Epr Study of Polarons In a Conducting Polymer With Nondegen-Erate Ground States: asF5 Complexes of Poly(p-phenylene)

EPR measurements of temperature-dependent susceptibility and linewidth are reported for poly(p-phenylene) as a function of exposure time to dopant. Most of the charge is in bipolarons, rather than in the spin-associated polarons. Observed decreases in high temperature spin concentration as a function of doping time and post-doping anneal likely arise from both polaron-polaron reaction and polaron ionization to form bipolarons. The susceptibility of all the investigated AsF₅-doped samples strongly deviate from Curie-Weiss behavior at low temperatures. The observed temperature dependence can be explained by the low-temperature condensation of isolated polarons to form intermolecular polaron pairs in singlet ground states. The interaction between spins is antiferro-magnetic and the energy of the singlet-state polaron pair relative to two isolated polarons (-2.4 to -4.8 meV) is similar to that for the n-doped polymer (-2.4 to -2.7 meV).     

Polyacetylene and Relativistic Field Theory Models

Connections between continuum, mean-field, adiabatic Peierls-fröhlich theory in the half-filled band limit and known field theory results are discussed. Particular attention is gi en to the Φ⁴ model and to the solvable N +2 Gross-Neveu model. The later is equivalent to the Peierls system at a static, semi- classical level. Based on this equivalence we note the prediction of both kink and polaron solitions in models of trans-(CH)_x. Polarons in cis-(CH) are compared with those in the trans isomer. Optical absorption from polarons is described, and general experimental consequences of polarons in (CH)_x and other conjugated polymers is discussed.     

Magnetism of the (TMTSF)2X Family of Organic Conductors

Abstract

The magnetic properties of the (TMTSF)₂X series of conductors have been studied using magnetic susceptibility, electron spin resonance and both CW and pulsed NMR measurements. The results imply the existence of a spin-density-wave ground state, at ambient pressure, in at least the PF₆ ^?, AsF₆ ^? and SbF₆ ^? salts. The amplitude of the SDW, the exchange interaction, the anisotropy energy and the single particle gap are extracted from the experimental data.     

Ferromagnetic Intermolecular Interaction of N-Protonated Pyridylphenylcarbene

Abstract

The HCl and DCl salts of pheny1-4-pyridyldiazomethane were prepared and irradiated at 6 K to generate the carbenes. Their ESR spectra indicated presence of plural triplet carbenes with slightly-different zero-field splitting constants. Besides the triplet species, we found a quintet species whose signal intensity decreases with increasing temperature. The quintet spin state was concluded to be a ground state which was produced by an intermolecular ferromagnetic interaction of the triplet carbenes. The ferromagnetic interaction would be explained by the McConnell mechanism, judging from the crystal structure of the HCl salt of phenyl-4-pyridylketone.     

Monte Carlo Study of Slot-waveguide Liquid Crystal Phase Shifters

Abstract

We present detailed Monte Carlo simulations of a simple model of a nematic liquid crystal slot waveguide shifter, investigating the effect of an applied electric external field. The simulations are based on the Lebwohl-Lasher lattice spin model with boundary conditions chosen to mimic the planar alignment as in Silicon Organic Hybrid waveguides and the homeotropic anchoring appropriate for Polydimethylsiloxane polymer walls. The external field is modeled by adding a term to the Hamiltonian which describes its coupling to the mesogenic molecules. We have investigated the effect of the external field on the optical transmission and the ordering across the cell.     

Spin-Spin Relaxation Process of 23Na-NMR in Na-Loaded NaY

Abstract

Spin-spin relaxation of ²³Na-NMR is observed by the spin echo method at room temperature for Y-type zeolite loaded with Na metal without hydration. For saturated and no levels of loading, the echo decay is well fitted by single exponential function. T ₂ decreases to be 0.25 times smaller by loading. This decrease of T ₂ is explained semiquantitatively with assuming nuclear dipole-dipole interaction between neighboring Na.     

Soliton excitations in liquid crystal polyacetylene

Abstract

A disubstituted polyacetylene with lyotropic liquid crystallinity showing SmA was synthesized. The number-average-molecular weight of the polymer was 5.2?×?10⁵ g/mol. Vapor phase iodine doping for the disubstituted polyacetylene in the form of liquid crystal order allows the generation of charge carriers. The ESR measurements show the charge carriers exhibit Pauli paramagnetism, indicating that these carriers are polarons. Mott-type variable range hopping (VRH) conduction of the polymer was observed. The combination of SmA molecular architecture and electrical conduction by polyene as a molecular wire in the polyacetylene derivative results in liquid crystal electrical conduction via electro-soliton movement.     

Low-temperature nuclear spin-lattice relaxation in glasses - homogeneous and inhomogeneous averaging

A phenomenological model is presented which is able to explain both temperature and magnetic field dependences of nuclear spin-lattice relaxation rates T₁^?1 observed recently in glasses at low temperatures. The model assumes randomly distributed defect centres, typical of glasses, producing internal fluctuating fields. The only restrictive assumption is a power law for the distance dependence of the responsible interaction. In accordance with experiment, homogeneous and inhomogeneous relaxation, corresponding to exponential and non-exponential relaxation behaviour, is predicted in the cases of rapid and slow spin diffusion, respectively. Describing the fluctuating interactions by a BPP-type spectral density and assuming an Arrhenius law for the correlation time, explicit expressions for T_-1^? are obtained. It is shown that by comparison of homogeneous and inhomogeneous relaxation data information on the type of interaction and the density of defect centres can be obtained.     

Microscopic Studies of 2H-NbSe2 Probed by Positive Muon

Abstract

The microscopic state of the positively charged light particle in the transition metal dichalcogenide 2H-NbSe₂ was studied using the muon spin relaxation method (μ⁺SR) and muon level crossing resonance method (μ-LCR). Muons are expected to stay at interlayer position and behaves as a hydrogen like intercalant. We discuss the relation between conduction electron properties and the muon's behavior.     

The Interaction of Light with Rb0.73Cr5Te8 and Polymer/Nanoferrite Composites

Abstract

The magnetic and photomagnetic characterizations of two materials are reported: ploymer/nanoferrite composites and the non-stoichiometric crystalline materials Rb0.73Cr5Te8. In the inorganic-organic composite material, the inorganic component is iron oxide and the organic component is the polymer poly(p-ethylphenol) (PEP). The Rb0.73Cr5Te8 material is a new non-stoichiometric formulation of the ACr5Te8 system that exhibits spin glass type magnetic behavior with a spin glass freezing temperature of 175K. The photomagnetism of both compounds has been measured at several temperatures below the spin blocking temperature.     

Measurements of Soliton Trapping and Motion in Trans-Polyacetylene Using Dynamic Nuclear Polarization

Abstract

The motion of solitons in undoped trans-(CH)_x is studied over the temperature range 1.5–300 K using dynamic nuclear polarization. The results of a theoretical model for the combined solid state and Overhauser effects in terms of the time dependence of the electron nuclear interaction due to motion of electrons is presented. Its application to the experiment shows that most of the solitons are trapped below 30 K and that the mobile solitons above 30 K have a diffusion coefficient that is approximately proportional to the square root of the temperature. At 300 K the time for a soliton to diffuse its length is greater than or equal to 1.7 × 10^?11 seconds.     

Role of Dimensionality in Dimerized Two-Band Systems

Abstract

We have studied the ground state properties of Pd(dmit)₂ salts using an effective dimer model. This model describes low-energy excitations of the two-band Hubbard model and is derived by a strong coupling expansion. Dimensionality of the Fermi surface, density-of-states singularity, and magnetic frustration in the dimer model are simultaneously controlled by substituting the cation.     

Localized Excitations in Competing Bond-Order-Wave,Charge-Density-Wave and Spin-Density Wave Systems

Abstract

The localized excitations in the competing bond-order-wave(BOW), charge-density-wave(CDW), and spin-density-wave(SDW) systems are investigated within the Bogoliubov-de Gennes formalism using an extended Peierls-Hubbard model. An extensive study of localized excitations over a wide range of the on-site e-ph coupling λ₂ and the Hubbard interaction U leads to the following observations: (a) As λ₂ increases at fixed U, the number of bound states inside the gap changes from two to four for the STE case and from two to three for the polaron case. (b) A non-monotonic dependence of the lattice relaxation energy on λ₂ is predicted within the lattice relaxation approach developed by two of us earlier, and is attributed to a crossover from the weak-coupling to strong-coupling behavior showing up as the emergence of new bound states inside the gap. Moreover, the non-radiative transition rate of STE is also calculated and is used to tentatively interpret the very short life-time of STE in PtCl complexes.     

Observation of Strong Antiferromagnetic Exchange Interactions in [1-(4′-methobenzyl)isoquinolinium][Ni(mnt)<Subscript>2</Subscript>]: Crystal Structures,Magnetic Properties

Abstract  

An new ion-pair compound, [CH₃BzQl][Ni(mnt)₂] (1), (CH₃BzQl = 1-(4′-methobenzyl)isoquinolinium and mnt²⁻ = maleonitriledithiolate), has been synthesized, structurally and magnetically characterized. The [Ni(mnt)₂]⁻ anions (A) and [CH₃BzIQl]⁺ cations (C) of 1 alternate to form a mixed columnar stack in the manner of ···ACAC··· along the a-axis, and the neighboring columns connect together to generate a three-dimensional (3D) network structure via intermolecular H-bonding interactions and van der Waals forces. Variable-temperature magnetic susceptibility measurements in the temperature range 2–300 K show that 1 is a spin gap system with strong antiferromagnetic interaction.     

Ultrafast Optical Response in Quasi-One Dimensional Halogen-Bridged Mixed-Valence Complexes [Pt(en)2][PtBr2(en)2](ClO4)4 and [Pd(en)2][PdCl2(en)2](ClO4)4

Abstract

Ultrafast optical response in two quasi-one-dimensional halogen-bridged mixed-valence complexs [Pt(en)₂][PtBr₂(en)₂](ClO₄)₄ (en=ethylenediamine) (abbreviated as PtBr) and [Pt(en)₂][PtBr₂(en)₂](ClO₄)₄ (as PdCl) has been investigated by femtosecond absorption spectroscopy at room temperature by pump-probe spectroscopy. The photo-induced absorption around 1.3 eV and the bleaching from 1.5 eV to 2.7 eV were observed in PtBr. Both consist of a fast-decay component due to STEs and a slow-decay component due probably to polaron pairs. The former decays exponentially with the time constant of 1.4 ± 0.2 ps. The latter decays as erf(t ^β) with β = -0.22 ± 0.02, indicating the geminate recombination of an electron polaron and a hole polaron after moving freely along the chain. The deviation of β from the ideal random-walk model (β = -0.5) is explained by introducing the effect of potential barrier between the polarons hindering the recombination. A pump-probe absorption spectrum of PdCl is obtained from the reflection spectrum by the Kramers-Kronig relations. The time dependence of the transient photoinduced absorption around 1.7 eV and the bleaching from 1.9 eV to 2.5 eV were calculated to be described with three components. They correspond to free excitons with lifetime of about 800 fs, self-trapped excitons with lifetime of about 3 ps, and polaron pairs which hardly relax within 100 ps.     

Semiconducting TiO2SiO2 glasses (I): Electronic conductivity

Semiconducting glasses, 70% TiO₂-30% SiO₂, with a Ti³⁺ content varying from zero to 7.8%, have been prepared by rf sputtering in argon/oxygen/hydrogen gas mixtures. Conduction in the Ti³⁺-containing glasses is by small polarons hopping adiabatically. The disorder energy is small (0.05 eV) compared to the hopping energy (0.37–0.51 eV). The variation of the hopping energy with Ti³⁺ content and temperature is ascribed to a polaron overlap effect and to effects arising from the random glass structure, respectively. At low temperatures conduction is a percolation process. The conduction in Ti³⁺-free glasses is not due to polarons; at low temperature these glasses exhibit variable-range hopping.     

Solitons in Polyacetylene Induced and Probed by Positive Mupons

Abstract

Positive muons were injected into both trans- and cis- (CH) as well as (CD)_x. In cis-polyacetylene, the μ⁺ was found to form a radical state with an unpaired electron localized near the μ⁺ In trans-polyacetylene, the longitudinal μ⁺ spin relaxation rate at 293 K showed H^?1/2 dependence on the applied field H. After considering possible mechanisms, we concluded that it is due to the soliton-like one-dimensional motion of induced unpaired electron.     

Magnetic Resonance on (QP)4(SbF6)3, a Radical Cation Salt of P-Quaterphenylene

Abstract

The unit cell of (QP)₄(SbF₆)₃ shows triads of QP in an arene stack and one QP in the anion sheet. The sum formula is therefore (QP)₃QP(SbF₆)₃. Electron-electron repulsion force le^?/site resulting in a semi-conducting state. This is in agreement with an activated conductivity of max 10^?1 Scm^?1 at 300 K.

Magnitude and activation energy of the paramagnetic susceptibility do not coincide with those of the conductivity; spins and charges are localized and decoupled due to the strong alternation of distances within the cation stack. Spin diffusion along the stacks governs, at least above 180 K, the magnetic properties: narrow ESR-lines and proton spin relaxation via hyperfine interaction with the relaxation dispersion T ^?1 _1H α ω^?1/2 above 200 K.

A lower value for the intrachain exchange rate τ₁ ^?1> 8.10¹¹ sec^?1 and a lower limit of 10³ for the anisotropy can be evaluated.

Keywords: quaterphenylene, molecular crystal, radical cation salt, low dimensionality, magnetic resonance     

Low-Temperature Electron Spin Resonance in (TMTSF)2PF6 in the High Pressure Metallic Phase

Abstract

The electron spin resonance of (TMTSF)₂PF₆ has been observed at low fields (H_o < 110 Oe) in the high pressure, metallic phase (p > 6.5 kbar) in the temperature range 1-4^?K. The anisotropy in the g value is similar to that observed at ambient pressure above the metal-insulator transition. The linewidth is very narrow and the spin susceptibility strongly decreases as the superconducting transition is approached from above. We interpret this as evidence for singlet-paired superconductivity. Superconductivity is observed at 1.1 K and the critical field has angular dependence in the be plane. These observations lead us to conclude that (TMTSF)₂PF₆ is a singlet paired superconductor.     

Electronic Properties of Ba-Intercalated Fullerides

Abstract

Raman spectra have been measured on Ba_xC₆₀ binary systems for x=3, 4 and 6. The pentagonal pinch Ag(2) mode exhibits a softening in an approximately proportional manner to the formal valence of C₆₀ molecule. This result suggests that the valence of C₆₀ in Ba doped systems are understood by the naive ionic crystal model based on Ba²⁺. The broader Raman peaks in superconducting Ba₄C₆₀ indicates that the electron-phonon interaction is considerably large comparing to those for Ba₃C₆₀ and Ba₆C₆₀.