Compositionally controlled semimetal to superconducting transition in NaF doped LaOFeAs: Enhancement in Tc due to Na-doping

We report the synthesis of LaOFeAs based oxypnictide superconductors (La_1?xNa_xO_1?xF_xFeAs) using sodium fluoride as a fluorinating agent. NaF doping leads to a systematic decrease in both the a and c lattice parameters. Resistivity measurements show the onset resistivity transition temperature at 30.9 (±0.05) K and corresponding Meissner transition at 28 (±0.05) K in La_0.85Na_0.15O_0.85F_0.15FeAs which is highest in LaOFeAs type superconductors synthesized at ambient pressure. Further increase of NaF content in LaOFeAs leads to suppression of T_c. The above superconductors show a negative value of the Seebeck coefficient which indicates that electrons are the dominant charge carriers.     

Magnetic field dependence of vortex activation energy: A comparison between MgB2, NbSe2 and Bi2Sr2Ca2Cu3O10 superconductors

The dissipative mechanism at low current density is compared in three different classes of superconductors. This is achieved by measuring the resistance as a function of temperature and magnetic field in clean polycrystalline samples of NbSe₂, MgB₂ and Bi₂Sr₂Ca₂Cu₃O₁₀ (BSCCO) superconductors. Thermally activated flux flow behaviour is seen in all the three systems and clearly identified in bulk MgB₂. While the activation energy at low fields for MgB₂ is comparable to Bi₂Sr₂Ca₂Cu₃O₁₀, its field dependence follows a parabolic behaviour unlike a power-law dependence seen in Bi₂Sr₂Ca₂Cu₃O₁₀. We analyse our results based on Kramer’s scaling for grain boundary pinning in MgB₂ and NbSe₂.      

Impurity pair modes in a diatomic linear chain: nearest neighbour pair

The behaviour of the impurity modes due to a pair of substitutional impurities characterized by both mass as well as force-constant changes and occupying nearest neighbour positions in a diatomic linear chain, is studied. The results are compared with those for the case of impurity pairs occupying next nearest neighbour sites discussed earlier as well as the existing three dimensional calculations of Elliott and Pfeuty. The nearest neighbour impurity pair gap and local modes can be interpreted in terms of two single impurities substituted in the two different sublattices unlike the next nearest neighbour pair modes. The inband resonant modes are totally new features characteristic of the pair. Finally, the predictions of the theory are compared with the available experimental data for Si-impurity-pair-complexes and qualitative agreement is shown.     

Leucascandrolide A: a second generation formal synthesis

A convergent, second generation formal synthesis of (+)-Leucascandrolide A (1) has been efficiently achieved by providing a flexible, enantiocontrolled strategy toward the bioactive macrolactone component. Advancements for stereocontrol in asymmetric allylation methodology are discussed. Efforts feature novel results for reductions using the Terashima hydride reagent. [structure: see text]     

Integral sum rules and inequalities for ππ partial wave amplitudes

Exact integral relations on all ππ partial waves are derived from crossing symmetry. As a corollary we also derive an infinite number of inequalities fors-wave amplitudes.     

Influence of periodic disturbances on inception of cavitation in sharp-edged orifices

The influence of periodic flow disturbances on the onset of cavitation in sharp-edged orifices was determined through a series of experiments. Different amplitudes and frequencies of disturbance were introduced in the flow. The threshold value of cavitation parameter for which transition from attached to cavitating flow took place was shown to depend on the amplitude of the disturbance. Cavitating flow was initiated more spontaneously as the amplitude of disturbance increased. Hysteresis in the flow was, however, observed to decrease. Variations in the frequency of the disturbance did not bring about any significant change in the cavitation parameter at transition. The results are interpreted in terms of the available dwell time for cavitation and the characteristic residence time of the fluid in the sharp-edged orifice and in the pipeline upstream of the orifice.     

Aggregation of a C60-didodecyloxybenzene dyad: structure, dynamics, and mechanism of vesicle growth

The mechanism of formation and the stability of spontaneously formed vesicles upon self-assembly of a partially ground-state charge-separated, nonpolar-polar-nonpolar fullerene(C60)-didodecyloxybenzene (DDB) dyad in binary solvent mixtures requiring a critical dielectric constant of approximately 30 are reported. Molecular interactions giving rise to defined vesicles with in-plane bilayer packing are detailed from the predominant van der Waals and electrostatic interactions existing on the dyad's framework. The vesicles are formed with a large bending rigidity of 18kBT, which on further extraction into a polar water medium resulted in uniform spheres that corroborated well with the theoretical predictions. Furthermore, the water-extracted spherical dyad aggregates at an increased dyad concentration, leading to the formation of giant micrometer-sized fractals following diffusion-limited cluster aggregation. These dyad aggregates act as efficient quenchers of fluorescent dyes with a quenching rate of 4.6 x 10(13) M(-1) s(-1).     

Synthesis and Characterisation of Tetraphenylarsonium Halochromates

Tetraphenylarsonium halochromate (halo=fluoro, chloro and bromo) complexes are synthesised and characterised by spectral and thermal studies. The effect of ionic size and electronegativity of halide ions on the infrared spectra, X-ray emission spectra, activation energy of the first decomposition step and thermal stability of the complexes are investigated. The complexes possess tetragonal unit cell with a=b=12.93 Å and c=7.68 Å.These complexes decompose in two exothermic stages. Cleavage of one Ph-As bond to give triphenylarsine and reduction of Cr(VI) to Cr(III) occur simultaneously. First step mass loss corresponds to the loss of two phenyl halide molecules and 3/2 moles of oxygen. The overall kinetics of the first step is described by diffusion controlled reaction mechanism with a function g(α)=[1?(1?α)^1/2]. The second step decomposition is due to the further degradation of triphenylarsine. The final product is Cr₂O₃.