bluuebunny/crossref_metadata_2025 huggingface dataset:
Development of a FIR interferometer for ATF: Part II—System design
Authors: Casson W. H. , Hutchinson D. P.
Published: March 2008

Because of the highly elliptical cross section of the Advanced Toroidal Facility (ATF) plasma, a large number of interferometer channels are required to determine the radial density profile by Abel inversion. A 15-channel system is being developed using a single fan-shaped beam through the plasma and observing the phase shift across the profile with a 15-element detector array. The laser output at 119 μ will be transmitted about 10 m from the laser to the plasma device via an oversized dielectric waveguide. A surface grating will be used to redistribute the Gaussian power distribution to a sinc-type distribution. The beam will then be expanded in one dimension and focused through the plasma. It will then be combined with the local oscillator and focused on the detector array. Commercial Schottky diodes mounted in corner cube reflectors will be used as mixers. The entire system will be controlled by computer including the lasers and the data-acquisition system. Research supported by the Office of Fusion Energy, U.S. DOE, under Contract No. DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc.
Atomic Transition Probabilities of Sodium and Magnesium. A Critical Compilation
Authors: Kelleher D. E. , Podobedova L. I.
Published: March 2008

This compilation is the first in a series of updates to a critical compilation published in 1969 [W. L. Wiese, M. W. Smith, and B. M. Miles, Atomic Transition Probabilities, Vol. II: Sodium through Calcium, NSRDS-NBS Vol. 2 (U.S. GPO, Washington, D.C., 1969)]. Atomic transition probabilities have been critically evaluated and compiled for about 11 400 spectral lines of sodium and magnesium (nuclear charge Z=11–12, respectively). The cited values and their estimated uncertainties are based on our consideration of all available theoretical and experimental literature sources. All ionization stages (except for hydrogenic) are covered, and the data are presented in separate tables for each atom and ion. Separate listings are given for “allowed” (electric dipole) transitions, on the one hand, and for “forbidden” (magnetic dipole plus electric and magnetic quadrupole) transitions, on the other. In each spectrum, lines are grouped into multiplets which are arranged in order of ascending lower and upper-level energies, respectively. For each line, the emission transition probability Aki, the line strength S, and (for allowed lines) the absorption oscillator strength fik are given, together with the spectroscopic designation, the wavelength, the statistical weights, and the energy levels of the lower and upper states. The estimated relative uncertainties of the line strength are also indicated, as are the source citations. We introduce a statistical method that we use to estimate these uncertainties for most of the cited transition rates. We only include those lines whose transition rates are deemed sufficiently accurate to qualify as reference values. Short introductions precede the tables for each ion. The general introduction contains a discussion of the principal criteria for our judgments and our method of data selection and evaluation.
Improved Critical Compilations of Selected Atomic Transition Probabilities for Neutral and Singly Ionized Carbon and Nitrogen
Authors: Wiese W. L. , Fuhr J. R.
Published: March 2008

We have undertaken new critical assessments and tabulations of the transition probabilities for important lines of neutral and singly ionized carbon and nitrogen. Our updates primarily address the persistent lower transitions as well as a greatly expanded number of forbidden lines (M1, M2, and E2 lines). For these transitions, sophisticated multiconfiguration Hartree–Fock calculations have been recently carried out, which have yielded data considerably improved and often appreciably different from our 1996 NIST compilation.
IUPAC-NIST Solubility Data Series. 84. Solubility of Inorganic Actinide Compounds
Authors: Miyamoto H.
Published: March 2008

This volume presents the solubility of inorganic compounds of actinides except for carbonates, which are included in Volume 74 of this series, and nitrates, which are covered in Volume 55. Also included are solubility data of compounds such as organosulfates, phosphates, and arsenates, which are not covered in Volume 74. The predominant part of this volume covers solubility data of thorium, uranium, neptunium, and plutonium compounds. Fewer data have been published for americium compounds and very few for compounds of actinium, protactinium, and transamericium elements. The literature has been covered up to the end of 2004. Documents which remained unavailable to the editor, and could not be included in the volume are listed in the Appendix. For some compounds it was not possible to show the Chemical Abstracts registry numbers since these have not been assigned.
Microwave Spectra of Molecules of Astrophysical Interest. XXVI. Acetic Acid (CH3COOH)
Authors: Ilyushin V. , Kleiner I. , Lovas F. J.
Published: March 2008

The microwave spectrum of acetic acid is critically reviewed and supplemented with spectral frequency calculations derived from the rotation-torsion analysis. A simultaneous analysis of the torsional ground state, vt=0, and first and second torsionally excited states, vt=1 and 2, was carried out using the so-called“rho axis method.” The primary objective of this review is to provide radio astronomers with complete spectral coverage over the 1–400 GHz range for the ground and vt=1 states, covering rotational quantum numbers J⩽30 and |Ka|⩽15.
Lineshape of rotational spectrum of CO in He4 droplets
Authors: Zillich Robert E. , Whaley K. Birgitta , von Haeften Klaus
Published: March 2008

In a recent experiment the rovibrational spectrum of CO isotopomers in superfluid helium-4 droplets was measured, and a Lorentzian lineshape with a large line width of 0.024K (half width at half maximum) was observed [von Haeften et al., Phys. Rev. B 73, 054502 (2006)]. In the accompanying theoretical analysis it was concluded that the broadening mechanism may be homogeneous and due to coupling to collective droplet excitations (phonons). Here we generalize the lineshape analysis to account for the statistical distribution of droplet sizes present in nozzle expansion experiments. These calculations suggest an alternative explanation for the spectral broadening, namely, that the coupling to phonons can give rise to an inhomogeneous broadening as a result of averaging isolated rotation-phonon resonances over a broad cluster size distribution. This is seen to result in Lorentzian lineshapes, with a width and peak position that depend weakly on the size distribution, showing oscillatory behavior for the narrower size distributions. These oscillations decrease with droplet size and for large enough droplets (∼104) the line widths saturate at a value equal to the homogeneous line width calculated for the bulk limit.
Magnetic field induced ferroelectricity in Mn0.9Fe0.1WO4
Authors: Chaudhury R. P. , Lorenz B. , Wang Y. Q. , Sun Y. Y. , Chu C. W.
Published: March 2008

We discovered that an external magnetic field does induce a ferroelectric phase in Mn0.9Fe0.1WO4 below T1=12K. The spontaneous polarization appears along the b axis upon applying a magnetic field above 4T. The temperature and magnetic field dependencies of the spontaneous polarization show a strong coupling between the magnetic and ferroelectric orders. We correlate the ferroelectric transitions with dielectric constant and magnetization anomalies. Our results are interpreted in terms of recent theories relating the magnetic order induced ferroelectricity in multiferroic compounds to the inversion symmetry breaking effects of unconventional magnetic structures.
Model of phase transitions in a copper spinel on alloying with antimony
Authors: Kuzmin E. V.
Published: March 2008

A theoretical study of the properties of the copper spinel Cu1+[Cr1+x3+Cr1−2x4+Sbx5+]S42− is carried out in the framework of the spin Hamiltonian. Only the cation sublattice B, containing the ions Cr3+ (S3=3∕2), Cr4+ (S4=1) and the diamagnetic sites Sb5+ (S=0), is considered. Nearest magnetic ions are coupled by effective exchange interactions via the intermediate anions: J34>0 (double exchange), J44>0, J33<0 (superexchange), where J34≫∣J33∣,J44. For a random distribution of ions the general physical behavior pattern of the system as a function of the concentration x of the alloying element is as follows. In the region x≤xcr≈0.3 there exists an infinite ferromagnetic cluster of intercoupled ionsCr3+–Cr4+ (percolation via strong 3–4 ferromagnetic bonds). At x>xcr finite ferromagnetic clusters appear, oriented “up” and “down” and coupled by antiferromagnetic bonds J33. The total magnetization vanishes, and a spin separation or a state of the cluster spin-glass type is realized. At x→0.5 percolation via antiferromagnetic bonds J33 appears, and practically only simple isolated ferromagnet clusters in an antiferromagnetic matrix remain, oriented “up” and “down” in equal numbers. The limit compound (x=0.5) consists 1/4 of “holes”—diamagnetic sites with broken bonds—and 3/4 of Cr3+ cations with antiferromagnetic bonds. Percolation via 3–4 bonds leads not only to ferromagnetism but also simultaneously to a metallic state owing to the double-exchange mechanism (the formation of a ferromagnetic half metal). In the absence of percolation via 3–4 bonds the system is an insulator. Thus as the copper spinel is alloyed with antimony (0≤x≤0.5), concentration phase transitions occur: a metallic ferromagnet (x<xcr) is transformed to an insulator with spin separation (xcr≈0.3), which in the limit x=0.5 becomes an antiferromagnetic insulator.
Theory of magnetic structure in the compound MnCoSi
Authors: Zavorotnev Ju. D. , Medvedeva L. I.
Published: March 2008

Possible mechanisms for the formation of long-period structure in the compound MnCoSi are considered. It is found that the main role in the theoretical treatment is played by invariants containing the first spatial derivative of the irreducible magnetic vectors to different powers. It is shown that in the formation of a spiral structure by only the antiferromagnetic vectors a spontaneous involvement of the ferromagnetic vector can occur, whereupon the orthogonality of the rotating vectors is broken. The analysis is carried out both with and without taking into account the approximate constancy of the moduli of the irreducible vectors. This makes it possible to consider the evolution of magnetic superstructure on changing temperature.
Kinetic equations in the theory of normal Fermi liquid
Authors: Kondratyev A. S. , Siddique I.
Published: March 2008

On the basis of an improved approximation for the spectral function of one-particle states, it is shown that the Landau-Silin kinetic equations for normal Fermi liquids of neutral and electrically charged particles are valid in a finite temperature region above the temperature of the superfluid transition.
Structure of the solid phases of SiH4
Authors: Prokhvatilov A. I. , Galtsov N. N. , Klimenko N. A. , Strzhemechny M. A.
Published: March 2008

X-ray studies of polycrystalline samples of monosilane SiH4 are done in the existence regions of the low-temperature and high-temperature phases with the use of photometric and diffractometric methods. It is found that the unit cells of both phases have monoclinic symmetry. The phase transition from the low-to the high-temperature phase is accompanied by an appreciable volume jump ΔV∕V=1.9% and a change of the number of molecules in the cell from 4 to 32. The unit cell in the low-temperature phase (at T=6K) has parameters a=8.198±0.002Å, b=4.601±0.001Å, c=7.364±0.002Å, β=90.32±0.08°, volume V=41.85cm3∕mole, and density 0.78±0.05g∕cm3. The high-temperature phase is substantially orientationally disordered, containing Z=32molecules per cell and has the parameters a=13.660±0.003Å, b=12.767±0.003Å, c=13.333±0.003Å, β=93.66±0.06°, volume V=43.67cm3∕mole, and density 0.735g∕cm3 at T=65K. The structural and a number of physical characteristics of silane are compared with the corresponding properties of other crystals made up of tetrahedral molecules.
Local vibrations in real crystals with combined defects
Authors: Gospodarev I. A. , Kotlyar A. V. , Manzhelii E. V. , Fedosyev S. B.
Published: March 2008

The conditions of formation and the main characteristics of local vibrations due to the presence of light impurities Al, Mg, and Mn in the bulk and on the close-packed [111] surface of the fcc crystal lattice of silver are investigated. The influence of surface roughness is analyzed. Analytical approximations are obtained which describe the frequency and intensity of the local vibrations at the impurity atom and its nearest neighborhood to high accuracy. The proposed analytical approximations provide a practical way of determining the parameters of the defect structure and the interatomic interactions from the experimentally determined characteristics of the local vibrations.
Simulations of the untying of molecular friction knots between individual polymer strands
Authors: Kirmizialtin Serdal , Makarov Dmitrii E.
Published: March 2008

The dynamics of molecular knots is implicated in a broad range of phenomena, from DNA replication to relaxation of polymer melts. Motivated by the recent experiments, in which biopolymer knots have been observed and manipulated at a single-molecule level, we have used computer simulations to study the dynamics of “friction knots” joining individual polymer strands. A friction knot splicing two ropes becomes jammed when the ropes are pulled apart. In contrast, molecular friction knots eventually become undone by thermal motion. We show that depending on the knot type and on the polymer structure, a microscopic friction knot can be strong (the time τ the knot stays tied increases with the force F applied to separate the strands) or weak (τ decreases with increasing F). The strong knot behavior is a microscopic analog of macroscopic knot jamming. We further describe a simple model explaining these behaviors.
Terahertz vibration-rotation-tunneling spectroscopy of the water tetramer-d8: Combined analysis of vibrational bands at 4.1 and 2.0THz
Authors: Lin Wei , Han Jia-Xiang , Takahashi Lynelle K. , Harker Heather A. , Keutsch Frank N. , Saykally Richard J.
Published: March 2008

We report the measurement of terahertz vibration-rotation-tunneling spectra of (D2O)4 in the spectral region near 4.13THz. The characterization of this perpendicular band extends a previously reported study [J. Chem. Phys. 111, 7801 (1999)]. We observed 239 new transitions, each being split into a doublet of constant (approximately 192MHz) spacing. These are included in a combined fit with the 113 previously measured transitions of the 2.03THz parallel band using an effective Hamiltonian similar to that used in the global fit of the water trimer. The detailed understanding of the water tetramer evolving from this work underlies our efforts to quantify the contribution of many-body forces to the hydrogen bonding interactions in condensed phase water.
Reversible tuning of lattice strain in epitaxial SrTiO3/La0.7Sr0.3MnO3 thin films by converse piezoelectric effect of 0.72Pb(Mg1/3Nb2/3)O3−0.28 PbTiO3 substrate
Authors: Levin A. A. , Pommrich A. I. , Weißbach T. , Meyer D. C. , Bilani-Zeneli O.
Published: March 2008

Thin SrTiO3/La0.7Sr0.3MnO3 (STO/LSMO) films of 30 nm thickness were epitaxially grown on a (001) Pb(Mg1/3Nb2/3)O3−28 mol % PbTiO3 piezoelectric single-crystalline plate and investigated by means of wide-angle x-ray scattering in situ under the influence of a direct current electric field with strength E up to ±18 kV/cm. The strain s in the films could by tuned dynamically due to a dominant converse piezoelectric effect of the substrate. The coefficient of coupling between electric field-induced strain alteration of the substrate and the buffer film (LSMO) is approximately 0.75, whereas for the substrate and top films (STO) it is about 0.35 related to a normalized strain alteration of the substrate of 1.
A kinetic theory analysis on the heat transfer in hard drive air bearing
Authors: Shen Sheng , Chen Gang
Published: March 2008

In this article, a generalized model for viscous heat generation and heat transfer in the air bearing of a hard drive is developed based on the linearized Boltzmann equation. It is found that the heat flux between the slider and the disk can be separated into four parts: heat conduction, viscous dissipation contributed by Couette flow, viscous dissipation from Poiseuille flow, and combined contribution from Couette flow and Poiseuille flow. Universal functions are obtained so that viscous dissipation and heat conduction can be determined accurately. Comparisons with continuum models reveal significant deficiencies existing in current models in use.
The effect of manganese ions in a MnCr1.9857Fe0.02O4 by Mössbauer spectroscopy
Authors: Choi Kang Ryong , Kim Sam Jin , Kim Chul Sung
Published: March 2008

Polycrystalline MnCr1.9857Fe0.02O4 compound was prepared by a wet-chemical process. The crystal structure was found to be a cubic spinel with a space group of Fd3m. The lattice constant a0 and the internal structural parameter x of the oxygen were determined to be 8.444Å and 0.263, respectively. Mössbauer spectra of MnCr1.9857Fe0.02O4 were taken from 4.2to295K. The absorption spectra slow well-developed two sextets, superposed with small difference in hyperfine fields of 482 and 472kOe below the Néel temperature of 50K, and a paramagnetic doublet above the Néel temperature. We have observed a sudden change in sextet around 22K, which corresponds to the spin structure transition temperature. Even though the values of the magnetic hyperfine fields of MnCr1.9857Fe0.02O4 on B sites are close to those of CoCr1.9857Fe0.02O4, there is a noticeable difference in the Néel temperature between them, which is due to the A-B superexchange interaction with spin-orbit coupling attributable to ionic bond lengths.
Scaling of the anomalous Hall effect in low Mn concentration (Ga,Mn)As
Authors: Shen S. , Liu X. , Ge Z. , Furdyna J. K. , Dobrowolska M. , Jaroszynski J.
Published: March 2008

We present magnetotransport in a series of Ga1−xMnxAs (x∼0.014) films. Both ordinary and anomalous Hall resistivities are analyzed in high magnetic field (up to 18.0T) at various temperatures. The unique scaling behavior n∼0.5 is found for the entire series, which is not compatible with existing scattering theories.
Effect of free layer thickness and shape anisotropy on the transfer curves of MgO magnetic tunnel junctions
Authors: Wiśniowski P. , Almeida J. M. , Cardoso S. , Barradas N. P. , Freitas P. P.
Published: March 2008

A study of the combined effect of CoFeB free layer thickness, shape anisotropy, and annealing temperature on the transfer curves of MgO based magnetic tunnel junctions is presented. The response of the free layer changes gradually as its thickness decreases and shows hysteresis-free response for CoFeB⩽15.5Å. For junctions with CoFeB=15.5Å, the onset of hysteresis-free response depends on junction area and annealing temperature. Linearized magnetic tunnel junctions with dimensions down to 1.5×3μm2 and sensitivity as high as 7.7%∕Oe were achieved.
Molecular orbital propagation to accelerate self-consistent-field convergence in an ab initio molecular dynamics simulation
Authors: Atsumi Teruo , Nakai Hiromi
Published: March 2008

Based on the idea of molecular orbital (MO) propagation, we propose a novel effective method for predicting initial guesses for the self-consistent-field calculations in direct ab initio molecular dynamics (AIMD) simulations. This method, called LIMO, adopts the Lagrange interpolation (LI) polynomial technique and predicts initial MO coefficients at the next AIMD step by using several previous results. Taking into account the crossing and/or mixing of MOs leads to orbital invariant formulas for the LIMO method. We also propose a simple method for determining the optimal degree of the LI polynomial, which corresponds to the number of previous steps. Numerical tests confirm that this proposed method is both effective and feasible.
Perpendicular magnetic tunnel junction with tunneling magnetoresistance ratio of 64% using MgO (100) barrier layer prepared at room temperature
Authors: Ohmori Hideto , Hatori Tomoya , Nakagawa Shigeki
Published: March 2008

MgO (100) textured films can be prepared by reactive facing targets sputtering at room temperature without postdeposition annealing process when they were deposited on (100) oriented Fe buffer layers. This method allows fabrication of perpendicular magnetic tunnel junction (p-MTJ) with MgO (100) tunneling barrier layer and rare-earth transition metal (RE-TM) alloy thin films as perpendicularly magnetized free and pinned layers. The 3-nm-thick MgO tunneling barrier layer in p-MTJ multilayer prepared on glass substrate revealed (100) crystalline orientation. Extraordinary Hall effect measurement clarified that the perpendicular magnetic components of 3-nm-thick Fe buffer layers on the two ends of MgO tunneling barrier layer were increased by exchange coupling with RE-TM alloy layers. The RA of 35kΩμm2 and tunneling magnetoresistance ratio of 64% was observed in the multilayered p-MTJ element by current-in-plane-tunneling.
Multipixel system for gigahertz frequency-domain optical imaging of finger joints
Authors: Netz Uwe J. , Beuthan Jürgen , Hielscher Andreas H.
Published: March 2008

Frequency-domain optical imaging systems have shown great promise for characterizing blood oxygenation, hemodynamics, and other physiological parameters in human and animal tissues. However, most of the frequency domain systems presented so far operate with source modulation frequencies below 150MHz. At these low frequencies, their ability to provide accurate data for small tissue geometries such as encountered in imaging of finger joints or rodents is limited. Here, we present a new system that can provide data up to 1GHz using an intensity modulated charged coupled device camera. After data processing, the images show the two-dimensional distribution of amplitude and phase of the light modulation on the finger surface. The system performance was investigated and test measurements on optical tissue phantoms were taken to investigate whether higher frequencies yield better signal-to-noise ratios (SNRs). It could be shown that local changes in optical tissue properties, as they appear in the initial stages of rheumatoid arthritis in a finger joint, are detectable by simple image evaluation, with the range of modulation frequency around 500MHz proving to yield the highest SNR.
Freezing of low energy excitations in charge density wave glasses
Authors: Starešinic D. , Zaitsev-Zotov S. V. , Baklanov N. I. , Biljaković K.
Published: March 2008

Thermally stimulated discharge current measurements were performed to study slow relaxation processes in two canonical charge density wave systems K0.3MoO3 and o-TaS3. Two relaxation processes were observed and characterized in each system, corroborating the results of dielectric spectroscopy. Our results are consistent with the scenario of the glass transition on the charge density wave superstructure level. In particular, the results directly prove the previously proposed criterion of charge density wave freezing based on the interplay of charge density wave pinning by impurities and screening by free carriers. In addition, we obtained new information on distribution of relaxation parameters, as well as on nonlinear dielectric response both below and above the threshold field for charge density wave sliding.
Photocurrent measurements for oxide charge characterization of high-κ dielectric metal oxide semiconductor capacitors
Authors: Felnhofer D. , Gusev E. P. , Buchanan D. A.
Published: March 2008

Photocurrent-voltage (photo-I-V) measurements for oxide charge characterization of high-κ metal-oxide-semiconductor (MOS) capacitors is a viable technique for quantifying both the oxide charge density and the centroid of charge. The latter information is not obtainable from conventional capacitance-voltage (C-V) measurements. In this paper, the theoretical background for photo-I-V measurements is reviewed and extended to address high-κ dielectric stacks. Experimental results comparing C-V and photo-I-V measurements for oxide charge characterization of Al gated, high-κ HfO2 MOS capacitors is given. For electron injection from Al, it was found that similar oxide charge densities were calculated using photo-I-V (between 5.5×1012 and 6.7×1012cm−2) and C-V (between 4.5×1012 and 5×1012cm−2) measurements. The centroid of trapped charge, x¯∕tox, as defined as a distance from the gate electrode normalized to the dielectric stack thickness was measured to be ∼0.2, which is near the Al electrode and suggests that significant charge trapping is occurring at defects within the HfO2 and not at the HfO2 interface with the thin oxide. Similar measurements using electron injection from the substrate shows that the centroid of trapped charge shifts from near the gate electrode (x¯∕tox=0.22) to the substrate electrode (x¯∕tox=1) as the applied stress bias increases from 0.75to2V. This result is consistent with increased field-assisted detrapping of oxide charge. From photocurrent measurements, it was also determined that the effective barrier height for electron photoinjection from the Al gate to the HfO2 conduction band was 2.6±0.1eV, which is consistent with current knowledge of the HfO2 band structure.
Correlating reduced fill factor in polymer solar cells to contact effects
Authors: Gupta Dhritiman , Bag Monojit , Narayan K. S.
Published: March 2008

A probable limiting factor for efficiency and fill factors of organic solar cells originates from the cathode-polymer interface. We utilize various forms of cathode layer such as Al, Ca, oxidized Ca, and low melting point alloys in model systems to emphasize this aspect in our studies. The current-voltage (JV) response in the fourth quadrant indicates a general trend of convex shaped JV characteristics (d2J∕dV2>0) for illuminated devices with good cathode-polymer interfaces and linear or concave JV responses (d2J∕dV2<0) for inefficient cathode-polymer interfaces.
Direct current electrical and microwave properties of polymer-multiwalled carbon nanotubes composites
Authors: Mdarhri A. , Carmona F. , Brosseau C. , Delhaes P.
Published: March 2008

We report the results of effective direct current (dc) resistivities and alternating current (ac) complex permittivity measurements carried out on two series of polymer∕multiwalled (MW) carbon nanotube (CNT) composite samples as function of the CNTs volume fraction and temperature. The CNTs have typical aspect ratio over 100:1 and are quasiuniformly dispersed in two types of polymer host [epoxy and polystyrene (PS)-cobutyl acrylate latex] according transmission electron microscopy characterization. A percolation threshold occurs in the composites with the PS latex matrix when the CNT volume fraction is ≈0.012. In contrast, the set of resistivity data for samples containing epoxy resin as host matrix is not representative of an intrinsically percolation transition. Atomic force microscopy, coupled to the measurement of the local electric resistances, permits us to study the agglomerate distribution in both types of composites. The differences in morphology between the two series of polymer∕MWCNTs are attributed to interface effects between the elongated filler nanoparticles and the polymer chains. Comparison of the measured effective dc and ac resistivities, at temperatures ranging from 30to300K, with Sheng’s model supports the fact that charge transport in the composites follows a thermal fluctuation induced tunneling mechanism, in which the tunneling of the electrons through the thermally induced fluctuating potential barrier formed by a thin insulating polymer layer separating MWCNTs aggregates. The spectral behavior of permittivity is consistent with a power-law form. Several mixing laws were tested to represent the filler fraction dependence of the effective permittivity in the microwave range of frequencies; however, none of them is able to quantitatively describe the sets of data. The basic deficiency of these formulas is that they make no explicit reference to the internal length scales in the composite samples.
Centrifugally driven electrostatic instability in extragalactic jets
Authors: Osmanov Z.
Published: March 2008

The stability problem of the rotation-induced electrostatic wave in extragalactic jets is presented. Solving a set of equations describing dynamics of a relativistic plasma flow of active galactic nuclei (AGN) jets, an expression of the instability rate has been derived and analyzed for typical values of AGNs. The growth rate was studied versus the wavelength and the inclination angle and it has been found that the instability process is very efficient with respect to the accretion disk evolution, indicating high efficiency of the instability.
Numerical simulation of the three-dimensional screech phenomenon from a circular jet
Authors: Li X. D. , Gao J. H.
Published: March 2008

Despite great advances on jet screech research during the past few decades, the prediction of jet screech amplitudes still remains a challenging task. The main objective of this paper is to develop an accurate three-dimensional computational aeroacoustic procedure for the simulation of screech phenomenon from an underexpanded supersonic circular jet. The three-dimensional Navier–Stokes equations and a modified two-equation standard k-ϵ turbulence model are solved in the generalized curvilinear coordinate system. A dispersion-relation-preserving scheme is utilized for space discretization. The 2N storage low-dissipation and low-dispersion Runge–Kutta scheme is applied for time marching. Numerical results are presented and compared with available experimental data over Mach number range from 1.17 to 1.60. The predicted shock cell structure and radial density profiles agree very well with the measured results by Panda and Seasholtz [“Measurement of shock structure and shock-vortex interaction in underexpanded jets using Rayleigh scattering,” Phys. Fluids 11, 3761 (1999)]. In particular, it is shown that not only the predicted wavelengths, but also the amplitudes of the flapping and helical modes are in good agreement with experimental data by Ponton et al. [“Near field pressure fluctuations in the exit plane of a choked axisymmetric nozzle,” NASA Tech. Memo TM-113137, 1997]. It is found that although the instantaneous flow fields accompanying the flapping and helical jet screech tones are not axisymmetric yet, the long time average mean flow field is still almost axisymmetric. This is because of the slow rotation of the flapping plane around the jet axis. The real time pressure signal of a Maj=1.30 screeching jet at r∕D=2 in the nozzle exit plane is analyzed. The results indicate that the acoustic field of the simulated flapping and rotating motions of the screeching jet agree well with Ponton and Seiner’s experimental measurements [“Acoustic study of B helical mode for choked axisymmetric nozzle,” AIAA J. 33, 413 (1995)].
Vortex shedding from a circular cylinder of finite length at low Reynolds numbers
Authors: Inoue Osamu , Sakuragi Akira
Published: March 2008

Flow past a circular cylinder of finite span-length with two free ends is investigated based on direct numerical solutions of the three-dimensional unsteady incompressible Navier–Stokes equations. Special attention is paid to the effect of length-to-diameter ratio L∕D on vortex shedding from the cylinder into its wake, where L is the span-length and D is the diameter of the cylinder. The length-to-diameter ratio L∕D is prescribed in the range of 0.5⩽L∕D⩽100, and the Reynolds number Re which is based on D is 40⩽Re⩽300. Results show that vortex shedding from, and thus wake pattern behind, the cylinder changes drastically depending on both L∕D and Re. Five basic patterns of vortex shedding are found to exist: (i) Periodic oblique vortex shedding at relatively large L∕D and at Re beyond a critical Reynolds number, (ii) quasiperiodic oblique vortex shedding also at relatively large L∕D but at Re below the critical Reynolds number, (iii) periodic shedding of hairpin-shaped vortices that occurs when L∕D is moderate, (iv) steady two counter-rotating vortex pairs that appear when both L∕D and Re are small, and (v) alternate shedding of a counter-rotating vortex pair from two flat ends, which occurs when L∕D is small but Re is high. Transition processes between the basic patterns are also examined. Three-dimensional vortical structures in the wake are clarified in some detail.
Hematite nanochain networks: Simple synthesis, magnetic properties, and surface wettability
Authors: Lu H. B. , Liao L. , Li J. C. , Shuai M. , Liu Y. L.
Published: March 2008

This paper describes a simple and effective method for the fabrication of hematite (α-Fe2O3) nanochain networks (NCNs) by using carbon nanotubes as templates. The α-Fe2O3 nanochains are made of numerous α-Fe2O3 nanocrystals, which exhibit weak ferromagnetic behavior at room temperature while showing antiferromagnetic behavior at low temperatures below the Morin temperature (180K). Furthermore, the α-Fe2O3 NCNs generate superhydrophilic surface displaying the contact angle of water of 0°. Therefore, the α-Fe2O3 NCNs may be applied in nanomagnetic filters to capture magnetic and paramagnetic micro- and nanocontaminations to obtain high purity liquid.
A note on oblique stagnation-point flow
Authors: Tooke R. M. , Blyth M. G.
Published: March 2008

Previous analyses of oblique stagnation-point flow at a plane wall are discussed and unified with reference to a free parameter. The oblique flow consists of orthogonal stagnation-point flow to which is added a shear flow whose vorticity is fixed at infinity. Physically the free parameter may be viewed as altering the structure of the shear flow component by varying the magnitude of the pressure gradient. For large adverse pressure gradients, the shear component has a region of reversed flow near the wall. Remarkably, combining an orthogonal flow with shear flows featuring different levels of reversed flow always produces the same oblique flow but with the stagnation-point of attachment shifted along the wall by a predictable amount.
Intense wideband terahertz amplification using phase shifted periodic electron-plasmon coupling
Authors: Shin Young-Min , Barnett Larry R.
Published: March 2008

We report efficient wideband amplification of terahertz frequency electromagnetic waves from sequential electron-plasmon coupling along a half-period staggered double grating structure. Numerical eigenmode calculations show that the fundamental plasmonic band has a strong symmetric TE-mode, with a significant longitudinal electric field component, that is confined to the electron beam channel. A particle-in-cell simulation analysis with a single frequency excitation, combined with a short pulse broad spectrum drive, confirms that this terahertz plasmonic circuit produces three or four orders of magnitude power amplification with up to 30% bandwidth and 3%–5.5% saturated energy conversion efficiency.
Frictional drag reduction in bubbly Couette–Taylor flow
Authors: Murai Yuichi , Oiwa Hiroshi , Takeda Yasushi
Published: March 2008

Frictional drag reduction due to the presence of small bubbles is investigated experimentally using a Couette–Taylor flow system; i.e., shear flow between concentric cylinders. Torque and bubble behavior are measured as a function of Reynolds number up to Re=5000 while air bubbles are injected constantly and rise through an array of vortical cells. Silicone oil is used to avoid the uncertain interfacial property of bubbles and to produce nearly monosized bubble distributions. The effect of drag reduction on sensitivity and power gain are assessed. The sensitivity exceeds unity at Re<2000, proving that the effect of the reduction in drag is greater than that of the reduction in mixture density. This is due to the accumulation of bubbles toward the rotating inner cylinder, which is little affected by turbulence. The power gain, which is defined by the power saving from the drag reduction per the pumping power of bubble injection, has a maximum value of O(10) at higher Re numbers around 2500. An image processing measurement shows this is because of the disappearance of azimuthal waves when the organized bubble distribution transforms from toroidal to spiral modes. Moreover, the axial spacing of bubble clouds expands during the transition, which results in an effective reduction in the momentum exchange.
A heater-integrated scanning probe microscopy probe array with different tip radii for study of micro-nanosize effects on silicon-tip/polymer-film friction
Authors: Bao Haifei , Li Xinxin
Published: March 2008

Electric-heated cantilever-tip probes fabricated by micromachining techniques can be used for high-density data storage, nanopatterning, etc., where contact-scanning and thermal-plastic nanowritings are frequently implemented on the surface of a polymer thin-film such as polymethylmethacrylate (PMMA). In such kind of applications, micro-nanofriction effects, e.g., contacting–size and temperature effects of the tip/film friction system, will largely influence the performance of the applications. To elucidate the effects, present research fabricates a monolithically integrated probe array that comprises three scanning probe microscopy cantilever-tip probes with different tip radii of tens of nanometers, submicrometers and microns, respectively. The tip is enabled an electric-heating function by integrating a heating resistors on the tip. Using the tips, the tip/film friction experiment shows an obvious contacting-area effect. Within a wide temperature range, the friction signal and the normal force load exhibit a nonlinear relationship for the nanoradius tip but a linear relationship for the submicron tip. With the heated tips, the experiment directly reveals significant size effects on friction and adhesion behaviors. It is found that the glassy transition of the PMMA film can be characterized using the submicron tip, while the nanotip is suited to detect the secondary β transition process. By fitting the experimental data into a power law with apparent friction coefficient included, the temperature-effect combined size effect of the micronano tip/polymer friction is modeled and discussed.
n
-type organic field effect transistors from perylene bisimide block copolymers and homopolymers
Authors: Hüttner Sven , Sommer Michael , Thelakkat Mukundan
Published: March 2008

We present organic field effect transistors (OFETs) based on solution-processable n-type polymers containing perylene bisimide as pendant groups. The OFET characteristics of a diblock copolymer consisting of polystyrene and poly(perylene acrylate) (PPerAcr) blocks and a PPerAcr homopolymer are compared. Thermal annealing improves the OFET performance by two to three orders of magnitude, which can be attributed to the improved order and interface properties in the transport layer, arising from the better alignment of the perylene bisimide moieties. Both polymers show excellent n-type performances with electron carrier mobility of 1.2×10−3cm2∕Vs and low threshold voltages of 4–7V.
A study on bending direction of sheet metal in laser forming
Authors: Shi Yongjun , Liu Yancong , Yao Zhenqiang , Shen Hong
Published: March 2008

Concave laser forming and convex forming are all required for the complicated curved surface plate forming. Concave laser forming can be readily obtained by temperature gradient mechanism, while convex forming may be achieved based on buckling mechanism. To achieve precise control of bending direction of plate, buckling critical condition is analyzed when the buckling mechanism plays a dominant role, and a judgment criterion of working conditions of buckling mechanism, Fbuckling value, is derived. To verify the validity of the Fbuckling value, the experiments and numerical simulations are carried out. The results suggest that the bending direction of the plate can be exactly judged according to the Fbuckling value. In addition, the effects of the heating location and starting point on the bending direction are discussed, which provide further insight into the convex forming process and is helpful for the parameter selection of future process planning.
Single-electron-based flexible multivalued logic gates
Authors: Lee C. K. , Kim S. J. , Shin S. J. , Choi J. B. , Takahashi Y.
Published: March 2008

Single-electron transistor (SET)-based multivalued (MV) not-AND (NAND) and not-OR (NOR) logic cells were implemented on a silicon-on-insulator chip. Depending on the ways of connecting two SETs with a field-effect transistor, the voltage transfer characteristics show typical NAND or NOR gate functions for various input voltages, which are binary, MV, and binary-MV mixed. Moreover, the switching functionality of our NAND (NOR) can convert to OR (AND) operation by simply adjusting their initial input voltages. These flexible two-input logic gates are expected to provide four basic arithmetic cells for the SET MV logic gate family.
Nanoaperture formation at metal covered tips by microspark optimized for near-field optical probes
Authors: Chaigneau M. , Louarn G. , Minea T. M.
Published: March 2008

A simple and reproducible method for nanoaperture opening of metal film covered tapered optical fibers is reported as the last step of the manufacturing process for near-field scanning optical microscopy (NSOM) probes. It uses a microspark between the biased tip and another plane electrode working as a coronalike discharge in point-to-plane configuration. The resultant high electric field is enhanced at the extremity of the metallized tip self-focusing the discharge ions. The opening process is performed in situ in a specially designed plasma reactor which also allows, for the optical fiber tips, their surface cleaning and silver coating. Nanoaperture diameter can be controlled between 30 and 100nm. These probes are able to generate subwavelength resolved images and are appropriate for high resolution NSOM.
The role of dislocations as nonradiative recombination centers in InGaN quantum wells
Authors: Abell Josh , Moustakas T. D.
Published: March 2008

InGaN multiple quantum wells (MQWs) were grown on atomically smooth c-GaN templates and identical c-GaN templates etched to reveal hexagonal pits associated with screw dislocations. We found that the room temperature internal quantum efficiency of the MQWs grown on the etched c-GaN templates is a factor of 2 higher than that of the smooth QWs. This finding is accounted for by the fact that the QWs on the nonplanar surfaces are thinner than the c-plane QWs, and thus the carriers are prevented from reaching the dislocations due to the energy barrier around each defect.
Photoluminescence from ZnS∕CdS:Mn∕ZnS quantum well quantum dots
Authors: Qian L. , Bera D. , Holloway P.
Published: March 2008

Enhanced photoluminescence is reported from ZnS∕CdS:Mn∕ZnS quantum well quantum dots (QWQDs) prepared with a reverse micelle process. Compared to CdS:Mn∕ZnS core/shell quantum dots, the QWQDs exhibited a much stronger Mn2+ T14-A16 yellow emission and the quantum yield (QY) was increased from 9% to 45%. This increased QY for QWQDs was postulated to result from the quantum well structure suppressing nonradiative relaxations from the Auger recombination at short times and reduced energy transfer between the Mn2+ dopants at longer times.
Sensitized photo- and electroluminescence from Er complexes mixed with Ir complex
Authors: Zhang Dongyu , Li Wenlian , Chu Bei , Li Xiao , Han Liangliang , Zhu Jianzhuo , Li Tianle , Bi Defeng , Yang Dongfang , Yan Fei , Liu Huihui , Wang Dan
Published: March 2008

We investigated the sensitized effect of fac-tris(2-phenylpyridine) iridium [Ir(ppy)3] on tris-(dibenzoylmethanato)-mono-(bathophenanthroline) erbium (Er-DB) for 1.5μm near-infrared emission. Compared with the neat Er-DB film, the blend film composed of Er-DB and Ir(ppy)3 with a 1:1 of the weight ratio shows 20 times and 4 times enhancement on intensity of photoluminescence (PL) and electroluminescence (EL), respectively. The improvement in both PL and EL intensities was conjectured to be the result of the energy transfer from Ir(ppy)3 to Er-DB complex, and the detailed mechanisms of the enhancement were also discussed.
Pretilt angle control for single-cell-gap transflective liquid crystal cells
Authors: Chen Tien-Jung , Chu Kuei-Lin
Published: March 2008

Doping a minute amount of photocurable monomers in a liquid crystal (LC) cell, we can stabilize the LC alignment after UV exposure by forming a LC pretilt layer on the substrate surface. By varying the UV exposure time, we can control the pretilt angle of the LCs continuously. A single-cell-gap transflective LC cell was fabricated through a photomask. The LC molecules at the transmissive and reflective pixels are pretilted at 54° and 65°, respectively, at which optimal phase retardations of half and quarter wavelengths are achieved. This type of transflective LC display shows an excellent electro-optical match between the two subpixels.
Application of channeling-enhanced electron energy-loss spectroscopy for polarity determination in ZnO nanopillars
Authors: Jasinski J. , Zhang D. , Parra J. , Katkanant V. , Leppert V. J.
Published: March 2008

Polarity in noncentrosymmetric crystals has been shown to affect a number of critical material properties. Here, we demonstrate the application of channeling-enhanced electron energy-loss spectroscopy for the determination of Zn polarity in ZnO nanopillars grown by the chemical vapor deposition method and detail important experimental parameters for the application of this technique. We also confirm ZnO polarity using the more common convergent beam electron diffraction technique.
Direct modulation of electroluminescence from silicon nanocrystals beyond radiative recombination rates
Authors: Carreras Josep , Arbiol J. , Garrido B. , Bonafos C. , Montserrat J.
Published: March 2008

We propose a light emitting transistor based on silicon nanocrystals provided with 200Mbits∕s built-in modulation. Suppression of electroluminescence from silicon nanocrystals embedded into the gate oxide of a field effect transistor is achieved by fast Auger quenching. In this process, a modulating drain signal causes heating of carriers in the channel and facilitates the charge injection into the nanocrystals. This excess of charge enables fast nonradiative processes that are used to obtain 100% modulation depths at modulating voltages of ∼1V.
Microwave guiding in air by a cylindrical filament array waveguide
Authors: Châteauneuf M. , Payeur S. , Dubois J. , Kieffer J.-C.
Published: March 2008

Microwave guiding was demonstrated over 16cm in air using a large diameter hollow plasma waveguide. The waveguide was generated with the 100TW femtosecond laser system at the Advanced Laser Light Source facility. A deformable mirror was used to spatially shape the intense laser pulses in order to generate hundreds of filaments judiciously distributed in a cylindrical shape, creating a cylindrical plasma wall that acts as a microwave waveguide. The microwaves were confined for about 10ns, which corresponds to the free electron plasma wall recombination time. The characteristics of the plasma waveguide and the results of microwave guiding are presented.
Correlation of growth temperature, photoluminescence, and resistivity in GaN nanowires
Authors: Talin A. Alec , Wang George T. , Lai Elaine , Anderson Richard J.
Published: March 2008

The photoluminescence and electrical transport of GaN nanowires grown by metal catalyzed metal-organic chemical vapor deposition were investigated as a function of substrate temperature during growth. As the growth temperature increased from 800to900°C, the electrical conduction mechanism changed from space-charge limited to ohmic transport, the nanowire resistivity dropped from ∼107to∼10−3Ωcm, and the band edge luminescence increased by more than two orders of magnitude. A strong correlation between the resistivity and the fraction of band edge luminescence for individual nanowires was observed.