Study of the peak effect phenomenon in single crystals of 2H-NbSe2

The weakly pinned single crystals of the hexagonal 2H-NbSe₂ compound have emerged as prototypes for determining and characterizing the phase boundaries of the possible order-disorder transformations in the vortex matter. We present here a status report based on the ac and dc magnetization measurements of the peak effect phenomenon in three crystals of 2H-NbSe₂, in which the critical current densities vary over two orders of magnitude. We sketch the generic vortex phase diagram of a weakly pinned superconductor, which also utilizes theoretical proposals. We also establish the connection between the metastability effects and pinning.     

The influence of a transverse magnetic field on a subnormal glow discharge in air

In subnormal glow discharge under d.c. excitation at different pressure in a varying transverse magnetic field (0 to 30 G) some measurements have been carried out for various initial average tube currents. The voltage across the discharge increases and average tube current and residual current decreases in the magnetic field. With the help of Beckman’s expression [4] for the axial field and the electron density distribution in a transverse magnetic field the observed variation of current and voltage can be satisfactorily explained. The variation of axial electric field with transverse magnetic field can be represented to a fair degree of accuracy by the derived equation. The behaviour of residual current with magnetic field has been observed in these oscillations.     

Integral equation formulation for buoyancy-driven convection problems

An integral equation formulation for buoyancy-driven convection problems is developed and illustrated. Buoyancy-driven convection in a bounded cylindrical geometry with a free surface is studied for a range of aspect ratios and Nusselt numbers. The critical Rayleigh number, the nature of the cellular motion, and the heat transfer enhancement are computed using linear theory. Green's functions are used to convert the linear problem into linear Fredholm integral equations. Theorems are proved which establish the properties of the eigenvalues and eigenfunctions of the linear integral operator which appears in these equations.     

CHARMM additive all-atom force field for carbohydrate derivatives and its utility in polysaccharide and carbohydrate-protein modeling

Monosaccharide derivatives such as xylose, fucose, N-acetylglucosamine (GlcNAc), N-acetylgalactosamine (GlaNAc), glucuronic acid, iduronic acid, and N-acetylneuraminic acid (Neu5Ac) are important components of eukaryotic glycans. The present work details development of force-field parameters for these monosaccharides and their covalent connections to proteins via O-linkages to serine or threonine sidechains and via N-linkages to asparagine sidechains. The force field development protocol was designed to explicitly yield parameters that are compatible with the existing CHARMM additive force field for proteins, nucleic acids, lipids, carbohydrates, and small molecules. Therefore, when combined with previously developed parameters for pyranose and furanose monosaccharides, for glycosidic linkages between monosaccharides, and for proteins, the present set of parameters enables the molecular simulation of a wide variety of biologically-important molecules such as complex carbohydrates and glycoproteins. Parametrization included fitting to quantum mechanical (QM) geometries and conformational energies of model compounds, as well as to QM pair interaction energies and distances of model compounds with water. Parameters were validated in the context of crystals of relevant monosaccharides, as well NMR and/or x-ray crystallographic data on larger systems including oligomeric hyaluronan, sialyl Lewis X, O- and N-linked glycopeptides, and a lectin:sucrose complex. As the validated parameters are an extension of the CHARMM all-atom additive biomolecular force field, they further broaden the types of heterogeneous systems accessible with a consistently-developed force-field model.     

Structures of nucleobases trapped within Au triangles and its effects on hydrogen bonding in base pairs of DNA

Nucleobases (adenine (A), thymine (T), cytosine (C), and guanine (G)) trapped within two metal clusters such as Au(3) undergo expansion. Our investigation reveals that this primarily arises due to the concomitant increase in all the bond lengths in molecules. Such expansion of the molecules can be qualitatively understood on the basis of classical harmonic potentials in the bonds and loss of aromaticity in the rings. Specifically, the highly electronegative O and N elements in the base pairs anchor to Au atoms and form X-Au bonds, which leads to charge redistribution within the molecules. As a very important consequence of this, the nature of the hydrogen bonds (in Au(3)-A...T-Au(3) and in Au(3)-G...C-Au(3)) change substantially within these electrodes in comparison to gas-phase structures. These hydrogen bonds have a single-well potential energy profile (of the type N...H...O and N...H...N) instead of double-well potentials (like N-H...O or N-H...N/ N...H-N types). A detailed energy calculation along the proton movement pathway supports our conclusions.     

Charge transport in polythiophene:fullerene:nanotube bulk heterojunction photovoltaic devices investigated by impedance spectroscopy

P3HT:PCBM bulk heterojunction devices incorporating SWNTs, which are predominantly metallic in character, have been analyzed using impedance spectroscopy to understand the effect of SWNTs on their charge carrier transport properties. SWNTs reduce the effective lifetime of injected charge carriers. Frequency dependence of capacitance and conductance of P3HT:PCBM devices show monotonic variations without any clear peak positions. Simulations of the complex admittance of the P3HT:PCBM devices under trap free space charge limited current within the framework of Scher–Montrol theory are used to qualitatively show that such characteristics are a signature of charge transport which is highly dispersive in nature. The position of peak τ_peak in the imaginary part of impedance Im(Z), which is essentially same as the first transition frequency of Cole–Cole plot, has a direct relation with the effective dc mobility of charge carriers which varies with dispersion parameters. Using the dc mobility values and the voltage variation of peak frequency of Im(Z), the ratio of τ_dc to τ_peak has been calculated. The magnitude of this ratio is indicative of the degree of dispersiveness in transport. It has been shown that, SWNTs at low concentrations tend to reduce the dispersiveness in charge transport.     

Aromatic superclusters from all-metal aromatic and antiaromatic monomers, [Al4]2- and [Al4]4-

Calculations on the structures of dimers of all-metal aromatic and anti-aromatic molecules such as (Al4(2-)) and (Al4(4-)) reveal that, unlike their organic counterparts such as benzene and cyclobutadiene which form pi-stacked complexes, these molecules form new clusters with no reminiscence of the original units. These clusters have a very large binding energy and can be further stabilized through charge-balance by counterions and solvents.     

Determination of isoniazid and hydrazine by capillary electrophoresis with amperometric detection at a Pt-particle modified carbon fiber microelectrode

Capillary electrpphoresis (CE)/electrochemical detection (EC) for the simultane-ous determination of hydrazine and isoniazid has been developed.The electrochemical method uses a novel modified electrode dispersed with ultrafine platinum particles on the surface of a 30μm carbon fiber microelectrode.The unique characteristic of the Pt-particles modified carbon fiber microelectrode is its excellent stability.The current measurement for hydrazine is more sensitive than that of isoniazid.Selective determination of trace amount of free hydrazine in isoniazid and its formulation can be achieved at applied potential of 0.5V.     

Investigating the vortex melting phenomenon in BSCCO crystals using magneto-optical imaging technique

Using a novel differential magneto-optical imaging technique we investigate the phenomenon of vortex lattice melting in crystals of Bi₂Sr₂CaCu₂O₈ (BSCCO). The images of melting reveal complex patterns in the formation and evolution of the vortex solid-liquid interface with varying field (H)/temperature (T). We believe that the complex melting patterns are due to a random distribution of material disorder/inhomogeneities across the sample, which create fluctuations in the local melting temperature or field value. To study the fluctuations in the local melting temperature/field, we have constructed maps of the melting landscape T _m(H, r), viz., the melting temperature (T _m) at a given location (r) in the sample at a given field (H). A study of these melting landscapes reveals an unexpected feature: the melting landscape is not fixed, but changes rather dramatically with varying field and temperature along the melting line. It is concluded that the changes in both the scale and shape of the landscape result from the competing contributions of different types of quenched disorder which have opposite effects on the local melting transition.     

Sequence dependent electron transport in wet DNA: ab initio and molecular dynamics studies

We combine molecular dynamics simulations and density functional theory to analyze the electrical structure and transmission probability in four different DNA sequences under physiological conditions. The conductance in these sequences is primarily controlled by interstrand and intrastrand coupling between low-energy guanine orbitals. Insertion of adenine-thymine base pairs between the guanine-cytosine rich domains acts as a tunneling barrier. Our theory explains recent length dependent conductance data for individual DNA molecules in water.