Polaron Density Research Articles

Electrochemical doping properties and electronic states of poly(3-phenylthiophene)

Electrochemical, optical, and magnetic properties in poly(3-phenylthiophene), as a function of doping and electronic states, are discussed in detail. The band gap of poly(3-phenylthiophene) was evaluated to be 2.0 eV, and similar to that of polythiophene. The evolution of localized states is evidenced by the spectral change with electrochemical doping. The spin density evaluated from the susceptibility was consistent with the polaron density assumed to be formed by a very small quantity of dopants. These results are interpreted in terms of the polaron and bipolaron models. The electronic band structure of poly(3-phenylthiophene) was determined. The top of the valence band of poly(3-phenylthiophene) is located at a higher energy state than that of polythiophene by about 0.1 eV and it became clear that the dopant in poly(3-phenylthiophene) is relatively stable compared with that in polythiophene. Successful n-type doping in poly(3-phenylthiophene) was also confirmed.

Friedel mechanism of formation of large bipolarons in high temperature superconductors

The interaction of large polarons in a degenerate gas of light electrons is investigated. Because of the Friedel oscillation of the electrostatic potential of large polarons these polarons may form a bipolaron molecule. Both the energy and the characteristic size of this bipolaron are calculated. The critical polaron density of the bipolaron liquid formation is presented.

The electrochemical properties of poly(2,5-thienylene vinylene) synthesized via water-soluble precursor polymer

The electrochemical, optical, and magnetic properties in poly(2,5-thienylene vinylene), PTV, in comparison with those of polythiophene, PT, during electrochemical p-type doping have been investigated by cyclic voltammetry, optical absorption spectrum, and electron spin resonance (ESR) measurements. The evolution of localized states is evidenced by the spectral change associating with doping. In the lightly doped state only two absorption peaks originating from the transitions between two gap states and the valence band appear within the gap region at about 0.6 and 1.2 eV. The spin susceptibility increased by about 1 order of magnitude from 3.7×10−7 to 2.6×10−6 emu/mol upon doping up to a dopant concentration of about 2.8 mol %. With further increasing dopants concentration, the spin susceptibility decreases slightly. The spin density evaluated from the susceptibility was inconsistent with the polaron density assumed to be formed by dopants of 2.8 mol %. Namely, even at lightly doped state, the newly developed species turn into p-type spinless bipolarons. The band gap of PTV was evaluated to be about 1.8 eV and smaller than that of PT by about 0.3 eV. This result can be interpreted in terms of a conformation change in the molecular structure, such as a change in the torsion angle between neighboring thiophene rings. The electronic band structure of PTV was determined. The bipolaron states in PTV appears in the band gap: about 0.6 eV below and above the conduction and the valence band, respectively. The bottom of the conduction band of PTV is in almost the same position as that of PT and successful n-type doping in PTV is expected.

In situ optical absorption spectra and electron spin resonance measurements of poly(1,4-naphthalene vinylene) during electrochemical n-type doping

The electrochemical, optical and magnetic properties in poly(1,4-naphthalene vinylene) (PNV) during electrochemical n-type doping have been investigated by cyclic voltammetry, optical absorption spectroscopy, and electron spin resonance (ESR) measurements. Evidence for electrochemical n-type doping of this polymer and evolution of localized states is obtained from measurements of a cyclic voltammogram and is demonstrated by the spectral change associated with doping. In the lightly doped state only two absortpion peaks originating from the transitions between two gap states and the conduction band appear within the gap region at 1.1 and 1.7 eV. The spin susceptibility increases by about two orders of magnitude from 5.9*10-8 to 6.1*10-6 emu mol-1 upon doping up to a dopant concentration of about 2.8 mol.%. With a further increase in dopant concentration the spin susceptibility decreases slightly. The spin density evaluated from the susceptibility was inconsistent with the polaron density assumed to be formed by dopants of 2.8 mol.%. That is, even at lightly doped slates the newly developed species turn into n-type spinless bipolarons. The ESR linewidth decreases by doping from about 9.9 G to 0.4 G. The g factor of 2.0031 at the neutral state shifts upon doping to the smaller value 2.0028, which corresponds to the nearly free electron g factor. These results are discussed in terms of bipolaron models.

Electrical, optical, and magnetic properties of poly(2,5-diethoxy-p-phenylene vinylene).

The electrochemical, optical, and magnetic properties in poly (2,5-diethoxy-p-phenylene vinylene), ${\mathrm{C}}_{2}$${\mathrm{H}}_{5}$O-PPV, during electrochemical p-type doping have been investigated by cyclic-voltammetry, optical-absorption-spectrum, and electron-spin-resonance (ESR) measurements. The evolution of localized states is evidenced by the spectral change with electrochemical doping. The band gap of ${\mathrm{C}}_{2}$${\mathrm{H}}_{5}$O-PPV was evaluated to be about 2.2 eV and smaller than that of poly (p-phenylene vinylene), PPV, by about 0.5 eV. The spin susceptibility increased by about three orders of magnitude from 2.8\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}8}$ to 1.6\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$ emu/mol up to a dopant concentration of about 2.2 mol %. With a further increasing dopant concentration, the spin susceptibility decreases slightly. The spin density evaluated from the susceptibility was consistent with the polaron density assumed to be formed by dopants of 2.2 mol %. The ESR linewidth decreases by doping from about 3.5 G to about 0.4 G and then again increases slightly with increasing dopant concentration. The g factor of 2.0036 at the neutral state shifts upon doping to the smaller value 2.0026, which corresponds to the nearly-free-electron g factor. These results are discussed in terms of polaron and/or bipolaron models.

Continuum studies of magnetic polarons and bipolarons in antiferromagnets.

We present a simple scaling model of magnetic-polaron and magnetic-bipolaron formation in antiferromagnetic systems. Within this picture, we determine the criteria for an excess electron forming a magnetic polaron and for a pair of excess electrons forming a (triplet) magnetic bipolaron. The dependence of the formation criteria on the dimensionality of the electronic system is specifically addressed. In addition, we consider the effect of the electron-lattice interaction on the size and energetic stability of magnetic polarons and bipolarons as a function of the system's dimensionality. Beyond the formation of isolated magnetic polarons and/or magnetic bipolarons, we consider how a system composed of magnetic polarons within an antiferromagnetic system is driven ferromagnetic as the density of magnetic polarons is increased. Finally, we discuss the large paramagnetic susceptibility associated with the formation of magnetic polarons. The magnetic susceptibility is composed of a Curie-like temperature-dependent component as well as a temperature-independent component. The temperature-dependent term arises from the reorienting of the magnetic polarons under the influence of a magnetic field. The temperature-independent contribution arises from the expansion and contraction of magnetic polarons which are, respectively, aligned parallel or antiparallel with the applied magnetic field.

Polarons and bipolarons in quasi-1-D conductor vanadium bronzes

The electronic state of V 4+ ions and the existence of bipolaron formed by the strong electron-phonon interaction in β -M xV 2O 5 are investigated by magnetic resonance studies. The ordering phenomena of these bipolarons were observed along the 1-D axis in the system of M = Na,Ag and Cu by X ray scattering studies. In β -Cu xV 2O 5 system of high polaron density x >0.54, the condensation of polarons in certain chains and the ordering of these chains within perpendicular plane to the 1-D axis were found. The phase diagram of β -Cu xV 2O 5 as a function of temperature and the polaron density x was obtained.

Polaron density of states of a hole in a narrow band of an alternate lattice of fermions

Studies the problem of a single hole propagating in an alternative lattice of fermions, in the atomic limit of the Hubbard model, for inhomogeneous spin configurations. An estimate of the band shape is deduced from the development of the Green function. It seems that in a spin polaron configuration, the band presents peaks at the extremities. Such peaks stabilise the polaron state.

Polaron interaction energies in reduced tungsten trioxide

Consideration of the properties of reduced tungsten trioxide suggest that the mobile charge carriers are polarons. As it is uncertain how the presence of polarons will influence the microstructures of the crystallographic shear ( CS) planes present in reduced tungsten trioxide we have calculated both the polaron- CS plane and polaron-polaron interaction energy for a variety of circumstances. Three CS plane geometries were considered, {102}, {103}, and {001} 1 CS plane arrays, and the nominal compositions of the crystals ranged from WO 2.70 to WO 3.0. The polarons were assumed to have radii from 0.6 to 1.0 nm and the polaron- CS plane electrostatic interaction was assumed to be screened. The results suggest that for the most part the total interaction energy is small and is unlikely to be of major importance in controlling the microstructures found in CS planes. However, at very high polaron densities the interaction energy could be appreciable and may have some influence on the existence range of CS phases.

Optical breakdown in alkali halides

A theory of optical breakdown in dielectrics is presented and applied to NaCl and NaBr for wavelengths 0.172 < or = lambda < or = 1.0642 ..mu..m. It is based on multiphoton carrier generation and energy transfer from the radiation field to the lattice via polaron-photon absorption. This mechanism represents a clear alternative to avalanche breakdown models as no lattice impact ionization is involved. Breakdown field strengths calculated for various laser pulse lengths and frequencies, are fully corrected for the nonlinear refractive index n/sub 2/ as well as for contributions to the refractive index resulting from a sharp increase in polaron density during a damaging pulse. The results are in good agreement with experimental data obtained with nanosecond laser pulses.

Effective mass and scattering mechanisms in semiconducting CdF 2 at high temperatures

We report the measurements, in semiconducting CdF 2, of Hall coefficient, thermoelectric power and mobility for temperatures in the vicinity of 300°K. Values of the polaron density of states mass m p ≅ 0.9 m e, the bare conduction mass m c ≅ 0.45 m e and the polar optical phonon coupling constant, α ≅ 3.3. are obtained. The electron polar optical phonon interaction does not appear to be the dominant scattering mechanism.