Dispersion theory and the low-energy constants for neutral-pion photoproduction

The relativistic amplitudes of pion photoproduction are evaluated by dispersion relations at t = const. The imaginary parts of the amplitudes are taken from the MAID model covering the absorption spectrum up to center-of-mass energies W = 2.2GeV. For sub-threshold kinematics the amplitudes are expanded in powers of the two independent variables ν and t related to energy and momentum transfer. Subtracting the loop corrections from this power series allows one to determine the counter terms of covariant baryon chiral perturbation theory. The proposed continuation of the amplitudes into the unphysical region provides a unique framework to derive the low-energy constants to any given order as well as an estimate of the higher-order terms by global properties of the absorption spectrum.     

A gauge-invariant chiral unitary framework for kaon photo- and electroproduction on the proton

We present a gauge-invariant approach to photoproduction of mesons on nucleons within a chiral unitary framework. The interaction kernel for meson-baryon scattering is derived from the chiral effective Lagrangian and iterated in a Bethe-Salpeter equation. Within the leading-order approximation to the interaction kernel, data on kaon photoproduction from SAPHIR, CLAS and CBELSA/TAPS are analyzed in the threshold region. The importance of gauge invariance and the precision of various approximations in the interaction kernel utilized in earlier works are discussed.     

Strong localization in conducting Langmuir-Blodgett films of quasi-one-dimensional charge-transfer complexes

The temperature dependence of intragrain conductivity of the Langmuir-Blodgett (LB) films of (C₁₆H₃₃-TCNQ)_0.4(C₁₇H₃₅-DMTTF)_0.6 has been studied by means of measuring the surface acoustic waves (SAW) attenuation in a piezocrystal resonator covered by the LB film. [(C₁₆H₃₃-TCNQ)_0.4(C₁₇H₃₅-DMTTF)_0.6 denotes the surface-active charge-transfer complex of hexadecyltetracyanoquinodimethane (C₁₆H₃₃-TCNQ) and heptadecyldimethyltetrathiofulvalene (C₁₇H₃₅-DMTTF)]. We have found that the intragrain conductivity decreases with decreasing temperature, following σ∝exp(−1/T^1/2) law. According to the theory of electron transport in quasi-one-dimensional disordered systems (Q1D DS) proposed by E.P. Nakhmedov et al. [Sov. Phys. Solid State 31 (1989) 368] and developed by Z.H. Wang et al. [Phys. Rev. B 43 (1991) 4373], this temperature behaviour of the conductivity points to the disorder-induced electron localization within the film grains. Fitting our experimental data to the relations proposed in these references, allowed us to evaluate the electron relaxation times due to scattering by phonons and impurities. Based on structural parameters of the conducting bilayer, we have also estimated the Fermi velocity and density of states at the Fermi level, which enabled us to determine the mean free path and localization length in the film studied.     

Electronic localization in an extreme 1-D conductor: the organic salt (TTDM-TTF) [Au(mnt) ]

This article reports the investigation of a new low-dimensional organic salt, (TTDM-TTF)₂ [Au(mnt)₂], by single crystal X-ray diffraction, static magnetic susceptibility, EPR, thermopower, electrical resistivity measurements under pressure up to 25 kbar and band structure calculations. The crystal structure consists in a dimerized head to tail stacking of TTDM-TTF molecules separated by layers of orthogonal Au(mnt)₂ anions. The absence of overlap between neighboring chains coming from this particular crystal structure leads to an extreme one-dimensionality (1-D) for which the carriers of the half-filled conduction band become strongly localized in a Mott-Hubbard insulating state. This material is the first 1-D conductor in which the Mott-Hubbard insulating character cannot be suppressed under pressure. Received 15 April 2002 / Received in final form 17 June 2002 Published online 17 September 2002     

An insight into the localization of charge carriers in conducting polyaniline by analyzing thermally stimulated depolarization signals

A dielectric relaxation mechanism, which is attributed to the localized motion of trapped polarons, has been recorded in conducting polyaniline by employing the thermally stimulated depolarization current technique. The signal was analyzed within the frame of the normal distribution in the activation energy value. The experimental dielectric relaxation results were manipulated in order to evaluate the length of the jump distance that the trapped polarons transfer along and the concentration of trap centers. The concentration of trapped carriers is calculated from two different viewpoints: the pair approximation that assumes phonon-assisted tunneling through the barrier separating two adjacent sites and the pinning model that considers the trapped polaron oscillating around its pinning point. Both models provide compatible results.     

Tuning the contact resistance in organic thin-film transistors with an organic-inorganic hybrid interlayer

We demonstrated the tunable contact resistance in pentacene thin film transistor (TFT) by inserting an organic-inorganic hybrid interlayer between Au electrode and pentacene layer. The contact resistance of pentacene-TFT varies with concentration of pentacene-TFT varies with concentration of MoO_x in organic-inorganic hybrid interlayer. MoO_x in organic-inorganic hybrid interlayer. The contact resistance of the device with 55 wt% MoO_x doped pentacene interlayer is about 7.8 times smaller than that of device without interlayer at the gate voltage of −20 V. Comparing the properties of pentacene-TFT without interlayer, the performance of the pentacene-TFT with 55 wt% MoO_x doped pentacene was significantly improved: saturation mobility increased from 0.39 to 0.87 cm²/V s, threshold voltage reduced from −21.3 to −7.2 V, and threshold swing varied from 3.75 to 1.39 V/dec. Our results indicated that the organic-inorganic hybrid interlayer is an effective way to improve the performance of p-channel OTFTs.     

Compositional dependence of serrated flow in nickel binary solid solutions during high-temperature microindentation

At 450?°C, pure nickel and nickel binary solid solutions, containing Ru, Rh, W, Re, Ir and Pt, have a smooth load–depth curves. In contrast, the alloying addition of Nb, Mo, Pd and Ta results in the occurrence of serrations in their load–depth profiles. Analyses show the load increment of the quasi-elastic segments scales linearly with the load at their starting point. The slope of such linear relationships was normalized by the solute concentration and then plotted against the diffusion rate of transition metal solutes within the nickel lattice and along dislocation cores. Linear relationships were observed in both cases. Results suggest that the solute diffusion along dislocation cores coupled with the atomic size misfit are mainly responsible for such serrations.     

Minimax efficient finite-difference stochastic gradient estimators using black-box function evaluations

Standard approaches to stochastic gradient estimation, with only noisy black-box function evaluations, use the finite-difference method or its variants. Though natural, it is open to our knowledge whether their statistical accuracy is the best possible. This paper argues so by showing that central finite-difference is a nearly minimax optimal zeroth-order gradient estimator for a suitable class of objective functions and mean squared risk, among both the class of linear estimators and the much larger class of all (nonlinear) estimators.     

Structural and electrical transport properties of Nb-doped TiO2 films deposited on LaAlO3 by rf sputtering

We have investigated the structural and electrical transport properties of Nb-doped TiO₂ films deposited on (1 0 0) LaAlO₃ substrates by rf magnetron sputtering at temperatures ranging from 873 K to 1073 K. Films deposited below 998 K are anatase, and mixed phases between anatase and rutile exist in the film grown at higher temperatures. We find that films deposited at low temperatures exhibit semiconductor behavior, while metallic conductivity is observed in the most conducting film deposited at 998 K. For this sample, compared to electron-phonon scattering mechanism, electron-phonon-impurity interference effect plays an important role in its electron transport process. Moreover, the temperature coefficient of the resistivity for the film deposited at 1073 K is negative from 2 K to 300 K. The temperature dependence of resistivity for the film is described by ∼exp(b/T)^1/2 at temperatures from 80 K down to 30 K, and by the fluctuation induced tunneling model from 80 K to 300 K.