Low temperature magneto-conductivity properties of micro-sized Ag/HD-PE metal/polymers

A composite material of a polyethylene matrix filled by a fine silver powder was prepared with different Ag contents and physical behaviours ranging from insulator to conductor. Ac differential magnetic susceptibility χ measurements show the samples are paramagnetic up to an Ag concentration of ∼65%. At low temperatures the composite is spin-glass type, whereas the transition from insulator to conductor corresponds to an abatement of χ at zero magnetizing field. Magneto-conductivity effects have been observed in resistivity measurements at low temperatures. They can be explained in terms of an effective exchange electronic scattering mechanism between the conduction electrons and the diluted magnetic moments arising from unpaired electron spins of boundary silver particles. Moreover, the presence of a broad minimum in the resistivity curve at T = ∼ 20 K, observed in samples with an Ag concentration just above the percolation threshold, addresses to possible interference effects similar to those reported in disordered materials. Received 17 October 2000 and Received in final form 22 February 2001     

Doping dependence of infrared conductivity of camphor-sulphonic-acid-doped polyaniline

The reflectivity spectrum of a polyaniline CSA-doped in presence of m-cresol has been measured over the wide wavenumber range 25-15,000 cm^-1 (0.003-1.9 eV) for three different doping levels. Since spectra cannot be fitted correctly with the conventional Drude model, several extensions are tested. A model derived from the factorized form of the dielectric response and including the effect of Anderson localization in disordered metals, is proposed and found to yield good fit to data with a satisfactory physical meaning. Data are reduced to a small number of parameters potentially useful for further comparison with other conducting polymers or even other non-Drude conducting media like oxides. Received 6 February 2002 / Received in final form 12 August 2002 Published online 27 January 2003 RID="a" ID="a"e-mail: gervais@delphi.phys.univ-tours.fr RID="b" ID="b"UMR 6157 CNRS/CEA     

Magnetic oscillations in the 2D network of compensated coupled orbits of the organic metal (BEDT-TTF) 8 Hg 4 Cl 12 (C 6 H 5 Br) 2

Interlayer magnetoresistance and magnetisation of the quasi-two dimensional organic metal (BEDT-TTF)₈Hg₄Cl₁₂(C₆H₅Br)₂ have been investigated in pulsed magnetic fields extending up to 60 T and 33 T, respectively. About fifteen fundamental frequencies, composed of linear combinations of only three basic frequencies, are observed in the oscillatory spectra of the magnetoresistance. The dependencies of the oscillation amplitude on the temperature and on the magnitude and orientation of the magnetic field are analyzed in the framework of the conventional two-dimensional Lifshitz-Kosevitch (LK) model. This model is implemented by damping factors which accounts for the magnetic breakthrough occurring between electron and hole orbits yielding conventional Shubnikov-de Haas closed orbits (model of Falicov and Stachowiak) and quantum interferometers. In particular, a quantum interferometer enclosing an area equal to the first Brillouin zone area is evidenced. The LK model consistently accounts for the temperature and magnetic field dependence of the oscillation amplitude of this interferometer. On the contrary, although this model formally accounts for almost all of the observed oscillatory components, it fails to give consistent quantitative data in most other cases. Received 4 September 2002 / Received in final form 14 November 2002 Published online 27 January 2003 RID="a" ID="a"e-mail: audouard@insa-tlse.fr RID="b" ID="b"UMR 5830: Unité Mixte de Recherche CNRS - Université Paul Sabatier - INSA de Toulouse RID="c" ID="c"UMS 5642: Unité Mixte de Service CNRS - Université Paul Sabatier - INSA de Toulouse     

Transport properties of La 1 - x Ca x MnO 3 ( 0.5 ? x < 1)

The transport properties of the La_1-xCa_xMnO₃ ( 0.5 ? x < 1) system in magnetic fields up to 14 T were studied. We found that the relationship between the charge ordering temperature T _CO and Mn⁴⁺ content n _{Mn4 +} obeys the formula T _CO / T _max = 1 - a ( n _{Mn4 +} - n ₀ ) ² , here n₀ and a are constants and T _max is the maximum of T _CO . For x = 0.65, T _CO arrives at the maximum value of 249.5 K in zero magnetic field, while the charge ordered (CO) state is most stable around x = 0.75. For x = 0.5 when H < 6 T the resistivity displays Mott's variable-range hopping (VRH) behavior, when 6 < H < 12 T it is suggested that two kinds of conduction mechanism, i.e., VRH and magnetic polarons, coexist in the material, and when H > 12 T the resistivity shows metallic-like behavior and the transport mechanism is attributed to coexistence of magnetic polarons and free carriers. For x = 0.95, the conduction mechanism accords with the coexistence of VRH and magnetic polarons. Received 4 January 2002 and Received in final form 28 January 2002     

Effective electron Hamiltonian for a quasi one-dimensional system. The (TMTSF)2X and the (TMTTF)2X systems

In this paper we have introduced a variational approach to investigate the ground state of a model which includes both the Holstein electron-phonon interaction and the extended Hubbard electron-electron interaction. We have considered a variational state for the phonon subsystem which generalizes the previous used forms. This state allows to take into account the possibility of extended phonon mediated correlations. The effective electron Hamiltonian, which we have obtained, includes first and second neighbor electron-electron interaction terms. We have treated exactly, through a Lanczos method, this effective model in the one-dimensional case. We have applied our method to two Bechgaard salts and in these cases we have estimated the correlation parameters. We have shown that the introduction of electron-phonon interaction allows an estimate of the on site U and nearest-neighbor V Coulomb repulsion, which are in agreement with the experimental optical spectra of the above mentioned two compounds. Received: 30 October 1997 / Revised: 28 January 1998 / Accepted: 10 April 1998     

Transport and magnetic properties of Pr1-xCaxCrO3 (x = 0.0–0.5): effect of t2g orbital degeneracy on the thermoelectric power

We report on the resistivity (ρ), thermoelectric power (S for Seebeck) and magnetic measurements of the series Pr_1-xCa_xCrO₃ (x=0.0–0.5). These orthochromites exhibit a T independent, large and positive S, from 100 K to 700 K which follows the Cr⁴⁺ concentration. Upon Ca²⁺ for Pr³⁺ substitution, a concomitant decrease of the resistivity and S values is found. The evolution of S as a function of hole concentration (Cr⁴⁺) can be nicely fitted by the modified Heikes formula which takes into account the orbital degeneracy associated to Cr³⁺/Cr⁴⁺. This is in good agreement with the model previously calculated by Marsh and Parris, in the case of weak magnetic coupling [Phys. Rev. B 54, 7720 (1996)]. The magnetic susceptibility measurements support the assumption of a weak magnetic coupling since the antiferromagnetic ordering T_N values are found to be lower than 250 K. For Pr_0.7Ca_0.3CrO₃, the power factor PF= S²/ρ measured at 700 K is equal to 1.9×10^-4 W m^-1K^-2. The present system, chemically stable in air up to T > 1000 °C, is promising for thermoelectric application at very high temperature.     

Hopping conductivity and activated transport in InxGa1-xAs quantum wells

We present measurements of the diagonal R_xx and off-diagonal R_xy magnetoresistance under quantum Hall conditions on several high electron mobility transistors (HEMT) based on In_xGa_1-xAs quantum wells. From the magnetoresistance tensor we obtain the longitudinal conductivity σ _xx . We study the transport mechanisms near the σ _xx minima at temperatures ranging between 2 K and 35 K; activated transport is the dominant mechanism for temperatures above 7 K while variable range hopping conductivity is significant for lower temperatures. We show that electron-electron correlations should be taken into account to explain the conductivity vs temperature behaviour below 5 K. Finally, we study the behaviour of the localization length as a function of Landau level filling and obtain a critical exponent γ = 3.45±0.15. Received 6 June 2001 and Received in final form 16 October 2001     

Influence of ambient atmosphere on charge transport in polycrystalline thin films of three simple aromatic hydrocarbons

Drift mobility of charge carriers in p-terphenyl, p-quaterphenyl and tetracene polycrystalline thin films was measured using the classical time-of-flight method. The measurements were carried out both in the atmosphere and in vacuum of the order of 10 ^{- 3} Torr. The mobility measured in the atmosphere is of the order of 10 ^-4cm²V^-1s^-1 and turns out to be about 1 order of magnitude higher than that measured in vacuum. It is suggested that the hopping transport of charge dominates in the structures investigated. The increase in the mobility for the measurements in the atmosphere probably results from the additional localized states due to water and other atmosphere molecules.     

Charge transfer and polarization screening in organic thin films: phthalocyanines on Au(100)

Core hole screening effects at organic/metal interfaces were studied by core level X-ray photoemission spectroscopy (XPS), X-ray excited Auger electron spectroscopy (XAES), and valence band ultraviolet photoemission spectroscopy (UPS). The comparison of energetic shifts in XPS and XAES enables the estimation of electronic relaxation energy (screening ability). Magnesium phthalocyanine (MgPc) and zinc phthalocyanine (ZnPc) evaporated on single crystalline Au(100) were used as model molecules. Two different features in the Mg KLL spectra can be clearly separated for (sub-)monolayer coverages, while only minor changes of the shape of Mg 1s are observed. Applying a dielectric continuum model, the major screening mechanism cannot be described sufficiently by polarization screening due to mirror charges, significant contributions by charge transfer screening have to be considered. In contrast, small screening effects in the bulk material can be explained by surface polarization.     

High temperature phase transitions in the crystal [4]of p-trimethylammoniumbenzenesulfonate zwitterion

Crystals of p-trimethylammoniumbenzenesulfonate (ZWT) have been obtained from the thermally induced soli d-state reaction of methyl p-dimethylaminobenzenesulfonate (MSE) or by recrystallization in aqueous solution. The characterization of four crystalline phases has been performed by calorimetric, birefringence, X-ray and Raman scattering studies. An unknown phase transition at room temperature yielding a new crystallographic phase has been found by calorimetric measurements. ZWT crystalline samples obtained from the solid state reaction are disordered as compared to ZWT single crystals obtained by growing from aqueous solution. This disorder is related to different arrangements of the phenyl ring orientations. Received 21 October 1998 and Received in final form 14 April 1999     

Analysis of infrared reflectivity of conducting polymers: example of camphor-sulphonic-acid-doped polyaniline

The reflectivity spectrum of a polyaniline CSA-doped in presence of m-cresol has been measured over the wide wavenumber range of 15- 9 000 cm ^-1 (0.002-1.1 eV) at room temperature. Experimental data compare well with similar experiments performed by another group. The conductivity spectrum of this conducting polymer has been deduced from the reflectivity spectrum by means of two methods, Kramers-Kronig transformation and best fit of an “extended Drude” model to the reflectivity spectrum. Whereas the deviation from Drude behavior was interpreted in terms of Anderson localization or by inhomogeneous disorder by other groups, it is shown here that a different model developed for conducting oxides that also exhibit non-Drude behavior, applies very well to this example of conducting polymer. Received 11 February 1999 and Received in final form 26 April 1999     

pH effects in the formation of interpolymer complexes between poly(N-vinylpyrrolidone) and poly(acrylic acid) in aqueous solutions

The critical pH values for hydrogen-bonded interpolymer complexes of poly(N-vinylpyrrolidone) and poly(acrylic acid) in aqueous solutions were determined. The effect of polymer concentrations, molecular weights and addition of inorganic salts (NaCl and KCl) on the complexation was studied. Received 29 April 2002 and Received in final form 24 June 2002 Published online: 21 January 2003     

Tensor polarization of 12C[2+ 1] in the 16O(13C,12C)17O reaction at 50 MeV

The ¹⁶O(¹³C,¹²C)¹⁷O reaction at 50 MeV has been investigated using the kinematical coincidence method. Polarization tensors t ₂₀ and t ₄₀ of ¹²C[2⁺ ₁] for the quantization axis taken along the direction of propagation have been measured by analyzing the energy spectrum of ¹²C[2⁺ ₁], modulated by the effect of γ ray emission. The deduced t ₄₀ values significantly deviate from zero, contrary to the prediction of the distorted-wave Born approximation theory based on one-step p shell neutron stripping without spin-dependent interactions. The phenomenological spin–orbit interaction necessary to reproduce the magnitude of measured t ₄₀ is found to be much larger than the folding model prediction. It is shown that the experimental polarization tensors as well as the cross sections can be reproduced by introducing multi-step processes involving excitations in ¹²C and ¹³C without introducing spin-dependent interactions. Received: 2 August 1999 / Revised version: 3 February 2000     

The spin-crossover complex [Fe(tpa)(NCS) 2 ]

The temperature-induced spin crossover of iron(II) in the [ Fe ( tpa )( NCS ) ₂ ] complex has been investigated by nuclear forward scattering (NFS), nuclear inelastic scattering (NIS), extended X-ray absorption fine structure (EXAFS) spectroscopy, conventional M?ssbauer spectroscopy (MS) and by measurements of the magnetic susceptibility (SQUID). The various measurements consistently show that the transition is complete and abrupt and exhibits a hysteresis between 102 and 110 K. The dependence of the hyperfine parameters of the high-spin (HS) and of the low-spin (LS) phase on temperature is gradual while the effective thickness (determined by the Lamb-M?ssbauer factor f _LM ) shows a step at the transition temperature. This step could be identified clearly because the effective thickness is measured directly by NFS. The Lamb-M?ssbauer factor, the Debye temperature and the mean-square displacement of iron(II) could be determined for the HS and for the LS phase. When comparing the NIS data with the results from density functional theory (DFT), the Fe-N stretching vibrations of both LS and HS phases could be unambiguously identified and the f _LM could be factorized for both phases into a lattice and a molecular part. The structural information from EXAFS and DFT geometry optimization are in reasonable agreement. Received 19 June 2001     

Onset of the nonlinear dielectric response of glasses in the two-level system model

We have calculated the real part of the nonlinear dielectric susceptibility of amorphous insulators in the kHz range, by using the two-level system model and a nonperturbative numerical quantum approach. At low temperature T, it is first shown that the standard two-level model should lead to a decrease of when the measuring field E is raised, since raising E increases the population of the upper level and induces Rabi oscillations cancelling the ones induced from the ground level. This predicted E-induced decrease of is at odds with experiments. However, a better, though still not perfect, agreement with low-frequency experimental nonlinear data is recovered if, in our fully quantum simulations, interactions between defects are taken into account by a new relaxation rate whose efficiency increases as , as was proposed recently by Burin et al. [Phys. Rev. Lett. 86, 5616 (2001)]. In this approach, the behavior of at low T is mainly explained by the efficiency of this new relaxation channel. Since a quantitative understanding of glasses is still missing, we finally discuss experiments whose results should yield a refined understanding of this new relaxation mechanism: i) a completely new nonlinear behavior should be found for samples whose thickness is ≃ 10 nm; ii) a decrease of nonequilibrium effects should be found when E is increased. Received 19 September 2002 / Received in final form 4 December 2002 Published online 14 March 2003     

Electron transport properties of MgB<Subscript>2</Subscript> in the normal state

We report measurements of the resistivity, ρ, and the Seebeck coefficient, S , of a MgB₂ sintered sample, and compare S with theoretical calculations based on precise electronic structure calculations. ρ is fitted well by a generalized Bloch-Grüneisen equation with a Debye temperature Θ _R of 1050 K. S is given by the sum of a diffusive and a phonon drag term and the behavior in the temperature region T _c < T < 0.1Θ _R follows the relationship AT+BT³. The phonon drag term indicates a strong electron-phonon interaction. The diffusive term, compared with calculations, suggests that σ bands give the main contribution to the Seebeck effect. Received 16 November 2001 and Received in final form 21 December 2001     

On the stoichiometry of the organic metal bis(ethylenedioxy)-tetrathiafulvalene chloride

We report on Shubnikov-de Haas and de Haas-van Alphen measurements of an organic metal based on the molecular donor bis(ethylenedioxy)-tetrathiavulvalene (= BEDO-TTF) and the anion Cl^- with H₂O molecules. The observed single oscillation frequency perfectly follows the two-dimensional dependence with T. The Fermi-surface area of of the first Brillouin zone proves a quarter-filled band, i.e., a 2:1 stoichiometry of the BEDO-TTF donor with respect to the anion. The apparent discrepancy to X-ray data which give a 1:1 ratio between BEDO-TTF and Cl^- is understood by replacement of H₂O molecules with (H₃O)⁺ ions. The proposed stoichiometry therefore is (BEDO-TTF)₂ ⁺(H₅O₂)⁺(Cl^-)₂. The cyclotron effective mass is when deduced from the temperature dependence of the fundamental oscillation amplitude, but strongly reduced when extracted from higher harmonics. This and the strong harmonic content of the oscillations signals an influence of the two-dimensional electronic structure and can be qualitatively understood by applying the concept of magnetic interaction to the effect of the oscillatory chemical potential. Received: 16 April 1998 / Revised: 7 July 1998 / Accepted: 9 July 1998     

Coexistence of superconductivity and spin density wave orderings in the organic superconductor (TMTSF) 2 PF 6

The phase diagram of the organic superconductor (TMTSF)₂PF₆has been revisited using transport measurements with an improved control of the applied pressure. We have found a 0.8 kbar wide pressure domain below the critical point (9.43 kbar, 1.2 K) for the stabilisation of the superconducting ground state featuring a coexistence regime between spin density wave (SDW) and superconductivity (SC). The inhomogeneous character of the said pressure domain is supported by the analysis of the resistivity between T _SDW and T _SC and the superconducting critical current. The onset temperature T _SC is practically constant ( 1.20±0.01 K) in this region where only the SC/SDW domain proportion below T _SC is increasing under pressure. An homogeneous superconducting state is recovered above the critical pressure with T _SC falling at increasing pressure. We propose a model comparing the free energy of a phase exhibiting a segregation between SDW and SC domains and the free energy of homogeneous phases which explains fairly well our experimental findings. Received 3 September 2001 and Received in final form 9 November 2001     

Pressure dependence of the Shubnikov-de Haas oscillation spectrum of \beta'-(BEDT-TTF)4(NH4)[ Cr(C2 O4)3] .DMF

The Shubnikov-de Haas (SdH) oscillation spectra of the -(BEDT-TTF)₄(NH₄)[ Cr(C₂O₄)₃] .DMF organic metal have been studied in pulsed magnetic fields of up to either 36 T at ambient pressure or 50 T under hydrostatic pressures of up to 1 GPa. The ambient pressure SdH oscillation spectra can be accounted for by up to six fundamental frequencies which points to a rather complex Fermi surface (FS). A noticeable pressure-induced modification of the FS topology is evidenced since the number of frequencies observed in the spectra progressively decreases as the pressure increases. Above 0.8 GPa, only three compensated orbits are observed, as it is the case for several other isostructural salts of the same family at ambient pressure. Contrary to other organic metals, of which the FS can be regarded as a network of orbits, no frequency combinations are observed for the studied salt, likely due to high magnetic breakdown gap values or (and) high disorder level evidenced by Dingle temperatures as large as ≃7 K.     

Balance-equation approach to impact ionization induced by an intense terahertz radiation: Application to InAs/AlSb heterojunctions

We have extended the balance equations to account for conduction-valence interband impact ionization (II) process induced by an intense terahertz (THz) electromagnetic irradiation in semiconductors, and applied them to study the II effect on electron transport and electron-hole pair generation-recombination rate in THz-driven InAs/AlSb heterojunctions (HJ). As many as needed multiphoton channels are self-consistently taken into account for yielding a given accuracy. The time evolution of transport state including THz-radiation-induced II process are monitored in details by an extensive time-dependent analysis. Two different physical stages, the quasi-steady state and the complete steady-state, are clearly identified from the present calculations. Intersubband electron transfer rate and net electron-hole generation rate are derived as functions of the THz radiation strength E _ac for various radiation frequencies from f _ac = 0.42 to 6 THz at lattice temperatures T = 6 K. It's indicated that the THz radiation with a larger E _ac or a lower f _ac, has a stronger effect on electron transport and II process. Qualitative agreement is obtained between the calculated electron-hole generation rate and the available experimental data for InAs/AlSb HJ's at T = 6 K. Received 24 May 2002 / Received in final form 26 August 2002 Published online 31 October 2002 RID="a" ID="a"e-mail: jccao8@hotmail.com