Showing papers on "Bipolaron published in 1990"

The spin-polaron theory of high-Tc superconductivity

Abstract: An outline is given of the model for some high-temperature superconductors which assumes that the carriers are holes in the (hybridized) oxygen 2p band and form ‘spin polarons’ with the moments on the copper atoms. A comparison is made with observations of spin polarons in Gd3-x v x S4 and with the properties of La1-x Sr x VO3 in relation to those of La2-x Sr x CuO4. It is assumed, following several authors, that in the superconductors the polarons form bipolarons, which are bosons, and a comparison is made with some other treatments of this hypothesis. It is proposed that in many such superconductors the boson, essentially a pair of these holes, moves in an impurity band, and that normally all the polarons (fermions) form bipolarons; the fermions repel each other on the same site (positive Hubbard U) but attract when on adjacent sites; the critical temperature T c is then that at which the Bose gas becomes non-degenerate. In such materials a non-degenerate gas of bosons would carry the current a...

Frequency dependent electrical transport in bismuth-modified amorphous germanium sulfide semiconductors

Abstract: AC conductivity of Bi-modified amorphous germanium sulfide semiconductors Ge20S80−xBix from 100 Hz to 10 kHz and in the temperature range 180–445 K is reported. The ac conductivity is proportional to ωs. In Ge20S80 composition, the value of σac(ω) depends linearly on the frequency and is nearly independent of temperature. In samples containing Bi, σac(ω) increases with Bi content, especially at high temperature and low frequencies. The temperature dependence of σac(ω) and S in all compositions examined is interpreted by a correlated barrier hopping model (CBH). Analysis of the results reveals that electronic conduction in unmodified semiconductors (x = 0, 4) takes place via bipolaron hopping. Addition of Bi in higher concentrations induces new types of defects which take part in a single polaron conduction process. Following Phillip's constraint theory and cluster model, the modified glass has microclusters of Bi2S3 with tetradymite structure, embedded in matrix of GeS2 and (S)n. These microclusters may have n-type defects situated at excess S− atoms on the cluster surface; these defect may be taking part in the single polaron hopping process.

Photoexcited states in poly(3-alkyl thienylenes)

Abstract: The authors present a comparative study of the properties of two solution-processible alkyl-substituted polythienylenes, poly(3-hexyl thienylene), P3HT and poly(3-dodecyl thienylene), P3DT. Both show the onset of pi - pi * interband absorption at 2 eV with the peak in absorption coefficient near 2.5 eV, and both show strong photoluminescence in an energy band extending from 1.3 to 2 eV. These properties are both due to intra-chain absorption across the pi - pi * gap, with the photoluminescence arising from the radiative decay of the singlet exciton. For the case of P3HT, photoinduced absorption measurements show the presence of two subgap absorption bands, which are associated with the formation of charged bipolarons which require charge separation between chains. P3DT, in contrast, shows a single induced absorption band, at 1 eV, which shows very different temperature and frequency dependence from that of the bipolaron bands in P3HT. The authors consider that a triplet-triplet transition of the triplet intrachain exciton is a possible assignment for this band, and consider that poorer contact between chains in P3DT over that in P3HT inhibits the transfer of charge between chains and thus prevents the formation of charged bipolarons in this polymer. They also discuss the role of the polymer structure in the stabilisation of the photogenerated bipolarons.

Explicit evidence for bipolaron formation: Cs-doped biphenyl

Abstract: Thin condensed layers of biphenyl have been studied by ultraviolet photoelectron spectroscopy (UPS) and electron-energy-loss spectroscopy (EELS) as a function of Cs doping in order to model the situation in a doped conducting polymer. The UPS spectra show the growth of gap states which are assigned to bipolaron formation. This is the first direct observation of emission from states thought to be important for conduction in conducting polymers. While EELS also implies the formation of gap states their evolution with doping cannot be simply interpreted. The valence-band emissions clearly show shifts in the Fermi level due to pinning by the bipolaron states and movements due to increased polarizability on the formation of the Cs-biphenyl complex.

The poly‐3‐hexylthiophene/NOPF6 system: A photoelectron spectroscopy study of electronic structural changes induced by the charge transfer in the solid state

Abstract: The electronic structure of poly‐3‐hexylthiophene (P3HT) is studied in the solid state with photoelectron spectroscopy, as the polymer is gradually doped from NOPF6. Solubility and processability of P3HT allow for the preparation of very clean and very thin films, which are then doped without air‐exposure. The evolution of the core level binding energies is related to the modification of the electron density on the conjugated backbone, due to the creation of polaron and bipolaron defects. Upon doping, valence spectra show a shift in the Fermi level of the system, and at saturation doping a finite density of states at the Fermi level is observed unambiguously for the first time in electrically conducting polymers with photoelectron spectroscopy. The experimental data on core and valence levels can be interpreted in terms of the formation of a polaron lattice at high doping.

Atomistic Simulation of High‐Tc Oxides

Abstract: Atomistic lattice simulations are reported of the normalstate crystal properties of high-Tc oxides. Nonstoichiometry in La2CuO4 and Nd2CuO4 associated with oxygen interstitial defects is examined in some detail. The lattice structure and defect chemistry of Ca2CuO3, Sr2CuO3, La2SrCu2O6, and Sr2CuO4 are also considered. Finally, Jahn-Teller effects and bipolaron formation are addressed in relation to high-Tc behavior.

Electrical properties of oxide superconductors above Tc

Abstract: Starting with the model of Prelovsek, Rice and Zhang according to which all carriers in, for instance, La2−xSrxCuO4 form bosons, assumed here to be of spin bipolaron type, a theory is proposed to account for the resistivity and thermopower above T c.

Multi-dimensional fröhlich bipolaron and dimensional scaling

Abstract: The formation and stability of the Frohlich bipolaron in a multi-dimensional polar crystal is investigated within the framework of strong coupling Landau-Pekar theory. The ground state energy, the effective mass and the size of the bipolaron are calculated. It is shown that Frohlich bipolarons can exist in both two and three dimensions, the bipolaronic binding being stronger in lower dimensions. The dimensional scaling relations satisfied by the ground state energy and the effective mass of the bipolaron are also obtained.

Electronic structure of polyazulene

Abstract: Electronic structure of polyazulene (PAz) has been studied based on its optimized structure of the neutral state using the one-dimensional tight-binding SCF-CO (self consistent field-crystal orbital) method. The oxidized states of polaron and bipolaron natures were also examined. It is found that the positive net charges of the oxidized states are mainly localized in the seven-membered ring and that in the polaron state the charge and the spin parts are separated in an azulene ring. It has been conjectured that confinement of the π hole in the seven-membered ring inherent in PAz is unfavourable for the intrachain electrical conduction.

Evidence for the Polaron State in Polyaniline from in situ Electron Spin Resonance and Resonance Raman Spectroscopy

Abstract: A combined investigation of the Raman cross-section for a benzene ring vibrational mode at 1630 cm−1 and of the spin concentration as a function of the e.c. potential in polyaniline identified the polaron state as the initially generated doping-induced species. The polaron transition energy is determined as 2.75 eV. Increasing oxidation quenches the Raman signal and decreases the spin concentration until, for oxidation potentials higher than 700 mV vs. SCE, both signals reappear indicating a reduction of the polaron lattice or of bipolaron states to individual polarons.

Forced Rayleigh scattering on a soluble conducting polymer

Abstract: Demonstration de l'application de la diffusion Rayleigh forcee a la mesure du coefficient de diffusion moyen Dz d'un polymere conducteur (poly (3-octyl thiophene)) photoexcite en solution diluee. Les resultats suggerent que les etats excites sont des bipolarons

Single polaron and bipolaron absorption in Ti4O7 above and below the metal-insulator transition

Abstract: The normal reflectivity of Ti 4 O 7 ( T = 290 K ) is measured in the range of 0.1 to 3 eV. The reflectivity shows a minimum which corresponds to the collective resonance of all Ti 3+ valences. The optical absorption as calculated from the diffuse reflectivity of Ti 4 O 7 is investigated in the spectral range of 0.5 to 2.5 eV and at temperatures between 10 and 300 K. A broad asymmetrical absorption band with a maximum at about 0.7 eV and an absorption sideband at about 1.5 eV is observed. The overall line shape is explained by the polaronic absorption mechanism. From room temperature down to the metal-insulator transition at T > m ≈ 150 K the total polaronic absorption cross section remains constant. It drops down in the intermediate phase (125–150 K) and then increases with decreasing temperature below the Verwey transition at T v ≈ 125 K. The increasing absorption cross section below T v as well as the shift of the peak position to lower energy is in accordance with the gap opening at ≈ 0.2 eV previously reported (Kaplan et al. [1]) and is explained by the separation of single polaronic from bipolaronic energy levels. The magnetic susceptibility of Ti 4 O 7 is discussed with respect to an exchange pairing mechanism above and below T m . It is suggested that a melting of the bipolaronic state during the metal-insulator transition results in a fluctuating valence bond system above T m .

Structural and energetic aspects of defects in high-Tc oxides from atomistic lattice simulations

Abstract: This paper reports recent atomistic lattice calculations of specific aspects of point defects related to high-Tc superconductivity. They are Jahn–Teller effects in La2CuO4, the defect chemistry of potential electron superconductors and the dielectric contribution to bipolaron formation in La2CuO4 and Nd2CuO4. It is shown that Jahn–Teller effects can be introduced empirically into defect calculations and that in the case of La2CuO4 these effects favour polaron formation by 0.2–0.4 eV. From an examination of the stability of holes and defect electrons in a range of ternary cuprates it is shown why La2CuO4 leads only to hole conductors and Nd2CuO4 and Pr2CuO4 only to electron conductors. Calculations show important differences between the interaction of Cu and O holes in La2CuO4. In both cases a confining potential is found for interplanar small polaron pairs and an attractive interaction for intraplanar O hole pairs in the small-polaron limit. The interaction of defect electrons in Nd2CuO4 shows neither of these characteristics.

Study of the metal-insulator transition in Ti4O7 by optical investigations

Abstract: Our reflectivity measurements of Ti 4 O 7 in the energy range between 1 meV and 12 eV and with temperature dependence are presented and discussed within a bipolaron model. The analogies with magnetite (Fe 3 O 4 ) are finally considered.

Atomistic study of polarons in YBa2Cu3O7

Abstract: This paper reports on simulation techniques applied to the orthorhombic YBa{sub 2}Cu{sub 3}O{sub 7} crystal in order to describe possible polaron and bipolaron species in the crystal. Perfect crystal properties are fitted well with a two- and three-body potential model. The O{sup {minus}} polaron state is significantly stabilized by the presence of holes in the CuO plane. A bipolaron O{sup 2 {minus}}{sub 2} is slightly stable on the O{sub 4} and O{sub 2} or O{sub 3} lattice ion sites of oxygen. The oxidation reaction leading to O{sup {minus}} and involving Cu{sup +} and oxygen vacancies is exothermic.

Synthesis of polypyrrole having an alkyl side chain

Abstract: Electrochemical polymerisation of 3-octylpyrrole (OCPY) was studied. Its conductivity was 5 S/cm at an optimal current density of 0.5 mA/cm2. Cyclic voltammograms of poly(3-octylpyrrole) (POCPY) films were measured in the presence of various electrolytes and their shapes were virtually independent of the electrolytes used. There was an anodic peak at 0.16 V (versus Ag) and a cathodic peak at 0.12 V. Optical absorption spectra were measured at various levels of doping. The neutral film showed one peak at 3.5 eV, corresponding to the π-π* transition. The oxidised film showed two peaks in the lower energy regions, suggesting it was in the bipolaron state. Scanning electron microscope (SEM) observation revealed a process of unique morphology formation.

MX chains: 1-D analog of CuD planes?

Abstract: We study a two-band Peierls-Hubbard model for halogen-bridged mixed-valence transition metal linear chain complexes (MX chains). We include electron-electron correlations (both Hubbard and PPP-like expressions) using several techniques including calculations in the zero-hopping limit, exact diagonalization of small systems, mean field approximation, and a Gutzwiller-like Ansatz for quantum phonons. The adiabatic optical absorption and phonon spectra for both photo-excited and doping induced defects (kinks, polarons, bipolarons, and excitons) are discussed. A long period phase which occurs even at commensurate filling for certain parameter values may be related to twinning. The effect of including the electron-phonon in addition to the electron-electron interaction on the polaron/bipolaron (pairing) competition is especially interesting when this class of compounds is viewed as a 1-D analog of high-temperature superconductors. 6 refs., 4 figs.

Electronic Structure of Processable Conducting Polymers

Abstract: The electronic structure of poly-3-hexylthiophene is studied in the solid state with photoelectron spectroscopy, as the polymer is gradually doped from NOPF6. The evolution of the core level binding energies is related to the modification of the electron density on the conjugated backbone, due to the creation of polaron and bipolaron defects. Upon doping, valence spectra show a shift in the Fermi level of the system, and at saturation doping a finite density of states is observed at the Fermi level. In the case of poly-para-phenylene vinylene, the experimental density of states of the neutral polymer is related to the results of Valence Effecttive Hamiltonian calculations. Comparison with gas phase data on styrene leads to an estimation of the polarization energy.

Optical evidence of the bipolaronic ground state in the low temperature phase of Ti4O7 and Fe3O4

Abstract: We discuss our thorough optical investigations on the 3d transition metal compounds Fe 3 O 4 and Ti 4 O 7 within the bipolaron model of Chakraverty. Of particular interest are the sharp lines overlapping the phonon modes. These are ascribed to the fingerprints of a charge ordering phase. Futhermore, we present the phonon spectrum of rutile, in order to review the vibrational states of magnetite and above all of Ti 4 O 7 .

Photoinduced Absorption and Nonlinear Optical Response in a Polycondensed Thiophene-Based Polymer (PTT)

Abstract: Nonlinear optical properties of a new thiophene based conjugated polymer (polythieno(3,2-b)thiophene, hereafter referred to as PTT) have been studied by two kinds of nonlinear spectroscopies: Photoinduced Absorption (PA) and Third Harmonic Generation (THG) PTT is a novel highly nonlinear optical material comparable to polythiophene, and can be prepared directly as an amorphous free standing film The electronic nonlinear response measured by means of THG gives an average value of resonant χ(3) (-3ω,ω1,ω1,ω1) = 2×10-11 esu in the 125–145 µm range of the fundamental laser wavelength The oscillations observed in χ(3) (-3ω,ω1,ω1,ω1) may be explained by three photon vibronic resonance enhancement with the C = C stretching mode Photoexcitation of electron-hole pairs in the neutral polymer generates long-lived bipolaron states within the semiconducting gap We relate the variation of the refractive index due to photoexcitations to the cubic susceptibility In this framework χ(3) (-3ω,ω1,ω1,ω1) = 13×10-2 esu at the maximum of the low energy bipolaronic band

The Electroactive Nature of Polyaniline. The Nature of the Chromaphores

Abstract: Thin films of polyaniline (PANi) prepared electrochemically can readily be investigated insitu. In this way, both the oxidation state and level of protonation can be precisely controlled. Insitu UV/VIS/NIR and Resonance Raman spectra are presented here. These results show that the usual polaron/bipolaron models applied to other conducting polymers are not consistent in the case of PANi. The structures of the chromaphores associated with the observed ‘dopant’ induced optical absorptions in PANi are quite different, which is inconsistent with polaron theory. Resonance Raman spectra show that the structure of the chromophore giving rise to absorption at ca. 3eV is that of a simple p-di-substituted phenylene moiety, whereas that of the absorption at ca. 1.6 eV is consistent with a mixed benizonoid/quinoid radical (eg. a semiquinone moiety). In both cases the chromophores are localised. Therefore a new model for PANi is proposed.