Showing papers on "Bipolaron published in 1999"

Electroactive and luminescent polymers: new fluorene-heterocycle-based hybrids

Abstract: The synthesis, characterization, and electrochromic properties of copolymers derived from 9,9-dialkyl-2,7-dibromofluorene (18a, alkyl=C 10 H 21 ; 24, alkyl=Et) and pyrrole, thiophene, 3,4-ethylenedioxythiophene, and furan are described. Two synthetic routes to 9,9-diethyl-2,7-bis(pyrrol-2-yl)fluorene (30) afford product in 30% and 20% yields, respectively. Monomer 30 undergoes electropolymerizationto yield electroactive polymer films. The lowest monomer oxidation potential (E p,m =0.4 V vs. Ag/Ag + ) is found in tetraethylammonium tosylate (TEATOS)-CH 3 CN, but film formation is slow. Spectroelectrochemical analysis of poly(30) reveals a band gap at 2.4 eV and upon polymer oxidation, two low energy absorptions peaking at 1.2 and 2.2 eV appear. This phenomenon is attributed to formation of bipolaron bands between the valence and conduction bands. Soluble fluorene-heterocycle polymers 34a-d have been synthesized by the Stille coupling reaction of 18a and 2,5-bis(trimethylstannyl)thiophene (21a), 5,5′-bis(trimethylstannyl)-2,2′-bithiophene (21b), 2,5-bis(trimethylstannyl)-3,4-ethylenedioxythiophene (21c), and 2,5-bis(trimethylstannyl)furan (22), respectively, in high yields. The NMR spectra are consistent with the proposed structures of the polymers 34a-d, and no evidence of ring opening of the furyl unit in 34d is seen in the NMR and IR spectra. The molecular weights of 34a-d are in the range of 8000 g mol –1 with polydispersity indices (PDI) of 2. Polymers 34a-c have band gaps measured at 2.4 eV, while polymer 34d has its gap at 2.6 eV. Polymers 34a-c undergo solution doping with SbCl 5 to form new low energy bipolaron bands at the expense of the absorption in the UV-VIS. However, polymer 34d does not oxidatively dope with SbCl 5 .

Theoretical investigation of the geometric and optical properties of neutral and charged oligophenylenes

Abstract: We have investigated the geometry and the nature of optically allowed transitions in neutral and charged phenylene-based oligomers by means of Hartree-Fock calculations. Geometry optimizations are performed using the semiempirical Austin Model 1 (AM1) method for oligomers containing from two $(2P)$ to twelve $(12P)$ benzene rings. The transition energies and related intensities of the optical-absorption spectra are calculated by means of the intermediate neglect of differential overlap Hamiltonian that is combined with a single configuration interaction technique in order to include electron correlation effects in the description of the excited states. The calculations show that two subgap absorption features appear in short oligomers carrying a single charge (polaron), whereas a single intense peak is observed in the presence of two charges (bipolaron). These results are consistent with a wide range of experimental and theoretical data obtained for various conjugated oligomers. Interestingly, the appearance of a second subgap feature is predicted in the spectra of long doubly oxidized chains as well as for chains supporting interacting bipolarons.

Dynamical properties of the one-dimensional Holstein model

Abstract: The spectral weight functions and the optical conductivity of the Holstein model are studied on a one-dimensional six-site lattice with periodic boundary conditions for three different electron concentrations: a single electron, two electrons of opposite spins, and half filling. A density matrix approach is used to obtain an optimal phonon basis and to truncate the phonon Hilbert space without significant loss of accuracy. This approach allows us to calculate spectral functions for electrons dressed locally by the optimal phonons as well as for bare electrons. We obtain evidence for a smooth crossover from quasifree electrons to a heavy itinerant small polaron (single-electron case) or bipolaron (two-electron case) as the electron-phonon coupling strength increases. At half filling, we observe a crossover from a quasifree-electron ground state to a quasidegenerate Peierls charge-density-wave ground state for a finite electron-phonon coupling. This crossover is marked by an abrupt drop of the Drude weight, which is vanishingly small in the Peierls phase.

Theory of colossal magnetoresistance in doped manganites

Abstract: The exchange interaction of polaronic carriers with localized spins leads to a ferromagnetic/paramagnetic transition in doped charge-transfer insulators with strong electron-phonon coupling. The relative strength of the exchange and electron-phonon interactions determines whether the transition is first or second order. A giant drop in the number of current carriers during the transition, which is a consequence of local bound-pair (bipolaron) formation in the paramagnetic phase, is extremely sensitive to an external magnetic field. Below the critical temperature of the transition, , the binding of the polarons into immobile pairs competes with the ferromagnetic exchange between polarons and the localized spins on Mn ions, which tends to align the polaron moments and, therefore, breaks up those pairs. The number of carriers abruptly increases below leading to a sudden drop in resistivity. We show that the carrier-density collapse explains the colossal magnetoresistance of doped manganites close to the transition. Below , transport occurs by polaronic tunnelling, whereas at high temperatures the transport is by hopping processes. The transition is accompanied by a spike in the specific heat, as experimentally observed. The gap feature in tunnelling spectroscopy is related to the bipolaron binding energy, which depends on the ion mass. This dependence explains the giant isotope effect of the magnetization and resistivity upon substitution of for . It is shown also that the localization of polaronic carriers by disorder cannot explain the observed huge sensitivity of the transport properties to the magnetic field in doped manganites.

Enhanced Seebeck coefficient from carrier-induced vibrational softening

Abstract: Through their electron-lattice interactions, localized carriers reduce vibrational stiffness constants in their surroundings. States with a large electronic polarizability (e.g., multiatomic localized states and states of a singlet bipolaron formed of degenerate electronic orbitals) induce exceptionally large softening. This carrier-induced softening augments the Seebeck coefficient of solids whose carriers hop between these localized states through two separate effects. One enhancement is due to a localized carrier inducing an increase in a solid's vibrational entropy. The other contribution is proportional to the vibrational energy transferred with a carrier as it hops. Both softening contributions are independent of the carrier density. The magnitudes and temperature dependencies of these contributions to the Seebeck coefficient indicate the electronic polarizabilities of the localized states and the vibrational modes to which they are coupled. Measured softening enhancements of Seebeck coefficients are sometimes large enough (e.g., g200 \ensuremath{\mu}V/K measured at 300 K in boron carbides) to significantly increase the efficiency of thermoelectric energy conversion.

Small bipolarons in the 2-dimensional Holstein-Hubbard model. I. The adiabatic limit

Abstract: The spatially localized bound states of two electrons in the adiabatic two-dimensional Holstein-Hubbard model on a square lattice are investigated both numerically and analytically. The interplay between the electron-phonon coupling g, which tends to form bipolarons and the repulsive Hubbard interaction v ≥ 0, which tends to break them, generates many different ground-states. There are four domains in the g, v phase diagram delimited by first order transition lines. Except for the domain at weak electron-phonon coupling (small g) where the electrons remain free, the electrons form bipolarons which can 1) be mostly located on a single site (small v, large g); 2) be an anisotropic pair of polarons lying on two neighboring sites in the magnetic singlet state (large v, large g); or 3) be a “quadrisinglet state” which is the superposition of 4 electronic singlets with a common central site. This quadrisinglet bipolaron is the most stable in a small central domain in between the three other phases. The pinning modes and the Peierls-Nabarro barrier of each of these bipolarons are calculated and the barrier is found to be strongly depressed in the region of stability of the quadrisinglet bipolaron.

The essential interactions in oxides and spectral weight transfer in doped manganites

Abstract: We calculate the value of the Frohlich electron-phonon interaction in manganites, cuprates, and some other charge-transfer insulators and show that this interaction is much stronger than any relevant magnetic interaction. A polaron shift due to the Frohlich interaction, which is about 1 eV, suggests that carriers in those systems are small (bi)polarons at all temperatures and doping levels, in agreement with the oxygen-isotope effect and other data. An opposite conclusion, recently suggested in the literature, is shown to be incorrect. The frequency and temperature dependence of the optical conductivity of ferromagnetic manganites is explained within the framework of the bipolaron theory.

Optical and electrical characterization of a conducting polypyrrole composite prepared by in situ electropolymerization

Abstract: A study of the optical and electrical properties of a conducting polypyrrole–polyoxyphenylene composite, PPy–POP, prepared by insitu electropolymerization is presented. Electropolymerization was performed potentiostatically in a solution of pH 9 which contained the monomers pyrrole, allylphenol and sodium 4-hydroxybenzenesulfonate (4HBS), by applying a potential of 1.25 V vs. SCE. The films obtained were characterized optically by UV/VIS and IR spectroscopy and electrically by measurements of the temperature dependence of the ac and dc conductivity. FTIR measurements indicated that the polymer blend obtained consists of PPy and the insulating polymer poly-2-allyloxyphenylene (POP), whereas the third monomer, 4HBS, is incorporated into the PPy–POP film as dopant for the conducting PPy. Furthermore, optical characterizations show a light degree of overoxidation of PPy in the PPy–POP composite. In the UV/VIS spectra, the formation of both polaron and bipolaron electronic states of the band structure of PPy can be seen, but the IR spectra demonstrate the transition of the PPy structure from a conducting quinoid to benzoid type with increasing polymerization potential. This is accompanied by the introduction of a carbonyl group into the PPy backbone and a reduction of the conjugation length of the polymer chain, which has a strong influence on the conductivity of the polymer composite. Despite this overoxidation process, the PPy–POP film retains a conductive character which allows the growth of thick films. The temperature dependence of the ac and dc conductivity of PPy–POP was investigated. The total ac conductivity, σtot(ω), in the frequency range 102–105 Hz, changes by approximately four orders of magnitude in the range from 77 to 300 K, showing a sub-linear dispersive behavior. The temperature dependence of the dc conductivity of such a polymer composite can be described by Mott's variable range hopping (VRH) model according to σ=σ0 exp[-(T0/T)γ], with γ=1/2

Bipolaron saturation in polypyrrole

Abstract: Saturation of bipolaron, the species responsible for conduction in polypyrrole (PPy), has been studied using positron annihilation lifetime (PAL) spectroscopy, conductivity, and electron spin resonance (ESR) measurement. It is observed that when strong oxidizing agents are used, the defects are predominantly bipolarons as is evident from the PAL spectroscopic studies coupled with ESR measurements and its saturation occurs right at the onset of oxidation. However, in the case of a mild oxidizing agent, the dominant defects are bipolarons only at higher oxidizing strengths $[g~1.0(N)]$ and no saturation is attained even at the highest strength studied. Conductivity data corroborate the findings.

A bipolaron in a spherical quantum dot with parabolic confinement

Abstract: A theory of bipolaron states in a spherical parabolic potential well is developed applying the Feynman variational principle. The basic parameters of the bipolaron ground state (the binding energy, the number of phonons in the bipolaron cloud, and the bipolaron radius) are studied as functions of the radius R of the potential well. Analytical expressions for bipolaron parameters are obtained at large and small sizes of the quantum well. It is shown that at R>>1 (where R is expressed in units of the polaron radius), the influence of confinement on the bipolaron binding energy W(R) is described by the function ~1/R2, while at small sizes this influence is more complicated: W(R) passes through a maximum in the region R<1.

High-spin polymeric arylamines

Abstract: High–spin p–doped polyarylamines have been created containing small clusters (tens) of ferromagnetically coupled unpaired electrons. In these doped polymers neighbouring spins couple through a pathway analogous to that found in meta –quinodimethane and the spin–carriers are triarylamine radical cations. The use of polymers in which the triarylamine radical cation centres are stabilized by Ar 2 N rather than RO substituents gives improved stability but much poorer spin–coupling properties. Furthermore, the much greater ease of dication (bipolaron) formation makes it difficult to dope these amine–stabilized polymers to the requisite level. Work by other groups exploiting phenoxy and triarlymethyl spin–carriers and the prospects for producing polymers with much larger clusters of ferromagnetically coupled spins are discussed.

Effect of valence electron spin polarization on the physical properties of CuCl2‐filled poly(vinylidene fluoride) as a microwave modulator

Abstract: Infrared and optical spectra, differential thermal analysis, dc electrical resistivity, magnetic susceptibility, electron spin resonance, and microwave response of CuCl2-filled poly(vinylidene fluoride) (PVDF) films, over the filler mass fraction range 0.05 # W # 0.4, were measured. The infrared spectra evidenced the presence of b-phase, for all of the filler levels with main deformations of 20% (for W 5 0.25) and 30% (for other filler levels) head-to-head and tail-to-tail units, which were considered as polaron and bipolaron defects. Optical activity was mainly influenced by PVDF struc- ture. Differential thermal analysis revealed dipole relaxation and premelting endother- mic peaks. A quasi- one-dimensional interpolaron hopping was thought to proceed in the direct current electric conduction, with a hopping distance less than the distance between two successive head-to-head sites. A temperature-independent Pauli paramag- netic behavior was observed, confirming the presence of induced energy bands due to CuCl2 filling. Most of the observed electron spin resonance signals were antisymmetric, with superimposed repels due to the hyperfine interactions characterizing PVDF. The obtained linear dependence of the isotropic hyperfine coupling constant (DA` ), for Cu(II), on average g-factor, implied that DAis a measure of the valence electron spin polarization. An octahedral or distorted octahedral configuration was suggested for Cu(II). The present system is a good microwave modulator. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 771-781, 1999

Colossal magneto-optical conductivity in doped manganites

Abstract: We show that the current carrier density collapse in doped manganites, which results from bipolaron formation in the paramagnetic phase, leads to a colossal change of the optical conductivity in an external magnetic field at temperatures close to the ferromagnetic transition. As with the colossal magnetoresistance itself, the corresponding magneto-optical effect is explained by the dissociation of bipolarons into polarons owing to the exchange interaction with the localized Mn spins in the ferromagnetic phase. The effect is positive at low frequencies and negative in the high-frequency region. The present results agree with available experimental observations.

Molecular and electronic structures of bipolaron in poly-para-phenylene in terms of molecular orbital symmetry

Abstract: The molecular structures of the model systems of the polaron and the bipolaron in poly-para-phenylene (PPP) were calculated by an ab initio molecular orbital (MO) method with fairly sophisticated approximations. The calculated models are monocations, dications, monoanions and dianions of biphenyl, para-terphenyl, para-quaterphenyl, para-quinquephenyl and para-hexaphenyl. The calculations show that the longer the PPP oligomer is, the stronger is the tendency to take on a non-planar twisting structure. This was accounted for by the combination of repulsions between proximate ortho-hydrogen atoms with resonance interactions between benzene π MOs. The magnitude of the resonance interactions was assessed by using the symmetry of benzene π MOs as well as an analytical Huckel solution of the π MO for polyene. In addition, negatively charged polarons and bipolarons were found to have a stronger tendency to take on a planar structure than positively charged ones. This result was also explained by comparing the benzene π HOMO with the benzene π LUMO.

Electronic structure of poly(1,10-phenanthroline-3,8-diyl) and its K-doped state studied by photoelectron spectroscopy

Abstract: Ultraviolet photoelectron spectra were measured using synchrotron radiation for thin films of poly(1,10-phenanthroline-3,8-diyl) (PPhen) and its potassium-doped state. Upon potassium doping of PPhen, two new states, which could be assigned to bipolaron bands, appear in the originally empty energy gap. The electronic structure of the neutral and potassium-doped states was theoretically analyzed using single-scattering approximation combined with semiempirical molecular orbital calculations.

Disorder Effects in the Bipolaron System Ti$_{4}$O$_{7}$ Studied by Photoemission Spectroscopy

Abstract: We have performed a photoemission study of Ti$_{4}$O$_{7}$ 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 Fe$_{3}$O$_{4}$, which exhibits a charge order-disorder transition (Verwey transition), can be interpreted in terms of a Coulomb gap.

Statistical physics of the solvated electron

Abstract: Statistical models of an electron solvated in a classical liquid are reviewed. The analogy between the behaviour of a quantum particle and a polymer chain results in that statistical physics methods can be applied to the solvated electron problem. We have derived basic relations for thermodynamic and structural characteristics of the solvated electron. The free energy of the solvated electron and the effective electron–solvent potential as well as the electron–solvent correlation function are studied in detail. The calculated results are compared with experimental data and a quantum molecular dynamics simulation for simple, polar, and Coulomb liquids. We also outline many-particle quantum effects, such as electron–electron interactions and dielectron (bipolaron) formation. The potentialities and restrictions of the statistical method are also discussed.

Study on microstructural lattice deformation of polypyrrole by positron annihilation lifetime spectroscopy

Abstract: Microstructural defects (polarons, bipolarons), responsible for conduction in polypyrrole (PPy) has been studied using positron annihilation lifetime (PAL) spectroscopy, conductivity and electron spin resonance (ESR) measurements. Pyrrole, oxidized at different oxidizing strengths [0.13(N)–2.0(N)] by ammonium ferric sulphate has been used for the preparation of PPy. It is observed from the PAL measurement that of the two lifetime components the longer one originates from annihilation of positron in the defect (polaron and bipolaron) sites and the corresponding intensity is a measure of the defect concentration. Further it is evident from the ESR spectra that the dominant defects are polarons at lower oxidizing strengths [⩽ 0.75(N)] and bipolarons at strengths ⩾ 1.0(N). Conductivity data corroborate the findings.

Large Bipolarons in Two and Three Dimensions

Abstract: The large bipolaron in two and three dimensions is investigated by combining the reasonable asymptotic relative wavefunctions and the Lee–Low–Pines–Huybrechts (LLP-H) variational method. The values of the critical coupling constant above which bipolarons may exist and the critical ratio of dielectric constants below which bipolarons may exist are evaluated, which are very close to the recent results obtained within totally different variational approaches. It is also found that low-dimensional materials are more favorable for the formation of bipolarons.

Theory of ferromagnetism and colossal magnetoresistance in doped manganites

Abstract: An exchange interaction of polaronic carriers with localized spins leads to a new ferromagnetic transition in doped charge-transfer insulators with strong electron–phonon coupling. The relative strength of the exchange and electron–phonon interactions determines whether the transition is first or second order. A giant drop in the number of current carriers during the transition, which is a consequence of bipolaron formation in the paramagnetic phase, is extremely sensitive to an external magnetic field. We show that the carrier density collapse describes the colossal magnetoresistance, anomalous specific heat, and tunneling gap of doped manganites close to the transition. The dependence of the binding energy on ion mass explains the giant isotope effect in magnetization and resistivity upon substitution of 16O by 18O.

Existence criteria for Landau-Pekar polarons

Abstract: We build the distribution function for a system with coexisting self-localized and delocalized fermions. The distribution function is used to study the behavior of the chemical potential of the carriers in such a system, which is found to differ substantially from the behavior of the chemical potential in a system of delocalized fermions. We also find that as the temperature changes, isostructural first-order phase transitions can emerge in the system of self-localized and delocalized fermions. These transitions, for which changes in the state of the macroscopic number of particles are responsible, manifest themselves in the electrical conductivity, in the contribution of carriers to the specific heat, and in the optical properties of such systems. Formulas are derived that approximate the dependence of the temperature of such a phase transition on the binding energy of the self-localized states of carriers and on the maximum group velocity of phonons participating in the formation of such states. Finally, we show that the special features of the behavior of the chemical potential of the carriers in a system with carrier self-localization lead to the possibility of Bose condensation in a system where bipolaron states are metastable.

Formation of bipolarons in the Holstein Hubbard model in one, two and three dimensions

Abstract: In this paper we have studied the properties of a two-fermion system interacting with the phonon field in the framework of the Holstein Hubbard model in one, two and three dimensions. We have chosen the modified Lang-Firsov variational wave function with on-site and nearest-neighbour lattice distortion for the phonon subsystem to obtain an effective electronic Hamiltonian. This effective electronic Hamiltonian is then solved exactly to obtain the binding energy and effective mass of the bipolaron. We investigated the stability of the bipolaronic phase by comparing the ground-state energy of a bipolaron and two free polarons. We observed a critical repulsive on-site Hubbard interaction Uc below which the bipolaronic phases are stable for a fixed electron-phonon coupling g. In the absence of on-site repulsion, bipolaronic phases are stable over the entire range of electron-phonon coupling for one dimension, whereas there is a critical electron-phonon coupling gc for formation of a stable bipolaron in two and three dimensions.