bluuebunny/crossref_metadata_2025 huggingface dataset:
Apolipoprotein D is Elevated in Oligodendrocytes in the Peri-Infarct Region after Experimental Stroke: Influence of Enriched Environment
Authors: Rickhag Mattias , Deierborg Tomas , Patel Shutish , Ruscher Karsten , Wieloch Tadeusz
Published: September 2007

Injury to the brain (e.g., stroke) results in a disruption of neuronal connectivity and loss of fundamental sensori-motor functions. The subsequent recovery of certain functions involves structural rearrangements in areas adjacent to the infarct. This remodeling of the injured brain requires trafficking of macromolecular components including cholesterol and phospholipids, a transport carried out by apolipoproteins including apolipoprotein D (apoD). We investigated the changes in the levels of apoD mRNA and protein, and its cellular localization during a recovery period up to 30 days after experimental stroke in the rat brain. In the core of the brain infarct, apoD immunoreactivity but not mRNA increased in dying pyramidal neurons, indicative of cellular redistribution of lipids. During 2 to 7 days of recovery after stroke, the apoD levels increased in the peri-infarct and white matter areas in cells identified as mature oligodendrocytes. The apoD expressing cells were conspicuously located along the rim of the infarct, suggesting a role for apoD in tissue repair. Furthermore, housing animals in an enriched environment improved sensori-motor function and increased the apoD levels. Our data strongly suggest that apoD is involved in regenerative processes and scar formation in the peri-infarct area presumably by enhancing lipid trafficking.
Interaction of Sulfur and Nitrogen Nutrition in Tobacco (Nicotiana tabacum) Plants: Significance of Nitrogen Source and Root Nitrate Reductase
Authors: Kruse J. , Kopriva S. , Hänsch R. , Krauss G.‐J. , Mendel R.‐R. , Rennenberg H.
Published: September 2007

Abstract: The significance of root nitrate reductase for sulfur assimilation was studied in tobacco (Nicotiana tabacum) plants. For this purpose, uptake, assimilation, and long‐distance transport of sulfur were compared between wild‐type tobacco and transformants lacking root nitrate reductase, cultivated either with nitrate or with ammonium nitrate. A recently developed empirical model of plant internal nitrogen cycling was adapted to sulfur and applied to characterise whole plant sulfur relations in wild‐type tobacco and the transformant. Both transformation and nitrogen nutrition strongly affected sulfur pools and sulfur fluxes. Transformation decreased the rate of sulfate uptake in nitrate‐grown plants and root sulfate and total sulfur contents in root biomass, irrespective of N nutrition. Nevertheless, glutathione levels were enhanced in the roots of transformed plants. This may be a consequence of enhanced APR activity in the leaves that also resulted in enhanced organic sulfur content in the leaves of the tranformants. The lack of nitrate reductase in the roots in the transformants caused regulatory changes in sulfur metabolism that resembled those observed under nitrogen deficiency. Nitrate nutrition reduced total sulfur content and all the major fractions analysed in the leaves, but not in the roots, compared to ammonium nitrate supply. The enhanced organic sulfur and glutathione levels in ammonium nitrate‐fed plants corresponded well to elevated APR activity. But foliar sulfate contents also increased due to decreased re‐allocation of sulfate into the phloem of ammonium nitrate‐fed plants. Further studies will elucidate whether this decrease is achieved by downregulation of a specific sulfate transporter in vascular tissues.
The Characteristic High Sulfate Content in Brassica oleracea is Controlled by the Expression and Activity of Sulfate Transporters
Authors: Koralewska A. , Posthumus F. S. , Stuiver C. E. E. , Buchner P. , Hawkesford M. J. , De Kok L. J.
Published: September 2007

Abstract: The uptake and distribution of sulfate in Brassica oleracea, a species characterised by its high sulfate content in root and shoot, are coordinated and adjusted to the sulfur requirement for growth, even at external sulfate concentrations close to the Km value of the high‐affinity sulfate transporters. Plants were able to grow normally and maintain a high sulfur content when grown at 5 or 10 μM sulfate in the root environment. Abundance of mRNAs for the high affinity sulfate transporters, BolSultr1;1 and BolSultr1;2, were enhanced at ≤ 25 μM sulfate, and this was accompanied with an up to three‐fold increase in the sulfate uptake capacity, whereas sulfate, organic sulfur, and thiol contents were only slightly affected. Upon sulfate deprivation, there was a much greater induction of the sulfate transporters, BolSultr1;1, BolSultr1;2, BolSultr1;3, BolSultr2;1, and BolSultr4;1, whilst the sulfate uptake capacity was only increased up to four‐fold. Plant growth and shoot to root biomass allocation were affected only upon sulfate‐deprivation and not at low external sulfate concentrations. From the current results it is suggested that the internal sulfate concentration may act as a determining factor in the regulation of activity and expression of sulfate transporters, and of shoot to root biomass allocation in B. oleracea.
Novel Insight into the Regulation of GSH Biosynthesis in Higher Plants
Authors: Rausch T. , Gromes R. , Liedschulte V. , Müller I. , Bogs J. , Galovic V. , Wachter A.
Published: September 2007

Abstract: In higher plants, the redox‐active tripeptide glutathione (GSH) fulfills a plethora of functions. These include its pivotal role for maintaining the cellular redox poise and its involvement in detoxification of heavy metals and xenobiotics. Intimately linked to these functions, GSH also acts as a cellular signal, mediating control of enzyme and/or regulatory protein activities, either directly or via glutaredoxins. The redox potential of the GSH/GSSG couple is not only affected by the GSH/GSSG ratio but also by changes in GSH synthesis and/or degradation. As this couple operates as redox buffer in several cellular compartments, the regulation of GSH biosynthesis and transport (both intra‐ and intercellularly) are fundamental to the maintenance of cellular redox homeostasis during plant development and, even more so, when plants are exposed to biotic or abiotic stress. This review highlights novel aspects of GSH biosynthesis and transport with a focus on the regulation of the GSH1 (=γ‐glutamylcysteine synthetase) enzyme. Interestingly, GSH1 appears to be exclusively confined to the plastids, whereas the second biosynthetic enzyme, GSH2, is predominantly localized in the cytosol. GSH1 expression and enzyme activity are under multiple controls, extending from transcriptional regulation to post‐translational redox control. Now that the plant GSH1 protein structure has been solved, the molecular basis of GSH1 function and redox regulation can be addressed. The review concludes with a discussion of the simultaneous changes observed for GSH synthesis, transport, and metabolism during Cd‐induced phytochelatin accumulation.
Potential energy surfaces and Jahn-Teller effect on CH4⋯NO complexes
Authors: Crespo-Otero Rachel , Suardiaz Reynier , Montero Luis Alberto , García de la Vega José M.
Published: September 2007

The potential energy surface of the CH4⋯NO van der Waals complexes was explored at the RCCSD(T)/aug-cc-pVTZ level including the full counterpoise correction to the basis set superposition error. The Jahn-Teller distortion of the C3v configurations for the CH bonded and CH3 face complexes was analyzed. From this distortion, two A′ and A″ adiabatic surfaces were considered. The estimated zero point energy of Cs configurations is above the barrier of the C3v ones. Therefore, the CH3 face complexes are dynamic Jahn-Teller systems. The D0 (140cm−1 for A″ state and 100cm−1 for A′) values obtained are in good agreement with the experimental values (103±2cm−1) recently reported.
Synchronization of two coupled self-excited systems with multi-limit cycles
Authors: Enjieu Kadji H. G. , Yamapi R. , Chabi Orou J. B.
Published: September 2007

We analyze the stability and optimization of the synchronization process between two coupled self-excited systems modeled by the multi-limit cycles van der Pol oscillators through the case of an enzymatic substrate reaction with ferroelectric behavior in brain waves model. The one-way and two-way couplings synchronization are considered. The stability boundaries and expressions of the synchronization time are obtained using the properties of the Hill equation. Numerical simulations validate and complement the results of analytical investigations.
Resistive effects on line-tied magnetohydrodynamic modes in cylindrical geometry
Authors: Delzanno Gian Luca , Evstatiev E. G. , Finn John M.
Published: September 2007

An investigation of the effect of resistivity on the linear stability of line-tied magnetohydrodynamic (MHD) modes is presented in cylindrical geometry, based on the method recently developed in the paper by Evstatiev et al. [Phys. Plasmas 13, 072902 (2006)]. The method uses an expansion of the full solution of the problem in one-dimensional radial eigenfunctions. This method is applied to study sausage modes (m=0, m being the poloidal wavenumber), kink modes (m=1), and m=2 modes. All these modes can be resistively unstable. It is found that m≠0 modes can be unstable below the ideal MHD threshold due to resistive diffusion of the field lines, with growth rates proportional to resistivity. For these resistive modes, there is no indication of tearing, i.e., current sheets or boundary layers due to ideal MHD singularities. That is, resistivity acts globally on the whole plasma column and not in layers. Modes with m=0, on the other hand, can exist as tearing modes if the equilibrium axial magnetic field reverses sign within the plasma.
Hindered settling velocity and microstructure in suspensions of solid spheres with moderate Reynolds numbers
Authors: Yin Xiaolong , Koch Donald L.
Published: September 2007

Lattice-Boltzmann simulations are employed to determine the mean settling velocity and pair distribution function for spheres settling in a liquid. The Reynolds number based on the terminal velocity ranges from 1 to 20, the solid-to-fluid density ratio is ρp∕ρf=2.0, and the solid volume fraction is varied from 0.005 to 0.40. At volume fractions larger than about 0.05, the ratio of the mean settling velocity to the terminal velocity u* can be fit by a power-law expression u*=k(1−ϕ)n, where k and n are functions of the Reynolds number based on the terminal velocity. The constant k is typically about 0.86–0.92 and u* deviates from the power-law behavior in dilute suspensions. The extent of this deviation increases with increasing Reynolds number. We show that the hindered settling velocity follows a power law when the particle microstructure is similar to that in a hard-sphere suspension. The deviation from the power-law behavior can be correlated with an anisotropic microstructure resulting from wake interactions among the spheres. This microstructure, which occurs in dilute suspensions and is most pronounced at the higher Reynolds numbers explored in our study, consists of a decreased pair distribution function for pairs with vertical separation vectors and a peak in the pair distribution function for horizontal separations corresponding to about two particle diameters.
Transient violations of the second law of thermodynamics in protein unfolding examined using synthetic atomic force microscopy and the fluctuation theorem
Authors: Paramore Sterling , Ayton Gary S. , Voth Gregory A.
Published: September 2007

The synthetic atomic force microscopy (AFM) method is developed to simulate a periodically replicated atomistic system subject to force and length fluctuations characteristic of an AFM experiment. This new method is used to examine the forced-extension and subsequent rupture of the α-helical linker connecting periodic images of a spectrin protein repeat unit. A two-dimensional potential of mean force (PMF) along the length and a reaction coordinate describing the state of the linker was calculated. This PMF reveals that the basic material properties of the spectrin repeat unit are sensitive to the state of linker, an important feature that cannot be accounted for in a one-dimensional PMF. Furthermore, nonequilibrium simulations were generated to examine the rupture event in the context of the fluctuation theorem. These atomistic simulations demonstrate that trajectories which are in apparent violation of the second law can overcome unfolding barriers at significantly reduced rupture forces.
Heat capacity of the liquid-liquid mixture nitrobenzene and dodecane near the critical point
Authors: Utt Nathan J. , Lehman S. Y. , Jacobs D. T.
Published: September 2007

The heat capacity of the liquid-liquid mixture nitrobenzene-dodecane has been measured for the first time near its upper critical consolute point using an adiabatic calorimeter. The theoretical expression for the heat capacity near the critical point was applied to our combined data runs. The critical exponent α was determined to be 0.124±0.006, which was consistent with theoretical predictions. When α was fixed at its theoretical value of 0.11, our value for the amplitude ratioA+∕A−=0.58±0.02 was consistent with experimental determinations and theoretical predictions. However, the two-scale-factor universality ratio X, now consistent among experiments and theories with a value between 0.019 and 0.020, was violated in this system when using a previously published value for the correlation length.
Dynamical effects in line shapes for coupled chromophores: Time-averaging approximation
Authors: Auer B. M. , Skinner J. L.
Published: September 2007

For an isolated resonance of an isolated chromophore in a condensed phase, the absorption line shape is often more sharply peaked than the distribution of transition frequencies as a result of motional narrowing. The latter arises from the time-dependent fluctuations of the transition frequencies. It is well known that one can incorporate these dynamical effects into line shape calculations within a semiclassical approach. For a system of coupled chromophores, both the transition frequencies and the interchromophore couplings fluctuate in time. In principle one can again solve this more complicated problem with a related semiclassical approach, but in practice, for large numbers of chromophores, the computational demands are prohibitive. This has led to the development of a number of approximate theoretical approaches to this problem. In this paper we develop another such approach, using a time-averaging approximation. The idea is that, for a single chromophore, a motionally narrowed line shape can be thought of as a distribution of time-averaged frequencies. This idea is developed and tested on both stochastic and more realistic models of isolated chromophores, and also on realistic models of coupled chromophores, and it is found that in all cases this approximation is quite satisfactory, without undue computational demands. This approach should find application for the vibrational spectroscopy of neat liquids, and also for proteins and other complicated multichromophore systems.
Rotating magnetic particle microrheometry in biopolymer fluid dynamics: Mucus microrheology
Authors: Besseris George J. , Yeates Donovan B.
Published: September 2007

The polymer properties of canine mucus were investigated through the method of rotating magnetic particle microrheometry. Mucus is visualized as a physically entangled biopolymer of low polydispersity in a water-based solution. Mucus was modeled according to the constitutive law of a Doi-Edwards fluid. The magnetic-particle equation of rotational motion is analytically solved in the linear viscoelastic limit rendering theoretical flow profiles which are used to fit the experimental trace signals of the particle remanent-magnetic-field decay. The zero-shear-rate viscosity was found to be 18000P and the relaxation time at about 42s. The molecular weight between entanglements for mucins was estimated at 1.7MDa rendering an estimation of about seven physical cross-links per molecule. Rheological investigations were extended also to diluted and concentrated rations of the normal mucus simulating the conditions found in more physiological extremes.
Intraband spectroscopy and band offsets of colloidal II-VI core/shell structures
Authors: Pandey Anshu , Guyot-Sionnest Philippe
Published: September 2007

The interband and intraband spectra of colloidal II-VI CdS and CdSe quantum dot cores and CdS∕ZnSe, CdS∕CdSe, CdSe∕CdS, and CdSe∕ZnSe core/shell systems are reported. Infrared absorption peaks between 0.5 and 0.2eV are observed. The slope of the intraband energy versus the first interband absorption feature is characteristic of the relative band alignments of the materials constituting the core and the shell and it is analyzed within an effective mass model. The analysis provides a new estimate of the band gap of zinc blende CdSe as well as the band offsets in zinc blende and wurtzite CdSe, CdS, and ZnSe.
Influence of dye molecules on the birefringence of liquid crystal mixtures at near infrared frequencies
Authors: Simpson S. H. , Richardson R. M. , Hanna S.
Published: September 2007

The optical properties of nematic liquid crystals have been extensively exploited in the production of devices working in the visible range of the spectrum. These same properties can be employed to make devices that function in the near infrared as required for telecommunications applications. However, it is generally observed that the birefringence of liquid crystal mixtures decreases with increasing wavelength, making it important to identify new materials, optimized for use in the near infrared region. One route to high birefringence is to operate close to an absorption band edge, which in the present context implies choosing highly conjugated materials which are potentially colored and, thus, not suited to traditional display applications. In this paper we explore the usefulness of dye molecules as birefringence enhancers in mixtures with conventional nematic liquid crystals. The optical properties, in particular, the absorption edge, polarizability, and birefringence, of families of known dyes are calculated at optical (589nm) and infrared (1550nm) wavelengths, using electronic density functional theory. We demonstrate the expected correlation between the proximity of the absorption edge and the magnitude of the birefringence, and estimate the birefringence enhancement occurring when each dye is incorporated in a guest-host system.
Cumulant reconstruction of the three-electron reduced density matrix in the anti-Hermitian contracted Schrödinger equation
Authors: DePrince A. Eugene , Mazziotti David A.
Published: September 2007

Differing perspectives on the accuracy of three-electron reduced-density-matrix (3-RDM) reconstruction in nonminimal basis sets exist in the literature. This paper demonstrates the accuracy of cumulant-based reconstructions, developed by Valdemoro (V) [F. Colmenero et al., Phys. Rev. A 47, 971 (1993)], Nakatsuji and Yasuda (NY) [Phys. Rev. Lett. 76, 1039 (1996)], Mazziotti (M) [Phys. Rev. A 60, 3618 (1999)], and Valdemoro–Tel–Pérez–Romero (VTP) [Many-electron Densities and Density Matrices, edited by J. Cioslowski (Kluwer, Boston, 2000)]. Computationally, we extend previous investigations to study a variety of molecules, including LiH, HF, NH3, H2O, and N2, in Slater-type, double-zeta, and polarized double-zeta basis sets at both equilibrium and nonequilibrium geometries. The reconstructed 3-RDMs, compared with 3-RDMs from full configuration interaction, demonstrate in nonminimal basis sets the accuracy of the first-order expansion (V) as well as the important role of the second-order corrections (NY, M, and VTP). Calculations at nonequilibrium geometries further show that cumulant functionals can reconstruct the 3-RDM from a multireferenced 2-RDM with reasonable accuracy, which is relevant to recent multireferenced formulations of the anti-Hermitian contracted Schrödinger equation (ACSE) and canonical diagonalization. Theoretically, we perform a detailed perturbative analysis of the M functional to identify its second-order components. With these second-order components we connect the M, NY, and VTP reconstructions for the first time by deriving both the NY and VTP functionals from the M functional. Finally, these 3-RDM reconstructions are employed within the ACSE [D. Mazziotti, Phys. Rev. Lett. 97, 143002 (2006)] to compute ground-state energies which are compared with the energies from the contracted Schrödinger equation and several wave function methods.
Ab initio embedded cluster study of the excitation spectrum and Stokes shifts of Bi3+-doped Y2O3
Authors: Réal Florent , Vallet Valérie , Flament Jean-Pierre , Schamps Joël
Published: September 2007

The ab initio embedded cluster method coupled with correlated spin-orbit calculations has been used to interpret the excitation spectrum of a Bi3+-doped yttria crystal. Our results indicate that the Bi3+ impurity can absorb light over a wider energy range in the C2 site than in the S6 site. Even if the computed absorption energies seem to be about 0.4eV too high with respect to the experimental peaks for both sites, it is noteworthy that the embedded cluster model renders 93% of the large crystal redshift, about 6eV. The determination of the geometry relaxation of the first shell of oxygen neighbors upon electronic excitation shows that the Stokes shift is smaller in the S6 site than in the C2 site. Combining all these results confirms the assignment of the violet emission to the S6 site and that of the green emission to the C2 site, as proposed by Boulon [J. Phys. (Paris) 32, 333 (1971)]. In addition, the nature of the metastable states which lie below the emitting ones and are responsible for the temperature dependence of the fluorescence lifetimes is discussed.
First-principles calculations of hydrogen diffusion on rutile TiO2(110) surfaces
Authors: Kajita Seiji , Minato Taketoshi , Kato Hiroyuki S. , Kawai Maki , Nakayama Takashi
Published: September 2007

Density functional calculations are performed to study the H-atom diffusion on titanium dioxide (110) surface in the cases of water-molecule dissociation and splitting of the adjacent hydroxyl OH pair. It is shown that, when a water molecule is adsorbed at a surface oxygen-vacancy site, a fragment H atom of the water molecule tends to diffuse toward the nearest-neighboring bridging-oxygen sites by using a straight-line or relay-point path. As the result, a pair of surface hydroxyl OH is formed on the same oxygen row. In a thermal process, on the other hand, such OH pair favorably splits only by using a relay-point path, i.e., by transferring one H atom from a bridging-oxygen site to a next-neighboring one along the same oxygen row by way of another in-plane oxygen site. We found that the latter splitting reaction is activated around room temperature.
Effect of burst ions on the excitation of ion-acoustic solitons in a drifting plasma
Authors: Xiao De-long , Ma J. X. , Li Yi-ren , Li Yang-fang , Yu M. Y.
Published: September 2007

The characteristics of fast burst ions and ion-acoustic solitons in a drifting plasma, as well as their interaction, are investigated experimentally. It is shown that the soliton evolves from the normal fast ion-beam mode excited locally in the presheath region of an excitation grid. The burst ions are created by applying a positive ramp voltage to the grid. Adjusting the rise time and amplitude of the ramp signal can lead to significant changes in the average speed of the burst ions, resulting in strong interaction (inverse Landau damping) between the burst ions and the soliton. The resonance conditions for solitons propagating along and against the plasma flow are shown to be different, since the soliton speed depends on the propagation direction.
Gain of a long-pulse KrCl excimer laser
Authors: Casper L. C. , Bastiaens H. M. J. , Peters P. J. M. , Boller K.-J. , Hofstra R. M.
Published: September 2007

The net gain is measured for various pump power densities in a discharge-pumped long-pulse KrCl laser (222 nm). For this, a three-electrode laser system is employed, which is operated at high gas pressures of around 3.5 bar with pump power densities of 300−650 kW cm−3. The net gain reaches a maximum of 1.6% cm−1. Using the experimental results, values are calculated for the small-signal gain, the losses, and the saturation intensity. The losses and the saturation intensity are found to increase much stronger with the excitation rate than the small-signal gain. Fluorescence measurements at 222 nm show that the upper laser level depopulates faster when the power density increases. The influence of collisional quenching and the halogen consumption on the laser performance are discussed.
Origin of photoresponse in heterophase ferroelectric Pt∕Pb(ZrTi)O3∕Ir capacitors
Authors: Delimova L. A. , Yuferev V. S. , Grekhov I. V. , Afanasjev P. V. , Kramar G. P. , Petrov A. A. , Afanasjev V. P.
Published: September 2007

A steady-state short-circuit photocurrent is measured in polycrystalline Pb(ZrTi)O3(PZT) films with Pb excess. It is found that although the photocurrent is directed opposite to ferroelectric polarization, it is not the depolarization current of the ferroelectric. To explain this, the authors assume that Pt/PZT/Ir capacitor consists of PZT grains and semiconductor PbO phase segregated on PZT grain boundaries during the PZT formation. In such medium, the current across the film flows through channels formed by PbO phase, where an electric field is generated by uncompensated polarization charge of PZT grains. Using this concept, the authors described the experimental dependence of photocurrent on polarization.
Path ensembles and path sampling in nonequilibrium stochastic systems
Authors: Harland Ben , Sun Sean X.
Published: September 2007

Markovian models based on the stochastic master equation are often encountered in single molecule dynamics, reaction networks, and nonequilibrium problems in chemistry, physics, and biology. An efficient and convenient method to simulate these systems is the kinetic Monte Carlo algorithm which generates continuous-time stochastic trajectories. We discuss an alternative simulation method based on sampling of stochastic paths. Utilizing known probabilities of stochastic paths, it is possible to apply Metropolis Monte Carlo in path space to generate a desired ensemble of stochastic paths. The method is a generalization of the path sampling idea to stochastic dynamics, and is especially suited for the analysis of rare paths which are not often produced in the standard kinetic Monte Carlo procedure. Two generic examples are presented to illustrate the methodology.
Characterization of LaVOx thin films by photoemission spectroscopy
Authors: Wadati H. , Hotta Y. , Takizawa M. , Fujimori A. , Susaki T. , Hwang H. Y.
Published: September 2007

We characterized the electronic structure of LaVOx thin films by x-ray photoemission spectroscopy. We could distinguish the photoemission spectra among the x=3 phase (LaV3+O3), the x=4 phase (LaV5+O4), and the mixed phase regime. In the mixed phase, the surface was found to be mostly LaVO4. We could observe the evolution of the electronic structure in the growth phase diagram by systematically measuring core-level photoemission spectra. The V4+ component present in the x=3 phase may be due to charge transfer to avoid the polar catastrophe of the polar LaVO3 thin-film surface and/or to the oxidation of the surface.
On the physics of moisture-induced cracking in metal-glass (copper-silica) interfaces
Authors: Card J. C. , Cannon R. M. , Saiz E. , Tomsia A. P. , Ritchie R. O.
Published: September 2007

Environmentally dependent subcritical crack growth, or stress-corrosion cracking, along ceramic-metal interfaces is studied for the silica glass-copper system. Tests were conducted in various gaseous and liquid environments in order to determine their relative effects on stress-corrosion cracking and to gain some insight into the mechanisms that control interfacial crack growth. In agreement with previous studies, interfacial crack-growth rates were found to vary by orders of magnitude depending on the moisture content in gaseous environments. Water and several organic liquids, namely n-butanol, methanol, and N-methylformamide, were also found to promote stress-corrosion cracking. Specifically, crack-growth behavior was found to be largely dependent on the molecular structure of the test environment. Crack growth at high velocities was limited by either transport of the reactive species to the crack tip or by viscous drag contributions. Results are discussed in the context of the current mechanistic models proposed for the stress corrosion of bulk silica.
Magnetophoresis of microspheres covered by magnetic nanoparticles
Authors: Zhao X. , Helseth L. E.
Published: September 2007

We fabricated magnetic microspheres by letting magnetic nanoparticles self-assemble on the surface of surfactant-covered oil droplets. The magnetophoretic velocities of individual magnetic microspheres were measured in a defined magnetic field gradient, and their susceptibilities were found to scale with the surface area.
Ion beam mixing by focused ion beam
Authors: Barna Árpád , Kotis László , Lábár János L. , Osváth Zoltán , Tóth Attila L. , Menyhárd Miklós , Zalar Anton , Panjan Peter
Published: September 2007

Si amorphous (41 nm)/Cr polycrystalline (46 nm) multilayer structure was irradiated by 30 keV Ga+ ions with fluences in the range of 25−820 ions∕nm2 using a focused ion beam. The effect of irradiation on the concentration distribution was studied by Auger electron spectroscopy depth profiling, cross-sectional transmission electron microscopy, and atomic force microscopy. The ion irradiation did not result in roughening on the free surface. On the other hand, the Ga+ irradiation produced a strongly mixed region around the first Si/Cr interface. The thickness of mixed region depends on the Ga+ fluence and it is joined to the pure Cr matrix with an unusual sharp interface. With increasing fluence the width of the mixed region increases but the interface between the mixed layer and pure Cr remains sharp. TRIDYN simulation failed to reproduce this behavior. Assuming that the Ga+ irradiation induces asymmetric mixing, that is during the mixing process the Cr can enter the Si layer, but the Si cannot enter the Cr layer, the experimental findings can qualitatively be explained.
Electronic properties of ultrathin (HfO2)x(SiO2)1−x dielectrics on Si (100)
Authors: Jin Hua , Oh Suhk Kun , Cho Young Joon , Kang Hee Jae , Tougaard Sven
Published: September 2007

Electronic properties of 6 nm (HfO2)x(SiO2)1−x dielectric thin films, deposited on Si(100) by atomic layer deposition with x equal to 0.75 and 0.25, were studied by means of x-ray photoelectron spectroscopy (XPS) and reflection electron energy loss spectroscopy (REELS). XPS measurements confirmed the formation of a silicate structure, and showed shifts of the Hf 4f and Si 2p peaks with varying Hf concentrations; these shifts are believed to be caused by changes in the amount of charge transfer and by the substitution of Si by Hf as second-nearest-neighbors. The band gap, Eg, was estimated from REELS. It increased from 5.52 eV for Hf silicate with a Hf/Si ratio of 3:1 to 6.61 eV for Hf silicate with a Hf/Si ratio of 1:3. For Hf silicate dielectrics, the band gap is mainly determined by Hf 5d conduction-band and O 2p valence-band states, and the increase with increasing Si concentration is due to mixing with the Si 3s electrons. By quantitative analysis of REELS spectra, the dielectric functions of the thin films were also determined and expressed as a sum of oscillators. The position, strength, and width of the oscillators were determined quantitatively for the different dielectric films.
Annealing of nanostructured silver films produced by supersonic deposition of nanoparticles
Authors: Huang Chong , Becker Michael F. , Keto John W. , Kovar Desiderio
Published: September 2007

Silver nanostructured films were produced by supersonic jet deposition of nanoparticles generated by laser ablation of microparticle aerosols (LAMA). The nano- and microscale morphologies of films were investigated and the electrical conductivities of films were measured in the as-deposited state and after annealing at temperatures from 100 to 400 °C. Scanning electron microscopy (SEM) and x-ray diffraction (XRD) were used to analyze the film morphologies and the grain sizes within the films. These analyses showed that the as-deposited grain size was 14–24 nm and the relative film densities ranged from 60% to 80%, depending on processing conditions. As a result of the extremely fine grain size and the lack of organics present on the surface of the grains within the films, the annealing temperatures required to achieve reasonable conductivities (20%−50% of bulk) were reduced dramatically compared to films produced from suspensions. For example, the grain size in the films began to increase at annealing temperatures of as low as 100 °C, and the corresponding conductivity was greater than 25% of the conductivity of bulk silver. Increases in annealing temperature resulted in greater coarsening and further increases in conductivity. Calculations showed that increases in conductivity during annealing at temperatures less than 400 °C resulted primarily from grain size coarsening rather than densification.
Ionic winds for locally enhanced cooling
Authors: Go David B. , Garimella Suresh V. , Fisher Timothy S. , Mongia Rajiv K.
Published: September 2007

Ionic wind engines can be integrated onto surfaces to provide enhanced local cooling. Air ions generated by field-emitted electrons or a corona discharge are pulled by an electric field and exchange momentum with neutral air molecules, causing air flow. In the presence of a bulk flow, ionic winds distort the boundary layer, increasing heat transfer from the wall. Experiments demonstrate the ability of ionic winds to decrease the wall temperature substantially in the presence of a bulk flow over a flat plate, corresponding to local enhancement of the heat transfer coefficient by more than twofold. Multiphysics simulations of the corona and flow describe the ability of the ionic wind to distort a bulk flow boundary layer and confirm the experimentally observed heat transfer enhancement trends.
Movement of oxygen vacancies in oxide film during annealing observed by an optical reflectivity difference technique
Authors: Wang Xu , Jin Kuijuan , Lu Hubin , Fei Yiyan , Zhu Xiangdong , Yang Guozhen
Published: September 2007

The monolayer growth of Nb-doped SrTiO3 on SrTiO3 (100) substrate is prepared by a pulsed laser deposition method. The growth and annealing of the film in vacuum and in oxygen ambient are monitored in real time by an oblique-incidence optical reflectivity difference (OIRD) technique and reflection high-energy electron diffraction technique. The films annealed in different ambient result in different optical annealing signals. From the comparison of experimental OIRD signals with the simulation of OIRD signals, we prove that the optical technique can easily tell whether the oxygen vacancies are moving into or moving out of the film during the annealing. The optical signals are found to be composed of contributions from step edges and terraces.
Physics of zinc vaporization and plasma absorption during CO2 laser welding
Authors: Dasgupta A. K. , Mazumder J. , Li P.
Published: September 2007

A number of mathematical models have been developed earlier for single-material laser welding processes considering one-, two-, and three-dimensional heat and mass transfers. However, modeling of laser welding of materials with multiple compositions has been a difficult problem. This paper addresses a specific case of this problem where CO2 laser welding of zinc-coated steel, commonly used in automobile body manufacturing, is mathematically modeled. The physics of a low boiling point material, zinc, is combined with a single-material (steel) welding model, considering multiple physical phenomena such as keyhole formation, capillary and thermocapillary forces, recoil and vapor pressures, etc. The physics of laser beam–plasma interaction is modeled to understand the effect on the quality of laser processing. Also, an adaptive meshing scheme is incorporated in the model for improving the overall computational efficiency. The model, whose results are found to be in close agreement with the experimental observations, can be easily extended for studying zinc-coated steel welding using other high power, continuous wave lasers such as Nd:YAG and Yb:YAG.
Formation of linear InAs quantum dot arrays on InGaAsP∕InP (100) by self-organized anisotropic strain engineering and their optical properties
Authors: Sritirawisarn N. , van Otten F. W. M. , Eijkemans T. J. , Nötzel R.
Published: September 2007

The formation of linear InAs quantum dot (QD) arrays based on self-organized anisotropic strain engineering of an InGaAsP∕InP (100) superlattice (SL) template in chemical beam epitaxy is demonstrated, and the optimized growth window is determined. InAs QD formation, thin InGaAsP capping, annealing, InGaAsP overgrowth, and stacking in SL template formation produce wirelike InAs structures along [001] due to anisotropic surface migration and lateral and vertical strain correlations. InAs QD ordering is governed by the corresponding lateral strain field modulation on the SL template surface. Careful optimization of InGaAsP cap layer thickness, annealing temperature, InAs amount and growth rate, and number of SL periods results in straight and well-separated InAs QD arrays. The InAs QD arrays exhibit excellent photoluminescence (PL) emission up to room temperature which is tuned into the 1.55μm telecommunications wavelength region through the insertion of ultrathin GaAs interlayers. Temperature dependent PL measurements and the linear polarization behavior indicate lateral electronic coupling of the QDs in the arrays.
Influence of the annealing temperature on the photoluminescence of Er-doped SiO thin films
Authors: Wora Adeola G. , Rinnert H. , Miska P. , Vergnat M.
Published: September 2007

Er-doped amorphous silicon suboxide thin films were prepared by the coevaporation method. The Er concentration was varied from 0.4to6at.% and the samples were annealed at different temperatures up to 900°C. The samples exhibit a broad photoluminescence band in the visible range. Both energy and intensity of this band were dependent on the annealing temperature. For as-deposited films and samples annealed below 500°C, this band was assigned to defects in the oxide films. For higher annealing temperatures, this photoluminescence band shifted to higher wavelengths and was correlated to the appearance of amorphous silicon clusters. Two narrow bands in the near-infrared range at 0.98 and 1.54μm were also observed for the annealed samples. The intensity of these Er-related luminescence was maximal for an annealing temperature equal to around 700°C. The effective absorption cross section of Er was dependent on the annealing temperature and was equal to 6.6×10−16cm2 for the sample annealed at 700°C. The strong Er-related photoluminescence is discussed in terms of a coupling phenomenon between Er3+ ions and spatially confined amorphous silicon clusters which act as sensitizers. The existence of a low annealing temperature to obtain the best Er-related photoluminescence is also discussed.
Comparative experimental analysis of the a-C:H deposition processes using CH4 and C2H2 as precursors
Authors: Peter S. , Graupner K. , Grambole D. , Richter F.
Published: September 2007

The plasma enhanced chemical vapor deposition of a-C:H films using methane and acetylene as precursors was studied. Noninvasive in situ techniques were used to analyze the plasma processes with respect to the self-bias voltage, the displacement currents to the grounded electrode, the neutral gas composition, the optical sheath thickness as well as current and energy of the ions hitting the powered electrode. The a-C:H films were characterized for their deposition rate, surface roughness, hardness, mass density, and hydrogen content. Ion mean free paths, suitable for low-pressure rf sheaths, have been quantified for both precursors. The film with the highest hardness of 25GPa was formed in the C2H2 discharge when the mean energy per deposited carbon atom was approximately 50eV. The hardness obtained with the CH4 discharge was lower at 17GPa and less sensitive to changes in the process parameters. It was found that the creation of hard (hardness >15GPa) a-C:H films from both precursors is possible if the mean energy per deposited carbon atom exceeds only ∼15eV. Further film characteristics such as surface roughness and hydrogen content show the interplay of ion flux and deposition from radicals to form the a-C:H structure and properties.
The behavior of the I-V-T characteristics of inhomogeneous (Ni∕Au)–Al0.3Ga0.7N∕AlN∕GaN heterostructures at high temperatures
Authors: Tekeli Z. , Altındal Ş. , Çakmak M. , Özçelik S. , Çalışkan D. , Özbay E.
Published: September 2007

We investigated the behavior of the forward bias current-voltage-temperature (I-V-T) characteristics of inhomogeneous (Ni∕Au)–Al0.3Ga0.7N∕AlN∕GaN heterostructures in the temperature range of 295–415K. The experimental results show that all forward bias semilogarithmic I-V curves for the different temperatures have a nearly common cross point at a certain bias voltage, even with finite series resistance. At this cross point, the sample current is temperature independent. We also found that the values of series resistance (Rs) that were obtained from Cheung’s method are strongly dependent on temperature and the values abnormally increased with increasing temperature. Moreover, the ideality factor (n), zero-bias barrier height (ΦB0) obtained from I-V curves, and Rs were found to be strongly temperature dependent and while ΦB0 increases, n decreases with increasing temperature. Such behavior of ΦB0 and n is attributed to Schottky barrier inhomogeneities by assuming a Gaussian distribution (GD) of the barrier heights (BHs) at the metal∕semiconductor interface. We attempted to draw a ΦB0 versus q∕2kT plot in order to obtain evidence of the GD of BHs, and the values of Φ¯B0=1.63eV and σ0=0.217V for the mean barrier height and standard deviation at a zero bias, respectively, were obtained from this plot. Therefore, a modified ln(I0∕T2)−q2σ02∕2(kT)2 versus q∕kT plot gives ΦB0 and Richardson constant A* as 1.64eV and 34.25A∕cm2K2, respectively, without using the temperature coefficient of the barrier height. The Richardson constant value of 34.25A∕cm2K2 is very close to the theoretical value of 33.74A∕cm2K2 for undoped Al0,3Ga0,7N. Therefore, it has been concluded that the temperature dependence of the forward I-V characteristics of the (Ni∕Au)–Al0.3Ga0.7∕AlN∕GaN heterostructures can be successfully explained based on the thermionic emission mechanism with the GD of BHs.
Electrical properties of Al2O3∕4H-SiC structures grown by atomic layer chemical vapor deposition
Authors: Avice Marc , Grossner Ulrike , Pintilie Ioana , Svensson Bengt G. , Servidori Marco , Nipoti Roberta , Nilsen Ola , Fjellvåg Helmer
Published: September 2007

Al 2 O 3 films have been deposited on n-type and p-type 4H-SiC by atomic layer chemical vapor deposition using trimethylaluminum as a precursor for aluminum and both H2O and O3 as an oxidant. After oxide deposition, annealing at different temperatures (800, 900, 1000°C) in argon atmosphere for different durations (1, 2, 3h) was performed. Bulk and interface properties of the oxide films were studied by capacitance-voltage, current-voltage, deep level transient spectroscopy, and thermally dielectric relaxation current (TDRC) measurements. The results reveal a decreasing flatband voltage with increasing annealing time, suggesting decrease of oxide charges and deep interface traps. After 3h annealing at 1000°C of the n-type samples, the flatband voltage is reduced to 6V compared to a value in excess of 40V for as-deposited samples. The TDRC measurements on annealed Al2O3∕SiC (n-type) capacitors showed substantially different spectra relative to conventional SiO2∕4H-SiC control samples; in the former ones no signal was recorded at temperatures less than 100K, demonstrating a low density of shallow electron traps below the conduction band edge of 4H-SiC and hence a prospect of obtaining a high electron channel mobility in 4H-SiC metal-oxide-semiconductor field-effect devices with Al2O3 as gate dielectric.
Carrier capture at the SiO2–Si interface: A physical model
Authors: Wang Y. , Cheung K. P.
Published: September 2007

The carrier (electron and hole) capture cross section of defects at the SiO2–Si interface is measured using trap-fill time in gigahertz charge-pumping experiment. The ability to use a highly asymmetric rectangular wave with very fast rise and fall times in our charge-pumping experiment greatly simplified data interpretation. Trap-fill times of less than or equal to 0.7ns are found for both electrons and holes. The corresponding capture cross section of 10−15cm2 or larger for both electron and hole cannot be explained by the usual multiphonon emission mechanism. A modified cascade capture model involving the strained Si–Si bonds around the Pb center is proposed to explain the large capture cross sections.
Wide-field Fourier transform spectral imaging
Authors: Atlan Michael , Gross Michel
Published: September 2007

The authors report experimental results of parallel measurement of spectral components of the light. The temporal fluctuations of an optical field mixed with a separate reference are recorded with a high throughput complementary metal oxide semiconductor camera (1Mpixel at 2kHz frame rate). A numerical Fourier transform of the time-domain recording enables wide-field coherent spectral imaging. Qualitative comparisons with frequency-domain wide-field laser Doppler imaging are provided.
Degradation and segregation: Thermal stability and highly ordered epitaxial thin films of large aromatic molecules
Authors: Wagner C. , Franke R. , Dienel T. , Forker R. , Jacob R. , Fritz T.
Published: September 2007

The authors present an investigation of highly ordered epitaxial thin films of large organic molecules, namely quaterrylene derivatives. Even though the raw material quaterrylene tetracarboxylic diimide (QTCDI) starts to decompose below its sublimation temperature, epitaxial monolayers could be grown. By modifying the preparation procedure, highly ordered domains of quaterrylene dicarboxylic monoimide, being the main decomposition product of QTCDI, could also be produced. Although both molecules bear strong similarities, they grow in a dissimilar fashion influenced by their substituents and molecular dipole and quadrupole moments. The structures are characterized by scanning tunneling microscopy and, in case of QTCDI, low energy electron diffraction.
Limit relation for quantum entropy and channel capacity per unit cost
Authors: Csiszár Imre , Hiai Fumio , Petz Dénes
Published: September 2007

In a thermodynamic model, Diósi et al. [Int. J. Quantum Inf. 4, 99–104 (2006)] arrived at a conjecture stating that certain differences of von Neumann entropies converge to relative entropy as system size goes to infinity. The conjecture is proven in this paper for density matrices. The analytic proof uses the quantum law of large numbers and the inequality between the Belavkin-Staszewski and Umegaki relative entropies. Moreover, the concept of channel capacity per unit cost is introduced for classical-quantum channels. For channels with binary input alphabet, this capacity is shown to equal the relative entropy. The result provides a second proof of the conjecture and a new interpretation. Both approaches lead to generalizations of the conjecture.