bluuebunny/crossref_metadata_2025 huggingface dataset:
Vibrational mode and dielectric function spectra of BGaP probed by Raman scattering and spectroscopic ellipsometry
Authors: Rogowsky S. , Baeumler M. , Wolfer M. , Kirste L. , Ostendorf R. , Wagner J. , Liebich S. , Stolz W. , Volz K. , Kunert B.
Published: March 2011

The influence of boron incorporation in BxGa1−xP (0 < x ≤ 6%) layers on the dielectric function spectrum and on the vibrational mode spectrum has been studied. BGaP layers were grown by metal-organic vapor phase epitaxy, using a GaP interlayer, on Si and, for reference purposes, also on GaP substrates. The boron content of the layers was determined by high-resolution x-ray diffraction. Two vibrational modes arising from the two boron isotopes 10BGa and 11BGa were observed in the Raman spectrum, increasing in scattering strength and shifting to higher frequencies with increasing boron content at a rate of (1.40 ± 0.20) cm−1/% and (1.41 ± 0.14) cm−1/% for the 10BGa and 11BGa modes, respectively. Spectroscopic ellipsometry was used to asses the pseudodielectric function of the BxGa1−xP layers. The main effect of boron incorporation is a strong broadening of the E1 interband transition with increasing boron content due to alloy disorder and inhomogeneous random strains introduced by the large size mismatch of boron. However, upon the addition of arsenic to BGaP, i.e., forming BxGa1−xAsyP1−y with y ≈ 10%, a clear low-energy shift of the E1 interband transition is resolved. A small low-energy shift (≈ 20 meV) of the lowest direct bandgap has also been observed for BxGa1−xP layers with x ≤ 0.7% grown on GaP substrates.
Metal–insulator transition induced by fluctuations of the magnetic potential in semiconductors with magnetic impurities
Authors: Meilikhov E. Z. , Farzetdinova R. M.
Published: March 2011

We investigate the metal–insulator transition occurring in semiconductors with magnetic impurities when lowering temperature. In contrast to the usually considered percolation transition in the nonuniform medium induced by the localization of charge carriers in the fluctuating electric potential, the studied transition is connected with their localization in the fluctuating magnetic potential produced by magnetized impurities (more accurately, in the combined fluctuating potential). When decreasing the temperature, the magnetization of the magnetic impurities in the semiconductor becomes higher and, even at the invariable (temperature-independent) amplitude of the electric potential, the magnetic component of the total potential increases. With increasing fluctuation amplitude, the Fermi level of charge carriers sinks deeper and deeper into the growing tail of density of states until it falls under the percolation level. For that, fluctuations of the total potential have to run up to some critical value. On reaching that value, the transition occurs from the metal conductivity to the activation one (the metal–insulator transition).
Electron density and temperature of gas-temperature-dependent cryoplasma jet
Authors: Noma Yuri , Hyuk Choi Jai , Muneoka Hitoshi , Terashima Kazuo
Published: March 2011

A microsize cryoplasma jet was developed and analyzed at plasma gas temperatures ranging from room temperature down to 5 K. Experimental results obtained from optical emission spectroscopy and current–voltage measurements indicate that the average electron density and electron temperature of the cryoplasma jet depend on the gas temperature. In particular, the electron temperature in the cryoplasma starts to decrease rapidly near 60 K from about 13 eV at 60 K to 2 eV at 5 K, while the electron density increases from about 109 to approximately 1012 cm−3 from room temperature to 5 K. This phenomenon induces an increase in the Coulomb interaction between electrons, which can be explained by the virial equation of state.
Tunneling current calculations for nonuniform and asymmetric multiple quantum well structures
Authors: Mukherjee Kasturi , Das N. R.
Published: March 2011

In this paper, we present our studies on current-voltage characteristics due to tunneling in nonuniform and asymmetric multiple quantum well (MQW) structures. First, the transmission coefficient is calculated by solving the Schrödinger equation with the piecewise-constant potential approximation and by considering the effects of nonuniformity and the asymmetry of layer dimensions and band-offsets. Then the tunneling current through the structure is calculated as a function of bias for different structural combinations of the MQW structure. The configurations suitable for some applications are indicated in the results.
Electrical properties of AlxGa1-xN/GaN heterostructures with low Al content
Authors: Köhler K. , Müller S. , Waltereit P. , Pletschen W. , Polyakov V. , Lim T. , Kirste L. , Menner H. P. , Brückner P. , Ambacher O. , Buchheim C. , Goldhahn R.
Published: March 2011

Electrical properties of AlxGa1−xN/GaN heterostructures with an Al content below 15% and carrier concentrations as low as 1.0 × 1012 cm−2 were investigated by Hall effect measurements and capacitance–voltage profiling. The nominally undoped GaN capped structures were grown by low-pressure metal-organic vapor-phase epitaxy. The threshold voltage of transistor devices follows the trend already found for high Al-containing structures, which are described by a model indicating a surface potential independent of Al content. Photoreflectance spectroscopy confirms the results for as-grown heterostructures. The Hall effect measured on the as-grown samples, however, shows a stronger decrease in carrier concentration than expected from the effect of polarization and constant surface potential. In contrast, Hall effect data determined on samples with Ni Schottky contacts and capacitance-voltage profiling on as-grown samples yield the expected behavior, with surface potentials of 0.86 eV and 0.94 eV, respectively. The inconsistency is eliminated by describing the results of the Hall effect on as-grown samples by a two-carrier model. Self-consistent Schrödinger–Poisson calculations support these considerations if we take into account a transition range at the AlxGa1−xN/GaN interface.
Magnetization jumps in a lead-porous glass composite: Experiment and simulation
Authors: Tien C. , Pirozerskii A. L. , Charnaya E. V. , Xing D. Y. , Ciou Y. S. , Lee M. K. , Kumzerov Yu. A.
Published: March 2011

Experimental and numerical studies of magnetization in a Pb-porous glass nanocomposite superconductor are reported. ZFC and FC magnetization revealed behavior typical for type-II superconductors with the critical temperature close to that in bulk lead. Magnetic instabilities were observed on the magnetization versus field hysteresis loops in the range 3.5 to 6 K. The magnetization jumps were successfully simulated using the adiabatic approximation which was developed for type-II superconductors.
Inconsistent temperature dependence in capacitance-voltage profiling of quantum wells
Authors: Panda Siddhartha , Biswas Dipankar
Published: March 2011

Carrier profiles of quantum wells, obtained from experimental capacitance-voltage (C-V) measurements match the theoretically simulated carrier profile quite closely but these are much different from the actual carrier profile. It is observed that the peaks of the experimental and simulated carrier profiles move in the opposite direction and at low temperature there nature changes drastically. These observations have been explained taking into account the two dimensional carrier confinement and its temperature dependence through self-consistent solutions of the Schrödinger and Poisson equations. The effect of the series resistance seems to be highly pronounced in experimental C-V measurements.
Spin current through double quantum dots with spin- and time-dependent interdot coupling
Authors: Wang Haixia , Yin Wen , Wang Fangwei
Published: March 2011

We analyze theoretically the characteristics of electron transport through double quantum dots between which time oscillating and spin-polarization-dependent tunneling occurs. General formulas for the time-averaged current (spin current and charge current) and its differential are obtained by Keldysh nonequilibrium, Green’s function, and the transport properties derived from those formulas, can be qualitatively explained by spin-dependent resonant tunneling images.
Direct observation of space charge field in tetracene field-effect transistor using time-resolved microscopic optical second harmonic generation
Authors: Satou Hideki , Ohshima Yuki , Kohn Hideki , Manaka Takaaki , Iwamoto Mitsumasa
Published: March 2011

Using electric field-induced second harmonic generation (EFISHG) measurements, we directly probed a time-varying space charge field responsible for carrier injection followed by radiative recombination (electroluminescence; EL). Experiments using tetracene as the active layer of a top-contact field-effect transistor with application of a pulsed voltage showed that a space charge field remained around the source electrode—caused by injected holes, but not by electrons. Observing the time-varying EFISHG intensity at the electrodes clearly showed that the space charge field formed in the vicinity of the electrodes made a dominant contribution to the opposite-polarity carrier injection, directly leading to EL around the electrodes by carrier recombination. We concluded that EFISHG measurement is useful for probing carrier behavior in luminescent materials, by eliminating two-photon excited photoluminescence from the material induced using an incident laser.
Non-Markovian shot noise spectrum of quantum transport through quantum dots
Authors: Jin Jinshuang , Li Xin-Qi , Luo Meng , Yan YiJing
Published: March 2011

The generalized quantum master equation with transport particle number resolution, similar to its conventional unconditioned counterpart, also has time-local and time-nonlocal configurations. The latter is found to be more suitable for the effect of an electrode’s bandwidth on quantum transport and noise spectrum for weak system-reservoir coupling, as calibrated with the exact results in the absence of a Coulomb interaction. We further analyze the effect of the Coulomb interaction on the noise spectrum of the transport current through quantum dot systems, and show that the realistic finite Coulomb interaction and finite bandwidth are manifested only with the non-Markovian treatment. We demonstrate a number of non-Markovian characteristics of the shot noise spectrum, including that which is due to finite bandwidth and that which is sensitive to and enhanced by the magnitude of the Coulomb interaction.
A posteriori analysis of numerical errors in subfilter scalar variance modeling for large eddy simulation
Authors: Kaul C. M. , Raman V.
Published: March 2011

Subfilter scalar variance is a critical indicator of small scale mixing in large eddy simulation (LES) of turbulent combustion and is an important parameter of conserved scalar based combustion models. Realistic combustion models have a highly nonlinear dependence on the conserved scalar, making the prediction of flow thermochemistry sensitive to errors in subfilter variance modeling, including errors due to numerical discretization. Large numerical errors can result from the use of grid-based filtering and the resulting under-resolution of the smallest filtered scales, which are a key to variance modeling. Hence, the development of variance models should take into account this sensitivity to numerical discretization. In this work, a novel coupled direct numerical simulation (DNS)-LES a posteriori method is used to study the role of discretization errors in variance prediction for the two most widely used types of models: algebraic dynamic models and transport equation-based models. Algebraic models are found to be ill-suited to discretization due to their dependence on filtered scalar gradient values. Additionally, the use of dynamic modeling procedures enhances their sensitivity to filtered scalar errors. The accuracy of transport equation models primarily rests on the accuracy of the scalar dissipation rate closure with numerical error having a secondary effect. The influence of dissipation rate modeling error is investigated using the unique information provided by the combined DNS-LES simulation method. Overall, transport equation models are found to offer a more powerful approach to variance modeling due to more complete model physics and reduced effects of discretization error.
Surface collision theory for suspension-based cleaning of particle-contaminated solid substrates
Authors: Andreev V. A. , Prausnitz J. M. , Radke C. J.
Published: March 2011

To quantify removal kinetics of contaminant particles on solid surfaces, we study collisions between nonspherical particles when one particle is suspended in laminar shear flow while the second is adhered to a solid surface. Based on kinetic theory of rigid nonspherical particles, we outline a theoretical framework for our previously developed binary-collision contaminant-removal model. We show that a distribution of adhered contaminant particles over orientation, size, and shape results in multiexponential decay of surface concentration of particles with time, in agreement with experimental findings [Andreev et al., J. Electrochem. Soc. 158, H55 (2011)]. Theory predicts a linear increase of removal rate constant with shear rate and with suspended solids concentration near the substrate surface, also in agreement with experiment [Andreev et al., J. Electrochem. Soc. 158, H55 (2011); Ind. Eng. Chem. Res. 49, 12461 (2010)]. To reveal the effect of geometry and size of colliding entrained particles on removal rates, an approximate singlet distribution function is derived for particles in flow at the level of the Smoluchowski theory for orthocoagulation. Two shapes of flow-suspended particles are considered: spheres and cuboids with high aspect ratio, while contaminant particles on the surface are small and spherical. Removal kinetic rate constants scale with contaminant particle size, aA, as aA3/2 for spheres and as aA for cuboids. Thus, rectangular platelet particles are effective for removal of small contaminant particles, confirming experimental observation [Andreev et al., J. Electrochem. Soc. 158, H55 (2011)]. The influence of platelet aspect ratio on removal rates is analyzed. Due to interplay between solids velocity and collision cross section, small aspect ratios improve cleaning efficiency when the size ratio of the entrained to contaminant particles is large.
The motion of a body in Newtonian theories
Authors: Weatherall James Owen
Published: March 2011

A theorem due to Bob Geroch and Pong Soo Jang [J. Math. Phys. 16(1), 65 (1975)] provides the sense in which the geodesic principle has the status of a theorem in general relativity (GR). Here we show that a similar theorem holds in the context of geometrized Newtonian gravitation (often called Newton–Cartan theory). It follows that in Newtonian gravitation, as in GR, inertial motion can be derived from other central principles of the theory.
Effects of chemical stoichiometry of channel region on bias instability in ZnO thin-film transistors
Authors: Kamada Yudai , Fujita Shizuo , Kimura Mutsumi , Hiramatsu Takahiro , Matsuda Tokiyoshi , Furuta Mamoru , Hirao Takashi
Published: March 2011

We investigated effects of chemical stoichiometry of ZnO channel, controlled by oxygen partial pressure during deposition, on bias instability for ZnO thin-film transistors. Parallel threshold voltage shifts were mainly enhanced under gate bias stresses due to charge trapping when O-rich ZnO was used for channel layer. On the contrary, negative threshold voltage shifts were observed under both gate and drain bias stresses when Zn-rich ZnO was used for channel layer. This degradation was enhanced regardless of the bias polarity and the direction, attributing to electrically activated trap generations.
Improvement of carrier ballisticity in junctionless nanowire transistors
Authors: Dehdashti Akhavan Nima , Ferain Isabelle , Razavi Pedram , Yu Ran , Colinge Jean-Pierre
Published: March 2011

In this work we show that junctionless nanowire transistor (JNT) exhibits lower degree of ballisticity in subthreshold and higher ballisticity above threshold compare to conventional inversion-mode transistors, according to quantum mechanical simulations. The lower degradation of the ballisticity above threshold region gives the JNT near-ballistic transport performance and hence a high current drive. On the other hand, lower ballisticity in subthreshold region helps reducing the off-current and improves the subthreshold slope. A three-dimensional quantum mechanical device simulator based on the nonequilibrium Green’s function formalism in the uncoupled mode-space approach has been developed to extract the physical parameters of the devices.
Two-dimensional vibronic spectroscopy of coherent wave-packet motion
Authors: Schubert Alexander , Engel Volker
Published: March 2011

We theoretically study two-dimensional (2D) spectroscopic signals obtained from femtosecond pulse interactions with diatomic molecules. The vibrational wave-packet dynamics is monitored in the signals. During the motion in anharmonic potentials the wave packets exhibit vibrational revivals and fractional revivals which are associated with particular quantum phases. The time-dependent phase changes are identified by inspection of the complex-valued 2D spectra. We use the Na2 molecule as a numerical example and discuss various pulse sequences which yield information about vibrational level structure and phase relationships in different electronic states.
Scaling behavior of quantum nanosystems: Emergence of quasi-particles, collective modes, and mixed exchange symmetry states
Authors: Shreif Zeina , Ortoleva Peter
Published: March 2011

Examples of quantum nanosystems are graphene nanoribbons, molecular wires, and superconducting nanoparticles. The objective of the multiscale theory presented here is to provide a new perspective on the coupling of processes across scales in space and time underlying the dynamics of these systems. The long range objective for this multiscale approach is to serve as an efficient computational algorithm. Long space-time dynamics is derived using a perturbation expansion in the ratio ɛ of the nearest-neighbor distance to a nanometer-scale characteristic length and a theorem on the equivalence of long time-averages and expectation values. This dynamics is shown to satisfy a coarse-grained wave equation (CGWE) which takes a Schrödinger-like form with modified masses and interactions. The scaling of space and time is determined by the orders of magnitude of various contributions to the N-body potential. If the spatial scale of the coarse-graining is too large, the CGWE would imply an unbounded growth of gradients; if it is too short, the system's size would display uncontrolled growth inappropriate for the bound states of interest, i.e., collective motion or migration within a stable nanoassembly. The balance of these two extremes removes arbitrariness in the choice of the scaling of space-time. Since the long-scale dynamics of each Fermion involves its interaction with many others, we hypothesize that the solutions of the CGWE have mean-field character to good approximation, i.e., can be factorized into single-particle functions. This leads to a coarse-grained mean-field approximation that is distinct in character from traditional Hartree–Fock theory. A variational principle is used to derive equations for the single-particle functions. This theme is developed and used to derive an equation for low-lying disturbances from the ground state corresponding to long wavelength density disturbances or long-scale migration. An algorithm for the efficient simulation of quantum nanosystems is suggested.
Flexible multilayer inverted polymer light-emitting diodes with a gravure contact printed Cs2CO3 electron injection layer
Authors: Chung Dae-Young , Leem Dong-Seok , Bradley Donal D. C. , Campbell Alasdair J.
Published: March 2011

Here we demonstrate high efficiency, flexible inverted polymer light-emitting diodes in which the bottom-contact cesium carbonate electron injection layer is gravure contact printed. The poly(9,9-dioctylfluorene-alt-benzothiadiazole) emissive/electron transport layer, the poly(9,9-dioctylfluorene-alt-N-(4-butylphenyl)-diphenylamine) hole transport/electron blocking layer and the poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) hole injection layer were sequentially spin-coated from solution using orthogonal solvent formulations with appropriate wetting properties. By switching from indium-tin-oxide (ITO) on glass to ITO on poly(ethylene terephthalate) and using gravure contact printing instead of spin-coating, Cs2CO3 smoothness and morphology was optimized, resulting in an approximately fivefold increase in current efficiency and power efficiency at 100 cd/m2.
Rheology of poly(methyl methacrylate) Langmuir monolayers: Percolation transition to a soft glasslike system
Authors: Maestro A. , Ortega F. , Rubio R. G. , Rubio M. A. , Krägel J. , Miller R.
Published: March 2011

An experimental study of the equilibrium properties and of the surface rheology of Langmuir monolayers of poly(methyl methacrylate) (PMMA) at the air/water interface has been carried out as a function of polymer concentration (Γ) and molecular weight (Mw). Dilational and shear complex elasticity moduli covering a frequency range from 10−3 to 0.2 Hz have been discussed. It was found that the air/water interface behaves as a poor solvent for PMMA monolayers, thus suggesting that the polymer coils take collapsed soft-disks (pancakes) shape at the interface. The equilibrium and dynamic results suggest a fluid-to-soft-glass transition as the polymer concentration increases above a critical packing fraction at constant temperature. This two-dimensional transition is in agreement with results previously discussed for the dilational rheology of poly(4-hydroxystyrene) [F. Monroy, F. Ortega, R. G. Rubio, H. Ritacco, and D. Langevin, J. Chem. Phys. 95, 056103 (2005)]. Furthermore, the Γ-dependence of the relaxation dynamics of the monolayers suggests that the gel state may be considered as a fragile soft glass.
Thermal transpiration flow: A circular cross-section microtube submitted to a temperature gradient
Authors: Rojas Cardenas Marcos , Graur Irina , Perrier Pierre , Meolans J. Gilbert
Published: March 2011

Thermal transpiration is the macroscopic movement of rarefied gas molecules induced by a temperature gradient. The gas moves from the lower to the higher temperature zone. An original method is proposed here to measure the mean macroscopic movement of gas in the case of a long circular cross-section glass microtube onto which a gradient of temperature is applied. The mass flow rate and the thermomolecular pressure difference have been measured by monitoring the absolute pressure evolution in time at both ends of the capillary using high-speed response pressure gauges. Two gases, nitrogen and helium, are studied and three different temperature differences of 50, 60, and 70 °C are applied to the tube. The analyzed gas rarefaction conditions vary from transitional to slip regime.
A simple theoretical approach to analyze polarization properties in semipolar and nonpolar InGaN quantum wells
Authors: Yamaguchi Atsushi A. , Kojima Kazunobu
Published: March 2011

By using a simple theoretical approach, the previously reported experimental results of the polarization properties in semipolar and nonpolar InGaN quantum wells (QWs) were analyzed. On the basis of the k⋅p-perturbation theory, we derived a useful analytical expression for describing the polarization properties of these QWs, and used this expression to analyze experimental data reported from various research groups. Based on these analyses, we predicted that the negative polarization degree, which is favorable for laser diodes with cleaved-facet cavity mirrors, would appear in the blue- or green-InGaN QWs on the lower-angle semipolar planes (30°–40° inclined from the c-plane).
Improved electrical properties of Pt/HfO2/Ge using in situ water vapor treatment and atomic layer deposition
Authors: Park In-Sung , Choi Youngjae , Nichols William T. , Ahn Jinho
Published: March 2011

The effects of water vapor treatment (WVT) on a Ge substrate were investigated in order to understand the improved electrical properties of Pt/HfO2/Ge metal-oxide-semiconductor (MOS) capacitors. The WVT and HfO2 deposition were performed in situ using an atomic layer deposition technique to avoid air exposure. As a result, the WVT on cleaned Ge substrates reduced the native oxide effectively and enhanced the initial growth of the HfO2 film. The improved interface qualities with WVT enhanced Ge-based device performance through a smoother capacitance-voltage curve, less increase in the inversion capacitance, and lower density of interface states.
Metastable BrO2+ and NBr2+ molecules in the gas phase
Authors: Aoto Yuri Alexandre , de Oliveira-Filho Antonio Gustavo S. , Franzreb Klaus , Ornellas Fernando R.
Published: March 2011

The doubly positively charged gas-phase molecules BrO2+ and NBr2+ have been produced by prolonged high-current energetic oxygen (17 keV 16O−) ion surface bombardment (ion beam sputtering) of rubidium bromide (RbBr) and of ammonium bromide (NH4Br) powdered ionic salt samples, respectively, pressed into indium foil. These novel species were observed at half-integer m/z values in positive ion mass spectra for ion flight times of roughly ∼12 μs through a magnetic-sector secondary ion mass spectrometer. Here we present these experimental results and combine them with a detailed theoretical investigation using high level ab initio calculations of the ground states of BrO2+ and NBr2+, and a manifold of excited electronic states. NBr2+ and BrO2+, in their ground states, are long-lived metastable gas-phase molecules with well depths of 2.73 × 104 cm−1 (3.38 eV) and 1.62 × 104 cm−1 (2.01 eV); their fragmentation channels into two monocations lie 2.31 × 103 cm−1 (0.29 eV) and 2.14 × 104 cm−1 (2.65 eV) below the ground state minimum. The calculated lifetimes for NBr2+ (v″ < 35) and BrO2+ (v″ < 18) are large enough to be considered stable against tunneling. For NBr2+, we predicted Re = 3.051 a0 and ωe = 984 cm−1; for BrO2+, we obtained 3.033 a0 and 916 cm−1, respectively. The adiabatic double ionization energies of BrO and NBr to form metastable BrO2+ and NBr2+ are calculated to be 30.73 and 29.08 eV, respectively. The effect of spin–orbit interactions on the low-lying (Λ + S) states is also discussed.
Bond lengths of diatomic molecules periodically driven by light: The p-LAMB scheme
Authors: Sola Ignacio R. , Shin Seokmin , Chang Bo Y.
Published: March 2011

A laser scheme using a periodically changing frequency is used to induce oscillations of the internuclear motion, which are quantum analogs of classical vibrations in diatomic molecules. This is what we call the periodic laser adiabatic manipulation of the bond, or p-LAMB scheme. In p-LAMB, the carrier frequency of the laser must vary periodically from the blue to the red of a photodissociation band and backwards, following for instance a cosine-dependent frequency of period τc. In the adiabatic regime the dynamics is fully time-reversible. The amplitude of the internuclear oscillation is controlled by the pulse frequency ω(t), while τc determines the duration (or period) of the bond oscillation. In the presence of efficient dipole coupling, the bandwidth of the pulse is the main constraint to the maximum bond stretch that can be obtained. Before the onset of the adiabatic regime the dynamics are more complex, showing dispersion of the vibrational wave packet and anharmonic deformation of the bond. However, the nonadiabatic effects are mostly canceled and full revivals are observed at certain multiples of τc.
Settling dynamics of asymmetric rigid fibers
Authors: Tozzi E. J. , Scott C. T. , Vahey D. , Klingenberg D. J.
Published: March 2011

The three-dimensional motion of asymmetric rigid fibers settling under gravity in a quiescent fluid was experimentally measured using a pair of cameras located on a movable platform. The particle motion typically consisted of an initial transient after which the particle approached a steady rate of rotation about an axis parallel to the acceleration of gravity, with its center of mass following a helical trajectory. Numerical and analytical methods were used to predict translational and angular velocities as well as the evolution of the fiber orientation as a function of time. A comparison of calculated and measured values shows that it is possible to quantitatively predict complex motions of particles that have highly asymmetric shape. The relations between particle shape and settling trajectory have potential applications for hydrodynamic characterization of fiber shapes and fiber separation.
A Lagrangian–Eulerian approach to modeling homogeneous condensation in high density gas expansions
Authors: Jansen Ryan , Gimelshein Natalia , Gimelshein Sergey , Wysong Ingrid
Published: March 2011

A computational approach to homogeneous nucleation is proposed based on Eulerian description of the gas phase expansion coupled with a Lagrangian approach to the cluster formation. A continuum, Euler/Navier–Stokes solver versatile advection code is used to model the gas transport, and a kinetic particle solver is developed in this work to simulate cluster nucleation and growth. Parameters in the new model were adjusted so as to match the known theoretical dimer formation equilibrium constants for the two gases under consideration, argon and water. Reasonable agreement between computed and available experimental data was found in terminal cluster size distributions for nozzle water expansions in a wide range of stagnation pressures. The proposed approach was found to be orders of magnitude faster than a comparable approach based on the direct simulation Monte Carlo method.
Study of the interplay between magnetic shear and resonances using Hamiltonian models for the magnetic field lines
Authors: Firpo M.-C. , Constantinescu D.
Published: March 2011

The issue of magnetic confinement in magnetic fusion devices is addressed within a purely magnetic approach. Using some Hamiltonian models for the magnetic field lines, the dual impact of low magnetic shear is shown in a unified way. Away from resonances, it induces a drastic enhancement of magnetic confinement that favors robust internal transport barriers (ITBs) and stochastic transport reduction. When low shear occurs for values of the winding of the magnetic field lines close to low-order rationals, the amplitude thresholds of the resonant modes that break internal transport barriers by allowing a radial stochastic transport of the magnetic field lines may be quite low. The approach can be applied to assess the robustness versus magnetic perturbations of general (almost) integrable magnetic steady states, including nonaxisymmetric ones such as the important single-helicity steady states. This analysis puts a constraint on the tolerable mode amplitudes compatible with ITBs and may be proposed as a possible explanation of diverse experimental and numerical signatures of their collapses.
Biaxial tensile strain effects on photoluminescence of different orientated Ge wafers
Authors: Lan H.-S. , Chan S.-T. , Cheng T.-H. , Chen C.-Y. , Jan S.-R. , Liu C. W.
Published: March 2011

The enhanced photoluminescence of direct transition is observed on (100), (110), and (111) Ge under biaxial tensile strain. The enhancement is caused by the increase in electron population in the Γ valley. The shrinkage of energy difference between the lowest L valleys and the Γ valley is responsible to the population increase on (100) and (110) Ge. For (111) Ge, the energy difference increases under biaxial tensile strain but the strain decreases energy difference between the electron quasi-Fermi level and the Γ valley due to the small density of state of the lowest L valleys, and thus enhances direct recombination.
Effect of humidity and thermal curing of polymer gate dielectrics on the electrical hysteresis of SnO2 nanowire field effect transistors
Authors: Hong Sahngki , Kim Daeil , Kim Gyu-Tae , Ha Jeong Sook
Published: March 2011

Electrical properties of SnO2 nanowire (NW) field effect transistor (FET) with polyimide gate dielectrics, prepared by thermal curing of polyamic acid, were investigated. In particular, the effect of humidity and the thermal curing on the electrical hysteresis was systematically studied by taking Fourier-transform infrared spectra of polymer films. Slow polarization of hydroxyl groups/water molecules in the polymer film due to the insufficient curing and the absorbed water molecules under high humidity during the device fabrication was attributed to the hysteresis in the direction opposite to that observed in SnO2 NW FET with SiO2 gate dielectrics.
Communication: Exciton–phonon information flow in the energy transfer process of photosynthetic complexes
Authors: Rebentrost Patrick , Aspuru-Guzik Alán
Published: March 2011

Non-Markovian and nonequilibrium phonon effects are believed to be key ingredients in the energy transfer in photosynthetic complexes, especially in complexes which exhibit a regime of intermediate exciton–phonon coupling. In this work, we utilize a recently developed measure for non-Markovianity to elucidate the exciton–phonon dynamics in terms of the information flow between electronic and vibrational degrees of freedom. We study the measure in the hierarchical equation of motion approach which captures strong coupling effects and nonequilibrium molecular reorganization. We propose an additional trace distance measure for the information flow that could be extended to other master equations. We find that for a model dimer system and for the Fenna–Matthews–Olson complex the non-Markovianity is significant under physiological conditions.
The slow dielectric Debye relaxation of monoalcohols in confined geometries
Authors: Jansson Helén , Swenson Jan
Published: March 2011

Broadband dielectric relaxation measurements have been performed on monoalcohols confined in the quasi-two-dimensional space between clay platelets and the quasi-one-dimensional pores of approximately 10 Å diameter in a molecular sieve. Interestingly, the results show that the slow Debye-like process is present even in these severe confinements, proving that structural models that are based on two-dimensional or three-dimensional cluster formations as the structural origin of the Debye-like process can be excluded. Rather, the insensitivity of its time-scale to confinements suggests that it is of local character and in some way related to the lifetime or breaking and reformation of hydrogen bonds.
Mode-filtered electron injection into a waveguide interferometer
Authors: Buchholz Sven S. , Kunze Ulrich , Reuter Dirk , Wieck Andreas D. , Fischer Saskia F.
Published: March 2011

Injection of mode-filtered electrons into a phase-sensitive four-terminal waveguide Aharonov–Bohm (AB) ring is studied. An individually tuneable quantum point contact (QPC) in a waveguide lead of the GaAs/AlGaAs-ring allows to selectively couple to one-dimensional modes in the ring. Thus, we demonstrate single-mode transport in a multimode waveguide structure. Coherent mode-filtering by the lowest QPC subband is verified by nonlocal bend resistance and phase-sensitive AB interference measurements.
Mass determination and sensitivity based on resonance frequency changes of the higher flexural modes of cantilever sensors
Authors: Parkin John D. , Hähner Georg
Published: March 2011

Micro- and nanocantilevers are increasingly employed as mass sensors. Most studies consider the first flexural mode and adsorbed masses that are either discretely attached or homogeneously distributed along the entire length of the cantilever. We derive general expressions that allow for the determination of the total attached mass with any mass distribution along the cantilever length and all flexural modes. The expressions are valid for all cantilevers whose flexural deflection can be described by a one-dimensional function. This approach includes the most common types of microcantilevers, namely, rectangular, picket, and V-shaped. The theoretical results are compared with experimental data up to the fourth flexural mode obtained from thermal noise spectra of rectangular and V-shaped cantilevers.
Spin and orbital magnetic moments of molecular beam epitaxy γ′-Fe4N films on LaAlO3(001) and MgO(001) substrates by x-ray magnetic circular dichroism
Authors: Ito K. , Lee G. H. , Harada K. , Suzuno M. , Suemasu T. , Takeda Y. , Saitoh Y. , Ye M. , Kimura A. , Akinaga H.
Published: March 2011

10-nm-thick γ′-Fe4N films were grown epitaxially on LaAlO3(001) and MgO(001) substrates by molecular beam epitaxy using solid Fe and a radio-frequency NH3 plasma. The lattice mismatch of these substrates to γ′-Fe4N is 0% and 11%, respectively. Spin and orbital magnetic moments of these γ′-Fe4N epitaxial films were deduced by x-ray magnetic circular dichroism measurements at 300 K. The total magnetic moments are almost the same for the two substrates, that is, 2.44±0.06 μB and 2.47±0.06 μB, respectively. These values are very close to those predicted theoretically, and distinctively larger than that for α-Fe.
Modeling bond dilutions in Josephson junction arrays
Authors: Lv Jian-Ping , Chen Qing-Hu
Published: March 2011

Using resistivity-shunted-junction dynamics, we model bond dilutions in Josephson junction arrays. The current-voltage characteristics are obtained by large-scale numerical simulations. By dynamic scaling analysis to the finite-temperature phase transition, we provide clear evidence for the existence of non-Kosterlitz–Thouless-type phase transition, as described by Kosterlitz et al. [J. Phys. C 6, 1181 (1973); J. Phys. C 7, 1046 (1974)] in a wide range of disorder strengths. Critical temperature and exponents of the phase transitions are estimated, which are further confirmed by the finite-size scaling analysis at the transition temperature.
Analysis of interband, intraband, and plasmon polariton transitions in silver nanoparticle films via in situ real-time spectroscopic ellipsometry
Authors: Little S. A. , Collins R. W. , Marsillac S.
Published: March 2011

The dielectric function of Ag nanoparticle films, deduced from an analysis of in situ real-time spectroscopic ellipsometry (RTSE) measurements, is found to evolve with time during deposition in close consistency with the film structure, deduced in the same RTSE analysis. In the nucleation regime, the intraband dielectric function component is absent and plasmon polariton behavior dominates. Only at nuclei contact, does the intraband amplitude appear, increasing above zero. Both intraband and plasmon amplitudes coexist during surface smoothening associated with coalescence. The intraband relaxation time increases rapidly after surface smoothening is complete, also in consistency with the thin film structural evolution.
Deep subnanosecond spin torque switching in magnetic tunnel junctions with combined in-plane and perpendicular polarizers
Authors: Rowlands G. E. , Rahman T. , Katine J. A. , Langer J. , Lyle A. , Zhao H. , Alzate J. G. , Kovalev A. A. , Tserkovnyak Y. , Zeng Z. M. , Jiang H. W. , Galatsis K. , Huai Y. M. , Amiri P. Khalili , Wang K. L. , Krivorotov I. N. , Wang J.-P.
Published: March 2011

We show that adding a perpendicular polarizer to a conventional spin torque memory element with an in-plane free layer and an in-plane polarizer can significantly increase the write speed and decrease the write energy of the element. We demonstrate the operation of such spin torque memory elements with write energies of 0.4 pJ and write times of 0.12 ns.
Theoretical study of polarization-doped GaN-based light-emitting diodes
Authors: Zhang L. , Ding K. , Liu N. X. , Wei T. B. , Ji X. L. , Ma P. , Yan J. C. , Wang J. X. , Zeng Y. P. , Li J. M.
Published: March 2011

Insufficient hole injection is a major impediment to the luminescence efficiency of III-nitride light-emitting diodes (LEDs). In our previous work by Zhang et al. [Appl. Phys. Lett. 97, 062103 (2010)], high-density mobile three-dimensional hole gas is obtained in Mg-doped Al composition graded AlGaN layer with Al composition linearly decreasing from a certain value to 0. In this paper, it is revealed by a theoretical study that the hole injection efficiency in blue-light GaN-based LEDs can be greatly enhanced by using this polarization-doped method. An increase in the electroluminescence intensity and the internal quantum efficiency in polarization-doped GaN-based LEDs is observed, in comparison with a conventional LED.
Multiferroic response to magnetic field in orthorhombic manganites
Authors: Qin M. H. , Tao Y. M. , Dong S. , Zhao H. B. , Gao X. S. , Liu J.-M.
Published: March 2011

The magnetoelectric coupling in Eu0.55Y0.45MnO3 is studied based on a microscopic spin model which includes the superexchange interaction, the single-ion anisotropy, the Dzyaloshinskii–Moriya interaction, and the cubic anisotropy. Our Monte Carlo simulation reproduces the experimentally observed multiferroic response to magnetic field B. It is demonstrated that the magnetic field can control the multiferroic behaviors by modulating the spin arrangements, leading to various flops of electric polarization. In addition, an interesting state in which both the electric polarizations along the a-axis and c-axis are activated under high B is predicted and discussed.
Anticipating π-Bond Dispositions in Cyclic, Even, Classical Hydrocarbons
Authors: Langler Richard F.
Published: March 2011

A new technique, which employs π-bond placement coefficients, is presented. That technique, in conjunction with a few parameters that are readily available from traditional Hückel theory, permits one to systematically anticipate π-bond placements for optimized lowest-lying singlet states. One may then foresee the relative magnitudes of calculated ΔHf values for selected sets of structural isomers. Structural predictions are compared with parameterization method 3 (PM3) calculations, density functional theory calculations and experimental results. Reasonable expectations for the most stable structure narrow the choice of molecules that may then be scrutinized by more exact computations or by experiment.
Assessment of topsoil properties in integrated crop - livestock and continuous cropping systems under zero tillage
Authors: Fernández P. L. , Alvarez C. R. , Taboada M. A.
Published: March 2011

A regional study was conducted in the northern Pampas of Argentina in order to compare soil quality at proximal cropland sites that are managed under either continuous cropping (CC) (n = 11) or integrated crop–livestock (ICL) (n = 11) systems under zero tillage. In the ICL system, samples were taken in the middle of the agricultural period. Although soil total and resistant organic carbon (TOC, ROC) were significantly higher in silt loam soils than in loam/sandy loam soils, variations in carbon concentration were not associated with differences in soil management. Soil relative compaction was the only property that was significantly (P < 0.05) affected by the soil type × management interaction. Soil relative compaction values were significantly lower with ICL in loam/sandy loam soils, but there were no significant differences in silt loam soils. Structural instability index showed little change from CC to ICL sites, indicating that there was no soil structural damage. Soil penetration resistance was significantly higher in ICL soils within the first 0.075 m of soil depth, slightly exceeding the critical threshold (2000 kPa). However, firmer topsoil under ICL was not due to shallow compaction, as evidenced by no increase in soil bulk density.
Antidiabetic and antisteatotic effects of the selective fatty acid synthase (FAS) inhibitor platensimycin in mouse models of diabetes
Authors: Wu Margaret , Singh Sheo B. , Wang Jun , Chung Christine C. , Salituro Gino , Karanam Bindhu V. , Lee Sang Ho , Powles Maryann , Ellsworth Kenneth P. , Lassman Michael E. , Miller Corey , Myers Robert W. , Tota Michael R. , Zhang Bei B. , Li Cai
Published: March 2011

Platensimycin (PTM) is a recently discovered broad-spectrum antibiotic produced by
Streptomyces platensis
. It acts by selectively inhibiting the elongation-condensing enzyme FabF of the fatty acid biosynthesis pathway in bacteria. We report here that PTM is also a potent and highly selective inhibitor of mammalian fatty acid synthase. In contrast to two agents, C75 and cerulenin, that are widely used as inhibitors of mammalian fatty acid synthase, platensimycin specifically inhibits fatty acid synthesis but not sterol synthesis in rat primary hepatocytes. PTM preferentially concentrates in liver when administered orally to mice and potently inhibits hepatic de novo lipogenesis, reduces fatty acid oxidation, and increases glucose oxidation. Chronic administration of platensimycin led to a net reduction in liver triglyceride levels and improved insulin sensitivity in
db/+
mice fed a high-fructose diet. PTM also reduced ambient glucose levels in
db/db
mice. These results provide pharmacological proof of concept of inhibiting fatty acid synthase for the treatment of diabetes and related metabolic disorders in animal models.
Expression, stability, and replacement of glucan-remodeling enzymes during developmental transitions inSaccharomyces cerevisiae
Authors: Rolli Eleonora , Ragni Enrico , de Medina-Redondo María , Arroyo Javier , de Aldana Carlos R. Vázquez , Popolo Laura
Published: March 2011

Sporulation is a developmental variation of the yeast life cycle whereby four spores are produced within a diploid cell, with proliferation resuming after germination. The GAS family of glycosylphosphatidylinositol-anchored glucan-remodeling enzymes exemplifies functional interplay between paralogous genes during the yeast life cycle. GAS1 and GAS5 are expressed in vegetative cells and repressed during sporulation while GAS2 and GAS4 exhibit a reciprocal pattern. GAS3 is weakly expressed in all the conditions and encodes an inactive protein. Although Gas1p functions in cell wall formation, we show that it persists during sporulation but is relocalized from the plasma membrane to the epiplasm in a process requiring End3p-mediated endocytosis and the Sps1 protein kinase of the p21-activated kinase family. Some Gas1p is also newly synthesized and localized to the spore membrane, but this fraction is dispensable for spore formation. By way of contrast, the Gas2–Gas4 proteins, which are essential for spore wall assembly, are rapidly degraded after spore formation. On germination, Gas1p is actively synthesized and concentrated in the growing part of the spore, which is essential for its elongation. Thus Gas1p is the primary glucan-remodeling enzyme required in vegetative growth and during reentry into the proliferative state. The dynamic interplay among Gas proteins is crucial to couple glucan remodeling with morphogenesis in developmental transitions.