bluuebunny/crossref_metadata_2025 huggingface dataset:
Surface preparation methods to enhance dynamic surface property measurements of shocked metal surfaces
Authors: Zellner M. B. , Vogan McNeil W. , Gray G. T. , Huerta D. C. , King N. S. P. , Neal G. E. , Valentine S. J. , Payton J. R. , Rubin J. , Stevens G. D. , Turley W. D. , Buttler W. T.
Published: April 2008

This effort investigates surface-preparation methods to enhance dynamic surface-property measurements of shocked metal surfaces. To assess the ability of making reliable and consistent dynamic surface-property measurements, the amount of material ejected from the free surface upon shock release to vacuum (ejecta) was monitored for shocked Al-1100 and Sn targets. Four surface-preparation methods were considered: Fly-cut machine finish, diamond-turned machine finish, polished finish, and ball rolled. The samples were shock loaded by in-contact detonation of HE PBX-9501 on the front side of the metal coupons. Ejecta production at the back side or free side of the metal coupons was monitored using piezoelectric pins, optical shadowgraphy, and x-ray attenuation radiography.
Optical properties of As33S67−xSex bulk glasses studied by spectroscopic ellipsometry
Authors: Orava J. , Šik J. , Wágner T. , Frumar M.
Published: April 2008

Variable angle spectroscopic ellipsometry (VASE) was employed to study the optical properties of As33S67−xSex (x=0, 17, 33.5, 50, and 67at.%) bulk glasses in the UV-vis-NIR (near infrared) spectral region for photon energies from 0.54to4.13eV (photon wavelengths from 2300to300nm). For data analysis, we employed Tauc–Lorentz (TL) dispersion model in the entire measured near bandgap spectral region and standard Cauchy dispersion model in the spectral region below the bandgap. With increasing Se content (x) in the bulk glass, we observed a linear decrease in optical bandgap energy Egopt from 2.52±0.02eV for As33S67 to 1.75±0.01eV for As33Se67 and linear increase in refractive index nTL in the NIR spectral region, e.g., at 0.80eV from 2.327 for As33S67 to 2.758 for As33Se67. The amplitude A decreased with increasing Se content. The peak transition energy E0 and broadening C had a maximum value for x=33.5at.% and systematically decreased for higher S or Se content in glasses. Our study showed that TL model is suitable to describe dielectric functions of studied chalcogenide bulk glasses in the broad spectral region. The bulk glasses had a higher refractive index compared to thin films of corresponding composition. The bulk glasses with high S content had higher value of optical bandgap energy than was previously reported for thin films. The optical bandgap energy of glasses with higher Se content was very similar to the thin films.
Resonance penetration of intense femtosecond laser pulses through ultrathin foils
Authors: Bordyuh N. V. , Krainov V. P.
Published: April 2008

Transmission coefficient for penetration of intense femtosecond laser pulse through ultrathin foils with the thickness of the order of 30–100nm has been derived using the Vlasov–Boltzmann equation as a function of foil thickness. It is found that this coefficient oscillates resonantly if the transverse (parallel to the foil boundary) electron energy is equal, or larger than the longitudinal (normal to the foil boundary) electron energy.
Effect of electrodiffusion current flow on electrostatic screening in aqueous pores
Authors: Liu Yang , Sauer Jon , Dutton Robert W.
Published: April 2008

A numerical study within the framework of the Poisson–Nernst–Planck equations is conducted to investigate electrostatic screening of charged biomolecules within synthetic pores having diameters of at least 10Debye lengths. We show that with external biases, the biomolecule charge is only partially screened due to the presence of electro-diffusion current flow. This is considerably different from the equilibrium Debye–Huckel screening behavior and will result in long-range electrostatic interactions. The potential application to direct biomolecule charge sensing is also discussed.
Thermal diffusivity of nonfractal and fractal nickel nanowires
Authors: Razeeb Kafil M. , Roy Saibal
Published: April 2008

The potential of using nanometallic wires inside a matrix as new generation of thermal interface material led us to study the thermal diffusivity of nickel nanowires embedded inside porous alumina template. Thermal diffusivity measurements using a laser flash method showed size dependence for nickel nanowires inside nanochannel alumina (NCA) templates having nominal pore diameters of 200, 100, and 20nm. Nickel nanowires embedded inside these templates showed decreasing diffusivity values of 10.7×10−6, 8.5×10−6, and 6.5×10−6m2s−1 at 300K with decreasing wire diameter when deposited at 40°C. Nanowires fabricated at 60°C showed similar decreasing diffusivity with wire diameter, and a further 42%–48% reduction was observed when compared to 40°C samples. The modified effective medium theory (MEMT) was employed to evaluate the experimental thermal diffusivity. Calculations based on MEMT resulted in mean thermal conductivities of 70.7 and 36.2Wm−1K−1 for nickel nanowires fabricated at 40 and 60°C respectively. These values are ∼20% and 60% lower than the thermal conductivity value of bulk nickel. A strong grain size dependence of thermal diffusivity in the nanowires was observed. It is believed that the decrease in diffusivity in lower temperature wires is associated with defects/dislocations in large single crystals and reduction in wire diameters according to pore diameters of NCA. Whereas in higher temperature wires, the drastic reduction in diffusivity is believed to arise from self-similar fractal morphology composed of nanogranules, close to the dimension of electron mean free path.
Deep-red to near-infrared electrophosphorescence based on bis(8-hydroxyquinolato) platinum(II) complexes
Authors: Xiang Hai-Feng , Xu Zong-Xiang , Roy V. A. L. , Yan Bei-Ping , Chan Siu-Chung , Che Chi-Ming , Lai P. T.
Published: April 2008

A series of neutral platinum(II) complexes with substituted 8-hydroxyquinoline ligands for use in organic electroluminescent devices have been prepared. These platinum(II) complexes give deep-red to near-infrared phosphorescence with λmax,peak from 650to695nm and λmax,shoulder from 705to755nm. High-performance organic light-emitting devices are obtained using these bis(8-hydroxyquinolato) platinum(II) complexes as a deep-red to near-infrared emitter with maximum external quantum efficiency, luminous efficiency, and turn-on voltage of 1.7%, 0.32cd∕A, and 3.2V, respectively.
Three-dimensional structure of confined swirling jets at moderately large Reynolds numbers
Authors: Sanmiguel-Rojas E. , Burgos M. A. , del Pino C. , Fernandez-Feria R.
Published: April 2008

We have performed a series of three-dimensional (3D) numerical simulations of the incompressible flow discharging from a rotating pipe into a coaxial static cylindrical container through a sudden expansion. We have considered several values of the Reynolds number based on the pipe flow rate ReQ between 50 and 300, and an expansion diameter ratio of 8, and have analyzed the emerging 3D flow structures in the swirling jet exiting from the rotating pipe as the swirl parameter S is increased. The results are compared to axisymmetric numerical simulations of the same problem. Three-dimensional, nonlinear instabilities are found in the swirling jet when ReQ≳98 above a critical value of S, which depends on ReQ, that obviously do not appear in the axisymmetric simulations. These nonlinear instabilities are initially triggered by the linear instabilities inside the rotating pipe, which are already present in the pipe from a much lower value of S, and are transformed in the jet. As S increases further, there exists another critical value above which the swirling jet undergoes vortex breakdown, producing a flow in the jet which is basically axisymmetric. This critical value of the swirl parameter for breakdown is significantly larger than that found in the axisymmetric simulations. Thus, one of the main results of the present work is that 3D instabilities delay the formation of vortex breakdown in the jet, in relation to the same axisymmetric flow, but once the vortex breakdown phenomenon occurs, the 3D instabilities coming from the rotating pipe appear to be suppressed in the jet, and the swirling flow becomes basically axisymmetric again. Finally, the axisymmetric simulations show that the jet becomes unstable to axisymmetric perturbations, when ReQ≳188, above another critical value of S. However, these axisymmetric instabilities do not appear in the 3D simulations because the flow becomes unstable to asymmetric perturbations at much lower values of S.
Turbulence and transport in two-dimensional magnetized electron plasmas
Authors: Shaikh Dastgeer , Shukla P. K.
Published: April 2008

Electron plasmas confined by an external magnetic field exhibit variations in a two-dimensional plane orthogonal to the confining magnetic field. A nonlinear fluid simulation code to investigate the properties of 2D electron plasma wave turbulence in a nonuniform magnetoplasma has been developed. It is found that the presence of the density gradient convection by mean electric fields considerably influence the characteristic nonlinear interaction processes, such as the energy cascades and the cross-field electron transport. The initial random turbulent state evolves towards an intermittent state where forward cascade of vorticity coexists with an inverse cascade of electric potential fluctuations. The latter lead to the formation of large scale entities in 2D electron plasmas and can be alternatively understood by seeking exact nonlinear coherent vortex solutions in the form of a dipolarlike configuration. The energy cascades are governed typically by the Kolmogorov-type k−5∕3 spectrum. In agreement with the experimental observations, we find that the electron transport is improved significantly by the application of an externally imposed electric field.
Electronic transport through a C60 molecular bridge: The role of single and multiple contacts
Authors: Saffarzadeh Alireza
Published: April 2008

The effects of different contact geometries, bond dimerization, and gate voltage on quantum transport through a C60 molecule are studied by the Landauer–Büttiker formula based on the Green’s function technique. It is shown that the number of contact points between the device electrodes and the molecule can play an important role in the electron conduction. The transmission is due to the resonant tunneling when the electrodes are contacted to one carbon atom of the molecule. In the case of multiple contacts, the interference effects are responsible for the change of the transmission through the C60. The bond dimerization and a gate voltage shift the molecular levels and by adjusting the related parameters the electron conduction can be controlled.
One-dimensional tunable ferroelectric photonic crystals based on Ba0.7Sr0.3TiO3/MgO multilayer thin films
Authors: Jim K. L. , Wang D. Y. , Leung C. W. , Choy C. L. , Chan H. L. W.
Published: April 2008

Tunable photonic crystals (PCs) have attracted much attention in the past decade because of their various applications, such as ultrafast optical filters and optical waveguides with add–drop functionalities. One way of achieving tunability is to make use of ferroelectric materials since the refractive index of ferroelectric materials can be electrically tuned through the electro-optic effect. In this paper, we present our work on developing a tunable one-dimensional (1D) PC based on a Ba0.7Sr0.3TiO3/MgO multilayer structure. The photonic band structures and band gap maps of the PC were calculated by using the plane-wave expansion (PWE) method. It is found that the gap center linearly shifts with the change in the refractive index of Ba0.7Sr0.3TiO3. A ferroelectric 1D PC consisting of a Ba0.7Sr0.3TiO3/MgO multilayer thin film was epitaxially deposited on a MgO (001) single-crystal substrate by pulsed laser deposition. A photonic band gap in the visible region is observed in the transmission spectrum of the multilayer thin film. The center wavelength of the band gap is ∼464 nm, which agrees with the simulation results obtained by the transfer matrix method. The band gap can be tuned by applying an electric field E. The band gap shifts by about 2 nm when the thin film is subjected to a dc voltage of 240 V (E∼12 MV/m). This shift corresponds to an ∼0.5% change in the refractive index of the Ba0.7Sr0.3TiO3 layer, as calculated by the PWE method.
Thermal behavior of the Au/c-Si3N4/Si(111) interface
Authors: Flammini Roberto , Wiame Frédéric , Belkhou Rachid , Taleb-Ibrahimi Amina , Spezzani Carlo , Moras Paolo , Crotti Corrado
Published: April 2008

Photoelectron spectroscopy and low energy electron diffraction have been used to investigate the thermal behavior of the Au/c-Si3N4/Si(111) interface in a temperature range extending from room temperature to 1000 °C. The interface consisted of 3.2×1015 atoms cm−2 of gold deposited at room temperature on a crystalline silicon nitride layer, which was previously grown on a Si(111) surface. At room temperature, no evidence of any reaction between gold and the substrate has been detected. Between 300 and 400 °C, Au–Si reaction takes place. A comparison of the spectra reveals that the Au–Si reaction occurs in the silicon matrix and on top of the nitride layer, due to silicon atom segregation. Nitrogen atoms do not participate in the reaction up to 600 °C, at which point a Au–Si–N ternary compound is formed. Subsequently, the silicon nitride desorption begins. Below 600 °C, the nitride layer appears to be cracked but not disrupted. In the range of 800–900 °C, a 3×3 pattern was recorded, and finally, gold and nitrogen disappear leading to the 5×1-reconstructed bare Si surface at 1000 °C.
Epitaxial growth of very large grain bicrystalline Cu(In,Ga)Se2 thin films by a hybrid sputtering method
Authors: Hall A. J. , Hebert D. , Lei C. , Rockett A. , Siebentritt S.
Published: April 2008

Epitaxial bicrystals of Cu(In,Ga)Se2 (CIGS) have been grown and characterized spanning high angle incoherent grain boundaries. The grain boundaries in the CIGS were generally found to be fully dense, although voids were occasionally observed as is typical in CIGS polycrystals. There was no significant difference in composition of the grains across the boundary. No reduction in dislocation density near the grain boundary was found. X-ray diffraction results were consistent with partially strained epitaxial layers with domains tilted relative to the surface normal and with different rotations in the surface plane. The tetragonal symmetry of the unit cell appears to have affected the residual strain in the layers and the amount of misfit observed. The surface morphology was found to reflect trends observed in growth of single well-oriented crystals. Differences in surface potential of air-exposed grains showed a morphology-dependent work function. However, no significant potential difference was observed that was related to the grain boundary. Surface morphology effects were much greater. The surface showed a clear rotation of morphology across twin boundaries in the surface plane of one grain. The twin boundaries were also fully dense.
Background removal in scanning tunneling spectroscopy of single atoms and molecules on metal surfaces
Authors: Wahl P. , Diekhöner L. , Schneider M. A. , Kern K.
Published: April 2008

Scanning tunneling spectroscopy has developed into a powerful spectroscopic technique that has found wide application in the atomic scale characterization of the electronic properties of clean surfaces as well as adsorbates and defects at surfaces. However, it still lacks the standard methods for data treatment and removal of artifacts in spectra as they are, e.g., common in photoemission spectroscopy. The properties of the atomic scale tip apex—the probe of the instrument—tend to introduce spurious background signals into tunneling spectra. We present and discuss two methods which permit to extract tip-independent information from low temperature tunneling spectra acquired on single atoms and molecules on single crystal surfaces by background subtraction. The methods rely on a characterization of the tip on the clean metal surface. The performance of both methods is demonstrated and compared for simulated and experimental tunneling spectra.
Mechanism of linear and nonlinear optical effects of chalcopyrites LiGaX2 (X=S, Se, and Te) crystals
Authors: Bai Lei , Lin Z. S. , Wang Z. Z. , Chen C. T.
Published: April 2008

The electronic band structures are calculated for the recently discovered infrared nonlinear optical crystals LiGaX2 (X=S, Se, Te) by using the ab initio plane-wave pseudopotential method. Based on the band structures, the linear and nonlinear optical coefficients for these crystals are obtained, both in reasonable agreement with the available experimental results. Moreover, the origin of the difference for the band gaps and optical properties between the Li-containing crystals and our previous studied infrared optical crystals AgGaX2 (X=S, Se, Te) family with the similar structure [L. Bai et al., J. Chem. Phys. 120, 8772 (2004)] is elucidated. The d-orbitals of silver ion entering into the very top of the valence band results in a narrower band gap for the Ag-containing crystals narrower than that of the Li-containing crystals where no Li orbital lies at the band gap edge. Meanwhile, due to the different sizes of Li and Ag cations, the structural distortion of the GaX4 groups in the Li-containing crystal is larger than that in the Ag-containing crystal, which explains the fact that the birefringences of the Li-containing crystals are larger than that of the Ag-containing crystals.
On the relationship between quantum control landscape structure and optimization complexity
Authors: Moore Katharine , Hsieh Michael , Rabitz Herschel
Published: April 2008

It has been widely observed in optimal control simulations and experiments that state preparation is surprisingly easy to achieve, regardless of the dimension N of the system Hilbert space. In contrast, simulations for the generation of targeted unitary transformations indicate that the effort increases exponentially with N. In order to understand such behavior, the concept of quantum control landscapes was recently introduced, where the landscape is defined as the physical objective, as a function of the control variables. The present work explores how the local structure of the control landscape influences the effectiveness and efficiency of quantum optimal control search efforts. Optimizations of state and unitary transformation preparation using kinematic control variables (i.e., the elements of the action matrix) are performed with gradient, genetic, and simplex algorithms. The results indicate that the search effort scales weakly, or possibly independently, with N for state preparation, while the search effort for the unitary transformation objective increases exponentially with N. Analysis of the mean path length traversed during a search trajectory through the space of action matrices and the local structure along this trajectory provides a basis to explain the difference in the scaling of the search effort with N for these control objectives. Much more favorable scaling for unitary transformation preparation arises upon specifying an initial action matrix based on state preparation results. The consequences of choosing a reduced number of control variables for state preparation is also investigated, showing a significant reduction in performance for using fewer than 2N−2 variables, which is consistent with the topological analysis of the associated landscape.
Towards fast computations of correlated vibrational wave functions: Vibrational coupled cluster response excitation energies at the two-mode coupling level
Authors: Seidler Peter , Hansen Mikkel Bo , Christiansen Ove
Published: April 2008

An efficient implementation of vibrational coupled cluster theory with two-mode excitations and a two-mode Hamiltonian is described. The algorithm is shown to scale cubically with respect to the number of modes which is identical to the scaling of the corresponding vibrational configuration interaction algorithm. This is achieved through the use of special intermediates. The same algorithm can also be used in vibrational Møller–Plesset calculations. To improve performance, screening techniques have been implemented as well. Test calculations on polyaromatic hydrocarbons with up to 264 coupled modes and model systems with up to 1140 modes are used to illustrate the various features of the algorithm.
Two-dimensional holographic polarization grating formed on azo-dye-doped polyvinyl alcohol films
Authors: Wu Wei Yen , Li Ming Shian , Lin Hui Chi , Fuh Andy Ying-Guey
Published: April 2008

This study describes an electrically tunable two-dimensional (2D) liquid crystal holographic polarization grating. It is a twisted nematic grating array, which is obtained by orthogonally overlaying two crossed one-dimensional (1D) polarization holograms. A 1D polarization hologram has a rotating linear polarization pattern, generated by the interference of two orthogonal circularly polarized beams on a substrate that is coated with an azo-dye-doped polyvinyl alcohol layer. The images under a polarized optical microscope and the diffraction patterns from the 2D grating are simulated using the Jones matrix and a Fourier transformation. The experimental results agree with the simulated results. This work represents a substantial advance toward the realization of highly functionalized passive optical devices in which both the beam propagation direction and the polarization state can be controlled in two dimensions.
Interface of atomic layer deposited HfO2 films on GaAs (100) surfaces
Authors: Hackley Justin C. , Demaree J. Derek , Gougousi Theodosia
Published: April 2008

Hf O 2 films have been deposited by using a tetrakis(dimethylamino)hafnium/H2O atomic layer deposition (ALD) process on GaAs. X-ray photoelectron spectroscopy measurements show that the HF and NH4OH predeposition surface treatment results in efficient removal of the Ga and As native oxides. No interface oxidation is detected after 15cycles of HfO2 ALD implying effective passivation of the GaAs surface. Spectroscopic ellipsometry confirms linear growth at 1.0Å∕cycle on both starting surfaces, while Rutherford backscattering spectrometry indicates steady-state coverage after about 10 ALD cycles. For films grown on native oxide GaAs, complete removal of the As oxide is observed after 20 ALD cycles.
Novel in situ setup to study the formation of nanoparticles in the gas phase by small angle x-ray scattering
Authors: Shyjumon I. , Rappolt M. , Sartori B. , Amenitsch H. , Laggner P.
Published: April 2008

An in-house built aerosol generator setup for in situ gas phase studies of aerosol and nanoparticles is described. The aerosol generator with an ultrasonic ceramic disk mist maker provides high enough particle concentrations for structural gas phase analysis by synchrotron small angle x-ray scattering (for water ∼4×108droplets∕s with a droplet size of ∼2.5μm). The working principle was proved by scattering of gold nanoparticles. For evaporation induced self-assembly studies of nanostructured particles, an additional thermal treatment chamber was included in the setup. The first on-line gas phase data with our setup for mesostructured silica particles are presented for different thermal treatments. Scanning electron microscope imaging revealed the average particle size to be ∼1μm. Furthermore, to quantify their internal nanostructure, diffraction experiments of deposited silica aerosols were carried out and the corresponding electron density map indicates a silica wall thickness of about 1nm.
Single-walled carbon nanotubes filled with bimetallic alloys: Structures and buckling behaviors
Authors: Wang L. , Zhang H. W. , Zheng Y. G. , Wang J. B. , Zhang Z. Q.
Published: April 2008

Structural and mechanical properties of single-walled carbon nanotubes (SWCNTs) completely filled with bimetallic alloys are investigated using classic molecular dynamics method. Progresses on both synthesis and property researches of metal-filled carbon nanotubes are also comprehensively reviewed. Different initial equilibrium structures and buckling behaviors are exhibited for tubes filled with alloys of different species and compositions. The critical buckling strain of a SWCNT filled with mixed metals can be higher than that of the tube encapsulated with any pure component metals. Strong dependencies of buckling deformations on the initial random distributions of the encapsulated alloy metals are also reported.
Highly efficient near-infrared hybrid organic-inorganic nanocrystal electroluminescence device
Authors: Bourdakos K. N. , Dissanayake D. M. N. M. , Lutz T. , Silva S. R. P. , Curry R. J.
Published: April 2008

We report the use of PbS nanocrystals within a hybrid device that emits 1.2μm electroluminescence with an external quantum efficiency of 1.15% corresponding to an internal quantum efficiency of ∼5%–12% thus demonstrating a viable, low-cost, highly efficient near infrared organic electroluminescent device. Direct generation of the excited state on the nanocrystal result in eliminating competing processes that have previously led to the low reported efficiencies in near-infrared light emitting devices. Furthermore, the emission wavelength can be tuned to cover a wide range of wavelengths including the 1.3–1.5μm region without significant change of the efficiency.
Nonlinear polymer-clad silicon slot waveguide modulator with a half wave voltage of 0.25V
Authors: Baehr-Jones Tom , Penkov Boyan , Huang Jingqing , Sullivan Phil , Davies Joshua , Takayesu Jocelyn , Luo Jingdong , Kim Tae-Dong , Dalton Larry , Jen Alex , Hochberg Michael , Scherer Axel
Published: April 2008

We report on an electro-optic modulator fabricated from a silicon slot waveguide and clad in a nonlinear polymer. In this geometry, the electrodes form parts of the waveguide, and the modulator driving voltage drops across a 120nm slot. As a result, a half wave voltage of 0.25V is achieved near 1550nm. This is one of the lowest values for any modulator obtained to date. As the nonlinear polymers are extremely resistive, our device also has the advantage of drawing almost no current, suggesting this type of modulator could operate at exceedingly low power.
Lifetime enhancement of organic thin-film transistors protected with organic layer
Authors: Jung Hoon , Lim Taehoon , Choi Youngill , Yi Mihye , Won Jongchan , Pyo Seungmoon
Published: April 2008

We investigated the long-term performance of pentacene organic thin-film transistors (OTFTs) passivated with a 200nm thick tetratetracontane (n-C44H90). The mobility of the passivated OTFTs in 50days was 0.143cm2∕Vs, which was almost the same as that (0.12cm2∕Vs) of OTFTs before passivation process. The performance of unpassivated OTFTs exponentially degraded and almost failed in 5days of exposure to an ambient condition. From the time dependence study for the performance of OTFTs, we found that the performance degradation mechanism of the two OTFTs is different and moisture is a more dominant factor for the performance degradation of the OTFTs.
Determination of Sm2O3∕GaAs heterojunction band offsets by x-ray photoelectron spectroscopy
Authors: Stewart A. D. , Gerger A. , Gila B. P. , Abernathy C. R. , Pearton S. J.
Published: April 2008

X-ray photoelectron spectroscopy was used to measure the energy discontinuity in the valence band (ΔEV) of Sm2O3∕GaAs heterostructures in which the Sm2O3 was grown by rf plasma-assisted molecular beam epitaxy on top of GaAs substrates. A value of ΔEV=2.63±0.10eV was obtained by using the Ga 3d energy level as a reference. Given the bandgap of 5.1eV for the Sm2O3, this would indicate a conduction band offset ΔEC of 1.13±0.10eV in this system.
A study of Hf vacancies at Si:HfO2 heterojunctions
Authors: Tang C. , Ramprasad R.
Published: April 2008

In this work we have investigated the formation and migration of Hf vacancies in a Si:HfO2 heterostructure based on first principles calculations. Our calculations indicate that Hf vacancies tend to diffuse from bulk HfO2 to the Si:HfO2 interface and that it is energetically favorable for Si atoms to fill the interfacial Hf vacancies. These results provide a plausible mechanism of the formation of interfacial Hf silicates.
High resolution x-ray transition-edge sensor cooled by tunnel junction refrigerators
Authors: Miller N. A. , O’Neil G. C. , Beall J. A. , Hilton G. C. , Irwin K. D. , Schmidt D. R. , Vale L. R. , Ullom J. N.
Published: April 2008

We demonstrate cooling of an x-ray transition-edge sensor (TES) using solid-state refrigerators based on normal-metal/insulator/superconductor (NIS) tunnel junctions. We are able to operate the NIS refrigerators at a starting temperature of 260mK, which is 75mK above the TES transition temperature (185mK), and still achieve high quality x-ray spectra with an energy resolution of 9.5±0.3eV full width at half maximum at 5.9keV. The maximum cooling achieved by the NIS refrigerators is 110mK, from a starting temperature of 300 down to 190mK.
The influence of mobility unbalance on GaN based vertical cavity surface emitting lasers
Authors: Petrolati E. , Di Carlo A.
Published: April 2008

In this work, we discuss the influence of the large mobility difference between electrons and holes on the electrical injection properties of GaN based vertical cavity surface emitting lasers. This mobility unbalance is mainly responsible for the unfocusing of the electron and hole radiative recombination in the central region of the device where the electromagnetic field is confined.
Low-temperature phase separation in GaN nanowires: An in situ x-ray investigation
Authors: Wu S. Y. , Ji J.-Y. , Chou M. H. , Li W.-H. , Chi G. C.
Published: April 2008

In this study, we in situ employed low-temperature x-ray diffraction to investigate phase separation in GaN nanowires. Our observations showed that a distinct phase separation developed below 260K, the zinc-blende phase, which is related to short range ordering. The correlation lengths of the zinc-blende phase reached their maximum at 140K but correlation length was still revealed at around 23nm. Our results may be understood using the finite size model and support the conclusion that the phase separation was reversible and occurred through the interaction of the characteristic size of the ordered domain of the GaN nanowires.
Tuning of magnetization dynamics by ultrathin antiferromagnetic layers
Authors: McCord Jeffrey , Kaltofen Rainer , Schmidt Oliver G. , Schultz Ludwig
Published: April 2008

The controlled adjustment of static and dynamic magnetic properties of ferromagnetic thin film by the use of ultrathin antiferromagnetic layers is demonstrated. Induced uniaxial anisotropy and thus precessional frequency is increased over more than one order of magnitude. Concurrently, the effective damping parameter is varied by which a significant decrease in magnetic settling time is achieved. Simple relations based on the interfacial antiferromagnetic layer contribution are derived. The use of ultrathin antiferromagnetic layers, below the thickness range for the onset of exchange bias, allows for tuning of magnetic properties beyond the established manipulation of the intrinsic ferromagnetic layer properties.
Microstructure of InN quantum dots grown on AlN buffer layers by metal organic vapor phase epitaxy
Authors: Chen J. Y. , Chi G. C. , Huang P. J. , Chen M. Y. , Hung S. C. , Nien C. H. , Chen M. C. , Lan S. M. , Pong B. J. , Pan C. J. , Tun C. J. , Ren F. , Chang C. Y. , Pearton S. J.
Published: April 2008

InN quantum dots (QDs) were grown over 2in. Si (1 1 1) wafers with a 300nm thick AlN buffer layer by atmospheric-pressure metal organic vapor phase epitaxy. When the growth temperature increased from 450to625°C, the corresponding InN QDs height increased from 16to108nm while the density of the InN QDs decreased from 1.6×109cm−2to3.3×108cm−2. Transmission electron microscopy showed the presence of a 2nm thick wetting layer between the AlN buffer layer and InN QDs. The growth mechanism was determined to be the Stranski–Krastanov mode. The presence of misfit dislocations in the QDs indicated that residual strain was introduced during InN QDs formation. From x-ray diffraction analysis, when the height of the InN QDs increased from 16to62nm, the residual strain in InN QDs reduced from 0.45% to 0.22%. The residual strain remained at 0.22% for larger heights most likely due to plastic relaxation in the QDs. The critical height of the InN QDs for releasing the strain was determined to be 62nm.
Precise determination of absolute coverage of thin films by layer-resolved surface states
Authors: Cheng Cheng-Maw , Tsuei Ku-Ding , Tsai Chi-Ting , Luh Dah-An
Published: April 2008

We report a technique to determine the coverage of metallic thin films by analyzing layer-resolved surface states with photoemission by using atomically flat Ag∕Au(111) thin films as a test system. We analyzed the surface state on Au(111) covered with Ag up to 4 ML with atomic resolution, and precisely determined the total Ag coverage of Ag∕Au(111) through a line-shape analysis. With precise measurements of the absolute coverage of thin films becoming possible, one can calibrate the tooling factor of evaporators with unprecedented accuracy.
Global versus local processing of frequency-modulated tones in gerbils: An animal model of lateralized auditory cortex functions
Authors: Wetzel Wolfram , Ohl Frank W. , Scheich Henning
Published: April 2008

Hemispheric asymmetries of speech and music processing might arise from more basic specializations of left and right auditory cortex (AC). It is not clear, however, whether such asymmetries are unique to humans, i.e., consequences of speech and music, or whether comparable lateralized AC functions exist in nonhuman animals, as evolutionary precursors. Here, we investigated the cortical lateralization of perception of linearly frequency-modulated (FM) tones in gerbils, a rodent species with human-like low-frequency hearing. Using a footshock-reinforced shuttle-box avoidance go/no-go procedure in a total of 178 gerbils, we found that (
i
) the discrimination of direction of continuous FM (rising versus falling sweeps, 250-ms duration) was impaired by right but not left AC lesions; (
ii
) the discrimination of direction of segmented FM (50-ms segments, 50-ms silent gaps, total duration 250 ms) was impaired by bilateral but not unilateral AC lesions; (
iii
) the discrimination of gap durations (10–30 ms) in segmented FM was impaired by left but not right AC lesions. AC lesions before and after training resulted in similar effects. Together, these experiments suggest that right and left AC, even in rodents, use different strategies in analyzing FM stimuli. Thus, the right AC, by using global cues, determines the direction of continuous and segmented FM but cannot discriminate gap durations. The left AC, by using local cues, discriminates gap durations and determines FM direction only when additional segmental information is available.
Prediction of human errors by maladaptive changes in event-related brain networks
Authors: Eichele Tom , Debener Stefan , Calhoun Vince D. , Specht Karsten , Engel Andreas K. , Hugdahl Kenneth , von Cramon D. Yves , Ullsperger Markus
Published: April 2008

Humans engaged in monotonous tasks are susceptible to occasional errors that may lead to serious consequences, but little is known about brain activity patterns preceding errors. Using functional MRI and applying independent component analysis followed by deconvolution of hemodynamic responses, we studied error preceding brain activity on a trial-by-trial basis. We found a set of brain regions in which the temporal evolution of activation predicted performance errors. These maladaptive brain activity changes started to evolve ≈30 sec before the error. In particular, a coincident decrease of deactivation in default mode regions of the brain, together with a decline of activation in regions associated with maintaining task effort, raised the probability of future errors. Our findings provide insights into the brain network dynamics preceding human performance errors and suggest that monitoring of the identified precursor states may help in avoiding human errors in critical real-world situations.
Suppression of interneuron programs and maintenance of selected spinal motor neuron fates by the transcription factor AML1/Runx1
Authors: Stifani Nicolas , Freitas Adriana R. O. , Liakhovitskaia Anna , Medvinsky Alexander , Kania Artur , Stifani Stefano
Published: April 2008

Individual spinal motor neuron identities are specified in large part by the intrinsic repertoire of transcription factors expressed by undifferentiated progenitors and maturing neurons. It is shown here that the transcription factor AML1/Runx1 (Runx1) is expressed in selected spinal motor neuron subtypes after the onset of differentiation and is both necessary and sufficient to suppress interneuron-specific developmental programs and promote maintenance of motor neuron characteristics. These findings show an important role for Runx1 during the consolidation of selected spinal motor neuron identities. Moreover, they suggest a requirement for a persistent suppression of interneuron genes within maturing motor neurons.
The mitotic kinesin CENP-E is a processive transport motor
Authors: Yardimci Hasan , van Duffelen Marilyn , Mao Yinghui , Rosenfeld Steven S. , Selvin Paul R.
Published: April 2008

In vivo
studies suggest that centromeric protein E (CENP-E), a kinesin-7 family member, plays a key role in the movement of chromosomes toward the metaphase plate during mitosis. How CENP-E accomplishes this crucial task, however, is not clear. Here we present single-molecule measurements of CENP-E that demonstrate that this motor moves processively toward the plus end of microtubules, with an average run length of 2.6 ± 0.2 μm, in a hand-over-hand fashion, taking 8-nm steps with a stall force of 6 ± 0.1 pN. The ATP dependence of motor velocity obeys Michaelis–Menten kinetics with
K
M,ATP
= 35 ± 5 μM. All of these features are remarkably similar to those for kinesin-1—a highly processive transport motor. We, therefore, propose that CENP-E transports chromosomes in a manner analogous to how kinesin-1 transports cytoplasmic vesicles.
A polycyclic terpenoid that alleviates oxidative stress
Authors: Bosak T. , Losick R. M. , Pearson A.
Published: April 2008

Polycyclic terpenoid lipids such as hopanes and steranes have been widely used to understand ancient biology, Earth history, and the oxygenation of the ocean–atmosphere system. Some of these lipids are believed to be produced only by aerobic organisms, whereas others actually require molecular oxygen for their biosynthesis. A persistent question remains: Did some polycyclic lipids initially evolve in response to certain environmental or metabolic stresses, including the presence of oxygen? Here, we identify tetracyclic isoprenoids in spores of the bacteriumBacillus subtilis. We call them sporulenes. They are produced by cyclization of regular polyprenes, a reaction that is more favorable chemically than the formation of terpenoids such as hopanoids and steroids from squalene. The simplicity of the reaction suggests that theB. subtiliscyclase may be analogous to evolutionarily ancient cyclases. We show that these molecules increase the resistance of spores to a reactive oxygen species, demonstrating a specific physiological role for a nonpigment bacterial lipid biomarker. Geostable derivatives of these compounds in sediments could thus be used as direct indicators of oxidative stress and aerobic environments.
Interactional pragmatics and court interpreting
Authors: Jacobsen Bente
Published: April 2008

This article reports on an investigation of face in a triadic speech event, a prosecutor’s interpreter-mediated questioning of a defendant in a criminal trial at a Danish district court. The power differential of this particular speech event makes it inherently threatening for the less powerful individual, the defendant, who by consenting to make a statement potentially puts his face at risk in multiple ways. Moreover, his face-protecting strategies may result in the prosecutor’s face being threatened. Simultaneously, while attending to the face-work of the primary participants, the interpreter has her own face to attend to as a professional. Consequently, the aim of the investigation was to explore face-work in the speech event and the interpreter’s strategies for translating and coordinating face-work. The analysis revealed that the interpreter frequently modified face-threatening and face-protecting utterances in an attempt to protect her own face and/or the face of one of the primary participants.
Intonation in Palenquero
Authors: Hualde José Ignacio , Schwegler Armin
Published: April 2008

The least understood aspect of Palenquero phonology is its intonational system. This is a serious gap, as it is precisely in the realm of prosody that the most striking phonological differences between Palenquero and (Caribbean) Spanish are apparent. Although several authors have speculated that African influence may be at the source of Palenquero’s peculiar intonation, to date published research offers no detailed information about the intonation of the creole.

The goal of this study is to remedy this situation. Here we identify several specific intonational features where conservative (or older-generation) Palenquero differs from (Caribbean) Spanish. One of these features is a strong tendency to use invariant word-level contours, with a H tone on the stressed syllable and L tones on unstressed syllables, in all sentential contexts, including prenuclear positions. A second feature that we have identified is the use of a sustained phrase-final high or mid level contour in declaratives accented on the final syllable, and a long fall in declaratives accented on the penult.

The final section addresses the issue of the possible origin of these intonational features. We point out similarities with Equatorial Guinea Spanish and conclude that, at some point in the history of Palenquero, the Spanish prosodic system was interpreted as involving lexical tone, in conformity with claims in the literature regarding several Atlantic creoles.
Aspect selection in adult L2 Spanish and the Competing Systems Hypothesis
Authors: Rothman Jason
Published: April 2008

Native-like use of preterit and imperfect morphology in all contexts by English learners of L2 Spanish is the exception rather than the rule, even for successful learners. Nevertheless, recent research has demonstrated that advanced English learners of L2 Spanish attain a native-like morphosyntactic competence for the Preterit/Imperfect contrast, as evidenced by their native-like knowledge of associated semantic entailments (Goodin-Mayeda and Rothman 2007, Montrul and Slabakova 2003, Slabakova and Montrul, 2003, Rothman and Iverson 2007). In addition to an L2 disassociation of morphology and syntax (e.g., Bruhn de Garavito 2003, Lardiere 1998, 2000, 2005, Prévost and White 1999, 2000, Schwartz 2003), I hypothesize that a system of learned pedagogical rules contributes to target-deviant L2 performance in this domain through the most advanced stages of L2 acquisition via its competition with the generative system. I call this hypothesis the Competing Systems Hypothesis. To test its predictions, I compare and contrast the use of the Preterit and Imperfect in two production tasks by native, tutored (classroom), and naturalistic learners of L2 Spanish.
A central function for perlecan in skeletal muscle and cardiovascular development
Authors: Zoeller Jason J. , McQuillan Angela , Whitelock John , Ho Shiu-Ying , Iozzo Renato V.
Published: April 2008

Perlecan's developmental functions are difficult to dissect in placental animals because perlecan disruption is embryonic lethal. In contrast to mammals, cardiovascular function is not essential for early zebrafish development because the embryos obtain adequate oxygen by diffusion. In this study, we use targeted protein depletion coupled with protein-based rescue experiments to investigate the involvement of perlecan and its C-terminal domain V/endorepellin in zebrafish development. The perlecan morphants show a severe myopathy characterized by abnormal actin filament orientation and disorganized sarcomeres, suggesting an involvement of perlecan in myopathies. In the perlecan morphants, primary intersegmental vessel sprouts, which develop through angiogenesis, fail to extend and show reduced protrusive activity. Live videomicroscopy confirms the abnormal swimming pattern caused by the myopathy and anomalous head and trunk vessel circulation. The phenotype is partially rescued by microinjection of human perlecan or endorepellin. These findings indicate that perlecan is essential for the integrity of somitic muscle and developmental angiogenesis and that endorepellin mediates most of these biological activities.
ErbB2 directly activates the exchange factor Dock7 to promote Schwann cell migration
Authors: Yamauchi Junji , Miyamoto Yuki , Chan Jonah R. , Tanoue Akito
Published: April 2008

The cellular events that precede myelination in the peripheral nervous system require rapid and dynamic morphological changes in the Schwann cell. These events are thought to be mainly controlled by axonal signals. But how signals on the axons are coordinately organized and transduced to promote proliferation, migration, radial sorting, and myelination is unknown. We describe that the axonal signal neuregulin-1 (NRG1) controls Schwann cell migration via activation of the atypical Dock180-related guanine nucleotide exchange factor (GEF) Dock7 and subsequent activation of the Rho guanine triphosphatases (GTPases) Rac1 and Cdc42 and the downstream c-Jun N-terminal kinase. We show that the NRG1 receptor ErbB2 directly binds and activates Dock7 by phosphorylating Tyr-1118. Dock7 knockdown, or expression of Dock7 harboring the Tyr-1118–to–Phe mutation in Schwann cells, attenuates the effects of NRG1. Thus, Dock7 functions as an intracellular substrate for ErbB2 to promote Schwann cell migration. This provides an unanticipated mechanism through which ligand-dependent tyrosine phosphorylation can trigger the activation of Rho GTPase-GEFs of the Dock180 family.
A cell-based reglucosylation assay demonstrates the role of GT1 in the quality control of a maturing glycoprotein
Authors: Pearse Bradley R. , Gabriel Luke , Wang Ning , Hebert Daniel N.
Published: April 2008

The endoplasmic reticulum (ER) protein GT1 (UDP-glucose: glycoprotein glucosyltransferase) is the central enzyme that modifies N-linked carbohydrates based upon the properties of the polypeptide backbone of the maturing substrate. GT1 adds glucose residues to nonglucosylated proteins that fail the quality control test, supporting ER retention through persistent binding to the lectin chaperones calnexin and calreticulin. How GT1 functions in its native environment on a maturing substrate is poorly understood. We analyzed the reglucosylation of a maturing model glycoprotein, influenza hemagglutinin (HA), in the intact mammalian ER. GT1 reglucosylated N-linked glycans in the slow-folding stem domain of HA once the nascent chain was released from the ribosome. Maturation mutants that disrupted the oxidation or oligomerization of HA also supported region-specific reglucosylation by GT1. Therefore, GT1 acts as an ER quality control sensor by posttranslationally reglucosylating glycans on slow-folding or nonnative domains to recruit chaperones specifically to critical aberrant regions.
The peroxisome: a production in four acts
Authors: Rout Michael P.
Published: April 2008

A cell regulates the number, size, and kind of each organelle it possesses in response to its particular role in an environment or tissue. Yet we still know little about how the molecular signaling networks within each cell perform such regulation. In this issue, Saleem et al. (Saleem, R.A., B. Knoblach, F.D. Mast, J.J. Smith, J. Boyle, C.M. Dobson, R. Long-O'Donnell, R.A. Rachubinski, and J.D. Aitchison. 2008. J. Cell Biol. 181:281–292) show for the first time how groups of kinases and phosphatases are organized to control when and how a cell assembles one kind of organelle, the peroxisome.