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Science and Technology of Synthetic Metals, 1994. ICSM '94. International Conference on

Date 24-29 July 1994

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Displaying Results 1 - 25 of 1338
  • International Conference on Science and Technology of Synthetic Metals - ICSM '94 [front matter]

    Page(s): i - iv
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  • Ultrafast optical probes of excited states in conducting polymers

    Page(s): 4
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    Summary form only given. The question whether the electronic states of 7/spl pi/-conjugated polymers can be described by a band model or exciton model, has been extensively debated over the last couple of years. We present an overview of relevant ultrafast optical data, such as transient photoluminescence, gain, and photomodulation, obtained from several luminescent (such as poly-(arylene vinylene) and polythiophene derivatives) and nonluminescent (such as poly(diethynyl-silane) and poly(thienylene vinylene)) conducting polymers, and we critically examine their interpretation in the context of both models. For laser excitation photon-energy close to the HOMO-LUMD energy-gap we observed in all cases the dominant role of excitons in the ultrafast response. The electronic dynamics is faster for polymers with E(2A/sub g/) < E(IB/sub u/), but the relaxed 2A/sub g/ state plays an equally important role in other polymers. We also report a new ultrafast optical technique to directly probe excited electronic states in conducting polymers, using picosecond strain waves in the 20 GHz range. We demonstrate its potential to detect various excitonic states with both A/sub g/ and B/sub u/ character. View full abstract»

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  • Nature of the primary photo-excitations in poly(arylene-vinylenes)

    Page(s): 4
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    Summary form only given. We present an overview of the optical properties and related experimental results obtained from poly(paraphenylene vinylene), PPV, and its soluble derivatives, and we critically examine the interpretation of these data in the context of both the band model and the exciton model of the electronic structure of these semiconducting and luminescent polymers. Results obtained from highly oriented and structurally ordered PPVs demonstrate that the observed disparities between the physical properties of PPV and the polydiacetylenes (PDAs) are not due to disorder; these differences are intrinsic and arise from the fundamental difference in the nature of the low energy photo-excited states: excitons in the PDAs and free carriers in the PPVs. View full abstract»

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  • Magnetic-field effects on the reentrance transition of the alloy of pristine and fully deuterated (DMeDCNQI)/sub 2/Cu

    Page(s): 5
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    Summary form only given. The title compound undergoes a metal-insulator-metal reentrance transition with decreasing temperature. The metal-insulator transition occurring at the higher transition temperature has been ascribed to the combination of a Peierls instability in the /spl pi/ -like band of DCNQI molecular stacks and the so-called 4K/sub F/ instability of d-like band of copper ions. We investigate possible origins of 'the reentrance transition to the metallic state with decreasing temperature. The electrical resistance of the title compound is measured in the metal-insulator-metal reentrance transition regime as functions of temperature and magnetic field under pressure up to 12 MPa (120 bar). Its heat capacity is measured in vacuum in the range of 0.1 K to evaluate the electronic density-of-states. Also the lattice parameter was measured at ambient pressure as functions of temperature. We found resistance enhancement by the application of magnetic fields in the vicinity of the insulating state. We propose that this transition is ascribed to the stabilization of Cu/sup 2+/ ionic state having paramagnetic spins. The idea is consistent with the lattice parameter change accompanying the reentrance transition. We found, in addition, the density-of-states is not anomalously large as an organic conductor. This suggests the reentrance transition is hardly explained in terms of the heavy electron model. We will discuss alternative models for the reentrance transition. View full abstract»

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  • Theory of metal-insulator transition in (DCNQI)/sup2/Cu

    Page(s): 5
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    Summary form only given. The metal-insulator transition of (DCNQI)/sub 2/Cu has various unique features; 1) The transition is of first order with very large hysterisis. 2) In the insulating phase there exists threefold superlattice distortions. 3) Although the temperature independent Pauli like spin susceptibility is observed in the metallic state, the localized spins are created in the insulating phase. 4) The average valence of Cu is 4/3 in both metallic and insulating state. 5) A reentrant metallic state is stabilized for some particular range of effective internal pressure. All of above mentioned experimental facts of this /spl pi/ - d system I)-4) are explained as a unique situation of the Peierls transition of the /spl pi/-band with the Mott transition of the d-band, and a possible mechanism of 5) has been proposed based on the quantum tunneling of the protons. View full abstract»

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  • Selectively deuterated molecular conducfor (DMe-DCNQI)/sub 2/-phase diagram and dimensionality

    Page(s): 6
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    Summary form only given. Conducting molecular system (R/sup 1/,R/sup 2/-DCNQI)/sub 2/Cu consists of one dimensional DCNQI columns, each of which is three-dimensionally linked by the coordination bond (Cu-N). P//sup/spl pi-d// interaction in this system provides unique electrical and magnetic properties. (DMe-DCNQI)/sub 2/Cu undergoes novel metal-insulator-metal (reentrant) transition under very low-pressure. Based on the discovery of the deuteration-induced metal-insulator transition/sup 1/), we present fine tuning of the effective pressure (1-500 bar) by selective deuteration of the DMe-DCNQI molecule. Structural and electrical aspects of the phase transition in the selectively deuterated system (including alloy system) are described. We also report that three-dimensional Fermi surface observed in the de-Haas van Alphen effect/sup2/) can be explained by a simple tight-binding band calculation. View full abstract»

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  • Properties of synthetic derivatives of buckminsterfullerene in conposites with conducting polymers

    Page(s): 6
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    Summary form only given. Studies of photoinduced electron transfer from a conducting polymer to C/sub 60/ and C/sub 61/ derivatives with 60 and 58 /spl pi/ electrons will be described. The molecules with 58 /spl pi/ electrons are the methanofullerenes; e.g., 1 and the molecules with 60 /spl pi/ electrons are the fulleroids (2). These molecules were made into composites with conjugated polymers such as MEH-PPV and the epi-cholestanoxy-PPV analogs. The photophysics of the various composites and the dramatic effect of electron count will be described. Other physicochernical phenomena related to fulleroid-methanofullerene isomerism will also be described. View full abstract»

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  • Photophysical properties of fullerene-conducting polymer system

    Page(s): 7
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    Summary form only given. Optical properties of conducting polymers such as absorption spectrum, photoluminescence, electroluminescence, photo-magnetic effect, photoconductivity and their dynamics change drastically upon doping of small amount of fullerene. Electrical and optical characteristics of junctions based on conducting polymers and fullerene are also anomalous. It should be mentioned that these unique photophysical properties and their dynamics are dependent on the molecular structure of conducting polymer (non-degenerated polymers such as polythiophene derivatives or degenerated polymers such as polyacetylene derivatives) and also on fullerenes (C60,C70,C76). These novel results are discussed in terms of photoexcitation in both conducting polymers and fullerene, photoinduced excitons, solitons, polarons and also their transfer between conducting polymers and fullerene by taking into account the electronic energy states of conducting polymers and fullerene. View full abstract»

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  • Addition reactions of fullerenes

    Page(s): 7
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    Summary form only given. In order to minimise strain in the the carbon cage, there is relatively little electron delocalisation in fullerenes, which therefore behave as giant (electron deficient) alkenes. They therefore undergo all the reactions typical of alkenes such as hydrogenation, halogenation, alkylation, alkylation, hydroxylation, metallation etc; the electron deficiency also makes them excellent dienophileg for the Diels-Alder reaction. The chemistry is of unparalleled complexity due to the myriad products that can be formed, and state-of-the-art techniques are needed to analyse them. Control of the extent of reaction is not yet possible in many cases, and progress has hardly begun on controlling the direction of subsequent additions. Steric hindrance is of overriding importance in these molecule which are rigid and therefore unable to flex to minimise steric interactions. Examples of the reactions, and analytical techniques employed will be described, together with some possible applications. View full abstract»

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  • Theory of bi-excitons in /spl pi/-conjugated polymers

    Page(s): 8
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    Summary form only given. Theoretical models of /spl pi/-conjugated polymers that emphasize the Coulomb electron-electron correlations predict that the lowest optidal excitation is to an exciton. We have extended our theoretical studies to an energy region that is much higher, to probe specifically the states that are reached by two-electron excitations from the ground state. Theoretical evidence for the biexciton, a bound state of two excitons that occurs below the two-electron continuum, is found. The lowest biexciton state is of even parity, can be reached by optical excitatioll from the IB/sub u/ exciton, and in principle also by two-photon excitation from the ground state. Interpretation of a recent picosecond photoinduced absorption experiment on polyparavinylene (PPV) in terms of the biexciton will be given. Very similar experimental results have been obtained also in pump-probe experiments with other conjugated polymers (polydiacety-enes, PTV, etc.) in the past. These results will be discussed within the context of our theory. Our earlier work on optical nonlinearity is /spl pi/-conjugated polymers already predicted stable biexcitons, and the present results therefore also confirm the validity of the mechanism of third order optical nonlinearity proposed by us. View full abstract»

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  • Metallic transitions of soliton lattice in conducting polymers randomly doped

    Page(s): 8
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    Summary form only given. Metallic transitions are theoretically investigated for a chain of trans- polyacetylene with an assumption of random impurity distributions, effects of excess charges by the dopants being represented by the introduction of the soliton lattice. Harigaya and Terai carried out a computer simulation work to find that the energy gap vanishes at the right impurity concentration. The energy gap was defined by the difference between the lowest energy among the lowest unoccupied states of a sample ensemble and the highest energy among the highest occupied states. Each sample has a non-vanishing energy gap. The purpose of this work is to see if their result can be reproduced using one of the traditional methods for random systems. The soliton lattice is resented by a modified order parameter with two adjustable parameters. One of them is associated with an amplitude variation and the other with a variation in the spatial periodicity. The method gives a set of selfconsistent conditions for the parameters. Numerical solutions show that they are satisfied fairly well, leading to the electronic density of states. We find that the gap vanishes at the right impurity concentration for a reasonable strength of the impurity potential. The order parameter still persists, indicating a gapless Peierls system. View full abstract»

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  • Pinning of /spl pi/ electrons in polyconjugated systems and frequency dispersion of the R mode (amplitude mode)

    Page(s): 9
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    Summary form only given. The correlative study of the Raman spectra (recorded in our laboratory) of series of oligomers of several classes of polyconjugated materials shows that the frequency and intensity dispersion of the R mode (amplitude mode) greatly changes in the different classes . This fact reflects the pinning of /spl pi/ electrons within the various chemical groups. This phenomenon is analyzed and its relevance in determining the localisation (confinment) length is discussed. View full abstract»

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  • Localization in quasi-one-dimensional systems

    Page(s): 9
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    Summary form only given. The conductance in a polymeric system of nearly macroscopic dimensions is calculated using the many-channel Buttiker-Landauer formula. The conductance is studied as a function of various types of irregularities including on-site disorder, disorder in the intrachain hopping caused by chain twisting and sp/sup3/ defects, and disorder due to variations in the interchain ordering. Calculations are performed on strictly linear systems, like trans-polyacetylene, as well as on chains build up totally or partially from cyclic monomers. From the results of these studies we are able to determine the most important factors that cause localization of the electronic wavefunctions in quasi-one-dimensional polymeric systems. A discussion about the extension of these results to curved graphitic structures is also included. View full abstract»

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  • Structural and physical properties of charge transfer complexes with [M(dto)/sub 2/ anions (M:Ni,Pt,Cu,Pd; dto:DITHIOOXALATE)

    Page(s): 10
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    Summary form only given. We report the structural, electrical and magnetic properties of the charge transfer complexes of [M(dto)/sub 2/](M=transition metals) as electron acceptors and TTF derivatives. Three dimensional alternating stacks of donor dimer and anion molecule are observed in the 2:1 complexes of BEDT-TTF and BEDO-TTF with M (dto)/sub 2/(M=Ni, Pd). While BEDT-TTF complex with Pt(dto)/sub2/ is 2:1 with DDADDA stacking. They are insulators (10/sup -4/-10/sup -5/Scm/sup -1/) and singlet-triplet type magnetic behaviors were detected in BEDT-TTF-Pt(dto)/sub 2/ and BEDO-TTF-Ni(dto)/sub 2/ complexes. BEDT-TTF and Pd(dto)/sub 2/ gave a metallic (ca.1Scm/sup -1/ at RT) 4:1 complex, whose crystal structure is similar to the oxalate analogues. BEDO-TTF and Cu(dto)/sub 2/ form a metallic complex (20OScm/sup -1/ at RT). View full abstract»

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  • Molecular metals based on pd(dddt)/sub 2/ complexes

    Page(s): 10
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    Summary form only given. The M(dddt)/sub 2/ complexes (dddt2- = 5,6-dihydro-1,4dithiin-2,3 dithio late) arouse great interest for they are isolobal analogs of the molecule BEDT-TTF [1] which is known to form superconducting cation radical salts [2]. In this work the first cation salts of Pd(dddt)/sub 2/ with the anions different in their geometry and electronic configuration (IBr/sub 2/, C10/sub 4/, ZnC1O/sub 4/, PF/sub 6/, SnC1/sub 6/ etc) were prepared by electrochemical oxidation of Pd(dddt)/sub 2/ neutral complex. Some of them were found to be molecular metals. Properties and structure of Pd(dddt)/sub 2/ cation salts were compared with those of BEDT-TTF and Pt(dddt)/sub 2/ ones. In contrast to the latters with the octahedral doubly charged anions, which are dielectric at Low temperatures, Pd(dddt)/sub 2/ salt with TeC1/sub 6//sup 2-/ has high conductivity at 4.2 K (20 /spl Omega//sup -1/cm/sup -1/). View full abstract»

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  • Recent developments in the chemistry of sulphiur-donor ligands

    Page(s): 11
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    Summary form only given. Over the past decade extensive studies have been made on the metal complexes of sulphur donor ligands. This has resulted in their use as building blocks in the synthesis of molecular metals and superconductors and as third order non-linear optical materials/sup 1,2/. To aid the understanding of the relationships between molecular structure and solid state properties and to develop materials with improved performance, extensive studies have taken place to chemically modify the ligands. We now wish to report on the synthesis and properties of metal complexes of the type (1) and (2). View full abstract»

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  • Magnetism in organic materials

    Page(s): 12
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    Summary form only given. Tailoring magnetic interactions of organic solids is currently an area of intense research activity. The cooperative efforts of chemists and physicists resulted in the preparation and analysis of a variety of new materials. Neutral monoradical solids and radical cation salts were used to study the variation of intermolecular exchange interaction with molecular packing and systematic molecular modifications (e.g. substituted triphenylverdazyls, perylene and tetraiodotetrathiafulvatene salts or charge transfer Complexes of C/sub 60). Intra and intermolecular exchange have been distinguished in di- and triradical systems (e.g. tri-triphenylverdazyl-benzene or molecules derived from well-known spin labels like nitronylnitroxides). Different metal-organic systems including charge transfer compounds were analysed; magnetism related to localized Manganese moments was observed in amorphous organic matrix, caused by Manganese in oxidic environment, or in charge transfer salts with TCNE and TCNQ. Besides chemical analysis and static magnetic susceptibility measurements in part ESR, NMR, ac susceptibility, specific heat and x-ray structural analysis were used to characterize these materials. View full abstract»

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  • A spectroscopic analysis of photochemical processes in fullerenes

    Page(s): 12
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    Summary form only given. Raman spectroscopy is employed as a probe of photochemical processes in both oxygen-free and oxygenated samples of C/sub 60/. The amplitude of the peaks are observed to be sensitive to light exposuro. Each of the initially observed peaks degrades while new peaks emerge approximately ten wavenumbers to the low energy side. In oxygenated films, the degradation process is observed to be apparently slower. The process is analysed in terms of the population of the triplet state leading to sample degradation competing with the ground state recovery in an attempt to elucidate the mechanism of fte photochemical process, proposed to be a 2+2 cycloaddition in which oxygen acts to inhibit the photochemical degradation via a triplet quenching. Particular attention is paid to the dependence of the process on input light intensity. The degradation is further reported to be highly dependent on the substrate employed. At high intensities, a further change in the Raman stectrum is observed, the peals abruptly shifting to a frequency intermediate -between those of the photoproduct and of pristine C/sub 60/. The shift is accompanied by a dramatic increase in the Raman intensity. Upon reduction of the intensity, the C/sub 60/ spectrum is recovered, indicating that the photochemical process is reversible under high intensity illumination. The nature of the high intensity intermediate is discussed. View full abstract»

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  • Electronic excited states in C/sub 60/ thin film as revealed by luminescence measurement: excitation spectra and temperature dependence

    Page(s): 13
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    Summary form only given. Important information pertaining to electronic excited states and their relax ation processes in solid C60 can be obtained by the measurement of luminescence and photoconductivity. We have recently succeeded in obtaining a hither to unknown luminescence excitation spectrum of C60 thin film exhibiting well resolved vibronic structures In the optical edge region. These structures can be assigned and compared with a corresponding absorption spectrum showing less resolution. A mirror-image like relationship was established between peaks in the emission and excitation spectrum, indicating a 0-0 transition around 670nm. We also found that the excitation spectrum showed strong peaks at 440-500nm, well corresponding to the solid-state specific absorption structures in the same wavelength region. These peaks exceeded that arising from the allowed transition at 350nm. Another newly found solid-state effect was that an emission intensity vs. temperature curve In 77-450K showed a change of slope at the phase transition point (250-260K), below which luminescence increased more steeply, the first example of an optical property of solid C60 perturbed by the phase transition. Relationship between photoconductivity and luminescence in C60 thin film will also be discussed. View full abstract»

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  • Magnetoresistance of carbon nanotube bundles

    Page(s): 13
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    Summary form only given. Since the discovery of carbon nanotubes a large number of theoretical studies predicted that their band gap changes significantly between zero and nonzero values depending on the diameter and degree of helicity of the nanotube. A magnetic field is expected either to reduce the band gap or to produce Aharonov-Bohm oscillations, when the tube axis is perpendicular or parallel to the magnetic field respectively. It was previously shown (1) that it was possible to realise a direct measurement of the electrical resistance of a single nanotube bundle. In the present work, we present more detailed results on the electrical resistance of single nanotube bundles. The results of an extensive study on the effect of magnetic field intensity and orientation with respect to sample axis as a function of temperature is reported. The electrical resistance measurements were performed from room temperature down to 10 mK in a magnetic field up to 14T. View full abstract»

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  • Photophysics of semiconducting polymer- C/sub 60/ composites: a comparative study

    Page(s): 14
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    Summary form only given. Experimental results on the metastable, reversible, ultrafast photoinduced electron transfer between semiconducting, conjugated polymers and buckminsterfullerene, C/sub 60/, are summarized. Comparative studies with different semiconducting polymers as donors demonstrate that in the degenerate ground state polymers soliton excitations form before the electron transfer can occur; thereby inhibiting charge separation. In non-degenerate ground state systems, photoinduced electron transfer occurs in less than 10/sup -12/ s, quenching the photoluminescence as well as the intersystem crossing. The hindrance of the photoelectron transfer by strongly Coulomb correlated photoexcitations (excitons) formed at the donor polymer is demonstrated with polydiacetylenes. The results are discussed in terms of opportunities for solar energy conversion, for photodiode detector devices, and for a variety of other applications which use photoinduced charge separation like in the photosynthesis. View full abstract»

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  • Role of the dopant counter-ions in the transport and magnetic properties of disordered conducting polymers

    Page(s): 14
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    Summary form only given. Disordered conducting polymers like polypyrroles or even some of the polyanilines contain generally short electroactive chains with conjugation lengths of a few ten monomers. Most of the electronic properties of these materials cannot be really understood in terms of quasi-one-dimensional models. i) Why in the fully doped materials the number of Curie spins is always lower than half the number of segments? ii) Why these disordered materials exhibit a Pauli-like component in their spin- susceptibility? iii) Why are the conductivities of all the fully doped materials roughly of the same order of magnitude (0.1 to 10 S/cm) independent of the micro structures and chain lengths? iv) Why the same value of the conductivity is found in thin films of doped oligomers? All these questions can be resolved if the interactions between the carriers and the dopant counter-ions are properly accounted for. It is then possible to derive a new interpretation of the spin-charge relation and of the so-called Curie and Pauli contributions to the susceptibility. Moreover the dopant ion is ultimately responsible for 3D localization through formation of transverse bipolarons connecting neighbour chains via enhanced tunnelling probability close to a counter-ion. Thus the hopping process through the disordered material can be revisited in relation with these new concepts. View full abstract»

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  • ESR in conducting polymers: effect of oxygen and air

    Page(s): 15
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    Summary form only given. It has been known since the fifties that the ESR signal of the free spins in various compounds is reversibly broadened in presence of air. In conducting polymers such an effect has been reported/sup 1/ to be a signature of the conducting state : in the conducting state, the spin/charge carriers can move to the surface and come into contact with the paramagnetic molecular oxygen. The mechanism of the line broadening will be discussed in relationship with the material properties, and new data obtained on emeraldine salt samples will be reported. In particular, in the case of powders or thin films, quite different behaviors are observed upon exposure to air or to oxygen gas. The kinetics appears to be at least 2 orders of magnitude faster with oxygen than with nitrogen. View full abstract»

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  • Kondo effect in a tomonaga-luttinger liquid

    Page(s): 15
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    Summary form only given. Kondo effect of the magnetic impurity in a interacting one-dimensional (ID) electron system, i.e., Toinonaga-Luttinger liquid, is studied. This problem is relevant to the /spl pi/ - d system, where the /spl pi/ band constitutes the 1D electron system. By using the poor rnan's scaling method, it is found that the Kondo coupling flows to strong coupling regime not only for antiferromagnetic but also for ferromagnetic case. The impurity spin is completely screened in the ground state, and the fixed-point Hamiltonian is that of two disconnected semi-infinite Tomonaga-Luttinger liquids and a spin singlet. Low temperature properties are also discussed by analyzing the leading irrelevant operators. View full abstract»

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  • Comparing the spin density distribution in radical anions of several different oligomeric /spl pi/-chains

    Page(s): 16
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    Summary form only given. We have studied the spin density distribution in a large number of radical ions, of oligomeric series of differently linked /spl pi/-systems as oligo-(ortho-, meta-, and paraphenylenevinylene)s la-c, oligoenes 2, -naphthylenes 3, -anthryienes 4, and -rylenos 5 by EPR/ENDOR and optical absorption spectroscopy. The results obtained show very different features depending on the geometry, the topology of the /spl pi/-systern, the chain ends and counter ion effects. Upon chain length extension both methods yield supplementary data allowing for a much deeper insight into characteristic molecular properties as limit of conjugation, bond length alternation patterns and structural reorganization upon charging. View full abstract»

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