Power dissipation in nanoscale conductors

A previous tight-binding model of power dissipation in a nanoscale conductor under an applied bias is extended to take account of the local atomic topology and the local electronic structure. The method is used to calculate the power dissipated at every atom in model nanoconductor geometries: a nanoscale constriction, a one-dimensional atomic chain between two electrodes with a resonant double barrier, and an irregular nanowire with sharp corners. The local power is compared with the local current density and the local density of states. A simple relation is found between the local power and the current density in quasiballistic geometries. A large enhancement in the power at special atoms is found in cases of resonant and anti-resonant transmission. Such systems may be expected to be particularly unstable against current-induced modifications.

Sub-mm wet etched linear to circular polarization fss based polarization converters

This communication investigates the potential for fabrication of micromachined silicon sub-millimeter wave periodic arrays of freestanding slot frequency selective surfaces (FSS) using wet etch KOH technology. The vehicle for this is an FSS for generating circularly polarized signals from an incident linearly polarized signal at normal incidence to the structure. Principal issues and fabrication processes involved from the initial design of the core FSS structures to be made and tested through to their final testing are addressed. Measured and simulated results for crossed and ring slot element shapes in single and double layer polarization convertor structures are presented for sub-mm wave operation. It is shown that 3 dB axial ratio (AR) bandwidths of 21% can be achieved with the one layer perforated screen design and that the rate of change is lower than the double layer structures. An insertion loss of 1.1 dB can be achieved for the split circular ring double layer periodic array. These results are shown to be compatible with the more specialized fabrication equipment dry reactive ion etching approach previously used for the construction of this type of structure. © 2011 IEEE.

The electrical characterization and response to hydrogen of Schottky diodes with a resistive metal electrode—rectifying an oversight in Schottky diode investigation

Schottky-barrier structures with a resistive metal electrode are examined using the 4-point probe method where the probes are connected to the metal electrode only. The observation of a significant decrease in resistance with increasing temperature (over a range of similar to 100 K) in the diode resistance-temperature (R(D)-T) characteristic is considered due to charge carrier confinement to the metal electrode at low temperature (high resistance), with the semiconductor progressively opening up as a parallel current carrying channel (low resistance) with increasing temperature due to increasing thermionic emission across the barrier. A simple model is constructed, based on thermionic emission at quasi-zero bias, that generates good fits to the experimental data. The negative differential resistance (NDR) region in the R(D)-T characteristic is a general effect and is demonstrated across a broad temperature range for a variety of Schottky structures grown on Si-, GaAs- and InP-substrates. In addition the NDR effect is harnessed in micro-scaled Pd/n-InP devices for the detection of low levels of hydrogen in an ambient atmosphere of nitrogen.

Comparison of frequency-selective screen-based linear to circular split-ring polarisation convertors

This study presents the use of periodic arrays of freestanding slot frequency-selective screens (FSS) as a means for generating circularly polarised signals from an incident linearly polarised signal at normal incidence to the structure. Measured and simulated results for crossed, linear and various ring slot element shapes in single and double-layer polarisation convertor structures are presented for 10 GHz operation. It is shown that 3 dB axial ratio (AR) bandwidths of 21% can be achieved with the one-layer perforated screen design and that the rate of change is lower than the double-layer structures. An insertion loss of 0.34 dB can be achieved for the split circular ring double-layer periodic array, and of the three topologies presented the hexagonal split-ring polarisation convertor gives the lowest variation of AR with angle of incidence 1.8 dB/45° and 3.6 dB/45° for the single and double-screen FSS, respectively. In addition, their tolerance to angle of incidence variation is presented. The capability of the surfaces reported here as twist polariser or spatial isolator components has been demonstrated with up to -30 dB isolation between incident and re-reflected signals for the double-layer designs being measured. © 2010 The Institution of Engineering and Technology.

INFRARED SURFACE-PLASMONS ON PLATINUM SILICIDE

It is shown that surface plasmons (SPs) are supported on thin PtSi films. Using a prism-air gap-sample configuration, p-polarised infra-red light (3.39-mu-m) has been coupled with approximately 95% efficiency to SPs on the silicide electrode of PtSi-Si Schottky barrier structures. Stimulating SPs offers both a means of optically characterising silicide films and of enhancing optical absorption with a view to significantly increasing the Schottky barrier photoresponse.

Barrier height variations and interface properties of PtSi/Si structures

To study some of the interfacial properties of PtSi/Si diodes, Schottky structures were fabricated on (100) crystalline silicon substrates by conventional thermal evaporation of Pt on Si followed by annealing at different temperatures (from 400 degrees C to 700 degrees C) to form PtSi. The PtSi/n-Si diodes, all yielded Schottky barrier (SB) heights that are remarkably temperature dependent. The temperature range (20-290 K) over which the I-V characteristics were measured in the present study is broader with a much lower limit (20 K), than what is usually reported in literature. These variations in the barrier height are adequately interpreted by introducing spatial inhomogeneity into the barrier potential with a Gaussian distribution having a mean barrier of 0.76 eV and a standard deviation of 30 meV. Multi-frequency capacitance-voltage measurements suggest that the barrier is primarily controlled by the properties of the silicide-silicon interface. The forward C-V characteristics, in particular, show small peaks at low frequencies that can be ascribed to interface states rather than to a series resistance effect.

Tight junctional abnormality in multiple sclerosis white matter affects all calibres of vessel and is associated with blood-brain barrier leakage and active demyelination

Blood-brain barrier (BBB) hyperpermeability in multiple sclerosis (MS) is associated with lesion pathogenesis and has been linked to pathology in microvascular tight junctions (TJs). This study quantifies the uneven distribution of TJ pathology and its association with BBB leakage. Frozen sections from plaque and normal-appearing white matter (NAWM) in 14 cases were studied together with white matter from six neurological and five normal controls. Using single and double immunofluorescence and confocal microscopy, the TJ-associated protein zonula occludens-1 (ZO-1) was examined across lesion types and tissue categories, and in relation to fibrinogen leakage. Confocal image data sets were analysed for 2198 MS and 1062 control vessels. Significant differences in the incidence of TJ abnormalities were detected between the different lesion types in MS and between MS and control white matter. These were frequent in oil-red O (ORO)+ active plaques, affecting 42% of vessel segments, but less frequent in ORO- inactive plaques (23%), NAWM (13%), and normal (3.7%) and neurological controls (8%). A similar pattern was found irrespective of the vessel size, supporting a causal role for diffusible inflammatory mediators. In both NAWM and inactive lesions, dual labelling showed that vessels with the most TJ abnormality also showed most fibrinogen leakage. This was even more pronounced in active lesions, where 41% of vessels in the highest grade for TJ alteration showed severe leakage. It is concluded that disruption of TJs in MS, affecting both paracellular and transcellular paths, contributes to BBB leakage. TJ abnormality and BBB leakage in inactive lesions suggests either failure of TJ repair or a continuing pathological process. In NAWM, it suggests either pre-lesional change or secondary damage. Clinically inapparent TJ pathology has prognostic implications and should be considered when planning disease-modifying therapy

Tetrahedra and vertex-sharing double tetrahedra in the ammonium iodomercurates(II) (NH4)(7)[HgI4](2)[Hg2I7](H2O) and (NH4)(3)[Hg2I7]

The new ammonium iodomercurates(II), (NH4)(7)[HgI4](2)[Hg2I7](H2O) (1) and (NH4)(3)[Hg2I7] (2) contain isolated tetrahedra and vertex-sharing double tetrahedra as the anions. The crystal structures were determined from single-crystal X-ray diffraction data: 1: orthorhombic, Pnma (no. 62), a = 2175.9(2), b = 1781.8(2), c = 1256.2(2) pm, Z = 4. R-1 [I-0 > 2 sigma(I-0)] = 0.0520; 2: monoclinic, P2(1)/c (no. 14), a = 1259.0(2), b = 773.2(1), c = 2172.4(3) pm, beta = 101.18(2)degrees, Z = 4, R, [I-0 > 2 sigma(I-0)] = 0.0308.

Facile biocatalytic reduction of the carbon-carbon double bond of 5-benzylidenethiazolidine-2,4-diones. Synthesis of ( )-5-(4-{2-[methyl(2-pyridyl)amino]ethoxy}benzyl)thiazolidine-2,4-dione (BRL 49653), its (R)-(+)-enantiomer and analogs.

Cantello, Barrier C. C.; Eggleston, Drake S.; Haigh, David; Haltiwanger, R. Curtis; Heath, Catherine M.; Hindley, Richard M.; Jennings, Keith R.; Sime, John T.; Woroniecki, Stefan R. SmithKline Beecham Pharmaceuticals, Surrey, UK. Journal of the Chemical Society, Perkin Transactions 1: Organic and Bio-Organic Chemistry (1994), (22), 3319-24. Publisher: Royal Society of Chemistry, CODEN: JCPRB4 ISSN: 0300-922X. Journal written in English. CAN 122:105736 AN 1995:237497 CAPLUS (Copyright (C) 2009 ACS on SciFinder (R)) Abstract A novel biotransformation system for the redn. of carbon-carbon double bonds in 5-benzylidenethiazolidine-2,4-diones to give the corresponding 5-benzylthiazolidine-1,4-diones, using whole cells of red yeasts, is described. These reduced compds., which are recovered in good yield, are of potential use in the treatment of non-insulin dependent diabetes mellitus. The mild reaction conditions developed allow redn. of 5-benzylidenethiazolidine-2,4-diones contg. other functionalities which are not compatible with alternative redn. methods. The biocatalytic redn. is enantioselective and the synthesis of R-(+)-5-(4-{2-[methyl(2-pyridyl)amino]ethoxy}benzyl)thiazolidine-2,4-dione by Rhodotorula rubra CBS 6469 and structure confirmation by X-ray crystallog. is detailed. Optimization of reaction conditions (including immobilization) for these whole cell redn. system is described.

Low-energy excitations of a boson pair in a double-well trap

The states of a boson pair in a one-dimensional double-well potential are investigated. Properties of the ground and lowest excited states of this system are studied, including the two-particle wave function, momentum pair distribution, and entanglement. The effects of varying both the barrier height and the effective interaction strength are investigated.

DNA double-strand break distributions in X-ray and alpha-particle irradiated V79 cells: Evidence for non-random breakage

Many studies have shown that with increasing LET of ionizing radiation the RBE (relative biological effectiveness) for dsb (double strand breaks) induction remains around 1.0 despite the increase in the RBE for cell killing. This has been attributed to an increase in the complexity of lesions, classified as dsb with current techniques, at multiply damaged sites. This study determines the molecular weight distributions of DNA from Chinese hamster V79 cells irradiated with X-rays or 110 keV/mu m alpha-particles. Two running conditions for pulsed-field gel-electrophoresis were chosen to give optimal separation of fragments either in the 225 kbp-5.7 Mbp range or the 0.3 kbp to 225 kbp range. Taking the total fraction of DNA migrating into the gel as a measure of fragmentation, the RBE for dsb induction was less than 1.0 for both molecular weight regions studied. The total yields of dsb were 8.2 x 10(-9) dsb/Gy/bp for X-rays and 7.8 x 10(-9) dsb/Gy/bp for a-particles, measured using a random breakage model. Analysis of the RBE of alpha-particles versus molecular weight gave a different response. In the 0.4 Mbp-57 Mbp region the RBE was less than 1.0; however, below 0.4 Mbp the RBE increased above 1.0. The frequency distributions of fragment sizes were found to differ from those predicted by a model assuming random breakage along the length of the DNA and the differences were greater for alpha-particles than for X-rays. An excess of fragments induced by a single-hit mechanism was found in the 8-300 kbp region and for X-rays and alpha-particles these corresponded to an extra 0.8 x 10(-9) and 3.4 x 10(-9) dsb/bp/Gy, respectively. Thus for every alpha-particle track that induces a dsb there is a 44% probability of inducing a second break within 300 kbp and for electron tracks the probability is 10%. This study shows that the distribution of damage from a high LET alpha-particle track is significantly different from that observed with low LET X-rays. In particular, it suggests that the fragmentation patterns of irradiated DNA may be related to the higher-order chromatin repealing structures found in intact cells.