bluuebunny/crossref_metadata_2025 huggingface dataset:
Spin-valve transistor
Authors: Huang Y. W. , Lo C. K. , Yao Y. D. , Hsieh L. C. , Huang J. H.
Published: May 2005

A spin transistor consisting of a metallic pseudo-spin-valve emitter, a copper base, and a p-n junction collector was prepared on a Si(100) wafer. This spin transistor can provide high magnetocurrent output, high magnetocurrent variation, and high transfer ratio. The common collector configuration with an emitter bias of 5.12V at 77K gives a large magnetocurrent variation of more than 95.5μA in the collector, and the change is more than 3400% with a transfer ratio of 2.59×10−3. At room temperature, these changes go down to 98.3μA and 55.3%, respectively, and the transfer ratio rises to 5.98×10−3. The high performance of this spin-valve transistor is due to the use of a p-n junction as an energy barrier to select spin electrons.
Micromagnetic investigation of resonance frequencies in ferromagnetic particles
Authors: Mohler G. , Harter A. W.
Published: May 2005

A micromagnetics study was performed to examine the dynamic magnetic susceptibility of isolated ferromagnetic nanoparticles. Cobalt and NiFe nanoparticles of dimensions L×30nm×15nm, and a Co nanoparticle of dimensions L×30nm×30nm were simulated. When L was less than or equal to one of the other dimensions, only one resonance peak was seen, with a frequency predicted using the Kittel equation and demagnetization factors calculated for a rectangular prism. However, when L was increased, a small second peak appeared with a frequency corresponding to the resonance frequency of an infinitely long needle. Simultaneously, the frequency of the first peak was shifted from the Kittel-predicted value. Additional increases in L did not change the two peak frequencies significantly but the strength of the first peak approached zero while the strength of the second peak approached a nonzero constant. At sufficiently large L, the second peak frequency converged to that of the Kittel prediction. A visual study of magnetization precession was then performed. For cubelike particles, the magnetization precession occurred uniformly throughout the particle, with the precession frequency matching the resonance frequency found in the susceptibility study. However, as L was increased, a decoupling in precession was observed. The magnetic moments in the interior volume of the particle precessed approximately uniformly at the frequency of the needlelike peak. A block of spins at each end of the particle precessed at a slower frequency corresponding to the first peak. An oscillator model was used to describe the decoupling and analyze the magnetization precession for large L.
Possible metamagnetic origin of ferromagnetism in transition-metal-doped SnO2
Authors: Punnoose A. , Hays J.
Published: May 2005

Room-temperature ferromagnetism has been observed recently in Co and Fe doped SnO2 powders. In Sn0.99Co0.01O2 powders prepared at 600 °C, ferromagnetism emerged following a sharp transition in the M vs H data at a critical field ∼25kOe. Broad susceptibility maxima were observed in ferromagnetic Sn1−xCoxO2 and Sn1−xFexO2 powders centered near 290 and 260 K, respectively, indicating spin fluctuation effects. High temperature magnetic measurements of Sn0.99Co0.01O2 and Sn0.99Fe0.01O2 obtained TC’s of 440 and 850 K respectively. However, unlike conventional ferromagnets, the magnetic transitions at the TC were associated with sharp changes in M. It is argued that these observed unusual magnetic features indicate a metamagnetic origin of the room-temperature ferromagnetism in Co and Fe doped SnO2.
Semi-epitaxial magnetic tunnel transistor: Effect of electron energy and temperature
Authors: Hagler Thomas , Bilzer Claus , Dumm Martin , Kipferl Wolfgang , Bayreuther Günther
Published: May 2005

A magnetic tunnel transistor with spin-valve metallic base and epitaxial Schottky barrier is used to probe the temperature and energy dependence of the magnetocurrent. The magnetocurrent ratio (MCR) reaches values up to 900% in our ultrathin microstructured three-terminal devices. The effect of hot-electron energy has been studied in the range of 0.6 to 2.2 eV. We observe a clear maximum of the MCR at about 1.5 eV electron energy. With increasing temperature (5K<T<185K) the MC remains almost constant, but a rapidly growing spin-independent additional current is measured, due to increasing collector leakage.
Mössbauer spectroscopy and neutron diffraction studies of the ferrimagnetic semiconductor on Ga-substituted FeGaxCr2−xS4
Authors: Son Bae Soon , Kim Sam Jin , Lee Bo Wha , Kim Chul Sung
Published: May 2005

The cation distribution and magnetic and transport properties of FeGaxCr2−xS4 are studied. Rietveld refinement of x-ray diffraction and Mössbauer spectroscopy lead to the conclusion that the samples are in inverse spinel type, where Ga ion is present at tetrahedral site (A). The neutron diffraction on FeGaxCr2−xS4(x=0.1) above 10 K shows that there is no crystallographic distortion and reveals an antiferromagnetic ordering, with the magnetic moment of Fe2+(−3.45μB) aligned antiparallel to Cr3+(+2.89μB) at 10 K. The resistance exhibits a strong dependence on Arrhenius model at temperature below 100 K, while it shows a strong correlation on small polaron model at temperature above 200 K. The electric quadrupole splittings of the A and B sites in Mössbauer spectra give a direct evidence that Ga ion stimulate asymmetric charge distribution of Fe ions in the A site.
Magnetic structure of V:TiO2 and Cr:TiO2 thin films from magnetic force microscopy measurements
Authors: Hong Nguyen Hoa , Ruyter Antoine , Gervais François , Prellier W. , Sakai Joe
Published: May 2005

Ferromagnetic V-doped TiO2 and Cr-doped TiO2 films were fabricated by the pulsed laser deposition technique on LaAlO3 substrates. All V∕Cr:TiO2 films are single phased anatase, well epitaxial, c-axis oriented, and strongly ferromagnetic at room temperature. Besides giving evidences for a great flatness and magnetic homogeneities of those films, magnetic force microscopy measurements implied that the V∕Cr-doped TiO2 films seem to have a diluted magnetic structure with the ferrmagnetism originated from the doped matrix rather than any type of magnetic clusters. The size of the ferromagnetic domains was assumed to be 5–10 μm.
Mn-doped V2VI3 semiconductors: Single crystal growth and magnetic properties
Authors: Choi Jeongyong , Lee Hee-Woong , Kim Bong-Seo , Choi Sungyoul , Choi Jiyoun , Song J. H. , Cho Sunglae
Published: May 2005

We have grown Mn-doped V2VI3 single crystals using the temperature gradient solidification method. We report on the structural and magnetic properties of Mn-doped Bi2Te3, Sb2Te3, Bi2Se3, and Sb2Se3 compound semiconductors. The lattice constants of several percent Mn-doped V2VI3 were slightly smaller than those of the undoped V2VI3 due to the smaller Mn atomic radius (1.40Å) than those of Bi (1.60Å) and Sb (1.45Å). Mn-doped Bi2Te3 and Sb2Te3 had ferromagnetic ordering at TC=10 and 17K, respectively. However, Mn-doped Bi2Se3 and Sb2Se3 showed spin glass and paramagnetic properties, respectively.
Magnetoelectronic device utilizing the Dresselhaus spin-orbit coupling
Authors: Tan S. G. , Jalil M. B. A. , Liew Thomas , Teo K. L. , Chong T. C.
Published: May 2005

We propose a ballistic device model which harnesses the Dresselhaus spin-orbit coupling effect to induce spin polarization ∣P∣ by constraining the transverse electron wave vector kx to specific, quantized values and aligning the crystalline c axis along the electron conduction path z. ∣P∣ induced in this magnetoelectronic device is further enhanced by applying a periodic system of delta magnetoelectric barriers that constitute a net “zero-A” periodic unit. Calculations for GaAs, GaSb, and InSb show a field enhancement of ∣P∣ from peak 2%–3% to 10%–80% near the conduction band. Magnetic and electric potentials have been shown to modulate ∣P∣ with few gate elements required.
Enhanced Hall voltage in a gate-controlled InSb Hall device
Authors: Kim W. Y. , Chang Joonyeon , Han S. H. , Lee W. Y. , Chang S. G.
Published: May 2005

We present an enhanced Hall voltage from the gate-controlled Hall device incorporating a micron-scaled InSb semiconductor cross junction and a single ferromagnetic element. Magnetic fringe field at an edge of the ferrogmanetic element gives rise to the Hall voltage, which shows hysteretic behavior upon magnetic-field sweep. The Hall effect is amplified by a factor of ∼40% when a gate voltage of −25V is applied. The increase is largely attributed to the reduction of carrier density affected by the gate confinement effect. The InSb Hall device controlled by gate voltage demonstrates a possible application for active nonvolatile memory cells and logic gate.
Ramping effects on dielectric tunnel capacitance of magnetic random access memory cell
Authors: Li Simon C. , Lee Jia-Mou , Shu Min-Fong , Canizo-Cabrera A. , Su J. P. , Wu Te-Ho
Published: May 2005

To characterize the dielectric tunnel capacitance of the thin insulating layer in magnetic random access memory cell under the various ramping rates, the examination of current images and current-voltage (I–V) characteristics of the magnetic tunnel junctions were carried out by conductive atomic force microscopy. It is confirmed that in all three contrasted regions, dielectric tunnel capacitance in the very low leaky state decreases with higher ramping rate. With more induced polarization charges in the dielectrics under slower ramping rate, the dielectric tunnel capacitance will increase due to a higher reversed polarized field formed within the dielectrics.
A method for an accurate T1 relaxation-time measurement compensating B1 field inhomogeneity in magnetic-resonance imaging
Authors: Mihara Hiroaki , Sekino Masaki , Iriguchi Norio , Ueno Shoogo
Published: May 2005

Two-point gradient echo methods are practical means for estimating T1 value in magnetic-resonance (MR) imaging. Meanwhile, the inhomogeneity of the transmitted B1 field and the imperfect slice profile severely distorts the estimated T1 value. We propose a method for an accurate T1 measurement compensating the inhomogeneity of the B1 field and the effects of the imperfect slice profile. Three MR images of a whole human brain were obtained by multisliced spoiled gradient echo sequences on a 1.5-T scanner with different repetition times and flip angles to calculate the quantitative T1 maps. The histograms of the T1 maps showed two distinguishable peaks of the white matter (672.9±15.5ms) and the gray matter (921.1±24.7ms) of the human brain. The results indicate that the T1 value was properly estimated compensating the B1 field inhomogeneity and the slice profile imperfection. The proposed method is applicable to images obtained on any scanners with different parameters, because the method needs neither the information of the flip angle calibrations nor the phantom data to compensate the B1 field inhomogeneity.
Semiconducting and magnetic properties of Zn1−xMnxO films grown by metalorganic chemical vapor deposition
Authors: Chikoidze E. , Dumont Y. , Jomard F. , Ballutaud D. , Galtier P. , Gorochov O. , Ferrand D.
Published: May 2005

Zn 1 − x Mn x O diluted magnetic semiconductor single phase thin films have been grown by the MOCVD technique. Depositions have been done on fused silica and (0001)-sapphire substrates. Layers on silica are polycrystalline with [001] preferential orientation, while Zn1−xMnxO films on c-sapphire are highly (0001) textured. Manganese content (x) in the samples varies in 0.00–0.44 range. Change of c lattice parameter follows Vegard’s law. Hall effect measurements show a decrease of mobility with Mn increase and no change of carrier concentration. Optical transmission results present the increase of band gap with manganese incorporation. Superconducting quantum interference device magnetometry shows Curie–Weiss behaviors of the susceptibility with antiferromagnetic coupling with an effective exchange constant of J1∕kB=−16K.
Rayleigh and Prandtl number scaling in the bulk of Rayleigh–Bénard turbulence
Authors: Calzavarini Enrico , Lohse Detlef , Toschi Federico , Tripiccione Raffaele
Published: May 2005

The Ra and Pr number scaling of the Nusselt number Nu, the Reynolds number Re, the temperature fluctuations, and the kinetic and thermal dissipation rates is studied for (numerical) homogeneous Rayleigh–Bénard turbulence, i.e., Rayleigh–Bénard turbulence with periodic boundary conditions in all directions and a volume forcing of the temperature field by a mean gradient. This system serves as model system for the bulk of Rayleigh–Bénard flow and therefore as model for the so-called “ultimate regime of thermal convection.” With respect to the Ra dependence of Nu and Re we confirm our earlier results [D. Lohse and F. Toschi, “The ultimate state of thermal convection,” Phys. Rev. Lett. 90, 034502 (2003)] which are consistent with the Kraichnan theory [R. H. Kraichnan, “Turbulent thermal convection at arbitrary Prandtl number,” Phys. Fluids 5, 1374 (1962)] and the Grossmann–Lohse (GL) theory [S. Grossmann and D. Lohse, “Scaling in thermal convection: A unifying view,” J. Fluid Mech. 407, 27 (2000); “Thermal convection for large Prandtl number,” Phys. Rev. Lett. 86, 3316 (2001); “Prandtl and Rayleigh number dependence of the Reynolds number in turbulent thermal convection,” Phys. Rev. E 66, 016305 (2002); “Fluctuations in turbulent Rayleigh–Bénard convection: The role of plumes,” Phys. Fluids 16, 4462 (2004)], which both predict Nu∼Ra1∕2 and Re∼Ra1∕2. However the Pr dependence within these two theories is different. Here we show that the numerical data are consistent with the GL theory Nu∼Pr1∕2, Re∼Pr−1∕2. For the thermal and kinetic dissipation rates we find ϵθ∕(κΔ2L−2)∼(RePr)0.87 and ϵu∕(ν3L−4)∼Re2.77, both near (but not fully consistent) the bulk dominated behavior, whereas the temperature fluctuations do not depend on Ra and Pr. Finally, the dynamics of the heat transport is studied and put into the context of a recent theoretical finding by Doering et al. [“Comment on ultimate state of thermal convection” (private communication)].
Ab initio molecular dynamics study of glycine intramolecular proton transfer in water
Authors: Leung Kevin , Rempe Susan B.
Published: May 2005

We use ab initio molecular-dynamics simulations to quantify structural and thermodynamic properties of a model proton transfer reaction that converts a neutral glycine molecule, stable in the gas phase, to the zwitterion that predominates in aqueous solution. We compute the potential of mean force associated with the direct intramolecular proton transfer event in glycine. Structural analyses show that the average hydration number (Nw) of glycine is not constant along the reaction coordinate, but rather progresses from Nw=5 in the neutral molecule to Nw=8 for the zwitterion. We report the free-energy difference between the neutral and charged glycine molecules, and the free-energy barrier to proton transfer. Finally, we identify the approximations inherent in our method and estimate the corresponding corrections to our reported thermodynamic predictions.
Towards the multireference Brillouin–Wigner coupled-clusters method with iterative connected triples: MR BWCCSDT-α approximation
Authors: Pittner Jiří , Demel Ondřej
Published: May 2005

We developed and implemented an approximation of the state-specific Brillouin–Wigner coupled-cluster method with singles, doubles, and triples, called MRBWCCSDT-α, for a general number of closed- and open-shell reference configurations. The accuracy of the method is assessed on the calculation of the oxygen molecule in the X3Σg−, a1Δg, and b1Σg+ states and the results of this multireference treatment are compared with previous MRBWCCSD results and with those obtained by the doubly ionized similarity transformed equation-of-motion CCSD and multireference configuration interaction methods and with experimental spectroscopic data. Explicit tests of the size-extensivity of the MRBWCCSDT-α method with iterative size-extensivity correction are also performed.
Heterostructure unipolar spin transistors
Authors: Flatté M. E. , Vignale G.
Published: May 2005

We extend the analogy between charge-based bipolar semiconductor electronics and spin-based unipolar electronics by considering unipolar spin transistors with different equilibrium spin splittings in the emitter, base, and collector. The current of base majority spin electrons to the collector limits the performance of “homojunction” unipolar spin transistors, in which the emitter, base, and collector are all made from the same magnetic material. This current is very similar in origin to the current of base majority carriers to the emitter in homojunction bipolar junction transistors. The current in bipolar junction transistors can be reduced or nearly eliminated through the use of a wide band-gap emitter. We find that the choice of a collector material with a larger equilibrium spin splitting than the base will similarly improve the device performance of a unipolar spin transistor. We also find that a graded variation in the base spin splitting introduces an effective drift field that accelerates minority carriers through the base towards the collector.
Distributed feedback lasing from dye-doped glass films using photopatterned gold nanoparticles
Authors: Fukushima Masanori , Yanagi Hisao , Hayashi Shinji , Suganuma Naotoshi , Taniguchi Yoshio
Published: May 2005

We found a tunable lasing action of dye-doped glass films using a distributed feedback (DFB) resonator composed of gold (Au) nanoparticles. The Au-DFB resonators having various periodic spacings of 400–600 nm were fabricated by photoreduction of chloroauric ions doped in silica/titania (SiO2∕TiO2) sol-gel films using two-beam interference of a He-Cd laser. After lamination of a SiO2∕TiO2 film containing Rhodamine B (RB), the device was pumped with a frequency-doubled yttrium aluminum garnet laser. As compared to the amplified spontaneous emission [full width at half maximum (FWHM) ∼20nm] from a RB-doped film without DFB, the laminated device indicated feedback-gained narrowing of the emission (FWHM∼1nm). Moreover, their peak wavelengths were shifted according to the tuned spacing of the Au-DFB resonator.
Cuboctahedral symmetry in the aggregation of [60]fullerene in polar organic media
Authors: Saha Avijit , Mukherjee Asok K.
Published: May 2005

[60]fullerene is known to aggregate in water and all experimental and theoretical evidences support that the aggregates contain (C60)13 units. No chemical kinetic study of the effect of solvent polarity on the aggregation of C60 has so far been reported. Here we show by simple kinetic study of the aggregation process and by scanning electron microscopy (SEM) that when methanol is added to a solution of [60]fullerene in CCl4, spontaneous aggregation starts immediately and the aggregation numbers (n) found to be dependent on the CCl4:CH3OH ratio (v∕v) of the medium. One particular ratio of the two liquids gives uniformly sized (C60)13 clusters for about 10 min. The values of n correspond to the minima of the previously reported energy calculations and in the present work, they have been shown to be a natural consequence of stacking of cuboctahedra made up of C60 molecules. A Young diagramlike method has been developed for counting the number of C60 molecules in these cuboctahedral stacks and the numbers obtained from this model and also from the present chemical kinetic and SEM studies agree very well with the “magic numbers” obtained by earlier mass spectrometric studies.
Adhesion forces between wetted solid surfaces
Authors: Charlaix Elisabeth , Crassous Jérôme
Published: May 2005

We investigate in this paper the influence of wetting films on the adhesion forces between macroscopic solid surfaces connected by a liquid bridge. We show that the capillary forces are dependent on the interactions governing the wetting layers, and that those interactions may be determined from the measurement of the capillary force in the presence of a condensable vapor. We illustrate those results with a surface force apparatus experiment where the capillary force between high-energy surfaces is measured for different liquid pressures.
Coherent control of the population transfer in complex solvated molecules at weak excitation. An experimental study
Authors: Prokhorenko Valentyn I. , Nagy Andrea M. , Miller R. J. Dwayne
Published: May 2005

We performed a series of successful experiments for the optimization of the population transfer from the ground to the first excited state in a complex solvated molecule (rhodamine 101 in methanol) using shaped excitation pulses at very low intensities (1 absorbed photon per 100–500 molecules per pulse). We found that the population transfer can be controlled and significantly enhanced by applying excitation laser pulses with crafted pulse shapes. The optimal shape was found in feedback-controlled experiments using a genetic search algorithm. The temporal profile of the optimal excitation pulse corresponds to a comb of subpulses regularly spaced by ∼150fs, whereas its spectrum consists of a series of well-resolved peaks spaced apart by approximately 6.5 nm corresponding to a frequency of 220cm−1. This frequency matches very well with the frequency modulation of the population kinetics (period of ∼150fs), observed by excitation with a short (∼20fs) transform-limited laser pulse directly after excitation. In addition, an antioptimization experiment was performed under the same conditions. The difference in the population of the excited state for the optimal and antioptimal pulses reaches ∼30% even at very weak excitation. The results of optimization are reproducible and have clear physical meaning.
Effects of fluorination on electronic and excited states of fused zinc oligoporphyrins
Authors: Yamaguchi Yoichi
Published: May 2005

Density functional theory (DFT) has been applied to study the effect of fluorination on the electronic and excited states of fused zinc oligoporphyrins in the search for new functionalizing materials, such as n-type organic semiconductors. The excitation spectra of meso-tetrafluoro, β-octafluoro, and perfluoro zinc porphyrins, and their triply meso-meso-, β-β, and β-β-linked fluorinated zinc oligoporphyrins were systematically examined using the time-dependent DFT method. The effect of the perfluorination on the zinc porphyrin (ZnP) causes the maximum 1.12eV and 1.42eV drops for the highest occupied and lowest unoccupied molecular orbital (HOMO and LUMO, respectively) levels, respectively. The electronic and excitation features of the fluorinated ZnPs are almost similar to the unfluorinated ones. However, the large antibonding contribution of the meso-fluorines disturbs the stabilization of the HOMO, resulting in a more effective reduction of both the HOMO-LUMO gaps and the lowest Q excitation energies with the increasing number of porphyrins compared to the unfluorination and the other types of fluorinations. It is found that the infinite fused fluorinated ZnP tapes with narrow gap (≈0.1eV–0.2eV) as predicted by using the periodic-DFT level are slightly inferior to the near-zero gap semimetallic unfluorinated ZnP tape as a conducting molecular wire. The combination of the condensation and the meso- and∕or β-fluorination of ZnP can finely tune the LUMO level to the Fermi level of the electrodes for fabrication of n-type conducting materials. The fused fluoro-oligoporphyrins may then become new n-type organic semiconductors, provided they are well crystallized with a high electron mobility, such as the recently synthesized perfluoropentacene.
Excitation of the centrifugally driven interchange instability in a plasma confined by a magnetic dipole
Authors: Levitt B. , Maslovsky D. , Mauel M. E. , Waksman J.
Published: May 2005

The centrifugally driven electrostatic interchange instability is excited for the first time in a laboratory magnetoplasma. The plasma is confined by a dipole magnetic field, and the instability is excited when an equatorial mesh is biased to induce a radial current that creates rapid axisymmetric plasma rotation. The observed instabilities appear quasicoherent in the lab frame of reference; they have global radial mode structures and low azimuthal mode numbers, and they are modified by the presence of energetic, magnetically confined electrons. The mode structure is measured using a multiprobe correlation technique as well as a novel 96-point polar imaging diagnostic which measures particle flux along field lines that map to the pole. Interchange instabilities caused by hot electron pressure are simultaneously observed at the hot electron drift frequency. Adjusting the hot electron fraction α modifies the stability as well as the structures of the centrifugally driven modes. In the presence of larger fractions of energetic electrons, m=1 is observed to be the dominant mode. For faster rotating plasmas containing fewer energetic electrons, m=2 dominates. Results from a self-consistent nonlinear simulation reproduce the measured mode structures in both regimes. The low azimuthal mode numbers seen in the experiment and simulation can also be interpreted with a local, linear dispersion relation of the electrostatic interchange instability. Drift resonant hot electrons give the instability a real frequency, inducing stabilizing ion polarization currents that preferentially suppress high-m modes.
Nonlinear evolution of the firehose instability in a magnetic dipole geotail geometry
Authors: Wong H. Vernon , Xu B.-Y. , Horton W. , Pratt J. , Van Dam J. W.
Published: May 2005

Bursty bulk flows increase the parallel pressure faster than the perpendicular pressure in the high-β central plasma sheet driving the firehose instability [S. Ji and R. A. Wolf, J. Geophys. Res. 108, 1191 (2003); S. Ji and R. A. Wolf, J. Geophys. Res.30, 2242 (2003)]. A nonlinear partial differential equation is derived and an initial-value code developed to investigate the firehose anisotropy-driven turbulence in the Earth’s geotail. It is essential to include dispersive ion kinetic effects in order to limit the range of linearly unstable parallel wave numbers and to achieve a stationary nonlinear turbulent state. The nonlinear dynamics of the firehose instability provides a possible explanation for ultralow-frequency Pi-2 fluctuations associated with bursty bulk flows and substorms.
Second virial coefficients of asymmetric top molecules
Authors: Wormer Paul E. S.
Published: May 2005

A short self-contained derivation is given for the second virial coefficient B2(T) of a gas consisting of identical interacting asymmetric rigid rotors. The resulting expression is correct through ℏ2. First, the canonical partition function is derived by means of an ℏ expansion of exp[−H∕(kBT)] due to Friedmann [Adv. Chem. Phys. 4, 225 (1962)]. The present work applies angular momentum operators and known facts from angular momentum theory. It is considerably more accessible than Friedmann’s exposition, which is not based on angular momentum operators, but instead on explicit derivatives with respect to Euler angles. The partition function obtained from the ℏ expansion is applied to the derivation of an expression for B2(T) that is identical in appearance to the expression for symmetric rotors of T Pack [J. Chem. Phys. 78, 7217 (1983)]. The final equation in this work is valid for rigid rotors of any symmetry.
Estimation of protein folding probability from equilibrium simulations
Authors: Rao Francesco , Settanni Giovanni , Guarnera Enrico , Caflisch Amedeo
Published: May 2005

The assumption that similar structures have similar folding probabilities (pfold) leads naturally to a procedure to evaluate pfold for every snapshot saved along an equilibrium folding-unfolding trajectory of a structured peptide or protein. The procedure utilizes a structurally homogeneous clustering and does not require any additional simulation. It can be used to detect multiple folding pathways as shown for a three-stranded antiparallel β-sheet peptide investigated by implicit solvent molecular dynamics simulations.
First-order mean spherical approximation for attractive, repulsive, and multi-Yukawa potentials
Authors: Tang Yiping , Lin Yang-Zheng , Li Yi-Gui
Published: May 2005

The first-order mean spherical approximation (FMSA) theory proposed by Tang et al. [Fluid Phase Equilib., 134, 21(1997)] is applied for studying several typical Yukawa fluids, including attractive, repulsive, and multi-Yukawa cases. The FMSA study is particularly advantageous in providing thermodynamics and structure information in a simple, analytical, and consistent manner. Comparisons with the latest reported computer simulation data for compressibility factor, internal energy, and radial distribution function show that FMSA performs very well and the performance is very close to the full MSA and to several other theories, developed individually for the above-mentioned cases or properties. The present study provides solid evidence to support FMSA applications to more complex fluids.
A time-dependent potential flow theory for the aerodynamics of vertical axis wind turbines
Authors: Ågren O. , Berg M. , Leijon M.
Published: May 2005

The Betz factor, i.e., the value 16∕27 for the power coefficient, is widely expected to give an upper limit for the performance of any wind turbine. In the present study, an analytical model of a vertical-axis wind turbine with straight vertical wings is developed. A goal of the work is to study if the one-dimensional Betz theory gives an upper limit of the performance of wind turbines when two-dimensional effects are included. The two-dimensional and time-dependent potential flow is solved by a conformal map of the wing sections to circles. The stagnation points are determined by the Kutta condition. The calculated power coefficient exceeds the Betz limit by a large factor. This is due to a completely different flow pattern compared to the one-dimensional Betz theory. In aerodynamic potential flow, the expanding flux tube of Betz is replaced by an asymptotic flow consisting of a superposition of homogeneous flow and a circulation around the wings. Moreover, the total torque on a turbine with three or more blades is found to be constant. The Betz theory does not take into account the two-dimensional flow effects and the velocity of the rotating wind turbine. By including these effects, far more optimistic theoretical results for the performance of vertical-axis wind turbines are obtained.
High-bootstrap, noninductively sustained electron internal transport barriers in the Tokamak à Configuration Variable
Authors: Coda S. , Goodman T. P. , Henderson M. A. , Sauter O. , Behn R. , Bottino A. , Camenen Y. , Fable E. , Martynov An. , Nikkola P. , Scarabosio A. , Zhuang G. , Zucca C. ,   the TCV team
Published: May 2005

Important ingredients of the advanced-tokamak route to fusion have been explored in depth in the Tokamak à Configuration Variable [F. Hofmann, J. B. Lister, M. Anton et al., Plasma Phys. Controlled Fusion 36, B277 (1994)] over the past two years. Using a uniquely powerful and flexible electron-cyclotron resonance heating (ECRH) system as the primary actuator, fully noninductive, steady-state electron internal transport barrier discharges have been generated with an electron-energy confinement time up to five times longer than in L mode, poloidal β up to 2.4, and bootstrap fraction up to 75%. Interpretative transport modeling confirms that the safety factor profile is nonmonotonic in these discharges. The formation of the barrier is a discrete event resulting in rapid and localized confinement improvement consistent with the time and location of magnetic-shear reversal. In steady state, however, the confinement quality appears to depend on the current gradient in a broader negative-shear region enclosed by the barrier, improving with increasing shear: in particular, the width and depth of the barrier can be controlled and finely tuned, along a magnetohydrodynamic-stable path, by manipulating the current profile with ECRH (six independently steerable 0.45 MW launchers). The crucial role of the current profile has been clearly demonstrated by applying small Ohmic current perturbations which dramatically alter the properties of the barrier, enhancing or reducing the confinement with negative and positive current, respectively, with negligible Ohmic heating. These results are in agreement with theoretical estimates: first-principle-based numerical simulations of microinstability dynamics and turbulence-driven transport predict a substantial suppression of turbulence and anomalous energy diffusivity near the location of the minimum in the safety factor.
Elliptic instability in the Lagrangian-averaged Euler–Boussinesq-α equations
Authors: Fabijonas Bruce R. , Holm Darryl D.
Published: May 2005

We examine the effects of turbulence on elliptic instability of rotating stratified incompressible flows, in the context of the Lagrangian-averaged Euler–Boussinesq-α (LAEB-α) model of turbulence. We find that the LAEB-α model alters the instability in a variety of ways for fixed Rossby number and Brunt–Väisälä frequency. First, it alters the location of the instability domains in the (γ,cosθ)-parameter plane, where θ is the angle of incidence the Kelvin wave makes with the axis of rotation and γ is the eccentricity of the elliptic flow, as well as the size of the associated Lyapunov exponent. Second, the model shrinks the width of one instability band while simultaneously increasing another. Third, the model introduces bands of unstable eccentric flows when the Kelvin wave is two dimensional. We introduce two similarity variables—one is a ratio of the Brunt–Väisälä frequency to the model parameter Υ0=1+α2β2 and the other is the ratio of the adjusted inverse Rossby number to the same model parameter. Here, α is the turbulence correlation length and β is the Kelvin wave number. We show that by adjusting the Rossby number and Brunt–Väisälä frequency so that the similarity variables remain constant for a given value of Υ0, turbulence has little effect on elliptic instability for small eccentricities (∣γ∣⪡1). For moderate and large eccentricities, however, we see drastic changes of the unstable Arnold tongues due to the LAEB-α model. Additionally, we find that introducing anisotropy in the vertical component of the transported velocity field merely alters the definition of the model parameter Υ0, which effectively reduces the original parameter value. When the similarity variables are viewed with the new definition, the results are similar to those for the isotropic case.
Surface passivation and electronic structure characterization of PbTiO3 thin films and Pt∕PbTiO3 interfaces
Authors: Kurasawa Masaki , Mclntyre Paul C.
Published: May 2005

The surface electronic structure, postdeposition surface passivation, and Schottky barrier height in contact with Pt of PbTiO3 thin films on (001) SrTiO3 were investigated by x-ray photoemission spectroscopy (XPS). Angle-resolved XPS analysis shows that an ∼10-Å-thick surface layer which consists of lead carbonate and lead oxide exists on high-quality PbTiO3 epitaxial films, although the layer can be removed by postdeposition aqueous HNO3 etching. Electronic states associated with this defective surface layer determine the position of the surface Fermi level relative to the band edges of the PbTiO3 film. In situ XPS measurements were carried out during the Pt deposition on as-grown and HNO3-treated PbTiO3 films. The Pb 4f, Ti 2p, and O 1s peaks were observed to shift to higher binding energies during the in situ Pt deposition, consistent with metallization-induced band bending. Although the initial Fermi energies for both Pt-uncoated as-grown and HNO3-treated PbTiO3 differ by ∼0.3eV, the postmetallization Fermi energy lies at 2.4eV above the valence-band maximum after 2 ML (monolayers) of the Pt deposition for both samples. These results suggest that the Fermi level is pinned by interface defect states because the resulting Pt∕PbTiO3 electron Schottky barrier (∼1eV) is substantially smaller than the value derived from recent electronic structure calculations (1.45eV). Consistent with this observation, angle-resolved XPS results indicate that the (001) surface of both as-deposited and HNO3-treated PbTiO3 films decomposes during the initial stages of the Pt deposition and that metallic Pb diffuses into the Pt layer during the Pt deposition, even at room temperature. The presence of the metallic Pb and the resultant formation of a defective interface layer at the Pt∕PbTiO3 (001) interface apparently produce the observed Fermi energy pinning.
Role of magnetic shear in large-scale structure formation in electron temperature gradient driven turbulence
Authors: Li Jiquan , Kishimoto Y.
Published: May 2005

Formation of secondary large-scale structures in electron temperature gradient (ETG) driven turbulence is investigated based on modeling analyses and three-dimensional gyrofluid turbulence simulations. What state the system evolves to, zonal flow dominated or streamerlike, depends on the spectral anisotropy of ETG turbulence. The radially elongated turbulent fluctuation tends to excite a zonal flow instability, while the poloidally elongated one favors to enhance the streamer formation. The magnetic shear is identified to play a key role in the structure selection through governing the anisotropy of ETG turbulent structures.
Excitation of the toroidicity-induced shear Alfvén eigenmode by toroidal ion-temperature-gradient mode turbulence
Authors: Marchenko V. S.
Published: May 2005

It is shown that the toroidicity-induced shear Alfvén eigenmode (TAE) with low mode numbers can be excited as a result of the modulational instability of the short wavelength toroidal ion-temperature-gradient mode turbulence. This instability seems to be responsible for the TAE excitation in Ohmically heated discharges at ASDEX Upgrade tokamak [M. Maraschek, S. Günter, T. Kass, B. Scott, and H. Zohm, Phys. Rev. Lett. 79, 4186 (1997)].
Dilute bidispersed tube flow: Role of interclass collisions at increased loadings
Authors: Borée J. , Caraman N.
Published: May 2005

This paper presents measurements and analysis of a bidispersed two-phase flow system at low (11%) and moderate (110%) mass loadings in a fully developed pipe flow. Our goal is to provide data and analysis showing the role of interclass collisions. The mass distribution of the particles mixture is therefore chosen in order to promote collisions between large 90μm and 60μm glass beads. Velocity statistics up to and including the third-order moments at both mass loadings are presented. The radial turbulent transport of longitudinal and radial fluctuating kinetic stresses is much higher for the particles than for the fluid phase. Dominant effects in the radial balance of the kinetic stresses are discussed. The signature of interparticle collisions is observed by comparing measurements at high- and low-mass loadings. In particular, at higher-mass loading, we measure a decrease of the streamwise kinetic stress of the large particles over the entire section of the tube. On the other hand, the streamwise kinetic stress of the 60μm particles decreases in the high shear region only. The most striking result is the strong increase of the radial fluctuating velocities of the glass beads and of the radial transport of the radial fluctuating kinetic energy. The expected phenomenology is an important contribution of interparticle collisions to the radial variance νp′ associated with a damping of the longitudinal kinetic stresses. This damping is more important for the 90μm glass beads than for the 60μm glass beads because the 60μm particles receive kinetic energy from 90μm particles. The redistribution mechanisms via collisions are expected to be largest in the near wall region where the anisotropy of the fluctuating motion is high. An analysis of the collision frequency in different regions of the flow shows that, even at Mj=110%, the mean time scale between collisions is of the order or larger than the particles aerodynamic time constant. Therefore, collisions cannot significantly decrease the mean free path of the particle fluctuating motion, and transport effects of both longitudinal and radial fluctuating kinetic stresses toward the core region are still high. For a stronger increase in mass loading, leading to dominant effects of collisions, one would expect these transport terms to be of minor importance.
Pulsed laser-deposited single-crystal LiZn-ferrite films with low microwave loss
Authors: Song Young-Yeal , Grinolds Michael S. , Krivosik Pavol , Patton Carl E.
Published: May 2005

Lithium zinc ferrite (LiZnF) films were prepared for a wide range of deposition parameters. Bulk LiZnF rather than pure Li ferrite targets were used to mediate the mismatch problem between the film and the MgO substrate. Preparation conditions were optimized to give single phase epitaxial films with half power ferromagnetic-resonance (FMR) linewidths of as low as 57 Oe at 9.5 GHz. The films were deposited on (100)-plane MgO substrates with a KrF excimer laser at a wavelength of 248 nm. The minimum linewidth films with a thickness of 0.70 μm were obtained for an oxygen partial pressure of 400 mTorr, a substrate temperature of 800 °C, and a laser fluence of 1.7J∕cm2. X-ray diffraction pattern indicated a pure LiZn phase with local normal axis deviations below 0.05°. The FMR profiles were measured at 9.5 GHz with the static field perpendicular and parallel to the film plane, and as a function of rotation angle for in-plane fields. The data yielded a saturation induction 4πMs value of 2.730 kG, about the same as for a bulk, and cubic anisotropy, with a first-order cubic anisotropy energy constant K1 of about 7.33×103ergs∕cm3, a factor of five or smaller than that for a bulk LiZn ferrite.
Eigenmode response to driven magnetic reconnection in a collisionless plasma
Authors: Egedal J. , Fox W. , Porkolab M. , Fasoli A.
Published: May 2005

The collisionless plasma of the Versatile Toroidal Facility experiment [J. Egedal, A. Fasoli, M. Porkolab, and D. Tarkowski, Rev. Sci. Instrum. 71, 3351 (2000)], applied to the study of magnetic reconnection, exhibits a global eigenmode response during which the reconnection rate and the current channel oscillate. The present paper describes experiments tailored for investigating this eigenmode response. It is found that the oscillatory plasma behavior is linked to ion-polarization currents and associated electron currents that flow to maintain quasineutrality within the active reconnection region. A theoretical model is developed which describes the eigenmode response and accounts for the temporal evolution of the measured profiles of key plasma parameters.
An analytical model for the thermal conductivity of silicon nanostructures
Authors: Chantrenne P. , Barrat J. L. , Blase X. , Gale J. D.
Published: May 2005

A simple model of thermal conductivity, based on the harmonic theory of solids, is used to study the heat transfer in nanostructures. The thermal conductivity is obtained by summing the contribution of all the vibration modes of the system. All the vibrational properties (dispersion curves and relaxation time) that are used in the model are obtained using the data for bulk samples. The size effect is taken into account through the sampling of the Brillouin zone and the distance that a wave vector can travel between two boundaries in the structure. The model is used to predict the thermal conductivity of silicon nanowires and nanofilms, and demonstrates a good agreement with experimental results. Finally, using this model, the quality of the silicon interatomic potential, used for molecular-dynamics simulations of heat transfer, is evaluated.
Characterization of core and edge turbulence in L- and enhanced Dα H-mode Alcator C-Mod plasmas
Authors: Basse N. P. , Edlund E. M. , Ernst D. R. , Fiore C. L. , Greenwald M. J. , Hubbard A. E. , Hughes J. W. , Irby J. H. , Lin L. , Lin Y. , Marmar E. S. , Mossessian D. A. , Porkolab M. , Rice J. E. , Snipes J. A. , Stillerman J. A. , Terry J. L. , Wolfe S. M. , Wukitch S. J. , Zhurovich K. , Kramer G. J. , Mikkelsen D. R.
Published: May 2005

The recently upgraded phase-contrast imaging (PCI) diagnostic is used to characterize the transition from the low (L) to the enhanced Dα (EDA) high (H) confinement mode in Alcator C-Mod [I. H. Hutchinson, R. Boivin, F. Bombarda et al., Phys. Plasmas 1, 1511 (1994)] plasmas. PCI yields information on line integrated density fluctuations along vertical chords. The number of channels has been increased from 12 to 32 and the sampling rate from 1 MHz to 10 MHz. This expansion of diagnostic capabilities is used to study broadband turbulence in L and EDA H mode and to analyze the quasicoherent (QC) mode associated with EDA H mode. Changes in broadband turbulence at the transition from L to EDA H mode can be interpreted as an effect of the Doppler rotation of the bulk plasma. Additional fluctuation measurements of Dα light and the poloidal magnetic field show features correlated with PCI in two different frequency ranges at the transition. The backtransition from EDA H to L mode, the so-called enhanced neutron (EN) mode, is investigated by new high frequency (132 and 140 GHz) reflectometer channels operating in the ordinary (O) mode. This additional hardware has been installed in an effort to study localized turbulence associated with internal transport barriers (ITBs). The EN mode is a suitable candidate for this study, since an ITB exists transiently as the outer density decreases much faster than the core density in this mode. The fact that the density decays from the outside inward allows us to study fluctuations progressing towards the plasma core. Our results mark the first localized observation of the QC mode at medium density: 2.2×1020m−3 (132 GHz). Correlating the reflectometry measurements with other fluctuating quantities provides some insight regarding the causality of the EN-mode development.
Optical second-harmonic imaging of PbxCd1−xTe ternary alloys
Authors: Scheidt T. , Rohwer E. G. , von Bergmann H. M. , Saucedo E. , Diéguez E. , Fornaro L. , Stafast H.
Published: May 2005

We employ femtosecond laser pulses (80 fs, 1.59 eV, and 80 MHz) to study the optical second-harmonic (SH) response of PbxCd1−xTe ternary alloys (x about 0.2) grown by the vertical Bridgman method. The alloy segregates into a Pb-rich and a Cd-rich phase, the latter dominating the SH response of the ternary alloy by at least two orders of magnitude. Several sample regions show a regular layer-by-layer accommodation of the Pb-rich and Cd-rich phases as seen by a periodic alternation of the alloy’s SH response on a ∼10-μm length scale. Furthermore, we employ polarization-resolved SH imaging as well as SH imaging at different azimuthal angles to obtain spatially resolved mappings of the sample, which are sensitive to the composition as well as the growth orientation of the PbxCd1−xTe material system. We observe an azimuthal phase shift of approximately 30° between coherent macroscopic regions (several mm2) in the Cd-rich phase of the ternary alloy. We interpret these regions as large area crystalline grains of (111) and (411) crystal orientations and approximately equal composition. Hence, SH imaging is shown to spatially resolve regions of different growth directions within the PbxCd1−xTe sample.
Perovskite-based heterostructures integrating ferromagnetic-insulating La0.1Bi0.9MnO3
Authors: Gajek M. , Bibes M. , Barthélémy A. , Varela M. , Fontcuberta J.
Published: May 2005

We report on the growth of thin films and heterostructures of the ferromagnetic-insulating perovskite La0.1Bi0.9MnO3. We show that the La0.1Bi0.9MnO3 perovskite grows single phased, epitaxially, and with a single out-of-plane orientation either on SrTiO3 substrates or onto strained La2∕3Sr1∕3MnO3 and SrRuO3 ferromagnetic-metallic buffer layers. We discuss the magnetic properties of the La0.1Bi0.9MnO3 films and heterostructures in view of their possible potential as magnetoelectric or spin-dependent tunneling devices.