bluuebunny/crossref_metadata_2025 huggingface dataset:
Temperature influence on hydrodynamic instabilities in a one-dimensional electron flow in semiconductors
Authors: Calderón-Muñoz Williams R. , Jena Debdeep , Sen Mihir
Published: April 2010

Hydrodynamic instabilities in one-dimensional electron flow in semiconductor and their dependency with the electron and lattice temperatures are studied here. The driving force for the electrons is imposed by a voltage difference, and the hydrodynamic and electrostatic equations are linearized with respect to the steady-flow solution. A two-temperature hydrodynamic model predicts a stable electron flow through the semiconductor. A one-temperature hydrodynamic model is obtained by neglecting the electron energy losses due to heat conduction and scattering. This model shows that the electron flow can become unstable and establishes a criterion for that. Applied voltage and temperature can play the role of tunable parameters in the stability of the electron flow.
Red-edge laser action from borondipyrromethene dyes
Authors: García-Moreno Inmaculada , Zhang Dakui , Costela Ángel , Martín Virginia , Sastre Roberto , Xiao Yi
Published: April 2010

Long-wavelength fluorescent dyes with improved efficiency and stability are required to realize new photoelectronic and biophotonic applications. We have designed and synthesized novel 2,6- or 3,5-substituted BODIPY (BDP) dyes by a simple protocol to reach wavelength-finely tunable laser action from 600 to 725 nm. The influence of the nature and position of substituents on the laser behavior of the new BDP dyes have been systematically characterized as a function of dye concentration and chemical character of the medium. Regarding efficiency and photostability, the new chromophores outperform the laser action of dyes presently commercialized and considered as benchmarks over this spectral region when pumped under identical conditions. The results obtained provide insight into the always complicated composition-structure-properties relationship of dyes, and suggest possible synthesis routes for new BDP derivatives with properties optimized for specific applications.
Hilbert-space treatment of incoherent, time-resolved spectroscopy. I. Formalism, a tensorial classification of high-order orientational gratings and generalized MUPPETS “echoes”
Authors: Berg Mark A.
Published: April 2010

Time-resolved spectroscopy that uses more than one incoherent excitation, and thus has multiple periods of time evolution, is becoming more common. A recent example is multiple population-period transient spectroscopy (MUPPETS), which is implemented as a high-order transient grating. In this paper, a formalism is developed to treat these types of incoherent spectroscopy in a manner that parallels the Liouville-pathway formalism used to treat multidimensional coherent spectroscopy. A Hilbert space of incoherent (population) states is defined and general expressions for transition and time-evolution operators acting on these states are derived from the corresponding quantum operators. This formalism describes incoherent experiments that involve an arbitrary number of temporal dimensions in terms of pathways through the Hilbert space. Each pathway is associated with a multiple-time rate-correlation function. Previous work has shown that these multiple-time correlation functions can measure heterogeneity in electronic-relaxation rates. Thus, they are an analog of coherent “echo” experiments, which measure heterogeneity in frequencies. We show that similar “MUPPETS echo” experiments can be done on any incoherent variable. For a dilute molecular solute, the Hilbert-space method leads to a systematic treatment of multidimensional transient gratings. The extension of irreducible-tensor methods to the incoherent Hilbert space results in a classification of orientational gratings of arbitrary order. The general methods developed in this paper are applied more specifically to single-photon, dipole transitions in the following paper.
Advanced tokamak research with integrated modeling in JT-60 Upgrade
Authors: Hayashi N. ,   JT-60 Team
Published: April 2010

Researches on advanced tokamak (AT) have progressed with integrated modeling in JT-60 Upgrade [N. Oyama et al., Nucl. Fusion 49, 104007 (2009)]. Based on JT-60U experimental analyses and first principle simulations, new models were developed and integrated into core, rotation, edge/pedestal, and scrape-off-layer (SOL)/divertor codes. The integrated models clarified complex and autonomous features in AT. An integrated core model was implemented to take account of an anomalous radial transport of alpha particles caused by Alfven eigenmodes. It showed the reduction in the fusion gain by the anomalous radial transport and further escape of alpha particles. Integrated rotation model showed mechanisms of rotation driven by the magnetic-field-ripple loss of fast ions and the charge separation due to fast-ion drift. An inward pinch model of high-Z impurity due to the atomic process was developed and indicated that the pinch velocity increases with the toroidal rotation. Integrated edge/pedestal model clarified causes of collisionality dependence of energy loss due to the edge localized mode and the enhancement of energy loss by steepening a core pressure gradient just inside the pedestal top. An ideal magnetohydrodynamics stability code was developed to take account of toroidal rotation and clarified a destabilizing effect of rotation on the pedestal. Integrated SOL/divertor model clarified a mechanism of X-point multifaceted asymmetric radiation from edge. A model of the SOL flow driven by core particle orbits which partially enter the SOL was developed by introducing the ion-orbit-induced flow to fluid equations.
Theoretical gain of strained GeSn0.02/Ge1−x−y′SixSny′ quantum well laser
Authors: Zhu Yuan-Hui , Xu Qiang , Fan Wei-Jun , Wang Jian-Wei
Published: April 2010

Using effective-mass Hamiltonian model of semiconductors quantum well structures, we investigate the electronic structures of the Γ-conduction and L-conduction subbands of GeSn/GeSiSn strained quantum well structure with an arbitrary composition. Our theoretical model suggests that the band structure could be widely modified to be type I, negative-gap or indirect-gap type II quantum well by changing the mole fraction of α-Sn and Si in the well and barrier layers, respectively. The optical gain spectrum in the type I quantum well system is calculated, taking into account the electrons leakage from the Γ-valley to L-valley of the conduction band. We found that by increasing the mole fraction of α-Sn in the barrier layer and not in the well layer, an increase in the tensile strain effect can significantly enhance the transition probability, and a decrease in Si composition in the barrier layer, which lowers the band edge of Γ-conduction subbands, also comes to a larger optical gain.
Kinetic Monte Carlo simulations of nanocrystalline film deposition
Authors: Ruan Shiyun , Schuh Christopher A.
Published: April 2010

A full diffusion kinetic Monte Carlo algorithm is used to model nanocrystalline film deposition, and study the mechanisms of grain nucleation and microstructure formation in such films. The major finding of this work is that new grain nucleation occurs predominantly on surface peaks. Consequently, development of a nanocrystalline structure is promoted by a growth surface with nanoscale roughness, on which new grains can nucleate and grow separately from one another. The grain minor dimension (in the plane of the film) is primarily dictated by surface peak spacing, which in turn is reduced at low temperatures and high deposition rates. The grain major dimension (in the growth direction) is related to the probability of nucleating new grains on top of pre-existing ones, with finer grains being formed at low temperatures and low grain boundary energies. Because vacancies kinetically pin grain boundaries, high vacancy content, which is obtained at high deposition rate, also favors nanograins. Consistent with empirical observations common in the experimental literature, it is found that as grains shrink, they transition from elongated to equiaxed.
Eight-band k⋅p calculations of the composition contrast effect on the linear polarization properties of columnar quantum dots
Authors: Andrzejewski Janusz , Sęk Grzegorz , O’Reilly Eoin , Fiore Andrea , Misiewicz Jan
Published: April 2010

We present eight-band k⋅p calculations of the electronic and polarization properties of columnar InzGa1−zAs quantum dots (CQD) with high aspect ratio embedded in an InxGa1−xAs/GaAs quantum well. Our model accounts for the linear strain effects, linear piezoelectricity, and spin-orbit interaction. We calculate the relative intensities of transverse-magnetic (TM) and transverse-electric (TE) linear polarized light emitted from the edge of the semiconductor wafer as a function of the two main factors affecting the heavy hole—light hole valence band mixing and hence, the polarization dependent selection rules for the optical transitions, namely, (i) the composition contrast z/x between the dot material and the surrounding well and (ii) the dot aspect ratio. The numerical results show that the former is the main driving parameter for tuning the polarization properties. This is explained by analyzing the biaxial strain in the CQD, based on which it is possible to predict the TM to TE intensity ratio. The conclusions are supported by analytical considerations of the strain in the dots. Finally, we present the compositional and geometrical conditions to achieve polarization independent emission from InGaAs/GaAs CQDs.
First principles multielectron mixed quantum/classical simulations in the condensed phase. I. An efficient Fourier-grid method for solving the many-electron problem
Authors: Glover William J. , Larsen Ross E. , Schwartz Benjamin J.
Published: April 2010

We introduce an efficient multielectron first-principles based electronic structure method, the two-electron Fourier-grid (2EFG) approach, that is particularly suited for use in mixed quantum/classical simulations of condensed-phase systems. The 2EFG method directly solves for the six-dimensional wave function of a two-electron Hamiltonian in a Fourier-grid representation such that the effects of electron correlation and exchange are treated exactly for both the ground and excited states. Due to the simplicity of a Fourier-grid representation, the 2EFG is readily parallelizable and we discuss its computational implementation in a distributed-memory parallel environment. We show our method is highly efficient, being able to find two-electron wave functions in ∼20 s on a modern desktop computer for a calculation this is equivalent to full configuration interaction (FCI) in a basis of 17 million Slater determinants. We benchmark the accuracy of the 2EFG by applying it to two electronic structure test problems: the harmonium atom and the sodium dimer. We find that even with a modest grid basis size, our method converges to the analytically exact solutions of harmonium in both the weakly and strongly correlated electron regimes. Our method also reproduces the low-lying potential energy curves of the sodium dimer to a similar level of accuracy as a valence CI calculation, thus demonstrating its applicability to molecular systems. In the following paper [W. J. Glover, R. E. Larsen, and B. J. Schwartz, J. Chem. Phys. 132, 144102 (2010)], we use the 2EFG method to explore the nature of the electronic states that comprise the charge-transfer-to-solvent absorption band of sodium anions in liquid tetrahydrofuran.
First principles multielectron mixed quantum/classical simulations in the condensed phase. II. The charge-transfer-to-solvent states of sodium anions in liquid tetrahydrofuran
Authors: Glover William J. , Larsen Ross E. , Schwartz Benjamin J.
Published: April 2010

Gas-phase atomic anions lack bound electronic excited states, yet in solution many of these anions exhibit intense absorption bands due to the presence of excited states, referred to as charge-transfer-to-solvent (CTTS) states that are bound only by the presence of the solvent. CTTS spectra thus serve as delicate probes of solute-solvent interactions, but the fact that they are created by the interactions of a solute with many solvent molecules makes them a challenge to describe theoretically. In this paper, we use mixed quantum/classical molecular dynamics with the two-electron Fourier-grid (2EFG) electronic structure method presented in the previous paper [W. J. Glover, R. E. Larsen, and B. J. Schwartz, J. Chem. Phys. 132, 144101 (2010)] to simulate the CTTS states of a sodium anion in liquid tetrahydrofuran, Na−/THF. Since our 2EFG method is based on configuration interaction with single and double excitations in a grid basis, it allows for an exact treatment of the two valence electrons of the sodium anion. To simulate Na−/THF, we first develop a new electron–THF pseudopotential, and we verify the accuracy of this potential by reproducing the experimental absorption spectrum of an excess electron in liquid THF with near quantitative accuracy. We also are able to reproduce the CTTS spectrum of Na−/THF and find that the CTTS states of Na− exhibit a Rydberg-like progression due to the pre-existing long-range solvent polarization around the anion. We also find that the CTTS states are highly mixed with the disjoint electronic states supported by naturally occurring solvent cavities that exist in liquid THF. This mixing explains why the solvated electrons that are ejected following CTTS excitation appear with their equilibrium absorption spectrum. The mixing of the CTTS and solvent-cavity states also explains why the recombination of the electron and its geminate Na0 partner occurs on slower time scales when photoexciting in the blue compared to in the red of the CTTS band: blue excitation accesses CTTS states whose charge densities lies further from the position of the anion, whereas red excitation accesses CTTS states that lie primarily within the anion’s first solvation shell. Finally, we see that the radial character of the CTTS states near the Na+ core matches that of Na0, explaining why the spectrum of this species appears instantly after photoexciting Na−.
A prototype chopper for synchrotron time-resolved crystallographic measurements
Authors: Husheer S. L. G. , Cole J. M. , d’Almeida T. , Teat S. J.
Published: April 2010

A mechanical x-ray chopper has been designed to perform microsecond time-resolved crystallographic studies at the DIAMOND synchrotron I19 beamline. It consists of two asymmetric absorbers rotating synchronously at frequencies from 0 to 50 Hz in the same direction around a rotation axis that is parallel to the x-ray beam. The duration of the x-ray pulses produced by the chopper is determined by the relative phase between the two blades, which can be adjusted. The chopper system presented in this paper offers a time resolution suitable for conducting in situ experiments that afford the crystal structure of materials while in their transient (>10 μs) photoactivated excited states.
Fractal dimension and surface topography on the diamond deposition of seeded WC–Co substrates
Authors: Chou C.-C. , Lin H.-H.
Published: April 2010

Diamond thin films were deposited on WC–Co substrates by hot filament chemical vapor deposition to improve the tribological performance. The influence of the substrate surface topography was found to play an important role during the nucleation stage and the later growth rate as well. In this study, we systematically investigated the relation between substrate surface irregularity, which was evaluated by fractal dimension as well as statistical roughness parameters and the quality of the later deposited diamond film. Preseeding processes, in diamond acetone suspensions with two particle diameters, by supersonic vibrator were also implemented to investigate the effect of particular size on diamond nucleation. The original surfaces were measured with a stylus profiler and contact-mode atomic force microscopy. The diamond deposited substrates were examined by scanning electron microscopy, x-ray diffractometry, Raman spectroscopy, and Rockwell-C indentation to study substrate topography, crystalline structure of the coating, the composition of diamond films, and adhesion between deposited layers and substrates, respectively. The synergetic influence of the substrate’s fractal dimension and the particular size of pre-seeding diamond suspension were studied and addressed. The deposited film of a WC–Co substrate with higher surface fractal dimension (>2.50), preseeded by fine diamond suspension (4–12 nm particle size) in advance, has a high diamond-rich composition and adhesion strength.
Theoretical analysis on Love waves in a layered structure with a piezoelectric substrate and multiple elastic layers
Authors: Liu Jiansheng , He Shitang
Published: April 2010

A method is developed to analyze the existence and behavior of piezoelectric Love waves in a multilayered structure consisting of a piezoelectric substrate and multiple elastic layers which are isotropic, nonpiezoelectric materials. The acoustic waves and electric fields in the substrate and the layers are investigated. A general dispersion equation is derived to describe the existence of Love surface waves with respect to phase velocity as a function of normalized layer thickness. An iteration formula for XN is introduced to describe the mechanical action between the layers and the substrate at the interface. Another formula for ε¯LN, the equivalent permittivity of the wave-guide layers, is produced to describe the electric fields in the layers. The dispersion equation including a mass loading on the surface of the top layer is deduced, and a formula for calculating the mass sensitivity of the phase velocity is presented. We also find the dispersion equation with an electric shorted interface and introduce a formula for calculating the electromechanical coupling coefficient K2. Numerical results illustrate the phase velocity, the mass sensitivity of the phase velocity and the electromechanical coupling coefficient as functions of the normalized layer thickness for the Love waves in a layered structure with a polymethylmethacrylate (PMMA) layer and a sputtered SiO2 layer on a 90° rotated ST-cut quartz (ST-quartz) substrate.
Electromechanical resonance in ferrite-piezoelectric nanopillars, nanowires, nanobilayers, and magnetoelectric interactions
Authors: Petrov V. M. , Bichurin M. I. , Srinivasan G.
Published: April 2010

A model is presented for magnetoelectric (ME) effects in the region of electromechanical resonance (EMR) for ferrite-piezoelectric nanobilayers, nanopillars, and nanowires on a substrate or a template. Influence of the lattice mismatch on the material parameters and ME interactions has been taken into account using the classical Landau–Ginsburg–Devonshire phenomenological thermodynamic theory. The clamping effect of the substrate for nanobilayers and nanopillars or template for nanowires have been considered in determining the ME voltage coefficient. The model is applied to nickel ferrite-lead zirconate titanate nanostructures on strontium titanate substrate or template. It is shown that the ME coupling strength decreases with increasing substrate or template volume fraction. For increasing thickness of substrate for a bilayer or the template radius for a nanowire (i) the ME coefficient drops exponentially and (ii) EMR frequency increases. For nanopillars on a substrate, a fine structure is predicted in the ME voltage versus frequency profile.
Experimental investigation of different structures of a radio frequency produced plasma column
Authors: Kumar Rajneesh , Bora Dhiraj
Published: April 2010

Experimental results on the observation of spatial structures appearing in a radio frequency (rf) produced plasma column are presented in this paper. A plasma column of length 30 cm is formed in a glass tube by applying rf power from a cw generator. It is observed that by changing the operating parameters, e.g., input power, working pressure, drive frequency, diameter of glass tube, background gases, etc., single plasma column transforms into stationary/moving striations and helical and spiral structures. For a better understanding of these observations, experiments are also carried out to study the electric field profile of these structures in the plasma. Findings of this study reveal that surface wave produced plasma column transversally bifurcates and transforms into stationary striations. With further increase in working pressure, stationary striations become moving striations with linear and rotational motions and finally rotating plasma column bifurcates longitudinally and a helical structure is formed, which can also be transformed into a spiral structure. Electric field profile of each structure is entirely different from that of the other structure.
Spin-valve sensor with an out-of-plane magnetic anisotropy: For small field sensing applications
Authors: Lee Kisu , Park Moon-Jung , Lee Seungkyo , Kim Ju-Young , Cho B. K.
Published: April 2010

This study investigates a spin-valve sensor, which consists of ferromagnetic layers with both an out-of-plane magnetic anisotropy (NiFe/Tb/NiFe layers) and an in-plane magnetic anisotropy (CoFe/IrMn layers). The out-of-plane magnetic anisotropy was able to be tuned by varying the thickness (tTb) of the Tb layer and applying an in-plane magnetic field during film deposition. In addition, the field sensitivity of the spin-valve sensor was also found to be a function of the degree of out-of-plane magnetic anisotropy. As a result, a sensor with tTb=3 nm showed a linear and reversible magnetoresistance (MR) response to an applied in-plane magnetic field with a higher sensitivity of 0.012%/Oe by one order of magnitude than that (∼0.000 75%/Oe) of a sensor with tTb=4 nm. This suggests that the spin-valve sensor can be optimized by changing the Tb thickness so that the magnetic properties of the sensing layer can meet the requirements of a small field sensing application, such as a biosensor.
Synthesis of SiOx–Ag core-shell nanostructures by oblique angle deposition
Authors: Singh Dhruv P. , Nagar Rupali , Singh J. P.
Published: April 2010

We report synthesis of SiOx–Ag core-shell nanostructures comprising of a uniform and patterned shell of Ag nanoparticles (67±9 nm) by oblique angle deposition (OAD) at room temperature. The Ag nanoparticles were observed to form hexagonal tessellation over the curved silica surface. The distribution of nanoparticles over silica spheres is explained in view of surface energy minimization by Euler’s characteristic for best coverage. The oblique angle of Ag vapor flux with respect to the substrate normal during growth was observed to be the control parameter in formation of SiOx–Ag core-shell nanostructures. Usually, OAD has been used to grow columnar nanostructures by exploiting the shadowing effect of adatoms during deposition but the application in surface modification at nanoscale by controlling the competing effect of shadowing and surface diffusion is the novelty in this work.
Spin phases and currents in ring shaped one-dimensional quantum dot arrays
Authors: Bellucci S. , Onorato P.
Published: April 2010

The electronic states of an array of Nd quantum dots (QDs) patterned in an isolated quantum ring are calculated. An external magnetic field B perpendicular to the ring’s plane is considered. The single particle spectra show the presence of an insulating pseudoband which can be totally filled by 2Nd electrons. A large gap is present between the insulating pseudoband and the metallic one. The damping of Aharonov–Bohm oscillations reflects the localization of the quantum states and can be justified in terms of metallic-insulating transition induced by the barriers. The energy spectra of low-lying states of Ne≤2Nd interacting electrons in the QDs array as a function of B are obtained. Phase diagrams are presented for the insulating band from half to integer filling. The diagrams indicate a rich variety of ground states revealing transitions in which the total spin changes.
Temperature dependence of strain–polarization coupling in ferroelectric ceramics
Authors: Weaver P. M. , Cain M. G. , Stewart M.
Published: April 2010

The temperature dependence of the strain, electric field, and polarization of lead zirconate titanate ceramic under unipolar electric fields up to 1.3 kV mm−1 from −50 °C up to +200 °C is reported. An electrostrictive model is shown to describe the response to high levels of induced polarization and electric field below and above the ferroelectric to paraelectric transition. This is despite nonlinearity and hysteresis in the strain–field response. A linear strain–polarization response is shown to be an approximation to a quadratic electrostriction which is invalid at higher temperatures while the electrostriction holds across the temperature range.
Origin of hydrogen-inclusion-induced critical current deviation in Nb/AlOx/Al/Nb Josephson junctions
Authors: Hinode Kenji , Satoh Tetsuro , Nagasawa Shuichi , Hidaka Mutsuo
Published: April 2010

We investigated the mechanisms that change the critical current density (Jc) of Nb/AlOx/Al/Nb Josephson junctions due to the inclusion of hydrogen in the Nb electrodes. Our investigations were performed according to three aspects: the superconductivity change, the change in thickness of the barrier layer, and the change in the barrier height due to the electronic effect. The results are as follows: (a) the hydrogen-inclusion-accompanied changes in the superconductivity parameters, such as the junction gap voltage, were much less than those of the critical current density, (b) the effect of hydrogen inclusion on Jc varied depending on the electrodes, i.e., the upper electrode above the barrier layer was the most affected, (c) the junctions with increased Ics due to hydrogen exclusion showed the identical amount of decrease in the junction resistance measured at room temperature, and (d) the hydrogen exclusion from the junction electrodes had no influence on the Nb/Al/AlOx/Al/Nb junctions, which had an extra Al layer. Based on these results we conclude that the Jc change is mainly caused by the change in junction resistance. A one order of magnitude smaller effect is caused by the superconductivity change. We believe the Jc change is caused by a Nb work function increase due to the hydrogen inclusion, resulting in an increase in barrier height.
On-off intermittent regulation of plasma turbulence
Authors: Bian N. H.
Published: April 2010

A standard low-dimensional model of the dynamical regulation of plasma turbulence including zonal flows is considered in a statistical sense by taking into account the fluctuating nature of the source driving the system. The probability distribution functions of the turbulent kinetic energy and the zonal flows energy are derived. The dynamics become on-off intermittent close to the bifurcation thresholds. In its low confinement mode, without zonal flows, the system can also display random finite amplitude burst close to marginal stability, a result reminiscent of the self-organized-criticality paradigm applied to flux-driven plasma turbulence.
Micromagnetic simulations of linewidths and nonlinear frequency shift coefficient in spin torque nano-oscillators
Authors: Carpentieri Mario , Torres Luis
Published: April 2010

The dependence of the linewidth on the temperature and the applied magnetic field angle is studied in spin torque nano-oscillators (STNOs) by means of full micromagnetic simulations. The analyzed spin valve is the experimental one by Sankey et al. [Phys. Rev. Lett. 96, 227601 (2006)] and the magnetic parameters are given by magnetoresistance fitting. Linewidth behavior increases with the temperature, in agreement with the analytical predictions by Tiberkevich et al. [Phys. Rev. B 78, 092401 (2008)], and its slope depends on the applied field angle. Also, the nonlinear frequency shift coefficient, which gives a measure of the nonlinearity degree of STNO and indicates the strength of the transformation of amplitude into phase fluctuations, is found. The understanding of the nonlinear frequency shift allows one to tune the generation frequency of the STNO, but, at the same time, creates an additional source of the phase noise, which leads to a significant broadening of the linewidth generation. Narrow linewidths (around 10 MHz at 0 K and 100 MHz at 300 K) are found in our shape-anisotropy nanopillars by applying close to in-plane magnetic field at an angle of 45° between in-plane easy and hard axes.
Investigation on critical failure thickness of hydrogenated/nonhydrogenated amorphous silicon films
Authors: Kim Jeehwan , Inns Daniel , Sadana Devendra K.
Published: April 2010

Thick films of polycrystalline silicon for solar cells can be made by crystallization of amorphous silicon. Due to the high internal stress of amorphous silicon films, deposition of such thick films is challenging. In this paper, we investigate the failure behavior of thick amorphous silicon films. Depending on the deposition method of amorphous silicon films, they can be cracked or delaminated. While nonhydrogenated amorphous silicon films deposited by an evaporator are cracked when the accumulated strain in thick films reaches a critical point, hydrogenated amorphous silicon films deposited by plasma enhanced chemical vapor deposition are delaminated from the substrate. Critical cracking/delamination thicknesses of amorphous silicon films are theoretically derived and excellent agreement with experimental results is shown.
Hodograph solutions for the Manakov–Santini equation
Authors: Chang Jen-Hsu , Chen Yu-Tung
Published: April 2010

We investigate the integrable (2+1)-dimensional Manakov–Santini equation from the Lax–Sato form. Several particular two- and three-component reductions are considered so that the Manakov–Santini equation can be reduced to systems of hydrodynamic type. Then one can construct infinitely many exact solutions of the equation by the hodograph method.
Dielectric tunability and electro-optic effect of Ba0.5Sr0.5TiO3 thin films
Authors: Takeda K. , Muraishi T. , Hoshina T. , Takeda H. , Tsurumi T.
Published: April 2010

We elucidated the relationship between the dielectric tunability and the electro-optic (EO) effect. A new measurement system for EO effect and dielectric permittivity (εr) was developed to investigate the accurate correlation between EO and dielectric properties of epitaxial barium strontium titanate, Ba0.5Sr0.5TiO3 (BST) thin films grown on SrTiO3 (STO) substrates. The BST films had a strained lattice with a large lattice parameter along the thickness direction. Small birefringence was induced in the as-deposited film by the alignment of the c-axis in the film plane. The tunability of dielectric permittivity calculated from the complex voltage and current with planer electrodes reached to 53.1%, and the tunability of birefringence by EO effect was 0.6%. The birefringence change from EO effect was much lower than the tunability of dielectric permittivity. Therefore, there is no strong correlation between the dielectric tunability and EO effect, and the materials with high tunability do not always exhibit high EO effect. Finally, we discussed how to obtain the materials with high EO effect considering these results.
Scanning nanoelectrometer based on a two-dimensional electron gas transistor with a probe-integrated gate electrode
Authors: Kawano Yukio , Ishibashi Koji
Published: April 2010

We report a scanning nanoelectrometer based on a two-dimensional electron gas (2DEG) transistor in GaAs/AlGaAs, whose gate electrode is combined with a metal-coated cantilever probe. In this system, local electric potential is detected through gate effects for the 2DEG detector caused by electrical polarization of the probe. Using this technique, we have demonstrated mapping of the electric potential distribution for 2DEG samples in a GaAs/AlGaAs interface and in a graphene surface. Time-resolved measurements of local potential are also presented.
Step-down piezoelectric transformer fabricated with (1−x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 single crystal
Authors: Wang Feifei , Shi Wangzhou , Luo Haosu
Published: April 2010

In present work, a ring-type unipoled step-down piezoelectric transformer with the dimensions of Φ16×0.7 mm3 was designed and fabricated using relaxor ferroelectric single crystal (1−x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 (PMN-PT). Both the input and output parts of this transformer were poled along the thickness direction, utilizing the radial vibration mode of PMN-PT. The electrical measurement results indicated that under the matching load resistance of 257 Ω, a voltage transformation ratio of 0.44 was obtained at the resonance frequency of 105 kHz. The frequency and load dependences of this transformer were also systematically studied and discussed. With the input voltage increasing, a maximum output power of about 0.9 W was obtained with the temperature rise less than 5 °C. This step-down single-crystal transformer show potential applications in compact-size, small-power ac-dc converters.
“Half-hydration” at the air/water interface revealed by heterodyne-detected electronic sum frequency generation spectroscopy, polarization second harmonic generation, and molecular dynamics simulation
Authors: Watanabe Hidekazu , Yamaguchi Shoichi , Sen Sobhan , Morita Akihiro , Tahara Tahei
Published: April 2010

A solute-solvent interaction at the air/water interface was investigated both experimentally and theoretically, by studying a prototypical surface-active polarity indicator molecule, coumarin 110 (C110), adsorbed at the air/water interface with heterodyne-detected electronic sum frequency generation (HD-ESFG) spectroscopy, polarization second harmonic generation (SHG), and a molecular dynamics (MD) simulation. The second-order nonlinear optical susceptibility (χ(2)) tensor elements of C110 at the air/water interface were determined experimentally by HD-ESFG and polarization SHG, and information on “intermediate” polarity sensed by C110 at the interface was obtained by HD-ESFG. An MD simulation and a time-dependent density functional theory calculation were used to theoretically evaluate the χ(2) tensor elements, which were in good agreement with the experimental results of HD-ESFG and polarization SHG. The microscopic “half-hydration” structure around C110 at the water surface was visualized on the basis of the MD simulation data, with which we can intuitively understand the microscopic origin of the surface activity of C110 and the intermediate polarity sensed by C110 at the air/water interface.
Micromagnetic simulations of a dual-injector spin transfer torque operated spin logic
Authors: Höink V. , Lau J. W. , Egelhoff W. F.
Published: April 2010

The application of spin transfer torque switching for the realization of the logic input in a reconfigurable spin logic based on two magnetic tunnel junctions is discussed. Two electrodes with orthogonal fixed magnetization directions are used to replace the magnetic fields generated by a current though leads close to the magnetic tunnel junctions which have been used so far. Initial results of micromagnetic simulations suggest the feasibility of this approach.
Separatrix shape of field-reversed configuration
Authors: Ohkuma Yasunori , Hiroi Masanori , Ikeyama Taeko , Nogi Yasuyuki
Published: April 2010

A method of determining three parameters of a separatrix shape on a field-reversed configuration plasma is described. The three parameters are center radius a, length b (half-distance between X-points), and shape index n when the separatrix radius rs is approximated by the radial position derived from the relationship r2/a2+|z|n/bn=1. To demonstrate the details of the method, magnetic fields in a vacuum region around the separatrix are numerically calculated for the assumed separatrix shapes in the ranges 4≤b/a≤10 and 1.2≤n≤4. Excluded flux radii rΔϕ(z) along the plasma are obtained using the calculated magnetic fields. First, it is confirmed that rΔϕ(0) at the axial midplane can be replaced by a with good precision. Second, a trial function including rΔϕ(z) and parameter n′ is defined. The parameters b and n are determined using the trial function that becomes b without any axial dependence when rΔϕ(z)≈rs and n′≈n. Errors in b and n arising from edge-layer plasmas are quantitatively estimated by introducing its width in the trial function. Finally, it is demonstrated, using experimental data, that the separatrix parameters are easily determined by the present method.
Propagation of three-dimensional ion-acoustic solitary waves in magnetized negative ion plasmas with nonthermal electrons
Authors: El-Labany S. K. , Sabry R. , El-Taibany W. F. , Elghmaz E. A.
Published: April 2010

Properties of small amplitude nonlinear ion-acoustic solitary waves in a warm magneto plasma with positive-negative ions and nonthermal electrons are investigated. For this purpose, the hydrodynamic equations for the positive-negative ions, nonthermal electron density distribution, and the Poisson equation are used to derive the corresponding nonlinear evolution equation; Zkharov–Kuznetsov (ZK) equation, in the small amplitude regime. The ZK equation is analyzed to examine the existence regions of the solitary pulses. It is found that compressive and rarefactive ion-acoustic solitary waves strongly depend on the mass and density ratios of the positive and negative ions as well as the nonthermal electron parameter. Also, it is found that there are two critical values for the density ratio of the negative-to-positive ions (υ), the ratio between unperturbed electron-to-positive ion density (μ), and the nonthermal electron parameter (β), which decide the existence of positive and negative ion-acoustic solitary waves. The present study is applied to examine the small amplitude nonlinear ion-acoustic solitary excitations for the (H+, O2−) and (H+, H−) plasmas, where they are found in the D- and F-regions of the Earth’s ionosphere. This investigation should be helpful in understanding the salient features of the nonlinear ion-acoustic solitary waves in space and in laboratory plasmas where two distinct groups of ions and non-Boltzmann distributed electrons are present.
Plasma fluctuation spectra as a diagnostic tool for submicron dust
Authors: Ratynskaia S. , De Angeli M. , Lazzaro E. , Marmolino C. , de Angelis U. , Castaldo C. , Cremona A. , Laguardia L. , Gervasini G. , Grosso G.
Published: April 2010

It is shown that the measurements of density fluctuation spectra in dusty plasmas can constitute a basis for in situ diagnostic of invisible submicron dust. The self-consistent kinetic theory that includes the charging processes and the natural density fluctuations of the dust particles predicts modifications of the spectra due to the presence of dust. A laboratory experiment was carried out where submicron dust was produced in a gas phase and diagnosed by surface analysis of samples and by measurements of its influence on the plasma density fluctuation spectra. Quantitative comparison of the latter with the theory yields information on dust density, size, and distribution in agreement with the results of the surface analysis. The method can be applied to various plasma environments in laboratory and space.
Passive magnetic field cancellation device by multiple high-Tc superconducting coils
Authors: Gu C. , Zou S. , Han Z. , Qu T.-M.
Published: April 2010

A passive magnetic field cancellation device (PMFCD) is designed. The PMFCD could automatically cancel the field as an active cancellation system did; however it requires no power sources and feedback systems. The capability of the PMFCD is based on the principle that a closed loop can resist flux variation and keep the flux constant inside. The closed loop in the PMFCD is formed by connecting two pairs of high temperature superconductor Helmholtz coils with different radii in series. More important thing is that the ratio of the radius and the turn number between the coils has to satisfy a number of conditions, with which 100% cancellation can be reached. Theoretical methods to obtain the turn number ratio and radius ratio are the major part of the paper. Numerical simulation was followed, aiming to evaluate field distribution under a cancellation state and correct the theoretical values.
Ultralow nonalloyed Ohmic contact resistance to self aligned N-polar GaN high electron mobility transistors by In(Ga)N regrowth
Authors: Dasgupta Sansaptak , Nidhi  , Brown David F. , Wu Feng , Keller Stacia , Speck James S. , Mishra Umesh K.
Published: April 2010

Ultralow Ohmic contact resistance and a self-aligned device structure are necessary to reduce the effect of parasitic elements and obtain higher ft and fmax in high electron mobility transistors (HEMTs). N-polar (0001¯) GaN HEMTs, offer a natural advantage over Ga-polar HEMTs, in terms of contact resistance since the contact is not made through a high band gap material [Al(Ga)N]. In this work, we extend the advantage by making use of polarization induced three-dimensional electron-gas through regrowth of graded InGaN and thin InN cap in the contact regions by plasma (molecular beam epitaxy), to obtain an ultralow Ohmic contact resistance of 27 Ω μm to a GaN 2DEG.
Electrical beam steering of Y-coupled quantum cascade lasers
Authors: Ahn S. I. , Mujagić E. , Nobile M. , Detz H. , Kalchmair S. , Andrews A. M. , Klang P. , Schrenk W. , Strasser G.
Published: April 2010

The authors describe electrical beam steering of mid-infrared quantum cascade lasers with a monolithically integrated Y-coupled cavity and electrically separated branches. The deflection of the laser beam is achieved without any additional components such as optic or mechanic systems. This is done by injecting additional direct current into one of the two emitting branches, locally increasing the temperature. We estimate that the required temperature difference between left and right branch is approximately 12 K to achieve 2° of the beam steering. This value is in a good agreement with heat transfer simulations.
Influence of secondary emission yield on the saturation properties of multipactor discharges between two parallel metal plates
Authors: Buyanova M. , Semenov V. E. , Anderson D. , Lisak M. , Puech J.
Published: April 2010

A detailed numerical simulation analysis is made of the saturation stage of multipactor discharges between two infinite parallel metal plates in vacuum exposed to a rf voltage. The main physical effect causing saturation of the multipactor discharge is increased space charge, as the electron density becomes large. It is found that the properties of the saturation stage depend crucially on the value of the secondary emission yield of the metal surfaces. Below a certain threshold value, the discharge has a two-sided character, but at this threshold the discharge makes an abrupt transition into two decoupled single-sided multipactor discharges containing significantly increased electron densities. The result of the numerical simulations gives a good picture of the saturation properties of the multipactor discharge and is also supplemented by an approximate analytical investigation that highlights and explains the characteristic properties of the observed saturation behavior.
Distinct C60 growth modes on anthracene carboxylic acid templates
Authors: Xu Bo , Zhu Erkuang , Lu Chao , Liu Yadi , Liu Zhongyuan , Yu Dongli , He Julong , Tian Yongjun
Published: April 2010

Two anthracene carboxylic acid (AnCA) self assembly structures on Ag(111) were employed to investigate the template effects on sequentially deposited C60 molecules using scanning tunneling microscope. The initial AnCA structures execute strong modulations on C60 growth. Either laterally separated AnCA–C60 elongated domain arrays or an “epitaxial” C60 dimer structure over AnCA can be formed depending on the selected AnCA template. These distinct C60 growth modes are closely related to the structural stability of the AnCA templates. Our studies suggest a pathway of molecular nanostructure fabrication through the choice of suitable template.
Dynamics of acoustic droplet vaporization in gas embolotherapy
Authors: Qamar Adnan , Wong Zheng Z. , Fowlkes J. Brian , Bull Joseph L.
Published: April 2010

Acoustic droplet vaporization is investigated in a theoretical model. This work is motivated by gas embolotherapy, a developmental cancer treatment involving tumor infarction with gas microbubbles that are selectively formed from liquid droplets. The results indicate that there exists a threshold value for initial droplet size below which the bubble evolution is oscillatory and above which it is smooth and asymptotic, and show that the vaporization process affects the subsequent microbubble expansion. Dampening of the bubble expansion is observed for higher viscosity and surface tension, with effects more pronounced for droplet size less than 6 μm in radius.
Thin film tandem photovoltaic cell from II-IV-V chalcopyrites
Authors: van Schilfgaarde Mark , Coutts Timothy J. , Newman Nathan , Peshek Timothy
Published: April 2010

Using quasiparticle self-consistent GW (QSGW) theory, we analyze materials properties of the II-IV-V family of chalcopyrite semiconductors consisting of compounds and alloys based on (Mg,Zn,Cd)(Si,Ge,Sn)(P,As)2, and show how they may offer excellent opportunities for the development of tandem thin-film solar cells. The constituent elements are abundant and nearly lattice-matched compounds can be found with near optimum band gaps. We show the close connection to band structures of other fourfold coordinated compounds that have led to the highest efficiency devices, and suggest potentially optimum alloys for tandem thin-film cells.
Basis set consistent revision of the S22 test set of noncovalent interaction energies
Authors: Takatani Tait , Hohenstein Edward G. , Malagoli Massimo , Marshall Michael S. , Sherrill C. David
Published: April 2010

The S22 test set of interaction energies for small model complexes [Phys. Chem. Chem. Phys. 8, 1985 (2006)] has been very valuable for benchmarking new and existing methods for noncovalent interactions. However, the basis sets utilized to compute the CCSD(T) interaction energies for some of the dimers are insufficient to obtain converged results. Here we consistently extrapolate all CCSD(T)/complete basis set (CBS) interaction energies using larger basis sets for the CCSD(T) component of the computation. The revised values, which we designate S22A, represent the most accurate results to date for this set of dimers. The new values appear to be within a few hundredths of 1 kcal mol−1 of the true CCSD(T)/CBS limit at the given geometries, but the former S22 values are off by as much as 0.6 kcal mol−1 compared to the revised values. Because some of the most promising methods for noncovalent interactions are already achieving this level of agreement (or better) compared to the S22 data, more accurate benchmark values would clearly be helpful. The MP2, SCS-MP2, SCS-CCSD, SCS(MI)-MP2, and B2PLYP-D methods have been tested against the more accurate benchmark set. The B2PLYP-D method outperforms all other methods tested here, with a mean average deviation of only 0.12 kcal mol−1. However, the consistent, slight underestimation of the interaction energies computed by the SCS-CCSD method (an overall mean absolute deviation and mean deviation of 0.24 and −0.23 kcal mol−1, respectively) suggests that the SCS-CCSD method has the potential to become even more accurate with a reoptimization of its parameters for noncovalent interactions.
Radiofrequency conical emission from femtosecond filaments in air
Authors: Forestier B. , Houard A. , Durand M. , André Y. B. , Prade B. , Dauvignac J.-Y. , Perret F. , Pichot Ch. , Pellet M. , Mysyrowicz A.
Published: April 2010

We show that the broadband conical emission associated with filaments in air extends down to the radiofrequency region. This rf emission which originates from the longitudinal oscillation of charged ions formed during filamentation is strongly enhanced by the presence of a longitudinal static electric field.