Preparation and characterization of multiwall carbon nanotube/polypyrrole coaxial fibrils

Multiwall carbon nanotube (MWNT)/polypyrrole (PPy) fibrils were fabricated by template-free in situ electrochemical deposition of PPy over MWNTs, and characterized by electron microscopy and electrical measurements. Scanning and transmission electron microscopy studies reveal that PPy coating on the surface of nanotube is quite uniform throughout the length, with the possibility of forming unique Y-junctions. Current (I)-voltage (V) characteristics at various temperatures show nonlinearity due to tunneling and hopping contributions to transport across the barriers. AC conductivity measurements (300-4.2 K) show that the onset frequency scales with temperature, and the nanoscale connectivity in MWNT/PPy fibrils decreases with the lowering of temperature. (C) 2011 Elsevier B.V. All rights reserved.

The role of Compton and Raman scattering in the quasar continuum

There are three ways in which an electromagnetic wave can undergo scattering in a plasma: (i) when the scattering of radiation occurs by a single electron, it is called Compton Scattering (CS); (ii) if it occurs by a longitudinal electron plasma mode, it is called Stimulated Raman Scattering (SRS), and (iii) if it occurs by a highly damped electron plasma mode, it is called Stimulated Compton Scattering (SCS). The non-thermal continuum of quasars is believed to be produced through the combined action of synchrotron and inverse Compton processes, which are essentially single-particle processes. Here, we investigate the role of SRS and SCS in the generation of continuum radiation from these compact objects. It is shown as an example that the complete spectrum of 3C 273 can be reproduced by suitably combining SCS and SRS. The differential contributions of SCS and SRS under different values of the plasma parameters are also calculated.

Raman Spectroscopy of the Charge Transfer Complex (TTF)x C60 Br8

We report Raman studies on powder samples of the charge transfer complex (TTF)(x)C60Br8 at room temperature. The phonons show considerable softening with respect to the frequencies observed in the Raman spectrum of solid C60Br8. The strongest mode at 1464 cm(-1) in C60Br8 is red shifted to a doublet with peaks at 1414 and 1421 cm(-1), implying an average phonon softening Delta omega of -47 cm(-1). A comparison with the phonon softening of the corresponding A(g)(2) mode in alkali-doped C-60 (Delta omega similar to -36 cm(-1) for A(6)C(60), A = K, Rb or Cs) suggests that 8 electrons are transferred per C60Br8 molecule in the charge transfer complex. The mode at 503 cm(-1) in C60Br8 is shifted upwards, similar to that in A(6)C(60) compounds.

Structure–property relationship in superconducting cuprates

An examination of the structure and superconducting properties of the various families of cuprates suggests several interesting structural commonalities. Relations between some of the structural parameters of the cuprates and the superconducting transition temperature, T-c, provide useful insights. Variations of T-c on the hole concentration, the in-plane Cu-O and the apical Cu-O distances, as well as the Madelung potentials and bond valence sums are particularly noteworthy. The Cu-O charge-transfer energy appears to be fundamental in determining the properties of cuprates.

Methods of nonlinear random vibration analysis

The various techniques available for the analysis of nonlinear systems subjected to random excitations are briefly introduced and an overview of the progress which has been made in this area of research is presented. The discussion is mainly focused on the basis, scope and limitations of the solution techniques and not on specific applications.

Pyridinidm Hexafluorotitanate and its Derivatives

Pyridinium hexafluorotitanate, (C5H5NH)(2)TiF6, has been prepared by the reaction of titanium metal with pyridinium poly(hydrogen fluoride), PPHF, at room temperature. Making use of (C5H5NH)(2)TiF6 as a precursor, ammonium and alkali metal hexafluorotitanates, M(2)TiF(6) (M = NH4, Na, K, Rb and Cs) have been synthesized by metathesis. These hexafluorotitanates have been characterized by chemical analyses, infrared and NMR (H-1 and F-19) spectroscopy and powder X-ray diffraction methods. Indexed powder X-ray diffraction data for Rb2TiF6 and Cs2TiF6 have been reported.

Photoinduced transparency of effective three-photon absorption coefficient for femtosecond laser pulses in Ge16As29Se55 thin films

We report a dramatic change in effective three-photon absorption coefficient of amorphous Ge16As29Se55 thin films, when its optical band gap decreases by 10 meV with 532 nm light illumination. This large change provides valuable information on the higher excited states, which are otherwise inaccessible via normal optical absorption. The results also indicate that photodarkening in chalcogenide glasses can serve as an effective tool to tune the multiphoton absorption in a rather simple way. (C) 2011 American Institute of Physics.

Chopped nonlinear magneto-optic rotation: A technique for precision measurements

We have developed a technique for precise measurement of small magnetic fields using nonlinear magneto-optic rotation (NMOR). The technique relies on the resonant laser beam being chopped. During the on time, the atoms are optically pumped into an aligned ground state (Delta m=2 coherence). During the off time, they freely precess around the magnetic field at the Larmor frequency. If the on-off modulation frequency matches (twice) the Larmor precession frequency, the rotation is resonantly enhanced in every cycle, thereby making the process like a repeated Ramsey measurement of the Larmor frequency. We study chopped-NMOR in a paraffin-coated Cs vapor cell. The out-of-phase demodulated rotation shows a Lorentzian peak of linewidth 85 mu G, corresponding to a sensitivity of 0.15nG/root Hz. We discuss the potential of this technique for the measurement of an atomic electric-dipole moment. Copyright (C) EPLA, 2011

Limiting ionic conductance of symmetrical, rigid ions in aqueous solutions: Temperature dependence and solvent isotope effects

Limiting ionic conductance (Lambda(0)) of rigid symmetrical unipositive ions in aqueous solution shows a strong temperature dependence. For example, Lambda(0) more than doubles when the temperature is increased from 283 to 318 K. A marked variation also occurs when the solvent is changed from ordinary water (H2O) to heavy water (D2O). In addition, Lambda(0) shows a nonmonotonic size dependence with a skewed maximum near Cs+. Although these important results have been known for a long time, no satisfactory theoretical explanation exists for these results. In this article we present a simple molecular theory which provides a nearly quantitative explanation in terms of microscopic structure and dynamics of the solvent. A notable feature of this theory is that it does not invoke any nonquantifiable models involving solvent-berg or clatherates. We find the strong temperature dependence of Lambda(0) to arise from a rather large number of microscopic factors, each providing a small but nontrivial contribution, but all acting surprisingly in the same direction. This work, we believe, provides, for the first time, a satisfactory explanation of both the anomalous size and temperature dependencies of Lambda(0) of unipositive ions in molecular terms. The marked change in Lambda(0) as the solvent is changed from H2O to D2O is found to arise partly from a change in the dielectric relaxation and partly from a change in the effective interaction of the ion with the solvent.

Piezoelectrically stimulated dielectric dispersion in KAP single crystals

The frequency response of the dielectric constant (epsilon(r)), the loss tangent (tan delta) and impedance Z of potassium acid phthalate (KAP) single crystals, monitored along the polar axis, exhibit strong resonances in the frequency range 50-200 kHz, depending on the dimensions of the sample. The observed resonance effect, which is strongly dependent on the geometric shape and size of the sample, is attributed to its piezoelectric nature. The resonance peak positions have been monitored as a function of both temperature and uniaxial pressure. The stiffness coefficient (C), computed based on the resonance data, is found to decrease with increasing temperature and increase with increasing pressure. The electro-mechanical coupling coefficient (k), obtained by resonance-anti-resonance method, has also been found to increase with rise in temperature. The epsilon(r) behaviour along the polar axis, as a function of temperature is consistent with that of k. The preliminary results on the influence, of partial replacement of K+ ions in the KAP crystal by Cs+ and Li+ ions, on the observed piezoelectric resonance effects are also included.

A finite element method for compressible viscous fluid flows

This work presents a mixed three-dimensional finite element formulation for analyzing compressible viscous flows. The formulation is based on the primitive variables velocity, density, temperature and pressure. The goal of this work is to present a `stable' numerical formulation, and, thus, the interpolation functions for the field variables are chosen so as to satisfy the inf-sup conditions. An exact tangent stiffness matrix is derived for the formulation, which ensures a quadratic rate of convergence. The good performance of the proposed strategy is shown in a number of steady-state and transient problems where compressibility effects are important such as high Mach number flows, natural convection, Riemann problems, etc., and also on problems where the fluid can be treated as almost incompressible. Copyright (C) 2010 John Wiley & Sons, Ltd.

Comparison of electrical resistivity by geophysical method and neutron probe logging for soil moisture monitoring in a forested watershed

Geophysical methods are becoming more popular nowadays in the field of hydrology due to their time and space efficiency. So an attempt has been made here to relate electrical resistivity with soil moisture content in the field. The experiments were carried out in an experimental watershed `Mulehole' in southern India, which is a forested watershed with approximately 80% red soil. Five auger holes were drilled to perform the soil moisture and electrical resistivity measurements in a toposequence having red and black soils, with sandy weathered soil at the bottom. Soil moisture was measured using neutron probe and electrical resistivity was measured using electrical logging tool. The results indicate that electrical resistivity measurements can be used to measure soil moisture content for red soils only.

Transient absorption studies on 3,6-dibromo polyvinylcarbazole and its model compounds

3,6-Dibromo-N-ethylcarbazole (DBNEC) and its polymeric analogue poly-3,6-dibromovinylcarbazole (PDBVCz) were studied by transient absorption spectroscopy. The transient absorption spectrum of the 3,6-dibromo-N-ethylcarbazole radical cation and decay rate constants of radical cations of 3,6-dibromo-N-ethylcarbazole and its polymeric analogue are presented. In the case of unsubstituted carbazole, the ratio of the yield of radical cation of monomer to polymer is 2.0, whereas in the case of PDBVCz, under the same experimental conditions, the yield of the radical cation is an order of magnitude less in comparison with the monomer model compound DBNEC. This drastic difference in yield has been correlated to the difference in the conformational structure of the polymer as evidenced by nuclear magnetic resonance spectroscopy. (C) 1997 Elsevier Science S.A.

The Equations of Equilibrium in Orthogonal Curvilinear Reference Coordinates

Analytical solutions to problems in finite elasticity are most often derived using the semi-inverse approach along with the spatial form of the equations of motion involving the Cauchy stress tensor. This procedure is somewhat indirect since the spatial equations involve derivatives with respect to spatial coordinates while the unknown functions are in terms of material coordinates, thus necessitating the use of the chain rule. In this classroom note, we derive compact expressions for the components of the divergence, with respect to orthogonal material coordinates, of the first Piola-Kirchhoff stress tensor. The spatial coordinate system is also assumed to be an orthogonal curvilinear one, although, not necessarily of the same type as the material coordinate system. We show by means of some example applications how analytical solutions can be derived more directly using the derived results.