bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental scaling law for mass ablation rate from a Sn plasma generated by a 1064 nm laser
Authors: Burdt Russell A. , Yuspeh Sam , Sequoia Kevin L. , Tao Yezheng , Tillack Mark S. , Najmabadi Farrokh
Published: August 2009

The ablation depth in planar Sn targets irradiated with a pulsed 1064 nm laser was investigated over laser intensities from 3×1011 to 2×1012 W/cm2. The ablation depth was measured by irradiating a thin layer of Sn evaporated onto a Si wafer, and looking for signatures of Si ions in the expanding plasma with spectroscopic and particle diagnostics. It was found that ablation depth scales with laser intensity to the (5/9)th power, which is consistent with analytical models of steady-state laser ablation, as well as empirical formulae from previous studies of mass ablation rate in overlapping parameter space. In addition, the scaling of mass ablation rate with atomic number of the target as given by empirical formulae in previous studies using targets such as C and Al, are shown to remain valid for the higher atomic number of the target (Z=50) used in these experiments.
Cancer cells (MCF-7, Colo-357, and LNCaP) viability on amorphous hydrogenated carbon nitride film deposited by dielectric barrier discharge plasma
Authors: Majumdar Abhijit , Ummanni Ramesh , Schröder Karsten , Walther Reinhard , Hippler Rainer
Published: August 2009

Atmospheric pressure dielectric barrier discharge plasma in CH4/N2 (1:1) gas mixture has been employed to deposit amorphous hydrogenated carbon nitride (aH–CNx) film. In vitro studies with three different cancer cell lines were carried out on the coated surfaces. Preliminary biocompatibility and effect of CH4/N2 films have been investigated by measuring cell proliferation. Three different cancer cell (MCF-7, Colo-357, and LNCaP) suspensions have been exposed on the surface of aH–CNx film to investigate the effect of deposited films on viability of cells. Results from the MTS (3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H- tetrazolium, inner salt) proliferation assays indicated that the deposited aH–CNx film is cytotoxic to cancer cell lines. Time course cell viability assay indicated maximum cell death at 24 h after seeding the cells. This effect is dependant on physicochemical and mechanical properties of the deposited films. The deposited film has been characterized by x-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. The results confirm the presence of C–N, C≡N, C–Hx, C–O, N–O, overlapping NH, and OH bonds in the film.
Annealing effects on the optical properties of semiconducting boron carbide
Authors: Billa R. B. , Hofmann T. , Schubert M. , Robertson B. W.
Published: August 2009

Infrared vibrations of as-deposited and annealed semiconducting boron carbide thin films were investigated by midinfrared spectroscopic ellipsometry. The strong boron-hydrogen resonance at ∼2560 cm−1 in as-deposited films reveals considerable hydrogen incorporation during plasma-enhanced chemical vapor deposition. Extended annealing at 600 °C caused significant reduction in film thickness, substantial reduction of boron-hydrogen bond resonance absorption, and development of distinct blue-shifted boron-carbon and icosahedral vibration mode resonances. Our findings suggest that annealing results in substantial loss of hydrogen and in development of icosahedral structure, accompanied by strain relaxation and densification.
Thermal conductivity suppression in bismuth nanowires
Authors: Moore Arden L. , Pettes Michael T. , Zhou Feng , Shi Li
Published: August 2009

The thermal conductivity of individual bismuth nanowires was characterized using a suspended microdevice and correlated with the crystal structure and growth direction obtained by transmission electron microscopy on the same nanowires. Compared to bulk bismuth in the same crystal direction perpendicular to the trigonal axis, the thermal conductivity of a single-crystal bismuth nanowire of 232 nm diameter was found to be three to six times smaller than bulk in the temperature range between 100 and 300 K, and those of polycrystalline bismuth nanowires of 74–255 nm diameter are reduced by factors of 18–78 over the same temperature range. The thermal conductivity suppression in the single-crystal nanowire can be explained by a transport model that considers diffuse phonon-surface scattering, partially diffuse surface scattering of electrons and holes, and scattering of phonons and charge carriers by ionized impurities such as oxygen and carbon of a concentration on the order of 1019 cm−3. The comparable thermal conductivity values measured for polycrystalline nanowires of different diameters suggests a grain boundary scattering mean free path for all heat carriers in the range of 15–40 nm, which is smaller than the nanowire diameters.
Nanoscale oxygen octahedral tilting in 0.90(Bi1/2Na1/2)TiO3–0.05(Bi1/2K1/2)TiO3–0.05BaTiO3 lead-free perovskite piezoelectric ceramics
Authors: Tai Cheuk W. , Lereah Y.
Published: August 2009

The oxygen octahedral tilted domains in 0.90(Bi1/2Na1/2)TiO3–0.5(Bi1/2K1/2)TiO3–0.5BaTiO3 lead-free perovskite piezoelectric ceramic have been studied by transmission electron microscopy (TEM). Selected-area electron diffraction patterns shows the 12ooo and 12ooe reflections, indicating the presence of antiphase (a−a−a−) and in-phase (aoaoc+) octahedral tilting, respectively. The morphology and distributions of these tilted domains are shown in the centered dark-field images. Further, the Bragg-filtered high-resolution TEM image reveals that the size of the in-phase tilted domains varies from 1 to 8 nm across. The ceramic contains the mixture of nontilted and variants of the antiphase and in-phase tilted domains.
Direct modulation of excited state quantum dot lasers
Authors: Stevens B. J. , Childs D. T. D. , Shahid H. , Hogg R. A.
Published: August 2009

The use of the excited state quantum dot lasers for high speed direct modulation is proposed and demonstrated. A direct comparison of lasers utilizing the ground state and excited state from the same laser material reveals a factor of two increase in the K-factor limited bandwidth. This is attributed to an increase in the saturated gain and reduced carrier scattering time of the excited state compared to the ground state.
A dual-channel, curved-crystal spectrograph for petawatt laser, x-ray backlighter source studies
Authors: Theobald W. , Stoeckl C. , Jaanimagi P. A. , Nilson P. M. , Storm M. , Meyerhofer D. D. , Sangster T. C. , Hey D. , MacKinnon A. J. , Park H.-S. , Patel P. K. , Shepherd R. , Snavely R. A. , Key M. H. , King J. A. , Zhang B. , Stephens R. B. , Akli K. U. , Highbarger K. , Daskalova R. L. , Van Woerkom L. , Freeman R. R. , Green J. S. , Gregori G. , Lancaster K. , Norreys P. A.
Published: August 2009

A dual-channel, curved-crystal spectrograph was designed to measure time-integrated x-ray spectra in the ∼1.5 to 2 keV range (6.2–8.2 Å wavelength) from small-mass, thin-foil targets irradiated by the VULCAN petawatt laser focused up to 4×1020 W/cm2. The spectrograph consists of two cylindrically curved potassium-acid-phthalate crystals bent in the meridional plane to increase the spectral range by a factor of ∼10 compared to a flat crystal. The device acquires single-shot x-ray spectra with good signal-to-background ratios in the hard x-ray background environment of petawatt laser-plasma interactions. The peak spectral energies of the aluminum Heα and Lyα resonance lines were ∼1.8 and ∼1.0 mJ/eV sr (∼0.4 and 0.25 J/Å sr), respectively, for 220 J, 10 ps laser irradiation.
Method for reducing response time in sensor measurement
Authors: Casas Oscar , Rillo Francesc I.
Published: August 2009

Transients are present in sensor and instrumentation systems. They are caused by energy transference and are typically modeled as first-order systems. If too long, transients may suppose a critical factor for these systems unless they are analyzed (e.g., in terms of consumption). This work presents a new method for estimating the final value of a first-order system transient. Since this problem may be harmful, mainly to autonomous systems, the method equations are composed of simple operations that can be implemented on microcontrollers or similar interfaces with low computational capacity. The experimental results validate the theoretical models of the method and prove that, when the system coexists with disturbances, it is possible to estimate the final value in around 60% of the time we would have to wait until reaching it, in the worst case scenario.
Orientational averaging of dye molecules attached to proteins in Förster resonance energy transfer measurements: Insights from a simulation study
Authors: Allen Lucy R. , Paci Emanuele
Published: August 2009

Förster resonance energy transfer is an increasingly popular method for studying protein folding at single molecule resolution. By attaching dye molecules to particular residues in a protein molecule and measuring the energy transfer to the acceptor dye on excitation of the donor dye, information about the separation of the dyes can be obtained. Here we use an atomistic coarse-grained molecular model of the protein and dyes to look at the assumption that the dyes rotate freely during the donor decay time. We find that although complete orientational averaging does not always occur, the consequences of this are not extreme. Even in the native state, the errors in efficiency, which result from incorrectly assuming κ2=2/3, are smaller than the typical experimental error of an efficiency measurement. The orientational freedom of the dyes originates both from the dynamics of the linker and dye molecules and also from the movements of the protein chain itself. In the unfolded state, the movements of the protein chain are sufficient to provide complete, or almost complete, orientational averaging within the donor lifetime. Increasing the rigidity of the dyes therefore has only a very small effect on the measured efficiencies in the unfolded state. In the native state the contribution of the linker and dye dynamics to orientational averaging is larger; nevertheless increasing the rigidity still has only a small effect on the measured efficiency.
Forced reptation revealed by chain pull-out simulations
Authors: Bulacu Monica , van der Giessen Erik
Published: August 2009

We report computation results obtained from extensive molecular dynamics simulations of tensile disentanglement of connector chains placed at the interface between two polymer bulks. Each polymer chain (either belonging to the bulks or being a connector) is treated as a sequence of beads interconnected by springs, using a coarse-grained representation based on the Kremer–Grest model, extended to account for stiffness along the chain backbone. Forced reptation of the connectors was observed during their disentanglement from the bulk chains. The extracted chains are clearly seen following an imaginary “tube” inside the bulks as they are pulled out. The entropic and energetic responses to the external deformation are investigated by monitoring the connector conformation tensor and the modifications of the internal parameters (bonds, bending, and torsion angles along the connectors). The work needed to separate the two bulks is computed from the tensile force induced during debonding in the connector chains. The value of the work reached at total separation is considered as the debonding energy G. The most important parameters controlling G are the length (n) of the chains placed at the interface and their areal density. Our in silico experiments are performed at relatively low areal density and are disregarded if chain scission occurs during disentanglement. As predicted by the reptation theory, for this pure pull-out regime, the power exponent from the scaling G∝na is a≈2, irrespective of chain stiffness. Small variations are found when the connectors form different number of stitches at the interface, or when their length is randomly distributed in between the two bulks. Our results show that the effects of the number of stitches and of the randomness of the block lengths have to be considered together, especially when comparing with experiments where they cannot be controlled rigorously. These results may be significant for industrial applications, such reinforcement of polymer-polymer adhesion by connector chains, when incorporated as constitutive laws at higher time/length scales in finite element calculations.
The free-volume structure of a polymer melt, poly(vinyl methylether) from molecular dynamics simulations and cavity analysis
Authors: Račko Dušan , Capponi Sara , Alvarez Fernando , Colmenero Juan , Bartoš Josef
Published: August 2009

In this work we analyze and compare the free volume of a polymer system poly(vinyl methylether) (PVME) at 300 K obtained by the two direct but different approaches: Positron annihilation lifetime spectroscopy (PALS) and computer simulations. The free volume is calculated from the simulated cells of PVME by means of numerical methods based on grid scanning and probing the structure with a probe of a given radius RP. The free-volume structure was found to be percolated for small probes at RP=0.53 Å. As the probe radius increases, the cavity structure breaks into isolated cavities, reaching a maximum of the cavity number at RP=0.78 Å. We further develop methods for a geometrical analysis of the free-volume cavities by considering their shape. The geometrical computations show that the cavities have elongated shape with side-to-length ratio corresponding to approximately 1:0.55 and with an average length of 6 Å. Based on the overlap between the computed cavities and simplified geometrical representations, the best match of the cavity shape is obtained for the approximation to the ellipsoidal shape (overlap on 84.4%). A match with other examined shapes follows the sequence: ellipsoid>cylinder>bar>sphere>cube. Finally, the computed geometrical parameters are used as input parameters into the quantum-mechanical models for the orthopositronium (o-Ps) lifetime in various free-volume hole geometries. Comparison with the experimental data gives support for two ideas about the existence of an o-Ps particle in the polymeric matrix: (i) the positronium cannot localize in a portion of very small cavities; (ii) and in the case of the percolated cavities, several o-Ps particles occupy some subcavities in the same cavity. Additionally, radial distribution functions of the free volume indicate the existence of two kinds of free volume, a structured one, corresponding to interstitial spaces along the polymer chain, and the so-called “bulk free volume,” distributed randomly in the structure. PALS measurements seem to be mainly related with this bulk free volume. The cavities represented by the idealized geometries are visualized in three-dimensional space providing a unique representation on the free-volume structures.
An open source/real-time atomic force microscope architecture to perform customizable force spectroscopy experiments
Authors: Materassi Donatello , Baschieri Paolo , Tiribilli Bruno , Zuccheri Giampaolo , Samorì Bruno
Published: August 2009

We describe the realization of an atomic force microscope architecture designed to perform customizable experiments in a flexible and automatic way. Novel technological contributions are given by the software implementation platform (RTAI-LINUX), which is free and open source, and from a functional point of view, by the implementation of hard real-time control algorithms. Some other technical solutions such as a new way to estimate the optical lever constant are described as well. The adoption of this architecture provides many degrees of freedom in the device behavior and, furthermore, allows one to obtain a flexible experimental instrument at a relatively low cost. In particular, we show how such a system has been employed to obtain measures in sophisticated single-molecule force spectroscopy experiments [Fernandez and Li, Science 303, 1674 (2004)]. Experimental results on proteins already studied using the same methodologies are provided in order to show the reliability of the measure system.
Monolithic time to amplitude converter for time correlated single photon counting
Authors: Resnati D. , Rech I. , Gallivanoni A. , Ghioni M.
Published: August 2009

Time correlated single photon counting techniques allow in depth examinations of very important chemical and biological processes involving complex interactions of biomolecules. Understanding of these processes is of the utmost importance to address vital medical issues such as the origin and growth of tumors. Modern developments of fluorescence analysis techniques require compact, low cost, and high performance instrumentation, and a major path to these goals is the successful integration of the electronics. In this paper we present a fully monolithic time to amplitude converter, built in standard 0.35 μm CMOS technology, characterized by good time resolution (60 ps), low differential nonlinearity (better than 0.5% rms), short dead time (80 ns), low power dissipation (60 mW), and low area occupation (1.8×1.4 mm2).
Design strategies for the creation of aperiodic nonchaotic attractors
Authors: Nandi Amitabha , Bhowmick Sourav K. , Dana Syamal K. , Ramaswamy Ram
Published: August 2009

Parametric modulation in nonlinear dynamical systems can give rise to attractors on which the dynamics is aperiodic and nonchaotic, namely, with largest Lyapunov exponent being nonpositive. We describe a procedure for creating such attractors by using random modulation or pseudorandom binary sequences with arbitrarily long recurrence times. As a consequence the attractors are geometrically fractal and the motion is aperiodic on experimentally accessible time scales. A practical realization of such attractors is demonstrated in an experiment using electronic circuits.
Double ferromagnetic resonance in nanowire arrays
Authors: Carignan Louis-Philippe , Boucher Vincent , Kodera Toshiro , Caloz Christophe , Yelon Arthur , Ménard David
Published: August 2009

Microstrip line measurements are used to determine the frequency dependent microwave response of 40 nm diameter CoFeB ferromagnetic nanowire arrays, with external static applied field parallel to the nanowire axis. The ferromagnetic resonance (FMR) response of the wires is obtained for applied fields below and above magnetization saturation. For applied magnetic fields above saturation, a single FMR peak is observed, while below saturation, two sets of peaks are obtained. The two FMR peaks below saturation are associated with two magnetization populations, one for nanowires with upward magnetization and one with downward magnetization. A model based on a Maxwell–Garnett homogenization procedure has been established and used to predict the frequency response of the FMR peaks. There is good agreement between the model and experimental results.
Properties of dielectric-barrier-free atmospheric pressure microplasma driven by submicrosecond dc pulse voltage
Authors: Ha Chang-Seung , Choi Joon-Young , Kim Dong-Hyun , Park Chung-Hoo , Lee Hae June , Lee Ho-Jun
Published: August 2009

Atmospheric pressure microplasma driven by dc pulse is developed. This device has a simple structure comprised of a flowing helium (He) feed gas and dielectric-free metal electrodes without an external current limiting resistor. It is shown that a stable glow mode plasma can be sustained without arc runaway by limiting the voltage pulse width to shorter than 300 ns. The properties of the device are reported in terms of discharge current waveforms, rotational temperature of N2+, and spatiotemporally resolved optical emission characteristics.
Enhancement in ordering of Fe50Pt50 film caused by Cr and Cu additives
Authors: Zhang W. Y. , Shima H. , Takano F. , Akinaga H. , Yu X. Z. , Hara T. , Zhang W. Z. , Kimoto K. , Matsui Y. , Nimori S.
Published: August 2009

A 100-nm-thick Fe50Pt50 film with high coercivity was fabricated by magnetron sputtering. On introducing a Cr top/bottom layer, the ordering temperature (Tor) of the film decreased considerably from 500 to 380 °C. On introducing both Cr top/bottom layer and a 0.2-nm-thick Cu interlayer, the Tor decreased even further to 340 °C. During the annealing process, the number of Cr atoms diffused into the Fe50Pt50 layer increased. This increased the Fe diffusivity, lowering the Tor in the process. The diffusion of the additive and the effects of annealing were verified by selected-area electron diffraction observations and by investigating the temperature dependence of the magnetization. The superlattice structure of the ordered Fe50Pt50 film containing Cr and Cu additives was confirmed by the transmission electronic microscopy. Because of the Cr and Cu additives, the average grain size in the ordered Fe50Pt50 film increased slightly; however, the coercivity of the ordered Fe50Pt50 film is improved considerably.
Low-voltage onset of electroluminescence in nanocrystalline-Si/SiO2 multilayers
Authors: Anopchenko A. , Marconi A. , Moser E. , Prezioso S. , Wang M. , Pavesi L. , Pucker G. , Bellutti P.
Published: August 2009

Thin film metal-oxide-semiconductor light emitting devices (LEDs) based on nanocrystalline silicon multilayer structure were grown by plasma-enhanced chemical vapor deposition. Room temperature electroluminescence was studied under direct current and time-resolved pulsed-current injection schemes. Multilayer LEDs operating at voltages below 5 V and electroluminescence turn-on voltage of 1.4–1.7 V are demonstrated. The turn-on voltage is less than 3.2 V which corresponds to the barrier height at the silicon oxide interface for electrons. Electrical injection in the multilayer LED is controlled by direct tunneling of electrons and holes among silicon nanocrystals. This injection regime is different than the Fowler–Nordheim tunneling that controls the electron injection in single thick layer LED operating at high voltages. A comparison of the power efficiency for the multilayer based LED and a similar single thick layer LED shows larger power efficiency for the former than for the second. Our results open new directions in the development of highly efficient room temperature silicon based LED.
Effect of threshold voltage instability on field effect mobility in thin film transistors deduced from constant current measurements
Authors: Ahnood A. , Chaji G. Reza , Sazonov A. , Nathan A.
Published: August 2009

The field effect (FE) mobility of thin film transistors is normally extracted using static measurement methods, which inherently rely on the assumption that the device remains stable during the measurement duration. However, these devices, particularly those based on emerging materials, can show large instability during the measurement, typically exhibiting hysteresis in the static characteristics. This letter looks at the effect of threshold voltage shift in FE mobility extracted using the conventional method, and introduces an alternative and more accurate technique of measuring device characteristics. The technique decouples the effect of transient phenomena, thus permitting extraction of the true device FE mobility, which turns out to be either over or underestimated depending on the magnitude and direction of threshold voltage shift.
Microwave assisted magnetization reversal in single domain nanoelements
Authors: Nembach H. T. , Bauer H. , Shaw J. M. , Schneider M. L. , Silva T. J.
Published: August 2009

We studied the microwave assisted magnetic reversal of 65×71 nm2 elliptical Ni80Fe20 nanomagnets. Hysteresis curves were measured by magneto-optical Kerr effect for a range of microwave frequencies and amplitudes. The coercive field Hc was reduced by 26% for an rf field of 0.08Hc when the microwave frequency coincided with the minimum of the experimentally determined ferromagnetic resonance frequency with varying dc field. The experimental results for the fractional reduction in Hc with rf field amplitude are in good agreement with numerical simulations for an array of interacting macrospins with a physically realistic shape anisotropy distribution.
Electrical behavior of nematic cells oriented by polypyrrole surface treatment
Authors: Alexe-Ionescu A. L. , Barbero G. , Dascalu C. , Scalerandi M.
Published: August 2009

We have studied the electrical response of nematic liquid crystal cells with uniform planar alignment imposed by ClO4− doped polypyrrole layers. From the measured electrical current on a resistance in series with the nematic sample when applying a triangular potential with different periods and amplitudes, the diffusion of the ions from the polypyrrole layer into the liquid crystal was evidenced. The sample is described as an insulating liquid containing a distribution of ions depending on the characteristics of the applied external field and also of the polymer layers. A simple model concerning the diffusion of the ions from the aligning doped polypyrrole film inside the liquid crystal is proposed. The agreement between experimental data and numerical results is good.
High-throughput indentational elasticity measurements of hydrogel extracellular matrix substrates
Authors: D’Sa Dexter J. , de Juan Pardo Elena M. , de las Rivas Astiz Rosalia , Sen Shamik , Kumar Sanjay
Published: August 2009

Much interest surrounds the effect of extracellular matrix (ECM) elasticity on cell behavior. Here we present a rapid method for measuring the elasticity of synthetic ECM substrates based on indentation of the substrate with a ferromagnetic sphere and optical tracking of the resulting deformation. We find that this method yields order-of-magnitude agreement with atomic force microscopy elasticity measurements, but that the degree of this agreement depends strongly on sphere density and gel elasticity. In its regime of greatest accuracy, we envision that this method may be used for high-throughput characterization of ECM substrates in cell biological studies.
Manipulation of the magnetic configuration of (Ga,Mn)As nanostructures
Authors: Haigh J. A. , Wang M. , Rushforth A. W. , Ahmad E. , Edmonds K. W. , Campion R. P. , Foxon C. T. , Gallagher B. L.
Published: August 2009

We have studied the magnetic reversal of L-shaped nanostructures fabricated from (Ga,Mn)As. The strain relaxation due to the lithographic patterning results in each arm having a uniaxial magnetic anisotropy. Our analysis confirms that the magnetic reversal takes place via a combination of coherent rotation and domain wall propagation with the domain wall positioned at the corner of the device at intermediate stages of the magnetic hysteresis loops. The domain wall energy can be extracted from our analysis. Such devices have found implementation in studies of current induced domain wall motion and have the potential for application as nonvolatile memory elements.
Semiquantum molecular dynamics simulation of liquid water by time-dependent Hartree approach
Authors: Hyeon-Deuk Kim , Ando Koji
Published: August 2009

Semiquantum liquid water molecular dynamics simulation was developed using the time-dependent Hartree approach. The classical intra- and intermolecular potential functions of water were extended to describe the wave packet (WP) hydrogen atoms. The equations of motion with an extended phase space including auxiliary coordinates and momenta representing the hydrogen WP widths were derived and solved. The molecular dynamics simulation of semiquantum water demonstrated that the semiquantum hydrogen atoms make the liquid water less structured and the hydrogen bonds weakened. The poor structurization in liquid water was inferred from the increased mobility of a water molecule and the redshift of OH stretching frequency. The zero-point energy introduced by the semiquantum hydrogens enhances the anharmonic potential effects and contributes to the redshifted OH stretching vibration. We found a significant peak around 4400 cm−1 in the absorption spectrum resulting from the energy exchange between the WP width dynamics and the coupling of the OH stretching mode and the rotational motion of each water. We proposed that a liquid free energy landscape is smoothed due to semiquantum hydrogen atoms, and influences the liquid structure and dynamics.
Characterization of boron charge traps at the interface of Si/SiO2 using second harmonic generation
Authors: Park H. , Qi J. , Xu Y. , Varga K. , Weiss S. M. , Rogers B. R. , Lüpke G. , Tolk N.
Published: August 2009

We report results from optical second harmonic generation studies of boron charge traps near the interface of Si/SiO2. Our data suggest that a static electric field at the interface is formed during the oxide growth process due to the presence of negative boron ions (B−) in the silicon substrate and positive boron ions (B+) in the oxide. We demonstrated that the B+ state traps could be filled through the creation of neutral boron states created by internal photoelectron emission. By fitting our data, we found that the effective interface susceptibility |χ(2)| depends on doping concentration.
Construction of basis sets for time-dependent studies
Authors: Guevara N. L. , Hall B. , Teixeira E. , Sabin J. R. , Deumens E. , Öhrn Y.
Published: August 2009

The common basis sets constructed for use in electronic structure calculations have been found inadequate for the representation of electrons participating in nonadiabatic time-dependent dynamics calculations. In this paper we outline an approach to construct electronic bases better suited for dynamical processes such as energy deposition and charge transfer in binary collisions of ions, atoms, and molecules. Since electrons of many-atom systems commonly are represented by orbitals formed as linear combinations of atomic orbitals, the focus is on atomic basis sets. The main idea is to construct basis sets that adequately reproduce the first few excitation energies of neutral atoms. In this paper we outline a method for such basis set construction of various levels of accuracy for first-row atoms and give a few illustrative examples.
Transient-mode multipactor discharge
Authors: Graves T. P. , Spektor R. , Stout P. , Axley A.
Published: August 2009

Multipactor discharge is a resonant condition in which electrons impact material surfaces in phase with an alternating rf or microwave electric field. A growing and/or sustained discharge requires the electrode secondary electron coefficient, δ(Ee), to be greater than unity at the impacting electron energy (Ee). E1, the minimum energy for δ=1, is highly dependent on electrode surface preparation and conditioning, and contaminated surfaces will generally experience a lower E1 than the same clean or “conditioned” surface. A transient mode of multipactor discharge can exist when the multipactor electron energy distribution occurs near E1. The transient nature of such a discharge is shown to result from dynamic changes in E1 due to multipactor conditioning and surface contamination. Experimental data depict transient-mode multipactor (TMM) discharges at a range of rf voltages and chamber pressures for both copper and aluminum electrodes. TMM was detected using a biased current probe, and these events were undetectable by third harmonic diagnostics at chamber pressures below 1×10−3 torr. Discharge parameters such as charge density, period, and duty cycle are shown to be related to both the rf voltage and the chamber pressure. Monte Carlo simulation results support the experimental findings, showing an exponential decrease in the resonant electron population with increasing E1.
Bistable device using anchoring transition of nematic liquid crystals
Authors: Kim Jin Ki , Araoka Fumito , Jeong Soon Moon , Dhara Surajit , Ishikawa Ken , Takezoe Hideo
Published: August 2009

The authors have demonstrated a novel bistable memory device writable by a laser beam. The device utilizes the hysteresis of a temperature-driven discontinuous anchoring transition (orientation change) in a dye-doped nematic liquid crystal. The laser light irradiation switches the stable orientation from homeotropic to planar in a liquid crystal on perfluoropolymer surface. The thickness of recorded lines comes down to as low as ∼20 μm. The recorded images were kept at least for a day, i.e., memory effect. We also showed that the temperature range of the hysteresis was lowered down to room temperature using a binary mixture system.
Fluorescence enhancement and quenching of Eu3+ ions by Au–ZnO core-shell and Au nanoparticles
Authors: Haldar Krishna Kanta , Patra Amitava
Published: August 2009

We investigate the enhancement and quenching of Eu3+ emission in presence of Au–ZnO core-shell nanoparticles and Au nanoparticles. The quenching of Eu3+-emission in presence of Au nanoparticles is caused by change of nonradiative rate due to energy transfer. However, the enhancement of Eu3+-emission in presence of Au–ZnO core-shell nanoparticles is due to local field, which modifies the radiative and nonradiative rate. Enhancement and quenching of Eu3+ emission reflect the change in environment of Eu3+ from Au nanoparticles to Au–ZnO core-shell nanoparticles which is confirmed by the Judd–Ofelt parameter (Ω2) analysis.
The behavior of subgrid-scale models near the turbulent/nonturbulent interface in jets
Authors: da Silva Carlos B.
Published: August 2009

The behavior of subgrid-scale models near the turbulent/nonturbulent interface in jets is analyzed by using direct numerical simulation and large-eddy simulation (LES). The subgrid scales of motion near this region are far from equilibrium and contain an important fraction of the total kinetic energy. The Smagorinsky constant CS needs to be corrected near the jet edge and the method used to obtain the dynamic Smagorinsky constant CD is not able to cope with the intermittent nature of this region. A priori tests and LES show that near the jet edge the Smagorinsky model is superior both to the dynamic Smagorinsky and to the gradient models.
Polarization and temperature dependence of photoluminescence of m-plane GaN grown on γ-LiAlO2 (100) substrate
Authors: Liu B. , Kong J. Y. , Zhang R. , Xie Z. L. , Fu D. Y. , Xiu X. Q. , Chen P. , Lu H. , Han P. , Zheng Y. D. , Zhou S. M.
Published: August 2009

We investigated the polarization and temperature dependence of photoluminescence (PL) of m-plane GaN grown on γ-LiAlO2 (100) substrate. The calculated electronic band structure with k•p Hamiltonian points out the energy splitting as well as polarization selection originate from the m-plane GaN epilayer under anisotropic strain. The polarization-angle dependence PL spectra are found to be selected from in-plane x- and z-polarized emission, corresponding to T1 and T2 transition. And the intensity distribution of the fitting peaks satisfies the Malus’ law. An S-shape energy evolution of near band edge peak on temperatures is observed, which originates from the transition between the localized holes and electrons in triangular potentials induced by basal stacking faults.
Is there a common orientational order for the liquid phase of tetrahedral molecules?
Authors: Rey Rossend
Published: August 2009

The title question is addressed with molecular dynamics simulations for a broad set of molecules: methane (CH4), neopentane (C(CH3)4), carbon tetrafluoride (CF4), carbon tetrachloride (CCl4), silicon tetrachloride (SiCl4), vanadium tetrachloride (VCl4), tin tetrachloride (SnCl4), carbon tetrabromide (CBr4), and tin tetraiodide (SnI4). In all cases the sequence of most populated relative orientations, for increasing distances, is found to be identical: The closest distances correspond to face-to-face followed by a dominant role of edge-to-face, while for larger distances the main configuration is edge-to-edge. The corner-to-face configuration plays an almost negligible role. The range of orientational order is also similar, with remnants of orientational correlation discernible up to the fourth solvation shell. The equivalence does not only hold in the qualitative terms just stated but is also quantitative to a large extent once the center-center distance is properly scaled.
The brain-specific factor FEZ1 is a determinant of neuronal susceptibility to HIV-1 infection
Authors: Haedicke Juliane , Brown Craig , Naghavi Mojgan H.
Published: August 2009

Neurons are one of the few cell types in the human body that do not support HIV type-1 (HIV-1) replication. Although the lack of key receptors is a major obstacle to infection, studies suggest that additional functions inhibit virus replication to explain the exquisite resistance of neurons to HIV-1. However, specific neuronal factors that may explain this resistance remain to be discovered. In a screen for antiviral factors using a fibroblast line chemically mutagenized and selected for resistance to retroviral infection, we recently identified induction of rat FEZ1 (fasciculation and elongation protein zeta-1), a brain-specific protein, as the cause of this resistance. When exogenously expressed in nonneuronal cell lines rat FEZ1 blocked nuclear entry of retroviral DNA. Here, we demonstrate that among human brain cells, neurons naturally express high levels of FEZ1 compared to astrocytes or microglia cells and are correspondingly less susceptible to infection with pseudotyped HIV-1 that bypasses receptor-mediated viral entry. Demonstrating that endogenous FEZ1 was functionally important in the resistance of neurons to HIV-1 infection, siRNA-mediated knockdown of endogenous FEZ1 increased the infectivity of neurons while sensitive brain cell types like microglia became more resistant upon FEZ1 overexpression. In addition, FEZ1 expression was not induced in response to IFN treatment. As such, in contrast to other widely expressed, IFN-inducible antiviral factors, FEZ1 appears to represent a unique neuron-specific determinant of cellular susceptibility to infection in a cell type that is naturally resistant to HIV-1.
Temporal changes of surface wave velocity associated with major Sumatra earthquakes from ambient noise correlation
Authors: Xu Zhen J. , Song Xiaodong
Published: August 2009

Detecting temporal changes of the medium associated with major earthquakes has implications for understanding earthquake genesis. Here we report temporal changes of surface wave velocity over a large area associated with 3 major Sumatra earthquakes in 2004, 2005, and 2007. We use ambient noise correlation to retrieve empirical Green's function (EGF) of surface waves between stations. Because the process is completely repeatable, the technique is powerful in detecting possible temporal change of medium. We find that 1 excellent station pair (PSI in Indonesia and CHTO in Thailand) shows significant time shifts (up to 1.44 s) after the 2004 and 2005 events in the Rayleigh waves at 10–20 s but not in the Love waves, suggesting that the Rayleigh time shifts are not from clock error. The time shifts are frequency dependent with the largest shifts at the period band of 11–16 s. We also observe an unusual excursion ∼1 month before the 2004 event. We obtain a total of 17 pairs for June, 2007 to June, 2008, which allow us to examine the temporal and spatial variation of the time shifts. We observed strong anomalies (up to 0.68 s) near the epicenter after the 2007 event, but not in the region further away from the source or before the event or 3 months after the event. The observations are interpreted as stress changes and subsequent relaxation in upper-mid crust in the immediate vicinity of the rupture and the broad area near the fault zone.
Organization of the autoantibody repertoire in healthy newborns and adults revealed by system level informatics of antigen microarray data
Authors: Madi Asaf , Hecht Inbal , Bransburg-Zabary Sharron , Merbl Yifat , Pick Adi , Zucker-Toledano Merav , Quintana Francisco J. , Tauber Alfred I. , Cohen Irun R. , Ben-Jacob Eshel
Published: August 2009

The immune system is essential to body defense and maintenance. Specific antibodies to foreign invaders function in body defense, and it has been suggested that autoantibodies binding to self molecules are important in body maintenance. Recently, the autoantibody repertoires in the bloods of healthy mothers and their newborns were studied using an antigen microarray containing hundreds of self molecules. It was found that the mothers expressed diverse repertoires for both IgG and IgM autoantibodies. Each newborn shares with its mother a similar repertoire of IgG antibodies, which cross the placental but its IgM repertoire is more similar to those of other newborns. Here, we took a system-level approach and analyzed the correlations between autoantibody reactivities of the previous data and extended the study to new data from newborns at birth and a week later, and from healthy young women. For the young women, we found modular organization of both IgG and IgM isotypes into antigen cliques—subgroups of highly correlated antigen reactivities. In contrast, the newborns were found to share a universal congenital IgM profile with no modular organization. Moreover, the IgG autoantibodies of the newborns manifested buds of the mothers' antigen cliques, but they were noticeably less structured. These findings suggest that the natural autoantibody repertoire of humans shows relatively little organization at birth, but, by young adulthood, it becomes sorted out into a modular organization of subgroups (cliques) of correlated antigens. These features revealed by antigen microarrays can be used to define personal states of autoantibody organizational motifs.
Ca
2+
regulation in the Na
+
/Ca
2+
exchanger features a dual electrostatic switch mechanism
Authors: Hilge Mark , Aelen Jan , Foarce Alice , Perrakis Anastassis , Vuister Geerten W.
Published: August 2009

Regulation of ion-transport in the Na
+
/Ca
2+
exchanger (NCX) occurs via its cytoplasmic Ca
2+
-binding domains, CBD1 and CBD2. Here, we present a mechanism for NCX activation and inactivation based on data obtained using NMR, isothermal titration calorimetry (ITC) and small-angle X-ray scattering (SAXS). We initially determined the structure of the Ca
2+
-free form of CBD2-AD and the structure of CBD2-BD that represent the two major splice variant classes in NCX1. Although the apo-form of CBD2-AD displays partially disordered Ca
2+
-binding sites, those of CBD2-BD are entirely unstructured even in an excess of Ca
2+
. Striking differences in the electrostatic potential between the Ca
2+
-bound and -free forms strongly suggest that Ca
2+
-binding sites in CBD1 and CBD2 form electrostatic switches analogous to C
2
-domains. SAXS analysis of a construct containing CBD1 and CBD2 reveals a conformational change mediated by Ca
2+
-binding to CBD1. We propose that the electrostatic switch in CBD1 and the associated conformational change are necessary for exchanger activation. The response of the CBD1 switch to intracellular Ca
2+
is influenced by the closely located cassette exons. We further propose that Ca
2+
-binding to CBD2 induces a second electrostatic switch, required to alleviate Na
+
-dependent inactivation of Na
+
/Ca
2+
exchange. In contrast to CBD1, the electrostatic switch in CBD2 is isoform- and splice variant-specific and allows for tailored exchange activities.
An active biopolymer network controlled by molecular motors
Authors: Koenderink Gijsje H. , Dogic Zvonimir , Nakamura Fumihiko , Bendix Poul M. , MacKintosh Frederick C. , Hartwig John H. , Stossel Thomas P. , Weitz David A.
Published: August 2009

We describe an active polymer network in which processive molecular motors control network elasticity. This system consists of actin filaments cross-linked by filamin A (FLNa) and contracted by bipolar filaments of muscle myosin II. The myosin motors stiffen the network by more than two orders of magnitude by pulling on actin filaments anchored in the network by FLNa cross-links, thereby generating internal stress. The stiffening response closely mimics the effects of external stress applied by mechanical shear. Both internal and external stresses can drive the network into a highly nonlinear, stiffened regime. The active stress reaches values that are equivalent to an external stress of 14 Pa, consistent with a 1-pN force per myosin head. This active network mimics many mechanical properties of cells and suggests that adherent cells exert mechanical control by operating in a nonlinear regime where cell stiffness is sensitive to changes in motor activity. This design principle may be applicable to engineering novel biologically inspired, active materials that adjust their own stiffness by internal catalytic control.
Single-molecule force spectroscopy distinguishes target binding modes of calmodulin
Authors: Junker Jan Philipp , Rief Matthias
Published: August 2009

The eukaryotic signaling protein calmodulin (CaM) can bind to more than 300 known target proteins to regulate numerous functions in our body in a calcium-dependent manner. How CaM distinguishes between these various targets is still largely unknown. Here, we investigate fluctuations of the complex formation of CaM and its target peptide sequences using single-molecule force spectroscopy by AFM. By applying mechanical force, we can steer a single CaM molecule through its folding energy landscape from the fully unfolded state to the native target-bound state revealing equilibrium fluctuations between numerous intermediate states. We find that the prototypical CaM target sequence skMLCK, a fragment from skeletal muscle myosin light chain kinase, binds to CaM in a highly cooperative way, while only a lower degree of interdomain binding cooperativity emerges for CaMKK, a target peptide from CaM-dependent kinase kinase. We identify minimal binding motifs for both of these peptides, confirming that affinities of target peptides are not exclusively determined by their pattern of hydrophobic anchor residues. Our results reveal an association mode for CaMKK in which the peptide binds strongly to only partially Ca
2+
-saturated CaM. This binding mode might allow for a fine-tuning of the intracellular response to changes in Ca
2+
concentration.
Ultrafast growth of wadsleyite in shock-produced melts and its implications for early solar system impact processes
Authors: Tschauner Oliver , Asimow Paul D. , Kostandova Natalya , Ahrens Thomas J. , Ma Chi , Sinogeikin Stanislas , Liu Zhenxian , Fakra Sirine , Tamura Nobumichi
Published: August 2009

We observed micrometer-sized grains of wadsleyite, a high-pressure phase of (Mg,Fe)
2
SiO
4,
in the recovery products of a shock experiment. We infer these grains crystallized from shock-generated melt over a time interval of <1 μs, the maximum time over which our experiment reached and sustained pressure sufficient to stabilize this phase. This rapid crystal growth rate (≈1 m/s) suggests that, contrary to the conclusions of previous studies of the occurrence of high-pressure phases in shock-melt veins in strongly shocked meteorites, the growth of high-pressure phases from the melt during shock events is not diffusion-controlled. Another process, such as microturbulent transport, must be active in the crystal growth process. This result implies that the times necessary to crystallize the high-pressure phases in shocked meteorites may correspond to shock pressure durations achieved on impacts between objects 1–5 m in diameter and not, as previously inferred, ≈1–5 km in diameter. These results may also provide another pathway for syntheses, via shock recovery, of some high-value, high-pressure phases.