bluuebunny/crossref_metadata_2025 huggingface dataset:
Reliable and repeatable characterization of optical streak cameras
Authors: Charest Michael R. , Torres Peter , Silbernagel Christopher T. , Kalantar Daniel H.
Published: November 2008

Optical streak cameras are used as primary diagnostics for a wide range of physics and laser experiments at facilities such as the National Ignition Facility. To meet the strict accuracy requirements needed for these experiments, the systematic nonlinearities of the streak cameras (attributed to nonlinearities in the optical and electrical components that make up the streak camera system) must be characterized. In some cases the characterization information is used as a guide to help determine how experiment data should be taken. In other cases, the characterization data are applied to the raw data images to correct for the nonlinearities. In order to characterize an optical streak camera, a specific set of data is collected, where the response to defined inputs are recorded. A set of analysis software routines has been developed to extract information such as spatial resolution, dynamic range, and temporal resolution from this data set. The routines are highly automated, requiring very little user input and thus provide very reliable and repeatable results that are not subject to interpretation. An emphasis on quality control has been placed on these routines due to the high importance of the camera characterization information.
Impact of calibration technique on measurement accuracy for the JET core charge-exchange system
Authors: Giroud Carine , Meigs A. G. , Negus C. R. , Zastrow K.-D. , Biewer T. M. , Versloot T. W. ,   JET-EFDA Contributors
Published: November 2008

The core charge-exchange diagnostic at the Joint European Torus (JET) provides measurements of the impurity ion temperature Ti, toroidal velocity VΦ, and impurity ion densities nimp, across the whole minor radius. A contribution to the uncertainty of the measured quantities is the error resulting from the multi-Gaussian fit and photon statistics, usually quoted for each measured data. Absolute intensity calibration and especially alignment of the viewing directions can introduce an important systematic error. The technique adopted at JET to reduce this systematic contribution to the error is presented in this paper. The error in Ti, VΦ, and nimp is then discussed depending on their use.
Ultrafast ion temperature and toroidal velocity fluctuation spectroscopy diagnostic design
Authors: McKee G. R. , Schlossberg D. J. , Shafer M. W.
Published: November 2008

High sensitivity measurements of localized, long-wavelength ion temperature, and toroidal velocity fluctuations (T̃i/Ti,ṽ∥/v∥) are required to address critical issues pertaining to turbulent transport. This diagnostic design exploits emission from charge exchange recombination between neutral beam atoms and the intrinsic carbon impurity. The n=8–7 transition of C VI at λ0=529.05 nm will be measured. The key difference between this diagnostic design and conventional charge exchange spectrometers is the use of high-efficiency prism-coupled transmission gratings, avalanche photodiode detectors, and high-throughput collection optics. The spectrometer achieves a spectral resolution of 0.25 nm, and observes 528.0–530.0 nm with eight discrete spectral channels, with an entrance throughput of 1.6 mm2 sr, two orders of magnitude larger than conventional charge exchange system. The diagnostic will achieve a turbulence-relevant time resolution of 1 μs. System modeling demonstrates a sensitivity of T̃i/Ti≤1%.
A novel compact high speed x-ray streak camera (invited)
Authors: Hares J. D. , Dymoke-Bradshaw A. K. L.
Published: November 2008

Conventional in-line high speed streak cameras have fundamental issues when their performance is extended below a picosecond. The transit time spread caused by both the spread in the photoelectron (PE) “birth” energy and space charge effects causes significant electron pulse broadening along the axis of the streak camera and limits the time resolution. Also it is difficult to generate a sufficiently large sweep speed. This paper describes a new instrument in which the extraction electrostatic field at the photocathode increases with time, converting time to PE energy. A uniform magnetic field is used to measure the PE energy, and thus time, and also focuses in one dimension. Design calculations are presented for the factors limiting the time resolution. With our design, subpicosecond resolution with high dynamic range is expected.
High heat flux Langmuir probe array for the DIII-D divertor plates
Authors: Watkins J. G. , Taussig D. , Boivin R. L. , Mahdavi M. A. , Nygren R. E.
Published: November 2008

Two modular arrays of Langmuir probes designed to handle a heat flux of up to 25 MW/m2 for 10 s exposures have been installed in the lower divertor target plates of the DIII-D tokamak. The 20 pyrolytic graphite probe tips have more than three times higher thermal conductivity and 16 times larger mass than the original DIII-D isotropic graphite probes. The probe tips have a fixed 12.5° surface angle to distribute the heat flux more uniformly than the previous 6 mm diameter domed collectors and a symmetric “rooftop” design to allow operation with reversed toroidal magnetic field. A large spring-loaded contact area improves heat conduction from each probe tip through a ceramic insulator into a cooled graphite divertor floor tile. The probe tips, brazed to molybdenum foil to ensure good electrical contact, are mounted in a ceramic tray for electrical isolation and reliable cable connections. The new probes are located 1.5 cm radially apart in a staggered arrangement near the entrance to the lower divertor pumping baffle and are linearly spaced 3 cm apart on the shelf above the in-vessel cryopump. Typical target plate profiles of Jsat, Te, and Vf with 4 mm spatial resolution are shown.
Radiation flux and spectral analysis of the multi-temperature Z dynamic hohlraum
Authors: Lockard T. E. , Idzorek G. C. , Tierney T. E. , Watt R. G.
Published: November 2008

Experiments performed at the Sandia National Laboratories (SNL) Z-machine, located in Albuquerque, New Mexico produce hot (∼220 eV) plasmas. X-ray emission from the plasma is used to drive radiation flow experiments. Our standard plasma diagnostic suite consists of x-ray diodes (XRDs), silicon photodiodes, and nickel thin film bolometers. Small diagnostic holes allow us to view the hot plasma from the side, top axial anode side, and bottom axial cathode side. Computer software has been written to process the raw data to calculate data quality, fold in detector spectral response and experiment geometry for emitted flux, calculate a multidetector spectral unfold, and yield an equivalent time-dependent Planckian temperature profile. Spectral unfolds of our XRD data generally yield a Planckian-like spectrum. In our presentation we will compare our diagnostic techniques, analysis, and results to more accurately characterize spectral unfolds in order to establish better drive conditions for our experiments.
Two-dimensional phase contrast imaging for local turbulence measurements in large helical device (invited)
Authors: Tanaka K. , Michael C. A. , Vyacheslavov L. N. , Sanin A. L. , Kawahata K. , Akiyama T. , Tokuzawa T. , Okajima S.
Published: November 2008

Two-dimensional phase contrast imaging (2D) installed on the large helical device (LHD) is a unique diagnostic for local turbulence measurements. A 10.6 μm infrared CO2 laser and 6×8 channel HgCdTe 2D detector are used. The length of the scattering volume is larger than plasma size. However, the asymmetry of turbulence structure with respect to the magnetic field and magnetic shear make local turbulence measurements possible. From a 2D image of the integrated fluctuations, the spatial cross-correlation function was estimated using time domain correlation analysis, then, the integrated 2D k-spectrum is obtained using maximum entropy method. The 2D k-spectrum is converted from Cartesian coordinates to cylindrical coordinates. Finally, the angle in cylindrical coordinate is converted to flux surface labels. The fluctuation profile over almost the entire plasma diameter can be obtained at a single moment. The measurable k-region can be varied by adjusting the detection optics. Presently, k=0.1–1.0 mm−1 can be measured which is expected region of ion temperature gradient modes and trapped electron mode in LHD. The spatial resolution is 10%–50% of the minor radius.
Hydrogen bond ordering in ice V and the transition to ice XIII
Authors: Knight Chris , Singer Sherwin J.
Published: November 2008

The proton ordered version of ice V, ice XIII, was recently identified using Raman spectroscopy and neutron diffraction techniques. The transformation, between 108 and 117K, only occurred in the presence of a small amount of dopant, similar to the proton ordering transition of ice Ih/XI. In this work, we investigate the hydrogen bond fluctuations in ice V and XIII with statistical mechanical techniques that use results from periodic electronic density functional theory calculations as input. We find a number of low-lying hydrogen bond configurations, ∼20 within 10K/water above the ground state state configuration, the structure of which agrees with fully ordered ice XIII. Using an analytic theory, graph invariants, we developed effective spin-lattice Hamiltonians governing hydrogen bond fluctuations to perform statistical mechanical calculations for a large simulation cell containing 6048 water molecules. Two models were constructed, one more elaborate than the first, to gauge the robustness of our methods when the unit cell is very complex and a large number of configurations lie close in energy to the ground state. The predicted proton ordering transitions, 62 and 72K for the two models, are in qualitative agreement with experiment. Occupation probabilities, obtained from our simulations, compare well with values from recent neutron diffraction experiments and help verify our effective Hamiltonians. In both models, we find that a second order phase transition intervenes about 10K above the transition to ice XIII, but its effect is negligible on the behavior of thermodynamic functions near the transition to ice XIII.
Similarity analysis of favorable pressure gradient turbulent boundary layers with eventual quasilaminarization
Authors: Cal Raúl Bayoán , Castillo Luciano
Published: November 2008

Using similarity analysis, the scales and similarity constraints for a favorable pressure gradient (FPG) turbulent boundary layer with eventual quasilaminarization are obtained. In order to achieve equilibrium in the boundary layer, the pressure parameter Λ must be a constant; thus, a power relation between the boundary layer thickness δ and the free-stream velocity U∞ exists. Consequently, the power is given by the pressure parameter Λ as δ∼U∞−1/Λ. Through an analysis using the pressure parameter, two quadrants are found: quadrant I describes FPG turbulent flows and quadrant II corresponds to quasilaminar flows. Moreover, a horizontal line exists for zero pressure gradient flows. Different values of the pressure parameter are found for equilibrium FPG flows, contrary to the findings of Castillo and George [“Similarity analysis for turbulent boundary layer with pressure gradient: Outer flow,” AIAA J. 39, 41 (2001)]. In the case of strong FPG flows with quasilaminarization, the pressure parameter reaches a maximum value of 0.47. At this point, a sudden reduction in the skin friction of about 57% is observed and a redistribution of the Reynolds stresses throughout the boundary layer is achieved. The mean velocity deficit profiles are also found to be attenuated when scaled using the free-stream velocity U∞ or U∞δ∗/δ. For flows in quadrant II, a reduction in the outer flow of the ⟨u2⟩ component of the Reynolds stress is observed, whereas the ⟨v2⟩ and −⟨uv⟩ components nearly vanish impending quasilaminarization. Due to the presence of the ⟨u2⟩ component in the boundary layer, the flow never reaches a full laminar state and a more uniform redistribution of the component is observed as the skin friction decreases due to the imposed FPG. Furthermore, the shape of the profile remains the same until a quasilaminar state is reached, where the profile no longer shows high values of the stress on the proximity to the wall. In addition, the production term −⟨uv⟩dU/dy is nearly zero for flows in quadrant II. Also, the boundary layer parameters such as the shape factor H and the ratio of the displacement thickness to the boundary layer thickness, δ∗/δ, increase as the flow achieves a quasilaminar state.
Development of microwave imaging reflectometry in large helical device
Authors: Yamaguchi S. , Nagayama Y. , Kuwahara D. , Yoshinaga T. , Shi Z. B. , Kogi Y. , Mase A.
Published: November 2008

Three key devices of the microwave imaging reflectometry (MIR) are under development in large helical device (LHD). The 2-D mixer array is developed by stacking the one-dimensional array of the planar Yagi–Uda antenna. The new type of the bandpass filter bank is modified to match the requirement of the MIR. The low-cost quadrature demodulator is also developed for the phase detection system. By using the low-price commercial wireless devices, the development cost becomes much lower than the expensive waveguide system. These devices enable the development of 2-D/3-D microwave imaging system for the plasma diagnostics and industrial applications.
Relativistic corrections to electrical first-order properties using direct perturbation theory
Authors: Stopkowicz Stella , Gauss Jürgen
Published: November 2008

Direct perturbation theory (DPT) is applied to compute relativistic corrections to electrical properties such as dipole moment, quadrupole moment, and electric-field gradient. The corrections are obtained as second derivatives of the energy and are given via method-independent expressions that involve the first derivative of the density matrix with respect to the relativistic perturbation as well as property integrals with additional momentum operators. Computational results obtained using Hartree–Fock (HF), second-order Møller–Plesset (MP2) perturbation theory, and the coupled-cluster singles and doubles approach augmented by a perturbative treatment of triple excitations are presented for the hydrogen halides HX with X=F, Cl, Br, (I, At) and the magnitude of relativistic effects, their basis-set dependence, and the limitations of DPT are discussed. We compare our results to those obtained using the second-order Douglas–Kroll method and benchmark them using four-component HF (Dirac–HF) and MP2 calculations. Relativistic effects are shown to be already important for elements of the third row (Na–Ar) when aiming at a high-accuracy quantum-chemical treatment. DPT provides reliable results for compounds containing elements up to the fourth period (K–Kr) and only breaks down when applied in lowest order to heavier elements. As a first application of the present DPT treatment for electrical properties, we report calculations for bromofluoromethane (CH2FBr) which was investigated using rotational spectroscopy by Cazzoli et al. [Mol. Phys. 106, 1181 (2008)] and for which consideration of relativistic effects turns out to be essential for good agreement between theory and experiment in the case of the bromine quadrupole-coupling constant.
Influence of polydispersity on the phase behavior of colloidal goethite
Authors: van den Pol E. , Thies-Weesie D. M. E. , Petukhov A. V. , Vroege G. J. , Kvashnina K.
Published: November 2008

The effect of fractionation on the phase behavior of colloidal goethite dispersions with different polydispersities (17%, 35%, and 55% in length) has been studied by small angle x-ray scattering and transmission electron microscopy. All systems show at least nematic and smectic phases. The occurrence of the latter phase at such a high polydispersity is remarkable. It is shown that in the highly polydisperse systems strong fractionation occurs, which is able to reduce the local length polydispersity up to a factor of 2. A columnar phase was only found in the 35% and 55% polydisperse systems. It seems that the columnar phase accommodates the particles that do not fit into the smectic layers and, thus, reduces the length polydispersity within the smectic phase even further. The fact that a columnar phase was not found in the system of lowest polydispersity indicates that the smectic phase is the most stable phase at higher concentrations.
Spontaneous luminescence polarizations of wurtzite InGaN∕GaN quantum wells
Authors: Jia Chuanyu , Yu Tongjun , Tao Renchun , Hu Xiaodong , Yang Zhijian , Qin Zhixin , Chen Zhizhong , Zhang Guoyi
Published: November 2008

In this paper, we analyze the polarization selection rules of wurtzite InGaN∕GaN quantum wells using the k⋅p perturbation method. It was found that the symmetry properties of the valence subbands’ wavefunctions at kt≠0 are quite different with those at kt=0. These symmetry properties of valence subbands’ wavefunctions influence the momentum matrix element for TE and TM modes, leading to a different polarization selection rules from the ones at kt=0 and the absence of spectra peak shift between TE and TM modes. It is suggested that the polarization selection rule at kt≠0 should be considered in the main transition process for wurtzite III-V semiconductors.
The multiplexed chemical kinetic photoionization mass spectrometer: A new approach to isomer-resolved chemical kinetics
Authors: Osborn David L. , Zou Peng , Johnsen Howard , Hayden Carl C. , Taatjes Craig A. , Knyazev Vadim D. , North Simon W. , Peterka Darcy S. , Ahmed Musahid , Leone Stephen R.
Published: November 2008

We have developed a multiplexed time- and photon-energy–resolved photoionization mass spectrometer for the study of the kinetics and isomeric product branching of gas phase, neutral chemical reactions. The instrument utilizes a side-sampled flow tube reactor, continuously tunable synchrotron radiation for photoionization, a multimass double-focusing mass spectrometer with 100% duty cycle, and a time- and position-sensitive detector for single ion counting. This approach enables multiplexed, universal detection of molecules with high sensitivity and selectivity. In addition to measurement of rate coefficients as a function of temperature and pressure, different structural isomers can be distinguished based on their photoionization efficiency curves, providing a more detailed probe of reaction mechanisms. The multiplexed three-dimensional data structure (intensity as a function of molecular mass, reaction time, and photoionization energy) provides insights that might not be available in serial acquisition, as well as additional constraints on data interpretation.
Electronic transport characterization of Sc@C82 single-wall carbon nanotube peapods
Authors: Cantone A. L. , Buitelaar M. R. , Smith C. G. , Anderson D. , Jones G. A. C. , Chorley S. J. , Casiraghi C. , Lombardo A. , Ferrari A. C. , Shinohara H. , Ardavan A. , Warner J. , Watt A. A. R. , Porfyrakis K. , Briggs G. A. D.
Published: November 2008

We present electrical transport and Raman measurements on individual single-wall carbon nanotubes filled with the paramagnetic metallofullerene Sc@C82. We find nearly all devices to be metallic p-type conductors, which we tentatively attribute to bandstructure modification of the nanotubes by the encapsulated Sc@C82 molecules. At low temperatures the peapod devices behave as quantum dots and transport is shown to be quantum coherent over distances of at least ∼100 nm. Kondo features are observed at the lowest measurement temperatures of 50 mK. Our results are of fundamental interest because of the long spin coherence times of the unpaired electrons on the Sc@C82 molecules and the possibility this offers for studying one-dimensional spin chains in carbon nanotubes.
Electron energy loss spectroscopy on alkylated silicon nanocrystals
Authors: Coxon Paul R. , Chao Yimin , Horrocks Benjamin R. , Gass Mhairi , Bangert Ursel , Šiller Lidija
Published: November 2008

Alkyl-passivated silicon nanoparticles have been studied via aberration-corrected scanning transmission electron microscopy (STEM) in conjunction with electron energy loss spectroscopy in order to characterize both their chemical composition and structural make-up. Energy loss spectra indicate a predominantly silicon core structure with some oxide species and Si–C surface bonds. Shifts in the Si L-edge onset to higher energies are attributed to quantum confinement effects in the material although the magnitudes are greater than theoretical values expected for silicon nanocrystals bound by alkyl chains. Nanocrystal STEM samples formed by direct evaporation and deposition of intact nanocrystals show evidence of crystalline planes and structural rearrangements, which can be observed under extended irradiation by the electron probe beam.
Hydrogenation and thermal stability of nano- and microcrystalline diamond films studied by vibrational electron spectroscopy
Authors: Michaelson Sh. , Akhvlediani R. , Hoffman A.
Published: November 2008

The influence of high temperature annealing of hydrogenated diamond films with average grain size of ∼300 and ∼5 nm on surface degradation by graphitization is reported. Ex situ microwave plasma hydrogenation was applied to obtain fully hydrogenated diamond surfaces. Hydrogen bonding and near surface phase composition of both films were studied by high resolution electron energy loss spectroscopy (HR-EELS) and electronic EELS. C–H vibrational modes, phonon losses, and their overtones were measured by HR-EELS and bulk and surface plasmons by EELS. In situ vacuum annealing at 1000 °C results in hydrogen desorption and reconstruction of both kinds of surfaces, detected by vanishing of C–H peaks and appearance of sp2 hybridized carbon features. Our results suggest that graphitization induced by hydrogen desorption occurs to a larger extent on the surface of ∼5 nm grain size films. Subsequent in situ atomic hydrogen exposure of both films’ surfaces results in hydrogen adsorption and recovery of the diamond surface, as evidenced by C–H vibrational modes and optical phonon losses. In both cases the film surface does not recover to its previous state, while the degree of degradation is larger for the ∼5 nm diamond grain size, where a clear mixed carbon sp2/sp3 near surface phase composition is evidenced from both HR-EEL and EEL spectra.
Excitons in conjugated polymers: Wavefunctions, symmetries, and quantum numbers
Authors: Barford William , Paiboonvorachat Nattapong
Published: November 2008

We introduce a mapping from configuration interaction singles wavefunctions, expressed as linear combinations of particle-hole excitations between Hartree–Fock molecular orbitals, to real-space exciton wavefunctions, expressed as linear combinations of particle-hole excitations between localized Wannier functions. The exciton wavefunction is a two-dimensional amplitude for the exciton center-of-mass coordinate, R, and the electron-hole separation (or relative coordinate), r, having an exact analogy to one-dimensional hydrogenlike wavefunctions. We describe the excitons by their appropriate quantum numbers, namely, the principle quantum number, n, associated with r and the center-of-mass pseudomomentum quantum number, j, associated with R. In addition, for models with particle-hole symmetry, such as the Pariser–Parr–Pople model, we emphasize the connection between particle-hole symmetry and particle-hole parity. The method is applied to the study of excitons in trans-polyacetylene and poly(para-phenylene).
Representing anisotropy of two-point second-order turbulence velocity correlations using structure tensors
Authors: Bhattacharya Amitabh , Kassinos Stavros C. , Moser Robert D.
Published: November 2008

A locally homogeneous representation for the two-point, second-order turbulent velocity fluctuation Rij(x,r)=⟨ui′(x)uj′(x+r)⟩ is formulated in terms of three linearly independent structure tensors [Kassinos et al., J. Fluid Mech. 428, 213 (2001)]: Reynolds stress Bij, dimensionality Dij, and stropholysis Qijk∗. These structure tensors are single-point moments of the derivatives of vector stream functions that contain information about the directional and componential anisotropies of the correlation. The representation is a sum of several rotationally invariant component tensors. Each component tensor scales like a power law in r, while its variation in r/r depends linearly on the structure tensors. Continuity and self-consistency constraints reduce the number of degrees of freedom in the model to 17. A finite Re correction is introduced to the representation for separations of the order of Kolmogorov’s length scale. To evaluate our representation, we construct a model correlation by fitting the representation to correlations calculated from direct numerical simulation (DNS) of homogeneous turbulence and channel flow. Comparison of the model correlation to the DNS data shows that the representation can capture the character of the anisotropy of two-point second-order velocity correlation tensors.
Direct numerical simulation of unsteady flow in channel with rough walls
Authors: Bhaganagar Kiran
Published: November 2008

A fundamental study has been performed to understand the effect of unsteady forcing on turbulence statistics in channel flow with rough walls using direct numerical simulation. Unsteady flows have been generated by applying an unsteady nonzero mean forcing in the form of time varying pressure gradient such that the amplitude of oscillations is between 19% and 26% of mean centerline velocity and covering a range of forcing frequencies. The analysis has revealed unsteady forcing, depending on the forcing frequency, results in enhanced roughness compared to steady channel flow. The rough-wall flow dynamics have been categorized into high-, intermediate-, and low-frequency regimes. In the regime of high-frequency forcing, unsteadiness alters the mean velocity and turbulence intensities only in the inner layer of the turbulent boundary layer. Further, the turbulence intensities are out of phase with each other and also with the external forcing. In the regime of intermediate-frequency forcing, mean velocity and turbulence intensities are altered beyond the inner layer. In the inner layer, the turbulence intensities are out of phase with each other. The Reynolds stress is in phase with the external forcing in the inner layer, but it is out of phase in the outer layer. In the regime of low-frequency forcing, the mean velocity and turbulence intensities are significantly altered throughout the turbulent boundary layer.
Does the sailfish skin reduce the skin friction like the shark skin?
Authors: Sagong Woong , Kim Chulkyu , Choi Sangho , Jeon Woo-Pyung , Choi Haecheon
Published: November 2008

The sailfish is the fastest sea animal, reaching its maximum speed of 110 km/h. On its skin, a number of V-shaped protrusions pointing downstream exist. Thus, in the present study, the possibility of reducing the skin friction using its shape is investigated in a turbulent boundary layer. We perform a parametric study by varying the height and width of the protrusion, the spanwise and streamwise spacings between adjacent ones, and their overall distribution pattern, respectively. Each protrusion induces a pair of streamwise vortices, producing low and high shear stresses at its center and side locations, respectively. These vortices also interact with those induced from adjacent protrusions. As a result, the drag is either increased or unchanged for most of the cases considered. Some of these cases show that the skin friction itself is reduced but the total drag including the form drag on the protrusion is larger than that of a smooth surface. In a few cases, the drag is decreased only slightly (∼1%) but this amount is within the experimental uncertainty. Since the shape of present protrusions is similar to that used by Sirovich and Karlsson [Nature (London) 388, 753 (1997)] where V-shaped protrusions pointing upstream were considered, we perform another set of experiments following their study. However, we do not obtain any drag reduction even with random distribution of those V-shaped protrusions.
An implicit algorithm for a rate-dependent ductile failure model
Authors: Zuo Q. H. , Rice Jeremy R.
Published: November 2008

An implicit numerical algorithm has been developed for a rate-dependent model for damage and failure of ductile materials under high-rate dynamic loading [F. L. Addessio and J. N. Johnson, J. Appl. Phys. 74, 1640 (1993)]. Over each time step, the algorithm first implicitly determines the equilibrium state on a Gurson surface, and then calculates the final state by solving viscous relaxation equations, also implicitly. Numerical examples are given to demonstrate the key features of the algorithm. Compared to the explicit algorithm used previously, the current algorithm allows significantly larger time steps that can be used in the analysis. As the viscosity of the material vanishes, the results of the rate-dependent model are shown here to converge to that of the corresponding rate-independent model, a result not achieved with the explicit algorithm.
Interface and bulk magnetization dynamics in biaxial Fe/Cr structures induced by ultrashort optical pulses
Authors: Rzhevsky A. A. , Krichevtsov B. B. , Bürgler D. E. , Schneider C. M.
Published: November 2008

The interface and bulk magnetization dynamics of single-crystalline, wedge-shaped Fe(001) thin films with Cr cap layers have been studied by time-resolved magneto-optical Kerr effect (MOKE) and time-resolved magnetization-induced second harmonic generation (MSHG) using an all-optical pump-probe technique. We observed long-lived (≈1 ns) MOKE and MSHG oscillations excited by ultrashort (≈150 fs) optical pulses. They exhibit the same main resonance frequency f and damping constant. However, a 90° phase shift was observed between linear and nonlinear responses proving that MOKE and MSHG oscillations are related to the temporal variations of different magnetization components Mz and My. Additionally, we found weak oscillations at the double frequency 2f. Comparing the results of static and dynamic MSHG measurements we evaluate the in-plane amplitude of the optically excited interfacial magnetization oscillations.
Growth of highly tensile-strained Ge on relaxed InxGa1−xAs by metal-organic chemical vapor deposition
Authors: Bai Yu , Lee Kenneth E. , Cheng Chengwei , Lee Minjoo L. , Fitzgerald Eugene A.
Published: November 2008

Highly tensile-strained Ge thin films and quantum dots have the potential to be implemented for high mobility metal-oxide-semiconductor field-effect transistor channels and long-wavelength optoelectronic devices. To obtain large tensile strain, Ge has to be epitaxially grown on a material with a larger lattice constant. We report on the growth of tensile-strained Ge on relaxed InxGa1−xAs epitaxial templates by metal-organic chemical vapor deposition. To investigate the methods to achieve high quality Ge epitaxy on III–V semiconductor surfaces, we studied Ge growth on GaAs with variable surface stoichiometry by employing different surface preparation processes. Surfaces with high Ga-to-As ratio are found to be necessary to initiate defect-free Ge epitaxy on GaAs. With proper surface preparation, tensile-strained Ge was grown on relaxed InxGa1−xAs with a range of In content. Low growth temperatures between 350 and 500 °C suppress misfit dislocation formation and strain relaxation. Planar Ge thin films with tensile strain as high as 0.5% were fabricated on relaxed In0.11Ga0.89As. For relatively high In-content (x>0.2) InxGa1−xAs templates, we observed an islanded growth morphology forming tensile-strained Ge quantum dots. Tensile strain as high as 1.37% was measured in these Ge quantum dots grown on In0.21Ga0.79As. The ability to grow these structures will enable us to further study the electronic and optoelectronic properties of tensile-strained Ge.
Improvements to a cryosystem to observe ice nucleating in a variable pressure scanning electron microscope
Authors: Waller D. , Stokes D. J. , Donald A. M.
Published: November 2008

The variable pressure scanning electron microscope (VPSEM) has expanded the scope of the SEM to allow the imaging of dynamic, electrically insulating systems. The use of water vapor as the imaging gas present in the chamber allows the successful imaging of hydrated samples. As awareness of the system capabilities becomes more well known, greater pressure has been put onto the microscopist to push the boundaries of both temperature and resolution for the study of diverse hydrated samples whose dynamics may not occur at the usual room temperatures in a VPSEM. In this article we discuss the stages in the development of a cryosystem that has led to the successful observation of the nucleation of ice from a solution in situ. This investigation also leads to further possibilities of imaging hydrated samples in the little explored temperature range of 188–238 K (from −85 to−35 °C). This study includes the exploration of how the temperature of various surfaces inside the microscope will change the system’s ability to keep a sample hydrated or in its native state.
A system for high-speed microinjection of adherent cells
Authors: Wang Wenhui , Sun Yu , Zhang Ming , Anderson Robin , Langille Lowell , Chan Warren
Published: November 2008

This paper reports on a semi-automated microrobotic system for adherent cell injection. Different from embryos/oocytes that have a spherical shape and regular morphology, adherent cells are flat with a thickness of a few micrometers and are highly irregular in morphology. Based on computer vision microscopy and motion control, the system coordinately controls a three-degrees-of-freedom microrobot and a precision XY stage, demonstrating an injection speed of 25 endothelial cells per minute with a survival rate of 95.7% and a success rate of 82.4% (n=1012). The system has a high degree of performance consistency. It is operator skill independent and immune from human fatigue, only requiring a human operator to select injection destinations through computer mouse clicking as the only operator intervention. The microrobotic system makes the injection of a large number of adherent cells practical for testing cellular responses to foreign molecules.
Possibility of spin device in a triple quantum well system
Authors: Cruz H. , Luis D.
Published: November 2008

We have numerically integrated in space and time the effective mass nonlinear Schrödinger equation for an electron wave packet in an InAs triple quantum well system. Considering the local spin-density approximation, we have calculated the tunneling dynamics in the triple quantum well system when an external bias is applied in the center quantum well. In such a device, the injected electronic current that is initially unpolarized could be divided into two spin-up and spin-down polarized currents at the same time and voltage obtaining a double efficiency.
Micromechanical observation of the kinetics of biomolecular interactions
Authors: Kwon Taeyun , Eom Kilho , Park Jinsung , Yoon Dae Sung , Lee Hong Lim , Kim Tae Song
Published: November 2008

Resonant microcantilevers have recently enabled the label-free detection of biomolecules. Here, we observed the kinetics of biomolecular interactions such as antigen-antibody interactions and/or DNA hybridization based on a resonant frequency shift, obeying Langmuir kinetics, measured in a buffer solution. It is shown that the kinetics of DNA adsorptions on the surface is governed by intermolecular interactions between adsorbed DNA molecules. It is also shown that the kinetics of DNA hybridization is determined by the intermolecular interaction. It is implied that resonant microcantilever in buffer solution may allow for gaining insights into the kinetics of various molecular interactions.
Role of chemical reactions of arylamine hole transport materials in operational degradation of organic light-emitting diodes
Authors: Kondakov Denis Y.
Published: November 2008

We report that the representative arylamine hole transport materials undergo chemical transformations in operating organic light-emitting diode (OLED) devices. Although the underlying chemical mechanisms are too complex to be completely elucidated, structures of several identified degradation products point at dissociations of relatively weak carbon-nitrogen and carbon-carbon bonds in arylamine molecules as the initiating step. Considering the photochemical reactivities, the bond dissociation reactions of arylamines occur by the homolysis of the lowest singlet excited states formed by recombining charge carriers in the operating OLED device. The subsequent chemical reactions are likely to yield long-lived, stabilized free radicals capable of acting as deep traps—nonradiative recombination centers and fluorescence quenchers. Their presence in the hole transport layer results in irreversible hole trapping and manifests as a positive fixed charge. The extent and localization of chemical transformations in several exemplary devices suggest that the free radical reactions of hole transporting materials, arylamines, can be sufficient to account for the observed luminance efficiency loss and voltage rise in operating OLEDs. The relative bond strengths and excited state energies of OLED materials appear to have a determining effect on the operational stability of OLED devices.
Resistance dependence of photovoltaic effect in Au/SrTiO3:Nb(0.5 wt %) Schottky junctions
Authors: Shang D. S. , Sun J. R. , Shi L. , Wang Z. H. , Shen B. G.
Published: November 2008

Photoresponse in the Au/SrTiO3:0.5 wt % Nb Schottky junction with an electric field–tunable resistance between ∼70 kΩ and ∼900 MΩ has been experimentally studied. The most remarkable observation is the strong dependence of the open-circuit photovoltage on junction resistance and the invariance of the short-circuit photocurrent during resistance switching. These results, combined with a theoretical calculation based on the equivalent circuit model consisting of a diode in parallel with a resistor, suggest the occurrence of filamentary conductive channels across the interface of the junction under the impact of electric pulses, whereas the remaining Schottky barrier keeps completely unchanged.
Ionic access through porous media with distributed electrodes
Authors: Auger Jérôme , Yssorche-Cubaynes Marie-Pierre , Lorente Sylvie , Cussigh François , Demilecamps Louis
Published: November 2008

This paper describes a proposal to position electrodes optimally in a porous material with the objective of distributing ionic species over a targeted territory. The ionic species are supplied by an external reservoir and are transported through the porous network to a targeted zone by means of an electrical potential difference. The constructal theory of generation of flow access configuration is used to discover the optimal configuration of electrodes. An optimal number of electrode layers is proposed for the example studied. The new concept of ionic lines is proposed as a means of visualizing, explaining, and designing ionic transport.
Plasmonic antenna array at optical frequency made by nanoapertures
Authors: Zhang Z. J. , Peng R. W. , Wang Z. , Gao F. , Huang X. R. , Sun W. H. , Wang Q. J. , Wang Mu
Published: November 2008

We show here that the plasmonic array based on nanoapertures in ultrathin silver film radiates at optical frequency and behaves as an optical antenna array (OAA). The far-field radiation originates from the coherent superposition of plasmonic emissions on each bank of the aperture. The radiation of OAA presents a strong directivity, which depends on the in-plane rotation of aperture array, and on the polarization and incidence angle of the excitation light as well. We suggest that these features have potential applications in photovoltaics, light-emitting devices, and optical sensors.
Zero-field spin splitting in AlxGa1−xN/GaN heterostructures with various Al compositions
Authors: Tang Ning , Shen Bo , Han Kui , Lu Fang-Chao , Xu Fu-Jun , Qin Zhi-Xin , Zhang Guo-Yi
Published: November 2008

The zero-field spin splitting in AlxGa1−xN/GaN heterostructures with various Al compositions has been investigated at low temperatures and high magnetic fields. The zero-field spin-splitting energy and the spin-orbit coupling parameter are obtained by means of beating pattern Shubnikov–de Haas measurements. It is found that the spin-orbit coupling parameter can be tuned by the polarization-induced electric field. The AlxGa1−xN/GaN heterostructure is one of the promising materials for the spin-polarized field effect transistor.
Substrate misorientation induced strong increase in the hole concentration in Mg doped GaN grown by metalorganic vapor phase epitaxy
Authors: Suski T. , Litwin-Staszewska E. , Piotrzkowski R. , Czernecki R. , Krysko M. , Grzanka S. , Nowak G. , Franssen G. , Dmowski L. H. , Leszczynski M. , Perlin P. , Łucznik B. , Grzegory I. , Jakieła R.
Published: November 2008

We demonstrate that relatively small GaN substrate misorientation can strongly change hole carrier concentration in Mg doped GaN layers grown by metalorganic vapor phase epitaxy. In this work intentionally misoriented GaN substrates (up to 2° with respect to ideal ⟨0001⟩ plane) were employed. An increase in the hole carrier concentration to the level above 1018 cm−3 and a decrease in GaN:Mg resistivity below 1 Ω cm were achieved. Using secondary ion mass spectroscopy we found that Mg incorporation does not change with varying misorientation angle. This finding suggests that the compensation rate, i.e., a decrease in unintentional donor density, is responsible for the observed increase in the hole concentration. Analysis of the temperature dependence of electrical transport confirms this interpretation.
Effects of Bi2Ti2O7 buffer layer on memory properties of BiFe0.95Mn0.05O3 thin film
Authors: Yang C. H. , Hu G. D. , Wen Z. , Yang H. L.
Published: November 2008

BiFe 0.95 Mn 0.05 O 3 (BFMO) thin films with and without Bi2Ti2O7 (BTO) buffer layer were fabricated on p-type Si (111) substrates by metal organic decomposition. The maximum memory window of BFMO/Si is only 0.3 V due to the severe charge injection. In contrast, the larger memory windows are 0.8 and 2.4 V, respectively, for BFMO deposited on as-deposited BTO/Si and annealed BTO/Si. More importantly, the memory window of BFMO/annealed BTO/Si is not affected by changing voltage ramp rate and frequency at ±6 V. The BFMO also shows much reduced leakage current by using an annealed BTO buffer layer.
An ultrathin Zr(Ge) alloy film as an exhaustion interlayer combined with Cu(Zr) seed layer for the Cu/porous SiOC:H dielectric integration
Authors: Liu B. , Song Z. X. , Li Y. H. , Xu K. W.
Published: November 2008

A highly reliable interface of an ultrathin Zr(Ge) exhaustion interlayer between Cu(Zr) film and porous SiOC:H (p-SiOC:H) dielectric has been developed in the present work. After being processed at a moderate elevated temperature (say, 450 °C), a self-formed nanomultilayer of CuGex/ZrOx(ZrSiyOx) was produced at the interface of Cu(Zr)/p-SiOC:H film stacks, which showed strong ability to effectively hinder Cu atoms diffusion into p-SiOC:H film and free Si atoms diffusion into Cu film. The mechanism involving the thermal stability of the films system is analyzed based on detailed characterization studies.
Ribonucleotide reduction is a cytosolic process in mammalian cells independently of DNA damage
Authors: Pontarin Giovanna , Fijolek Artur , Pizzo Paola , Ferraro Paola , Rampazzo Chiara , Pozzan Tullio , Thelander Lars , Reichard Peter A. , Bianchi Vera
Published: November 2008

Ribonucleotide reductase provides deoxynucleotides for nuclear and mitochondrial (mt) DNA replication and repair. The mammalian enzyme consists of a catalytic (R1) and a radical-generating (R2 or p53R2) subunit. During S-phase, a R1/R2 complex is the major provider of deoxynucleotides. p53R2 is induced by p53 after DNA damage and was proposed to supply deoxynucleotides for DNA repair after translocating from the cytosol to the cell nucleus. Similarly R1 and R2 were claimed to move to the nucleus during S-phase to provide deoxynucleotides for DNA replication. These models suggest translocation of ribonucleotide reductase subunits as a regulatory mechanism. In quiescent cells that are devoid of R2, R1/p53R2 synthesizes deoxynucleotides also in the absence of DNA damage. Mutations in human p53R2 cause severe mitochondrial DNA depletion demonstrating a vital function for p53R2 different from DNA repair and cast doubt on a nuclear localization of the protein. Here we use three independent methods to localize R1, R2, and p53R2 in fibroblasts during cell proliferation and after DNA damage: Western blotting after separation of cytosol and nuclei; immunofluorescence in intact cells; and transfection with proteins carrying fluorescent tags. We thoroughly validate each method, especially the specificity of antibodies. We find in all cases that ribonucleotide reductase resides in the cytosol suggesting that the deoxynucleotides produced by the enzyme diffuse into the nucleus or are transported into mitochondria and supporting a primary function of p53R2 for mitochondrial DNA replication.
NMDA receptor-dependent switching between different gamma rhythm-generating microcircuits in entorhinal cortex
Authors: Middleton Steven , Jalics Jozsi , Kispersky Tilman , LeBeau Fiona E. N. , Roopun Anita K. , Kopell Nancy J. , Whittington Miles A. , Cunningham Mark O.
Published: November 2008

Local circuits in the medial entorhinal cortex (mEC) and hippocampus generate gamma frequency population rhythms independently. Temporal interaction between these areas at gamma frequencies is implicated in memory—a phenomenon linked to activity of NMDA-subtype glutamate receptors. While blockade of NMDA receptors does not affect frequency of gamma rhythms in hippocampus, it exposes a second, lower frequency (25–35 Hz) gamma rhythm in mEC. In experiment and model, NMDA receptor-dependent mEC gamma rhythms were mediated by basket interneurons, but NMDA receptor-independent gamma rhythms were mediated by a novel interneuron subtype—the goblet cell. This cell was distinct from basket cells in morphology, intrinsic membrane properties and synaptic inputs. The two different gamma frequencies matched the different intrinsic frequencies in hippocampal areas CA3 and CA1, suggesting that NMDA receptor activation may control the nature of temporal interactions between mEC and hippocampus, thus influencing the pathway for information transfer between the two regions.
Mumijo Traditional Medicine: Fossil Deposits from Antarctica (Chemical Composition and Beneficial Bioactivity)
Authors: Aiello Anna , Fattorusso Ernesto , Menna Marialuisa , Vitalone Rocco , Schröder Heinz C. , Müller Werner E. G.
Published: November 2008

Mumijo is a widely used traditional medicine, especially in Russia, Altai Mountains, Mongolia, Iran Kasachstan and in Kirgistan. Mumijo preparations have been successfully used for the prevention and treatment of infectious diseases; they display immune‐stimulating and antiallergic activity as well. In the present study, we investigate the chemical composition and the biomedical potential of a Mumijo(‐related) product collected from the Antarctica. The yellow material originates from the snow petrels,Pagodroma nivea. Extensive purification and chemical analysis revealed that the fossil samples are a mixture of glycerol derivatives.In vitroexperiments showed that the Mumijo extract caused in cortical neurons a strong neuroprotective effect against the apoptosis‐inducing amyloid peptide fragmentβ‐fragment 25–35 (Aβ25–35). In addition, the fraction rich in glycerol ethers/wax esters displayed a significant growth‐promoting activity in permanent neuronal PC12 cells. It is concluded that this new Mumijo preparation has distinct and marked neuroprotective activity, very likely due to the content of glycerol ether derivatives.