Showing papers on "Bipolaron published in 1983"

Electron-spin-resonance studies of pyrrole polymers: Evidence for bipolarons

Abstract: ESR measurements have been performed on samples of neutral polypyrrole, polypyrrole-perchlorate, oxygen-doped polypyrrole, and poly($\ensuremath{\beta}$,${\ensuremath{\beta}}^{\ensuremath{'}}$-dimethylpyrrole perchlorate). A narrow Lorentzian line is observed in oxidized, highly conducting films, but no correlation between conductivity and either linewidth or intensity is found. Electrochemically cycled films, which remain highly conducting, show little or no detectable ESR absorption. It is deduced that the ESR signal seen in the as-prepared material does not arise from the current-carrying species, but rather from accidental neutral $\ensuremath{\pi}$-radical defects. The absence of paramagnetism in the metallic state is discussed within the framework of bipolaron formation.

Solitons and polarons in polyacetylene: Self-consistent-field calculations of the effect of neutral and charged defects on molecular geometry

Abstract: Semiempirical self-consistent-field calculations are carried out on various polyenes with emphasis on defect configurations designed to simulate soliton and polaron excitations in polyacetylene. For defect-free polyenes, the calculated bond-length alternation ($\ensuremath{\Delta}r=0.106$ \AA{}) is in close agreement with generally accepted values for polyacetylene. Solitons and polarons are simulated on polyene radicals, ions, and radical ions by the introduction of appropriate symmetries and by charging; these are viewed as neutral (radical) and charged (cation and anion) defects in the polyene structure. The equilibrium geometries and charge distributions of varying-length polyenes and defect-containing polyenes are calculated. The extent of disruption of the bond-length alternation pattern of the carbon backbone is predicted to be about half as large as previously thought for neutral defects (solitons) and even smaller for anionic defects. Positively charged solitons are of larger extent. These results are consistent with known conductivity---doping-level relationships. A spatially damped charge-density wave is associated with the charged solitons and a spin-density wave with the neutrals; in both cases the wave extends farther than the lattice kink itself. Polarons and bipolarons, studied as soliton-antisoliton pairs (radical cations and radical anions) on the same model molecule, are also found to exhibit charge-density-wave solutions. The bipolaron is shown to be unstable to the formation of a like-charge soliton-antisoliton pair. Strong interchain coupling is expected as a result of the large dipole moments associated with the unique form of the lattice polarization due to the damped charge-density wave in charged solitons and polarons.

Bipolarons in polypyrrole chains

Abstract: First-principles calculations on the geometric and electronic structures of undoped and doped polypyrrole chains indicate the appearance upon high doping of bipolaron states in the gap. A picture of the band-structure evolution upon doping is presented, showing the presence of bipolaron bands in the gap for experimentally achieved doping levels. A conductivity mechanism based on motion of spinless bipolarons is consistent with the absence of ESR signal in electro-chemically cycled highly conducting polypyrrole.

As F 5 -Doped Polyparaphenylene: Evidence for Polaron and Bipolaron Formation

Abstract: Both elastic and inelastic electron scattering have been used to characterize and investigate the electronic properties of As${\mathrm{F}}_{5}$-doped polyparaphenylene. Evidence is presented for the existence of a polaron state slightly above the top of the valence band at low doping levels. An increase in the doping level first leads to the formation of bipolaron states which broaden and finally form a band in the gap in agreement with recent theoretical predictions.

Polaron states in polydiacetylenes

Abstract: The ground state and lowest-lying excited states of polydiacetylene chains are investigated using a tight-binding Hamiltonian which includes electron-phonon interactions but does not explicitly include electron-electron repulsion. The parameters of the model are obtained by comparison with experiment and the character and properties of the low-lying charge states are determined. This calculation shows that localised polaron states will dominate charge conduction in these systems. In contrast to the situation for polyacetylene, bipolaron states are stable in the polydiacetylenes. Moreover, they are so strongly bound that they may survive the inclusion of electron-electron repulsion.

EPR Relaxation and Transport Phenomena in the Quasi-One-Dimensional Conductor β-Na x V 2 O 5

Abstract: The dynamical behaviors of the 3 d -electrons in β-Na x V 2 O 5 are studied by means of EPR relaxation, in the temperature range between 19 and 303 K. The anisotropy and the temperature dependence of the linewidth are found to originate mainly from the dipolar interaction between the 3 d -electrons of the excited state of the bipolaron at the V 1 site and the delocalized dilute paramagnetic state at the V 3 site. The correlation time of the fluctuating local field is estimated to be of the order of 10 -11 s. On the basis of this correlation time, the transverse conductivity is found in terms of the diffusive electron hopping motion between the V 1 and V 3 sites perpendicular to the chain axis, while the longitudinal one is interpreted as the collective motion of the bipolaron parallel to the chain axis.

Properties of Bipolaron System

Abstract: The idealized bipolaron model proposed by Alexandrov and Ranninger is studied within the molecular field and the random phase approximations. It is shown that the incommensurate charge density wave (CDW) appears in the presence of the second neighbor interaction. Its period is determined both by the ratio of the interaction constants and by the number of bipolarons. The phase diagrams are shown to consist of four phases, i.e., the normal, the superconductive, the normal CDW and the condensed CDW (CCDW) where the ODLRO coexists with the DLRO. The lowest excitation mode is shown to have the phonon-like spectrum in both the superconductive and the CCDW phases. The CCDW phase, therefore, is expected to be superconductive contrary to the results by Alexandrov and Ranninger.

Nuclear Magnetic Relaxation in the Quasi-One-Dimensional Conductor β-Na 0.40 V 2 O 5

Abstract: Spin-lattice relaxation times of 23 Na and 51 V nuclei in the quasi-one-dimensional conductor β-Na 0.40 V 2 O 5 were measured in the temperature range between 77 K and 300 K. On the basis of bipolaron model, we discuss the fluctuation of local field at the nuclei and determine the correlation time of electron hopping motion to be about 1.0×10 -11 sec. As far as we are concerned with this comparatively slow hopping rate of electron, the observed high conductivity along chain of 10 2 \(\varOmega^{-1}\)cm -1 is not understood without taking into consideration of the collective motion of bipolarons. In the higher temperature, the observed broad maximum of the spin-lattice relaxation rate of 23 Na nuclei around 220 K indicates the existence of the motion of sodium ions.

Vibrational spectra and structure of undoped polyparaphenilene

Abstract: Infrared and Raman spectroscopic data on undoped and AsF5-doped polyparaphenylene (PPP), are presented. The results are discussed in terms of the normal mode calculation of Rakovic et al and the bipolaron defect mode1 of the doped polymer proposed by Bredas et al.

Specific heat at low temperature due to negative U centres in disordered solids

Abstract: The negativeU-Anderson model is considered and energy spectrum is obtained using the Gorkov’s decoupling scheme for one-electron Green’s function. The correlation of localized electron pair (bipolaron) is explicitly taken into account in this scheme. The electronic specific heat of disordered solids with negativeU-centres and having a distribution of negativeU is then calculated. At low temperature the specific heat shows linear temperature dependence, and this linearT-term is a combined effect of distributedU and of the existence of localized electron pairs.