bluuebunny/crossref_metadata_2025 huggingface dataset:
Statins Prevent β-Amyloid Inhibition of Sympathetic α7-nAChR-Mediated Nitrergic Neurogenic Dilation in Porcine Basilar Arteries
Authors: Si Min-Liang , Long Chen , Yang Ding-I , Chen Mei-Fang , Lee Tony Jer-Fu
Published: September 2005

The exact mechanism underlying regional cerebral hypoperfusion in the early phase of Alzheimer's disease (AD) is not understood. We have shown in isolated porcine cerebral arteries that stimulation of sympathetic α7-nicotinic acetylcholine receptors (α7-nAChRs) causes release of nitric oxide in parasympathetic nitrergic nerves and vasodilation. We therefore examined if β-amyloid peptides (Aβs), which play a key role in pathogenesis of AD, blocked sympathetic α7-nAChRs leading to reduced neurogenic nitrergic dilation in isolated porcine basilar arteries, using in vitro tissue bath, calcium image, and patch clamping techniques. The results indicated that Aβ1–40, but not Aβ40–1, blocked relaxation of endothelium-denuded basilar arterial rings induced by nicotine (100μ;mol/L) and choline (1 mmol/L) without affecting that induced by sodium nitroprusside or isoproterenol. In cultured superior cervical ganglion (SCG) cells, Aβ1–40, but not Aβ40–1, blocked choline- and nicotine-induced calcium influx and inward currents. The Aβ blockade of the nitrergic vasodilation and inward currents, but not that of calcium influx, was prevented by acute pretreatment with 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors mevastatin and lovastatin. These results suggest that Aβ1–40blocks cerebral perivascular sympathetic α7-nAChRs, resulting in the attenuation of cerebral nitrergic neurogenic vasodilation. This effect of Aβ may be responsible in part for cerebral hypoperfusion occurred in the early phase of the AD, which may be prevented by statins most likely because of their effects independent of cholesterol lowering. Statins may offer an alternative strategy in the prevention and treatment of AD.
Surface barrier for flux entry and exit in mesoscopic superconducting systems
Authors: Berdiyorov G. R. , Cabral L. R. E. , Peeters F. M.
Published: September 2005

The energy barrier which has to be overcome for a single vortex to enter or exit the sample is studied for thin superconducting disks, rings, and squares using the nonlinear Ginzburg–Landau theory. The shape and the height of the nucleation barrier is investigated for different sample radii and thicknesses and for different values of the Ginzburg–Landau parameter κ. It is shown that the London theory considerably overestimates (underestimates) the energy barrier for vortex expulsion (penetration).
Time-dependent Ginzburg–Landau equations with charged boundaries
Authors: Berger Jorge
Published: September 2005

We build a discrete form for the Ginzburg–Landau thermodynamic potential for an infinite film and find how the TDGL equations are related to it. We discuss why the usual superconductor-insulator boundary condition, which prohibits the passage of superconducting current, should be questioned in dynamic problems; nevertheless, we conclude that this condition remains valid. The formalism we develop enables us to deal with situations in which surface charge is present at the boundaries. These situations include the Hall configuration when the influence of the normal electrons is not negligible and the case of an electromagnetic wave parallel to the film. In the case of the electromagnetic wave, we evaluate the electromagnetic field inside the superconductor and follow the motion of the vortices.
Bifurcation and stability of superconductivity
Authors: Ma Tian , Wang Shouhong
Published: September 2005

In this article, we present a bifurcation and stability analysis on time-dependent Ginzburg–Landau model of superconductivity. It is proved in particular that there are two different phase transitions from the normal state to superconducting states or vice versa: one is continuous, and the other is jump. These two transitions are precisely determined by a simple nondimensional parameter, which links the superconducting behavior with the geometry of the material, the applied field and the physical parameters. The rigorous analysis is conducted using a bifurcation theory newly developed by the authors, and provides some interesting physical predictions.
Emergence of the triangular lattice near a flat wall
Authors: Almog Y.
Published: September 2005

The bifurcation of periodic solutions near a flat wall for applied magnetic fields which are slightly weaker than HC2 is considered for a reduced Ginzburg–Landau model obtained in the large κ limit. We formally demonstrate that following the bifurcation of the first mode, when the applied magnetic field is further decreased, there is a second bifurcation, after which the solution develops continuously into the well-known triangular lattice.
Ginzburg–Landau description of confinement and quantization effects in mesoscopic superconductors
Authors: Chibotaru L. F. , Ceulemans A. , Morelle M. , Teniers G. , Carballeira C. , Moshchalkov V. V.
Published: September 2005

An approach to the Ginzburg–Landau problem for superconducting regular polygons is developed making use of an analytical gauge transformation for the vector potential A which gives An=0 for the normal component along the boundary line of different symmetric polygons. As a result the corresponding linearized Ginzburg–Landau equation reduces to an eigenvalue problem in the basis set of functions obeying Neumann boundary condition. Such basis sets are found analytically for several symmetric structures. The proposed approach allows for accurate calculations of the order parameter distributions at low calculational cost (small basis sets) for moderate applied magnetic fields. This is illustrated by considering the nucleation of superconductivity in squares, equilateral triangles and rectangles, where vortex patterns containing antivortices are obtained on the Tc–H phase boundary. The calculated phase boundaries are compared with the experimental Tc(H) curves measured for squares, triangles, disks, rectangles, and loops. The stability of the symmetry consistent solutions against small deviations from the phase boundary line deep into the superconducting state is investigated by considering the full Ginzburg–Landau functional. It is shown that below the nucleation temperature symmetry-switching or symmetry-breaking phase transitions can take place. The symmetry-breaking phase transition has the same structure as the pseudo-Jahn-Teller instability of high symmetry nuclear configurations in molecules. The existence of these transitions is predicted to be strongly dependent on the size of the samples.
A configurational temperature Nosé-Hoover thermostat
Authors: Braga Carlos , Travis Karl P.
Published: September 2005

We propose two new thermostats which can be employed in computer simulations to ensure that two different variants of the configurational temperature fluctuate around their equilibrium values. These new thermostats differ from one previously introduced by Delhommelle and Evans [Mol. Phys. 99, 1825 (2001)] in several important ways. First, our thermostats are derived in the same spirit as the Nosé-Hoover thermostat and therefore generate the canonical phase-space distribution. Second, our thermostats involve simpler equations of motion, which do not involve spatial gradients of the configurational temperature. They do not suffer from problems stemming from stiff equations of motion and furthermore, in large temperature perturbation simulations, the measured temperature follows the set-point temperature without any overshoot, and with good damping of oscillations. We show that both of our configurational thermostats are special cases of a more general set of Nosé-Hoover equations proposed by Kusnezov et al. [Ann. Phys. 204, 155 (1990)]. The new thermostats are expected to be highly useful in nonequilibrium simulations, particularly those characterized by spatial inhomogeneities. They should also find applicability in simulations involving large changes in temperature over small time scales, such as temperature quench molecular dynamics and radiation damage modeling.
Effects of C–H stretch excitation on the H+CH4 reaction
Authors: Camden Jon P. , Bechtel Hans A. , Brown Davida J. Ankeny , Zare Richard N.
Published: September 2005

We have investigated the effects of C–H stretching excitation on the H+CH4→CH3+H2 reaction dynamics using the photo-LOC technique. The CH3 product vibrational state and angular distribution are measured for the reaction of fast H atoms with methane excited in either the antisymmetric stretching fundamental (ν3=1) or first overtone (ν3=2) with a center-of-mass collision energy of Ecoll ranging from 1.52 to 2.20 eV. We find that vibrational excitation of the ν3=1 mode enhances the overall reaction cross section by a factor of 3.0±1.5 for Ecoll=1.52eV, and this enhancement factor is approximately constant over the 1.52–2.20-eV collision energy range. A local-mode description of the CH4 stretching vibration, in which the C–H oscillators are uncoupled, is used to describe the observed state distributions. In this model, the interaction of the incident H atom with either a stretched or an unstretched C–H oscillator determines the vibrational state of the CH3 product. We also compare these results to the similar quantities obtained previously for the Cl+CH4→CH3+HCl reaction at Ecoll=0.16eV [Z. H. Kim, H. A. Bechtel, and R. N. Zare, J. Chem. Phys. 117, 3232 (2002); H. A. Bechtel, J. P. Camden, D. J. A. Brown, and R. N. Zare, ibid. 120, 5096 (2004)] in an attempt to elucidate the differences in reactivity for the same initially prepared vibration.
Density-functional-based molecular-dynamics simulations of molten salts
Authors: Hazebroucq Sandrine , Picard Gérard S. , Adamo Carlo , Heine Thomas , Gemming Sibylle , Seifert Gotthard
Published: October 2005

The physicochemical properties of two molten salts, namely, KCl and NaCl, have been studied with a molecular-dynamics approach using a density-functional-based tight-binding (DFTB) model. The obtained results have been compared with a number of previously reported simulations, carried out on smaller systems and using classical force-field techniques. A good agreement has been found for both structural parameters and macroscopic properties, such as self-diffusion coefficients. Furthermore, our DFTB results are very close to the available experimental data. From a more general point of view, our results demonstrate the applicability of DFTB as an efficient tool in the modeling of melts. At the same time, the quality of the obtained results supports the use of this as a reliable alternative to the more expensive ab initio dynamics approaches, if accurate parameters are provided.
Interphase exchange coupling in Fe∕Sm–Co bilayers with gradient Fe thickness
Authors: Yu Ming-hui , Hattrick-Simpers Jason , Takeuchi Ichiro , Li Jing , Wang Z. L. , Liu J. P. , Lofland S. E. , Tyagi Somdev , Freeland J. W. , Giubertoni D. , Bersani M. , Anderle M.
Published: September 2005

We have fabricated Fe∕Sm–Co bilayers with gradient Fe thicknesses in order to systematically study the dependence of exchange coupling on the thickness of the Fe layer. The Fe layer was deposited at two different temperatures (150 and 300°C) to study the effect of deposition temperature on the exchange coupling. Magneto-optical Kerr effect and x-ray magnetic circular dichroism (XMCD) have been employed as nondestructive rapid characterization tools to map the magnetic properties of the gradient samples. Systematic enhancement in exchange coupling between the soft layer and the hard layer is observed as the soft layer thickness is decreased. Separate exchange couplings of the Fe layer with Co and Sm in the hard layer are revealed through measuring the element-specific hysteresis curves using XMCD. The single-phase-like magnetization reversal critical thickness increases from 12nm for Fe deposited at 150°Cto24nm for Fe deposited at 300°C, indicating an important role of the state of the interface in the exchange coupling.
Diffusion coefficients of small gas molecules in amorphouscis-1,4-polybutadiene estimated by molecular dynamics simulations
Authors: Meunier M.
Published: October 2005

Molecular dynamics (MD) simulations were employed to estimate the diffusion coefficients of small gas molecules (Ar, O2, N2, CO2, and CH4) in amorphous cis-1,4-polybutadiene in the temperature range of 250–400K. The VT diagram and solubility parameter of the amorphous polymer have been successfully reproduced using a full atomistic potential. Diffusion coefficients were calculated from long NPT MD runs (up to 3ns) at temperature ranging from 250 up to 400K. Calculated diffusion coefficients compare well with experimental data as well as previous published work, though a systematic overestimation is found due to the finite-size effect of the model. The influence of various physical and computational parameters on the results is discussed. The diffusion mechanism is examined at the different temperatures of study.
An anomaly in the isotopomer shift of the hyperfine spectrum of LiI
Authors: Cederberg J. , Nichol J. , Frodermann E. , Tollerud H. , Hilk G. , Buysman J. , Kleiber W. , Bongard M. , Ward J. , Huber K. , Khanna T. , Randolph J. , Nitz D.
Published: October 2005

A high-precision examination of the hyperfine spectrum of Li6I in comparison with Li7I shows a shift in the iodine nuclear electric quadrupole moment that cannot be accounted for by a model in which the electric field gradient at the iodine site is assumed to depend only upon the internuclear distance between Li and I. The other hyperfine interactions are consistent between the two isotopomers, including the previously reported electric hexadecapole interaction of the iodine nucleus.
Electronic structures of dangling-bond states on the Si nanoisland and the Si(111) 7×7 substrate
Authors: Negishi R. , Suzuki M. , Shigeta Y.
Published: September 2005

The electronic structure of the fabricated surface, on which uniform Si nanoislands are formed by a fine control of the growth conditions, has been studied in detail by using angle-resolved photoelectron spectroscopy. We found two surface components at ≈−0.57 and −0.35eV by decomposition of the photoelectron spectra in addition to three well-known dangling-bond states (center and corner adatoms and rest atom) in the 7×7 dimer-adatom stacking (DAS) fault unit cell. The energy of the additional surface components relative to wave number shows no dispersion, suggesting a localized surface state. By comparison with our scanning tunneling microscopy and scanning tunneling spectroscopy results, the additional two surface states are assigned to the dangling-bond states localized on the inner and outer adatoms of the rounded Si nanoislands.
Near-field focusing by a photonic crystal concave mirror
Authors: Saado Y. , Golosovsky M. , Davidov D. , Frenkel A.
Published: September 2005

We have studied the electric-field distribution in the photonic crystal (PC) concave mirror. This was done numerically and was verified in experiment. The field mapping in our experiments was performed using the Slater’s perturbation technique which was modified for nonresonant structures. The PC consists of a planar array of dielectric rods with a trapezoidal depression that forms the concave mirror. At the midgap frequency of the first stopband of the underlying photonic crystal, the mirror allows focusing to a subwavelength spot size of 0.2λ×0.2λ. The intensity is enhanced by 16 times at the focal point, as compared with the incident wave.
Ripening during magnetite nanoparticle synthesis: Resulting interfacial defects and magnetic properties
Authors: Barker Alex J. , Cage Brant , Russek Stephen , Stoldt Conrad R.
Published: September 2005

The structure and magnetic properties of magnetite (Fe3O4) nanoparticles synthesized by a solvothermal processing route are investigated. The nanoparticles are grown from the single organometallic precursor Fe(III) acetylacetonate in trioctylamine (TOA) solvent at 260°C, with and without the addition of heptanoic acid (HA) as a stabilizing agent. From the temporal particle size distributions, x-ray-diffraction patterns, high-resolution transmission electron microscope tilt series experiments, and superconducting quantum interference device magnetometry, we demonstrate that HA, a strong Lewis acid stabilizing agent, slows growth processes during ripening thus reducing the formation of interfacial defects, which we observe in the TOA-only synthesis. Nanoparticles grown with HA remain single crystalline for long growth times (up to 24h), show a focused particle size distribution for intermediate growth times (3h), and possess a higher magnetic anisotropy (15.8×104J∕m3) than particles grown without the additional stabilizing agent. The reduced magnetic anisotropy value for the magnetite nanoparticles grown in TOA only (1.29×104J∕m3) is attributed to polycrystallinity induced by the uncontrolled ripening process. This work may have significance for contrast enhancement in magnetic resonance imaging.
Low-temperature upconversion spectroscopy of nanosized Y2O3:Er,Yb phosphor
Authors: Pires Ana Maria , Serra Osvaldo Antonio , Heer Stephan , Güdel Hans Ulrich
Published: September 2005

This work reports on the structural characterization and on the low-temperature upconversion spectroscopy of the Y2O3:2%Er,1%Yb nanophosphor prepared by thermal decomposition of a polymeric resin (Pechini’s method [U.S. Patent No. 3,330,697 (July 11 1967)]). The average particle size evaluated from transmission electron microscopy lies in the range of 05–25nm. The high-resolution upconversion luminescence spectrum at 10K in the ultraviolet to near infrared (UV-NIR) spectral regions shows narrow lines, characteristic of Er3+ transitions occupying both Y3+ sites with point symmetries C2 and C3i in the oxide cubic system. The excitation spectrum at 10K in the IR region was used to monitor the green upconversion and IR luminescences and it displayed Er and Yb lines. Power dependence measurements at 298 and 10K indicate that the main upconversion mechanism is a two-photon excitation process. The temperature dependence of the upconversion luminescence shows a decrease in the emission intensity with increasing temperature, and such decrease is much more evident for the emission in the violet than in the green and the red regions.
Temperature measurement in a Paris-Edinburgh cell by neutron resonance spectroscopy
Authors: Stone H. J. , Tucker M. G. , Meducin F. M. , Dove M. T. , Redfern S. A. T. , Le Godec Y. , Marshall W. G.
Published: September 2005

Neutron resonance spectroscopy is demonstrated as a viable and reliable method for the determination of sample temperatures in high-pressure, high-temperature neutron diffraction studies at neutron spallation sources. The technique operates by characterizing the thermally induced Doppler broadening of neutron absorption resonances of heavy nuclei present within the sample assembly observed in transmission. Accuracies of ±10K have been readily achieved with data acquisition times of approximately 40 min. It is shown that the temperatures determined by this technique are independent of the sample pressure provided that the sample temperature is kept above the Debye temperature of the material whose resonances are being examined. A short list of candidate resonances for temperature measurement by neutron resonance spectroscopy is presented and suggestions volunteered for the combinations of elements most suitable for temperature measurement based on the experience of the authors.
Photoluminescence from as-grown and annealed GaN0.027As0.863Sb0.11∕GaAs single quantum wells
Authors: Kudrawiec R. , Motyka M. , Misiewicz J. , Yuen H. B. , Bank S. R. , Wistey M. A. , Bae H. P. , Harris James S.
Published: September 2005

We have investigated characteristics of photoluminescence (PL) spectra obtained from as-grown and annealed GaN0.027As0.863Sb0.11∕GaAs single quantum wells (SQWs). For the as-grown SQW at low temperature (<150K), a broadband emission at ∼0.9eV has been observed in addition to the band-gap-related recombination at ∼0.95eV. After annealing, this broad emission disappears and the band-gap-related PL peak blueshifts ∼55meV. The Stokes shift for this peak is 11meV at 10K. Thus, the low-temperature PL peak is mainly associated with the recombination of localized excitons. A rise in temperature leads to a continuous change in this peak; the contribution associated with localized excitons decreases while the contribution associated with free-excitons increases. Above 150K the line shape gradually loses the Gaussian profile and a high-energy tail appears due to a thermal band filling effect. In this temperature range (>150K), the PL peak is attributed to free-carrier recombination.
Theoretical analysis of two-dimensional buckling patterns of thin metal-polymer bilayer on the substrate
Authors: Joon Kwon S. , Lee Hong H.
Published: September 2005

Theoretical analysis based on a nondegenerate perturbation method is presented for two-dimensional buckling patterns of a thin metal-capped polymer bilayer on a rigid substrate. The analysis reveals that isotropic one-dimensional buckling mode has the lowest free energy, which contrasts previous work on the buckling of a thin metal film on the semi-infinitely thick polymer layer studied by Chen and Hutchinson [J. Appl. Mech. 71, 597 (2004)]. In the low stress region, however, checkerboard buckling mode has the lowest energy. Herringbone buckling mode is shown to be represented by a linear combination of the checkerboard mode and the isotropic one-dimensional mode. Disorder parameter values are given for these various two-dimensional buckling modes on the basis of an envelope function.
Faraday rotation, ferromagnetism, and optical properties in Fe-doped BaTiO3
Authors: Rajamani A. , Dionne G. F. , Bono D. , Ross C. A.
Published: September 2005

The structural, magneto-optical, optical, and magnetic properties of BaTi1−xFexO3 films grown by pulsed laser deposition onto MgO (100) have been investigated for Fe content, x=0.15, 0.2, 0.3, and 0.5. Saturation Faraday rotation varies from 0.033deg∕μm for x=0.2 to 0.109deg∕μm for x=0.5 at a 1.55-μm wavelength, with corresponding saturation magnetizations of 11.3emu∕cm3 and 18.3emu∕cm3. The optical absorption drops with a reduction in iron content and the BaTi0.85Fe0.15O3 sample has an optical absorption of 1.9dB∕mm. The magneto-optic figure of merit, which is the ratio of Faraday rotation to absorption, peaks at x=0.2 among the samples considered. The magnetic properties are interpreted in terms of the electronic structure of Fe4+ ions, and their exchange coupling with Fe3+ ions associated with oxygen vacancies.
Nanostructural characterization and two-dimensional electron-gas properties in high-mobility AlGaN∕AlN∕GaN heterostructures grown on epitaxial AlN/sapphire templates
Authors: Miyoshi Makoto , Egawa Takashi , Ishikawa Hiroyasu , Asai Kei-Ichiro , Shibata Tomohiko , Tanaka Mitsuhiro , Oda Osamu
Published: September 2005

Al 0.26 Ga 0.74 N ∕ Al N ∕ Ga N heterostructures with a 1nm-thick AlN interfacial layer were grown on epitaxial AlN/sapphire templates by metal-organic vapor phase epitaxy, and they exhibited excellent film qualities and very high electron mobilities, such as over 2100cm2∕Vs at room temperature and over 25 000cm2∕Vs at 15K with a two-dimensional electron-gas (2DEG) density of approximately 1×1013∕cm2. Cross-sectional transmission electron microscopy images revealed that the thin AlN layer with a thickness of 1nm is continuously grown between AlGaN and GaN layers with atomically abrupt and flat interfaces. The experimental and calculated results for 2DEG transport properties indicated that an AlN interfacial layer between AlGaN and GaN layers effectively suppresses alloy disorder scattering and that epitaxial AlN/sapphire templates largely contribute to the high electron mobility because they allow for the realization of a high-quality GaN channel with a low dislocation density and a smooth interface.
A comprehensive study of boron and carbon diffusion models in SiGeC heterojunction bipolar transistors
Authors: Sibaja-Hernandez Arturo , Decoutere Stefaan , Maes Herman
Published: September 2005

Three different diffusion models in SiGeC layers are compared and evaluated for their implementation in the process simulation program Taurus TSUPREM-4™ (TSUPREM-4 computer code from Synopsys, Inc.). The models considered in the evaluation are verified for actual process conditions of modern SiGeC heterojunction bipolar transistors in bipolar complementary metal-oxide-semiconductor technologies. Rapid thermal annealing temperatures in the range of 1020–1070°C were used for the present study. In this study it is shown that the compared models are similar to each other despite the different physical approach. A common set of coefficients was used for the simulations of the three models under study.
Characterization of Si nano-polycrystalline films at the nanometer level using resonant Raman scattering
Authors: Yoshikawa M. , Murakami M. , Matsuda K. , Matsunobe T. , Sugie S. , Okada K. , Ishida H.
Published: September 2005

We have measured a resonant Raman scattering from polycrystalline silicon (poly-Si) films with thicknesses of 24–381nm at the depth resolution of approximately 5nm and found that poly-Si films are under compressive stress. The main Raman peak in poly-Si films tends to shift to a higher frequency as the thickness of the underlayer of the silicon dioxide (SiO2) film becomes greater. From this result, it has been considered that the compressive stress becomes larger with an increase in the thickness of the underlayer of the SiO2 film. We have tried to separate the observed Raman shifts into those caused by the crystallite size effect and those caused by stress in the poly-Si films. We did this using the resonant Raman scattering technique and calculation by the phonon confinement model. It has been found that the crystallite size obtained from the Raman measurement roughly agrees with the size obtained from the x-ray measurement. This result suggests that the phonon-dispersion curve does not change significantly under the compressive stress of 102MPa order.
Phosphorescence response to excitonic interactions in Ir organic complex-based electrophosphorescent emitters
Authors: Kalinowski J. , Mȩżyk J. , Meinardi F. , Tubino R. , Cocchi M. , Virgili D.
Published: September 2005

The phosphorescence (PH) response to increasing excitation intensity (I) has been studied from an efficient electrophosphorescent iridium (III) complex, fac tris(2-phenylpyridine) iridium [Ir(ppy)3], dispersed in a diamine derivative (TPD)-doped polycarbonate (PC) hole-transporting matrix and in the form of neat vacuum-evaporated films. It is demonstrated that the observed decrease in relative PH efficiencies at increasing I is principally due to triplet-triplet (T-T) interactions that include mutual annihilation of the TPD host, Ir(ppy)3 guest, and host-guest triplets. The effective annihilation rate constants [γTTeff] fall in the range (1–3)×10−12cm3s−1 depending slightly on the matrix composition. The lower and upper limits of γTTeff correspond to TPD-free Ir(ppy)3-doped PC samples and high-content TPD or neat Ir(ppy)3 solid films, respectively. A deviation from the second-order kinetics of Ir(ppy)3 triplets observed with neat films is attributed to a saturation of nonradiative excited sites (e.g., molecular aggregates) populated by energy transfer from the triplets. From extrapolation of Icrit at which T-T interactions become the triplet lifetime controlling process to electrical excitation in Ir(ppy)3-based light-emitting-diodes, the onset current of the roll off in electrophosphorescence (EPH) quantum efficiency (QE) is calculated. Its values exceed at least one order of magnitude the experimental data, supporting previous suggestions of the large current density EPH QE roll off to be substantially underlain by the field-assisted dissociation of emissive states and their precursors.
Terahertz generation in submicron GaN diodes within the limited space-charge accumulation regime
Authors: Sokolov V. N. , Kim K. W. , Kochelap V. A. , Woolard D. L.
Published: September 2005

The conditions for microwave power generation with hot-electron transport are investigated in a submicron GaN diode when it operates in the limited space-charge accumulation (LSA) mode. Applying a transport model based on the local quasistatic approximation, the analysis shows that the nitride diodes can support the LSA mode of oscillation in the terahertz-frequency range. For a 100nm n-GaN diode with a cross section of 500μm2 and the electron density of 1×1017cm−3, the generated microwave power is estimated to be as high as ≈0.6W with the corresponding dc-to-rf conversion efficiency of ≈9% and the negative differential resistance of ≈−1.3Ω; which thus provides an efficient mechanism to achieve very high-frequency microwave generation in the nitrides.
All-epitaxial growth of Ba0.6Sr0.4(Ti0.94Al0.06)O3–Si heterostructures and their leakage current characteristics
Authors: Chen Tong Lai , Li Xiao Min , Wu Wen Biao
Published: September 2005

6 at. % Al-doped Ba0.6Sr0.4TiO3 (BSTA) thin films were grown on Ir∕MgO-buffered Si(001) substrates by in situ pulsed-laser deposition techniques. All-epitaxial growth of BSTA∕Ir∕MgO∕Si(001) heterostructures with layer-by-layer mode was evidenced by in situ reflection high-energy electron-diffraction observation and x-ray diffraction. The epitaxy relationship was determined as: BSTA⟨100⟩‖Ir⟨100⟩MgO⟨100⟩‖Si⟨100⟩ (in-plane) and BSTA(001)‖Ir(001)MgO(001)‖Si(001) (out-of-plane). The BSTA thin films exhibit an extremely smooth surface with a roughness of RMS=0.89nm. The largely reduced leakage current for BSTA thin films, which was dominated by the Schottky emission mechanism, might be attributed to combination effects of the crystal-structure-perfection and acceptor Al doping. Moreover, the BSTA thin films show good dielectric properties at low-frequency regime.
Leakage and performance of zero-Schottky-barrier carbon nanotube transistors
Authors: Alam Khairul , Lake Roger K.
Published: September 2005

Undoped carbon nanotube field-effect transistors (CNTFETs) with zero-Schottky-barrier contacts have the current-voltage response of field-effect transistors (FETs), but the physics of their operation is that of voltage-controlled tunnel barriers. The carbon nanotube (CNT) body itself provides the tunnel barrier. The leakage current in CNTFETs is a combination of both interband and intraband tunneling and this current can be significantly reduced by changing the CNT diameter as well as the CNT length and source/drain asymmetry. Source and drain extensions significantly reduce the leakage current and increase the ON/OFF current ratio. Asymmetry with the gate closer to the source further reduces leakage, improves the ON/OFF current ratio, decreases the switching time, and increases the cutoff frequency despite the higher gate capacitance. An ON/OFF current ratio of >104 can be obtained from a 50-nm-long, 1.5-nm-diameter CNT with a 2nm gate. The switching time is very small in the 0.1ps range and the cutoff frequency is very high in the 4THz range. Coulomb blockade is expected to block the interband resonant tunneling (ambipolar) leakage current so that the CNTFETs become effectively unipolar devices. Poisson’s equation is solved self-consistently with the nonequilibrium Green’s-function equations using a π-bond model for the CNT.
The size-dependent elastic properties of nanofilms with surface effects
Authors: Guo Jian-Gang , Zhao Ya-Pu
Published: October 2005

Size-dependent elastic constants are investigated theoretically with reference to a nanoscale single-crystal thin film. A three-dimensional (3D) model is presented with the relaxation on the surface of the nanofilm taken into consideration. The constitutive relation of the 3D model is derived by using the energy approach, and analytical expressions for the four nonzero elastic constants of the nanofilm are obtained. The size effects of the four elastic constants are then discussed, and the dependence of these elastic constants on the surface relaxation and the ambiguity in the definition of the thickness of the nanofilm are also analyzed. In addition, the elastic moduli of the nanofilm in two kinds of plane problem are obtained and discussed in the case of a special boundary condition.
Observation of the lamellar phase in a Zr-free Sm(Co0.45Fe0.15Cu0.4)5 alloy
Authors: Zhang Y. , Gabay A. M. , Hadjipanayis G. C.
Published: September 2005

A lamellar phase is reported in a Zr-free Sm(Co0.45Fe0.15Cu0.4)5 alloy after it had been solutionized and aged at 650°C for 50h. The lamellar phase has a composition of Sm(Co,Fe,Cu)3 with a Be3Nb structure type, which is similar to that in the fully heat-treated Sm(Co,Cu,Fe,Zr)z 2:17 magnets containing Zr. This observation is contrary to the traditional thinking that associates the lamellar phase to the presence of Zr. This present result provides additional information that can be used to elaborate the origin of coercivity in precipitation Sm-Co magnets.
Field emission from high aspect ratio tubular carbon cones grown on gold wire
Authors: Li J. J. , Gu C. Z. , Wang Q. , Xu P. , Wang Z. L. , Xu Z. , Bai X. D.
Published: September 2005

Tubular carbon cones (TCCs) with nanometer-sized tips and micrometer-sized roots, having a herringbone hollow interior surrounded by helical sheets of graphite coiling around, were grown on Au wires by hot filament chemical vapor deposition (HFCVD). These TCCs exhibit excellent field emission properties with a very low threshold field of 0.27V∕μm and a corresponding current density of about 1μA∕cm2; and a stable emitting current density of 1.9mA∕cm2 can be obtained at only 0.6V∕μm. Their low effective work function of ∼0.0056eV and their conical bases—which effectively reduce the screening effect due to sufficient distance between adjacent tubular cones—are both favorable to field emission enhancement.
Strain relaxation in InAs∕InGaAs quantum dots investigated by photoluminescence and capacitance-voltage profiling
Authors: Chen J. F. , Hsiao R. S. , Chen Y. P. , Wang J. S. , Chi J. Y.
Published: September 2005

We present detailed studies of the onset of strain relaxation in InAs∕InGaAs quantum dots. We show that the ground-state photoluminescence (PL) emission redshifts with increasing the InAs coverage before relaxation and blueshifts when relaxation occurs. PL spectra of the relaxed samples show two predominant families of dots with very different temperature-dependent efficiency. By comparison we show that the dots emitting at long wavelength are degraded by relaxation while the dots emitting at short wavelength remain coherently strained. Consequently, the PL spectra are dominated by the dots emitting at short wavelength, leading to the observed blueshift. This result suggests that the relaxation does not occur uniformly. In addition, we show that the relaxation occurs in the dot bottom interface.
Fabrication of C60 field-effect transistors with polyimide and Ba0.4Sr0.6Ti0.96O3 gate insulators
Authors: Kubozono Yoshihiro , Nagano Takayuki , Haruyama Yusuke , Kuwahara Eiji , Takayanagi Toshio , Ochi Kenji , Fujiwara Akihiko
Published: September 2005

A flexible C60 field-effect transistor (FET) device has been fabricated with a polyimide gate insulator on the poly(ethylene terephthalate) substrate, and n-channel normally off FET properties are observed in this FET device. The field-effect mobility, μ, is estimated to be ∼10−2cm2V−1s−1 at 300K. Furthermore, the C60 FET has been fabricated with a high-dielectric Ba0.4Sr0.6Ti0.96O3 (BST) gate insulator, showing n-channel properties; the μ value is estimated to be ∼10−4cm2V−1s−1 at 300K. The FET device operates at very low gate voltage, VG, and low drain-source voltage, VDS. Thus these C60 FET devices possess flexibility and low-voltage operation characteristic of polyimide and BST gate insulators, respectively.
Nonlinear oscillations and collapse of elongated bubbles subject to weak viscous effects
Authors: Tsiglifis Kostas , Pelekasis Nikos A.
Published: October 2005

The weak viscous oscillations of a bubble are examined, in response to an elongation that perturbs the initial spherical shape at equilibrium. The flow field in the surrounding liquid is split in a rotational and an irrotational part. The latter satisfies the Laplacian and can be obtained via an integral equation. A hybrid boundary-finite element method is used in order to solve for the velocity potential and shape deformation of axisymmetric bubbles. Weak viscous effects are included in the computations by retaining first-order viscous terms in the normal stress boundary condition and satisfying the tangential stress balance. An extensive set of simulations was carried out until the bubble either returned to its initial spherical shape, or broke up. For a relatively small initial elongation the bubble returned to its initial spherical state regardless of the size of the Ohnesorge number; Oh=μ∕(ρRσ)1∕2. For larger initial elongations there is a threshold value in Oh−1 above which the bubble eventually breaks up giving rise to a “donut” shaped larger bubble and a tiny satellite bubble occupying the region near the center of the original bubble. The latter is formed as the round ends of the liquid jets that approach each other from opposite sides along the axis of symmetry, coalesce. The size of the satellite bubble decreased as the initial elongation or Oh−1 increased. This pattern persisted for a range of large initial deformations with a decreasing threshold value of the Oh−1 as the initial deformation increased. As its equilibrium radius increases the bubble becomes more susceptible to the above collapse mode. The effect of initial bubble overpressure was also examined and it was seen that small initial overpressures, for the range of initial bubble deformations that was investigated, translate the threshold of Oh−1 to larger values while at the same time increasing the size of the satellite bubble.
ZnO extended-gate field-effect transistors as pH sensors
Authors: Batista P. D. , Mulato M.
Published: September 2005

The objective of this work is the study and characterization of zinc oxide (ZnO) as pH sensor. We used an extended-gate field-effect transistor (EGFET) to obtain the response of ZnO as a function of pH. Sol-gel was used for the production of ZnO films because this is a low cost and easy fabrication procedure. The ZnO powder was obtained at different temperatures of calcination, from 150 up to 500°C. The samples were investigated by x-ray diffraction, infrared spectroscopy, thermogravimetric analysis and differential thermal analysis. The films were investigated as pH sensors (range 2–12) and the ZnO EGFET shows a sensitivity of 38mV∕pH.
Chemical turnstile
Authors: Lee W. T. , Salje E. K. H.
Published: September 2005

A chemical turnstile is a device for transporting small, well-characterized doses of atoms from one location to another. A working turnstile has yet to be built, despite the numerous technological applications available for such a device. The key difficulty in manufacturing a chemical turnstile is finding a medium which will trap and transport atoms. Here we propose that ferroelastic twin walls are suitable for this role. Previous work shows that twin walls can act as two-dimensional trapping planes within which atomic transport is fast. We report simulations showing that a stress-induced reorientation of a twin wall can occur. This behavior is ideal for chemical turnstile applications.
High-mobility Sb-doped p-type ZnO by molecular-beam epitaxy
Authors: Xiu F. X. , Yang Z. , Mandalapu L. J. , Zhao D. T. , Liu J. L. , Beyermann W. P.
Published: October 2005

Reproducible Sb-doped p-type ZnO films were grown on n-Si (100) by electron-cyclotron-resonance-assisted molecular-beam epitaxy. The existence of Sb in ZnO:Sb films was confirmed by low-temperature photoluminescence measurements. An acceptor-bound exciton (A°X) emission was observed at 3.358 eV at 8 K. The acceptor energy level of the Sb dopant is estimated to be 0.2 eV above the valence band. Temperature-dependent Hall measurements were performed on Sb-doped ZnO films. At room temperature, one Sb-doped ZnO sample exhibited a low resistivity of 0.2Ωcm, high hole concentration of 1.7×1018cm−3 and high mobility of 20.0cm2∕Vs. This study suggests that Sb is an excellent dopant for reliable and reproducible p-type ZnO fabrication.
Clusters of interacting receptors can stabilize synaptic efficacies
Authors: Shouval Harel Z.
Published: September 2005

Synaptic weights store memories that can last a lifetime. Yet, memory depends on synaptic protein receptors that are recycled in and out of the membrane at a fast rate, possibly several times an hour. Several proposals to bridge this vast gap in time scales between memory and its molecular substrate have relied on bistable molecular switches. Here, we propose an alternative to this approach based on clusters of interacting receptors in the synaptic membrane. We show that such clusters can be metastable and that the lifetime of such clusters can be many orders of magnitude larger than the lifetime of the receptors of which they are composed. We also demonstrate how bidirectional synaptic plasticity can be implemented in this framework.
Developmental regulation of Foxp3 expression during ontogeny
Authors: Fontenot Jason D. , Dooley James L. , Farr Andrew G. , Rudensky Alexander Y.
Published: October 2005

Thymectomy of neonatal mice can result in the development of autoimmune pathology. It has been proposed that thymic output of regulatory T (T reg) cells is delayed during ontogeny and that the development of autoimmune disease in neonatally thymectomized mice is caused by the escape of self-reactive T cells before thymectomy without accompanying T reg cells. However, the kinetics of T reg cell production within the thymus during ontogeny has not been assessed. We demonstrate that the development of Foxp3-expressing T reg cells is substantially delayed relative to nonregulatory thymocytes during ontogeny. Based on our data, we speculate that induction of Foxp3 in developing thymocytes and, thus, commitment to the T reg cell lineage is facilitated by a signal largely associated with the thymic medulla.
Augmented cell-graphs for automated cancer diagnosis
Authors: Demir Cigdem , Gultekin S. Humayun , Yener Bülent
Published: October 2005

Summary: This work reports a novel computational method based on augmented cell-graphs (ACG), which are constructed from low-magnification tissue images for the mathematical diagnosis of brain cancer (malignant glioma). An ACG is a simple, undirected, weighted and complete graph in which a node represents a cell cluster and an edge between a pair of nodes defines a binary relationship between them. Both the nodes and the edges of an ACG are assigned weights to capture more information about the topology of the tissue. In this work, the experiments are conducted on a dataset that is comprised of 646 human brain biopsy samples from 60 different patients. It is shown that the ACG approach yields sensitivity of 97.53% and specificities of 93.33 and 98.15% (for the inflamed and healthy, respectively) at the tissue level in glioma diagnosis.

Contact:  demir@cs.rpi.edu
A framework for computational and experimental methods: Identifying dimerization residues in CCR chemokine receptors
Authors: de Juan David , Mellado Mario , Rodríguez-Frade José Miguel , Hernanz-Falcón Patricia , Serrano Antonio , del Sol Antonio , Valencia Alfonso , Martínez-A Carlos , Rojas Ana María
Published: October 2005

Summary: Solving relevant biological problems requires answering complex questions. Addressing such questions traditionally implied the design of time-consuming experimental procedures which most of the time are not accessible to average-sized laboratories. The current trend is to move towards a multidisciplinary approach integrating both theoretical knowledge and experimental work. This combination creates a powerful tool for shedding light on biological problems. To illustrate this concept, we present here a descriptive example of where computational methods were shown to be a key aspect in detecting crucial players in an important biological problem: the dimerization of chemokine receptors. Using evolutionary based sequence analysis in combination with structural predictions two CCR5 residues were selected as important for dimerization and further validated experimentally. The experimental validation of computational procedures demonstrated here provides a wealth of valuable information not obtainable by any of the individual approaches alone.

Contact:  arojas@cnb.uam.es

Supplementary information: Supplementary data are available at Bioinformatics online.
A query language for biological networks
Authors: Leser Ulf
Published: October 2005

Motivation: Many areas of modern biology are concerned with the management, storage, visualization, comparison and analysis of networks, but no appropriate query language for such complex data structures yet exists.

Results: We have designed and implemented the pathway query language (PQL) for querying large protein interaction or pathway databases. PQL is based on a simple graph data model with extensions reflecting properties of biological objects. Queries match subgraphs in the database based on node properties and paths between nodes. The syntax is easy to learn for anybody familiar with SQL. As an important feature, a query may require a certain structure in the database to exist but return a different subgraph. We have tested PQL queries on networks of up to 16 000 nodes and found it to scale very well.

Availability: The code is available on request from the author.

Contact:  leser@informatik.hu-berlin.de
Adding Some SPICE to DAS
Authors: Prlić Andreas , Down Thomas A. , Hubbard Tim J. P.
Published: October 2005

Summary: The distributed annotation system (DAS) defines a communication protocol used to exchange biological annotations. It is motivated by the idea that annotations should not be provided by single centralized databases but instead be spread over multiple sites. Data distribution, performed by DAS servers, is separated from visualization, which is carried out by DAS clients. The original DAS protocol was designed to serve annotation of genomic sequences. We have extended the protocol to be applicable to macromolecular structures. Here we present SPICE, a new DAS client that can be used to visualize protein sequence and structure annotations.

Availability:  http://www.efamily.org.uk/software/dasclients/spice/

Contact:  ap3@sanger.ac.uk
Predicting protein stability changes from sequences using support vector machines
Authors: Capriotti Emidio , Fariselli Piero , Calabrese Remo , Casadio Rita
Published: October 2005

Motivation: The prediction of protein stability change upon mutations is key to understanding protein folding and misfolding. At present, methods are available to predict stability changes only when the atomic structure of the protein is available. Methods addressing the same task starting from the protein sequence are, however, necessary in order to complete genome annotation, especially in relation to single nucleotide polymorphisms (SNPs) and related diseases.

Results: We develop a method based on support vector machines that, starting from the protein sequence, predicts the sign and the value of free energy stability change upon single point mutation. We show that the accuracy of our predictor is as high as 77% in the specific task of predicting the ΔΔG sign related to the corresponding protein stability. When predicting the ΔΔG values, a satisfactory correlation agreement with the experimental data is also found. As a final blind benchmark, the predictor is applied to proteins with a set of disease-related SNPs, for which thermodynamic data are also known. We found that our predictions corroborate the view that disease-related mutations correspond to a decrease in protein stability.

Availability:  http://gpcr2.biocomp.unibo.it/cgi/predictors/I-Mutant2.0/I-Mutant2.0.cgi

Contact:  casadio@alma.unibo.it