bluuebunny/crossref_metadata_2025 huggingface dataset:
Bite Block Vowel Production in Apraxia of Speech
Authors: Jacks Adam
Published: July 2008

Purpose

This study explored vowel production and adaptation to articulatory constraints in adults with acquired apraxia of speech (AOS) plus aphasia.

Method

Five adults with acquired AOS plus aphasia and 5 healthy control participants produced the vowels [
i
], [ɛ], and [æ] in four word-length conditions in unconstrained and bite block conditions. In addition to acoustic and perceptual measures of vowel productions, individually determined idealized vowels based on each participant’s best performance were used to assess vowel accuracy and distinctiveness.

Results

Findings showed (a) clear separation of vowel formants in speakers with AOS; (b) impaired vowel production in speakers with AOS, shown by perceptual measures of vowel quality and acoustic measures of vowel accuracy and contrastivity; and (c) incomplete compensation to bite block compensation both for individuals with AOS and for healthy controls.

Conclusions

Although adults with AOS were less accurate overall in vowel production than unimpaired speakers, introduction of a bite block resulted in similar patterns of decreased vowel accuracy for the two groups. Findings suggest that feedback control for vowel production is relatively intact in these individuals with AOS and aphasia. Predominant use of feedback control mechanisms is hypothesized to account for characteristic vowel deficits of the disorder.
Self-Esteem, Shyness, and Sociability in Adolescents With Specific Language Impairment (SLI)
Authors: Wadman Ruth , Durkin Kevin , Conti-Ramsden Gina
Published: July 2008

Purpose

To determine if lower global self-esteem, shyness, and low sociability are outcomes associated with SLI in adolescence. Possible concurrent predictive relationships and gender differences were also examined.

Method

Fifty-four adolescents with SLI, aged between 16 and 17 years, were compared with a group of 54 adolescents with typical language abilities on the Rosenberg Self-Esteem scale (Rosenberg, 1965) and the Cheek and Buss Shyness and Sociability scales (Cheek & Buss, 1981).

Results

The SLI group had significantly lower global self-esteem scores than the group with typical language abilities. The adolescents with SLI were more shy than their peers, but the groups did not differ in their sociability ratings. Regression analysis found that language ability was not concurrently predictive of self-esteem but shyness was. Mediation analysis suggested that shyness could be a partial but significant mediator in the relationship between language ability and global self-esteem.

Conclusions

Older adolescents with SLI are at risk of lower global self-esteem and experience shyness, although they want to interact socially. The relationship between language ability and self-esteem at this point in adolescence is complex, with shyness potentially playing an important mediating role.
Maturation of Visual and Auditory Temporal Processing in School-Aged Children
Authors: Dawes Piers , Bishop Dorothy V. M.
Published: July 2008

Purpose

To examine development of sensitivity to auditory and visual temporal processes in children and the association with standardized measures of auditory processing and communication.

Methods

Normative data on tests of visual and auditory processing were collected on 18 adults and 98 children aged 6–10 years of age. Auditory processes included detection of pitch from temporal cues using iterated rippled noise and frequency modulation detection at 2 Hz, 40 Hz, and 240 Hz. Visual processes were coherent form and coherent motion detection. Test–retest data were gathered on 21 children.

Results

Performance on perceptual tasks improved with age, except for fine temporal processing (iterated rippled noise) and coherent form perception, both of which were relatively stable over the age range. Within-subject variability (as assessed by track width) did not account for age-related change. There was no evidence for a common temporal processing factor, and there were no significant associations between perceptual task performance and communication level (Children’s Communication Checklist, 2nd ed.; D. V. M. Bishop, 2003) or speech-based auditory processing (SCAN-C; R. W. Keith, 2000).

Conclusions

The auditory tasks had different developmental trajectories despite a common procedure, indicating that age-related change was not solely due to responsiveness to task demands. The 2-Hz frequency modulation detection task, previously used in dyslexia research, and the visual tasks had low reliability compared to other measures.
Attentional Modulation of Word Recognition by Children in a Dual-Task Paradigm
Authors: Choi Sangsook , Lotto Andrew , Lewis Dawna , Hoover Brenda , Stelmachowicz Patricia
Published: July 2008

Purpose

This study investigated an account of limited short-term memory capacity for children’s speech perception in noise using a dual-task paradigm.

Method

Sixty-four normal-hearing children (7–14 years of age) participated in this study. Dual tasks were repeating monosyllabic words presented in noise at 8 dB signal-to-noise ratio and rehearsing sets of 3 or 5 digits for subsequent serial recall. Half of the children were told to allocate their primary attention to word repetition and the other half to remembering digits. Dual-task performance was compared to single-task performance. Limitations in short-term memory demands required for the primary task were measured by dual-task decrements in nonprimary tasks.

Results

Results revealed that (a) regardless of task priority, no dual-task decrements were found for word recognition, but significant dual-task decrements were found for digit recall; (b) most children did not show the ability to allocate attention preferentially to primary tasks; and (c) younger children (7- to 10-year-olds) demonstrated improved word recognition in the dual-task conditions relative to their single-task performance.

Conclusions

Seven- to 8-year-old children showed the greatest improvement in word recognition at the expense of the greatest decrement in digit recall during dual tasks. Several possibilities for improved word recognition in the dual-task conditions are discussed.
Visually driven analysis of movement data by progressive clustering
Authors: Rinzivillo Salvatore , Pedreschi Dino , Nanni Mirco , Giannotti Fosca , Andrienko Natalia , Andrienko Gennady
Published: July 2008

The paper investigates the possibilities of using clustering techniques in visual exploration and analysis of large numbers of trajectories, that is, sequences of time-stamped locations of some moving entities. Trajectories are complex spatio-temporal constructs characterized by diverse non-trivial properties. To assess the degree of (dis)similarity between trajectories, specific methods (distance functions) are required. A single distance function accounting for all properties of trajectories, (1) is difficult to build, (2) would require much time to compute, and (3) might be difficult to understand and to use. We suggest the procedure of progressive clustering where a simple distance function with a clear meaning is applied on each step, which leads to easily interpretable outcomes. Successive application of several different functions enables sophisticated analyses through gradual refinement of earlier obtained results. Besides the advantages from the sense-making perspective, progressive clustering enables a rational work organization where time-consuming computations are applied to relatively small potentially interesting subsets obtained by means of ‘cheap’ distance functions producing quick results. We introduce the concept of progressive clustering by an example of analyzing a large real data set. We also review the existing clustering methods, describe the method OPTICS suitable for progressive clustering of trajectories, and briefly present several distance functions for trajectories.
Circular and noncircular subsonic jets in cross flow
Authors: Gutmark Ephraim J. , Ibrahim Irene M. , Murugappan Shanmugam
Published: July 2008

Experiments were conducted to investigate the impact of circular and noncircular injector nozzle geometries on the penetration, mixing, and structure of a jet injected perpendicular into subsonic cross flow. Circular, triangle, and elongated slot nozzles were studied including varying their orientations relative to the cross flow, to comprise a matrix of five different test cases. The cross-sectional areas of all geometries were equal. The interaction between the jets and the cross flow was strongly dependent on the different geometries and their orientations. Each of the injector geometries resulted in a distinctive region of backflow at the leeward side of the jet. The location and size of this region and the reverse flow magnitude were largely dependent on the aspect ratio and the jet cross section lateral dimension change in the axial direction. A combination of instantaneous and time-averaged particle image velocimetry images taken in the axial and lateral cross sections of the jet near the exit plane revealed several large scale turbulent structures downstream of the jets for two different blowing ratios r=3.2 and 8. Detailed flow field data are presented for the higher blowing ratio due to the better spatial resolution of the large scale structures in the near field. A counter-rotating vortex pair was identified in the near field of the jets resulting in a reverse flow region. These structures had an upright nature similar to the wake vortices with the exception that the former were attached to and constantly fed from jet fluid mixed with the cross flow. The latter was formed by the cross flow, fed from its boundary layer vorticity, and remained separate from the jet. It was attached to the surface of the flat plate at one end and to the trajectory of the bent jet at the other end. Both the shape and orientation of the nozzle geometries had a strong effect on the jet spread and penetration. Compared to the baseline circular nozzle, the slot major (slot nozzle’s major axis oriented along the cross stream) exhibited the highest penetration, whereas the slot minor penetrated the least. The triangular jets’ characteristics were significantly affected by their orientation relative to the cross flow. Several different scaling techniques were explored in order to compare the present jet trajectories with previously published data. Commonly used length scaling parameters of rd and r2d were shown to be inadequate to scale different experimental data. Newly proposed scaling that was based on the velocity profiles of the cross flow boundary layer and the jet, prior to their interaction, yielded better results.
Electron spin-spin coupling from multireference configuration interaction wave functions
Authors: Gilka Natalie , Taylor Peter R. , Marian Christel M.
Published: July 2008

We present the implementation of two-electron spin-spin coupling as a quasidegenerate perturbative treatment of the Breit–Pauli spin-spin Hamiltonian. The evaluation is based on a multireference CI treatment and constitutes one of the first efforts in the calculation of this effect within a highly sophisticated consideration of both nondynamical and dynamical correlation. The extension of existing schemes for efficient calculation, in particular, of the spin-coupling elements necessitated some involved derivations, the outline of which is presented herein. Application of the program to calculations of diagonal as well as off-diagonal spin-coupling elements is illustrated with the test cases O2 and NH.
Deformation twin formed by self-thickening, cross-slip mechanism in nanocrystalline Ni
Authors: Wu X. L. , Narayan J. , Zhu Y. T.
Published: July 2008

We report the observation of a deformation twin formed by a recently proposed self-thickening, cross-slip twinning mechanism. This observation verifies one more twinning mechanism, in addition to those reported before, in nanocrystalline face-centered-cubic metals. In this mechanism, once the first Shockley partial is emitted from a grain boundary, and cross slips onto another slip plane, a deformation twin could nucleate and grow in both the primary and cross-slip planes without requiring the nucleation of additional Shockley partials from the grain boundary.
Thermocapillary manipulation of droplets using holographic beam shaping: Microfluidic pin ball
Authors: Cordero Maria Luisa , Burnham Daniel R. , Baroud Charles N. , McGloin David
Published: July 2008

We demonstrate that holographically generated optical patterns offer greater flexibility for the thermocapillary control of water droplets than Gaussian spots; droplets can be stopped in faster flows while using less optical intensity when the surface tension variations are created by line patterns instead of single spots. Further, experiments are performed making use of variable light patterns to achieve controlled droplet routing in a four-way cross microfluidic channel. Finally, multiple droplet storage is demonstrated as well as changing drop order.
Local ordering of polymer-tethered nanospheres and nanorods and the stabilization of the double gyroid phase
Authors: Iacovella Christopher R. , Horsch Mark A. , Glotzer Sharon C.
Published: July 2008

We present results of Brownian dynamics simulations of tethered nanospheres and tethered nanorods. Immiscibility between tether and nanoparticle facilitates microphase separation into the bicontinuous, double gyroid structure (first reported by Iacovella et al. [Phys. Rev. E 75, 040801(R) (2007)] and Horsch et al. [J. Chem. Phys. 125, 184903 (2006)], respectively). We demonstrate the ability of these nanoparticles to adopt distinct, minimal energy local packings, in which nanospheres form icosahedral-like clusters and nanorods form splayed hexagonal bundles. These local structures reduce packing frustration within the nodes of the double gyroid. We argue that the ability to locally order into stable structures is key to the formation of the double gyroid phase in these systems.
Quantum Markov chains
Authors: Gudder Stanley
Published: July 2008

A new approach to quantum Markov chains is presented. We first define a transition operation matrix (TOM) as a matrix whose entries are completely positive maps whose column sums form a quantum operation. A quantum Markov chain is defined to be a pair (G,E) where G is a directed graph and E=[Eij] is a TOM whose entry Eij labels the edge from vertex j to vertex i. We think of the vertices of G as sites that a quantum system can occupy and Eij is the transition operation from site j to site i in one time step. The discrete dynamics of the system is obtained by iterating the TOM E. We next consider a special type of TOM called a transition effect matrix. In this case, there are two types of dynamics, a state dynamics and an operator dynamics. Although these two types are not identical, they are statistically equivalent. We next give examples that illustrate various properties of quantum Markov chains. We conclude by showing that our formalism generalizes the usual framework for quantum random walks.
Diffusion in linear porous media with periodic entropy barriers: A tube formed by contacting spheres
Authors: Vazquez Marco-Vinicio , Berezhkovskii Alexander M. , Dagdug Leonardo
Published: July 2008

The problem of transport in quasi-one-dimensional periodic structures has been studied recently by several groups [D. Reguera et al., Phys. Rev. Lett.96, 130603 (2006); P. S. Burada et al., Phys. Rev. E75, 051111 (2007); B. Q. Ai and L. G. Liu, ibid.74, 051114 (2006); B. Q. Ai et al., ibid.75, 061126 (2007); B. Q. Ai and L. G. Liu, J. Chem. Phys.126, 204706 (2007); 128, 024706 (2008); E. Yariv and K. D. Dorfman, Phys. Fluids19, 037101 (2007); N. Laachi et al., Europhys. Lett.80, 50009 (2007); A. M. Berezhkovskii et al., J. Chem. Phys.118, 7146 (2003); 119, 6991 (2003)]. Using the concept of “entropy barrier” [R. Zwanzig, J. Phys. Chem.96, 3926 (1992)] one can classify such structures based on the height of the entropy barrier. Structures with high barriers are formed by chambers, which are weakly connected with each other because they are connected by small apertures. To escape from such a chamber a diffusing particle has to climb a high entropy barrier to find an exit that takes a lot of time [I. V. Grigoriev et al., J. Chem. Phys.116, 9574 (2002)]. As a consequence, the particle intrachamber lifetime τesc is much larger than its intrachamber equilibration time, τrel, τesc⪢τrel. When the aperture is not small enough, the intrachamber escape and relaxation times are of the same order and the hierarchy fails. This is the case of low entropy barriers. Transport in this case is analyzed in the works of Schmid and co-workers, Liu and co-workers, and Dorfman and co-workers, while the work of Berezhkovskii et al. is devoted to diffusion in the case of high entropy barriers.
Formation and diffusion of oxygen-vacancy pairs on TiO2(110)-(1×1)
Authors: Cui Xuefeng , Wang Bing , Wang Zhuo , Huang Tian , Zhao Yan , Yang Jinlong , Hou J. G.
Published: July 2008

We present the measurements for the diffusion of bridging oxygen vacancy (OV) crossover Ti rows via OV pairs (OVPs). Using a high-resolution scanning tunneling microscope (STM), we show that the OVs can be moved along the bridging oxygen rows driven by the STM tip at voltages higher than 3.0 V on TiO2(110)-(1×1) surface. It is found that the combination of OVPs leads to the formation of OVPs, which can diffuse crossover Ti rows under the mediation of OVs in adjacent bridging oxygen rows. The deduced diffusion activation energy for the diffusive OVPs from experiments is in agreement with first-principles calculations. The reaction activation energy of the OVPs with O2 is lower than that of the OVs by 82 meV.
An extended multireference study of the electronic states of para-benzyne
Authors: Wang Evan B. , Parish Carol A. , Lischka Hans
Published: July 2008

A state-averaged, multireference complete active space (CAS) approach was used for the determination of the vertical excitation energies of valence and Rydberg states of para-benzyne. Orbitals were generated with a 10- and 32-state averaged multiconfigurational self-consistent field approach. Electron correlation was included using multireference configuration interaction with singles and doubles, including the Pople correction for size extensivity, multireference averaged quadratic coupled cluster (MR-AQCC), and MR-AQCC based on linear response theory. There is a very high density of electronic states in this diradical system—there are more than 17 states within 7eV of the ground state including two 3s Rydberg states. All excitations, except 2Ag1, are from the π system to the σσ* system. Of the 32 states characterized, 15 were multiconfigurational, including the ground Ag1 state, providing further evidence for the necessity of a multireference approach for p-benzyne. The vertical singlet-triplet splitting was also characterized using a two-state averaged approach. A CAS(2,2) calculation was shown to be inadequate due to interaction with the π orbitals.
Solvent and lipid dynamics of hydrated lipid bilayers by incoherent quasielastic neutron scattering
Authors: Swenson J. , Kargl F. , Berntsen P. , Svanberg C.
Published: July 2008

The microscopic dynamics of the planar, multilamellar lipid bilayer system 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) has been investigated using quasielastic neutron scattering. The DMPC was hydrated to a level corresponding to approximately nine water molecules per lipid molecule. Selective deuteration has been used to separately extract the dynamics of the water, the acyl chains, and the polar head groups from the strong incoherent scattering of the remaining hydrogen atoms. Furthermore, the motions parallel and perpendicular to the bilayers were probed by using two different sample orientations relative to the incident neutron beam. For both sample orientations, the results showed an onset of water motions at 260 K on the experimental time scale of about 100 ps. From lack of wave-vector dependence of the onset temperature for water motions, it is evident that the observed water dynamics is of mainly rotational character at such low temperatures. At 290 K, i.e., slightly below the gel-to-liquid transition around 295 K, the nature of the water dynamics had changed to a more translational character, well described by a jump-diffusion model. On the limited experimental time and length (about 10 Å) scales, this jump-diffusion process was isotropic, despite the very anisotropic system. The acyl chains exhibited a weak onset of anharmonic motions already at 120 K, probably due to conformational changes (trans-gauche and/or syn-anti) in the plane of the lipid bilayers. Other anharmonic motions were not observed on the experimental time scale until temperature had been reached above the gel-to-liquid transition around 295 K, where the acyl chains start to show more substantial motions.
Kinetic mechanism of the thermal-induced self-organization of Au/Si nanodroplets on Si(100): Size and roughness evolution
Authors: Ruffino F. , Canino A. , Grimaldi M. G. , Giannazzo F. , Roccaforte F. , Raineri V
Published: July 2008

Very thin Au layer was deposited on Si(100) using the sputtering technique. By annealing at 873 K Au/Si nanodroplets were formed and their self-organization was induced changing the annealing time. The evolution of droplet size distribution, center-to-center distance distribution, and droplet density as a function of the annealing time at 873 K was investigated by Rutherford backscattering spectrometry, atomic force microscopy (AFM), and scanning electron microscopy. As a consequence of such study, the droplet clustering is shown to be a ripening process of hemispherical three-dimensional structures limited by the Au surface diffusion. The application of the ripening theory allowed us to calculate the surface diffusion coefficient and all other parameters needed to describe the entire process. Furthermore, the AFM measurements allowed us to study the roughness evolution of the sputtered Au thin film and compare the experimental data with the dynamic scaling theories of growing interfaces.
On the doublet states of the potassium trimer
Authors: Hauser Andreas W. , Callegari Carlo , Soldán Pavel , Ernst Wolfgang E.
Published: July 2008

The potassium trimer is investigated in its lowest electronic doublet states, employing several high-level ab initio methods (coupled cluster with single, double, and noniterative triple excitations, multiconfiguration self-consistent field, and multireference Rayleigh–Schrödinger perturbation theory of second order). One-dimensional cuts through the lowest 12 electronic states at C2v symmetry give insight in the complex electronic structure of the trimer, showing several (pseudo-)Jahn–Teller distortions that involve two or three excited states. Contour plots of the involved molecular orbitals are shown to prove the validity of the shell model frequently used for a qualitative description of metallic clusters.
On the heat capacities of Ta2AlC, Ti2SC, and Cr2GeC
Authors: Drulis Monika K. , Drulis H. , Hackemer A. E. , Leaffer O. , Spanier J. , Amini S. , Barsoum M. W. , Guilbert T. , El-Raghy T.
Published: July 2008

Herein we report on the heat capacities cp of bulk predominantly single-phase polycrystalline samples of Ti2SC and Cr2GeC in the 3–1500K temperature range and Ta2AlC in the 3–260K range. At temperatures up to 10K the main contributors to cp for Ta2AlC and Cr2GeC are electronic, with electronic coefficients γ of 7.13 and 26.12mJ∕molK2, respectively. The latter is exceptionally high and is a record for this family of layered ternary carbides and nitrides also known as the MAX phases. In Ti2SC another low-temperature contribution—in addition to a γ of 3.8mJ∕molK2—is manifested by an upturn in cp∕T observed at the lowest temperatures. This feature, appearing as a Schottky-like anomaly, has a local maximum near 4.5K and an intensity of ∼1.9×10−2J∕molK. A defect concentration of ∼3×1021∕mol presumably on the S-sublattice, is proposed as the origin of a two-level energy system responsible for this anomaly. As in previous work on these compounds, the lattice contributions to cp in all compounds are analyzed using the Debye and Einstein model approximations. The main effect of increasing the atomic number of the transition metal is a reduction in Debye temperature.
Quantum simulation of noise in silicon nanowire transistors
Authors: Park Hong-Hyun , Jin Seonghoon , Park Young June , Min Hong Shick
Published: July 2008

The noise phenomena of silicon nanowire transistors are investigated through quantum transport simulations. Under the assumption of phase-coherent transport, the scattering approach and the nonequilibrium Green’s function formalism are employed. We present the drain current and noise characteristics at various bias conditions and interpret these properties. We also present conditions for generating shot noise, which explains the bias dependence of shot noise suppression in silicon nanowire transistors.
Surface and size effects on the electrical properties of Cu nanowires
Authors: Huang Qiaojian , Lilley Carmen M. , Bode Matthias , Divan Ralu
Published: July 2008

Copper nanowires were patterned with e-beam lithography and fabricated with a copper film deposited by e-beam evaporation. Various electrical properties of these nanowires (including resistivity, temperature coefficient of resistance, and failure current density) were characterized. It was experimentally found that surface and size have apparent effects on the electrical properties. Smaller values for the temperature coefficient of resistance and higher failure current density were found for Cu nanowires with decreasing wire width. The experimental finding of width dependent failure current density also agrees with finding for theoretical heat transfer of the nanowire and substrate system as calculated with the finite element method.
Integral invariants of two-dimensional and quasigeostrophic shallow-water turbulence
Authors: Fox S. , Davidson P. A.
Published: July 2008

We use direct numerical simulations to investigate the low-wavenumber behavior of (i) two-dimensional turbulence and (ii) shallow-water, quasigeostrophic (SWQG) turbulence. For two-dimensional turbulence there are three canonical cases: E(k→0)∼Jk−1, E(k→0)∼Lk, and E(k→0)∼Ik3, where J, L, and I are various integral moments of the two-point vorticity correlation ⟨ωω′⟩. Our simulations confirm that in line with earlier theoretical predictions, J and L are invariants, but that I is time dependent. This, in turn, is consistent with the idea that the triple correlations in two-dimensional turbulence fall off as ⟨ux2ux′⟩∼r−3 at large separation, r, something that we confirm directly in our simulations. We also confirm that a random sea of monopoles leads to E∼Jk−1 turbulence, while a random sea of dipoles yields E∼Lk. Finally, we observe that the integral scale in two-dimensional turbulence grows approximately as ℓ∼t in all three cases, i.e., E∼Jk−1, E∼Lk, and E∼Ik3. The earlier theoretical work is extended to SWQG turbulence. In particular we note that in contrast to two-dimensional turbulence, there are unlikely to be any long-range triple correlations of the form ⟨ux2ux′⟩∼r−3. If this is indeed the case, then Loitsyansky’s integral I is an invariant of SWQG turbulence, as is M, the prefactor in front of k5 in the low-k expansion E∼Ik3+Mk5+O(k7). This suggests that if SWQG turbulence is started with a spectrum steeper than E∼k7, then it will revert to E∼k7, whereas a spectrum shallower than E∼k7 will be invariant at low k. All of these predictions are confirmed by direct numerical simulation.
A metal-dielectric cathode
Authors: Fan Yu-Wei , Zhong Hui-Huang , Li Zhi-Qiang , Yang Han-Wu , Shu Ting , Zhou Heng , Yuan Cheng-Wei , Zhang Jun , Luo Ling
Published: July 2008

In order to improve the pulse repetition rate and the maintenance-free lifetime of an improved magnetically insulated transmission line oscillator (MILO) previously developed in our laboratory, a metal-dielectric cathode is investigated experimentally. It consists of three components: a stainless steel base, bronze foils, and double-sided printed boards. The experimental results show that the shot-to-shot reproducibility of the diode voltage and current is very good and the performances of the improved MILO are steady. In addition, no observable degradation could be detected in the emissive characteristic of the metal-dielectric cathode after 350 shots. The experimental results prove that the metal-dielectric cathode is a promising cathode for repetitively pulsed MILO operation. However, the leading edge of the radiated microwave pulse is gradually delayed during the repetition rate. A likely reason is that high pressure results in gas ionization in the beam-microwave interaction region, and plasma formation delays the time that the improved MILO achieves nonlinear steady state.
Comparison of 4H-SiC impact ionization models using experiments and self-consistent simulations
Authors: Akturk Akin , Goldsman Neil , Aslam Shahid , Sigwarth John , Herrero Fred
Published: July 2008

We report comparisons of measured photocurrent versus voltage curves of avalanche photodiodes (APDs) with those calculated using different 4H-SiC hole and electron impact ionization coefficients. As the published impact ionization coefficients result in ionization rates that differ greatly in magnitude, the predicted breakdown voltages using these models vary by many volts. To this end, we investigate the breakdown voltage prediction capability of three prevailing impact ionization models in conjunction with several experiments. To obtain APD performance numerically, we developed a device simulator, which shows that the inclusion of proper electric field-dependent impact ionization rates can accurately predict a variety of measured current-voltage curves, breakdown voltages, and current multiplication rates.
Convective diffusion in steady flow through a tube with a retentive and absorptive wall
Authors: Ng Chiu-On , Rudraiah N.
Published: July 2008

This is a study on the mass transport, accomplished by reaction, advection, and dispersion, of a solute in steady Poiseuille flow through a circular tube with a reactive wall layer. The reaction consists of a reversible component due to phase exchange between the flowing fluid and the wall layer and an irreversible component due to absorption into the wall. First, the generalized dispersion model is employed to deduce asymptotic steady-state values of the first three transport coefficients in terms of the strengths and kinetics of the two reactions, which can be of any magnitude. Second, a numerical simulation is performed to examine the time development of the fluid- and wall-phase concentration profiles starting from the initial release of the solute into the tube. The analytical deduction brings out not only results relevant to the asymptotic state when the transport coefficients become independent of time but also criteria that can be used to estimate the significance of the asymptotic steady state in the whole course of mass transport. The numerical simulation generates time-developing concentration profiles that can be used to explain some paradoxical behaviors exhibited by the transport coefficients under certain conditions.
Observation of spectral anisotropy of gold nanoparticles
Authors: Cang Hu , Montiel Daniel , Xu C. Shan , Yang Haw
Published: July 2008

Metallic nanoparticles synthesized by solution-phase chemistry usually exhibit various polygonal morphologies. The shape is known to have a great impact on a nanoparticle’s optical properties, for instance, the surface plasmon resonance frequency. It remains unclear, however, whether the scattering spectrum of nanoparticles is generally anisotropic in the far field as a result. This simple question turns out to be extremely challenging to address because of the particle-to-particle shape inhomogeneity in a bulk sample, and the high sensitivity of surface plasmon resonance to local environments. We report the observation of scattering angle-dependent spectra using a newly developed single-particle tracking spectroscopy (SPS). Furthermore, we show that SPS has provided a way to directly visualize the rotational random walk of individual gold nanoparticles in water for the first time.
Analysis and control of cavity flow
Authors: Kourta A. , Vitale E.
Published: July 2008

A flow above a cavity leads to an unsteady separated flow. This configuration exhibits an intense aeroacoustic coupling, where very intense aerodynamic noise can be emitted. Moreover, a majority of tangential flow above a cavity has an oscillatory character, resulting from a strong coupling between the acoustic and the flow dynamics. In the present work, we are interested in characterizing the dynamics and the frequency distribution of a cavity flow. First, the dynamics of the cavity are analyzed and the frequency distribution is established, which is followed by a study of nonlinear interaction. An open loop control using a synthetic jet is then applied in order to reduce noise generation. Finally, by choosing suitable jet parameters a significant noise reduction is obtained.
Mechanical twinning and omega transition by ⟨111⟩ {112} shear in a metastable β titanium alloy
Authors: Xing H. , Sun J.
Published: July 2008

{112} ⟨111⟩ mechanical twinning and stress-induced omega transition were observed by high-resolution transmission electron microscope in a metastable β titanium alloy with chemical composition of Ti-23Nb-0.7Ta-2Zr-1.2O at. % after deformation. The orientation relationships between the ω phase and β parent matrix are (1¯010)ω‖(211)β, [12¯10]ω‖[01¯1]β and [0001]ω‖[1¯11]β, and the habit plane of (1¯010)ω‖(211)β for the stress-induced ω transition is different from that of (0001)ω‖(111)β often observed for the thermal ω transition. Both mechanical twinning and ω transition arise from the shear along ⟨111⟩ {112}. A dislocation mechanism for mechanical twinning and stress-induced ω transition was discussed additionally.
Driving force for formation of a surface relief grating on an azobenzene-containing polymer
Authors: Inoue Naoki , Nozue Manabu , Yamane Osamu , Umegaki Shinsuke
Published: July 2008

The driving force for formation of a surface relief grating on an azobenzene-containing polymer is theoretically analyzed and experimentally confirmed by polarization analyses. We formulate the driving force based on the Coulomb interaction between an optical electric field and its induced polarization charges. The formula derived includes not only the optically induced gradient force but also the force due to the spatial modulation of the refractive index. The latter stems from the reorientation of the azobenzene molecules in the polymer through the cyclic trans-cis photoisomerization caused by the Ar+ laser used to construct the interference pattern.
Anisotropic biaxial stresses in diamond films by polarized Raman spectroscopy of cubic polycrystals
Authors: Gries T. , Vandenbulcke L. , Simon P. , Canizares A.
Published: July 2008

The anisotropic stresses in diamond films induced by permanent deformations of titanium tensile substrates are studied by polarized Raman spectroscopy. The influence of the in-plane orientation of each grain relative to the stress directions is shown experimentally and theoretically. Considering the dynamical equations under small strains, the solutions of the secular equation permit the determination of the triply degenerate phonon frequency of polycrystalline diamond, which splits into three singlets for each crystallite. The calculation is carried out here for six growth directions, along [001], [110], [111], [112], [113], and [331], and for all in-plane orientations normal to these textures. In the case of anisotropic stresses, it is shown how the relative values of the two stresses induce different shapes in the Raman spectra. For an isotropic polycrystalline diamond film, this Raman spectroscopy of strained cubic polycrystals approximates the complete solution for all crystallite orientations relative to the anisotropic biaxial stresses. It allows calculating theoretical Raman spectra that fairly well compare to the experimental ones. The determination of the anisotropic stresses is obtained with a better accuracy when the influence of all orientations is taken into account than with the previous average procedures used for isotropic cubic polycrystalline materials. Partially or completely textured films along one of the growth directions can also be studied. If the stresses along the two directions are identical, the particular case of isotropic biaxial stresses is also studied with a better accuracy.
Ultralow dielectric constant pSiCOH films prepared with tetramethylcyclotetrasiloxane as skeleton precursor
Authors: Grill A. , Patel V.
Published: July 2008

Ultralow dielectric constant pSiCOH films have been prepared using tetramethylcyclotetrasiloxane (TMCTS) as the skeleton precursor and two different porogen precursors. The porogen has been removed from the deposited films by thermal annealing at 400 °C, obtaining films with dielectric constants down to 1.95. The films have been investigated by Fourier transform infrared spectroscopy, and n&k optical measurements of the refractive index (n) and the electrical characteristics have been measured on metal-insulator-semiconductor structures. It was found that the properties of the annealed films depend on the deposition temperature and used porogen, different concentrations of porogens in the plasma feed being required to obtain the same final dielectric constant. The efficiency of porogen incorporation in the films prepared from TMCTS was very low and was dependent on the selected porogen precursor.
Evidence of low interface trap density in GeO2∕Ge metal-oxide-semiconductor structures fabricated by thermal oxidation
Authors: Matsubara Hiroshi , Sasada Takashi , Takenaka Mitsuru , Takagi Shinichi
Published: July 2008

We have fabricated GeO2∕Ge metal-oxide-semiconductor (MOS) structures by direct thermal oxidation of Ge substrates. The interface trap density (Dit) of Al∕GeO2∕Ge MOS structures, measured by the low temperature conductance method including the effect of the surface potential fluctuation, is found to be reduced as the oxidation temperature increases. The minimum values of Dit can be obtained for the oxidation around 575°C, which is in the maximum temperature range where GeO volatilization does not occur under atmospheric pressure of O2. It is also found that the hydrogen annealing before Al gate formation is effective for the passivation of GeO2∕Ge interface states. It is clarified, as a result, that the minimum Dit value lower than 1011cm−2eV−1 can be obtained for GeO2∕Ge MOS interfaces fabricated by direct oxidation of Ge substrates.
Precise in situ thickness analysis of epitaxial graphene layers on SiC(0001) using low-energy electron diffraction and angle resolved ultraviolet photoelectron spectroscopy
Authors: Riedl C. , Zakharov A. A. , Starke U.
Published: July 2008

We demonstrate an easy and practical method for the thickness analysis of epitaxial graphene on SiC(0001) that can be applied continuously during the preparation procedure. Fingerprints in the spot intensity spectra in low energy electron diffraction (LEED) allow for the exact determination of the number of layers for the first three graphene layers. The LEED data have been correlated with the electronic bandstructure around the K¯-point of the graphene Brillouin zone as investigated by laboratory based angle resolved ultraviolet photoelectron spectroscopy using He II excitation. The morphology and homogeneity of the graphene layers can be analyzed by low energy electron microscopy.
Development of a trielectrode plasma curtain at atmospheric pressure
Authors: Zastawny H. , Sosa R. , Grondona D. , Márquez A. , Artana G. , Kelly H.
Published: July 2008

The development of a nonequilibrium, low-power, trielectrode plasma curtain at atmospheric pressure is presented. The discharge is based on the combination of an ac dielectric barrier discharge with a dc corona discharge in a three electrode system, and can be sustained for large time periods and over interelectrode air gaps up to 20mm and with an electrode length of ∼10cm in the transversal direction. The discharge is composed of a train of streamers, with a repetition frequency in the range 50–200kHz, and carrying an average current in the range 0.1–0.4mA. The geometry of the discharge makes it appropriate for gas decontamination.
Multiple substrate microwave plasma-assisted chemical vapor deposition single crystal diamond synthesis
Authors: Asmussen J. , Grotjohn T. A. , Schuelke T. , Becker M. F. , Yaran M. K. , King D. J. , Wicklein S. , Reinhard D. K.
Published: July 2008

A multiple substrate, microwave plasma-assisted chemical vapor deposition synthesis process for single crystal diamond (SCD) is demonstrated using a 915MHz reactor. Diamond synthesis was performed using input chemistries of 6–8% of CH4∕H2, microwave input powers of 10–11.5kW, substrate temperatures of 1100–1200°C, and pressures of 110–135Torr. The simultaneous synthesis of SCD over 70 diamond seeds yielded good quality SCD with deposition rates of 14–21μm∕h. Multiple deposition runs totaling 145h of deposition time added 1.8–2.5mm of diamond material to each of the 70 seed crystals.
Resistance-pressure sensitivity and a mechanism study of multiwall carbon nanotube networks/poly(dimethylsiloxane) composites
Authors: Hu C. H. , Liu C. H. , Chen L. Z. , Peng Y. C. , Fan S. S.
Published: July 2008

In this work, we explored the electrical resistance-pressure sensitivity of multiwall carbon nanotube (MWNT) networks/poly(dimethylsiloxane) (PDMS) composites and proposed a deformation-induced property transition mechanism of the nanotubes to explain this behavior. The thermoelectric coefficients of the MWNT networks/PDMS composites and MWNT mat under pressure were also measured and discussed to support our proposition. Our results revealed that the relative resistances of MWNT networks/PDMS composites with lower MWNT loadings are more sensitive on the applied pressure. Furthermore, the I-V characteristic of the MWNT networks/PDMS composites modulated with pressure on one side has shown a well rectified behavior.
Negative-bias temperature instability induced electron trapping
Authors: Campbell J. P. , Cheung K. P. , Suehle J. S. , Oates A.
Published: July 2008

Despite four decades of research, the physics responsible for the negative-bias temperature instability (NBTI) in p-channel metal-oxide-silicon field-effect transistors is still unresolved. The current NBTI debate focuses on the dominance of either a hole trapping/detrapping mechanism or a hydrogen depassivation mechanism. In this study, we present NBTI-induced changes in the peak transconductance which indicate the presence of a third mechanism involving electron trapping/detrapping. The presence of this electron trapping/detrapping component adds further complexity to the very complicated NBTI phenomenon.
Evolution of the neutron quasielastic scattering through the ferroelectric phase transition in 93%PbZn1∕3Nb2∕3O3–7%PbTiO3
Authors: Rotaru G.-M. , Gvasaliya S. N. , Roessli B. , Kojima S. , Lushnikov S. G. , Günter P.
Published: July 2008

We show that the neutron diffuse scattering in relaxor ferroelectric (1−x)PZN−xPT (x=0.07) consists of two components. The first component is strictly elastic but extended in q-space and grows below 600K. The second component, which was not reported before for the (1−x)PZN−xPT relaxor ferroelectrics, is quasielastic with a linewidth that has a similar temperature dependence as the width of the central peak observed by Brillouin spectroscopy. The temperature dependence of the susceptibility of the quasielastic scattering has a maximum at the ferroelectric transition.
Bias-controlled spin polarization in quantum wires
Authors: Chen T.-M. , Graham A. C. , Pepper M. , Farrer I. , Ritchie D. A.
Published: July 2008

We demonstrate that a source-drain bias creates a fully spin-polarized current as the 0.25(2e2∕h) plateau in quantum wires even in zero magnetic field. When a source-drain bias lifts the momentum degeneracy, the dc measurements show that it is possible to achieve a unidirectional ferromagnetic order and this ordered spin array is destroyed once transport in both directions commences. The spin polarization of currents, between full spin polarization and partial spin polarization (or spin degeneracy), is thus simply controlled by source-drain bias and split-gate voltage, something of considerable value for spintronics.
Metallization contacts to nonpolar a-plane n-type GaN
Authors: Kim Hyunsoo , Lee Sung-Nam , Park Yongjo , Kwak Joon Seop , Seong Tae-Yeon
Published: July 2008

We have investigated the electrical characteristics of metallization contacts to nonpolar a-plane and polar c-plane n-type GaN. Pd Schottky diodes and x-ray photoemission spectroscopy measurements show that the Schottky barrier height of the a-plane GaN is lower than that of the c-plane GaN by 0.24 and 0.30eV, respectively. Ti∕Al Ohmic contacts to the a-plane n-GaN produce lower contact resistivity than that of the c-plane samples when annealed at 500°C. However, Ti∕Al contacts to the c-plane and a-plane GaN show opposite electrical behavior when annealed at temperatures above 500°C, which is attributed to the absence of polarization-induced surface charges for a-plane GaN.
Voltage dependent director of a homeotropic negative liquid crystal cell
Authors: Ruan L. Z. , Yang Fuzi , Sambles J. R.
Published: July 2008

Thin layers of obliquely (60° to normal) thermally evaporated SiOx lead to homeotropic alignment of a nematic liquid crystal (LC) having negative dielectric anisotropy. Under application of an ac voltage the director, as characterized by the fully leaky waveguide technique, is found to realign with a voltage controlled tilt along the evaporation direction. This behavior is in complete contrast with that of a LC having positive dielectric anisotropy and may have important implications for modern LC display technology.
Carrier dynamics of type-II InAs∕GaAs quantum dots covered by a thin GaAs1−xSbx layer
Authors: Chang Wen-Hao , Liao Yu-An , Hsu Wei-Ting , Lee Ming-Chih , Chiu Pei-Chin , Chyi Jen-Inn
Published: July 2008

Carrier dynamics of InAs∕GaAs quantum dots (QDs) covered by a thin GaAs1−xSbx layer were investigated by time-resolved photoluminescence (PL). Both the power dependence of PL peak shift and the long decay time constants confirm the type-II band alignment at the GaAsSb–InAs interface. Different recombination paths have been clarified by temperature dependent measurements. At lower temperatures, the long-range recombination between the QD electrons and the holes trapped by localized states in the GaAsSb layer is important, resulting in a non-single-exponential decay. At higher temperatures, optical transitions are dominated by the short-range recombination with the holes confined to the band-bending region surrounding the QDs.
Electrostatically actuated silicon-based nanomechanical switch at room temperature
Authors: Guerra Diego N. , Imboden Matthias , Mohanty Pritiraj
Published: July 2008

We demonstrate a silicon-based high-frequency nanomechanical device capable of switching controllably between two states at room temperature. The device uses a nanomechanical resonator with two distinct states in the hysteretic nonlinear regime. In contrast to prior work, we demonstrate room-temperature electrostatic actuation and sensing of the switching device with 100% fidelity by phase modulating the drive signal. This phase-modulated device can be used as a low-power, high-speed mechanical switch integrated on-chip with silicon circuitry.