structures lead to a high quasicrystalline structural order and the metallic conduction. Other single organic crystals that can act as dopants could also be grown using this approach, which will also enable the fabrication of complex micro/nanostructures on organic single crystals.

Bipolarons from long-range interactions: Singlet and triplet pairs in the screened Hubbard-Fröhlich model on the chain

Abstract: We present details of a continuous-time quantum Monte Carlo algorithm for the screened Hubbard-Frohlich bipolaron. We simulate the bipolaron in one dimension with arbitrary interaction range in the presence of Coulomb repulsion, computing the effective mass, binding energy, total number of phonons associated with the bipolaron, mass isotope exponent, and bipolaron radius in a comprehensive survey of the parameter space. We discuss the role of the range of the electron-phonon interaction, demonstrating the evolution from Holstein to Frohlich bipolarons and we compare the properties of bipolarons with singlet and triplet pairing. Finally, we present simulations of the bipolaron dispersion. The bandwidth of the Frohlich bipolaron is found to be broad and the decrease in bandwidth as the two polarons bind into a bipolaron is found to be far less rapid than in the case of the Holstein interaction. The properties of bipolarons formed from long-range electron-phonon interactions, such as light strongly bound bipolarons and intersite pairing when Coulomb repulsion is large, are found to be robust against screening, with qualitative differences between Holstein and screened Frohlich bipolarons found even for interactions screened within a single lattice site.

Correlation effects on the dynamics of bipolarons in nondegenerate conjugated polymers.

Abstract: By employing an adaptive time-dependent density-matrix-renormalization-group method, we investigate the dynamics of a charged bipolaron in the presence of both electron-phonon and electron-electron interactions. We use a Su–Schrieffer–Heeger model modified to include electron-electron interactions via a Hubbard Hamiltonian, a Brazovskii–Kirova symmetry-breaking term, and an external electric field. Our results show that the velocity of the bipolaron increases first and then decreases with the increasing of the on-site Coulomb interaction, U. Furthermore, the dependence of the bipolaron velocity, bipolaron effective mass, and bipolaron stability on the lattice structures is discussed.

Enhancement of the Hall-Lorenz number in optimally doped YBa2Cu3O7−d

Abstract: Electronic heat transport in the normal state of a high-quality single crystal of the optimally doped superconductor YBa2Cu3O6.95 was studied by measurements of longitudinal and transverse transport coefficients. For the temperature range from 100 to 300 K, the Hall-Lorenz number (Lxy) depends weakly on temperature and is about two times larger than the Sommerfeld value of the Lorenz number L0=π2/3. Our results can be interpreted using a Fermi liquid model when effects of the pseudogap that opens at the Fermi level are included. However, we find that the bipolaron model can also explain both the enhanced value and the weak temperature dependence of the Hall-Lorenz number.

Bipolaron Jahn–Teller Pairing and Charge Transport in Cuprates

Abstract: Recently the mechanism for an intersite pairing was proposed for cuprates, where the coupling of two fold electronic degenerated levels to local lattice models at finite wave vector was introduced. The model is able to describe the stripe phase and offers a possible explanation of the pseudogap. Moreover, we argue that the single particle and pair conductivity may differ. For a better analysis on this issue, we compute the charge mobility arising from single particle using a variable range hopping model. The general trend is that the mobility has a crossover from 1/T temperature behavior at high temperature to a strong reduction of the mobility at low temperatures.

Enhancement of the Hall-Lorenz number in optimally doped YBa2Cu3O_7-d

Abstract: Electronic heat transport in the normal state of a high-quality single crystal of optimally-doped superconductor YBa2Cu3O6.95 was studied by measurements of longitudinal and transverse transport coefficients. For the temperature range from 100 to 300 K, the Hall-Lorenz number (Lxy) depends weakly on temperature and is about two times larger than the Sommerfeld value of the Lorenz number Lo = (pi^2)/3. Our results can be interpreted using a Fermi liquid model when effects of the pseudogap that opens at the Fermi level are included. However, we find that the bipolaron model can also explain both the enhanced value and the weak temperature dependence of the Hall-Lorenz number.

Ground-state properties of the two-site Hubbard-Holstein model: an exact solution.

Abstract: We study the two-site Hubbard-Holstein model by using an extended phonon coherent state. For the nontrivial singlet bipolarons, the double occupancy probability, the fidelity and the entanglement entropy are calculated to characterize the ground-state properties in both two-site and single-site bipolaron-dominated regimes. We use the localized minimum of the fidelity to define a crossover and plot the bipolaron phase diagram, which separates the large and small entanglement region. Furthermore, the relation between the bipolaron entanglement and the correlation functions demonstrates that the large entanglement corresponds to the large magnitude of lattice deformations induced by electrons.

Bipolaron formation and interpolaron interactions in dielectric layers

Abstract: The Bogolyubov–Tyablikov canonical transformation is used to separate the electron coordinates in the polaron Hamiltonian from the coordinates of the polaron center of mass. The conditions are identified under which the electrostatic image forces at an interface between the insulating phases result in spatial confinement of a polaron. It is shown that motion of a polaron in the vicinity of an interface separating dielectric phases is quantized. It is shown that oscillations of polaron center mass lead to an effective long-range interpolaron attraction. The dielectric properties of adjacent phases required for complete compensation of Coulomb interpolaron repulsion are determined. Numerical estimates are obtained for a specific case of the polaron states in the ammonia system.

Formation of Intermediate Coupling Optical Polarons and Bipolarons in Two-Dimensional Systems

Abstract: he formation of the optical polaron and bipolaron in two-dimensional (2D) systems are studied in the intermediate electron-phonon coupling regime. The total energies of 2D polaron and bipolaron are calculated by using the Buimistrov-Pekar method of canonical transformations and analyzed in the weak, intermediate and strong coupling regimes. It is shown that the electron-phonon correlation significantly reduces the total energy of 2D polaron in comparison with the energy of the strong-coupling (adiabatic) polaron. A charge carrier in polar crystals remains localized in a 2D potential well when the electron-phonon coupling constant $\alpha$ is greater than the critical value $\alpha_{c}\simeq2.94$, which is much lower than a critical value of the electron-phonon coupling constant $\alpha$ for a 3D system. The critical values of the electron-phonon coupling constant $\alpha$ and the parameter of Coulomb repulsion between two carriers $\beta=1/(1-\epsilon_{\infty}/\epsilon_{0})$ (where $\epsilon_{\infty}$ and $\epsilon_{0}$ are the high frequency and static dielectric constants, respectively), which determine the bipolaron stability region, are numerically calculated. The obtained results are compared with the ones obtained by using the Feynman path integral method and the modified Lee-Low-Pines unitary transformation method.

Bipolaron formation in the presence of magnetic impurities

Abstract: The formation of bipolarons in the presence of magnetic impurities is studied theoretically. We use the extended Hubbard Su–Schrieffer–Heeger (SSH) model with Brazovskii–Kirova and Kondo interaction terms. Parameters are chosen suitably for cis-polyacetylene. A Quantum Monte Carlo (QMC) algorithm is used to study the equilibrium lattice structure and charge distribution as a function of doping level and Kondo exchange integral. The magnetic impurities can have a destructive effect on bipolaron stability, instead favouring the two polaron configuration. However by suitably adjusting the doping level, bipolarons can be stabilized over a wide range of impurity strength.

In-situ Chemically Polymerized PANi-SWNTs Composites: Characterizations and Gas Sensing Feature

Abstract: An experiment has been made in order to study the impact of SWNTs in in-situ chemically polymerized PANi-SWNTs composites It is found that the SWNTs surface plays a role of nucleation sites for PANi growth The presence of SWNTs enhances the degree of polymerization and increase the conductivity (polaron or bipolaron lattice) of PANi When radiated by a near-infrared laser (1064 nm) the dried materials emit strong fluorescent bands in mid-infrared region which are assumed to be the transitions from the π-orbital (highest occupied molecular orbital) to polaron and bipolaron lower levels The bands are speculated to arise from charge transfer interactions between the PANi and SWNTs, which brings new polaron and bipolaron levels into the band gap of PANi The PANi-SWNTs also exhibits an improvement in NH3 gas sensing characterization in comparison to neat SWNTs and PANi The results are interpreted as a modification in the chemical and electronic structure of PANi

Spin transport and bipolaron density in organic polymers

Abstract: We present a theory for spin-polarized transport through a generic organic polymer connected to ferromagnetic leads with arbitrary angle theta between their magnetization directions, taking into account the polaron and bipolaron states as effective charge and spin carriers. Within a diffusive description of polaron-bipolaron transport including polaron-bipolaron conversion, we find that the bipolaron density depends on the angle theta. This is remarkable, given the fact that bipolarons are spinless quasiparticles, and opens a new way to probe spin accumulation in organic polymers.

Theoretical prediction of a new noncluster lithium boride Li1 − xB9 with one-dimensional ionic and two-dimensional electronic conductivity in mutually perpendicular directions

Abstract: Based on the features of the structure of B5H11 and other known boranes, the possibility of the existence of a new structure type—LiB9 (hexagonal, space group P63cm, a = 0.565 nm, c = 0.504 nm, Z = 2, d = 2.49 g/cm3)—was predicted. The basal plane contains perforated deltahedral layers of boron atoms with delocalized electrons combined into a framework by fixed 3c2e bonds. Discrete, almost cylindrical channels accommodating Li+ cations are perpendicular to the layers. Thermal or electrochemical removal of part of lithium should be favorable for the appearance or buildup of the cationic conductivity with the possible intermediate formation of lithium incommensurate phase. Valence-scheme analysis of boride layers revealed low-barrier hole bipolaron conductivity within the layers and considerable hindrance to interlayer electron transport.

Spin transport and bipolaron density in organic polymers.

Abstract: We present a theory for spin-polarized transport through a generic organic polymer connected to ferromagnetic leads with arbitrary angle ? between their magnetization directions, taking into account the polaron and bipolaron states as effective charge and spin carriers. Within a diffusive description of polaron?bipolaron transport including polaron?bipolaron conversion, we find that the bipolaron density depends on the angle ?. This is remarkable, given the fact that bipolarons are spinless quasiparticles, and opens a new way to probe spin accumulation in organic polymers.

Response Equation Based Thermochemical Analysis of Singlet Bipolaron Structures in Oligo(3-Methyl-Thiophenes)

Abstract: A thermochemical investigation of singlet dication (bipolaron) structures in a set of 3-methyl-thiophene oligo- mers (up to the length of 24-mers) was carried out. Equlibrium structures and heats of formation were calculated with the semiempirical quantum chemical method RHF/PM3 for the doubly oxidized forms. All-cisoid planar structural variants of the form An1Qn2An3 were considered, where A and Q are thiophene monomer units in aromatic and quinoidal chain segments, respectively. The types of segments were determined from their bond-length pattern. At this level of theory, di- verse structures combining both types of segments were found stable. The so-called "response equation based quantitative structure-property relationship" (REQ-QSPR) method, recently pro- posed by Fishtik et al. was used to perform and interpret regression data analysis. The REQ-based approach rationalizes mathematical treatment of stoichiometric equations. It provides a simple concept for breaking down a dataset for heat of formation into linear combination of terms that are easy to interpret in the framework of chemical groups. REQ-QSPR in- cludes arbitrary structural descriptors rather than being restricted to using only the number of chemical constituents. Ap- plied to our system, it is demonstrated that modelling nonlinear effects becomes possible as well. Supra-linear dependence of the energy with respect to the bipolaron localization length is indicated by REQ-QSPR, and detailed analysis suggests specific further calculations to carry out for improving the model.

Evolution of the bipolaron structure in oligo-diacetylene films: a semiempirical study

Abstract: The formation of polaron and/or bipolaron species upon chemical doping and photo- or electro-chemical generation is a scientific and technological problem which is still object of discussion. Here we use a simple electrostatic model to study the role of the interchain interaction in stabilizing or destabilizing the bipolaronic structure, which is usually neglected in theoretical studies. The results show that, as a consequence of the interchain interactions in the bulk system, a bipolaron generated on a single oligomer chain more than 12 units long will soon split into two polarons on adjacent chains, this process being maximized for specific interchain distances.

Bipolaronic excitations of interacting electron(hole) gas in one-dimensional lattice model

Abstract: By developing a diagonalization scheme we observe that the dynamics of interacting electrons or holes locally coupled to dispersionless phonon mode in one-dimensional lattice can be mapped into that of paired electron or hole states (bipolaronic states), which then gives a physically appealing picture of excitation modes of the interacting electron or hole gas. As a result, the bipolaronic model of the interacting electron or hole gas, obtained from Holstein–Hubbard Hamiltonian in one-dimensional lattice, exhibits normalization of the hopping terms and leads to a reduced effective mass of the bipolaron, which essentially gives support to the bipolaronic theory of high T c superconductivity.