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© 1992-2012 IEEE. Given the benefits of its low storage requirements and high retrieval efficiency, hashing has recently received increasing attention. In particular, cross-modal hashing has been widely and successfully used in multimedia similarity search applications. However, almost all existing methods employing cross-modal hashing cannot obtain powerful hash codes due to their ignoring the relative similarity between heterogeneous data that contains richer semantic information, leading to unsatisfactory retrieval performance. In this paper, we propose a triplet-based deep hashing (TDH) network for cross-modal retrieval. First, we utilize the triplet labels, which describe the relative relationships among three instances as supervision in order to capture more general semantic correlations between cross-modal instances. We then establish a loss function from the inter-modal view and the intra-modal view to boost the discriminative abilities of the hash codes. Finally, graph regularization is introduced into our proposed TDH method to preserve the original semantic similarity between hash codes in Hamming space. Experimental results show that our proposed method outperforms several state-of-the-art approaches on two popular cross-modal data sets.

Cross-modal retrieval based on hashing has attracted much attention for its low storage cost and fast query speed. Most cross-modal hashing methods arbitrarily embed the heterogeneous features into a common Hamming space, which is unsuitable for many real-world applications and makes the search results unsatisfactory. In this paper, we propose a novel cross-modal hashing method, which embeds heterogeneous features into their respective Hamming spaces. The proposed method has two advantages: 1) reconstructive embedding in each individual modality represented by matrix decomposition is performed to enhance the discriminative ability of the binary codes; 2) the correlations between different modalities are maximized by joint coupled-hashing representation. Experimental results on two public datasets demonstrate that our method gains better retrieval performance than several state-of-the-art approaches.

Electrical conductivity, current-voltage (I-V) characteristics, and low-temperature magnetoresistance of template-synthesized poly(3,4-ethylenedioxythiophene) (PEDOT) nanowires are reported. The room-temperature conductivity of the semiconducting nanowire with a diameter of 82 nm is about 80 S/cm. It is found that with lowering temperature, the I-V curves show a transition from linear to nonlinear around 50 K and a clear Coulomb-gap-like structure appears on the differential conductance (dl/dV) curves possibly due to enhanced electron-electron interaction. The resistance below 70 K is dependent on the applied bias voltage used in the resistance measurements due to the nonlinear I-V characteristics. The magnetoresistance below 10 K is positive and increases with lowering temperature due to the wavefunction shrinkage effect on hopping conduction. Copyright (C) 2009 John Wiley & Sons, Ltd.

The present article investigates a reversible structural phase transition, and related physical properties change in a new quasi-one-dimensional (1D) organic semiconductor crystal, DBA(TCNQ)2 [DBA = dibutylammonium, TCNQ = 7,7,8,8-tetracyanoquinodimethane]. Differential scanning calorimetry traces reveal the DBA(TCNQ)2 single crystal undergoes a first-order reversible phase transition at around 260−270 K, which is accompanied by a dramatic change in both conductivity and magnetic susceptibility. A direct correlation of physical properties with crystal structure is established. Detailed X-ray structure analyses indicate a reversible structural change related to dimer−tetramer transition along the TCNQ chain and disruption of the hydrogen-bonding chains as the temperature decreases from 270 to 253 K, which is the origin of the electronic and magnetic transition.

PANI nanotubes with diameters of 100-150 nm with conductivities of single nanotubes as high as 30.6 S . cm(-1) were directly oxidized with APS in the absence of hard templates and acidic dopants. The high conductivity was shown to result from protons produced during the polymerization. The pH decreases with increasing polymerization time, and the morphology changes from hollow spheres to short and long tubes. It is proposed that the micelles formed by the aniline monomer in aqueous solution serve as a soft template for forming hollow spheres at the initial stage. As the polymerization proceeds, these hollow spheres are linearly aggregated and elongated to form short and long tubes.

Core–shell structured and electro-magnetic functionalized polyaniline (PANI) composites were chemically prepared by micro-spherical hydroxyl iron (Fe(OH)) magnet as the hard-template associated with a self-assembly process in the presence of β-naphthalene sulfonic acid (β-NSA) as the dopant. Field emitting scanning electron microscope (SEM) and transmission electron microscope (TEM) measurements indicate that the electro-magnetic functionalized PANI–β-NSA/Fe(OH) composites have a novel core–shell structure, in which micro-spherical Fe(OH) magnet (0.5–5 μm in diameter) is as the core, and self-assembled PANI–β-NSA nanofibers (30–50 nm in diameter) formed on the surface of the Fe(OH) microspheres is as the shell (60 nm in thickness). Moreover, the electro-magnetic properties of the core–shell micro/nanostructured composites are adjustable by changing the ratio of Fe(OH) to aniline monomer.

Gold_poly(3,4-ethylenedioxythiophene) (Au-PEDOT) nanocables were synthesized at room temperature by one-step interfacial reaction of EDOT (in organic phase) and HAuCl4 (in aqueous phase) without the assistance of a surfactant, a template, or a stabilizer. The coaxial nanocables have lengths of several micrometers, outer diameters around 50 nm, and central cores around 30 nm. The room-temperature conductivity of the as-prepared nanocable pallet was measured to be about 770 S cm-1, and it changed from negative to positive temperature coefficient of resistivity (TCR) at around 50 K. The single Au-PEDOT nanocable exhibits as a semiconductor in the temperature range of 100−300 K with room-temperature conductivity as high as 2.3 × 103 S cm-1. Dissolution of the Au component from the nanocables using saturated I2 solution resulted in the formation of hollow PEDOT nanotubes with diameters around 50 nm, and gold nanowires were also produced by oxygen plasma decomposition of the PEDOT skins of the nanocables.

A "Chemical one step method" (COSM) is proposed to prepare electromagnetic functional composite nanofibers of polyaniline (PANI/gamma-Fe2O3) having a diameter of approximate to 20 nm. In this approach FeCl3 is acts as the oxidant either for polymerization of aniline or for preparation of gamma-Fe2O3 magnets. Besides, it also provides protons produced by the hydrolysis process for doping PANI. It is found that the composite nanofibers have a high conductivity (10(-1) similar to 10(0) S . cm(-1)) and super-paramagnetic properties (M-s = 0.46 similar to 6.03 emu . g(-1) and H-c = 0) at room temperature, where the conductivity is mainly affected by the molar ratio of FeCl3 to aniline monomer whereas the magnetic properties are dominated by the amount of FeCl2.

Conductivity and magnetoresistance (MR) measurements on pellets of conducting polyaniline and polypyrrole nanotubes/wires are presented. A transition from small negative MR to large positive MR was measured below 60 K. The positive and negative MR has been discussed in terms of a wavefunction shrinkage effect and a quantum interference effect on hopping conduction. In addition, further studies show that the MR of single polymer nanotubes/wires is very small even at 2 K (MR < 5% at 10 T) compared with that of the pellets (40%-100% at 10 T), and no evident and stable negative MR is observed above 50 K. The results indicate that the MR in the bulk pellet samples made of polymer nanotubes/wires is dominated by a random network of inter-fibril contacts.

Core-shell micro/nanostructured and electromagnetic functionalized polypyrrole (PPy) composites were prepared by a self-assembly process associated with the template method in the presence of p-toluenesulfonate acid (p-TSA) as the dopant, in which the spherical hydroxyl iron (Fe[OH], 0.5-5 microm in diameter) functioned not only as the hard template, but also as the "core" of the micro/nanostructure, and the self-assembled PPy-p-TSA nanofibers (20-30 nm in diameter) acted as the "shell" (50-100 nm in thickness) of the microspheres. We found that the core-shell micro/nanostructures exhibit controllable electromagnetic properties by adjusting the mass ratio of Fe[OH] to pyrrole monomer. The micelle model was proposed to interpret the self-assembly of the core-shell micro/nanostructured composites.

Polypyrrole nanowires have been electrosynthesized by direct oxidation of 0.1 mol l(-1) pyrrole in a medium of 75% isopropyl alcohol + 20% boron trifluoride diethyl etherate + 5% poly (ethylene glycol) (by volume) using porous alumina membranes as the templates. The as-prepared nanowires had a smooth surface and uniform diameter and were arranged in an orderly manner in a high density. The conductivity of a single nanowire was measured by the four-electrode technique to be 23.4 S cm(-1) at room temperature. The field emission devices based on the nanowire array were fabricated and their operations were explored. The experimental results indicated that the field emission characteristics of the devices fitted well to the Fowler-Nordheim model of emission. The turn-on electric field was only 1.2 V microm(-1) and the current density reached 200 microA cm(-2) at 2.6 V microm(-1).

We report on electrical and magnetic properties of polyaniline (PANI) nanotubes (∼150 nm in diameter) and PANI/Fe 3 O 4 nanowires (∼140 nm in diameter) containing Fe 3 O 4 nanoparticles with a typical size of 12 nm. These systems were prepared by a template-free method. The conductivity of the nanostructures is 10 −1 –10 −2 S/cm; and the temperature dependent resistivity follows a ln ρ∼T −1/2 law. The composites (6 and 20 wt% of Fe 3 O 4 ) show a large negative magnetoresistance compared with that of pure PANI nanotubes and a considerably lower saturated magnetization ( M s =3.45 emu/g at 300 K and 4.21 emu/g at 4 K) compared with the values measured from bulk magnetite ( M s =84 emu/g) and pure Fe 3 O 4 nanoparticles ( M s =65 emu/g). AC magnetic susceptibility was also measured. It is found that the peak position of the AC susceptibility of the nanocomposites shifts to a higher temperature (>245 K) compared with that of pure Fe 3 O 4 nanoparticles (190–200 K). These results suggest that interactions between the polymer matrix and nanoparticles take place in these nanocomposites.

The electrical resistivity, thermal conductivity, and thermopower of α -TiAl alloy samples with Al content from 0 at.% to 10 at.% are measured from room temperature down to liquid helium temperature. It is found that with the increase in Al content the single phonon scattering contribution to the resistivity remains nearly constant, but the multi-phonon scattering contribution monotonously increases. This provides an alternative explanation of resistivity saturation to the shunt-resistor model. The Wiedemann–Franz law holds for the whole temperature range, allowing the separation of the electron and phonon contributions to the thermal conductivity. The data from samples with different doping levels enable us to separate the band and the scattering terms in the thermopower.

A 1D trimetallic coordination polymer containing mixed molybdenum-vanadium heteropolyoxometalate bridges, {[Ni(phen)(2)(H2O)](2) [Ni(phen)(2)] [(V8Mo6Mo2O40)-Mo-IV-Mo-VI-O-V(PO4)[Ni(phen)(2)(H2O)](2)[(V8Mo6Mo2O40)-Mo-IV-Mo-VI-O-V(PO4)(2)]}.5H(2)O.2EtOH (1) (phen=1,10-phenanthroline, EtOH = ethanol), has been synthesised hydrothermally. An X-ray crystallographic study revealed that I is composed of ID zigzag chains of {[Ni(phen)(2)(H2O)](2)[Ni(phen)(2)][(V8Mo6Mo2O40)-Mo-IV-Mo-VI-O-V(PO4)]}(n)(n+) cations, {[Ni(phen)(2)(H2O)](2) [(V8Mo6Mo2O40)-Mo-IV-Mo-VI-O-V(PO4)(2)]}(-) anions and solvent molecules. Both the cation and the anion in 1 are mixed molybdenum-vanadium heteropolyoxometalates with covalently linked nickel complex fragments. Magnetic investigations of I indicate spin glass behaviour and several magnetic transitions occur at low temperatures (below 20 K). ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004).

A multi-walled carbon nanotube/polypyrrole (CNT/PPy) nanocomposite with cable-like morphology has been synthesized using an in situ chemical oxidative polymerization method. The temperature dependences of the conductivity and magnetoresistance (MR) have been studied. We found that the room temperature conductivity increases by two orders of magnitude with increasing carbon nanotube weight percentage. The temperature dependence of the conductivity follows a charge–energy-limited tunnelling model (lnσ(T) T−1/2) and indicates clearly that the CNT weight percentages of 9.1 and 13.04 wt% are below the percolation threshold and 23.1 wt% is beyond the percolation threshold. At low temperatures the 23.1 wt% composite shows a negative MR whereas the pure PPy shows a positive one. The results reveal that at the percolation threshold the complete CNT conductive paths can change and enhance dramatically the electrical properties of the CNT/PPy composites.

Polyaniline (PANT) nanotubes with an outer diameter of 165-240 nm and an inner diameter of 10-70 nm were prepared by a self-assembly process in the presence of six different carboxylic acids-propionic acid (PA), lactic acid (LA), succinic acid (SA), malic acid (MA), tartaric acid (TA), and citric acid (CA)-as the dopants. These nanotubes aggregated to form nanotube dendrites when the carboxylic acids contain an OH group. Moreover, the number of OH and COOH groups of the carboxylic acids affected the size, aggregated dendrite morphology, and thermal and electrical properties of the nanotubes. It was proposed that the micelle formed by the carboxylic acids acts as a template in the formation of the nanotubes, while the hydrogen bonds between the polymer chain of PANI and the OH group of the carboxylic acids supply a driving force to form the aggregated nanotube dendrites.

We report on the specific heat and magnetic susceptibility of polyaniline (PANI) nanotubes, which were self-assembled by a template-free method. It is interestingly found that an electronic specific heat is not observed from present data, and the lattice specific heat of PANI nanotubes can be interpreted in terms of the contributions from both the crystalline and the amorphous phases. The magnetic susceptibility includes a Pauli-like susceptibility and a Curie-type susceptibility. The results reveal an inhomogeneous disorder picture of PANI nanotubes (i.e., PANI nanotubes are composed of crystalline regions and amorphous regions). The crystallinity fraction Dg obtained from the specific heat is about 15% for the measured PANI nanotubes, which indicates that most of the polymer chains in the nanotubes are disordered.

New polyaniline (PANI) blends, PANI-camphor sulfonic acid (PANI-CSA) and PANI-dodecylbenzene sulfonic acid (PANI-DBSA) composite films with high room-temperature conductivity (∼200 S/cm) were prepared. The experimental data show a transition from negative temperature coefficient of resistivity (TCR) to positive TCR at around 210 K. An equation ρ(T)=a·ρ m exp( −T m /T)+b·ρ 0 exp( T 0 /T) 1/4 combining 3D variable-range hopping in disordered regions with quasi-one-dimensional metallic conduction in ordered regions has been used, which are consistent well with the resistivity data even from 1.8 up to 370 K. It is found that the PANI blends show positive magnetoresistance (MR) from 240 K down to 1.8 K and show negative MR from 240 K up to 350 K, which can be attributed to different dominant conduction mechanisms in different temperature ranges.

The temperature dependence of the resistivity of nanotubular polyaniline (PANI) doped with H3PO4 and β-naphthalene sulfonic acid (NSA) synthesized by a “template-free method” has been investigated. The molar ratios of H3PO4 (NSA) to aniline monomer (represented by H3PO4/An and NSA/An) of the samples are H3PO4/An=0.01, 0.05, 0.3, 0.5 and NSA/An=1, 2, respectively. It is found that ln ρ(T) is linear in T−1/2 and the resistivity is sensitive to the doping concentration. For NSA/An=1 sample, a crossover at 57 K in the plot of ln ρ(T)∼T−1/2 has been observed. On the basis of detailed analysis of the structure characteristics of the PANI nanotubes, we suggest a hopping–tunneling model to explain the experimental results.

Nanotubular polyaniline (PANI) doped with protonic acids have been synthesized by a "template-free" method. The temperature dependence of resistivity and the influences of doping concentration, dopant, morphology and crystallinity have been studied. A model has been proposed to explain the T-1/2 resistivity. Porous PANI-CSA (camphor sulfonic acid) and PANI-DBSA (dodecylbenzene sulfonic acid) composite films also have been investigated, we have suggested a new equation rho(T) = rho(1)exp(T-0/T)(1/4) + rho(2)(T/T-0)(2) to interpret the transition from negative temperature coefficient of resistivity (TCR) to positive TCR. (C) 2002 Elsevier Science B.V. All rights reserved.

Magnetic relaxation and I?V characteristics of YBCO films with a thickness ranging from 2.4 to 200 nm have been studied. Magnetic relaxation measurements show strong quantum tunnelling effects at temperatures as high as Tcr = 8.5?15 K in ultra-thin films (2.4?40 nm), whereas in 200 nm film it is dominated by thermal activation down to 4.2 K. The high Tcr of ultra-thin films is interpreted using Blatter's dissipative tunnelling theory. The strong quantum fluctuations in the ultra-thin films are explained by their large normal state resistivity and higher activation energy compared to thicker films.

Measurements of the magnetic relaxation rates S(T) and transport characteristics have been performed on Y1Ba2Cu3O7-delta (YBCO) films with thickness between 2.4 and 200nm. Strong quantum fluctuations of the vortex movements which occur at temperatures as high as 8.5 similar to 15 K are detected in all ultra-thin films (2.4 similar to 40nm), while the thick sample (200nm) does not show any quantum fluctuations at the experimental temperatures down to 4.2K. The experimental results are discussed with the theory based on dissipative tunneling of collectively pinned vortex. It is suggested that the strong quantum fluctuations in the ultra-thin films are due to the higher zero temperature normal state resistivity and/or the higher active energy of them compared to those of the thick sample.

The magnetoresistance oscillation of the charge density wave (CDW) material NbSe3 whisker has been studied under strain at different temperatures (4.2 K, 30 K, 40 K). With a constant measuring current, the magnetoresistance (MR) curves oscillate at 30 K while they are smooth at 4.2 K and become a series of zigzag curves at 40 K. These oscillation periods are Delta H, rather than Delta(1/H). It is found that either increasing the temperature to 40 K, or straining the sample to a displacement as large as epsilon = 3.1%, makes the regular oscillations disappear. By analysing the data, we found our results can be interpreted as the quantum coherence effect of a moving CDW.

High resolution specific heat measurements have been performed on K 0.3 MoO 3 and K 0.15 Tl 0.15 MoO 3 in the temperature region between 90 and 220K. Specific heat anomalies are observed at 178K and 164.8K for K 0.3 MoO 3 and K 0.15 Tl 0.15 MoO 3 respectively. The specific heat jump, entropy and enthalpy changes associated with the CDW phase transition are estimated. The results suggest that perfect order of K polyhedral chains is important for the phase transition of K blue bronze.

Magnetization of single crystals ofD-,L-alanine andD-valine were measured as a function of temperature using the SQUID magnetometer. An obvious lambda transition at 270 ± 1 K was shown in the specific heat measurement of alanine and valine enantiomers by differential scanning calorimetry (DSC). Magnetic transition temperature seems coincident with that of lambda transition. Temperature dependence of the X-ray powder diffraction forD-valine showed no crystal lattice change under the temperature cooling down from 293 K to 123 K. We propose that the difference of theX ∼T curve between theD-alanine andL-alanine is attributable to the variation of intramolecular geometry of chirality density in the presence of an external magnetic field. The chirality characteristics of alanine and valine enantiomers by the specific heat and susceptibility behaviour are discussed.

A multiwalled carbon nanotube∕polyaniline composite with cablelike morphology has been synthesized by an in situ chemical oxidative polymerization directed with cationic surfactant cetyltrimethylammonium bromide. It is interestingly found that with increasing carbon nanotube loading from 0 to 24.8wt%, the conductivity increases by two orders of magnitude and the Mott’s characteristic temperature T0 which depends on the hopping barrier decreases by three orders of magnitude. Furthermore, the low-temperature magnetoresistance has also changed the sign from positive to negative. The results reveal a strong coupling between the carbon nanotube and the tightly coated polymer chains, which enhances the average localization length and the electronic properties of the composites.

In this letter, we report the electrical properties of hollow polyaniline (PANI) microspheres. β-naphthalene sulfonic acid (NSA) and salicylic acid (SA) doped PANI microspheres were synthesized by a self-assembly method. The room-temperature conductivity is 8.6×10−2 S/cm for PANI–NSA microspheres (0.8–2 μm in outer diameter) and 5.6×10−4 S/cm for PANI–SA microspheres (3–7 μm in outer diameter). The conductivity of an individual PANI–SA microsphere is measured directly by a two-probe technique, about 8×10−2 S/cm (which is two orders of magnitude higher than that of a PANI–SA microsphere’s pellet). The measurements of conductivity, I–V curve, and magnetoresistance demonstrate that the electrical properties of PANI microspheres are dominated by the intersphere contacts due to the sample’s microscopic inhomogeneity.

We report on the electrical properties of a conducting polyaniline (PANI) derivative film containing azobenzene side chain. The freestanding thin film of HCl-doped PANI derivative was synthesized through an N-alkyl-substituted reaction. We found that the side of the film irradiated by a weak ultraviolet light during the preparation is a poor conductor at room temperature and shows abnormal V–I characteristics. Its resistance decreases sharply with increasing temperature. However, the other side of the film without irradiation is highly conductive. The room-temperature conductivity is about 1.2 S/cm, which is much higher than that of other N-alkyl-substituted polyanilines (10−2–10−7 S/cm). The temperature dependence of resistance follows one-dimensional variable-range hopping model.

Two kinds of methods such as spontaneous nucleation from flux-free stoichiometric melt and CuO flux method have been used for the growth of Pb0.5Sr2.5Y1−xCaxCu2Oy single crystals. The morphologies, phases, and compositions occurring in the crystals grown from the two kinds of methods were compared systematically. Optimum conditions for the growth of larger 1212 phase single crystals are x =0.15, 0.25, and 0.35. The lattice parameters of the crystals with the 1212 phase were found to be increasing upon Ca doping. The resistivity behavior of the PbSrYCaCuO single crystals with the 1212 phase both in the state of as-grown and after extended annealings under various oxygen partial pressure was also discussed briefly.

Bi2Sr2Ca1-xCexCu2Oy single crystals with in-plane dimensions up to 8X8 mm(2) were successfully grown by spontaneous nucleation from eutectic melts. For x less than or equal to 0.8, all the grown crystals possess the pure 2212 phase. However, when x = 1.0

The low temperature specific heat ofD-Valine andL-Valine has been measured by differential scanning calorimetry in the temperature region between 77–300K. It was found that an obvious lambda transition at 272±1K. X-ray diffraction crystallographic data showed that no crystal lattice changed C,H,N element analysis proved the high purity of the sample ofD andL-Valine. We propose that the shape of the jump forD-Valine is contributed by the specific heat of electron coupling.

We report the electrical properties of a single conducting polyaniline nanotube measured by a standard four-terminal technique. Camphor sulfonic acid doped polyaniline nanotubes were self-assembled by a template-free method. The directly measured conductivity of the single polyaniline nanotube is very high (∼31.4 S/cm), and its temperature dependence follows the three-dimensional variable range hopping model. However, the bulk conductivity of the polyaniline nanotube pellets is much smaller than the nanotube itself (only 3.5×10−2 S/cm) and ln ρ(T) is linear in T−1/2, which is due to the large intertubular contact resistance. These results will help us to understand the conduction mechanism in conducting polymers.

We report measurements of the electrical resistivity, thermoelectric power, and Hall coefficient on La- and Pb-doped Bi2Sr2CuOy compounds as a function of temperature. Both insulating and overdoped nonsuperconducting-metal samples have been obtained. Anal

During the past 15 years, significant progress has been made in synthesis, structural and electrical characterizations, and applications of conducting polymer nanotubes/wires. In this chapter, a brief review of the recent advances in electronic transport properties of individual conducting polymer nanotubes/wires prepared by both the template-free selfassembly method and the template method is presented. Results with broad interest have been discussed. For example, it was found that the electrical conductivity of the individual polymer tubes/wires increases by several times of magnitudes with decreasing outer diameter (size effect in electrical conductivity). The crossover from Mott to Efros-Shklovskii variable-range hopping conduction was observed at a relatively high transition temperature in single nanotubes/wires (enhanced Coulomb interaction effect). The low-temperature magnetoresistance of a single polymer tube/wire is positive and quite smaller than those of the nanotube/wire pellets (small magnetoresistance effect). The intrinsic resistance of an individual nanotube is much smaller than the contact resistance of two crossed nanotubes (nanocontact resistance effect). In addition, individual polymer tubes/wires show obvious transition from linear to nonlinear I-V curves at low temperature, and a clear zero-bias anomaly with Coulomb gap-like structure appeared on the differential conductance curves at low temperatures. These results indicate that the electrical properties of isolated conducting polymer tubes/wires are different from those of bulk polymer pellets or films in some cases due to their nanoscale diameters. However, in order to eliminate nanocontact resistance and reveal the intrinsic electronic transport properties of an individual nanotube/ wire, it is still quite important to develop new or improved conductivity measurement approaches on a single nanofibre. Furthermore, due to the complicated microstructures of conducting polymers, there are still problems and challenges to fulfill their applications in nanoscale devices, such as whether fully metallic conducting polymer nanotubes/wires, which show metallic behavior from room-temperature down to low temperatures, can be prepared through improving their molecular or supramolecular ordering. In addition, reproducibility and/or controllability of individual polymer nanotubes/wires are also a problem, since their electrical properties are sensitive to many factors such as doping level, extent of disorder, diameter, temperature, aging effect, etc.

The critical current density and n -values on four types of Bi 2 Sr 2 Ca 2 CuO x tapes have been studied from 40 K to the critical temperature. The results showed that the critical current density appears in three periods with decreasing temperature. Compared to Ag alloy-sheathed tapes, the critical current density of Ag-sheathed tape is more affected by temperature. It also appeared that n -values have a maximized value near the temperature of liquid nitrogen. When the temperature is lower than 60 K, although the critical current density still shows apparent improvement, n -values are very low.

Electron paramagnetic behaviors of BiSrCaCu 2 O y high-T c superconductor are studied at different temperatures by observation of electron paramagnetic resonance (EPR). No EPR was detected below superconducting transition temperature T c besides a signal near zero field. However, two types of EPR signals exist in this sample above T c . One of them (at the higher field) is almost independent of temperature and its behavior is similar to that of the free electron. Another one (at the lower field) may be related to copper and oxygen ions and its average Lande factor ḡ-value, EPR linewidth and integrated intensity seem to change anomalously at around 240 K. We propose that the anomaly observed may be due to that the sample undergoes a displacive phase transition at around 240 K.

A nearly pure 110 K phase of the ceramic superconductor Bi1.92Pb0.38,0.3SbxSr2Ca2Cu3.2O10- delta was obtained by the technique of co-decomposition of co-precipitated oxalate. Those samples with Sb=0.1, Pb=0.38 had a zero-resistance temperature of 102 K. For those samples with Sb=0.3, Pb=0.3, Tco was equal to 90 K. Their magnetic susceptibilities all dropped sharply at 110 K and did not change at 80 K. X-ray diffraction revealed that there was no 80 K or 20 K phase but a 110 K phase and a new phase. This new phase itself is not superconducting above 77 K, and it belongs to the monoclinic system. The average composition of the new phase is given as Bi32Pb3Sb7Sr29Ca23Cu7Ox.

A nearly pure 110 K phase of the ceramic superconductor Bi1.92Pb0.38,0.3SbxSi2Ca2Cu3.2O10-δ was obtained by codecomposition of coprecipatated oxalate technique. These samples with Sb=0.1, Pb=0.38 had zero resistance temperature at 102 K. For those samples with Sb=0.3, Pb=0.3, Tco was equal to 90 K. Their magnetic susceptibility all dropped sharply at 110 K and it does not change at 80 K. X-ray diffraction revealed that there was no 80 K or 20 K phase but the 110 K and an unknown phase. This unknown phase itself is not superconducting above 77 K. The average composition of the unknown phase is given as Bi32Pb3Sb7Sr29Cu7Ox.

A nearly pure 2223 phase of the ceramic superconductor Bi1.8Pb0.3Sb0.1Sr2Ca2Cu3O10−δ was obtained by solid-state reaction in air. The samples showed a semiconducting behavior when the temperature was higher than 140 K. The electrical resistance began to drop at 140 K and went to zero at 92 K. Their a.c. magnetic susceptibility showed two transitions at 130 and 89 K, but the 110 K transition was absent. These transitions were stable during cycling for three times from 77 K to room temperature. X-ray diffraction revealed that there was no 2212 or 2201 phase in these samples but only a small amount of monoclinic phase before and after cycling from 77 K to room temperature. SEM images showed that the grains of the monoclinic phase segregated on the surface of the sample and at the grain boundaries inside the sample, which is the reason for the sample's poor superconductivity and the decrease in zero resistance. Evidence from TEM and SEM equipped with a wavelength dispersive meter indicated distortion of the 2223 phase. These distorted 2223 crystals may be responsible for the 130 K superconductivity.

The resistance and ac susceptibility measurements show that cooling rate of the cast-annealing samples in heat treatment process has some effect on the 110 K superconducting phase in Bi–Pb–Sr–Ca–Cu–O system. Rapid quenching of the sample in air from 845°C causes oxygen deficiency in lattice and brings about a trifling change of unit cell size along c-axis direction. The dc magnetization and specific heat anomaly ∆c measurements demonstrate that fast cooling rate can reduce the transition temperature of high T c phase and the lower critical field, and weaken the pinning forces for vertex lines. The peak value of specific heat anomaly of the sample with nominal composition of Bi 1.7 Pb 0.3 Sr 2 Ca 2 Cu 4.5 O y is still small in comparison with YBa 2 Cu 3 O 7. From the magnetization curve we can estimate that the superconducting volume fraction is about 20%.

The role of the nanowire diameter on the electrical properties of isolated poly(3,4-ethylenedioxythiophene) nanowires has been studied systematically by a four-terminal technique. A transition from an insulating to a metallic regime is observed when the diameter decreases from 190to35nm and a transition from a metallic to an insulating regime takes place for the smaller diameters. These results are of importance for the different potential applications based on polymer nanowires or nanotubes.

There are two standpoints to explain the formation of the peak of magnetic susceptibility versus temperature in CeNiSn. One is the presence of the pseudo-gapped density of states (DOS) while the other is the development of an antiferromagnetic correlation. To check which one is more reasonable, an experiment is designed on measuring magnetic susceptibility under different magnetic fields. It is found that magnetic field can shift the peak of the magnetic susceptibility to lower temperatures while by calculation the presence of the pseudo-gapped DOS cannot shift the peak, which gives another evidence of the existence of antiferromagnetic correlations and is discussed in the light of some recent theoretical model.

International audience; We report the measurements of conductivity, I-V curve, and magnetoresistance of a single Au/polyaniline microfiber with a core-shell structure, on which a pair of platinum microleads was attached by focused ion beam. The Au/polyaniline microfiber shows a much higher conductivity (~110 S/cm at 300 K) and a much weaker temperature dependence of resistance [R(4 K)/R(300 K)=5.1] as compared with those of a single polyaniline microtube [sigmaRT=30–40 S/cm and R(4 K)/R(300 K)=16.2]. The power-law dependence of R(T)[proportional]T–beta, with beta=0.38, indicates that the measured Au/polyaniline microfiber is lying in the critical regime of the metal-insulator transition. In addition, the microfiber shows a H2 dependent positive magnetoresistance at 2, 4, and 6 K.

International audience; Magnetic susceptibility measurements on conducting polyaniline and polypyrrole nanostructures with different dopant type and doping level as functions of temperature and magnetic field are reported. The susceptibility data cannot be simply described as Curie-like susceptibility at lower temperatures and temperature-independent Pauli-like susceptibility at higher temperatures; some unusual transitions are observed in the temperature dependence of susceptibility, for example, paramagnetic susceptibility decreases gradually with lowering temperature, which suggests the coexistence of polarons and spinless bipolarons and possible formation of bipolarons with changing temperature or doping level. In particular, it is found that the direct current magnetic susceptibilities are strongly dependent on applied magnetic field, dopant type, and doping level