bluuebunny/crossref_metadata_2025 huggingface dataset:
Density‐driven current between reed belts and open water in a shallow lake
Authors: Lövstedt Charlotta Borell , Bengtsson Lars
Published: October 2008

Differential heating of surface waters of lakes caused by shading from vegetation can induce convection, which could be significant for the ecosystem because the chemistry and biology can differ in the reed belt zone compared to the open lake. This process has been given little attention in the literature. In the present study surface currents just outside a reed belt as well as radiation and water temperature profiles within and outside the vegetated belt were measured in a shallow lake in southern Sweden to determine the lateral flux between the two zones. The measured surface current directed toward the vegetation was about 1 cm/s. Net radiation at the water surface was 85% lower within the vegetation than in the open lake. The water surface temperature during sunny days was on average 0.5°C warmer outside the reed belt, and at maximum 1°C warmer. The lateral heat flux from the open water into the water within the reeds was estimated to ∼200 W/m2 during midday on sunny days, which corresponded to a calculated maximum current of 1.5 cm/s. This current between the open water and the reed vegetation is presumably the main water exchange process between the two zones during sunny summer days with low wind speeds.
Influence of pressure on the stability of the magnetically ordered states in alloys of the system Mn2−xFexAs0.5P0.5
Authors: Val’kov V. I. , Varyukhin D. V. , Golovchan A. V. , Gribanov I. F. , Sivachenko A. P. , Kamenev V. I. , Todris B. M.
Published: October 2008

The phase diagram of the system Mn2−xFexAs0.5P0.5 under pressure is investigated experimentally and theoretically. It is found that the spontaneous and magnetic-field-induced low-temperature phase in the region 0.5⩽x<0.8 does not suffer significant changes under hydrostatic pressure up to 2kbar. Based on ab initio calculations of the electronic structure of the alloys Mn1.5Fe0.5As0.5P0.5, it is established that the degree of filling of the 3d electron band changes upon ferromagnetic polarization and compression of the crystal lattice. A model is proposed by which one can take into account the main features of the antiferromagnetic and canted ferromagnetic structures. The parameters of the model are the degree of filling of the d band, the nonmagnetic electronic density of states, and the intra-atomic exchange integral. Their values are estimated directly from the data of the first-principles electronic structure calculations. It is shown in the framework of the model that the stability of the magnetic characteristics of the canted ferromagnetic structure with respect to pressure is due to an increase of the number of electrons in the magnetically active band upon a decrease of the unit cell volume.
Modeling the hysteretic properties of (LaSr)MnO3 nanostructure samples
Authors: Krivoruchko V. N. , Marchenko M. A.
Published: October 2008

A modeling of the hysteretic properties of an ensemble of interacting La0.7Sr0.3MnO3 (LSMO) magnetic nanoparticles is carried out. The experimental data are processed in the framework of the Preisach model. In this approach the system is modeled as an ensemble of two-level elements (hysterons) characterized by a magnetic moment and effective fields hsw and hint, which specify the “energy profile” of the hysteron in configuration space. Each particular magnetic sample is distinguished by a unique hysteron distribution described by the Preisach function P(hsw,hint). A feature of the theory is that it incorporates effects due to thermal fluctuations and superparamagnetic behavior of the magnetic nanoparticles at temperatures higher than the blocking temperature. The theory faithfully reproduces the magnetostatic properties of the system of LSMO nanoparticles in the whole investigated range of temperatures T=130–300K and magnetic fields H=0–10kOe.
Magnetoresistive properties of p-La0.78Mn0.99O3.5 and p-La0.80Mn1.04O3.5 nanocomposites at temperatures 4.2–300K and magnetic fields to 140kOe
Authors: Dmitriev V. M. , Khlybov E. P. , Kondrashev D. S. , Shevchenko A. D. , Uvarov V. N.
Published: October 2008

A study is made of the magnetoresistive properties and structure of nanocomposite manganites La0.78Mn0.99O3.5 and La0.80Mn1.04O3.5 formed at different temperatures and durations of the high-pressure sintering. The structural features of the samples are established by x-ray structure and x-ray phase analyses and scanning atomic-force microscopy. The temperature and field dependences of the resistivity of nanostructure LaMnO manganites are obtained. These and other experimental results indicate that the magnetoresistive properties of nanomanganites are influenced materially by phase separation and frozen spin clusters; since those properties depend strongly on the preparation technology, it should be possible to create nanomaterials with specified properties.
Study of the structural nonuniformity and low-temperature micromechanical properties of ultrafine-grain aluminum
Authors: Éstrin Yu. Z. , Fomenko L. S. , Lubenets S. V. , Shumilin S. É. , Pustovalov V. V.
Published: October 2008

The optimal conditions for measuring the microhardness of aluminum, concerning the surface preparation of the samples (electropolishing) and the load on an indenter (at least 0.5N), are determined. The degree of structural uniformity of aluminum after deformation by equal-channel angular pressing (ECAP) is studied by the microindentation method. It is found that the microhardness of an extruded blank varies over the cross-section, and it reaches its maximum value in the central part. The nonuniformity decreases as the number of passes increases. The main structural changes giving rise to hardening occur during the first pass. The temperature dependence of the microhardness in the interval 77–295K intensifies as the number of ECAP passes increases. The Hall-Petch law describes the hardening of aluminum as result of grain-size reduction during ECAP well, and the Hall-Petch coefficient increases as temperature decreases. For ultrafine-grain aluminum the microhardness and yield stress with strain ε=0.076 are related as HV=(3.7–4.2)σ0.076.
Fast simulation of phase-change processes in chalcogenide alloys using a Gillespie-type cellular automata approach
Authors: Ashwin Peter , Patnaik B. S. V. , Wright C. David
Published: October 2008

A stochastic cellular automata simulator capable of spatiotemporal modeling of the crystallization and amorphization behavior of phase-change materials during the complex annealing cycles used in optical and electrical memory applications is presented. This is based on consideration of bulk and surface energies to generate rates of growth and decay of crystallites built up from “monomers” that may themselves be quite complex molecules. The approach uses a stochastic Gillespie-type time-stepping algorithm to deal with events that may occur on a very wide range of time scales. The simulations are performed at molecular length scale and using an approximation of local free energy changes that depend only on immediate neighbors. The approach is potentially capable of spanning the length scales between ab initio atomistic modeling methods, such as density functional theory, and bulk-scale methods, such the Johnshon–Mehl–Avrami–Kolmogorov formalism. As an example the model is used to predict the crystallization behavior in the chalcogenide Ge2Sb2Te5 alloy commonly used in phase-change memory devices. The simulations include annealing cycles with nontrivial spatial and temporal variations in temperature, with good agreement to experimental incubation times at low temperatures while modeling nontrivial crystal size distributions and melting dynamics at higher temperatures.
Influence of substrate morphology on the growth of gold nanoparticles
Authors: Grochola Gregory , Snook Ian K. , Russo Salvy P.
Published: October 2008

We have simulated the vacuum deposition and subsequent growth of gold nanoparticles on various substrates in order to explore the effects that substrate morphology has on the resultant morphology of gold nanoparticles. The substrates and conditions explored included, the three low index faces, namely, (111), (100), and (110) for both fcc and bcc crystalline substrate structures, including various substrate lattice constants and temperatures. Firstly, we cataloged the major nanoparticle morphologies produced overall. While some substrates were found to produce a mixture of the main nanoparticle morphologies we were successful in identifying certain substrates and temperature conditions for which only Ih, Dh, or certain fcc crystalline nanoparticles can be grown almost exclusively. The substrate characteristics, temperature conditions, and governing growth dynamics are analyzed. We shed light on the balance between substrate influences and vacuum growth tendencies. From observations we can speculate that a substrate alters both the free energy stability of gold nanoparticles and/or the free energy barriers to transformation between certain morphologies. As such we find that substrates are an effective tool in templating the selective growth of desired nanoparticles or surface nanostructures.
Theoretical rotation-torsion energies of HSOH
Authors: Ovsyannikov Roman I. , Melnikov Vladlen V. , Thiel Walter , Jensen Per , Baum Oliver , Giesen Thomas F. , Yurchenko Sergei N.
Published: October 2008

The rotation-torsion energies in the electronic ground state of HSOH are obtained in variational calculations based on a newly computed ab initio CCSD(T)/aug-cc-pV(Q+d)Z potential energy surface. Using the concept of the reaction path Hamiltonian, as implemented in the program TROVE (theoretical rovibrational energies), the rotation-vibration Hamiltonian is expanded around geometries on the torsional minimum energy path of HSOH. The calculated values of the torsional splittings are in excellent agreement with experiment; the root-mean-square (rms) deviation is 0.0002 cm−1 for all experimentally derived splittings (with J≤40 and Ka≤4). The model provides reliable predictions for splittings not yet observed. The available experimentally derived torsion-rotation term values (with J≤40 and Ka≤4) are reproduced ab initio with an rms deviation of 1.2 cm−1 (0.7 cm−1 for J≤20), which is improved to 1.0 cm−1 (0.07 cm−1 for J≤20) in an empirical adjustment of the bond lengths at the planar trans configuration. The theoretical torsional splittings of HSOH are analyzed in terms of an existing semiempirical model for the rotation-torsion motion. The analysis explains the irregular variation of the torsional splittings with Ka that has been observed experimentally.
On the consistency, extremal, and global properties of counterdiabatic fields
Authors: Demirplak Mustafa , Rice Stuart A.
Published: October 2008

The control of population transfer can be affected by the adiabatic evolution of a system under the influence of an applied field. If the field is too rapidly varying or too weak, the conditions for adiabatic transfer are not satisfactorily met. We report the results of an analysis of properties of counterdiabatic fields (CDFs) that restore the adiabatic dynamics of a system by suppressing diabatic effects as they are generated. We observe that a CDF is not unique and find the one that has minimum intensity, and we provide natural upper and lower bounds to the integrated intensity of a CDF in terms of integrals of the eigenvalues of the system Hamiltonian. For Hamiltonians that are separable with respect to their parameters, we prove that the time integral of an associated CDF is path independent. Finally we explain why and when, in the neighborhood of an avoided crossing, a CDF can be approximated by Lorentzian pulses.
Structure of cylindrical electric double layers: A systematic study by Monte Carlo simulations and density functional theory
Authors: Goel Teena , Patra Chandra N. , Ghosh Swapan K. , Mukherjee Tulsi
Published: October 2008

We present a systematic study of the structure of cylindrical double layers to envisage the distribution of small ions around a cylindrical polyion through canonical Monte Carlo simulation and density functional theory. The polyion is modeled as an infinite, rigid, and impenetrable charged cylinder surrounded by charged hard spheres of equal diameter modeled for small ions of the electrolyte. The solvent is considered as dielectric continuum. The theory is partially perturbative where the hard sphere contribution to the total excess free energy is evaluated using weighted density approximation, and the ionic interactions are calculated using quadratic Taylor expansion with respect to a uniform fluid. The system is studied over a wide range of parameters, viz., ionic concentrations, valences, and ionic sizes as well as for varying axial charge densities of the polyion. The theoretical predictions are observed to be in good agreement with that of simulation results. Some interesting phenomena relating to the width of the diffuse layer, mean electrostatic potential, and charge inversion have been observed to be dependent on different parametric conditions.
A polarizable force-field model for quantum-mechanical-molecular-mechanical Hamiltonian using expansion of point charges into orbitals
Authors: Biswas P. K. , Gogonea Valentin
Published: October 2008

We present an ab initio polarizable representation of classical molecular mechanics (MM) atoms by employing an angular momentum-based expansion scheme of the point charges into partial wave orbitals. The charge density represented by these orbitals can be fully polarized, and for hybrid quantum-mechanical-molecular-mechanical (QM/MM) calculations, mutual polarization within the QM/MM Hamiltonian can be obtained. We present the mathematical formulation and the analytical expressions for the energy and forces pertaining to the method. We further develop a variational scheme to appropriately determine the expansion coefficients and then validate the method by considering polarizations of ions by the QM system employing the hybrid GROMACS-CPMD QM/MM program. Finally, we present a simpler prescription for adding isotropic polarizability to MM atoms in a QM/MM simulation. Employing this simpler scheme, we present QM/MM energy minimization results for the classic case of a water dimer and a hydrogen sulfide dimer. Also, we present single-point QM/MM results with and without the polarization to study the change in the ionization potential of tetrahydrobiopterin (BH4) in water and the change in the interaction energy of solvated BH4 (described by MM) with the P450 heme described by QM. The model can be employed for the development of an extensive classical polarizable force-field.
Hydrodynamic type integrable equations on a segment and a half-line
Authors: Gürses Metin , Habibullin Ismagil , Zheltukhin Kostyantyn
Published: October 2008

The concept of integrable boundary conditions is applied to hydrodynamic type systems. Examples of such boundary conditions for dispersionless Toda systems are obtained. The close relation of integrable boundary conditions with integrable reductions in multifield systems is observed. The problem of consistency of boundary conditions with the Hamiltonian formulation is discussed. Examples of Hamiltonian integrable hydrodynamic type systems on a segment and a semiline are presented.
Potential conservation laws
Authors: Kunzinger Michael , Popovych Roman O.
Published: October 2008

We prove that potential conservation laws have characteristics depending only on local variables if and only if they are induced by local conservation laws. Therefore, characteristics of pure potential conservation laws have to essentially depend on potential variables. This statement provides a significant generalization of results of the recent paper by Bluman et al. [J. Math. Phys. 47, 113505 (2006)]. Moreover, we present extensions to gauged potential systems, Abelian and general coverings, and general foliated systems of differential equations. An example illustrating possible applications of these results is given. A special version of the Hadamard lemma for fiber bundles and the notions of weighted jet spaces are proposed as new tools for the investigation of potential conservation laws.
Is it possible to determine rigorous magnetic Hamiltonians in spin s=1 systems from density functional theory calculations?
Authors: Labèguerie Pierre , Boilleau Corentin , Bastardis Roland , Suaud Nicolas , Guihéry Nathalie , Malrieu Jean-Paul
Published: October 2008

The variational energies of broken-symmetry single determinants are frequently used (especially in the Kohn–Sham density functional theory) to determine the magnetic coupling between open-shell metal ions in molecular complexes or periodic lattices. Most applications extract the information from the solutions of msmax and msmin eigenvalues of Ŝz magnetic spin momentum, assuming that a mapping of these energies on the energies of an Ising Hamiltonian is grounded. This approach is unable to predict the possible importance of deviations from the simplest form of the Heisenberg Hamiltonians. For systems involving s=1 magnetic centers, it cannot provide an estimate of neither the biquadratic exchange integral nor the three-body operator interaction that has recently been proven to be of the same order of magnitude [Phys. Rev. B 70, 132412 (2007)]. The present work shows that one may use other broken-symmetry solutions of intermediate values of ms to evaluate the amplitude of these additional terms. The here-derived equations rely on the assumption that an extended Hubbard-type Hamiltonian rules the interactions between the magnetic electrons. Numerical illustrations on a model problem of two O2 molecules and a fragment of the La2NiO4 lattice are reported. The results obtained using a variable percentage of Fock exchange in the BLYP functional are compared to those provided by elaborate wave function calculations. The relevant percentage of Fock exchange is system dependent but a mean value of 30% leads to acceptable amplitudes of the effective exchange interaction.
Optimization of expanded ensemble methods
Authors: Escobedo Fernando A. , Martinez-Veracoechea Francisco J.
Published: October 2008

Expanded ensemble methods, designed to sample a range of an order parameter λ of interest, can be optimized to overcome the difficulties associated with traversing large free-energy barriers or rugged landscapes. The optimization strategy of Trebst et al. [Phys. Rev. E 70, 046701 (2004)] is based on finding suitable biasing weights for inter-λ transitions that maximize the number of round trips that the system performs between the lower and upper λ bounds. In this work, this optimized-ensemble methodology is extended by finding weights that minimize the mean round-trip time τ (between the λ end states) for a Markovian walk. Applications are presented for an atomistically detailed model and for systems where one needs to sample a wide range of concentrations or compositions. A less rigorous method that implements a dual τ minimization (for both upward and downward trajectories) is found to be harder to converge but produce more round trips than a method based on a single τ minimization for all trajectories. While the proposed methods do not always minimize the true τ, they have performances that are either similar or better than those of the original optimized-ensemble method and provide useful information to characterize deviations from Markovian dynamics in the sampling of the λ space.
Influence of water content on structure and mobility of polyvinyl alcohol: A molecular dynamics simulation
Authors: Bermejo Javier Sacristan , Ugarte Carmen Mijangos
Published: October 2008

Fully atomistic molecular dynamics simulations have been performed on aqueous solutions of polyvinyl alcohol (PVA) to get some insight into the effect of water content on PVA structure and dynamics. Simulations were carried out on mixtures of water/atactic PVA at 300 K covering a composition range (water weight fraction Xw=0.1–0.8. The analysis focuses on polymer segmental dynamics, which is monitored by the mean square displacement, Van Hove self correlation function, non-Gaussian parameter, and the intermediate incoherent scattering function. It was found that PVA dynamics is strongly affected by addition of water. Both, PVA carbon and hydrogen (hydroxyl and main chain hydrogens) motions are faster with further dilution. Differences on atom mobility (dynamic heterogeneity) decreases as water content increases allowing more isotropic segmental motions. Moreover, the non-Gaussian parameter shows a maximum at Xw=0.25 followed by a continuous decrease with dilution. The non-Gaussian effects become more important as the water content decreases and dynamic heterogeneity is enhanced. These results are explained in terms of the plasticization effects of water on the polymer and the intrinsic dynamic heterogeneity observed on PVA dynamics
Effects of pressure on structure and dynamics of model elastomers: A molecular dynamics study
Authors: Liu Jun , Wu Sizhu , Cao Dapeng , Zhang Liqun
Published: October 2008

On the basis of an idealized model of an elastomer, we use molecular dynamics simulations to explore the effects of pressure on the glass transition, structure, and dynamics of the model elastomer. The simulated results indicate that with the pressure increasing, the glass transition temperature Tg increases while the glass transition strength decreases, which is in accordance with the experimental result from Colucci et al. [J. Polym. Sci., B: Polym. Phys. 35, 1561 (1997)] For the structure of the elastomer, it is found that the intramolecular packing remains nearly unchanged over the pressure range studied, also validated by the independence of the chain size and shape on the pressure, while the intermolecular distribution exhibits a more efficient packing effect at high pressures. By analyzing the end-to-end vector correlation and incoherent intermediate dynamic structure factor, which are well fitted by a stretched exponential Kohlrauch–William–Watts (KWW) function, we observe that the time-pressure superposition principle (TPSP) takes effect at the chain length scale, while at the segmental length scale the TPSP does not completely hold, attributed to the enhanced dynamic heterogeneity with the pressure increasing, which is evidenced by the beta values in stretched exponential fitting over the pressure range studied. Extracting the characteristic relaxation time from the KWW function, and then plotting the logarithm of the characteristic relaxation time versus the pressure, we observe a good linear relationship and find that the pressure exerts nearly the same effect on the relaxation behavior at both the segmental and chain length scales. This point is further validated by almost the same dependence of the α-relaxation time for three representative q wave vectors, indicating that the segmental and chain relaxations of the elastomer are influenced similarly by the pressure variation and the same physical processes are responsible for relaxation at the probed length scales. The calculated activation volume is independent of pressure at fixed temperature but increases with the temperature decreasing at fixed pressure. Finally, the pressure effect on the stress autocorrelation function is also examined, and a more difficult trend for stress relaxation and dissipation of the elastomer at high pressure is found. It is expected that all these simulated results would shed some light on the relevant experimental and theoretical studies.
Full-dimensional quantum dynamics of Ã-state photodissociation of ammonia: Absorption spectra
Authors: Lai Wenzhen , Lin Shi Ying , Xie Daiqian , Guo Hua
Published: October 2008

Full-dimensional quantum dynamics of the Ã-state photodissociation of NH3 and ND3 is characterized on potential energy surfaces recently developed by Li et al. [Theor. Chem. Acc. 118, 9 (2007)]. The Ã←X̃ absorption spectra of the two isotopomers were obtained for several vibrational levels on the ground electronic state. The calculated positions and lifetimes of low-lying umbrella resonances on the excited Ã electronic state are found to reproduce experimental trends and shed light on the multidimensional dynamics. The agreement with measured spectra is reasonable, but assignment of some individual features remains incomplete.
Excess electron states in reduced bulk anatase TiO2: Comparison of standard GGA, GGA+U, and hybrid DFT calculations
Authors: Finazzi Emanuele , Di Valentin Cristiana , Pacchioni Gianfranco , Selloni Annabella
Published: October 2008

The removal of lattice O atoms, as well as the addition of interstitial H atoms, in TiO2 is known to cause the reduction in the material and the formation of “Ti3+” ions. By means of electronic structure calculations we have studied the nature of such oxygen vacancy and hydrogen impurity states in the bulk of the anatase polymorph of TiO2. The spin polarized nature of these centers, the localized or delocalized character of the extra electrons, the presence of defect-induced states in the gap, and the polaronic distortion around the defect have been investigated with different theoretical methods: standard density functional theory (DFT) in the generalized-gradient approximation (GGA), GGA+U methods as a function of the U parameter, and two hybrid functionals with different admixtures of Hartree–Fock exchange. The results are found to be strongly dependent on the method used. Only GGA+U or hybrid functionals are able to reproduce the presence of states at about 1 eV below the conduction band, which are experimentally observed in reduced titania. The corresponding electronic states are localized on Ti 3d levels, but partly delocalized solutions are very close in energy. These findings show the limited predictive power of these theoretical methods to describe the electronic structure of reduced titania in the absence of accurate experimental data.
A rocket-borne mass analyzer for charged aerosol particles in the mesosphere
Authors: Knappmiller Scott , Robertson Scott , Sternovsky Zoltan , Friedrich Martin
Published: October 2008

An electrostatic mass spectrometer for nanometer-sized charged aerosol particles in the mesosphere has been developed and tested. The analyzer is mounted on the forward end of a rocket and has a slit opening for admitting a continuous sample of air that is exhausted through ports at the sides. Within the instrument housing are two sets of four collection plates that are biased with positive and negative voltages for the collection of negative and positive aerosol particles, respectively. Each collection plate spans about an order of magnitude in mass which corresponds to a factor of 2 in radius. The number density of the charge is calculated from the current collected by the plates. The mean free path for molecular collisions in the mesosphere is comparable to the size of the instrument opening; thus, the analyzer performance is modeled by a Monte Carlo computer code that finds the aerosol particles trajectories within the instrument including both the electrostatic force and the forces from collisions of the aerosol particles with air molecules. Mass sensitivity curves obtained using the computer models are near to those obtained in the laboratory using an ion source. The first two flights of the instrument returned data showing the charge number densities of both positive and negative aerosol particles in four mass ranges.
Low-temperature synthesis of Si nanowires using multizone chemical vapor deposition methods
Authors: Qi Pengfei , Wong William S. , Zhao Huaizhou , Wang Dunwei
Published: October 2008

With a multitemperature zone chemical vapor deposition reactor, silicon nanowires (SiNWs) were synthesized at temperatures below the Au–Si eutectic point, with Au nanoparticles as the growth seeds. Nanoparticle seeds with diameters less than 60nm were used to grow NWs at temperatures below 350°C. A strong dependence on the growth rate with the synthesis temperature and the size of the Au nanoparticle seeds was observed. With 10nm particles, SiNWs were reproducibly synthesized at a temperature of 290°C. We suggest that the governing role for the synthesis lies in the diffusion of Si feeding into the growth seeds.
Optical beam profile monitor and residual gas fluorescence at the relativistic heavy ion collider polarized hydrogen jet
Authors: Tsang T. , Bellavia S. , Connolly R. , Gassner D. , Makdisi Y. , Russo T. , Thieberger P. , Trbojevic D. , Zelenski A.
Published: October 2008

A gas fluorescence beam profile monitor has been implemented at the relativistic heavy ion collider (RHIC) using the polarized atomic hydrogen gas jet, which is part of the polarized proton polarimeter. RHIC proton beam profiles in the vertical plane of the accelerator are obtained as well as measurements of the width of the gas jet in the beam direction. For gold ion beams, the fluorescence cross section is sufficiently large so that profiles can be obtained from the residual gas alone, albeit with long light integration times. We estimate the fluorescence cross sections that were not known in this ultrarelativistic regime and calculate the beam emittance to provide an independent measurement of the RHIC beam. This optical beam diagnostic technique, utilizing the beam induced fluorescence from injected or residual gas, offers a noninvasive particle beam characterization and provides visual observation of proton and heavy ion beams.
A transistor based air flow transducer for thermohygrometric control of neonatal ventilatory applications
Authors: Schena Emiliano , Silvestri Sergio
Published: October 2008

An air flow transducer for controlling heated humidifiers used in neonatal artificial ventilation, suitable for in-line application in monopatient breathing circuits, is described here. The sensor is built with two nominally identical bipolar junction transistors, with different packages, as hot elements operated at a regulated constant voltage. The operation principle is based on the differential convective heat power exchanged with the fluid stream due to the different thermal conductivities of the transistors’ packages. The underlying theory is described in mathematical terms and the theoretical model is validated with experimental data in the mass flow rate range from 4 to 215 mg s−1. The nonlinear behavior allows sensitivities from −5 mV/(mg s−1) at flow rates in the range of 4–130 mg s−1 to −2 mV/(mg s−1) at higher flow rates up to 215 mg s−1. The linear range extends from 40 to 130 mg s−1, with constant sensitivity equal to −5 mV/(mg s−1). The differential nature of the output allows to obtain repeatabilities in the order of 2% for fluid temperatures between 20 and 25 °C and of about 6% if the fluid temperature lies in the range of 15–35 °C. The relatively long time constant, in the order of 20±5 s, makes the sensor suitable for average flow rate measurements. Using the sensor’s output as a control variable of a heated humidifier for artificial ventilation, the relative humidity of gases varies by only 20% in the flow rate range of the sensor (from 95% to 75%), whereas the same parameter shows a variation of about 40% (from 100% to 60%) with the same humidifier without flow control.
Capillary rise of a liquid between two vertical plates making a small angle
Authors: Higuera F. J. , Medina A. , Liñán A.
Published: October 2008

The penetration of a wetting liquid in the narrow gap between two vertical plates making a small angle is analyzed in the framework of the lubrication approximation. At the beginning of the process, the liquid rises independently at different distances from the line of intersection of the plates except in a small region around this line where the effect of the gravity is negligible. The maximum height of the liquid initially increases as the cubic root of time and is attained at a point that reaches the line of intersection only after a certain time. At later times, the motion of the liquid is confined to a thin layer around the line of intersection whose height increases as the cubic root of time and whose thickness decreases as the inverse of the cubic root of time. The evolution of the liquid surface is computed numerically and compared with the results of a simple experiment.
Low divergence single-mode surface emitting quantum cascade ring lasers
Authors: Mujagić E. , Hoffmann L. K. , Schartner S. , Nobile M. , Schrenk W. , Semtsiv M. P. , Wienold M. , Masselink W. T. , Strasser G.
Published: October 2008

We describe the fabrication and operation of surface emitting second-order distributed feedback quantum cascade ring lasers. The devices exhibit single-mode emission at a wavelength of 3.95μm with a side mode suppression ratio of 25dB. A linear tuning coefficient of 0.13cm−1∕K is observed. A single longitudinal mode in the ring shaped resonator results in a highly symmetric far-field pattern and a low beam divergence, represented by a full width at half maximum of ∼3°. Based on these characteristics the presented compact coherent light source may find its way into today’s midinfrared spectroscopy applications.
The flapping-flag instability as a nonlinear eigenvalue problem
Authors: Alben Silas
Published: October 2008

We reconsider the classical problem of the instability of a flapping flag in an inviscid background flow with a vortex sheet wake, and reformulate it as a nonlinear eigenvalue problem. We solve the problem numerically for the 20 lowest wave number modes. We find that the lowest wave number mode is the first to become unstable, and has the fastest growth rate within 3.5 decades of the stability boundary in the parameter space of flag mass and flag rigidity. This and subsequent modes become unstable by merging into complex-conjugate pairs, which then lose conjugacy further into the region of instability. Eigenmodes exhibit sharp transitions in shape across the stability boundary. In the corresponding initial value problem, we show a correlation between the wave number of unstable modes in the small-amplitude (growth) regime and the large-amplitude (saturation) regime. Wave number increases with decreasing rigidity and shows a combination of discrete and continuous change in the shape of unstable modes. Using an infinite flag model we compute the parameters of the most unstable flag and show that a classical mechanism for the instability correlating pressure lows to flag amplitude peaks does not hold.
Discrete approximation of quantum stochastic models
Authors: Bouten Luc , Van Handel Ramon
Published: October 2008

We develop a general technique for proving convergence of repeated quantum interactions to the solution of a quantum stochastic differential equation. The wide applicability of the method is illustrated in a variety of examples. Our main theorem, which is based on the Trotter–Kato theorem, is not restricted to a specific noise model and does not require boundedness of the limit coefficients.
Synthesis and conductance measurement of periodic arrays of gold nanoparticles
Authors: Kakefuda Y. , Narita K. , Komeda T. , Yoshimoto S. , Hasegawa S.
Published: October 2008

We report a formation method and an electrical characterization of aligned arrays of Au nanoparticles (∼20–40 nm). We synthesized the structure by immersing a substrate vertically into a solvent with nanoparticle and letting the solvent evaporate. The periodicity of the arrays has been controlled in 10–40 μm range by tuning the evaporation rate of solvent. The resistivity shows a drastic decrease (∼10−4) when annealed at ∼170 °C. While it is considerably due to a breakdown of ligands surrounding the Au particles, the large change in conductivity should be used for sensors to examine the interface molecules between Au nanoparticles.
Self-organized metal nanowire arrays with tunable optical anisotropy
Authors: Toma A. , Chiappe D. , Massabò D. , Boragno C. , Buatier de Mongeot F.
Published: October 2008

Here we report on the development of an unconventional approach for the physical synthesis of laterally ordered self-organized arrays of metallic nanowires supported on nanostructured dielectric templates. The method, based on a combination of nanoscale patterning of the glass substrate by ion beam sputtering with shadow deposition of the metal nanoparticles, provides a viable alternative to time consuming serial nanopatterning approaches. Far-field optical characterization demonstrates that the nanowire arrays exhibit tunable anisotropic properties in the visible range due to the excitation of localized plasmon resonances.
Spectral diffusion and line broadening in single self-assembled GaAs∕AlGaAs quantum dot photoluminescence
Authors: Abbarchi M. , Troiani F. , Mastrandrea C. , Goldoni G. , Kuroda T. , Mano T. , Sakoda K. , Koguchi N. , Sanguinetti S. , Vinattieri A. , Gurioli M.
Published: October 2008

We experimentally and theoretically investigate the photoluminescence broadening of different excitonic complexes in single self-assembled GaAs∕AlGaAs quantum dots. We demonstrate that the excitonic fine-structure splitting leads to a sizable line broadening whenever the detection is not resolved in polarization. The residual broadening in polarized measurements is systematically larger for the exciton with respect to both the trion and the biexciton recombination. The experimental data agree with calculations of the quantum confined Stark effect induced by charge defects in the quantum dot (QD) environment, denoting the role of the QD spectator carrier rearrangement in reducing the perturbation of the fluctuating environment.
Visible and ultraviolet light alternative photodetector based on ZnO nanowire/n-Si heterojunction
Authors: Guo Zhen , Zhao Dongxu , Liu Yichun , Shen Dezhen , Zhang Jiying , Li Binghui
Published: October 2008

Closely packed ZnO nanowire array was fabricated on a n-type Si (100) substrate by a magnetron cosputtering method. The ZnO nanowire/n-Si heterojunction showed good diode characteristics with rectification ratio of above 1.6×102 at 4V in the dark. Experiments demonstrated that the diode could be used to detect either visible or ultraviolet light by easily controlling the polarity of the voltage applied on the heterojunction. The spectral response of the device will be discussed in terms of the band diagrams of the heterojunction and the carrier diffusion process.
High efficiency photodetectors fabricated by electrostatic layer-by-layer self-assembly of CdTe quantum dots
Authors: Tu Chang-Ching , Lin Lih Y.
Published: October 2008

We demonstrate high-performance photodetectors from multilayers of CdTe quantum dots (QDs). The QDs are synthesized and dispersed in aqueous solution with either 2-mercaptoethylamine (positively charged) or thioglycolic acid (negatively charged) as capping stabilizers. By electrostatic attraction, the charged QDs are self-assembled layer by layer on an indium tin oxide substrate modified with (3-aminopropyl)triethoxysilane. This process allows control of active layer thickness by self-assembly, and can in principle be applied to a wide range of substrates. The photodetector exhibits high responsivity (0.18A∕W) under 0.1V bias due to extremely short capping ligands of QDs, which have high internal quantum efficiency, and the densely packed multilayer structure.
Time scale for point-defect equilibration in nanostructures
Authors: Millett Paul C. , Wolf Dieter , Desai Tapan , Yamakov Vesselin
Published: October 2008

Molecular dynamics simulations of high-temperature annealing are performed on nanostructured materials enabling direct observation of vacancy emission from planar defects (i.e., grain boundaries and free surfaces) to populate the initially vacancy-free grain interiors on a subnanosecond time scale. We demonstrate a universal time-length scale correlation that governs these re-equilibration processes, suggesting that nanostructures are particularly stable against perturbations in their point-defect concentrations, caused for example by particle irradiation or temperature fluctuations.
Study of the characteristics of 1.55μm quantum dash/dot semiconductor lasers on InP substrate
Authors: Zhou D. , Piron R. , Grillot F. , Dehaese O. , Homeyer E. , Dontabactouny M. , Batte T. , Tavernier K. , Even J. , Loualiche S.
Published: October 2008

InAs quantum dash (QDH) and quantum dot (QD) lasers grown by molecular beam epitaxy on InP substrate are studied. The laser active zones with multiple stacked layers exhibit lasing wavelength at 1.55μm. On these devices, the experimental threshold current density reaches its minimum value for a double stacked QDH/QD structure. Other basic laser properties such as gain and quantum efficiency are compared. QD lasers exhibit better threshold current densities but equivalent modal gain per layer than QDH. Finally, the analysis of the modal gain on QD lasers shows a promising potential for improvement.
Band edge tunability of M-structure for heterojunction design in Sb based type II superlattice photodiodes
Authors: Nguyen Binh-Minh , Hoffman Darin , Delaunay Pierre-Yves , Huang Edward Kwei-Wei , Razeghi Manijeh , Pellegrino Joe
Published: October 2008

We present theoretically and experimentally the effect of the band discontinuity in type II misaligned InAs∕GaSb superlattice heterodiodes. Calculations using the empirical tight binding method have shown the great flexibility in tuning the energy levels of the band edge in M-structure superlattice as compared to the standard InAs∕GaSb superlattice. Through the experimental realization of several p-π-M-n photodiodes, the band discontinuity alignment between the standard binary-binary superlattice and the M-structured superlattice was investigated via optical characterization. The agreement between the theoretical predictions and the experimental measurement confirms the capability of controlling the M-structure band edges and suggests a way to exploit this advantage for the realization of heterostructures containing an M-structured superlattice without bias dependent operation.
Spin transfer torque switching of cobalt nanoparticles
Authors: Wang X. J. , Zou H. , Ji Y.
Published: October 2008

Magnetization reversals of cobalt nanoparticles induced by spin transfer torque are demonstrated at 4.2K by a mechanical point-contact made on a Co∕Cu∕Co trilayer thin film. The top Co layer (∼0.5nm) is discontinuous and consists of isolated cobalt nanoparticles with a diameter of <5nm. The number of nanoparticles underneath a point contact can be controlled by varying the size of the contact. The characteristics of spin transfer switching are qualitatively different for contacts involving a few particles (5–10) and those involving many particles (40–50).
High efficiency domain wall gate in ferromagnetic nanowires
Authors: Petit Dorothée , Jausovec Ana-Vanessa , Zeng Huang T. , Lewis Emma , O’Brien Liam , Read Dan , Cowburn Russell P.
Published: October 2008

A transverse domain wall (DW) switchable gate with a very high efficiency is experimentally demonstrated in Permalloy nanowires using a transverse T-shaped structure. DWs are found to either travel undisturbed through the open gate or to be strongly trapped in front of the closed gate only able to travel backwards. The opening and closing of the gate depends on the magnetic configuration of the gate and is controlled using externally applied magnetic fields. Micromagnetic simulations confirm the experimental results.
Selective coupling of type 6 adenylyl cyclase with type 2 IP3 receptors mediates direct sensitization of IP3 receptors by cAMP
Authors: Tovey Stephen C. , Dedos Skarlatos G. , Taylor Emily J.A. , Church Jarrod E. , Taylor Colin W.
Published: October 2008

Interactions between cyclic adenosine monophosphate (cAMP) and Ca2+ are widespread, and for both intracellular messengers, their spatial organization is important. Parathyroid hormone (PTH) stimulates formation of cAMP and sensitizes inositol 1,4,5-trisphosphate receptors (IP3R) to IP3. We show that PTH communicates with IP3R via “cAMP junctions” that allow local delivery of a supramaximal concentration of cAMP to IP3R, directly increasing their sensitivity to IP3. These junctions are robust binary switches that are digitally recruited by increasing concentrations of PTH. Human embryonic kidney cells express several isoforms of adenylyl cyclase (AC) and IP3R, but IP3R2 and AC6 are specifically associated, and inhibition of AC6 or IP3R2 expression by small interfering RNA selectively attenuates potentiation of Ca2+ signals by PTH. We define two modes of cAMP signaling: binary, where cAMP passes directly from AC6 to IP3R2; and analogue, where local gradients of cAMP concentration regulate cAMP effectors more remote from AC. Binary signaling requires localized delivery of cAMP, whereas analogue signaling is more dependent on localized cAMP degradation.
Membrane microdomain switching: a regulatory mechanism of amyloid precursor protein processing
Authors: Sakurai Takashi , Kaneko Kumi , Okuno Misako , Wada Koji , Kashiyama Taku , Shimizu Hideaki , Akagi Takumi , Hashikawa Tsutomu , Nukina Nobuyuki
Published: October 2008

Neuronal activity has an impact on β cleavage of amyloid precursor protein (APP) by BACE1 to generate amyloid-β peptide (Aβ). However, the molecular mechanisms underlying this effect remain to be elucidated. Cholesterol dependency of β cleavage prompted us to analyze immunoisolated APP-containing detergent-resistant membranes from rodent brains. We found syntaxin 1 as a key molecule for activity-dependent regulation of APP processing in cholesterol-dependent microdomains. In living cells, APP associates with syntaxin 1–containing microdomains through X11–Munc18, which inhibits the APP–BACE1 interaction and β cleavage via microdomain segregation. Phosphorylation of Munc18 by cdk5 causes a shift of APP to BACE1-containing microdomains. Neuronal hyperactivity, implicated in Aβ overproduction, promotes the switching of APP microdomain association as well as β cleavage in a partially cdk5-dependent manner. We propose that microdomain switching is a mechanism of cholesterol- and activity-dependent regulation of APP processing in neurons.
Dual inhibition of SNARE complex formation by tomosyn ensures controlled neurotransmitter release
Authors: Sakisaka Toshiaki , Yamamoto Yasunori , Mochida Sumiko , Nakamura Michiko , Nishikawa Kouki , Ishizaki Hiroyoshi , Okamoto-Tanaka Miki , Miyoshi Jun , Fujiyoshi Yoshinori , Manabe Toshiya , Takai Yoshimi
Published: October 2008

Neurotransmitter release from presynaptic nerve terminals is regulated by soluble NSF attachment protein receptor (SNARE) complex–mediated synaptic vesicle fusion. Tomosyn inhibits SNARE complex formation and neurotransmitter release by sequestering syntaxin-1 through its C-terminal vesicle-associated membrane protein (VAMP)–like domain (VLD). However, in tomosyn-deficient mice, the SNARE complex formation is unexpectedly decreased. In this study, we demonstrate that the N-terminal WD-40 repeat domain of tomosyn catalyzes the oligomerization of the SNARE complex. Microinjection of the tomosyn N-terminal WD-40 repeat domain into neurons prevented stimulated acetylcholine release. Thus, tomosyn inhibits neurotransmitter release by catalyzing oligomerization of the SNARE complex through the N-terminal WD-40 repeat domain in addition to the inhibitory activity of the C-terminal VLD.