bluuebunny/crossref_metadata_2025 huggingface dataset:
Mode decomposition scheme for ideal magnetohydrodynamic plane waves in space‐time coordinates
Authors: Podesta J. J. , Goldstein M. L. , Roberts D. A.
Published: June 2007

Exact nonsinusoidal plane wave solutions of the linearized equations of ideal magnetohydrodynamics are used to develop a decomposition scheme for extracting the forward and backward propagating components of the fast, slow, and Alfvén modes from measured data. The decomposition technique is formulated in the space‐time domain for waves propagating in one direction. The different wave modes are extracted by means of projection operators that are expressed in matrix form. Because the elements of these matrices are constants, the same projection operators can be used to obtain the mode decomposition in the frequency (Fourier) domain. The projection operators are identical to those obtained by Glassmeier et al. (1995) although they are derived here by different means. In the case of wave propagation parallel or perpendicular to the background magnetic field the wave modes are degenerate and require separate treatment. These special cases are included in the present analysis so the resulting decomposition scheme encompasses all possible propagation directions.
Off‐fault damage patterns due to supershear ruptures with application to the 2001 Mw 8.1 Kokoxili (Kunlun) Tibet earthquake
Authors: Bhat Harsha S. , Dmowska Renata , King Geoffrey C. P. , Klinger Yann , Rice James R.
Published: June 2007

We extend a model of a two‐dimensional self‐healing slip pulse, propagating dynamically in steady state with slip‐weakening failure criterion, to the supershear regime in order to study the off‐fault stressing induced by such a slip pulse and investigate features unique to the supershear range. Specifically, we show that there exists a nonattenuating stress field behind the Mach front that radiates high stresses arbitrarily far from the fault (practically this would be limited to distances comparable to the depth of the seismogenic zone), thus being capable of creating fresh damage or inducing Coulomb failure in known structures at large distances away from the main fault. We allow for both strike‐slip and dip‐slip failure induced by such a slip pulse. We show that off‐fault damage is controlled by the speed of the slip‐pulse, scaled stress drop, and principal stress orientation of the prestress field. We apply this model to study damage features induced during the 2001 Kokoxili (Kunlun) event in Tibet, for which it has been suggested that much of the rupture was supershear. We argue that an interval of simultaneous induced normal faulting is more likely due to a slip partitioning mechanism suggested previously than to the special features of supershear rupture. However, those features do provide an explanation for otherwise anomalous ground cracking at several kilometers from the main fault. We also make some estimates of fracture energy which, for a given net slip and dynamic stress drop, is lower than for a sub‐Rayleigh slip pulse because part of the energy fed by the far‐field stress is radiated back along the Mach fronts.
Power variations of Schumann resonances related to El Niño and La Niña phenomena
Authors: Yang Heng , Pasko Victor P.
Published: June 2007

A three dimensional Finite Difference Time Domain (FDTD) model of the Earth‐ionosphere cavity with the realistic conductivity profile is employed to study the intensity variations of Schumann resonances (SR) associated with the El Niño and La Niña phenomena. Comparison of the results derived from our FDTD model and the previous studies by other authors on related subjects shows that the intensity of the Schumann resonances varies with the spatial shifts of the thunderstorm regions under El Niño and La Niña conditions. Due to the different spatial field distributions of SR electrical and magnetic components in the Earth‐ionosphere cavity, the different power variation patterns are clearly observed in the electrical and magnetic components with the motion of the thunderstorm center in our FDTD results. A new method is proposed to detect the shifts of the thunderstorm regions related to the El Niño and La Niña phenomena using the combination of both electrical and magnetic components of Schumann resonances at a single station.
Parenteral long-acting amoxicillin reduces intestinal bacterial community diversity in piglets even 5 weeks after the administration
Authors: Janczyk Pawel , Pieper Robert , Souffrant Wolfgang Bernhard , Bimczok Diane , Rothkötter Hermann-Josef , Smidt Hauke
Published: June 2007

We investigated the long-term effects of a single intramuscular administration of amoxicillin (15 mg kg−1) 1 day after birth, on piglet intestinal microbiota. Animals received no creep feed before weaning on day 28 of age. For the next 11 days, the piglets received a wheat–barley-based diet. Colon digesta samples were collected on day 39 and subjected to denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S rRNA gene fragments. DGGE fingerprint diversity indices differed between the group treated with amoxicillin and the untreated group (0.8±0.19 and 1.03±0.17, respectively, P=0.012). Reamplification and sequencing of two bands present in all samples revealed that a Roseburia faecalis-related population was strongly reduced in relative abundance (98% identity) in the treated group, while an enterobacterial population with 100% identity to Shigella spp., Escherichia coli and Salmonella enterica serovar Typhi was enriched. A band corresponding to Lactobacillus sobrius was present only in the control group. The protective effect of prophylactic antibiotic administration may be outweighed by the long-lasting disturbance of the gut ecosystem.
Metabolism, cytoskeleton and cellular signalling in the grip of protein Nϵ ‐ and O‐acetylation
Authors: Yang Xiang‐Jiao , Grégoire Serge
Published: June 2007

Acetylation of the ϵ‐amino group of lysine residues (Nϵ‐acetylation) is a reversible post‐translational modification with the potential to rival phosphorylation. In addition to histones and many transcription factors such as p53, regulators of DNA repair, replication and recombination are subject to Nϵ‐acetylation. This modification is also important for governing the activities of various enzymes, including histone acetyltransferases, histone deacetylases, bacterial and mammalian acetyl‐CoA synthases, kinases, phosphatases, the ubiquitin ligase murine double minute 2 and the chaperonin heat shock protein 90. Furthermore, lysine acetylation occurs in cellular structure proteins such as α‐tubulin, actin, cortactin and p120 catenin. Strikingly, the Yersinia outer protein YopJ promotes O‐acetylation of crucial serine and threonine residues that are required for activation of the MAPK/ERK kinase and IκB kinase families, which precludes their phosphorylation and blocks signal transduction. Thus, Nϵ‐ and O‐acetylation are becoming recognized as two prominent mechanisms for regulating protein functions in diverse organisms.
Role of the placenta in fetal programming: underlying mechanisms and potential interventional approaches
Authors: Jansson Thomas , Powell Theresa L.
Published: June 2007

Adverse influences during fetal life alter the structure and function of distinct cells, organ systems or homoeostatic pathways, thereby ‘programming’ the individual for an increased risk of developing cardiovascular disease and diabetes in adult life. Fetal programming can be caused by a number of different perturbations in the maternal compartment, such as altered maternal nutrition and reduced utero–placental blood flow; however, the underlying mechanisms remain to be fully established. Perturbations in the maternal environment must be transmitted across the placenta in order to affect the fetus. Here, we review recent insights into how the placenta responds to changes in the maternal environment and discuss possible mechanisms by which the placenta mediates fetal programming. In IUGR (intrauterine growth restriction) pregnancies, the increased placental vascular resistance subjects the fetal heart to increased work load, representing a possible direct link between altered placental structure and fetal programming of cardiovascular disease. A decreased activity of placental 11β-HSD-2 (type 2 isoform of 11β-hydroxysteroid dehydrogenase) activity can increase fetal exposure to maternal cortisol, which programmes the fetus for later hypertension and metabolic disease. The placenta appears to function as a nutrient sensor regulating nutrient transport according to the ability of the maternal supply line to deliver nutrients. By directly regulating fetal nutrient supply and fetal growth, the placenta plays a central role in fetal programming. Furthermore, perturbations in the maternal compartment may affect the methylation status of placental genes and increase placental oxidative/nitrative stress, resulting in changes in placental function. Intervention strategies targeting the placenta in order to prevent or alleviate altered fetal growth and/or fetal programming include altering placental growth and nutrient transport by maternally administered IGFs (insulin-like growth factors) and altering maternal levels of methyl donors.
The collision-induced polarizability of a pair of hydrogen molecules
Authors: Li X. , Ahuja C. , Harrison J. F. , Hunt K. L. C.
Published: June 2007

Collision-induced light scattering, impulsive stimulated scattering, and subpicosecond-induced birefringence all depend on the transient changes Δα in molecular polarizabilities that occur when molecules collide. Ab initio results for Δα are needed to permit comparisons with accurate experimental results for these spectra and for refractive index virial coefficients and dielectric virial coefficients. In this work, we provide results for Δα for a pair of hydrogen molecules, treated at CCSD(T) level, with an aug-cc-pV5Z (spdf) basis set. Our values replace the best previous ab initio results for the variation of Δα with intermolecular separation, the self-consistent-field results obtained by Bounds [Mol. Phys. 38, 2099 (1979)] with a relatively small (3s2p) basis set for H2. For the six geometrical configurations studied by Bounds, the inclusion of correlation and improvements in the basis tend to increase both the trace Δα00 and the anisotropy Δα2m of the pair polarizability. The change in the anisotropy is relatively small, but our values for the trace differ by factors of 2 or more from Bounds’ results. For use in computing experimental line shapes, intensities, and virial coefficients, we have calculated Δα for 18 different relative orientations of a pair of H2 molecules, with the intermolecular separation R ranging from 2a.u. (3a.u. for a linear pair) to 10a.u. The H2 bond length is fixed at the vibrationally averaged internuclear separation in the ground state r=1.449a.u. Our results agree well with the CCSD(T) results for Δα obtained by Maroulis [J. Phys. Chem. A 104, 4772 (2000)] for two pair configurations of H2⋯H2 (linear and T-shaped) at a fixed internuclear distance of R=6.5a.u. in a [6s4p1d] basis. As the intermolecular distance increases (for R⩾8a.u.), the spherical-tensor components of Δα converge to the results from a long-range model that includes dipole-induced-dipole (DID) interactions, higher-multipole induction, nonuniformity of the local field, hyperpolarization, and van der Waals dispersion. Deviations from the first-order DID model are still evident for R between 8 and 10a.u. in most orientations of the pair. At shorter range, overlap damping, exchange, and orbital distortion reduce both Δα00 and Δα20 below their long-range limiting forms.
Spectroscopic Data for Neutral Francium (FrI)
Authors: Sansonetti J. E.
Published: June 2007

Energy levels and hyperfine structure constants have been compiled for the sixteen longest lived isotopes of francium (Z=87). For most isotopes with atomic weights in the range 199 ≤A≤232 the only measurements made are for the 7sS1∕22 and 7pP1∕2,3∕2°2 levels. Additional energy-level data are available for Fr210, Fr212, and Fr221. Wavelengths with classifications and transition probabilities are tabulated for Fr212. In addition, the ionization energy is included for isotopes for which a sufficient number of levels have been measured, Fr212 and Fr221.
Half-Skyrmions and spike-vortex solutions of two-component nonlinear Schrödinger systems
Authors: Lin Tai-Chia , Wei Juncheng
Published: June 2007

Recently, Skyrmions with integer topological charges have been observed numerically but have not yet been shown rigorously on two-component systems of nonlinear Schrödinger equations (NLSEs) describing a binary mixture of Bose-Einstein condensates [see Battye et al., Phys. Rev. Lett. 88, 080401 (2002) and Savage and Ruostekoski, Phys. Rev. Lett. 91, 010403 (2003)] Besides, half-Skyrmions characterized by half-integer topological charges can also be found in the nonlinear σ model which is a model of the Bose-Einstein condensate of the Schwinger bosons [see Morinari, Phys. Rev. B 72, 104502 (2005)]. Here we prove rigorously the existence of half-Skyrmions which may come from a new type of soliton solutions called spike-vortex solutions of two-component systems of NLSE on the entire plane R2. These spike-vortex solutions having spikes in one component and a vortex in the other component may form half-Skyrmions. By Liapunov-Schmidt reduction process, we may find spike-vortex solutions of two-component systems of NLSE.
Blow up of solutions to the nonlinear Schrödinger equations on manifolds
Authors: Ma Li , Zhao Lin
Published: June 2007

In this paper, we partially settle down the long-standing open problem of the finite time blow-up property about the nonlinear Schrödinger equations on some Riemannian manifolds such as the standard 2-sphere S2 and the hyperbolic 2-space H2(−1). Using the similar idea, we establish such blow-up results on higher dimensional standard sphere and hyperbolic n-space. Extensions to n-dimensional Riemannian warped product manifolds with n⩾2 are also given.
Andreev reflection with spin-flip scattering through a T-shaped double quantum dot
Authors: Li Yu-Xian , Choi Han-Yong , Lee Hyun-Woo , Liu Jian-Jun
Published: June 2007

The spin-dependent Andreev reflection (AR) tunneling through a ferromagnet double-quantum-dot superconductor (F-DQD-S) is studied using the nonequilibrium Green’s function method. The F-DQD-S is a T-shaped hybrid structure where one of the dots is coupled to the F and S electrodes and the other dot is coupled to the first dot only. On each dot the intradot spin-flip process is present. The behavior of the F-DQD-S tunneling conductance is determined by the competition among the interdot coupling, the intradot spin-flip process, and the coupling of the first dot to the electrodes. When the interdot coupling is present, the quantum interference effects between the two dots cause secondary peaks in the tunneling conductance besides the primary one, indicating a structure in the AR spectrum. On the other hand, the spin-flip scattering process suppresses the AR peaks. With the spin-flip scattering strength increasing, the resonance peaks are split.
Analytic solution of Hedin’s equations in zero dimensions
Authors: Pavlyukh Y. , Hübner W.
Published: June 2007

Feynman diagrams for the many-body perturbational theory are enumerated by solving the system of Hedin’s equations in zero dimension. We extend the treatment of Molinari [Phys. Rev. B 71, 113102 (2005)] and give a complete solution of the enumeration problem in terms of Whittaker functions. An important relation between the generating function of the electron propagator and anomalous dimension in quantum field theory of massless fermions and mesons in four dimensions (Yukawa theory) is found. The Hopf algebra of undecorated rooted trees yields the anomalous field dimension in terms of the solution of the same differential equation. Its relation to the mathematical problem of combinatorics of chord diagrams is discussed; asymptotic expansions of the counting numbers are obtained.
Spin-pair correlation dependent dynamic properties in magnetic relaxor ferroelectrics
Authors: Xia Q. S. , Jiang Q.
Published: June 2007

In magnetic relaxor ferroelectrics, for the coupling interaction between the relaxor ferroelectricity and magnetic order, the dielectric constant exhibits a sharp increase around the magnetic phase-transition temperature. Based on the consideration that a polarization process is associated with the thermally activated flips of the polar regions, the reorientable polar regions have the main contribution for the dielectric behavior. Considering the specialty of relaxor ferroelectrics, a distribution function for the relaxation times is introduced to simulate the dielectric behavior of the magnetic relaxor ferroelectrics. It is indicated that the dielectric anomaly below the magnetic phase-transition temperature is attributed to the fact that the activation energy against dipole reorientation is amended by the spin-pair correlation via the magnetoelectric coupling. The obtained real part of dielectric constants and the magnetocapacitance are in good agreement with the experimental results.
Electronic quenching of OH AΣ+2 radicals in single collision events with molecular hydrogen: Quantum state distribution of the OH XΠ2 products
Authors: Cleary Patricia A. , Dempsey Logan P. , Murray Craig , Lester Marsha I. , Kłos Jacek , Alexander Millard H.
Published: June 2007

We report a combined experimental and theoretical investigation of the nonreactive quenching channel resulting from electronic quenching of OH AΣ+2 by molecular hydrogen. The experiments utilize a pump-probe scheme to determine the OH XΠ2 population distribution following collisional quenching in a pulsed supersonic expansion. The pump laser excites OH AΣ+2 (ν′=0, N′=0), which has a significantly reduced fluorescence lifetime due to quenching by H2. The probe laser monitors the OH XΠ2 (ν″, N″) population via laser-induced fluorescence on various A-X transitions under single collision conditions. The experiments reveal a high degree of rotational excitation (N″) of the quenched OH XΠ2 products observed in ν″=1 and 2 as well as a pronounced propensity for quenching into the Π(A′) Λ-doublet level. These experiments have been supplemented by extensive multireference, configuration-interaction calculations aimed at exploring the topology of the relevant potential energy surfaces. Electronic quenching of OH AΣ+2 by H2 proceeds through conical intersections between two potentials of A′ reflection symmetry (in planar geometry) that correlate with the electronically excited AΣ+2 and ground XΠ2 states of OH. The conical intersections occur in high-symmetry geometries, in which the O side of OH points toward H2. Corroborating and extending earlier work of Hoffman and Yarkony [J. Chem. Phys. 113, 10091 (2000)], these calculations reveal a steep gradient away from the OH–H2 conical intersection as a function of both the OH orientation and interfragment distance. The former will give rise to a high degree of OH rotational excitation, as observed for the quenched OH XΠ2 products.
Qualitative distinction in enhancement of the piezoelectric response in PbTiO3 in proximity of coercive fields: 90° versus 180° switching
Authors: Budimir Marko , Damjanovic Dragan , Setter Nava
Published: June 2007

Piezoelectric properties of ferroelectric crystals can be greatly enhanced in the proximity of electric and mechanical coercive fields. While the thermodynamic origin of the enhancement in both cases can be traced to the flattening of a free energy profile, we show using the Landau–Ginzburg–Devonshire theory that there is a qualitative difference in how the proximity of 90° and 180° polarization switching affects the piezoelectric response and its anisotropy. Taking as an example PbTiO3 monodomain single crystals biased by near-coercive electric and stress fields applied along the polar axis, it is shown that the antiparallel electric field softens the material along the polar direction while the compressive stress softens the crystal along nonpolar directions. In the former cases the piezoelectric enhancement is dominated by the polarization contraction along the polar axis while in the latter case it is dominated by the polarization rotation away from the polar axis. An analogy is made between the effects of polarization switching and thermally induced phase transitions on the piezoelectric anisotropy.
The effect of alkali niobate addition on the phase stability and dielectric properties of Pb(Zn1∕3Nb2∕3)O3 based ceramic
Authors: Park Jong-Sung , Lee Jung-Kun , Hong Kug Sun
Published: June 2007

While Pb(Zn1∕3Nb2∕3)O3–PbTiO3 (PZN-PT) single crystals have shown superior ferroelectric properties, less scientific and technical interests have been placed on PZN-PT polycrystalline ceramics due to their poor thermodynamic stability and the difficult processing conditions. Here, we stabilized the PZN-PT based ceramics by adding alkali niobates such as NaNbO3 (NN) and KNbO3 (KN) and investigated their structure and dielectric properties. Two stabilization mechanisms are suggested in alkali niobate added PZN-PT ceramics, increased tolerance factor and enhanced electronegativity difference. KN stabilized the perovskite structure of PZN-PT based ceramics more effectively than NN. Both PZN-PT-KN and PZN-PT-NN ceramics showed the typical behavior of relaxor ferroelectrics. The temperature of maximum dielectric constant of PZN-PT-NN was slightly higher than that of the PZN-PT-KN, which was explained by the difference in ionic size and B-site ordering.
Surface-morphology evolution during growth-interrupt in situ annealing on GaAs(110) epitaxial layers
Authors: Yoshita Masahiro , Akiyama Hidefumi , Pfeiffer Loren N. , West Ken W.
Published: June 2007

Temperature and surface-coverage dependence of the evolution of surface morphology during growth-interrupt in situ annealing on GaAs epitaxial layers grown on the singular (110) cleaved edges by the cleaved-edge overgrowth method with molecular-beam epitaxy has been studied by means of atomic force microscopy. Annealing at substrate temperatures below 630 °C produced atomically flat surfaces with characteristic islands or pits, depending on the surface coverage. The atomic flatness of the surfaces is enhanced with increasing annealing temperature owing to the enhanced adatom migration. At a higher annealing temperature of about 650 °C, however, 2-monolayer-deep triangular pits with well-defined step edges due to Ga-atom desorption from the crystal appeared in the atomically flat surface. The growth-interrupt annealing temperature optimal for the formation of atomically flat GaAs(110) surfaces is therefore about 630 °C.
Magnetic interactions in ε-Fe3N–GaN nanocomposites
Authors: Gajbhiye N. S. , Bhattacharyya Sayan
Published: June 2007

ε - Fe 3 N – Ga N nanocomposites (∼25nm) are synthesized by the precursor technique according to the ε-Fe3N∕GaN mass ratios of 85∕15, 73∕27, 62∕38, and 54∕46. For ε-Fe3N∕GaN 85∕15 system, single phase ε-Fe2.8Ga0.2N is obtained. For the nanocomposites, GaN forms the shell over the ε-Fe3N core, with a large interface region. The GaN shell protects the ferromagnetic (FM) ε-Fe3N core from surface oxidation effects. The ε-Fe2.8Ga0.2N system represents the magnetically diluted system, and significant surface oxidation is observed resulting in antiferromagnetic (AF) oxynitride/oxide surface layer. Exchange bias is observed in ε-Fe2.8Ga0.2N system, and the hysteresis loop shift of 19Oe is observed in the positive direction of the cooling field indicating antiparallel/perpendicular alignment of spins across the AF-FM interface. For the ε-Fe3N–GaN nanocomposites, spin-glass-like ordering is observed below 50K in the zero-field cooled curves which is confirmed from relaxation experiments and ac magnetic susceptibility measurements. Spin-glass-like ordering results from the random exchange anisotropy due to the random distribution of ε-Fe3N in the GaN matrix in the core/shell interface region.
Thermochemistry of Oxabicyclo-Heptanes, Oxabicyclo-Heptene: Enthalpy of Formation, Entropy, Heat Capacity, and Group Additivity
Authors: Bozzelli Joseph W. , Rajasekaran Indumathi
Published: June 2007

Thermochemical properties, ΔfH°(298), S°(298) and [Cp°(T)] (10K⩽T⩽5000) of the seven target bicyclic epoxides are calculated using the density functional methods. Molecular structures and vibration frequencies for 6-oxabicyclo[3.1.0]hexane, 7-oxabicyclo[4.1.0]heptane, 8-oxabicyclo[5.1.0]octane, 7-oxabicyclo[2.2.1]heptane, endo-2-methyl-7-oxabicyclo[2.2.1]heptane, exo-2-methyl-7-oxabicyclo[2.2.1]heptane and 7-oxabicyclo[4.1.0]hept-2,4-ene are calculated at the B3LYP∕6-31G(d,p) level of theory. Enthalpies of formation are determined at B3LYP∕6-31G(d,p) calculation level using isodesmic and homodesmic working reactions with the ΔrxnH° and known enthalpies of reference species in each of three different work reactions. Entropy (298) and heat capacity [Cp°(T)] values along with Benson Group additivity parameters are reported for each ring system. Data previously reported on oxybicyclo-epoxides are summarized.
Metamaterials for ballistic electrons
Authors: Dragoman D. , Dragoman M.
Published: June 2007

This article presents a metamaterial for ballistic electrons, which consists of a quantum barrier formed in a semiconductor with negative effective electron mass. This barrier is the analog of a metamaterial for electromagnetic waves in media with negative electrical permittivity and magnetic permeability. Besides applications similar to those of optical metamaterials, a nanosized slab of a metamaterial for ballistic electrons, sandwiched between quantum wells of positive effective mass materials, reveals unexpected conduction properties, e.g., single or multiple room temperature negative differential conductance regions at very low voltages and with considerable peak-to-valley ratios, while the traversal time of ballistic electrons can be tuned to larger or smaller values than in the absence of the metamaterial slab. Thus, slow and fast electrons, analogous to slow and fast light, occur in metamaterials for ballistic electrons.
Localized exchange-correlation potential from second-order self-energy for accurate Kohn-Sham energy gap
Authors: Fabiano E. , Della Sala F.
Published: June 2007

A local Kohn-Sham (KS) exchange-correlation potential is derived by localizing the second-order self-energy operator, using approximations to the linear response Sham-Schlüter equation. Thanks to the use of the resolution-of-identity technique for the calculation of the self-energy matrix elements, the method is very efficient and can be applied to large systems. The authors investigate the KS energy gaps and lowest excitation energies of atoms and small- and medium-size molecules. Reference KS energy gaps (from accurate densities) of atoms and small molecules can be reproduced with great accuracy. For larger systems they found that the KS energy gap is smaller than the one obtained from the local-density approximation, showing the importance of an ab initio correlation in the Kohn-Sham potential.
Embedded clustering in Cr-doped AlN: Evidence for general behavior in dilute magnetic III-nitride semiconductors
Authors: Cui X. Y. , Fernandez-Hevia D. , Delley B. , Freeman A. J. , Stampfl C.
Published: June 2007

We report a systematic density-functional theory investigation of the “structure-property relationship” of Cr:AlN by doping up to 5 Cr atoms in large supercells, for which exhaustive structural and magnetic configurations have been calculated—including full atomic relaxation. Our results demonstrate that the Cr atoms tend to segregate to form Cr-N-Cr bonded clusters, which are embedded in the AlN host wurtzite structure. Significantly, while the ferromagnetic state with a spin moment close to 3 μB∕Cr is the ground state for both isolated “single” and “pair” doping configurations, for larger cluster configurations states containing antiferromagnetic or ferrimagnetic coupling with net spin in the range of 0−1.53 μB∕Cr are found to be energetically more favorable. Electrical conductivity (half-metallic or insulating) is predicted to be sensitively dependent on the dopant concentration. We propose a picture that various sized Cr-N-Cr bonded clusters coexist and the statistical distribution and associated magnetic properties will depend sensitively on the growth conditions. Such a scenario is in agreement with recent experiments and can help understand a number of hitherto puzzling experimental observations, notably the low mean saturation magnetic moment, the contracted lattice constants, and the highly insulating behavior.
Exact solution of Dirac equation in 2+1 dimensional gravity
Authors: Sucu Yusuf , Ünal Nuri
Published: June 2007

We find exact solutions of the Dirac equation in the 2+1 dimensional curved background by separation of variables. These solutions are given in terms of hypergeometric functions. We also perform the Gordon decomposition for the Dirac current to discuss the time dependence of the polarization densities and the magnetization density, and to show that the polarization densities are more effective than the magnetization density in the pair production in finite time intervals.
Complementary neutron and x-ray reflectivity studies of “near-frictionless” carbon films
Authors: Johnson Jacqueline A. , Park Sungkyun , Roy Sujoy , Sinha Sunil K. , Erdemir Ali , Eryilmaz Osman L. , Woodford John B.
Published: June 2007

“Near-frictionless carbon,” a form of amorphous hydrogenated diamond-like-carbon developed at Argonne National Laboratory, has been investigated by neutron and x-ray reflectivity under ambient conditions, in vacuum, and at elevated temperature. A series of films was made with different ratios of hydrogen to carbon in the plasma; the deuterated counterparts were also synthesized. The details of scattering length density profiles, thickness, and surface and interfacial roughness of the films were obtained from neutron and x-ray reflectivity profiles. This allowed us to calculate the mass density and hydrogen composition of the bulk carbon film, leading to insight into the frictional properties. From the analysis we found that the magnitude of the scattering length density of the top surface layer was greatly affected by the experimental atmosphere in which the films were contained although the trend with hydrogen in the plasma remained the same. All samples were fitted with a three-layer model in addition to the substrate.
Electrowetting on plasma-deposited fluorocarbon hydrophobic films for biofluid transport in microfluidics
Authors: Bayiati P. , Tserepi A. , Petrou P. S. , Kakabakos S. E. , Misiakos K. , Gogolides E.
Published: June 2007

The present work focuses on the plasma deposition of fluorocarbon (FC) films on surfaces and the electrostatic control of their wettability (electrowetting). Such films can be employed for actuation of fluid transport in microfluidic devices, when deposited over patterned electrodes. Here, the deposition was performed using C4F8 and the plasma parameters that permit the creation of films with optimized properties desirable for electrowetting were established. The wettability of the plasma-deposited surfaces was characterized by means of contact angle measurements (in the static and dynamic mode). The thickness of the deposited films was probed in situ by means of spectroscopic ellipsometry, while the surface roughness was provided by atomic force microscopy. These plasma-deposited FC films in combination with silicon nitride, a material of high dielectric constant, were used to create a dielectric structure that requires reduced voltages for successful electrowetting. Electrowetting experiments using protein solutions were conducted on such optimized dielectric structures and were compared with similar structures bearing commercial spin-coated Teflon® amorphous fluoropolymer (AF) film as the hydrophobic top layer. Our results show that plasma-deposited FC films have desirable electrowetting behavior and minimal protein adsorption, a requirement for successful transport of biological solutions in “digital” microfluidics.
Conservation laws for the Maxwell-Dirac equations with dual Ohm’s law
Authors: Ibragimov Nail H. , Khamitova Raisa , Thidé Bo
Published: June 2007

Using a general theorem on conservation laws for arbitrary differential equations proved by Ibragimov [J. Math. Anal. Appl. 333, 311–320 (2007)], we have derived conservation laws for Dirac’s symmetrized Maxwell-Lorentz equations under the assumption that both the electric and magnetic charges obey linear conductivity laws (dual Ohm’s law). We find that this linear system allows for conservation laws which are nonlocal in time.
Quantum layers over surfaces ruled outside a compact set
Authors: Lin Christopher , Lu Zhiqin
Published: June 2007

In this paper, we prove that quantum layers over a surface which is ruled outside a compact set, nonplanar but asymptotically flat, admit a ground state for the Dirichlet Laplacian. The work here also represents some technical progress toward resolving the conjecture that a quantum layer over any nonplanar, asymptotically flat surface with integrable Gauss curvature must possess ground state.
Deep level transient spectroscopy study of defects in hydrogen implanted p-type 4H-SiC
Authors: Alfieri Giovanni , Kimoto Tsunenobu
Published: June 2007

p -type 4H-SiC epitaxial layers grown by chemical vapor deposition have been implanted with 200 and 100keV protons at five different implantation temperatures. An isochronal annealing series was performed from 100to1800°C, and Al-doped epitaxial layers have been characterized by means of deep level transient spectroscopy (DLTS) after each annealing step. DLTS measurements were carried out in the 150–670K temperature range and revealed the presence of eight hole traps located in the 0.18–1.8eV range above the valence band (EV). Heat treatments for temperatures above 700°C showed the progressive reactivation of the Al doping in the implanted region, which is completed after a 1500°C annealing treatment. Two traps located at EV+0.44eV and EV+1.8eV are persistent even after annealing at 1800°C, while the other traps anneal out after heat treatments at ⩽1700°C. An activation energy for dissociation of 6.2eV is estimated for the hole trap at EV+0.79eV, and the nature of this defect is discussed on the basis of previous experimental results and theoretical calculations. Furthermore, the study of the annealing behavior as a function of the implantation temperature shows that the detected traps display an increase of concentration for increasing implantation temperatures.
Thermochemistry of new molecular species: SBr and HSBr
Authors: Ornellas Fernando R.
Published: June 2007

Total energies, optimized geometries, and vibrational frequencies of SBr and HSBr have been evaluated at the coupled cluster level of theory with the correlation consistent basis sets. Extrapolated to the complete basis set limit and with corrections for core-valence, scalar relativistic, and spin-orbit effects, atomization energies were computed and then combined with the experimental heats of formation of the atomic species to generate very accurate heats of formation for the species SBr and HSBr. For SBr, we predict 37.45 and 36.07kcal∕mol for ΔHf(0K) and ΔHf (298.15K), respectively, in very good agreement with the inferred experimental values of 37.98 and 36.15kcal∕mol. For HSBr, the estimate turns out to be 10.38 and 8.29kcal∕mol for ΔHf (0K) and ΔHf (298.15K), respectively. Using the more recent HBrO experimental heat of formation at 298.15K of Lock et al., [J. Phys. Chem. 100, 7972 (1996)] the inferred experimental value for HSBr is predicted to be 8.15kcal∕mol, compared with 8.65kcal∕mol derived from the data of Ruscic and Berkowitz [J. Chem. Phys. 101, 7795 (1994)]. Considering the better agreement of the result with that predicted using the experimental value of ΔHf(298.15K) of Lock et al., the author also supports the suggestion made by Denis [J. Phys. Chem. A. 110, 5887 (2006)] that the result of Lock et al. should be preferred over the one of Ruscic and Berkowitz. For ΔHf(0K), the author found 10.38 and 10.56kcal∕mol, respectively, for the theoretical and inferred experimental estimates.
Effects of dimensional nanoscaling on the optical and electrical properties of crystalline Si thin films
Authors: Sharma A. K. , Logofãtu P. C. , Mayberry C. S. , Brueck S. R. J. , Islam N. E.
Published: June 2007

Thin film Si structures between 10 and 200nm in thickness and configured into two terminal metal-semiconductor-metal structures have been characterized for optical and electrical properties. Dark currents, spectral response, dc quantum efficiency, and ultrafast time response up to 400nm femtosecond laser illuminations at low fields have been studied. Dark currents and dc photocurrent measurements showed an increase in the film conductivity between 75 and 35nm, suggesting an increase in the carrier effective velocities due to confinement. An increase in the carrier effective velocity below 75nm was also confirmed through the transient response analysis. The measured spectral responses are in good agreement with Fresnel’s theoretical model for thin film coupling. The electron-limited transient signal has a full width at half maximum (FWHM) approximately 40ps for the 10nm Si film as compared to 490ps for a 200nm structure. For a hole-limited transit time signal the FWHM was about 82ps for the 10nm film as compared to 2.5ns for the 200nm film reduction in FWHM for both electrons and holes for the 10nm film, signifying that carriers travel much faster as the film thickness is reduced.
Maximally symmetric vector propagator
Authors: Tsamis N. C. , Woodard R. P.
Published: June 2007

We derive the propagator for a massive vector field on a de Sitter background of arbitrary dimension. This propagator is de Sitter invariant and possesses the proper flat space-time and massless limits. Moreover, the retarded Green’s function inferred from it produces the correct classical response to a test source. Our result is expressed in a tensor basis which is convenient for performing quantum-field-theory computations using dimensional regularization.
Nonlinearity and dynamic phase transition of charge-density-wave lattice
Authors: Du Chao-hung , Lo Chung-Yu , Lin Hsiu-Hau , Chang Shih-Lin
Published: June 2007

We report the investigation of the dynamic behavior of charge-density waves (CDWs) in a quasi-one-dimensional material K0.3MoO3 using x-ray scattering and multiple x-ray diffraction. Under the application of voltages, we demonstrate that the occurrence of nonlinear conductivity caused by CDW is through the internal deformation of the CDW lattice, i.e., a phase jump of 2π, as the applied voltage exceeds the threshold. By measuring the evolution of peak width of satellite reflections as a function of the field strength, we also report that the CDW lattice can be driven to move and undergo a dynamic phase transition, i.e., from the disordered pinning state to ordered moving solid state, and finally, to disordered moving liquid.
A ray-tracing analysis of the absorption of light by smooth and rough metal surfaces
Authors: Bergström D. , Powell J. , Kaplan A. F. H.
Published: June 2007

Ray tracing has been employed to investigate the absorption of light by smooth and random rough metal surfaces. For normally incident light the absorptance of the surface increases with surface roughness. However, for light incident at a tangent to the surface the absorptance-surface roughness relationship is more complex. For example, in certain cases the absorptance can rise, fall, and rise again as the surface roughness increases. In this paper this complex absorptance-roughness relationship is defined and explained. The wavelengths of the light chosen for this study correspond to the primary and secondary output wavelengths of Nd:YAG lasers.
Structural and compositional characterization of a Co∕Re multilayer and superlattice
Authors: Xu Wentao , De Long Lance E. , Charlton Timothy , Chisholm Matthew , Lederman David
Published: June 2007

The structure and composition of a Co∕Re trilayer and a 19-period superlattice were characterized by high-resolution transmission electron microscopy (TEM) and scanning TEM (STEM). Low-angle x-ray reflectivity measurements were performed and compared with the TEM results. The Re and Co layers are epitaxial with their (101¯0) planes parallel to the (112¯0) plane of Al2O3, and the [001] direction of Re and Co layers coincides with that of the Al2O3. The in-plane lattice parameters for Co, Re, and Al2O3 are approximately 0.24 and 0.43nm, 0.26 and 0.44nm, and 0.24 and 0.44nm, respectively, in the superlattice. The lattice spacing of Al2O3 corresponds to a∕2 and c∕3, where a and c are lattice parameters of the Al2O3. High-angle and low-angle annular-dark-field STEM and nanoscale electron energy loss spectroscopy line analysis exhibit very weak interdiffusion between Co and Re layers; therefore, very sharp interfaces are maintained in the superlattice. The initial interface roughness between the Re buffer layer and the first Co layer is amplified during the subsequent growth of the superlattices. Layer thickness fluctuations are much smaller in the Re∕Co superlattice than the interface roughness, which suggests that interface roughness plays a more important role in the giant magnetoresistance effect than thickness fluctuations of the spacer layer.
Micromechanical sensor for studying heats of surface reactions, adsorption, and cluster deposition processes
Authors: Antonietti Jean-Marie , Gong Jiong , Habibpour Vahideh , Röttgen Martin A. , Abbet Stéphane , Harding Christopher J. , Arenz Matthias , Heiz Ulrich , Gerber Christoph
Published: June 2007

We present a newly designed highly sensitive micromechanical sensor devoted to thermodynamic studies involving supported clusters. The thermally sensitive element of the sensor consists of a micromachined silicon cantilever array, onto which a thin metal film is evaporated. Due to the difference between the thermal expansion coefficients of silicon and the metal employed, thermal bending is observed when heat is exchanged with the cantilever. The sensitivity and the response time of the cantilever are studied as a function of the film material (gold or aluminum) and the thickness of the metal film. With our routinely prepared cantilevers, a minimum power of 120nW is measurable with a submillisecond response time, corresponding to a limit of detection in the femtojoule range. The high sensitivity of the sensor is demonstrated by measuring the heat exchange which occurs during the deposition of clusters on the cantilever. Experimentally, we illustrate the 1,3-butadiene hydrogenation reaction using a cluster model catalysts created by soft-landing palladium clusters onto the cantilever surface.
Magnetic frustration in the site ordered Mg3Fe4(VO4)6 vanadate
Authors: Guskos N. , Likodimos V. , Glenis S. , Zolnierkiewicz G. , Typek J. , Szymczak R. , Blonska-Tabero A.
Published: June 2007

The magnetic properties of the site ordered multicomponent vanadate Mg3Fe4(VO4)6 are studied using dc magnetization and electron paramagnetic resonance (EPR) measurements. The static susceptibility shows antiferromagnetic interactions between Fe3+ spins with a Curie-Weiss temperature Θ=−111(1)K, while a transition to a spin-glass-like state is observed at T≈8.5K, indicating appreciable spin frustration. EPR measurements corroborate the presence of antiferromagnetically coupled Fe3+ spins from high temperatures, while a distinct change in the temperature variation of the EPR parameters is observed at T<80K, complying with the mean-field energy scale provided by the Curie-Weiss temperature of the system. The resulting magnetic inhomogeneity that persists despite the absence of cation disorder between magnetic and diamagnetic metal ions is attributed to the presence of small amounts of oxygen deficiency that modifies the connectivity and superexchange coupling between Fe3+ spins and leads to a disordered magnetic ground state.
On the role of chemical reactions in initiating ultraviolet laser ablation in poly(methyl methacrylate)
Authors: Prasad Manish , Conforti Patrick F. , Garrison Barbara J.
Published: June 2007

The role of chemical reactions is investigated versus the thermal and mechanical processes occurring in a polymer substrate during irradiation by a laser pulse and subsequent ablation. Molecular dynamics simulations with an embedded Monte Carlo based reaction scheme were used to study ultraviolet ablation of poly(methyl methacrylate) at 157nm. We discuss the onset of ablation, the mechanisms leading to ablation, and the role of stress relaxation of the polymer matrix during ablation. Laser induced heating and chemical decomposition of the polymer substrate are considered as ablation pathways. It is shown that heating the substrate can set off ablation via mechanical failure of the material only for very short laser pulses. For longer pulses, the mechanism of ejection is thermally driven limited by the critical number of bonds broken in the substrate. Alternatively, if the photon energy goes towards direct bond breaking, it initiates chemical reactions, polymer unzipping, and formation of gaseous products, leading to a nearly complete decomposition of the top layers of substrates. The ejection of small molecules has a hollowing out effect on the weakly connected substrates which can lead to lift-off of larger chunks. Excessive pressure buildup upon the creation of gaseous molecules does not lead to enhanced yield. The larger clusters are thermally ejected, and an entrainment of larger polymer fragments in gaseous molecules is not observed.
Solvent effects on two-photon absorption of dialkylamino substituted distyrylbenzene chromophore
Authors: Zhao Ke , Ferrighi Lara , Frediani Luca , Wang Chuan-Kui , Luo Yi
Published: June 2007

Solvent effects on the two-photon absorption of a symmetrical diamino substituted distyrylbenzene chromophore have been studied using the density functional response theory in combination with the polarizable continuum model. It is shown that the dielectric medium has a rather small effect both on the bond length alternation and on the one-photon absorption spectrum, but it affects significantly the two-photon absorption cross section. It is found that both one- and two-photon absorptions are extremely sensitive to the planarity of the molecule, and the absorption intensity can be dramatically reduced by the conformation distortion. It has led to the conclusion that the experimentally observed anomalous solvent effect on the two-photon absorption of dialkylamino substituted distyrylbenzene chromophores cannot be attributed to the intrinsic properties of a single molecule and its interaction with solvents.
In situ real-time monitoring of biomolecular interactions based on resonating microcantilevers immersed in a viscous fluid
Authors: Kwon Tae Yun , Eom Kilho , Park Jae Hong , Yoon Dae Sung , Kim Tae Song , Lee Hong Lim
Published: June 2007

The authors report the precise (noise-free) in situ real-time monitoring of a specific protein antigen-antibody interaction by using a resonating microcantilever immersed in a viscous fluid. In this work, they utilized a resonating piezoelectric thick film microcantilever, which exhibits the high quality factor (e.g., Q=15) in a viscous liquid at a viscosity comparable to that of human blood serum. This implies a great potential of the resonating microcantilever to in situ biosensor applications. It is shown that the microcantilever enables them to monitor the C reactive protein antigen-antibody interactions in real time, providing an insight into the protein binding kinetics.