Showing papers on "Bipolaron published in 2002"
TL;DR: In this paper, the geometric and electronic structures of oligothiophene dications were revisited using the spin-unrestricted broken symmetry hybrid density functional B3LYP method, and it was found that there exists a transition region of bipolaron to two-polaron structure conversion in the moderately sized oligomers, as that had been reported earlier in an AM1-CI calculation.
Abstract: The geometric and electronic structures of oligothiophene dications (with 6 to 12 monomers) have been revisited using the spin-unrestricted broken symmetry hybrid density functional B3LYP method. It is found that there exists a transition region of bipolaron to two-polaron structure conversion in the moderately sized oligomers, as that had been reported earlier in an AM1-CI calculation. According to our calculation, the transition region should be from hexamer to octamer. TD-DFT simulation led to a different rationalization of the experimental UV/visible spectra, which suggested the coexistence of bipolaron and two-polaron state in the transition region.
70 citations
TL;DR: In this article, the authors have carried out symmetrized density-matrix renormalization-group calculations to study the nature of excited states of long polyacene oligomers within a Pariser-Parr-Pople Hamiltonian.
Abstract: We have carried out symmetrized density-matrix renormalization-group calculations to study the nature of excited states of long polyacene oligomers within a Pariser-Parr-Pople Hamiltonian. We have used the ${C}_{2}$ symmetry, the electron-hole symmetry, and the spin parity of the system in our calculations. We find that there is a crossover in the lowest dipole forbidden two-photon state and the lowest dipole allowed excited state with size of the oligomer. In the long system limit, the two-photon state lies below the lowest dipole allowed excited state. The triplet state lies well below the two-photon state and energetically does not correspond to its description as being made up of two triplets. These results are in agreement with the general trends in linear conjugated polymers. However, unlike in linear polyenes wherein the two-photon state is a localized excitation, we find that in polyacenes, the two-photon excitation is spread out over the system. We have doped the systems with a hole and an electron and have calculated the charge excitation gap. Using the charge gap and the optical gap, we estimate the binding energy of the ${1}^{1}{B}^{\ensuremath{-}}$ exciton to be 2.09 eV. We have also studied doubly doped polyacenes and find that the bipolaron in these systems, to be composed of two separated polarons, as indicated by the calculated charge-density profile and charge-charge correlation function. We have studied bond orders in various states in order to get an idea of the excited state geometry of the system. We find that the ground state, the triplet state, the dipole allowed state, and the polaron excitations correspond to lengthening of the rung bonds in the interior of the oligomer while the two-photon excitation corresponds to the rung bond lengths having two maxima in the system.
57 citations
TL;DR: In this article, a modified model for bipolaron correlated barrier hopping (CBH) is proposed, since the usual bipolaron CBH model has been found to be inconsistent with experimental data for ac conductivity for chalcogenide glasses at high temperatures.
Abstract: In this study, a modified model for bipolaron correlated barrier hopping (CBH) is proposed, since the usual bipolaron CBH model has been found to be inconsistent with experimental data for ac conductivity for chalcogenide glasses at high temperatures. The present model is based on the existence of a focal point when the experimental data are extrapolated to high temperatures on the σ(ω)–1/T plot. The correct analysis must introduce the concept of the Meyer–Neldel rule in the expression of the relaxation time. The results of the present model are in quantitative agreement with reported ac conductivity measurements.
42 citations
TL;DR: In this paper, the nature of the charge carriers in conducting polypyrrole highly doped with p-toluenesulphonate and their behavior under pressure was investigated by high-pressure Raman spectroscopy.
Abstract: The nature of the charge carriers in conducting polypyrrole highly doped with p-toluenesulphonate and their behavior under pressure was investigated by high-pressure Raman spectroscopy. Changes of band positions and relative intensities of bands assigned to the radical cation (polaron) or the dication (bipolaron) were observed. The results are discussed in the context of structural and electronic aspects by comparing the pressure effects with those observed in the Raman spectrum obtained from oxidized or reduced samples. The changes of the relative number of both charge-carrier species indicated in the Raman spectra under pressure are explained by a shift in the character of the dominant charge carriers from bipolarons to polarons.
29 citations
TL;DR: The electrochemical oxidation of a dicyanovinylidene-bridged CPDT dimer gave a dication that had a quinoid-like structure.
22 citations
TL;DR: In this paper, the Rice-Sneddon model was used to calculate the activation energy of the polarons and bipolarons in a three-dimensional version of a Peierls insulator.
Abstract: ${\mathrm{BaBiO}}_{3}$ is a prototype ``charge ordering system'' forming interpenetrating sublattices with nominal valence ${\mathrm{Bi}}^{3+}$ and ${\mathrm{Bi}}^{5+}.$ It can also be regarded as a three-dimensional version of a Peierls insulator, the insulating gap being a consequence of an ordered distortion of oxygen atoms. When holes are added to ${\mathrm{BaBiO}}_{3}$ by doping, it remains insulating until a very large hole concentration is reached, at which point it becomes superconducting. The mechanism for insulating behavior of more lightly doped samples is formation of small polarons or bipolarons. These are self-organized point defects in the Peierls order parameter, which trap carriers in bound states inside the Peierls gap. We calculate properties of the polarons and bipolarons using the Rice-Sneddon model. Bipolarons are the stable defect; the missing pair of electrons come from an empty midgap state built from the lower Peierls band. Each bipolaron distortion also pulls down six localized states below the bottom of the unoccupied upper Peierls band. The activation energy for bipolaron hopping is estimated.
16 citations
TL;DR: In this article, the dielectric relaxation in SrTiO3, K0.989Li 0.011TaO3 was analyzed and it was shown that such a behavior is a consequence of the dynamic properties of bipolarons, which undergo a transition from thermally activated hopping to a tunneling regime.
Abstract: The dielectric relaxation in SrTiO3, Sr0.942Ca0.058TiO3, and K0.989Li0.011TaO3 was analyzed. While at higher temperatures the characteristic relaxation frequency follows the Arrhenius behavior, its value is temperature independent at low temperatures. It is shown that such a behavior is a consequence of the dynamic properties of bipolarons, which undergo a transition from thermally activated hopping to a tunneling regime.
15 citations
TL;DR: In this paper, a new analysis of electronic transport in chalcogenide glasses, based on bipolaron hopping in the extended pair approximation, is presented, where the relaxation time of the carrier when hopping has a Meyer-Neldel type of temperature dependence instead of a simple activated form.
Abstract: A new analysis of electronic transport in chalcogenide glasses, based on bipolaron hopping in the extended pair approximation, is presented. It is assumed that the relaxation time of the carrier when hopping has a Meyer-Neldel type of temperature dependence instead of a simple activated form. In this way, the experimental data for both dc conductivity and ac conductivity can be fitted over a wide range of temperatures, and for a number of glasses using the same set of parameters.
14 citations
TL;DR: In this article, the canonical transformation of coordinates for a strong-coupled system of electron and phonon field was formulated for a bipolaron, taking into account the rigorous fulfillment of the conservation laws.
Abstract: The method of canonical transformation of coordinates is formulated for a strong-coupled system of electron and phonon field, taking into account the rigorous fulfillment of the conservation laws. The system of electronically excited terms of a bipolaron has been established. The sequence of the excited terms and their dependence on the distance between polarons have been obtained.
12 citations
TL;DR: In this article, a photo-emission study of Ti4O7 around its two transition temperatures was performed to cover the metallic, high-temperature insulating ("bipolaron-liquid") and low-temperate insulating (bipolar-on-crystal) phases.
Abstract: We have performed a photoemission study of Ti4O7 around its two transition temperatures so as to cover the metallic, high-temperature insulating ("bipolaron-liquid") and low-temperature insulating (bipolaron-crystal) phases. While the spectra of the low-temperature insulating phase show a finite gap at the Fermi level, the spectra of the high-temperature insulating phase are gapless, which is interpreted as a soft Coulomb gap due to dynamical disorder. We suggest that the spectra of the high-temperature disordered phase of Fe3O4, which exhibits a charge order-disorder transition (Verwey transition), can be interpreted in terms of a Coulomb gap.
11 citations
TL;DR: In this paper, a model of the disordered phases of selenium based upon ideas akin to small-world networks is presented, which accounts for the description of defect centers of liquid and amorphous Se, which can be introduced in controllable manner in a broad interval of concentrations.
Abstract: In this work, we present a model of the disordered phases of selenium based upon ideas akin to small-world networks. This model accounts for the description of defect centers of liquid and amorphous Se, which can be introduced in a controllable manner in a broad interval of concentrations. The electronic structure is described by a tight-binding Hamiltonian, which is extended by a Hubbard term to take the influence of an effective electron-electron interaction into account. The resulting electronic structure problem is solved self-consistently. The electronic structure is analyzed in terms of the density of states, the charge order originating from defect states, the charge distribution function, and the localization properties of the eigenfunctions. Only bipolaron states---which compensate the charge of anionic dangling bonds---and positively charged centers with a threefold coordination give rise to impurity states within the band gap of the unperturbed system: The latter species corresponds to one of the defects of the Kastner, Adler, and Fritzsche model of selenium. Within the models studied here, states corresponding to negatively charged singly coordinated atoms lie deep within the bulk of the density of states, and their positively charged counterparts are not observed at all.
TL;DR: In this paper, the interface between poly (9,9-dioctylfluorene) (PFO) and different alkali metals (Cs, K, Na, and Li) was studied by photoelectron spectroscopy.
Abstract: In this article we study the interface between poly (9,9-dioctylfluorene) (PFO) and different alkali metals (Cs, K, Na, and Li) by photoelectron spectroscopy. The low work-function alkali metals led to low or no electron injection barrier at the PFO interface. From the ultraviolet photoelectron spectroscopy, alteration of electronic structures upon Cs, K, Na, or Li doping into PFO represented a charge transfer process among them. Two new gap states known as bipolaron states were found above the highest-occupied molecular orbital of PFO. Variations in the intensity and feature of these gap states with increasing coverage of the alkali metals were correlated with changes of C 1s shakeup peaks acquired from x-ray photoelectron spectroscopy. From the deduced energy level diagram, it is suggested that the new gap states may reduce the radiative recombination of holes and electrons in the polymer light-emitting devices. Films exposed either to residual gases at a pressure of 2.0×10−9 mbar for 3 h or to small am...
TL;DR: In this article, the stability criterion for bipolaron formation in a spherical quantum dot was retrieved in the strong-electron-phonon-coupling regime, where a pair of electrons were immersed in a reservoir of bulk LO phonons and confined within an isotropic parabolic potential box.
Abstract: In the strong-electron-phonon-coupling regime, we retrieve the stability criterion for bipolaron formation in a spherical quantum dot. The model that we use consists of a pair of electrons immersed in a reservoir of bulk LO phonons and confined within an isotropic parabolic potential box. In this particular quasi-zero-dimensional geometry, where the electrons do not have any free spatial direction to expand indefinitely, a plausible approach would be to treat the electrons either to form a bipolaronic bound state or enter a state of two close, but individual polarons inside the same dot. The confined two-polaron model adopted here involves the polaron-polaron separation introduced as an adjustable parameter to be determined variationally. It is found that the fundamental effect of imposing such a variational flexibility is to modify the phase diagram to a considerable extent and to sustain the bipolaron phase in a broader domain of stability.
TL;DR: In this paper, the bipolaron energy of a triplet bipolaron was calculated for various distances between the centers of polarization wells of two polarons with accounting the electron correlations, and a prediction was made about a possibility of Wigner crystallization of a polaron gas which occurs with antiferromagnetic ordering in the polaron svstem.
Abstract: Bipolaron energy is calculated for various distances between the centers of polarization wells of two polarons with accounting the electron correlations. A singlet bipolaron is stable at a rather high energy of ion binding η < η m 0.143, η = e/e 0 (e x and e 0 are the high-frequency and static dielectric constants, respectively). The unique energy minimum corresponds to a one-center bipolaron (analog of a helium atom). The bipolaron binding energy constitutes up to 25.8 % of a double polaron energy at η → 0. A triplet bipolaron (analog of ortho-helium) is energetically disadvantageous. The one-center configuration of a triplet bipolaron corresponds to a maximum on the distance dependence of the total energy J Bp (R). The exchange interaction between polarons has antiferromagnetic character. A prediction is made about a possibility of Wigner crystallization of a polaron gas which occurs with antiferromagnetic ordering in the polaron svstem.
TL;DR: In this article, the electronic structure of poly(9,9-dioctylfluorene) (PFO) coated with potassium has been studied by X-ray photoelectron spectroscopy (XPS) and ultraviolet photo-electron (UPS).
TL;DR: In this paper, the long-range electron-phonon interaction was proposed to explain high values of the superconducting critical temperature T c without adjustable parameters, and the model quantitatively explains high T c values without any fitting parameters and other key features of the cuprates.
Abstract: Over the last decade, several competing models of high-temperature superconductivity (HTSC) were proposed, none of which have succeeded to explain high values of the superconducting critical temperature T c without adjustable parameters. Most of the proposed models are based on the short-range electron-electron interaction or/and on a short-range electron-phonon interaction. Here we propose a model of HTSC which explicitly takes into account the long-range origin of both types of interaction. Remarkably, the long-range electron-phonon interaction binds carriers into superlight real-space pairs-bipolarons-with a relatively low mass but sufficient binding energy. The model quantitatively explains high T c values without any fitting parameters and other key features of the cuprates.
TL;DR: In this article, far-infrared, midinfrared and Raman spectroscopy were used to characterize iodine-doped poly(isothianaphthene) (PITN) films and powders.
Abstract: Far-infrared, mid-infrared, and Raman spectroscopy were used to characterize iodine-doped poly(isothianaphthene) (PITN) films and powders. The far-infrared and mid-infrared results show changes from absorption mode to reflective mode as the doping level increases, consistent with the iodine-doped PITN becoming more metallic and more conductive at higher doping levels. The far-IR and Raman (514.5-nm laser excitation) results show that I3- is dominant in iodine-doped PITN. The Raman spectral changes observed using 1064-nm excitation are different from those measured using 514.5-nm excitation. The spectra recorded with 514.5-nm excitation show features due to the undoped parts of the polymer, and these indicate that the effective conjugated chain length decreases with increased doping. The Raman spectra obtained by using 1064-nm excitation show features due to polaron and bipolaron states in the doped polymer.
TL;DR: In this article, the stability of two-dimensional bipolarons in a two-axis elliptic potential well of harmonic boundaries was analyzed in the limit of strong electron-phonon coupling, where the mean polaron-polaron separation was treated as an adjustable parameter to be determined variationally.
Abstract: In the limit of strong electron-phonon coupling, we analyze the stability of two dimensional bipolarons in a two-axis elliptic potential well of harmonic boundaries. The confined two-polaron wavefunction adopted here makes the electrons to form either a bipolaronic bound state or go into a composite state of two separated polarons bounded inside the same potential well. The methodology involves the mean polaron-polaron separation treated as an adjustable parameter to be determined variationally. By tuning the barrier slopes of the confining potential we obtain an explicit tracking of the criterion for bipolaron stability encompassing the particular cases of a two dimensional circular dot or a planar strip-like quantum well wire. We observe that, while an increased degree of confinement enhances bipolaronic stability, the effect of anisotropy is to inhibit bipolaron formation.
TL;DR: In this article, the photoexcitation process in a conducting polymer with a non-degenerate ground state, where the carrier without spin is a bipolaron, was theoretically studied and it was predicted that this photoinduced phenomenon can be detected experimentally by observing the change of magnetic susceptibility.
Abstract: We theoretically study the photoexcitation process in a conducting polymer with a non-degenerate ground state, where the carrier without spin is a bipolaron. The dynamical simulation reveals a photoinduced phenomenon in which a positive bipolaron is split into two polarons by absorbing one photon. The relaxation time of this photoinduced bipolaron split is about 40 fs. It is predicted that this photoinduced phenomenon can be detected experimentally by observing the change of magnetic susceptibility.
TL;DR: In this article, the stability condition of large bipolarons confined in a parabolic potential containing certain parameters and a uniform magnetic field was investigated, and an analytical expression for the bipolaron energy was found, from which the ground and excited-state energies were obtained numerically by minimization procedure.
Abstract: We investigate the stability condition of large bipolarons confined in a parabolic potential containing certain parameters and a uniform magnetic field. The variational wave function is constructed as a product form of electronic parts, consisting of center of mass and internal motion, and a part of coherent phonons generated by Lee-Low-Pines transformation from the vacuum. An analytical expression for the bipolaron energy is found, from which the ground and excited-state energies are obtained numerically by minimization procedure. The bipolaron stability region is determined by comparing the bipolaron energy with those of two separate polarons, which is already calculated within the same approximation. It is shown that the results obtained for the ground state energy of bipolarons reduce to the existing works in zero magnetic field. In the presence of a magnetic field, the stability of bipolarons is examined, for three types of low-dimensional system, as function of certain parameters, such as the magnetic-field, the electron-phonon coupling constant, Coulomb repulsion and the confinement strength. Numerical solutions for the energy levels of the ground and first excited states are examined as functions of the same parameters.
TL;DR: In this article, the collision processes between two acoustic polarons, two acoustic bipolarons, and a polaron and a bipolaron in a one-dimensional electron-lattice system are analyzed by numerical simulations based on Su, Schrieffer and Heeger's model extended to include short-ranged Coulombic electron-electron interactions.
Abstract: The collision processes between two acoustic polarons, two acoustic bipolarons, and a polaron and a bipolaron in a one-dimensional electron-lattice system are analyzed by numerical simulations based on Su, Schrieffer and Heeger's model extended to include short-ranged Coulombic electron-electron interactions. The collisions between two polarons with the same spin and between two bipolarons are essentially elastic because of a rather strong repulsion due to the Pauli principle. On the other hand, those between two polarons with opposite spins and between a polaron and a bipolaron show some inelastic behaviors depending on the relative velocity of the two objects before the collision. This knowledge will serve to understand the dynamics of the system when many polarons and/or bipolarons are created in the system e.g. by photoexcitation.
TL;DR: In this article, the first bound state of two polarons was found by solving an integral equation, which corresponds to a Schrodinger equation describing internal vibrations of a bipolaron, and the Hamiltonian describing these vibrations was separated from the complete Hamiltonian of the electron-phonon system by using the Bogoliubov-Tyablikov method of canonical transformations of coordinates.
Abstract: The ranges of ɛ*/ɛ∞ and of the electron-phonon coupling constant in which the three-dimensional bipolaron exists are determined. The limits of these ranges correspond to the emergence of the first bound state of two polarons. The criteria for the first bound state to arise are found by solving an integral equation, which corresponds to a Schrodinger equation describing internal vibrations of a bipolaron. The Hamiltonian describing these vibrations is separated from the complete Hamiltonian of the electron-phonon system by using the Bogoliubov-Tyablikov method of canonical transformations of coordinates.
TL;DR: In this article, the electron spin resonance (ESR) and electron detected magnetic resonance (EDMR) were applied in MH-PPV and MEHPPV, respectively.
Abstract: In this work the electron spin resonance (ESR) and electron detected magnetic resonance (EDMR) are applied in MH-PPV and MEH-PPV, respectively. The ESR measurements revealed that there is a change of lineshape and a clear reduction of spin density on MH-PPV degraded by light and air. This result is in agreament with the fact that the spin density resides predominantly on vinylic carbon. The EDMR measurements in MEH-PPV LEDs provided evidences that there is a spin-dependent fusion of two polarons to spinless bipolaron.
27 Feb 2002
TL;DR: In this article, the electronic structure of poly(9,9-dioctylfluorene)(PFO) in different molecular weights coated with alkali metals (K and Cs) has been studied by x-ray photoelectron spectroscopy (XPS) and ultraviolet photo-electron spectrum analysis (UPS).
Abstract: The electronic structure of poly(9,9-dioctylfluorene)(PFO) in different molecular weights coated with alkali metals (K and Cs) has been studied by x-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). No significant differences on the electronic structures were observed for all combinations of alkali metals and PFO in different molecular weights. These metals led to low injection barrier at the K/PFO or Cs/PFO interfaces and induced bipolaron gap states in the PFO. With increasing coverage of the alkali metals, the bipolaron states and the highest-occupied molecular orbitals gradually broadened, and the two shake-up peaks of Cls XPS peaks associated with the lowest-unoccupied molecular orbitals of PFO also broadened and greatly diminished. Upon slight oxygen exposure, the two bipolaron states disappeared and the deformed features in the UPS and XPS spectra were partially recovered.
TL;DR: In this article, the geometrical and electronic structure for an open chain with finite length of an emeraldine-base polymer was investigated, and the numerical results were very similar to those in the Born-von Karman condition.
Abstract: Adding a revision term H' to the Baranowski-Buttner--Voit (BBV) model, we investigate the geometrical and electronic structure for an open chain with finite length of an emeraldine-base polymer. The numerical results are very similar to those in the Born-von Karman condition. On this basis, excitons and biexcitons are explored. Our solutions show that the exciton and biexciton in the period-4 EB polymer are not self-trapped like those in the LPPP or PNB, but form discrete polaron pairs and bipolaron pairs, respectively.
01 Jan 2002
TL;DR: The ground state of cuprates is thought to be a charged Bose-liquid of intersite bipolarons with single polarons existing only as thermal excitations as discussed by the authors, and it is the bipolaron energy dispersion rather than a particular pairing interaction that is responsible for the d wave symmetry.
Abstract: We argue that local pairs (bipolarons), formed by any short-range attraction are localized and cannot give rise to high-Tc no matter whether they are hybridized with Fermions. However, a long-range Frohlich electron-phonon interaction can provide mobile intersite bipolarons in the CuO2 plane condensing at high Tc. The ground state of cuprates is thought to be a charged Bose-liquid of intersite bipolarons with single polarons existing only as thermal excitations. We show that some bipolaron configurations lead to a d wave charged Bose-Einstein condensate in cuprates. It is the bipolaron energy dispersion rather than a particular pairing interaction that is responsible for the d wave symmetry. Single-particle spectral density is derived, taking into account realistic band structure and disorder. The tunneling and photoemission (PES) spectra of cuprates are described.
01 Jan 2002
TL;DR: In this article, a finite-range Frohlich interaction with optical phonons was proposed to evaluate the role of different interactions in high-temperature superconductors, where the electron-phonon (elph) interaction is long-range in the cuprates.
Abstract: Over the last decade a few competing models of high-temperature superconductivity were proposed, most of them with short-range interactions due to electron repulsive correlations. However, assessing the role of different interactions in novel superconductors one has to take into account that these materials are highly polarizable ionic lattices, where the Frohlich electron-phonon interaction with optical phonons should be strong. Indeed, a parameter-free estimate[1] based on the measured dielectric constants shows that the polaron binding energy, Ep is about 0.5 eV or larger in oxides. Hence, the Frohlich interaction should play an important role. Also, because of a poor screening, the direct unscreened Coulomb repulsion is important. There are extensive experimental [2, 3, 4, 5, 6, 7] and theoretical studies[8, 9, 10, 11], which prove that the electron-phonon (el-ph) interaction in high-Tc superconductors is exceptionally strong. Electron correlations are strong as well shaping the Mott-Hubbard insulating state of many parent (undoped) compounds [12]. Hence, the theory of high- Tc superconductors must treat both interactions on equal footing as was suggested some time ago [8]. Motivated by the fact that el-ph interaction is long-ranged in the cuprates because of poor screening, we have proposed a new approach to the high-Tc problem introducing a finite-range (Frohlich) interaction [13, 14, 15]. The analytical [13] and Monte-Carlo [15] studies of a simple chain model with a long-range el-ph coupling revealed a several order lower effective mass of the small Frohlich polaron compared with the small Holstein polaron. Later the single-polaron and bipolaron cases of the chain model were analyzed in more detail in Refs.[16] and [17], respectively. These