bluuebunny/crossref_metadata_2025 huggingface dataset:
X-ray conversion efficiency in vacuum hohlraum experiments at the National Ignition Facility
Authors: Olson R. E. , Suter L. J. , Kline J. L. , Callahan D. A. , Rosen M. D. , Dixit S. N. , Landen O. L. , Meezan N. B. , Moody J. D. , Thomas C. A. , Warrick A. , Widmann K. , Williams E. A. , Glenzer S. H.
Published: May 2012

X-ray fluxes measured in the first 96 and 192 beam vacuum hohlraum experiments at the National Ignition Facility (NIF) were significantly higher than predicted by computational simulations employing XSN average atom atomic physics and highly flux-limited electron heat conduction. For agreement with experimental data, it was found that the coronal plasma emissivity must be simulated with a detailed configuration accounting model that accounts for x-ray emission involving all of the significant ionization states. It was also found that an electron heat conduction flux limit of f = 0.05 is too restrictive, and that a flux limit of f = 0.15 results in a much better match with the NIF vacuum hohlraum experimental data. The combination of increased plasma emissivity and increased electron heat conduction in this new high flux hohlraum model results in a reduction in coronal plasma energy and, hence, an explanation for the high (∼85%-90%) x-ray conversion efficiencies observed in the 235 < Tr < 345 eV NIF vacuum hohlraum experiments.
Effective theories of connections and curvature: Abelian case
Authors: Díaz-Marín Homero G. , Zapata José A.
Published: May 2012

We introduce a notion of measuring scales for quantum Abelian gauge systems. At each measuring scale a finite dimensional affine space stores information about the evaluation of the curvature on a discrete family of surfaces. Affine maps from the spaces assigned to finer scales to those assigned to coarser scales play the role of coarse graining maps. This structure induces a continuum limit space which contains information regarding curvature evaluation on all piecewise linear surfaces with boundary. The evaluation of holonomies along loops is also encoded in the spaces introduced here; thus, our framework is closely related to loop quantization and it allows us to discuss effective theories in a sensible way. We develop basic elements of measure theory on the introduced spaces which are essential for the applicability of the framework to the construction of quantum Abelian gauge theories.
Accurate ab initio potential energy surfaces for the 3A′′ and 3A′ electronic states of the O(3P)+HBr system
Authors: de Oliveira-Filho Antonio G. S. , Ornellas Fernando R. , Peterson Kirk A.
Published: May 2012

In this work, we report the construction of potential energy surfaces for the 3A′′ and 3A′ states of the system O(3P) + HBr. These surfaces are based on extensive ab initio calculations employing the MRCI+Q/CBS+SO level of theory. The complete basis set energies were estimated from extrapolation of MRCI+Q/aug-cc-VnZ(-PP) (n = Q, 5) results and corrections due to spin-orbit effects obtained at the CASSCF/aug-cc-pVTZ(-PP) level of theory. These energies, calculated over a region of the configuration space relevant to the study of the reaction O(3P) + HBr → OH + Br, were used to generate functions based on the many-body expansion. The three-body potentials were interpolated using the reproducing kernel Hilbert space method. The resulting surface for the 3A′′ electronic state contains van der Waals minima on the entrance and exit channels and a transition state 6.55 kcal/mol higher than the reactants. This barrier height was then scaled to reproduce the value of 5.01 kcal/mol, which was estimated from coupled cluster benchmark calculations performed to include high-order and core-valence correlation, as well as scalar relativistic effects. The 3A′ surface was also scaled, based on the fact that in the collinear saddle point geometry these two electronic states are degenerate. The vibrationally adiabatic barrier heights are 3.44 kcal/mol for the 3A′′ and 4.16 kcal/mol for the 3A′ state.
Attraction between like-charged monovalent ions
Authors: Zangi Ronen
Published: May 2012

Ions with like-charges repel each other with a magnitude given by the Coulomb law. The repulsion is also known to persist in aqueous solutions albeit factored by the medium's dielectric constant. In this paper, we report results from molecular dynamics simulations of alkali halides salt solutions indicating an effective attraction between some of the like-charged monovalent ions. The attraction is observed between anions, as well as between cations, leading to the formation of dimers with lifetimes on the order of few picoseconds. Two mechanisms have been identified to drive this counterintuitive attraction. The first is exhibited by high-charge density ions, such as fluoride, at low salt concentrations, yielding effective attractions with magnitude up to the order of 1–2 kT. In this case, the stronger local electric field generated when the two ions are in contact augments the alignment of neighboring waters toward the ions. This results in a gain of substantial favorable ion-water interaction energy. For fluorides, this interaction constitutes the major change among the different energy components compensating for the anion-anion repulsion, and therefore, rendering like-charge association possible. The second mechanism involves mediation by counterions, the attractions increase with salt concentration and are characterized by small magnitudes. In particular, clusters of ion triplets, in which a counterion is either bridging the two like-charged ions or is paired to only one of them, are formed. Although these two mechanisms may not yield net attractions in many cases, they might still be operational and significant, explaining effective repulsions between like-charged ions with magnitudes much smaller than expected based on continuum electrostatics.
Fluctuations and micro-heterogeneity in aqueous mixtures
Authors: Perera A. , Mazighi R. , Kežić B.
Published: May 2012

The problem as to why water-water density correlations are systematically overestimated in computer simulation of aqueous mixtures is examined through an extensive molecular dynamics study of mixtures of the extended single point charge water model with a fully miscible weaker version of it, obtained by scaling down the site partial charges by a factor 2/3, thereby eliminating solute-solvent size differences. The study reveals that enhanced water correlations is a genuine physical effect, and are not an artifact of the simulations or the models, as previously suggested in the context of realistic aqueous mixtures. Rather, they correspond to the existence of strongly correlated water domains, for “weak-water” mole fraction x > 0.4, that modulate the spatial decay of the density correlations. These domains produce a prepeak in the structure factor, suggesting that simple aqueous mixture might behave just like micro-emulsions. The overestimated long range water correlations result from incorrect predictions of the asymptote of these correlations, which themselves arise from size limitations of the simulation box. However, by requiring consistency between thermodynamical and structural expressions of the concentration fluctuations, a method to predict the proper decay of the correlation function is obtained herein, inspired by the formal analogy with micro-emulsions. This study provides a new insight for the large values of the experimental Kirkwood-Buff integrals for many aqueous mixtures: these mixtures are in a Lifshitz-type regime, where concentration fluctuations compete with water domain formation.
Nucleation of Ge clusters at high temperatures on Ge/Si(001) wetting layer
Authors: Arapkina Larisa V. , Yuryev Vladimir A.
Published: May 2012

Difference in nucleation of Ge quantum dots during Ge deposition at low (<600 °C) and high (≳600 °C) temperatures on the Si(001) surface is studied by high resolution scanning tunneling microscopy and in situ reflected high-energy electron diffraction. Two processes resulting in appearance of {105}-faceted clusters on the Ge wetting layer have been observed at high temperatures: Pyramids have been observed to nucleate via the previously described formation of strictly determined structures, resembling blossoms, composed by 16 dimers grouped in pairs and chains of 4 dimers on top of the wetting layer M×N patches, each on top of a separate single patch, just like it goes on at low temperatures; an alternative process consists in faceting of shapeless heaps of excess Ge atoms which arise in the vicinity of strong sinks of adatoms, such as pits or steps. The latter process is not observed at low temperatures; it is typical only for the high-temperature deposition mode.
Excitation-dependent recombination and diffusion near an isolated dislocation in GaAs
Authors: Gfroerer T. H. , Crowley C. M. , Read C. M. , Wanlass M. W.
Published: May 2012

In low-magnification, plan-view photoluminescence images of a nominally lattice-matched, undoped GaAs/GaInP heterostructure, we observe a random distribution of isolated dark spots. We attribute the dark spots to crystal dislocations, where nonradiative recombination is augmented by transitions utilizing defect-related energy levels between the conduction and valence bands. We note that, when the laser excitation intensity is reduced, the darkened regions expand. At lower excitation, the density of photogenerated electrons and holes is reduced, and they are more likely to reach the defective region before encountering a partner for radiative recombination. When we model the behavior with a simulation that allows for Laplacian diffusion and defect-related recombination only through mid-bandgap energy levels, we do not obtain good agreement between experimental and simulated images. But if we allow for an arbitrary distribution of defect levels, such that the occupation of the levels and bands can change independently, we have more flexibility for fitting the density-dependent recombination rates. The more sophisticated model produces results that are more consistent with experimental images.
A digitally configurable measurement platform using audio cards for high-resolution electronic transport studies
Authors: Gopman D. B. , Bedau D. , Kent A. D.
Published: May 2012

We report on a software-defined digitally configurable measurement platform for determining electronic transport properties in nanostructures with small readout signals. By using a high-resolution audio analog-to-digital/digital-to-analog converter in a digitally compensated bridge configuration we significantly increase the measurement speed compared to established techniques and simultaneously acquire large and small signal characteristics. We characterize the performance (16 bit resolution, 100 dB dynamic range at 192 kS/s) and demonstrate the application of this measurement platform for studying the transport properties of spin-valve nanopillars, a two-terminal device that exhibits giant magnetoresistance and whose resistance can be switched between two levels by applied magnetic fields and by currents applied by the audio card. The high resolution and fast sampling capability permits rapid acquisition of deep statistics on the switching of a spin-valve nanopillar and reduces the time to acquire the basic properties of the device – a state-diagram showing the magnetic configurations as function of applied current and magnetic field – by orders of magnitude.
Effect of Coulomb interactions on the vibronic couplings in ${\rm C}_{60}^-$C60−
Authors: Iwahara Naoya , Sato Tohru , Tanaka Kazuyoshi
Published: May 2012

Vibronic couplings in \documentclass[12pt]{minimal}\begin{document}${\rm C}_{60}^-$\end{document}C60− anion are discussed on the basis of the concept of the vibronic coupling density (VCD) [T. Sato, K. Tokunaga, and K. Tanaka, J. Chem. Phys. 124, 024314 (2006); K. Tokunaga, T. Sato, and K. Tanaka, J. Chem. Phys. 124, 154303 (2006); and T. Sato, K. Tokunaga, and K. Tanaka, J. Phys. Chem. A 112, 758 (2008)]. The VCD analysis clearly reveals that the coupling to the bending hg(2) mode is weaker than the coupling to the stretching hg(7) and hg(8) modes. For the vibronic couplings with the stretching modes, polarizations of the electron density difference on the bonds play a crucial role in the vibronic couplings. Such a polarized electron density difference appears as a result of the Coulomb interactions between the electrons in the lowest unoccupied molecular orbital and relevant doubly-occupied orbitals.
Influence of annealing and Al2O3 properties on the hydrogen-induced passivation of the Si/SiO2 interface
Authors: Dingemans G. , Einsele F. , Beyer W. , van de Sanden M. C. M. , Kessels W. M. M.
Published: May 2012

Annealing at moderate temperatures is required to activate the silicon surface passivation by Al2O3 thin films while also the thermal stability at higher temperatures is important when Al2O3 is implemented in solar cells with screenprinted metallization. In this paper, the relationship between the microstructure of the Al2O3 film, hydrogen diffusion, and defect passivation is explored in detail for a wide range of annealing temperatures. The chemical passivation was studied using stacks of thermally-grown SiO2 and Al2O3 synthesized by atomic layer deposition. Thermal effusion measurements of hydrogen and implanted He and Ne atoms were used to elucidate the role of hydrogen during annealing. We show that the passivation properties were strongly dependent on the annealing temperature and time and were significantly influenced by the Al2O3 microstructure. The latter was tailored by variation of the deposition temperature (Tdep = 50 °C–400 °C) with hydrogen concentration [H] between 1 and 13 at.% and mass density ρmass between 2.7 and 3.2 g/cm3. In contrast to films with intermediate material properties, the passivation by low- and high density films showed a reduced thermal stability at relatively high annealing temperatures (∼600 °C). These observations proved to be in good agreement with thermal effusion results of hydrogen and inert gas atoms that were also strongly dependent on film microstructure. We demonstrate that the temperature of maximum effusion decreased for films with progressively lower density (i.e., with increasing [H]). Therefore, the reduced thermal stability of the passivation for low-density hydrogen-rich ([H] >∼5 at. %) films can be attributed to a loss of hydrogen at relatively low annealing temperatures. In contrast, the lower initial [H] for dense Al2O3 films can likely explain the lower thermal stability associated with these films. The effusion measurements also allowed us to discuss the role of molecular- and atomic hydrogen during annealing.
Molecular beam epitaxy growth of high quality p-doped SnS van der Waals epitaxy on a graphene buffer layer
Authors: Wang W. , Leung K. K. , Fong W. K. , Wang S. F. , Hui Y. Y. , Lau S. P. , Chen Z. , Shi L. J. , Cao C. B. , Surya C.
Published: May 2012

We report on the systematic investigation of optoelectronic properties of tin (IV) sulfide (SnS) van der Waals epitaxies (vdWEs) grown by molecular beam epitaxy (MBE) technique. Energy band simulation using commercial CASTEP code indicates that SnS has an indirect bandgap of size 0.982 eV. Furthermore, our simulation shows that elemental Cu can be used as a p-type dopant for the material. Growth of high quality SnS thin films is accomplished by MBE technique using graphene as the buffer layer. We observed significant reduction in the rocking curve FWHM over the existing published values. Crystallite size in the range of 2–3 μm is observed which is also significantly better than the existing results. Measurement of the absorption coefficient, α, is performed using a Hitachi U-4100 Spectrophotometer system which demonstrate large values of α of the order of 104 cm−1. Sharp cutoff in the values of α, as a function of energy, is observed for the films grown using a graphene buffer layer indicating low concentration of localized states in the bandgap. Cu-doping is achieved by co-evaporation technique. It is demonstrated that the hole concentration of the films can be controlled between 1016 cm−3 and 5 × 1017cm−3 by varying the temperature of the Cu K-cell. Hole mobility as high as 81 cm2V−1s−1 is observed for SnS films on graphene/GaAs(100) substrates. The improvements in the physical properties of the films are attributed to the unique layered structure and chemically saturated bonds at the surface for both SnS and the graphene buffer layer. Consequently, the interaction between the SnS thin films and the graphene buffer layer is dominated by van der Waals force and structural defects at the interface, such as dangling bonds or dislocations, are substantially reduced.
Improving the quality factor of microwave compact resonators by optimizing their geometrical parameters
Authors: Geerlings K. , Shankar S. , Edwards E. , Frunzio L. , Schoelkopf R. J. , Devoret M. H.
Published: May 2012

Applications in quantum information processing and photon detectors are stimulating a race to produce the highest possible quality factor on-chip superconducting microwave resonators. We have tested the surface-dominated loss hypothesis by systematically studying the role of geometrical parameters on the internal quality factors of compact resonators patterned in Nb on sapphire. Their single-photon internal quality factors were found to increase with the distance between capacitor fingers, the width of the capacitor fingers, and the resonator impedance. Quality factors were improved from 210 000 to 500 000 at T = 200 mK. All of these results are consistent with our starting hypothesis.
Observation of infrared free-induction decay and optical nutation signals from nitrous oxide using a current modulated quantum cascade laser
Authors: Duxbury Geoffrey , Kelly James F. , Blake Thomas A. , Langford Nigel
Published: May 2012

Free induction decay (FID), optical nutation, and rapid passage induced signals in nitrous oxide, under both optically thin and optically thick conditions, have been observed using a rapid current pulse modulation, or chirp, applied to the slow current ramp of a quantum cascade (QC) laser. The variation in optical depth was achieved by increasing the pressure of nitrous oxide in a long path length multipass absorption cell. This allows the variation of optical depth to be achieved over a range of low gas pressures. Since, even at the highest gas pressure used in the cell, the chirp rate of the QC laser is faster than the collisional reorientation time of the molecules, there is minimal collisional damping, allowing a large macroscopic polarization of the molecular dipoles to develop. This is referred to as rapid passage induced polarization. The resultant FID signals are enhanced due to the constructive interference between the field within the gas generated by the slow ramp of the laser (pump), and that of the fast chirp of the laser (probe) signal generated by pulse modulation of the continuously operating QC laser. The FID signals obtained at large optical depth have not been observed previously in the mid-infrared regions, and unusual oscillatory signals have been observed at the highest gas pressures used.
The ac Stark effect in nitric oxide induced by rapidly swept continuous wave quantum cascade lasers
Authors: Duxbury Geoffrey , Kelly James F. , Blake Thomas A. , Langford Nigel
Published: May 2012

A large ac Stark effect has been observed when nitric oxide, at low pressure in a long optical path (100 m) Herriot cell, is subjected to infrared radiation from a rapidly swept, continuous wave infrared quantum cascade laser. As the frequency sweep rate of the laser is increased, an emission signal induced by rapid passage occurs after the laser frequency has passed through the resonance of 1-0 R(11.5)3/2 /molecular absorption line. At very high sweep rates a laser field-induced splitting of the absorptive part of the signal is observed, due to the ac Stark effect. This splitting is related to the Autler-Townes mixing of the e, f lambda doublet components of the 1-0 R(11.5)3/2 transition, which lie under the Doppler broadened envelope.
Sub-Doppler spectra of infrared hyperfine transitions of nitric oxide using a pulse modulated quantum cascade laser: Rapid passage, free induction decay, and the ac Stark effect
Authors: Duxbury Geoffrey , Kelly James F. , Blake Thomas A. , Langford Nigel
Published: May 2012

Using a low power, rapid (nsec) pulse-modulated quantum cascade (QC) laser, collective coherent effects in the 5 μm spectrum of nitric oxide have been demonstrated by the observation of sub-Doppler hyperfine splitting and also Autler-Townes splitting of Doppler broadened lines. For nitrous oxide, experiments and model calculations have demonstrated that two main effects occur with pulse-modulated (chirped) quantum cascade lasers: free induction decay signals, and signals induced by rapid passage during the laser chirp. In the open shell molecule, NO, in which both Λ-doubling splitting and hyperfine structure occur, laser field-induced coupling between the hyperfine levels of the two Λ-doublet components can induce a large ac Stark effect. This may be observed as sub-Doppler structure, field-induced splittings, or Autler-Townes splitting of a Doppler broadened line. These represent an extension of the types of behaviour observed in the closed shell molecule nitrous oxide, using the same apparatus, when probed with an 8 μm QC laser.
Conformal growth of Mo/Si multilayers on grating substrates using collimated ion beam sputtering
Authors: Voronov D. L. , Gawlitza P. , Cambie R. , Dhuey S. , Gullikson E. M. , Warwick T. , Braun S. , Yashchuk V. V. , Padmore H. A.
Published: May 2012

Deposition of multilayers on saw-tooth substrates is a key step in the fabrication of multilayer blazed gratings (MBG) for extreme ultraviolet and soft x-rays. Growth of the multilayers can be perturbed by shadowing effects caused by the highly corrugated surface of the substrates, which results in distortion of the multilayer stack structure and degradation of performance of MBGs. To minimize the shadowing effects, we used an ion-beam sputtering machine with a highly collimated atomic flux to deposit Mo/Si multilayers on saw-tooth substrates. The sputtering conditions were optimized by finding a balance between smoothening and roughening processes in order to minimize degradation of the groove profile in the course of deposition and at the same time to keep the interfaces of a multilayer stack smooth enough for high efficiency. An optimal value of energy of 200 eV for sputtering Kr+ ions was found by deposition of test multilayers on flat substrates at a range of ion energies. Two saw-tooth substrates were deposited at energies of 200 eV and 700 eV for the sputtering ions. It was found that reduction of the ion energy improved the blazing performance of the MBG and resulted in a 40% gain in the diffraction efficiency due to better replication of the groove profile by the multilayer. As a result of the optimization performed, an absolute diffraction efficiency of 28.8% was achieved for the 2nd blaze order of the MBG with a groove density of 7350 lines/mm at a wavelength of 13.5 nm. Details of the growth behavior of the multilayers on flat and saw-tooth substrates are discussed in terms of the linear continuous model of film growth.
High magnetic field study of the Gd-Co exchange interactions in GdCo12B6
Authors: Isnard O. , Skourski Y. , Diop L. V. B. , Arnold Z. , Andreev A. V. , Wosnitza J. , Iwasa A. , Kondo A. , Matsuo A. , Kindo K.
Published: May 2012

Pulsed magnetic fields up to 68 Tesla have been used to determine the intersublattice coupling strength and its temperature dependence of GdCo12B6 compound. This compound exhibits ferrimagnetic behaviour below 163 ± 2 K. Two antiferromagnetically coupled sublattices cancel out at compensation temperature at about 48 K. They are carrying magnetization of typically 0.42 μB/Co atom and 7 μB/Gd. The intrinsic magnetic properties of the GdCo12B6 compound have been determined by combining low temperature magnetic measurements in both steady and pulsed magnetic field, as well as isofield studies in steady field. At 4.2 K, the magnetization curve of GdCo12B6 is found to reach the full saturation with sum of both sublattice magnetizations for an applied magnetic field of about 68 T. In addition a detailed study is presented in the whole ordered temperature range on the basis of magnetization curves recorded using pulsed magnetic field up to 60 T. This has enabled to investigate the intersublattice coupling strength and its temperature dependence, a value JCo-Gd/kB = −5.3 ± 0.3 K is derived from the magnetization curves whereas one gets much larger value for JCo-Co/kB = 108 K.
Crystal structure and thermoelectric properties of KxBa8−xZnyGe46−y clathrates
Authors: Kishimoto Kengo , Sasaki Yuta , Koyanagi Tsuyoshi , Ohoyama Kenji , Akai Koji
Published: May 2012

Polycrystalline samples of degenerate n-type KxBa8−xZnyGe46−y (y∼8-x/2) with the type-I clathrate structure (No. 223, Pm3¯n) were prepared by powder metallurgy to obtain a high-efficiency Ge-based clathrate. Their Zn atoms preferred to exist at the 6c site in the framework, and consequently, the samples with x around 4, such as K4Ba4Zn6Ge40, possessed highly ordered Zn/Ge atom frameworks whose 6c, 16i, and 24k sites were occupied almost solely by Zn, Ge, and Ge atoms, respectively. In spite of such ordered structures and small numbers of substituting Zn atoms, these samples exhibited carrier mobilities lower than those of Ba8Zn8Ge38 and Ba8Ga16Ge30. Band structure calculations implied that the combination of the rattler K and Ba atoms in the cages considerably modified the conduction band edge of the corresponding clathrates; such a modification is considered to strengthen alloy disorder scattering, which reduces carrier mobility. The maximum dimensionless figure-of-merit ZT was 0.51 at 1000 K for the K2Ba6Zn7Ge39 sample, which is similar to that of 0.50 at 900 K for the Ba8Zn8Ge38 sample.
The generalized Buckley-Leverett and the regularized Buckley-Leverett equations
Authors: Hong John Meng-Kai , Wu Jiahong , Yuan Juan-Ming
Published: May 2012

This paper studies solutions of the generalized Buckley-Leverett (BL) equation with variable porosity and solutions of the regularized BL equation with the Burgers-Benjamin-Bona-Mahony-type regularization. We construct global in time weak solutions to the Cauchy problem and to the initial- and boundary-value problem for the generalized BL equation and global classical solutions for the regularized BL equation. Solutions of the regularized BL equation are shown to converge to the corresponding solution of the generalized BL equation when the coefficient γ of the BBM term and the coefficient ν of the Burgers term obey γ = O(ν2).
Intrinsic electrocaloric effect in ultrathin ferroelectric capacitors
Authors: Liu Yang , Peng Xing-ping , Lou Xiaojie , Zhou Hu
Published: May 2012

Electrocaloric effect in ultrathin SrRuO3/BaTiO3/SrRuO3 capacitors is calculated using a multiscale thermodynamic model. It is found that the electrocaloric coefficient vs working temperature is shifted to higher temperatures with increasing the BaTiO3 layer thickness (above the critical thickness). The electrocaloric effect above the critical thickness is much stronger than that below it. Furthermore, it is demonstrated that a giant electrocaloric effect (3.5 K under 0.24 V) in such capacitors (six BaTiO3 unit cells) at 300 K can be achieved, which suggests that ultrathin ferroelectric capacitors may be promising candidates for room temperature solid-state refrigeration.
Magnetic vortices in single crystalline Fe-V disks with four folds magnetic anisotropy
Authors: Mitsuzuka K. , Lacour D. , Hehn M. , Andrieu S. , Montaigne F.
Published: May 2012

Magnetic vortices in circular disks made from Fe-V single crystal film have been studied by magnetic force microscopy. At remanent state, all the disks host a single vortex structure. The magnetic contrast differs drastically from the one usually observed in "traditional" materials and reveals the influence of the four folds anisotropy originating from the bcc structure of Fe-V lattice. Field dependence of vortex position has been studied. The vortex displacement is observed to be perpendicular to the field and depends on the field direction. Micromagnetic simulations reproduce quantitatively this behavior and explain the influence of cubic magnetic anisotropy.
Mixing properties of coaxial jets with large velocity ratios and large inverse density ratios
Authors: Alexander Schumaker S. , Driscoll James F.
Published: May 2012

An experimental study was conducted to better understand the mixing properties of coaxial jets as several parameters were systematically varied, including the velocity ratio, density ratio, and the Reynolds number. Diameters of the inner and outer jet were also varied. Coaxial jets are commonly used to mix fluids due to the simplicity of their geometry and the rapid mixing that they provide. A measure of the overall mixing efficiency is the stoichiometric mixing length (Ls), which is the distance along the jet centerline where the two fluids have mixed to some desired concentration, which was selected to be the stoichiometric concentration for H2/O2 and CH4/O2 in this case. For 56 cases, the profiles of mean mixture fraction, rms mixture fraction fluctuations (unmixedness), and Ls were measured using acetone planar laser induced fluorescence diagnostics. Results were compared to three mixing models. The entrainment model of Villermaux and Rehab showed good agreement with the data, indicating that the proper non-dimensional scaling parameter is the momentum flux ratio M. The work extends the existing database of coaxial jet scalar mixing properties because it considers the specific regime of large values of both the velocity ratio and the inverse density ratio, which is the regime in which rocket injectors operate. Also the work focuses on the mixing up to Ls where previous work focused on the mixing up to the end of the inner core. The Reynolds numbers achieved for a number of cases were considerably larger than previous gas mixing studies, which insures that the jet exit boundary conditions are fully turbulent.
Spin-polarized two-dimensional electron gas in undoped MgxZn1−xO/ZnO heterostructures
Authors: Han K. , Tang N. , Ye J. D. , Duan J. X. , Liu Y. C. , Teo K. L. , Shen B.
Published: May 2012

Quantum transport properties of two-dimensional electron gas (2DEG) in undoped MgxZn1−xO/ZnO heterostructures grown by metal organic vapor phase epitaxy have been investigated. A large zero-field spin-splitting energy more than 15 meV in the 2DEG is determined at 1.6 K. Meanwhile, ferromagnetism is observed in the heterostructures. The findings reveal that the 2DEG is spin polarized at zero magnetic fields. It is believed that the exchange interaction between the itinerant electrons in the two-dimensional channel and the magnetic polarons in the MgxZn1−xO barrier around the interface results in the spin polarization of the 2DEG.
Optimization of AlAs/AlGaAs quantum well heterostructures on on-axis and misoriented GaAs (111)B
Authors: Herzog F. , Bichler M. , Koblmüller G. , Prabhu-Gaunkar S. , Zhou W. , Grayson M.
Published: May 2012

We report systematic growth optimization of high Al-content AlGaAs, AlAs, and associated modulation-doped quantum well (QW) heterostructures on on-axis and misoriented GaAs (111)B by molecular beam epitaxy. Growth temperatures TG > 690 °C and low As4 fluxes close to group III-rich growth significantly suppress twin defects in high-Al content AlGaAs on on-axis GaAs (111)B, as quantified by atomic force and transmission electron microscopy as well as x-ray diffraction. Mirror-smooth and defect-free AlAs with pronounced step-flow morphology was further achieved by growth on 2° misoriented GaAs (111)B toward [01¯1] and [21¯1¯] orientations. Successful fabrication of modulation-doped AlAs QW structures on these misoriented substrates yielded record electron mobilities (at 1.15 K) in excess of 13 000 cm2/Vs at sheet carrier densities of 5 × 1011 cm−2.
Growth rate and start current in Smith–Purcell free-electron lasers
Authors: Li D. , Hangyo M. , Tsunawaki Y. , Yang Z. , Wei Y. , Miyamoto S. , Asakawa M. R. , Imasaki K.
Published: May 2012

This letter reports a theory to calculate the growth rate and start current of a Smith–Purcell free-electron laser, which is a promising radiation source in the terahertz domain. A two-dimensional model was used to investigate the interaction between a sheet electron beam and the surface wave above a lamellar grating. After deriving the growth rate from the dispersion equation, the start current was carefully estimated by considering the power flow above the grating. The agreement between the predictions of our theory and the results from the particle-in-cell simulations is acceptable.
Experimental investigation of reshocked spherical gas interfaces
Authors: Si Ting , Zhai Zhigang , Yang Jiming , Luo Xisheng
Published: May 2012

The evolution of a spherical gas interface under reshock conditions is experimentally studied using the high-speed schlieren photography with high time resolutions. A number of experimental sets of helium or SF6 bubble surrounded by air for seven different end wall distances have been performed. Distinct flow structures are observed due to the additional vorticity and wave configuration caused by the reshock. In the air/helium case, the deformation of the reshocked bubble is dependent on the development of the penetrating air jet along the symmetry axis of the bubble. In general, two separate vortex rings can be observed, i.e., one develops slowly, and the other approaches and eventually impinges on the shock tube end wall. In the air/SF6 case, two SF6 jets moving in opposite directions are generated and the oscillation of the interface is observed for small end wall distances, while small scale vortex morphologies on the gas interface are found for large end wall distances. The physical mechanisms of the baroclinic vorticity generation and the pressure perturbation are highlighted in the interface evolution process. Based on the sequence of the schlieren images obtained during a single run for each case, the x-t diagrams of the shock and reshock interacting with the helium or SF6 bubble are plotted and the velocities estimated in linear stages are compared with those calculated from one-dimensional gas dynamics. The changes with time in the characteristic bubble sizes including the interface length, height, and vortex diameter are also measured.
Study of the pair correlations between p-nitroaniline molecules in solution by depolarized hyper-Rayleigh scattering
Authors: Chan Y. C. , Wong K. Y.
Published: May 2012

The concentration dependence of the hyper-Rayleigh scattering depolarization ratios of p-nitroaniline in solution was obtained and the results were compared with theory. It was found that the experimental data can be theoretically accounted for by using a pair distribution function that includes only direct correlation, with the molecules interact through a dipolar hard-sphere potential. The results show that short-range dipole-dipole interactions are responsible for the correlation between pairs of p-nitroaniline molecules in solution.
Enthalpy recovery in glassy materials: Heterogeneous versus homogenous models
Authors: Mazinani Shobeir K. S. , Richert Ranko
Published: May 2012

Models of enthalpy relaxations of glasses are the basis for understanding physical aging, scanning calorimetry, and other phenomena that involve non-equilibrium and non-linear dynamics. We compare models in terms of the nature of the relaxation dynamics, heterogeneous versus homogeneous, with focus on the Kovacs-Aklonis-Hutchinson-Ramos (KAHR) and the Tool-Narayanaswamy-Moynihan (TNM) approaches. Of particular interest is identifying the situations for which experimental data are capable of discriminating the heterogeneous from the homogeneous scenario. The ad hoc assumption of a single fictive temperature, Tf, is common to many models, including KAHR and TNM. It is shown that only for such single-Tf models, enthalpy relaxation of a glass is a two-point correlation function in reduced time, implying that experimental results are not decisive regarding the underlying nature of the dynamics of enthalpy relaxation. We also find that the restriction of the common TNM model to a Kohlrausch-Williams-Watts type relaxation pattern limits the applicability of this approach, as the particular choice regarding the distribution of relaxation times is a more critical factor compared with isothermal relaxation experiments. As a result, significant improvements in fitting calorimetry data can be achieved with subtle adjustments in the underlying relaxation time distribution.
Temperature dependence of fluid transport in nanopores
Authors: Xu Baoxing , Wang Binglei , Park Taehyo , Qiao Yu , Zhou Qulan , Chen Xi
Published: May 2012

Understanding the temperature-dependent nanofluidic transport behavior is critical for developing thermomechanical nanodevices. By using non-equilibrium molecular dynamics simulations, the thermally responsive transport resistance of liquids in model carbon nanotubes is explored as a function of the nanopore size, the transport rate, and the liquid properties. Both the effective shear stress and the nominal viscosity decrease with the increase of temperature, and the temperature effect is coupled with other non-thermal factors. The molecular-level mechanisms are revealed through the study of the radial density profile and hydrogen bonding of confined liquid molecules. The findings are verified qualitatively with an experiment on nanoporous carbon.
Non-volatile organic memory devices comprising SiO2 and C60 showing 104 switching cycles
Authors: Siebeneicher P. , Kleemann H. , Leo K. , Lüssem B.
Published: May 2012

We present a non-volatile organic memory device comprising a thin SiO2 layer, the organic semiconductor C60, and an organic n-type doped layer between two metallic electrodes. The memory device shows a stable hysteresis in the current-voltage characteristics with an ON/OFF ratio in the range of three or higher and reasonable switching behavior with 104 write-read-erase-read cycles. The data retention time reaches from several hours up to several days and depends on the read out frequency. We exclude a filamentary conduction mechanism as cause of the memory effect and propose that the presence of charge carrier traps at the interface of the C60 layer with the oxide causes the hysteresis of this organic non-volatile memory device.
Flow and force inducement using micron size dielectric barrier discharge actuators
Authors: Zito Justin C. , Durscher Ryan J. , Soni Jignesh , Roy Subrata , Arnold David P.
Published: May 2012

Micron size dielectric barrier discharge actuators, designed for minimal footprint area and weight penalty, show a wall jet up to 2.0 m/s consuming 15 W/m of electrode. A torsional balance measures force up to 3 mN/m of electrode and demonstrates equivalent “thrust effectiveness” (induced force/power) to macroscale actuators. Compared with reported macroscale data, the microscale actuator shows a 31% increase in energy conversion efficiency. Per unit actuator mass, both the force and the velocity induced by microscale actuators show an order of magnitude (22.1 and 18.5 times, respectively) increase over macroscale actuators, making them suitable for distributed flow control applications.
Ultrafast oxygen exchange kinetics on highly epitaxial PrBaCo2O5+δ thin films
Authors: Liu Jian , Collins Gregory , Liu Ming , Chen Chonglin , He Jie , Jiang Jiechao , Meletis Efstathios I.
Published: May 2012

Symmetric half cell structures of highly epitaxial PrBaCo2O5+δ (PBCO)/Gd0.8Ce0.2.2O2:Y0.08Zr0.92O2/PrBaCo2O5+δ thin films were designed and fabricated on (001) LaAlO3. Microstructural studies indicate that the multilayer films have excellent single-crystal quality and epitaxial nature. The electrochemical impedance spectroscopy measurements reveal that the PBCO film electrodes have a polarization resistance as low as 0.109 Ω cm2 at 597 °C in air and an ultrafast surface exchange coefficient (0.006 cm/s at 598 °C) with low activation energy (0.77 eV). The low polarization resistance and rapid surface oxygen exchange kinetics suggest that the PBCO can be an excellent cathode material for intermediate-temperature solid oxide fuel cells.
Role of the electric waveform supplying a dielectric barrier discharge plasma actuator
Authors: Benard N. , Moreau E.
Published: May 2012

The paper compares the influence of different waveforms as input for a dielectric barrier discharge plasma actuator investigated in context of plasma-assisted flow control. The electrical aspects, the plasma morphology, the body force production, and the two-component time-resolved electric wind produced over a single ac period of signal are investigated. Results shown that square waveform is optimal in terms of body force and mean electric wind production, but the velocity fluctuations are enhanced by using a sinusoidal waveform. The time-resolved measurements of the produced velocity demonstrate that the electromechanical conversion mechanism is quasi-linear in the vicinity of the discharge.
Grating duty-cycle induced enhancement of substrate emission from ring cavity quantum cascade lasers
Authors: Schwarzer Clemens , Mujagić Elvis , Ahn Sang Il , Andrews Aaron Maxwell , Schrenk Werner , Charles William , Gmachl Claire , Strasser Gottfried
Published: May 2012

We report on experimental and theoretical investigation of grating parameters to promote one specific emission direction from ring-cavity quantum cascade lasers. The devices show single-mode behavior at room temperature and pulsed operation. A maximized surface output power was found for grating duty cycles around 70%, while for substrate emission, the optimized grating duty cycle is ≈50%. All tested devices emit more power through the substrate, when accounting for absorption and the reflection at the substrate air interface.
An efficient way to enhance output strain for shear mode Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 crystals: Applying uniaxial stress perpendicular to polar direction
Authors: Li Fei , Zhang Shujun , Xu Zhuo , Lin Dabin , Gao JunJie , Li Zhenrong , Wang Linghang
Published: May 2012

The shear piezoelectric behavior of [001] poled tetragonal and [011] poled rhombohedral Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PIN-PMN-PT) crystals, with “1T” and “2R” domain configurations, respectively, were investigated under uniaxial stress perpendicular to polar direction. The shear piezoelectric coefficient d15 was found to decrease with increasing compressive stress for both “1T” and “2R” crystals. Based on thermodynamic analysis, the phase structure can be stabilized by applying compressive stress perpendicular to polar direction, resulting in a “harder” polarization rotation process, accounts for the reduced shear piezoelectric coefficient. Of particular importance is that the allowable drive electric field was greatly increased and transverse dielectric loss was drastically reduced under compressive stress, leading to the improved maximum-shear-strain.
Giant magnetoelectric coupling in laminate thin film structure grown on magnetostrictive substrate
Authors: Park Chee-Sung , Khachaturyan Armen , Priya Shashank
Published: May 2012

Highly dense 1 μm-thick piezoelectric film was deposited on magnetostrictive substrate [platinized nickel-zinc ferrite (NZF)]. A strong magnetic coupling between the piezoelectric film and magnetostrictive NZF substrate was measured exhibiting the maximum magnetoelectric (ME) coefficient on the order of 140 mV/cm Oe at the conditions of HDC = 50 Oe and HAC = 1 Oe at f = 1 kHz. This giant ME coupling under low DC magnetic field condition is attributed to effective elastic coupling. A rotation-type dynamic strain distribution was observed on the PZT film surface which provides information about the nature of elastic coupling.
Insight into the vertical detachment energy oscillation of NanC60− clusters
Authors: Wang H. B. , Li S. J. , Xiu S. L. , Gong L. , Chen G. , Mizuseki H. , Kawazoe Y.
Published: May 2012

We have performed a detailed density functional theory study on the structural and electronic properties of NanC60− (n = 1–12) clusters. The calculated vertical detachment energies show good agreement with the experimental data, which confirms the 3p (n = 3p) oscillation rule. The oscillation can be attributed to the combination of the charge depletion distribution induced by removing electrons and the number of the sodium atoms in direct contact with the fullerene. Based on the structural and electronic properties, the Na atoms can be categorized into two groups, one is for the metal atoms directly bonded to the fullerene surface, and the other one is for those without bonding to the fullerene. The Na atoms in group one would donate electrons to both the fullerene and the Na atoms in group two. As the total number of the sodium atoms increases, the number of Na atoms in group one would continue increasing till the size n = 3p − 1 to meet a shoulder from n = 3p − 1 to n = 3p, which accounts for the maximum vertical detachment energy at the size of n = 3p as drawn from the detailed electronic property studies.
Note: A simple spring-pressed heat link for low temperature vacuum environment
Authors: Fan Jie , Su Shaokui , Liu Ruiyuan , Song Xiaohui , Zhang Dian-lin
Published: May 2012

A simple spring-pressed design aimed at improving low temperature solid-solid heat link is reported. The essence of the design is to separate the contact pressure control from heat conduction. This design is especially useful for quick sample exchange in cryogen-free cryostat with demountable sample holder.
Tuning of nanogap size in high tensile stress silicon nitride thin films
Authors: Günay-Demirkol Anıl , Kaya İsmet İ.
Published: May 2012

High tensile stress suspended structures are demanded for high mechanical quality factor applications. However, high tensile stress causes distortion of the original shapes by contracting, buckling, and bending the suspended structures. We demonstrate a method to compensate for the shape deformation of suspended structures due to intrinsic tensile stress after they are released. With a new design, the distance between two suspended structures after wet etch can easily be tuned by a single fabrication beyond the lithographic resolution limits. The technique is simulated by finite element analysis and experimentally implemented to demonstrate a gap tuning capability with 2.4 nm standard error.
Superconductivity in fluorine and yttrium co-doped SmFeAsO
Authors: Lai K. T. , Kwong F. L. , Ng Dickon H. L.
Published: May 2012

Polycrystalline fluorine and yttrium co-doped SmFeAsO samples are synthesized by solid state sintering and their physical properties are studied. The lattice parameters of the Sm1−yYyFeAsO0.8F0.2 samples decrease with the increasing y due to the smaller Y ions and the stiffness of the Y-O bond. The maximum critical temperature Tc of the samples is at y = 0.05. This may be due to the fact that the strong interaction between Sm and Fe of the Fe-As bond is being re-disturbed by the doped Y ions.
Metastable phase formation in the Au-Si system via ultrafast nanocalorimetry
Authors: Zhang M. , Wen J. G. , Efremov M. Y. , Olson E. A. , Zhang Z. S. , Hu L. , de la Rama L. P. , Kummamuru R. , Kavanagh K. L. , Ma Z. , Allen L. H.
Published: May 2012

We have investigated the stability and solidification of nanometer size Au-Si droplets using an ultrafast heating/cooling nanocalorimetry and in situ growth techniques. The liquid can be supercooled to very low temperatures for both Au-rich (ΔT ∼ 95 K) and Si-rich (ΔT ∼ 220 K) samples. Solidification of a unique metastable phase δ1 is observed with a composition of 74 ± 4 at. % Au and a b-centered orthorhombic structure (a = 0.92, b = 0.72, and c = 1.35 nm; body-center in the a-c plane), which grows heteroepitaxially to Aus. Its melting temperature Tm is 305 ± 5  °C. There is competition during formation between the eutectic and δ1 phases but δ1 is the only metastable alloy observed. For small size droplets, both the δ1 and eutectic phases show considerable depression of the melting point (size-dependent melting).