bluuebunny/crossref_metadata_2025 huggingface dataset:
Circulation and energy of the leading vortex ring in a gravity-driven starting jet
Authors: Gao L. , Yu S. C. M. , Ai J. J. , Law A. W. K.
Published: September 2008

The gravity-driven starting jet characteristics over the range of Reynolds numbers from 1179 to 10 611 have been investigated using planar laser induced fluorescence and particle image velocimetry. Of the 17 cases investigated, pinch off for the leading vortex was found to appear at Reynolds number above 3000. For those cases involving pinch off, limiting values for the nondimensional circulation for the leading vortex could be identified at around 1.26 and the nondimensional energy was at about 0.35±0.01, respectively. The former agrees with the universal value proposed by Shusser et al. [Phys. Fluids 18, 033601 (2006)] to within 9%, while the latter lies within the analytical result range (from 0.27 to 0.4) proposed by Gharib et al. [J. Fluid Mech. 360, 121 (1998)]. The results thus suggested that the pinched-off vortex ring would achieve similar characteristics and that may also be independent of the generation mechanism.
Diamond’s elastic stiffnesses from 322 K to 10 K
Authors: Migliori Albert , Ledbetter Hassel , Leisure Robert G. , Pantea C. , Betts J. B.
Published: September 2008

Using resonant-ultrasound spectroscopy, we measured diamond’s monocrystal elastic-stiffness coefficients C11, C12, and C44, between 322 and 10 K. Changes are small and smooth: The bulk modulus B=(C11+2C12)/3 increases about 1 part in 1000, describable by a quasiharmonic Einstein-oscillator model. Zero-temperature Cij correspond to a 2244-K Debye characteristic temperature. Using a low-temperature form of the Grüneisen–Debye model, we calculated an overall thermodynamic Grüneisen parameter of γ=1.26; using a high-temperature form we calculated 0.71; the lattice specific heat yields γ=1.10.
Switching kinetics of ferroelectric polymer nanomesas
Authors: Othon C. M. , Kim Jihee , Ducharme Stephen , Fridkin V. M.
Published: September 2008

The switching dynamics and switching time of ferroelectric nanomesas grown from the paraelectric phase of ultrathin Langmuir–Blodgett vinylidene fluoride and trifluoroethylene copolymer films are investigated. Ferroelectric nanomesas are created through heat treatment and self-organization and have an average height of 10 nm and an average diameter of 100 nm. Ferroelectric nanomesas are highly crystalline and are in the ferroelectric phase and switch faster than 50 μs. The dependence of switching time on applied voltage implies an extrinsic switching nature.
Steplike magnetocapacitance and dielectric relaxation in spin frustrated Ca3Co2O6
Authors: Li P. L. , Yao X. Y. , Wang K. F. , Lu C. L. , Gao F. , Liu J.-M.
Published: September 2008

The dielectric relaxation and magnetocapacitance effect of one-dimensional spin frustrated compound Ca3Co2O6 are investigated. The steplike magnetocapacitance effect is observed and one to one corresponds to the steplike magnetization. We explain this phenomenon from the spin configuration dependent dielectric response. The simulation results using the Monte Carlo method are in good agreement with experimental data at low temperature. The close correspondence between the magnetic and dielectric properties indicates that the coupling is the intrinsic character of Ca3Co2O. The steplike magnetocapacitance effect may find potential applications in data storage and sensors.
Local modifications of magnetism and structure in FePt (001) epitaxial thin films by focused ion beam: Two-dimensional perpendicular patterns
Authors: Albertini F. , Nasi L. , Casoli F. , Fabbrici S. , Luches P. , Gazzadi G. C. , di Bona A. , Vavassori P. , Valeri S. , Contri S. F.
Published: September 2008

Focused ion beam was utilized to locally modify magnetism and structure of L10 FePt perpendicular thin films. As a first step, we have performed a magnetic, morphological, and structural study of completely irradiated FePt films with different Ga+ doses (1×1013–4×1016 ions/cm2) and ion beam energy of 30 keV. For doses of 1×1014 ions/cm2 and above a complete transition from the ordered L10 to the disordered A1 phase was found to occur, resulting in a drop of magnetic anisotropy and in the consequent moment reorientation from out-of-plane to in-plane. The lowest effective dose in disordering the structure (1×1014 ions/cm2) was found not to affect the film morphology. Taking advantage of these results, continuous two-dimensional (2D) patterns of perpendicular magnetic structures (250 nm dots, 1 μm dots, 1 μm-large stripes) were produced by focused ion beam without affecting the morphology. The 2D patterns were revealed by means of magnetic force microscopy, that evidenced peculiar domain structures in the case of 1 μm dots.
Three-dimensional observations of polar domain structures using a confocal second-harmonic generation interference microscope
Authors: Kaneshiro Junichi , Kawado Satoshi , Yokota Hiroko , Uesu Yoshiaki , Fukui Tatsuo
Published: September 2008

The necessary conditions for nondestructive and three-dimensional (3D) observations of inverted polar domain structures using the second-harmonic generation interference microscope (SHGIM) are examined experimentally and theoretically using MgO doped congruent LiNbO3 and periodically poled (PP) MgO doped near stoichiometrical LiNbO3. The results can be explained well by the Boyd and Kleinman theory of strongly focused optical systems. To obtain clear 3D images of domain structures by SHGIM, the sign of the wave number Δk offset of the fundamental and SH waves should be negative and the SHG tensor components d31 should be used for both samples when they are observed with a wavelength of 1064 nm at room temperature. Under these conditions, 3D PP domain structures are clearly revealed by SHGIM. General conditions for the 3D visualization of polar domain structures are discussed for the cases of sum-frequency and difference-frequency generations.
Isotropic non-Heisenberg terms in the magnetic coupling of transition metal complexes
Authors: Bastardis Roland , Guihéry Nathalie , de Graaf Coen
Published: September 2008

This paper analyzes the different contributions to the magnetic coupling in systems with more than one unpaired electron per center. While in S=1∕2 spin systems the Heisenberg Hamiltonian involving only bilinear exchange interactions is reliable for the description of the magnetic states, biquadratic exchange interactions must be sometimes introduced for S=1 (or higher) spin systems to account for isotropic deviations to Heisenberg behavior. The analysis establishes that the excited atomic states, the so-called non-Hund states, are responsible for the main contribution to the deviations. The kinetic exchange contribution and the spin, hole, and particle polarizations increase the magnetic coupling but essentially maintain the Heisenberg pattern. The importance of the different contributions has been studied for a series of Ni2 compounds with a polarizable double azido bridge. The coupling between two Fe3+ ions in the molecular crystal Na3FeS3, which is known experimentally to present large deviations to Heisenberg behavior, has also been investigated.
0.3
Pb
(
Fe
1
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2
Nb
1
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2
)
O
3
–
0.7
Pb
(
Mg
1
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2
W
1
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2
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O
3
: A magnetic and electric relaxor
Authors: Levstik A. , Filipič C. , Bobnar V. , Holc J. , Drnovšek S. , Trontelj Z. , Jagličić Z.
Published: September 2008

0.3 Pb ( Fe 1 / 2 Nb 1 / 2 ) O 3 – 0.7 Pb ( Mg 1 / 2 W 1 / 2 ) O 3 solid solution is a magnetic and electric relaxor which shows broad and frequency dependent maxima in both the electric and magnetic susceptibilities. In spite of its multiferroic properties, i.e., irrespective of magnetic and electric ordering, the magnetoelectric coupling in this system is negligibly small.
Mean field theory with only a few transverse Fourier components of EM fields for low-frequency two-dimensional photonic bands
Authors: Chang L. , Lee K.-C. , Wu G. Y.
Published: September 2008

We propose a mean field band structure theory for low-lying two-dimensional photonic states based on the principle of plane wave expansion. Our theory reduces the complexities of a two-dimensional problem into that of an effective one-dimensional crystal, which provides two key advantages: (a) simplification of dimensions assists in the ease of calculation and (b) delineation of the photonic state physics leads to a gain in its physical insights. Our method distinguishes itself from previous known mean field theories in its capability to include more than one Fourier component of EM fields decomposed along the direction perpendicular to propagation. Furthermore, the method applies to virtually any crystal structure and direction of propagation, and it was discovered to function well for both E-polarization and H-polarization modes of states. We also attempt to demonstrate a systematic improvement of the calculation with the increasing number of Fourier components. Satisfactory numerical accuracy is obtained particularly for the states of the two lowest bands.
Uniform formation of high-density InAs quantum dots by InGaAs capping growth
Authors: Tonomura Shinichi , Yamaguchi Koichi
Published: September 2008

We studied the capping growth process of an InGaAs on high-density InAs/GaAs(001) quantum dots (QDs) with a bimodal size distribution grown by molecular beam epitaxy. Atomic force microscopy and reflection high energy electron beam diffraction observations revealed that the bimodal size distribution is composed of larger QDs with {110} facets and smaller QDs with {136} facets. In addition, it was found that, during the InGaAs capping growth, the height of larger QDs decreased, while the height of smaller QDs increased due to the incorporation of indium adatoms. As a result, the size fluctuation of the QDs was suppressed as compared to GaAs capping growth. We achieved a narrow photoluminescence (PL) linewidth of 24 meV and an enhanced PL peak intensity.
Influence of the band structure of BiSb alloy on the magneto-Seebeck coefficient
Authors: Teramoto Takayuki , Komine Takashi , Yamamoto Shinji , Kuraishi Masahiro , Sugita Ryuji , Hasegawa Yasuhiro , Nakamura Hiroaki
Published: September 2008

The influence of the band structure of BiSb alloys on the magneto-Seebeck coefficient was estimated by the Boltzmann equation using an energy-dependent relaxation time approximation at 100 K. The magneto-Seebeck coefficient along the trigonal direction was calculated under a magnetic field along the bisectrix direction. The scattering factor was supposed as being an acoustic deformation potential scattering. In order to estimate the influence of band structure on the magneto-Seebeck coefficient, both two band and three band models were utilized. The two band model consists of an electron surface at the L-point and a hole surface at the T-point. The influence of the change in band overlap and the effective electron mass was investigated using the two band model. The three band model consists of an electron surface at the L-point and a hole surface at the T-point. The influence of change in the Sb concentration of BiSb alloy was also investigated using the three band model. As a result, the Seebeck coefficient was observed to increase as the band overlap decreased. The band overlap has little influence on the improvement of the magneto-Seebeck coefficient. The Seebeck coefficient decreases as the effective mass decreases; however, improvement in the magneto-Seebeck coefficient increases as the effective mass decreases. The magneto-Seebeck coefficient of Bi95Sb5 was effectively improved under a low magnetic field.
Temperature dependence of localized exciton transitions in AlGaN ternary alloy epitaxial layers
Authors: Murotani Hideaki , Yamada Yoichi , Taguchi Tsunemasa , Ishibashi Akihiko , Kawaguchi Yasutoshi , Yokogawa Toshiya
Published: September 2008

The optical properties of Ga-rich AlxGa1−xN (x=0.019, 0.038, 0.057, 0.077, and 0.092) ternary alloy epitaxial layers have been studied by means of temperature-dependent photoluminescence (PL) and time-resolved PL spectroscopy. The luminescence intensity of excitons in five epitaxial layers indicated a thermal quenching process with two activation energies. The two quenching activation energies were attributed to the delocalization of excitons and thermal dissociation of excitons. Anomalous temperature dependence of the PL peak energy was also observed in the epitaxial layers, which enabled the evaluation of the localization energy of the excitons. The localization energy increased as the 1.7th power of the PL linewidth, which reflected a broadening of the density of localized exciton states. In addition, the luminescence decay of the localized excitons for the five epitaxial layers became longer with decreasing emission energy. These observations suggest that the decay of excitons is caused not only by radiative recombination, but also by transfer to lower energy states.
Ultrafast melting and resolidification of gold particle irradiated by pico- to femtosecond lasers
Authors: Zhang Yuwen , Chen J. K.
Published: September 2008

Ultrafast melting and resolidification of a submicron gold particle subject to pico- to femtosecond laser pulse are studied in this paper. The nonequilibrium heat transfer in the electrons and lattice is described using a two-temperature model, and the locations of the solid-liquid interface are determined using an interfacial tracking method. The interfacial velocity, as well as elevated melting temperature and depressed solidification temperature, is obtained by considering the interfacial energy balance and nucleation dynamics. The results showed that the maximum melting depth, peak interfacial temperature, and velocity increase with the decreasing particle size and pulse width or with the increasing laser fluence.
Dependence of mobility on density of gap states in organics by GAMEaS-gate modulated activation energy spectroscopy
Authors: So Woo-young , Lang David V. , Butko Vladimir Y. , Chi Xiaoliu , Lashley Jason C. , Ramirez Arthur P.
Published: September 2008

We develop a broadly applicable transport-based technique, gate modulated activation energy spectroscopy (GAMEaS), for determining the density of states (DOS) in an energy gap. GAMEaS is applied to field-effect transistors (FETs) made from different single crystal oligomer semiconductors. We find that there are two distinct types of band tails, deep and shallow, depending on the crystallization process. The exponential band tails of the localized DOS are characterized by their slope with the highest mobility FETs having a value of 29 eV−1 close to 1/kBT at 300 K.
Laser generation of regular wavy patterns by nonlinear instability of a metal nanolayer
Authors: Lugomer S. , Geretovszky Zs. , Szörényi T.
Published: September 2008

We report on an experimental study on the formation of structures caused by surface-tension-driven instability of a molten chromium layer generated by a single KrF laser pulse of nanosecond duration. The molten metal layer is formed in a confined configuration in a 500 nm thick Cr film encapsulated between a glass substrate and a quartz cover plate. The large lateral thermal gradient ∇Ty generates a surface tension gradient, which causes the formation of instabilities on a liquid Cr film with a very high aspect ratio, Γ=13 000, and of a low Prandtl number of Pr∼0.01. Two types of patterns were observed, the low and high wave-number wavylike instabilities, inclined left and right with respect to the direction of ∇Ty, respectively. As the laser energy increases from 38 to 100 mJ, the inclination angle of the waves alternates, indicating transitions between the right and left inclined structures, and vice versa. The analysis shows that the formation of inclined wavy structures occurs because the ratio W of the Rayleigh number Ra and the Marangoni number Ma (W=Ra/Ma) takes values W>1. The oscillation of inclined waves is governed by the Biot number, which decreases from the intermediate values of Bi∼49 to the low values Bi∼3–4, as the energy of the laser pulse is increased.
Thermal annealing effects of dangling bonds in hydrogenated polymorphous silicon
Authors: Takeda K. , Morigaki K. , Hikita H. , Roca i Cabarrocas P.
Published: September 2008

We investigate the effect of thermal annealing on the density of thermally induced native dangling bonds existing in the amorphous network and created during sample preparation in hydrogenated polymorphous silicon (pm-Si:H) by means of electron spin resonance (ESR). The decay curve of the ESR signal intensity associated with dangling bonds is fitted by a stretched exponential function. The dispersion parameter β and the characteristic time τ are deduced and compared to those for hydrogenated amorphous silicon (a-Si:H). The results are discussed in terms of rapid hydrogen motion associated with the peculiar structure of pm-Si:H.
Acoustic phonon transport and ballistic thermal conductance through a three-dimensional double-bend quantum structure
Authors: Xie Fang , Chen Ke-Qiu , Wang Y. G. , Wan Qing , Zou B. S. , Zhang Yan
Published: September 2008

Based on scalar model of elasticity, we investigate the acoustic phonon transport and ballistic thermal conductance in a three-dimensional double-bend quantum structure at low temperatures using scattering matrix method. The results show that the quantum steps of the acoustic phonon transport probability is observed only with an assumption that the quantum wire is perfect, and the characteristics of quantum steps are different from those of two-dimensional systems, also different from those of electronic transport in quantum wire. The results also show that the thermal conductance depends on geometric parameters sensitively and displays different behaviors from those presented in two-dimensional systems.
Approximate similarity reduction for singularly perturbed Boussinesq equation via symmetry perturbation and direct method
Authors: Jiao Xiaoyu , Yao Ruoxia , Lou S. Y.
Published: September 2008

We investigate the singularly perturbed Boussinesq equation in terms of the approximate symmetry perturbation method and the approximate direct method. The similarity reduction solutions and similarity reduction equations of different orders display formal coincidence for both methods. Series reduction solutions are consequently derived. For the approximate symmetry perturbation method, similarity reduction equations of the zero order are equivalent to the Painlevé IV, Painlevé I, and Weierstrass elliptic equations. For the approximate direct method, similarity reduction equations of the zero order are equivalent to the Painlevé IV, Painlevé II, Painlevé I, or Weierstrass elliptic equations. The approximate direct method yields more general approximate similarity reductions than the approximate symmetry perturbation method.
Experimental setup for characterization of self-actuated microcantilevers with piezoresistive readout for chemical recognition of volatile substances
Authors: Filenko Denis , Ivanov Tzvetan , Volland Burkhard E. , Ivanova Katerina , Rangelow Ivo W. , Nikolov Nikolay , Gotszalk Teodor , Mielczarski Jerzy
Published: September 2008

This paper summarizes our achievements in the development of an advanced microcantilever-based platform for the detection and recognition of various volatile analytes. The implemented microcantilevers include integrated piezoresistive readout, integrated thermally driven bimorph actuator, and a gold pad at the cantilever apex for functionalization toward the detection of specific substances. Up to eight single microcantilevers can be installed and investigated quasisimultaneously in either gas flow or gas/vapor single injection mode. The experimental setup enables the detection of the microcantilever bending via surface stress changes, characterization of either amplitude or phase spectra of the microcantilever, and also calibration of its sensitivity.
Quantifying the linear stability of a flowing electrified two-fluid layer in a channel for fast electric times for normal and parallel electric fields
Authors: Uguz A. Kerem , Aubry N.
Published: September 2008

Motivated by the destabilization of a two-fluid layer flowing in a microchannel for efficient mixing or droplet formation, we study quantitatively the linear stability of the interface between two liquids subjected to an electric field parallel or normal to the flat interface. In the case of fast electric charge relaxation times, the equations for the perturbation can be significantly reduced [A. K. Uguz, O. Ozen, and N. Aubry, Phys. Fluids 20, 031702 (2008)]. Using a simple argument and without solving the equations, Uguz et al. determined the range of parameters over which the electric field is destabilizing, which is narrower for the parallel compared to the normal electric field. However, the argument of Uguz et al. was not amenable to the calculation of growth rates and neutral stability curves. In this paper, by solving the equations, we not only confirm the previous findings but also determine the quantitative linear stability properties, namely, the growth rates and neutral stability curves. Depending on the value of the physical parameters and when both the normal and parallel electric fields lead to instability, it is found that for the same electric potential gradient either the normal or the parallel electric field leads to the largest maximum growth rate. This result should be of interest for experimental purposes.
Intersubband transitions in InGaAsN/GaAs quantum wells
Authors: Liu W. , Zhang D. H. , Fan W. J. , Hou X. Y. , Jiang Z. M.
Published: September 2008

The dependences of intersubband transitions on well width and nitrogen (N) content in n-type In0.23Ga0.77As1−xNx/GaAs quantum wells (QWs) are investigated using a ten-band k⋅p model. The absorption peak energy is found to increase first with the well width starting from 2 nm. It becomes insensitive from about 2.5 to 4.5 nm although the absorption intensity increases and bandwidth decreases monotonically, and then keeps decreasing with the well width beyond 4.5 nm. The peak energy is much larger than that of the N-free structure for narrower wells, but the difference decreases quickly with increasing well width. In the case of wider wells, the absorption peak energy shows relatively slow monotonic increase with increasing N content up to 3% because of the N-band and conduction-band coupling. In the nearly lattice-matched GaAsN/AlGaAs QWs the absorption peak energy shows a redshift with increasing N content from 0% to 0.4% and then increases gradually. The theoretical results are consistent with the reported experimental data.
Model for current drive stabilization of neoclassical tearing modes
Authors: Woodby Jennifer , Schuster Eugenio , Bateman Glenn , Kritz Arnold H.
Published: September 2008

A model derivation is presented for the effect of current drive on the saturated width of magnetic islands driven by the neoclassical tearing mode instability in axisymmetric plasmas. The derivation is carried out for continuous current drive as well as for modulated current that is driven at the same angle as the island O-point. The results of the derivation are implemented in a revision of the ISLAND module to compute saturated magnetic island widths. It is found that the greatest stabilizing effect of both modulated and continuous current drive on the island width is achieved when current is driven at the island center. In addition, narrow current drive is more effective at stabilizing the magnetic islands than wide current drive, for which more current falls outside the island. When modulated and continuous current drives are compared for equal total driven current, the modulated current is shown to be more effective, particularly as the offset from the island center increases.
Influence of cis double-bond parametrization on lipid membrane properties: How seemingly insignificant details in force-field change even qualitative trends
Authors: Martinez-Seara Hector , Róg Tomasz , Karttunen Mikko , Reigada Ramon , Vattulainen Ilpo
Published: September 2008

We have employed atomistic molecular dynamics simulations to investigate the effect of double-bond parametrization on lipid membrane properties. As models, we use one-component membranes composed of glycerol-based phosphatidylcholines (PCs) with monounsaturated acyl chains, and we complement these studies by additional PC/cholesterol simulations. We compare differences between double-bond parametrizations by varying the position of the double bond systematically along the lipid hydrocarbon chains. The results give rise for concern: They indicate that the double-bond description may change not only the quantitative but also the qualitative nature of membrane behavior. In particular, we find that the double-bond description which accounts for skew states in the vicinity of a double bond predicts a maximum in membrane disorder, when the double bond resides at the middle of an acyl chain, in agreement with experiments. The more commonly used description which does not accommodate skew states, however, predicts membrane disorder to decrease monotonically as the double bond is shifted from the glycerol backbone to the end of an acyl chain. The results highlight the importance of properly describing double bonds especially in many-component membranes, where the interplay of different molecule types is difficult to predict on intuitive grounds.
Comparison of Lagrangian approach and method of moments for reducing dimensionality of soliton dynamical systems
Authors: Ankiewicz Adrian , Akhmediev Nail
Published: September 2008

For equations that cannot be solved exactly, the trial function approach to modelling soliton solutions represents a useful approximate technique. It has to be supplemented with the Lagrangian technique or the method of moments to obtain a finite dimensional dynamical system which can be analyzed more easily than the original partial differential equation. We compare these two approaches. Using the cubic-quintic complex Ginzburg–Landau equation as an example, we show that, for a wide class of plausible trial functions, the same system of equations will be obtained. We also explain where the two methods differ.
Measurement of the gamma-ray sensitivity and signal-to-noise ratio of a new scattered-electron detector
Authors: Xia Liangbin , Ouyang Xiaoping , Wang Qunshu , Kang Kejun , Tan Xinjian
Published: September 2008

A new detector suitable for measuring high intensity pulsed gamma-ray sources and based upon scattered-electron method is proposed. The detector has a relatively flat energy response in the range of 0.4–5 MeV and works in current mode. The performances of the detector under several conditions were studied by Monte Carlo simulation using the MCNP code. A comparison between calculations and measurements performed using the 1.25 MeV line of Co-60 is also addressed. The experimental signal produced by the detector was thus studied and decomposed into its main components in order to establish the signal-to-noise ratio (SNR). The latter is compared to SNR calculated for other type of detectors.
An extended two-dimensional Toda lattice hierarchy and two-dimensional Toda lattice with self-consistent sources
Authors: Liu Xiaojun , Zeng Yunbo , Lin Runliang
Published: September 2008

We extend the two-dimensional Toda lattice hierarchy (2DTLH) by its squared eigenfunction symmetries. This extended 2DTLH (ex2DTLH) includes the two-dimensional Toda lattice equation with self-consistent sources (2DTLSCS) as its first nontrivial equation. Lax representation of ex2DTLH is also presented. With the help of the Lax representation, we construct a nonauto-Bäcklund Darboux transformation (DT) for 2DTLSCS by applying the variation of constants to 2DTLSCS auto-Bäcklund DT. This DT enables us to find many solutions to 2DTLSCS, including solitons, rational solutions, positons, negatons, and complexitons.
The recurrence spectra of Rydberg HeH+ molecule ion in a magnetic field
Authors: Wang Dehua , Xu Qiang , Yang Chuanlu , Wang Meishan , Ma Xiaoguang
Published: September 2008

Making use of the molecular closed-orbit theory and a new model potential for the Rydberg molecule, we calculated the recurrence spectra of HeH+ molecule ion in a magnetic field for different quantum defects. The Fourier transformed spectra of HeH+ molecule ion have allowed direct comparison between peaks in the spectra and the scaled action values of closed orbits of the excited electron in the external fields. We find that the spectral modulations can be analyzed in terms of the scattering of the excited electron on the molecular core. Unlike the case of the Rydberg atom where the elastic scattering is predominant, modulations produced by inelastic scattering are also vital to the photoabsorption spectrum of Rydberg molecule. Our results are in good agreement with the quantum results, which suggest that our method is correct.
Relationship between symmetry and scaling of spin torque thermal switching barrier
Authors: Wang Xiaobin , Zhu Wenzhong , Xi Haiwen , Dimitrov Dimitar
Published: September 2008

This letter shows that the scaling relationship between the thermal reversal barrier and the (spin polarized) current density in the limit of low current density strongly depends on magnetization dynamics symmetry. This is quite different from magnetic field induced thermal switching. Our analysis demonstrates that the widely used scaling relationship between switching time and current density needs to be modified to include magnetization dynamics symmetry effects. A formula for a thermal reversal barrier symmetry correction factor is given in the limit of low current density. The implications for the estimation of magnetic element thermal stability are discussed.
Electron propagator calculations on C60 and C70 photoelectron spectra
Authors: Zakrzewski V. G. , Dolgounitcheva O. , Ortiz J. V.
Published: September 2008

Vertical ionization energies of C60 and C70 fullerenes are calculated with semidirect implementations of electron propagator methods and a triple-ζ plus polarization basis set. These predictions are in close agreement with photoelectron spectra for final states in which the Koopmans description is qualitatively valid. Many correlation states, where the latter description fails, are predicted by methods with nondiagonal self-energies.
Magnetization switching behavior with microwave assistance
Authors: Okamoto S. , Kikuchi N. , Kitakami O.
Published: September 2008

Microwave assisted switching of magnetization has become one of the key techniques for future advanced recording. We have studied the magnetization switching behaviors excited by circularly and linearly polarized microwaves based on the Landau–Lifshitz–Gilbert equation. The precessional motion of magnetization is excited by the microwave, resulting in significant reduction of the irreversible reversal field. The reversal takes place through very complicated precessional motion, which proceeds via nearly steady precession at an early stage, subsequent unstable motion, and finally abrupt irreversible switching. We discuss the detailed effect of microwave polarization on magnetization reversals.
Absolute intensity calibration of the Wendelstein 7-X high efficiency extreme ultraviolet overview spectrometer system
Authors: Greiche Albert , Biel Wolfgang , Marchuk Oleksandr , Burhenn Rainer
Published: September 2008

The new high effiency extreme ultraviolet overview spectrometer (HEXOS) system for the stellarator Wendelstein 7-X is now mounted for testing and adjustment at the tokamak experiment for technology oriented research (TEXTOR). One part of the testing phase was the intensity calibration of the two double spectrometers which in total cover a spectral range from 2.5 to 160.0 nm with overlap. This work presents the current intensity calibration curves for HEXOS and describes the method of calibration. The calibration was implemented with calibrated lines of a hollow cathode light source and the branching ratio technique. The hollow cathode light source provides calibrated lines from 16 up to 147 nm. We could extend the calibrated region in the spectrometers down to 2.8 nm by using the branching line pairs emitted by an uncalibrated pinch extreme ultraviolet light source as well as emission lines from boron and carbon in TEXTOR plasmas. In total HEXOS is calibrated from 2.8 up to 147 nm, which covers most of the observable wavelength region. The approximate density of carbon in the range of the minor radius from 18 to 35 cm in a TEXTOR plasma determined by simulating calibrated vacuum ultraviolet emission lines with a transport code was 5.5×1017 m−3 which corresponds to a local carbon concentration of 2%.
An accurate analytical approximation to the capacitance transient amplitude in deep level transient spectroscopy for fitting carrier capture data
Authors: Lauwaert J. , Van Gheluwe J. , Clauws P.
Published: September 2008

An accurate analytical approximation to the capacitance transient amplitude measured by deep level transient spectroscopy as a function of the filling pulse duration is derived. The equation includes the influence of a defect concentration profile and the nonexponential trapping in the Debye tail of the depletion region. Because this equation gives an accurate approximation for all filling pulse lengths it is possible to use it for the fitting of experimental data in capture cross-section measurements. The method is implemented in a fitting routine and is demonstrated for the E1-level in Fe-implanted n-type germanium.
High resolution study of spin-orbit mixing and the singlet-triplet gap in chlorocarbene: Stimulated emission pumping spectroscopy of CH35Cl and CD35Cl
Authors: Tao Chong , Mukarakate Calvin , Terranova Zack , Ebben Carlena , Judge Richard H. , Reid Scott A.
Published: September 2008

We report on high resolution studies of spin-orbit mixing and the singlet-triplet gap in a prototypical halocarbene, CHCl, using stimulated emission pumping (SEP) spectroscopy from the Ã A1″ state. Results are reported for two isotopomers, CH35Cl and CD35Cl. We have obtained rotationally resolved spectra for the majority of X̃ A1′ levels lying between 0 and 6000 cm−1 above the zero-point level that were previously observed under low resolution in single vibronic level emission studies and several new levels that were previously unobserved or unresolved. In addition, SEP spectra were obtained for six ã A3″ levels in CH35Cl and three levels in CD35Cl. The derived term energies and rovibrational parameters of the X̃ A1′ and ã A3″ states are in good agreement with theory. The ã A3″ triplet spin-spin parameter is vibrational state dependent, and dominated by a second-order contribution from spin-orbit coupling with nearby X̃ A1′ levels; it therefore provides a sensitive probe of spin-orbit mixing in this system. An analysis of three pairs of interactions between specific ã A3″ and X̃ A1′ levels in CH35Cl affords a pure electronic spin-orbit coupling element of 150 cm−1, in good agreement with theoretical expectations. The derived singlet-triplet gaps, which are the most precise determined to date for any carbene, are compared with the predictions of high level ab initio theory.
Parameter study of acceleration of externally injected electrons in the linear laser wakefield regime
Authors: van Dijk W. , van der Geer S. B. , van der Wiel M. J. , Brussaard G. J. H.
Published: September 2008

A parameter study for laser wakefield acceleration is presented, in which externally injected electrons are accelerated in low amplitude plasma waves, represented by an analytical two-dimensional description. Results have been obtained for plasma densities up to 2.6×1024m−3, plasma lengths up to 300mm, laser intensities up to 3.5×1021W∕m2, and injection of Gaussian model bunches at energies up to 12MeV. For the range of parameters studied, effects of laser depletion and the influence of the electron bunch on the plasma can be ignored. In the parameter space, a region is identified where final energies of over 100MeV are reached, at an energy spread of less than 5% and a rms emittance of a few micrometers.
Gyrokinetic particle simulations of toroidal momentum transport
Authors: Holod I. , Lin Z.
Published: September 2008

Simulations of toroidal angular momentum transport have been carried out using global toroidal gyrokinetic particle-in-cell code. The significant redistribution of toroidal momentum is observed, driven by the ion temperature gradient turbulence with adiabatic electrons, resulting in a peaked momentum profile in the central region of the radial domain. Cases with rigid and sheared plasma rotation are considered. Diffusive and off-diagonal (pinchlike) fluxes are identified. Toroidal momentum diffusivity is calculated by subtracting pinch contribution from the total momentum flux, and compared to quasilinear estimates. It is found that the ratio of momentum to heat conductivity is smaller than unity even after subtracting pinch contribution when wave-particle resonance energy is larger than thermal energy.
Photoresponse of the Schottky junction Au/SrTiO3:Nb in different resistive states
Authors: Shang D. S. , Sun J. R. , Shi L. , Shen B. G.
Published: September 2008

A systematic study on photovoltaic effects has been performed for the Schottky junction Au/SrTiO3:0.05 wt %Nb, the resistance of which can be tuned, by applied electric pulses, between ∼1 and ∼200 MΩ. It is found that, despite the great change in junction resistance, the photocurrent across the junction is constant when the power and wavelength of incident light are fixed. The corresponding Schottky barrier, deduced from the photoresponse data is ∼1.5 eV, independent of junction resistance. This result suggests the invariance of the interfacial barrier during resistance switching and the occurrence of filamentary conduction channels.
Improved contact for thermal and electrical transport in carbon nanofiber interconnects
Authors: Saito Tsutomu , Yamada Toshishige , Fabris Drazen , Kitsuki Hirohiko , Wilhite Patrick , Suzuki Makoto , Yang Cary Y.
Published: September 2008

We study the performance and reliability of carbon nanofiber (CNF) interconnects under high-current stress by examining CNF breakdown for four test configurations, suspended/supported with/without tungsten deposition. The use of W is to improve the CNF-electrode contact. The supported cases show a larger current density just before breakdown than the suspended ones, suggesting an effective heat dissipation to the substrate. The W-deposited contacts reduce the initial total resistance from megaohm range without W to kilo-ohms. High-current stress does not change the total resistance of the test structures with W unlike those without W deposition.
Performance improvement of phase-change memory cell with cup-shaped bottom electrode contact
Authors: Wu L. C. , Song Z. T. , Rao F. , Gong Y. F. , Liu B. , Wang L. Y. , Liu W. L. , Feng S. L.
Published: September 2008

Cup-shaped tungsten (W) bottom electrode contact (BEC) was proposed for phase-change memory (PCM) cell and compared with the same size solid BEC. The reset voltage of 2.5V and the shortest reset time of 10ns were obtained for cup-shaped W-BEC based PCM cell, which is much better than those of the solid BEC based PCM cell. The improved performance of the cup-shaped W-BEC based PCM cell is attributed to the mushroom heating region with high heating efficiency, the inner edge contact of W-BEC and Ge2Sb2Te5, and the reduced thermal diffusion from the bottom electrode, which is further verified by theoretical simulation.
Combinatorial investigation of magnetostriction in Fe–Ga and Fe–Ga–Al
Authors: Hattrick-Simpers Jason R. , Hunter Dwight , Craciunescu Corneliu M. , Jang Kyu Sung , Murakami Makoto , Cullen James , Wuttig Manfred , Takeuchi Ichiro , Lofland Samuel E. , Benderksy Leonid , Woo Noble , Van Dover Robert Bruce , Takahashi Toshiya , Furuya Yasubumi
Published: September 2008

A high-throughput high-sensitivity optical technique for measuring magnetostriction of thin-film composition-spread samples has been developed. It determines the magnetostriction by measuring the induced deflection of micromachined cantilever unimorph samples. Magnetostriction measurements have been performed on as-deposited Fe–Ga and Fe–Ga–Al thin-film composition spreads. The thin-film Fe–Ga spreads display a similar compositional variation of magnetostriction as bulk. A previously undiscovered peak in magnetostriction at low Ga content was also observed and attributed to a maximum in the magnetocrystalline anisotropy. Magnetostrictive mapping of the Fe–Ga–Al ternary system reveals the possibility of substituting up to 8at.% Al in Fe70Ga30 without significant degradation of magnetostriction.
Enhancement in output power of blue gallium nitride-based light-emitting diodes with omnidirectional metal reflector under electrode pads
Authors: Sheu J. K. , Hung I-Hsiu , Lai W. C. , Shei S. C. , Lee M. L.
Published: September 2008

In this study, we demonstrate a GaN-based light-emitting diode (LED) with nonalloyed metal contacts onto the n+-GaN surface and transparent contact layer (indium tin oxide) to serve as the n-type electrode (cathode) and the p-type electrode pad (anode), respectively. Comparing with the conventional LEDs, which the electrode pads and/or Ohmic contacts form through conventional Cr∕Au metal contacts, the nonalloyed metal contacts (Ag∕Cr∕Au or Al∕Cr∕Au) used in the present experimental blue LEDs also play the role of reflector to prevent the emitted light from absorption by the opaque electrode pads with low reflectivity (Cr∕Au). With an injection current of 20mA, the enhancement in the light output power has approximately a 14% magnitude compared to the GaN-based LEDs without Ag or Al reflectors under the Cr∕Au electrode pads.
Elastic softening and central peaks in BaTiO3 single crystals above the cubic-tetragonal phase-transition temperature
Authors: Ko J.-H. , Kojima S. , Koo T.-Y. , Jung J. H. , Won C. J. , Hur N. J.
Published: September 2008

A strong relaxation mode was observed in the paraelectric phase of barium titanate (BaTiO3) single crystals by Brillouin scattering study and was found to correlate with the softening of the longitudinal acoustic mode and the increase in the hypersonic damping. These observations support the existence of polar percursors and their electrostrictive coupling with the strain caused by the acoustic waves, consistent with former studies evidencing off-centered Ti ions in the high-symmetry cubic phase. A critical slowing down has been clearly observed in the vicinity of the cubic-tetragonal phase transition, indicating order-disorder component contributes to the phase transition of BaTiO3.
Detaching speed of a fibrillar interface
Authors: Chen B. , Wu P. D. , Huang Y.
Published: September 2008

Considering elastic energy stored in the fibrillar structure recoverable, we provide different initial and dynamic fracture toughnesses for a fibrillar interface to reconcile the contradiction existing in literature. A theory on the speed of a semi-infinite crack propagating along a fibrillar interface is then developed, which is used to estimate the speed of gecko toe detachment. The predicted time interval for gecko toe detachment is less than a few milliseconds.
High figures of merit and natural nanostructures in Mg2Si0.4Sn0.6 based thermoelectric materials
Authors: Zhang Q. , He J. , Zhu T. J. , Zhang S. N. , Zhao X. B. , Tritt T. M.
Published: September 2008

Mg 2 ( Si , Sn ) compounds have shown great promise for thermoelectric applications due to good thermoelectric properties, nontoxicity, and abundantly available constituent elements. Herein we report on the thermoelectric properties and microstructure of high performance Mg2Si0.4−xSn0.6Sbx (0⩽x⩽0.015) alloys. The state-of-the-art ZT value of ∼1.1 has been attained in the samples with x=0.0075 due to the relatively low thermal conductivity. In light of the simple cubic structure and mostly light constituent elements, the reduction in lattice thermal conductivity has been discussed in connection with a fairly large amount of in situ formed nanostructures in these samples.
Germanium substrate loss during low temperature annealing and its influence on ion-implanted phosphorous dose loss
Authors: Ioannou N. , Skarlatos D. , Tsamis C. , Krontiras C. A. , Georga S. N. , Christofi A. , McPhail D. S.
Published: September 2008

In this work we demonstrate germanium loss from uncapped substrates during low temperature drive-in annealing in inert ambient. An Arrhenius law with an activation energy of 2.03 eV describes the measured loss rate of germanium as a function of temperature. Accurate simulations of implanted phosphorous profiles in nonpassivated substrates have been performed considering the extracted germanium loss rate. A capping layer on the germanium surface reduces phosphorous dose loss, with Si3N4 being more efficient than SiO2. The capping layer material also affects the extent of dopant concentration-dependent diffusion for high dose phosphorous implantation.
The short-sequence designs of isochores from the human genome
Authors: Costantini Maria , Bernardi Giorgio
Published: September 2008

The human genome, a typical mammalian genome, is made up of long (≈1-Mb, on average) regions, the isochores, that are fairly homogeneous in base composition and belong in five families characterized by different GC levels. An analysis of di- and tri-nucleotide densities in the isochores from the five families has shown large differences. These different “short-sequence designs:” (
i
) account for the fractionation of human DNA (and vertebrate DNA in general) when using sequence-specific ligands in density gradients, (
ii
) are very similar in whole isochores and in the corresponding intergenic sequences and introns, (
iii
) are reflected in different codon usages, (
iv
) lead to amino acid differences that increase the thermal stability of the proteins encoded by genes located in increasingly GC-rich isochore families, and (
v
) correspond to different chromatin structures.
Benefits of using multiple first-line therapies against malaria
Authors: Boni Maciej F. , Smith David L. , Laxminarayan Ramanan
Published: September 2008

Despite the availability of many drugs and therapies to treat malaria, many countries' national policies recommend using a single first-line therapy for most clinical malaria cases. To assess whether this is the best strategy for the population as a whole, we designed an evolutionary-epidemiological modeling framework for malaria and compared the benefits of different treatment strategies in the context of resistance evolution. Our results show that the population-wide use of multiple first-line therapies (MFT) against malaria yields a better clinical outcome than using a single therapy or a cycling strategy where therapies are rotated, either on a fixed cycling schedule or when resistance levels or treatment failure become too high. MFT strategies also delay the emergence and slow the fixation of resistant strains (phenotypes), and they allow a larger fraction of the population to be treated without trading off future treatment of cases that may be untreatable because of high resistance levels. Earlier papers have noted that cycling strategies have the disadvantage of creating a less temporally variable environment than MFT strategies, making resistance evolution easier for the parasite. Here, we illustrate a second feature of parasite ecology that impairs the performance of cycling policies, namely, that cycling policies degrade the mean fitness of the parasite population more quickly than MFT policies, making it easier for new resistant types to invade and spread. The clinical benefits of using multiple first-line therapies against malaria suggest that MFT policies should play a key role in malaria elimination and control programs.