Magnetic excitations in the Ising-Heisenberg chain at low temperatures

The spectral weight of transverse spin fluctuations in the Ising-Heisenberg chain is calculated in the limit of low temperatures. The spin Hamiltonian contains three forms of anisotropy; an external magnetic field and exchange and single-site anisotropy. Single spin-wave and bound-state contributions to the spectral weight are studied as functions of the various types of anisotropy, with particular attention to features of the spectrum that can be measured by inelastic neutron scattering.     

The transmission loss and vibration of glass panels in non-reverberative conditions

In the usual method of measurement, the structure is placed between two semi-reverberant rooms and diffuse sound fields are used. This paper describes measurements made in conditions of high absorption, with a single directional source. Thus the results should be of value where information on the insulation of glass panels is required for use in high absorption environments, such as recording studios or anechoic laboratories. Microphone measurements are compared with the predicted transmission loss from a capacitive transducer and the effect of sealing the panels is described. Both the microphone measurements and the corrected transducer results are in good agreement with the mass law. The improvement obtained by sealing is an average of about 6dB but large increases in insulation are found at low frequencies. The velocity amplitude is lower than the theoretical value.     

Quantum Monte Carlo calculations of nanostructure optical gaps: application to silicon quantum dots

Quantum Monte Carlo (QMC) calculations of the optical gaps of silicon quantum dots ranging in size from 0 to 1.5 nm are presented. These QMC results are used to examine the accuracy of density functional (DFT) and empirical pseudopotential based calculations. The GW approximation combined with a solution of the Bethe-Salpeter equation performs well but is limited by its scaling with system size. Optical gaps predicted by DFT vary by 1-2 eV depending on choice of functional. Corrections introduced by the time dependent formalism are found to be minimal in these systems.     

Electrochemically programmed, spatially selective biofunctionalization of silicon wires

A method for the spatially selective biofunctionalization of silicon micro- and nanostructures is reported, and results are presented for both single-crystal silicon (111) or (100) surfaces. An electroactive monolayer of hydroquinone was formed on the surface of H-terminated silicon working electrodes via an olefin reaction with UV-generated surface radicals. Molecules presenting either cyclopentadiene or a thiol group can be immobilized onto the regions where the hydroquinone has been oxidized. Molecular size and crystal orientation are evaluated as important factors that dictate the electrode stability in aqueous solution under anodic potentials. Monolayers composed of smaller molecules on (111) surfaces exhibit the highest packing density and are more effective in preventing anodic oxidation of the underlying substrate. Voltammetry, X-ray photoelectron spectroscopy, and atomic force and fluorescence microscopy are utilized to interrogate the kinetic rates of biofunctionalization, the extent of surface coverage, monolayer quality, and the spatial selectivity of the process.     

The prediction of noise levels L10 due to road traffic

The authors have developed an improved procedure for predicting noise levels L₁₀ from road traffic. The new method has been adopted for use within England and Wales in connection with the Noise Insulation Regulations 1975 and for other aspects of planning. The formulation of the new procedure is discussed, and its overall performance assessed by reference to a comprehensive data bank specifically assembled for that purpose.     

Annoyance and noise from road traffic

The cause of insufficient privacy in open plan offices is identified as the reduction of background noise level resulting from the installation of highly absorbent ceilings intended to enhance the performance of portable acoustic screens. The V-form ceiling is suggested as a ceiling design which may increase both screen performance and background level. The extent to which these variables are affected by the use of such a design is investigated by using an acoustic scale model technique. The experimental approach used enables the measurement of attenuation curves for different ceiling configurations and allows the full appraisal of screen performance. It is concluded from the investigation that a V-form ceiling can contribute only marginally to increased privacy in open plan offices by decreasing the Articulation Index for speaker/listener situations by about 0·1.     

Properties of a soft-sphere liquid from non-Newtonian molecular dynamics

A soft-sphere, inverse-12 liquid is simulated in both the isokinetic-isochoric and the isokinetic-isobaric ensemble using nonequilibrium molecular dynamics. The simulation for the isobaric ensemble is discussed in detail. The non-Newtonian characteristics of the liquid are clearly demonstrated; namely, the shear-rate-dependent pressure and density (shear dilatancy), the shear-rate-dependent shear viscosity (shear thinning), and evidence of normal pressure differences. For the first time, it is clearly shown that a significant component of isobaric shear thinning is due to shear dilatancy. The isochoric and isobaric results are checked for consistency. Simple empirical relations for the equation of state and transport properties of the fluid are presented.This is a publication in part of the National Institute of Standards and Technology (formerly NBS) and is not subject to copyright.     

Effect of electric field switching on the electrophoretic mobility of single-stranded DNA molecules in polyacrylamide gels

We have examined the effects of pulsed electric fields on the separation of single-stranded DNA molecules in polyacrylamide sequencing gels. Using different electric field pulsing regimens, the mobilities of single-stranded DNA molecules can be retarded or increased as compared to conventional electrophoresis. These results indicated that pulsed field techniques can be applied to gel electrophoresis of small single-stranded DNA molecules.     

The crystal engineering of radiation-sensitive diacetylene cocrystals and salts

In this work we develop photoreactive cocrystals/salts of a commercially-important diacetylene, 10,12-pentacosadiynoic acid (PCDA, 1) and report the first X-ray crystal structures of PCDA based systems. The topochemical reactivity of the system is modified depending on the coformer used and correlates with the structural parameters. Crystallisation of 1 with 4,4′-azopyridine (2), 4,4′-bipyridyl (3), and trans-1,2-bis(4-pyridyl)ethylene (4) results in unreactive 2 : 1 cocrystals or a salt in the case of 4,4′-bipiperidine (5). However, salt formation with morpholine (6), diethylamine (7), and n-butylamine (8), results in highly photoreactive salts 12·7 and 1·8 whose reactivity can be explained using topochemical criteria. The salt 1·6 is also highly photoreactive and is compared to a model morpholinium butanoate salt. Resonance Raman spectroscopy reveals structural details of the photopolymer including its conformational disorder in comparison to less photoactive alkali metal salts and the extent of solid state conversion can be monitored by CP-MAS NMR spectroscopy. We also report an unusual catalysis in which amine evaporation from photopolymerised PCDA ammonium salts effectively acts as a catalyst for polymerisation of PCDA itself. The new photoreactive salts exhibit more reactivity but decreased conjugation compared to the commercial lithium salt and are of considerable practical potential in terms of tunable colours and greater range in UV, X-ray, and γ-ray dosimetry applications.

In this work we develop photoreactive cocrystals/salts of a commercially-important diacetylene, 10,12-pentacosadiynoic acid (PCDA, 1) and report the first X-ray crystal structures of PCDA based systems.