Design and development of a superconducting magnet system for study of magnetoresistance in the temperature range 1.5–300 K

The design and fabrication of an all-metal cryostat, superconducting magnet and variable temperature sample holder insert for the measurement of magnetoresistance at temperatures in the range 1.5–300 K are described. A field of 50 kilogauss has been achieved at the centre of a one-inch bore superconducting solenoid and has a uniformity of 0.2% over an axial distance of 20 mm around the centre of the solenoid. Also, the design and construction of vapour cooled current leads for the magnet are discussed.     

Group actions on arrangements of linear subspaces and applications to configuration spaces

For an arrangement of linear subspaces in that is invariant under a finite subgroup of the general linear group we develop a formula for the -module structure of the cohomology of the complement . Our formula specializes to the well known Goresky-MacPherson theorem in case , but for the formula shows that the -module structure of the complement is not a combinatorial invariant. As an application we are able to describe the free part of the cohomology of the quotient space . Our motivating examples are arrangements in that are invariant under the action of by permuting coordinates. A particular case is the ``-equal' arrangement, first studied by Björner, Lovász, and Yao motivated by questions in complexity theory. In these cases and are spaces of ordered and unordered point configurations in many of whose properties are reduced by our formulas to combinatorial questions in partition lattices. More generally, we treat point configurations in and provide explicit results for the ``-equal' and the ``-divisible' cases.

     

Ortho effects in organic molecules on electron impact. Part 15. Oxygen transfers from the nitro group to both sulphur and olefinic double bonds in allyl o-nitrophenyl sulphide

An oxygen transfer from the nitro group to the C?C group, followed by a simple cleavage, afford intense fragments corresponding to o-nitrosothiophenol at m/z 139 and o-nitrosothiophenoxy cation at m/z 138 during mass spectral fragmentations of allyl o-nitrophenyl sulphide. Further, a concerted double oxygen transfer from the nitro group to the sulphur is proposed for the ejection of ?O₂H from the molecular ion of this compound, leading to the quinolinium cation at m/z 130. These processes are supported by the high-resolution data, collision-induced dissociation linked-scan spectra and chemical evidence.     

Rapid grid-based construction of the molecular surface and the use of induced surface charge to calculate reaction field energies: applications to the molecular systems and geometric objects

This article describes a number of algorithms that are designed to improve both the efficiency and accuracy of finite difference solutions to the Poisson-Boltzmann equation (the FDPB method) and to extend its range of application. The algorithms are incorporated in the DelPhi program. The first algorithm involves an efficient and accurate semianalytical method to map the molecular surface of a molecule onto a three-dimensional lattice. This method constitutes a significant improvement over existing methods in terms of its combination of speed and accuracy. The DelPhi program has also been expanded to allow the definition of geometrical objects such as spheres, cylinders, cones, and parallelepipeds, which can be used to describe a system that may also include a standard atomic level depiction of molecules. Each object can have a different dielectric constant and a different surface or volume charge distribution. The improved definition of the surface leads to increased precision in the numerical solutions of the PB equation that are obtained. A further improvement in the precision of solvation energy calculations is obtained from a procedure that calculates induced surface charges from the FDPB solutions and then uses these charges in the calculation of reaction field energies. The program allows for finite difference grids of large dimension; currently a maximum of 571(3) can be used on molecules containing several thousand atoms and charges. As described elsewhere, DelPhi can also treat mixed salt systems containing mono- and divalent ions and provide electrostatic free energies as defined by the nonlinear PB equation.     

n-Heptane under pressure: structure and dynamics from molecular simulations

Atomistic molecular dynamics simulations have been performed in the isothermal-isobaric ensemble to explore the phase behavior of n-heptane. Motivated by recent high-pressure spectroscopic experiments on n-heptane, the present work aims at understanding the liquid-solid and the alluded to solid-solid transitions upon increasing pressure. Starting from the stabilized solid phase at 300 K and 10 kbar, we have investigated the range of these two transitions by a gradual decrease and increase of pressure, respectively. Although the solid-liquid transition has clear signatures such as the formation of gauche defects along the molecular backbone, the present model does not show any sign of a first-order solid-solid transition at high pressures. However, interesting changes in the environment around methyl groups and in their dynamics are observed. These have been substantiated by calculations of the vibrational density of states obtained from a normal-mode analysis and from the simulation trajectory.     

Investigation of the effect of finite-sized ions on the nanowire field-effect transistor in electrolyte concentration using a modified Poisson–Boltzmann model

One-dimensional (1D) nanowire field-effect transistors (FETs) have recently played a major role in sensing applications. Due to charging of the surface functional chemical groups with protonation and deprotonation, the transport properties of these nanowire transistors affect the aqueous environment, altering the electrical double layer (EDL) potential drops and charge distributions in the electrolyte concentration. In this work, we have implemented the simple modified Poisson–Boltzmann (MPB) theory in a 1D silicon nanowire FET, and the effect of the various finite sizes of ions in z:z symmetric electrolyte concentration was investigated. For a given ionic concentration and surface charge, the EDL potential drop, accumulation of charges and the charge distributions of NaCl and CsCl ions were studied. From the MPB model results with the nanowire FET, it was observed that the potential drop of the EDL depends on the size of the ions in the electrolyte. The study of various electrostatic investigations of finite-sized ions was successfully done by implementing the MPB model on a silicon nanowire FET. It can be used in both chemical and biological sensors.