bluuebunny/crossref_metadata_2025 huggingface dataset:
Micro-eukaryotic diversity of the human distal gut microbiota: qualitative assessment using culture-dependent and -independent analysis of faeces
Authors: Scanlan Pauline D , Marchesi Julian R
Published: July 2008

Molecular ecological surveys of the human gut microbiota to date have focused on the prokaryotic fraction of the community and have revealed a remarkable degree of bacterial diversity and functionality. However, there is a dearth of information on the eukaryotic composition of the microbiota, and no culture-independent sequence-based surveys of human faeces are available. Culture-independent analyses based on DNA extraction and polymerase chain reaction targeting both the total eukaryotic 18S rRNA genes and fungal internal transcribed regions (ITS), together with culture-dependent analyses of fungi, were performed on a group of healthy volunteers. Temporal analysis was also included wherever possible. Collectively, the data presented in this study indicate that eukaryotic diversity of the human gut is low, largely temporally stable and predominated by different subtypes of Blastocystis. Specific analyses of the fungal populations indicate that a disparity exists between the cultivable fraction, which is dominated by Candida sp, and culture-independent analysis, where sequences identical to members of the genera Gloeotinia/Paecilomyces and Galactomyces were most frequently retrieved from both fungal ITS profiles and subsequent clone libraries. Collectively, these results highlight the presence of unprecedented intestinal eukaryotic inhabitants whose functional roles are as yet unknown in healthy individuals. Furthermore, differences between results obtained from traditionally employed culture-based methods and those obtained from culture-independent techniques highlight similar anomalies to that encountered when first analysing the bacterial diversity of the human faecal microbiota using culture-independent surveys.
Exploring the spatio-temporal dynamics of geographical processes with geographically weighted regression and geovisual analytics
Authors: Demšar Urška , Fotheringham A. Stewart , Charlton Martin
Published: July 2008

The paper examines the potential for combining a spatial statistical methodology – Geographically Weighted Regression (GWR) – with geovisual analytical exploration to help understand complex spatio-temporal processes. This is done by applying the combined statistical – exploratory methodology to a simulated data set in which the behaviour of regression parameters was controlled across space and time. A variety of complex spatio-temporal processes was captured through space-time (i.e. as spatio-temporal) varying parameters whose values were known. The task was to see if the proposed methodology could uncover these complex processes from the data alone. The results of the experiment confirm that the combined methodology can successfully identify spatio-temporal patterns in the local GWR parameter estimates that correspond to the controlled behaviour of the original parameters.
Investigation of blind tip estimation
Authors: Bakucz P. , Krüger-Sehm R. , Koenders L.
Published: July 2008

In this work we study functions for maximum likelihood estimation in blind tip estimation. We will implement the expectation maximization (EM), the stochastic EM, and stochastic approximation EM algortithms to estimate the unknown tip geometry. To demonstrate the functionality of the algorithms we applied it to dilated artificial input signal.
A model of diffusion in a potential well for the dynamics of the large-scale circulation in turbulent Rayleigh–Bénard convection
Authors: Brown Eric , Ahlers Guenter
Published: July 2008

Experimental measurements of properties of the large-scale circulation (LSC) in turbulent convection of a fluid heated from below in a cylindrical container of aspect ratio 1 are presented and used to test a model of diffusion in a potential well for the LSC. The model consists of a pair of stochastic ordinary differential equations motivated by the Navier–Stokes equations. The two coupled equations are for the azimuthal orientation θ0 and for the azimuthal temperature amplitude δ at the horizontal midplane. The dynamics is due to the driving by Gaussian distributed white noise that is introduced to represent the action of the small-scale turbulent fluctuations on the large-scale flow. Measurements of the diffusivities that determine the noise intensities are reported. Two time scales predicted by the model are found to be within a factor of 2 or so of corresponding experimental measurements. A scaling relationship predicted by the model between δ and the Reynolds number is confirmed by measurements over a large experimental parameter range. The Gaussian peaks of probability distributions p(δ) and p(θ̇0) are accurately described by the model; however, the non-Gaussian tails of p(δ) are not. The frequency, angular change, and amplitude behavior during cessations are accurately described by the model when the tails of the probability distribution of δ are used as experimental input.
Conditional statistics of passive-scalar mixing in a confined wake flow
Authors: Feng Hua , Olsen Michael G. , Fox Rodney O. , Hill James C.
Published: July 2008

Experimental results for conditional statistics in a confined turbulent wake were compared to the linear model and the transported probability density function (PDF) model for predicting the velocity conditioned on scalar mixture fraction and to a linear model for predicting the scalar mixture fraction conditioned on velocity. For velocity conditioned on scalar mixture fraction, the linear model was found to accurately predict the conditional transverse velocity, but for the conditional streamwise velocity, agreement between the experiments and the linear model was poor. The transverse conditional velocity was accurately predicted by the transported PDF model, but for the streamwise conditional velocity, a modified transported PDF model that incorporated two components of the scalar diffusivity tensor had to be used. Finally, it was found that a linear model for predicting the fluctuating scalar conditioned on velocity performed poorly due to the strongly non-Gaussian scalar PDF at the downstream locations investigated.
Drift instabilities in current sheets with guide field
Authors: Yoon P. H. , Lui A. T. Y.
Published: July 2008

Drift instabilities in current sheets with or without the guide field are investigated with a newly developed improved electrostatic dispersion relation. Traditional (local) theories of lower-hybrid drift instability typically assumes small electron drift speed, and expand the electron distribution function in Taylor series. This approximate treatment is removed in this paper. The resulting formalism is uniformly valid for an arbitrary magnitude of relative ion and electron drift speeds, and is valid for an arbitrary strength of the guide field.
A particle simulation of current sheet instabilities under finite guide field
Authors: Wang X. Y. , Lin Y. , Chen L. , Lin Z.
Published: July 2008

The instability of a Harris current sheet under a broad range of finite guide field (BG) is investigated using a linearized (δf) gyrokinetic electron and fully kinetic ion particle simulation code. The simulation is carried out in the two-dimensional plane containing the guide field along y and the current sheet normal along z. In this particle model, the rapid electron cyclotron motion is removed, while the realistic mass ratio mi∕me, finite electron Larmor radii, and wave-particle interactions are kept. It is found that for a finite BG∕Bx0⩽1, where Bx0 is the asymptotic antiparallel component of magnetic field, three unstable modes, i.e., modes A, B, and C, can be excited in the current sheet. Modes A and C, appearing to be quasielectrostatic modified two-stream instability/whistler mode, are located mainly on the edge of the current sheet. Mode B, on the other hand, is confined in the current sheet center and carries a compressional magnetic field (δBy) perturbation along the direction of electron drift velocity. Our new finding suggests that mode B may contribute directly to the electron anomalous resistivity in magnetic reconnection. In the cases with extremely large BG∕Bx0⪢1, the wave modes evolve to a globally propagating instability. The simulation shows that the presence of finite BG modifies the physics of the current sheet significantly.
Influence of suprathermal background electrons on strong auroral double layers: Observations
Authors: Andersson L. , Newman D. L. , Ergun R. E. , Goldman M. V. , Carlson C. W. , McFadden J. P.
Published: July 2008

This study examines the properties and evolution of strong, laminar double layers in the downward current region of the aurora and present several previously unpublished events from the Fast Auroral SnapshoT (FAST) satellite. Analysis of an event is presented for which the FAST satellite appears to have dwelled on the high-potential side (high-altitude side) of a double layer for an extended period, suggesting that a small (few percent of density) background of suprathermal electrons can strongly influence the structure and stability of double layers. From these observations, it is inferred that the accelerated electrons can be accelerated by either of two classes of double layers: (1) self-regulated double layers (varying field-aligned potential, with weak or no suprathermal electron background) or (2) externally regulated double layers (less variable field-aligned potential, with significant suprathermal electron background). Elements of the interpretation presented here are supported by numerical simulations, as reported in two accompanying papers.
Influence of suprathermal background electrons on strong auroral double layers: Vlasov-simulation parameter study
Authors: Newman D. L. , Andersson L. , Goldman M. V. , Ergun R. E. , Sen N.
Published: July 2008

Recent observations by the Fast Auroral SnapshoT (FAST) satellite [Andersson et al., Phys. Plasmas 15, 072901 (2008), this issue] suggest that characteristics of particles and waves on the high-potential side of strong double layers in Earth’s auroral downward current region are influenced by properties of the suprathermal electron component. Current-driven Vlasov simulations in one spatial dimension with suprathermal electrons differing in density and temperature were performed to study their influence on double-layer evolution. A sufficiently dense and hot suprathermal population is found to stabilize strong laminar double layers over long periods while regulating their strength and velocity. Synthetic electron energy-flux spectra from simulations with different suprathermal electron populations reveal certain similarities to the corresponding FAST observations.
A novel method for analyzing the global stability of inviscid columnar swirling flow in a finite pipe
Authors: Wang S.
Published: July 2008

We developed a general strategy to study the stability problem of the inviscid columnar swirling flow in a finite pipe based on the perturbation method of linear operators. By virtue of the columnar base state, we were able to derive all the necessary formulas used in perturbation method in a manner of separation of variables. We then conduct a necessary benchmark case study based on the solid body rotation flow. We then applied the general method to the Lamb–Oseen vortex and the q-vortex and found their approximated growth rate functions. The perturbation method proved effective and robust in application to these vortex flows. We also extended the method by using the analytic continuation to find the unstable modes with complex growth rates. This analytic continuation method reflects the global nature of the perturbation method. Using the new method, we investigated the energy transfer mechanism of the Lamb–Oseen vortex in a finite pipe and revealed a new energy transfer mechanism related to the flow stability.
Numerical study on the Eulerian–Lagrangian analysis of Navier–Stokes turbulence
Authors: Ohkitani K. , Constantin P.
Published: July 2008

Detailed analyses of the Navier–Stokes equations on the basis of the Euler–Lagrangian formalism are presented with the use of numerical simulations. A singular perturbation property arising in the limit of vanishing viscosity is one issue of this paper. By using the connection matrix, which is related to the geometry of particle paths, we introduce “connection anomaly” for the characterization of the property and confirm it numerically. As a characterization in physical space, we show how regions with small values of a determinant of a derivative of diffusive labels are spatially correlated with vortex structures. Two kinds of initial conditions are examined: (i) Decaying isotropic turbulence developing from a random initial condition and (ii) the orthogonally offset two vortex tubes. For (i), it is found that when turbulence is fully developed, the resetting process occurs very frequently, which defines a short time scale associated with small-scale motion. For (ii), we confirm our previous finding that resetting of diffusive label captures successfully reconnection of vortex at higher Reynolds numbers. Even for this special initial condition, turbulence is developed after the phase of prominent reconnection and frequent resettings are associated with it.
Mechanical monolithic horizontal sensor for low frequency seismic noise measurement
Authors: Acernese Fausto , Giordano Gerardo , Romano Rocco , De Rosa Rosario , Barone Fabrizio
Published: July 2008

This paper describes a mechanical monolithic horizontal sensor for geophysical applications developed at the University of Salerno. The instrument is basically a monolithic tunable folded pendulum, shaped with precision machining and electric discharge machining, that can be used both as seismometer and, in a force-feedback configuration, as accelerometer. The monolithic mechanical design and the introduction of laser interferometric techniques for the readout implementation makes it a very compact instrument, very sensitive in the low frequency seismic noise band, with a very good immunity to environmental noises. Many changes have been produced since last version (2007), mainly aimed to the improvement of the mechanics and of the optical readout of the instrument. In fact, we have developed and tested a prototype with elliptical hinges and mechanical tuning of the resonance frequency together with a laser optical lever and a new laser interferometer readout system. The theoretical sensitivity curve for both laser optical lever and laser interferometric readouts, evaluated on the basis of suitable theoretical models, shows a very good agreement with the experimental measurements. Very interesting scientific result is the measured natural resonance frequency of the instrument of 70mHz with a Q=140 in air without thermal stabilization. This result demonstrates the feasibility of a monolithic folded pendulum sensor with a natural resonance frequency of the order of millihertz with a more refined mechanical tuning.
Mode shape reconstruction of an impulse excited structure using continuous scanning laser Doppler vibrometer and empirical mode decomposition
Authors: Kyong Yongsoo , Kim Daesung , Dayou Jedol , Park Kyihwan , Wang Semyung
Published: July 2008

For vibration testing, discrete types of scanning laser Doppler vibrometer (SLDV) have been developed and have proven to be very useful. For complex structures, however, SLDV takes considerable time to scan the surface of structures and require large amounts of data storage. To overcome these problems, a continuous scan was introduced as an alternative. In this continuous method, the Chebyshev demodulation (or polynomial) technique and the Hilbert transform approach have been used for mode shape reconstruction with harmonic excitation. As an alternative, in this paper, the Hilbert–Huang transform approach is applied to impact excitation cases in terms of a numerical approach, where the vibration of the tested structure is modeled using impulse response functions. In order to verify this technique, a clamped-clamped beam was chosen as the test rig in the numerical simulation and real experiment. This paper shows that with additional innovative steps of using ideal bandpass filters and nodal point determination in the postprocessing, the Hilbert–Huang transformation can be used to create a better mode shape reconstruction even in the impact excitation case.
General formulation of the resistive wall mode coupling equations
Authors: Pustovitov V. D.
Published: July 2008

A theoretical framework to describe the magnetic coupling of the toroidal plasma with the resistive wall and other sources of the field asymmetry is formulated. This is done for general toroidal geometry without restrictions on the plasma, while the wall is considered as a thin shell. Mathematically, the approach here exploits the Chance concept [M. S. Chance, Phys. Plasmas 4, 2161 (1997)], but with an essential difference: each source of the magnetic perturbation b (plasma, wall, external currents) is treated separately with account of their differences in space and nature. This allows much simpler formulation of the problem than was known before. The final equation couples the normal component of ∂b∕∂t at the wall to the perturbation at the plasma surface. Step by step reduction of this first-principle equation is performed with demonstration of its main features, starting from the known problem of singularities. This also includes the reduction to axially symmetric geometry, large-aspect-ratio, and the cylindrical limits. In the latter case, the known “cylindrical” equation is reproduced, but now from the full “toroidal” equations.
Weyl–Wigner formulation of noncommutative quantum mechanics
Authors: Bastos Catarina , Bertolami Orfeu , Dias Nuno Costa , Prata João Nuno
Published: July 2008

We address the phase-space formulation of a noncommutative extension of quantum mechanics in arbitrary dimension, displaying both spatial and momentum noncommutativities. By resorting to a covariant generalization of the Weyl–Wigner transform and to the Darboux map, we construct an isomorphism between the operator and the phase-space representations of the extended Heisenberg algebra. This map provides a systematic approach to derive the entire structure of noncommutative quantum mechanics in phase space. We construct the extended star product and Moyal bracket and propose a general definition of noncommutative states. We study the dynamical and eigenvalue equations of the theory and prove that the entire formalism is independent of the particular choice of the Darboux map. Our approach unifies and generalizes all the previous proposals for the phase-space formulation of noncommutative quantum mechanics. For concreteness we rederive these proposals by restricting our formalism to some two-dimensional spaces.
Fluid model of the boundary of a one-dimensional plasma under the influence of an oblique magnetic field for a wide range of collisionality
Authors: Zimmermann T. M. G. , Coppins M. , Allen J. E.
Published: July 2008

The effect of a magnetic field on the boundary of a plasma is studied using a one-dimensional fluid model based on the work of K.-U. Riemann [Contrib. Plasma. Phys. 34, 127 (1994)]. The model takes into account the effects of both collisions and ionization. Two limiting regimes are identified: the collisional presheath and the (highly) magnetized presheath. Results from this model demonstrate that a highly magnetized presheath may be treated in terms of two regions: The B-aligned presheath and a Chodura layer [R. Chodura, Phys. Fluids 25, 1628 (1982)]. The properties of this Chodura layer are explored in some detail and it is found that the size of this layer, for example, follows a simple expression in the highly magnetized regime. Finally, an attempt is made to recover the singular behavior of the Chodura layer as the magnetic field becomes very strong and use a pseudo two-scale approach to resolve both scale lengths of the magnetized presheath.
Steady-state electron trajectories and growth rate in electromagnetically pumped free-electron laser with specific nonuniform magnetic field
Authors: Mehdian H. , Jafari S. , Hasanbeigi A.
Published: July 2008

A theory of the dispersion relation for electromagnetically pumped free-electron laser in the presence of a special tapered axial guide magnetic field is presented. An analysis of the steady-state electron trajectories is obtained by solving the equations of motion. Next an eleventh-degree polynomial equation for electromagnetic and space-charge wave is derived. Numerical solution of the polynomial equation of the dispersion relation yield the complex wave number as a function of the frequency of the waves. These results are used to illustrate the dependence of growth rate curves on the axial guide field frequency. It is found that the tapered guide field shifts electron trajectories and enhances the growth rate in the comparison of employing uniform axial magnetic field, without needing a strong guide magnetic field.
Dispersion relation and growth rate in electromagnetically pumped free-electron lasers with ion-channel guiding
Authors: Mehdian H. , Hasanbeigi A. , Jafari S.
Published: July 2008

A theory of electromagnetic-wave wiggler with ion-channel guiding is presented. Steady-state electron trajectories are obtained by solving the equations of motion for a single electron in a free-electron laser with electromagnetic-wave wiggler and ion-channel guiding field. The eighth-degree polynomial dispersion equation for electromagnetic and space charge waves in this configuration is derived. The characteristics of dispersion relation are analyzed in detail by numerical solutions. It is found that the growth rate for group I and II orbits has been affected by the presence of the ion-channel guiding and the growth rate for group II orbits is more affected.
Beat of sound generated by flow past two side-by-side square cylinders of different sizes
Authors: Inoue Osamu , Suzuki Yoshinori
Published: July 2008

The sound generated by two square cylinders of different sizes placed in a side-by-side arrangement in a uniform flow at low Mach numbers is studied by direct solution of the two-dimensional unsteady compressible Navier–Stokes equations. Special attention is paid to the effect of the size difference between the two cylinders on the generation mechanism of the sound. Results show that synchronization of vortex sheddings from the two cylinders occurs when the size difference is small and that it is caused by strong flow interference between the two cylinders. It is found that the effect of the flow interference appears in characteristic frequencies St1 and St2, which are defined by St1=2StL−StS and St2=2StS−StL. Here, StL is the vortex shedding frequency of the larger cylinder and StS is that of the smaller cylinder. The results also show that two types of a beat of sound appear depending on the size difference. The first type of the beat appears when the size difference is small and thus the vortex shedding frequency of the smaller cylinder is close to the shedding frequency of the larger cylinder; that is, StS≈StL. The second type of the beat appears when the size difference is large so that the shedding frequency of the smaller cylinder is close to the first high harmonic of the frequency of the larger cylinder; that is, StS≈2StL.
Ideal magnetohydrodynamic simulation of magnetic bubble expansion as a model for extragalactic radio lobes
Authors: Liu Wei , Hsu Scott C. , Li Hui , Li Shengtai , Lynn Alan G.
Published: July 2008

Nonlinear ideal magnetohydrodynamic (MHD) simulations of the propagation and expansion of a magnetic “bubble” plasma into a lower density, weakly magnetized background plasma, are presented. These simulations mimic the geometry and parameters of the Plasma Bubble Expansion Experiment (PBEX) [A. G. Lynn, Y. Zhang, S. C. Hsu, H. Li, W. Liu, M. Gilmore, and C. Watts, Bull. Am. Phys. Soc. 52, 53 (2007)], which is studying magnetic bubble expansion as a model for extragalactic radio lobes. The simulations predict several key features of the bubble evolution. First, the direction of bubble expansion depends on the ratio of the bubble toroidal to poloidal magnetic field, with a higher ratio leading to expansion predominantly in the direction of propagation and a lower ratio leading to expansion predominantly normal to the direction of propagation. Second, a MHD shock and a trailing slow-mode compressible MHD wavefront are formed ahead of the bubble as it propagates into the background plasma. Third, the bubble expansion and propagation develop asymmetries about its propagation axis due to reconnection facilitated by numerical resistivity and to inhomogeneous angular momentum transport mainly due to the background magnetic field. These results will help guide the initial experiments and diagnostic measurements on PBEX.
Current multiplication by using multiple thyristors
Authors: Liu Z. , Pemen A. J. M. , Van Heesch E. J. M. , Winands G. J. J.
Published: July 2008

This paper presents a circuit topology to obtain current multiplication by using multiple thyristors. To gain insight into this technique, an equivalent circuit model is introduced. Proper operation of the topology was demonstrated by experiments on a small-scale setup including three thyristors. One thyristor is triggered by a trigger circuit; the other two are autotriggered and require no external trigger circuit. The three thyristors could be synchronized automatically in sequence. During the closing process, the discharging of the energy storage capacitors via the thyristors is prevented. The discharging starts when all thyristors are closed, and the currents through each thyristor are simultaneous and identical. The output current is exactly three times the switching current.
Ion acoustic shock waves in electron-positron-ion quantum plasma
Authors: Masood W. , Mirza Arshad M. , Hanif M.
Published: July 2008

Ion acoustic shock waves (IASWs) are studied in an unmagnetized quantum plasma consisting of electrons, positrons, and ions employing the quantum hydrodynamic (QHD) model. Nonlinear quantum IASWs are investigated by deriving the Korteweg–deVries–Burger equation under the small amplitude perturbation expansion method. The dissipation is introduced by taking into account the kinematic viscosity among the plasma constituents. It is found that the strength of the ion acoustic shock wave is maximum for spherical, intermediate for cylindrical, and minimum for planar geometry. The temporal evolution of the shock for a quantum e-p-i plasma in a spherical geometry is also investigated. It is found that the strength and the steepness of the quantum ion acoustic shock wave increases with decreasing stretched time coordinate (representing slow time scale) ∣τ∣. It is also found that an increase in the quantum Bohm potential decreases the strength as well as the steepness of the shock. The temporal evolution of the quantum ion acoustic solitons in an e-p-i plasma for cylindrical and spherical geometries is also explored by substituting the dissipative coefficient C equal to zero. The relevance of the present study with regard to the dense astrophysical environments is also pointed out.
A novel method to construct stationary solutions of the Vlasov-Maxwell system: The relativistic case
Authors: Suzuki Akihiro
Published: July 2008

A method to derive stationary solutions of the relativistic Vlasov-Maxwell system is explored. In the nonrelativistic case, a method using the Hermite polynomial series to describe the deviation from the Maxwell-Boltzmann distribution is found to be successful in deriving a few stationary solutions, including a two-dimensional one. Instead of the Hermite polynomial series, two special orthogonal polynomial series, which are appropriate to expand the deviation from the Maxwell-Jüttner distribution, are introduced in this paper. By applying this method, a new two-dimensional equilibrium is derived, which may provide an initial setup for investigations of three-dimensional relativistic collisionless reconnection of magnetic fields.
Direct drive heavy-ion-beam inertial fusion at high coupling efficiency
Authors: Logan B. G. , Perkins L. J. , Barnard J. J.
Published: July 2008

Issues with coupling efficiency, beam illumination symmetry, and Rayleigh-Taylor instability are discussed for spherical heavy-ion-beam-driven targets with and without hohlraums. Efficient coupling of heavy-ion beams to compress direct-drive inertial fusion targets without hohlraums is found to require ion range increasing several-fold during the drive pulse. One-dimensional implosion calculations using the LASNEX inertial confinement fusion target physics code shows the ion range increasing fourfold during the drive pulse to keep ion energy deposition following closely behind the imploding ablation front, resulting in high coupling efficiencies (shell kinetic energy/incident beam energy of 16% to 18%). Ways to increase beam ion range while mitigating Rayleigh-Taylor instabilities are discussed for future work.
Improving the electrical performance of a conductive atomic force microscope with a logarithmic current-to-voltage converter
Authors: Aguilera L. , Lanza M. , Porti M. , Grifoll J. , Nafría M. , Aymerich X.
Published: July 2008

A new configuration of conductive atomic force microscope (CAFM) is presented, which is based in a standard CAFM where the typical I-V converter has been replaced by a logI-V amplifier. This substitution extends the current dynamic range from 1–100pAto1pA–1mA. With the broadening of the current dynamic range, the CAFM can access new applications, such as the reliability evaluation of metal-oxide-semiconductor gate dielectrics. As an example, the setup has been tested by analyzing breakdown spots induced in SiO2 layers.
Simultaneous sensing and actuation with a piezoelectric tube scanner
Authors: Moheimani S. O. Reza , Yong Yuen K.
Published: July 2008

Piezoelectric tube scanners with quartered external electrodes are the most widely used nanopositioning technology in modern scanning probe microscopes. There has been increasing interest in utilizing feedback control techniques to improve bandwidth and accuracy of these nanopositioners. The use of feedback requires a sensor to be incorporated into the nanopositioning device. Noncontact displacement sensors, e.g., capacitive and inductive sensors, have been used for this purpose. However, their measurements contain a significant noise component if operated over large bandwidths. The piezoelectric voltage induced in a tube nanopositioner has been proposed recently as an alternative measure of displacement with a much improved noise figure, up to three orders of magnitude better than capacitive sensors. In this arrangement, an electrode is used to actuate the tube, while the opposite electrode is used as a sensor. This approach has two drawbacks: (i) the operating range of the tube is reduced to half and (ii) the tube is not driven symmetrically, thus the opposite sides of the tube experience asymmetric stresses, i.e., in this mode of operation, the scanner is not a perfectly collocated system. In this paper, we present a new electrode pattern for piezoelectric tube scanners which addresses the above problems and allows simultaneous sensing and actuation of the tube in an efficient way.
Influence of exciting frequency on gas and ion rotational temperatures of nitrogen capacitively coupled plasma
Authors: Huang X. J. , Xin Y. , Yuan Q. H. , Ning Z. Y.
Published: July 2008

By using optical emission spectroscopy, the nitrogen gas and ion rotational temperatures in capacitively coupled plasma discharges with different exciting frequencies are investigated. The rotational temperatures are acquired by comparing the measured and calculated spectra of selected transitions with a least-square procedure. It is found that N2 gas rotational temperature minimum around 13MHz is the combined effect of ion-dominated heating and electron-dominated heating in the plasma. The influence of exciting frequency on N2+ rotational temperature is much more than that of the N2 molecule, the lower frequency, the higher N2+ rotational temperature. Also, N2+ rotational temperature is much higher than the corresponding N2 gas rotational temperature in the plasma driven by low frequencies. These experimental phenomena may be attributed to the effective ion heating and/or possible resonant heating in the bulk plasma under the low-frequency field.
A dual beam photonic crystal fiber trap for microscopic particles
Authors: Gherardi David M. , Carruthers Antonia E. , Čižmár Tomáš , Wright Ewan M. , Dholakia Kishan
Published: July 2008

The dual beam counterpropagating optical trap has found increased use in studies such as optical stretching, optical binding, Raman spectroscopy, and the trapping of high index particles. In this letter we demonstrate the use of photonic crystal fiber to realize a long range dual beam trap that may support multiple wavelengths simultaneously. We develop a dual wavelength conveyor belt for trapped particles and realize the first ever dual beam white light (supercontinuum) trap. This low coherence light trap permits long range longitudinal optical binding of microparticles in the trap with no deleterious interference effects.
Thermal conductivity of ultrathin tetrahedral amorphous carbon films
Authors: Balandin A. A. , Shamsa M. , Liu W. L. , Casiraghi C. , Ferrari A. C.
Published: July 2008

We investigate the thermal conductivity of ultrathin tetrahedral amorphous carbon (ta-C) films on silicon, down to subnanometer thickness. For films with an initial sp3 content of 60%, the thermal conductivity reduces from 1.42to0.09W∕mK near room temperature as the thickness decreases from 18.5to∼1nm. The variation in ta-C film thickness is accompanied by changes in Young’s modulus, density, and sp3 content. The thermal resistance of the finite-thickness interface layer, which forms between ta-C and silicon, is ∼10−8m2K∕W near room temperature, thus producing a noticeable effect on thermal transport in ultrathin ta-C films.
Polarization sensitive detection of 100 GHz radiation by high mobility field-effect transistors
Authors: Sakowicz M. , Łusakowski J. , Karpierz K. , Grynberg M. , Knap W. , Gwarek W.
Published: July 2008

Detection of 100 GHz electromagnetic radiation by a GaAs/AlGaAs high electron mobility field-effect transistor was investigated at 300 K as a function of the angle α between the direction of linear polarization of the radiation and the symmetry axis of the transistor. The angular dependence of the detected signal was found to be A0 cos2(α−α0)+C with A0, α0, and C dependent on the electrical polarization of the transistor gate. This dependence is interpreted as due to excitation of two crossed phase-shifted oscillators. A response of the transistor chip (including bonding wires and the substrate) to 100 GHz radiation was numerically simulated. Results of calculations confirmed experimentally observed dependencies and showed that the two oscillators result from an interplay of 100 GHz currents defined by the transistor impedance together with bonding wires and substrate related modes.
Parametrization of analytic interatomic potential functions using neural networks
Authors: Malshe M. , Narulkar R. , Raff L. M. , Hagan M. , Bukkapatnam S. , Komanduri R.
Published: July 2008

A generalized method that permits the parameters of an arbitrary empirical potential to be efficiently and accurately fitted to a database is presented. The method permits the values of a subset of the potential parameters to be considered as general functions of the internal coordinates that define the instantaneous configuration of the system. The parameters in this subset are computed by a generalized neural network (NN) with one or more hidden layers and an input vector with at least 3n−6 elements, where n is the number of atoms in the system. The Levenberg–Marquardt algorithm is employed to efficiently affect the optimization of the weights and biases of the NN as well as all other potential parameters being treated as constants rather than as functions of the input coordinates. In order to effect this minimization, the usual Jacobian employed in NN operations is modified to include the Jacobian of the computed errors with respect to the parameters of the potential function. The total Jacobian employed in each epoch of minimization is the concatenation of two Jacobians, one containing derivatives of the errors with respect to the weights and biases of the network, and the other with respect to the constant parameters of the potential function. The method provides three principal advantages. First, it obviates the problem of selecting the form of the functional dependence of the parameters upon the system’s coordinates by employing a NN. If this network contains a sufficient number of neurons, it will automatically find something close to the best functional form. This is the case since Hornik et al., [Neural Networks 2, 359 (1989)] have shown that two-layer NNs with sigmoid transfer functions in the first hidden layer and linear functions in the output layer are universal approximators for analytic functions. Second, the entire fitting procedure is automated so that excellent fits are obtained rapidly with little human effort. Third, the method provides a procedure to avoid local minima in the multidimensional parameter hyperspace. As an illustrative example, the general method has been applied to the specific case of fitting the ab initio energies of Si5 clusters that are observed in a molecular dynamics (MD) simulation of the machining of a silicon workpiece. The energies of the Si5 configurations obtained in the MD calculations are computed using the B3LYP procedure with a 6-31G** basis set. The final ab initio database, which comprises the density functional theory energies of 10 202 Si5 clusters, is fitted to an empirical Tersoff potential containing nine adjustable parameters, two of which are allowed to be the functions of the Si5 configuration. The fitting error averaged over all 10 202 points is 0.0148eV (1.43kJmol−1). This result is comparable to the accuracy achieved by more general fitting methods that do not rely on an assumed functional form for the potential surface.
Depletion-induced surface alignment of asymmetric diblock copolymer in selective solvents
Authors: Wang Rong , Chen Yeng-Long , Hu Jinglei , Xue Gi
Published: July 2008

Phase separation of asymmetric diblock copolymer near surfaces in selective solvents is theoretically investigated by using the real-space version of self-consistent field theory (SCFT). Several morphologies are predicted and the phase diagram is constructed by varying the distance between two parallel hard surfaces (or the film thickness) W and the block copolymer concentration fP. Morphologies of the diblock copolymer in dilute solution are found to change significantly with different film thicknesses. In confined systems, stable morphologies found in the bulk solution become unstable due to the loss of polymer conformation entropy. The vesicle phase region contracts when the repulsive interaction between the blocks is strong (strong segregation regime). The mixture of vesicles, rodlike and spherelike micelles and the mixture of vesicles and sphere-like micelles disappear in contrast to the weakly segregating regime. The walls strongly affect the phase separation of block copolymer in selective solvent, and the depletion layer near the surface contributes much to the micelle formation of the block copolymer. Interestingly, the self-assembled morphologies stay near the walls with the distance on the order of the radius of gyration of the block copolymer. The oscillation of the polymer distribution near the walls allows the surface phase separation to be observed due to the strong repulsion between the blocks A and B.
Strong dependence of tunneling transport properties on overdriving voltage for room-temperature-operating single electron/hole transistors formed with ultranarrow [100] silicon nanowire channel
Authors: Lee Sejoon , Hiramoto Toshiro
Published: July 2008

The single electron/hole transistor (SET/SHT) consisting of a Si nanowire channel with a self-assembled Si quantum dot showed a strong dependence of Coulomb blockade (CB) characteristics on the overdriving voltage (Vover). The SET/SHT requiring a low Vover due to the moderate dot size compared to the nanowire size showed good device performances, for example, very sharp CB oscillation and long extension of blockade regime. However, the SET/SHT requiring a high Vover due to the unduly shrunken of size, which leads to an increase in the valence band offset between the dot and the nanowire, showed the drastic degradation of the device performances.
Method for characterizing self-assembled monolayers as antirelaxation wall coatings for alkali vapor cells
Authors: Yi Y. W. , Robinson H. G. , Knappe S. , Maclennan J. E. , Jones C. D. , Zhu C. , Clark N. A. , Kitching J.
Published: July 2008

We describe a method for characterizing self-assembled monolayers (SAMs) in terms of their performance as antirelaxation wall coatings for alkali atom vapor cells. A combination of initial surface analysis and subsequent laser spectroscopy is used to provide insight into the quality of the coating, as well as its performance under the exposure to alkalis as it occurs, for example, when used in applications such as atomic magnetometers or clocks. Fused silica plates coated with octadecyltrichlorosilane SAMs were used to make cubic R87b gas cells. The surface was characterized by x-ray diffraction, contact angle measurements, and atomic force microscopy. Measurements of hyperfine resonance linewidths and frequency shifts show that the rubidium vapor atoms collide up to 40 times with the walls of the cells before their coherence relaxes and their adsorption energy is around 0.065 eV. Chemical analysis of the cell indicates some weak reactions between the coating and the rubidium atoms.
Distinguishing between dipoles and field effects in molecular gated transistors
Authors: Shaya O. , Shaked M. , Doron A. , Cohen A. , Levy I. , Rosenwaks Y.
Published: July 2008

We combine Kelvin probe force microscopy and current-voltage measurements in order to characterize silicon-on-insulator bioFETs. The measurements were conducted on monolayer of (3-aminopropyl)-trimethoxysilane, which was deposited on ozone activated silicon oxide surface covering the transistor channel. The work function of the modified surface decreased by more than 2eV, and the threshold voltage measured on the same devices showed a very large increase (∼10V) following the chemical modification. A detailed analysis enables us to distinguish between electron affinity and field effects in such devices, and in molecular gated transistors in general.
Glycosidic linkage conformation of methyl-α-mannopyranoside
Authors: Coskuner Orkid , Bergeron Denis E. , Rincon Luis , Hudgens Jeffrey W. , Gonzalez Carlos A.
Published: July 2008

We study the preferred conformation of the glycosidic linkage of methyl-α-mannopyranoside in the gas phase and in aqueous solution. Results obtained utilizing Car–Parrinello molecular dynamics (CPMD) simulations are compared to those obtained from classical molecular dynamics (MD) simulations. We describe classical simulations performed with various water potential functions to study the impact of the chosen water potential on the predicted conformational preference of the glycosidic linkage of the carbohydrate in aqueous solution. In agreement with our recent studies, we find that results obtained with CPMD simulations differ from those obtained from classical simulations. In particular, this study shows that the trans (t) orientation of the glycosidic linkage of methyl-α-mannopyranoside is preferred over its gauche anticlockwise (g−) orientation in aqueous solution. CPMD simulations indicate that this preference is due to intermolecular hydrogen bonding with surrounding water molecules, whereas no such information could be demonstrated by classical MD simulations. This study emphasizes the importance of ab initio MD simulations for studying the structural properties of carbohydrates in aqueous solution.
Seebeck coefficients of n-type (Ga,In)(N,As), (B,Ga,In)As, and GaAs
Authors: Homm G. , Klar P. J. , Teubert J. , Heimbrodt W.
Published: July 2008

We measured the Seebeck coefficients of n-type (Ga,In)(N,As), (B,Ga,In)As, and GaAs epitaxial layers with doping concentrations ranging from 1017to1019cm−3 in the temperature range between 50 and 290K. Despite the significant differences in electronic structure between the nonamalgamation type quaternary alloys and the binary GaAs, the temperature dependence of the Seebeck coefficient for samples of similar doping concentration is almost the same for all three semiconductor systems. The finding can be explained by the similarity of the dispersions of the extended phonon states of the three semiconductor systems in conjunction with a dominant phonon drag contribution to the Seebeck coefficient in the temperature range studied.
Experimental study and modeling of the electron-attracting sheath: The influence of secondary electron emission
Authors: Schiesko L. , Carrère M. , Cartry G. , Layet J.-M.
Published: July 2008

A copper sample facing an energy-resolved mass spectrometer is biased positively beyond plasma potential in low-pressure argon plasma. Some ions are created in the sheath by electrons extracted from plasma and accelerated toward the mass spectrometer where they are detected according to their energy. Ion energy is related to local electrical sheath potential at which the ion has been created. Therefore, the Ion Distribution Function (IDF) allows us to probe in a nonperturbative way electron-attracting sheath potential. We observe a strong decrease of ion signal a few volts before sample bias. We attribute this effect to the presence of secondary electrons shielding positive sample potential. Potential profile and IDFs are computed using a model including secondary emission. The fit of IDFs provides an estimation of secondary emission yield and secondary electron temperature.
Statistical properties of wall pressure fluctuations over a forward-facing step
Authors: Camussi R. , Felli M. , Pereira F. , Aloisio G. , Di Marco A.
Published: July 2008

This work describes an experimental study of the flow field and wall pressure fluctuations induced by quasi-two-dimensional incompressible turbulent boundary layers overflowing a forward-facing step (FFS). Pressure fluctuations are measured upstream and downstream of an instrumented FFS step model installed inside a large scale recirculation water tunnel, while two-dimensional (2D) velocity fields are measured close to the step via 2D particle image velocimetry (PIV). The overall flow physics is studied in terms of averaged velocity and vorticity fields for different Reynolds numbers based on the step’s height. The wall pressure statistics are analyzed in terms of several indicators, including the root mean squares and probability density functions of the pressure fluctuations, demonstrating that the most relevant flow structure is the unsteady recirculation bubble formed at the reattachment region downstream of the step. Pressure spectra and cross correlations are computed as well, and the convection velocity characterizing the propagation of hydrodynamic perturbations is determined as a function of the distance to the vertical side of the step. The simultaneous measurement of time-resolved PIV fields and wall pressure signals enabled us to compute the pressure/velocity cross correlations in the region downstream of the step and substantiated the relevant role played by the recirculation bubble.
Surface Fermi level in GaAsSb structures grown by molecular beam epitaxy on InP substrates
Authors: Chouaib Houssam , Bru-Chevallier Catherine , Apostoluk Aleksandra , Rudno-Rudzinski Wojciech , Lijadi Melania , Bove Philippe
Published: July 2008

Photoreflectance (PR) spectroscopy is performed to investigate the Fermi level pinning at the surface of GaAsSb, in a series of epitaxial structures with different Sb concentration. PR spectra exhibit Franz–Keldysh oscillations, originating from the built-in electric field in the GaAsSb layer. Experimental results indicate that the surface Fermi level is pinned in the lower half bandgap. The surface Fermi level is determined versus the Sb concentration between 38% and 52%.