Showing papers by "National Physical Laboratory published in 2005"

Experimental observation of the spin-Hall effect in a two-dimensional spin-orbit coupled semiconductor system.

Abstract: We report the experimental observation of the spin-Hall effect in a 2D hole system with spin-orbit coupling. The 2D hole layer is a part of a $p\mathrm{\text{\ensuremath{-}}}n$ junction light-emitting diode with a specially designed coplanar geometry which allows an angle-resolved polarization detection at opposite edges of the 2D hole system. In equilibrium the angular momenta of the spin-orbit split heavy-hole states lie in the plane of the 2D layer. When an electric field is applied across the hole channel, a nonzero out-of-plane component of the angular momentum is detected whose sign depends on the sign of the electric field and is opposite for the two edges. Microscopic quantum transport calculations show only a weak effect of disorder, suggesting that the clean limit spin-Hall conductance description (intrinsic spin-Hall effect) might apply to our system.

Theory, developments and applications of optical coherence tomography

Abstract: In this paper, we review the developments in optical coherence tomography (OCT) for three-dimensional non-invasive imaging. A number of different OCT techniques are discussed in some detail including time-domain, frequency-domain, full-field, quantum and Doppler OCT. A theoretical treatment is given and some relevant comparisons made between various implementations. The current and potential applications of OCT are discussed, with close attention paid to biomedical imaging and its metrological issues.

A study of the effect of annealing on Fe‐doped LiNbO3 by HRXRD, XRT and FT–IR

Abstract: The annealing effect on the structural perfection of Fe-doped LiNbO3 single crystals has been studied by high-resolution X-ray diffractometry (HRXRD), X-ray topography (XRT) and Fourier transform infrared (FT–IR) spectroscopy. The single crystals, prepared by mixing Li2CO3 and Nb2O5 powders in the molar ratio 48.6:51.4 with 0.05 mol% of iron at 1415 (1) K, were grown by the Czochralski (CZ) method along the [001] direction in air and poled during crystal growth by the application of a DC field. Two low-angle (tilt angle ∼1 arc minute) structural grain boundaries were observed in as-grown specimens. FT–IR spectra revealed that these crystals contain OH− and CO32− ionic defects. Grain boundaries and CO32− ionic defects were successfully removed, while the concentration of OH− ions was considerably reduced by post-growth thermal annealing at elevated temperatures.

Low Field Magnetotransport in Manganites

Abstract: The perovskite manganites of general formula RE_1-xAe_xMnO_3 (RE= rare earth,AE=Ca, Sr, Ba and Pb)have drawn considerable attention, especially following the discovery of colossal magnetoresistance (CMR). They exhibit extraordinary large magnetoresistance pronounced as CMR in the vicinity of insulator-metal/paramagnetic-ferromagnetic transition at a relatively large applied magnetic fields. However, for applied aspectes, occurence of significant CMR at low applied magnetic fields would be required. This review consists of of two sections: In the first section we have extensively reviewed the salient features e.g. structure, phase diagram, double exchange mechansim, Jahn Teller effect, different types of ordering and phase separation of CMR mangnaites. The second is devoted to an overview of experimental results on CMR and related magnetotransport characteristics at low magnetic fields for doped manganites such as polycrystalline La_0.67Ca_0.33MnO_3 films, Ag admixed La_0.67Ca_0.33MnO_3 films, polycrystalline (La_0.7Ca_0.2Ba_0.1MnO_3)and epitaxial (La_0.67Ca_0.33MnO_3) films on different substrates, nanophasic La_0.7Ca_0.3MnO_3, mangnaite-polymer composites (La_0.7Ba_0.2Sr_0.1MnO_3-PMMA and La_0.67Ca_0.33MnO_3-PMMA)and double layered polycrystalline (La_1.4Ca_1.6-xBa_xMn_2O_7) and films (La_1.4Ca_1.6Mn_2O_7). Some other potential magnetoresistive materials e.g. pyrochlores, chalcogenides, ruthenates, diluted magnetic semiconductors, magnetic tunnel junctions, nanocontacts etc have aslo been briefly dealt with. The review concludes with the summary of results for low field magnetotransport behaviour and prospectes for applications.

Analytical techniques for trace element analysis: an overview

Abstract: Techniques for the analysis of trace elements have developed rapidly in response to the increasing need for accurate measurements at extremely low amount of contents in diverse matrices. We review potentiometric, voltammetric, atomic spectrometric, X-ray and nuclear techniques and discuss the extent to which they have been shown to give comparable results in typical applications.

Development of nanophosphors—A review

Abstract: Nanophosphors have been extensively investigated during the last decade due to their application potential for various high-performance displays and devices. These act as a strategic component in almost all displays. Synthesis of nanophosphors can be accomplished in two ways namely, chemical and physical methods. Under chemical methods, different routes such as colloidal, capping, cluster formation, sol–gel, electrochemical, etc., are being followed. Physical methods widely used are molecular beam epitaxy, ionised cluster beam, liquid metal ion source, consolidation, sputtering and gas aggregation of monomers. Chemical precipitation in presence of capping agents, reaction in microemulsions, sol–gel reaction and autocombustion are commonly used techniques for synthesis of nanophosphors. However, the particle size has to be restricted to 3–5 nm to get the real advantage of quantum confinement. In other words, the particle size must be less than twice of Bohr radii of exciton as quantum confinement regime is limited to that size. A brief review of different synthesis techniques employed all over the world for the development of industrially important nanophosphors and extent of particle size reduction achieved is discussed.

Aerosols over Delhi during pre‐monsoon months: Characteristics and effects on surface radiation forcing

Abstract: [1] The surface fluxes in the wavelength range 280-2800 nm were measured during the pre-monsoon period, April-June 2003 along with the spectral distribution of aerosol optical depth (AOD) in the visible and near infrared wavelengths. The Angstrom exponent alpha retrieved from the data showed abundance of desert aerosols over Delhi during this period. The aerosol composition constructed using the OPAC model indicated a typical mixture of two aerosol types: urban and desert. Due to this the aerosol mixture had a very low value of single scattering albedo ∼0.67. The average total radiative forcing efficiency observed at the surface in the broad wavelength band (280-2800 nm) was estimated and compared with the SBDART model calculated values. Citation: Singh, S., S. Nath, R. Kohli, and R. Singh (2005), Aerosols over Delhi during pre-monsoon months: Characteristics and effects on surface radiation forcing.

Observational study of surface ozone at New Delhi, India

Abstract: Surface ozone has been measured over New Delhi, an urban site, a region of intensive anthropogenic activity since 1997. Seasonal variations in ozone concentration show pronounced maxima in the summer and autumn seasons and minima in monsoon and winter seasons. Diurnal patterns in ozone concentration show daytime in situ photochemical production throughout the year. The high ozone episode days were associated with meteorological parameters such as sunny and warm weather, stagnant wind patterns and low relative humidity. The monthly average maximum concentration in summer was found to be in the range of 62–95 ppb whereas, it was found to be 50–82 ppb in the autumn (October and November). The analysis of hourly averaged surface ozone data illustrates that on a large number of days the surface ozone values at Delhi exceed the World Health Organization (WHO) ambient air quality standards (hourly average of 80 ppb) for ozone. On some occasions, night‐time increases of surface ozone concentration have been obser...

The measurement of viscosity of alloys—a review of methods, data and models

Abstract: Values of the viscosities of liquid metals are important in the prediction of fluid flow in many metallurgical manufacturing processes This paper describes a number of methods used to measure the viscosity of liquid metals, including capillary, oscillating vessel, rotational bob or crucible, oscillating plate, draining vessel, levitation using the damping of surface oscillations and acoustic methods A number of models used to estimate viscosity for elements, the temperature dependence of viscosity, and viscosity of multicomponent systems are also given, including the Andrade equation, Arrhenius equation, Hildebrand's free volume theory, Chhabra models, Moelwyn-Hughes model and thermodynamic models The scatter of data available in the literature are highlighted by comparing two reviews of data for elements

Development of a lactate biosensor based on conducting copolymer bound lactate oxidase

Abstract: An amperometric enzyme electrode was developed for determination of lactate in serum. To prepare this electrode, commercial lactate oxidase from Pediococcus species has been immobilized through glutaraldehyde coupling onto polyaniline-co-fluoroaniline film deposited on an Indium tin oxide (ITO) coated glass plate. This plate acted as working electrode when combined with Pt electrode as counter electrode to the electrometer for the development of a biosensor. The method is based on generation of electrons from H 2 O 2 , which is formed from lactic acid by immobilized lactate oxidase. The concentration of lactic acid is directly proportional to the current measured. The enzyme electrode showed optimum response when operated at 42 °C in 0.05 M, sodium phosphate buffer pH 6.5 for 50 s. The biosensor showed a good performance with a linear response range from 0.1 to 5.5 mM/l. The minimum detection limit of the method is 0.1 mM/l and sensitivity of the sensor is 1.18 μA/mM/l lactate. This electrode was employed for determination of lactate in serum. The serum values in healthy and diseased persons were in the range 0.51–2.9 and 5.0–15.0 mM, respectively. The analytical recovery of added lactic acid was 71%. Within batch and between batch CV were

Long exposure steam oxidation testing and mechanical properties of slurry aluminide coatings for steam turbine components

Abstract: Important efforts to develop new steels or to protect high creep strength steels in order to allow operation of steam turbines at 650 °C are being carried out world-wide to increase efficiency. Within the European Project “SUPERCOAT” (Coatings for Supercritical Steam Cycles), work has been concentrated in the development of coatings to withstand 50,000–100,000 h of operation at 650 °C under high pressure steam. Aluminide coatings on ferritic–martensitic steels produced by applying an Al slurry followed by a diffusion heat treatment, have shown to be protective at 650 °C under steam for at least 32,000 h of laboratory steam exposure under atmospheric pressure. Although the “as diffused” coatings present through thickness cracks, these do not propagate during exposure to steam or thermal cycling and no new cracks seem to develop. Moreover, no changes in residual stresses could be observed after thermal cycling. Microstructural characterization of samples at different periods of exposures has been carried out by SEM-EDS and XRD. The principal mechanism of coating degradation is loss of Al at the surface due to inwards diffusion. Microhardness as well as Young's modulus and fracture strength were measured using well established techniques. The coatings show reasonable ductility (∼1.6%) when stressed in tension between room temperature and 400 °C which further increases at higher temperatures providing evidence that the coatings should withstand the mechanical conditions likely to be encountered in service.

Traceable measurements of the static permittivity of dielectric reference liquids over the temperature range 5–50 °C

Abstract: Measurements of the static permittivity of pure samples of eight polar liquids—acetone, butan-1-ol, dimethyl sulphoxide, ethanediol, ethanol, methanol, propan-1-ol and water—and two non-polar liquids—cyclohexane and silicone oil—were measured in a shielded micrometer-driven, parallel-plate admittance cell at temperatures from 5 to 50 °C. The experimental method, which requires measurements of capacitance using a four-terminal-pair impedance analyser, is described in detail. The full measurement results have been tabulated separately in a National Physical Laboratory report. The present paper provides a comparative study of these latest results with previously published data and also describes the details of the uncertainty analyses employed. Admittance cell measurement techniques are also briefly reviewed to put this work into context.

Traceable calibration of transfer standards for scanning probe microscopy

Abstract: A Metrological Atomic Force Microscope (MAFM) has been constructed for the traceable calibration of transfer standards for scanning probe microscopy. It uses optical interferometry to generate image scales with direct traceability to the national standard of length. Three interferometers monitor the relative displacements of the AFM tip and sample in the x, y and z directions and the interferometer data is used directly to construct 3D images of sample surfaces. Traceable dimensional measurement of surface features may then be derived from the image data. This paper describes the MAFM instrument and presents a measurement uncertainty budget. Examples are given of measurements of pitch and step height on calibration transfer standards for scanning probe microscopy.

Evaluation of the primary frequency standard NPL-CsF1

Abstract: A new caesium fountain frequency standard (NPL-CsF1) at the National Physical Laboratory is described. Procedures for evaluation of the systematic frequency shifts are presented. The NPL-CsF1 has a short-term stability σy(τ) = 1.4 × 10−13τ−1/2, predominantly due to the local oscillator phase noise. The accuracy of 1 part in 1015 is limited by the uncertainty of the frequency shift due to collisions between cold atoms.

Carbon monoxide sensitivity of vacuum deposited polyaniline semiconducting thin films

Abstract: By utilizing the semiconducting polymeric thin films on various substrates a new simple and quick technique has been developed for sensing carbon monoxide gas The thin films of polyaniline were prepared by vacuum deposition The particular doping combination in the polymer makes the sensor specific for detection of CO Polyaniline was prepared by copolymerization of aniline and formaldehyde Metal halides with specific concentrations were used as dopants to make polyaniline film specific for CO sensing The thickness of the film was measured by using quartz thickness monitor The thickness of the film was found the order of 1000 A Sensitivity of the film is measured by using the relation S=(Ie−Io)/Io, where Ie is current after exposure and Io is the current before exposure the sensor to CO, respectively The sensor, investigated in the environment of various gases as CO, HCN and NH3, showed the high sensitivity of order 800 for carbon monoxide gas and response time of the sensor is 5 s for carbon monoxide gas The fabrication process, the morphological, structural and electrical characterization of the sensor in regard to detection of CO has been described

Room-temperature ferromagnetism in Ge 1 − x Mn x nanowires

Abstract: We report an observation of room temperature ferromagnetism in Ge nanowires doped with Mn. High-density arrays of ${\mathrm{Ge}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\phantom{\rule{0.2em}{0ex}}(x=1%--5%)$ nanowires with the smallest diameter of 35 nm have been synthesized within the pores of anodized aluminium oxide membranes using a supercritical fluid inclusion-phase technique. Structural analysis of the nanowires proved the existence of a highly crystalline germanium host lattice containing discrete manganese atoms. All of the nanowires studied displayed ferromagnetic properties at room temperature. Ferromagnetic ordering reaches a maximum at intermediate Mn concentrations. The magnetic properties of the nanowires can be understood by considering the influence of co-dopant nonmagnetic impurities and nanowire/membrane interfaces.

Edge spin accumulation in semiconductor two-dimensional hole gases

Abstract: The controlled generation of localized spin densities is a key enabler of semiconductor spintronics In this work, we study spin Hall effect induced edge-spin accumulation in a two-dimensional hole gas with strong spin orbit interactions. We argue that it is an intrinsic property, in the sense that it is independent of the strength of disorder scattering. We show numerically that the spin polarization near the edge induced by this mechanism can be large, and that it becomes larger and more strongly localized as the spin-orbit coupling strength increases, and is independent of the width of the conducting strip once this exceeds the elastic scattering mean-free-path. Our experiments in two-dimensional hole gas microdevices confirm this remarkable spin Hall effect phenomenology. Achieving comparable levels of spin polarization by external magnetic fields would require laboratory equipment whose physical dimensions and operating electrical currents are a million times larger than those of our spin Hall effect devices.

An accurate semi-empirical equation for sputtering yields, II: for neon, argon and xenon ions

Abstract: A new general, predictive semi-empirical equation for the sputtering yields of mono-elemental solids, using 250–10,000 eV ions incident normally on the surface, has been developed. This equation is based on the approach of Matsunami et al. but includes a new atomic density term and avoids their arbitrary Q term. By analysing published experimental data for Ne, Ar and Xe sputtering yields, the accuracy of this approach may be evaluated. The standard deviations of the data for Ne, Ar and Xe are 14%, 9% and 14%, respectively and represent a very significant improvement on the semi-empirical approaches of Matsunami et al. and of Yamamura and Tawara. General predictive relationships are established to allow the approach to be used for any incident ions, however, it is expected that accuracies for low reactivity (inert gas) incident ions with masses in the range from Ne to Xe will be comparable to the above figures but extrapolation to lighter elements than Ne or heavier than Xe, or to reactive primary ions, should be treated with considerable caution.

Mesopic models—from brightness matching to visual performance in night-time driving: a review

Abstract: At present, suitable methods to evaluate the visual effectiveness of lighting products in the mesopic region are not available. The majority of spectral luminous efficiency functions obtained to date in the mesopic range have been acquired by heterochromatic brightness matching. However, the most recent studies in the mesopic field have adopted a task performance-based approach. This paper summarizes the major mesopic models proposed so far, presenting in detail the experimental conditions of these studies. The authors represent a research consortium which has adopted the task performance-based approach for night-time driving in which mesopic visual performance has been divided into three subtasks. Data for each sub-task will be generated by using a set of common parameter values and 120 observers. The approach and methods used by the consortium are presented. © The Chartered Institution of Building Services Engineers 2005.