bluuebunny/crossref_metadata_2025 huggingface dataset:
Self potential improves characterization of hydraulically‐active fractures from azimuthal geoelectrical measurements
Authors: Wishart DeBonne N. , Slater Lee D. , Gates Alexander E.
Published: September 2006

We examined the sensitivity of azimuthal self potential (ASP) and azimuthal resistivity surveys (ARS) to hydraulic anisotropy in a laboratory fracture block model. Both ASP and ARS measurements are influenced by anisotropy (due to the strike of major fracture sets) and heterogeneity (due to variable fracture density) of the block model. The polarity of the self potential defines the flow direction and the self potential magnitude within a single fracture set is observed to increase with flow rate. Whereas the ARS anisotropy is primarily controlled by fracture density/connectivity (and hence presumably hydraulic conductivity) ASP anisotropy appears diagnostic of (1) hydraulic gradient driving flow within fracture sets, and (2) fracture density (presumably controlling the strength of the streaming potential coefficient). Preliminary field data from the New Jersey Highlands illustrate that ASP surveys can define hydraulic anisotropy in fractured rock environments. Our results suggest that combined interpretation of ASP and ARS can lead to improved geoelectrical characterization of fracture rock anisotropy.
Transoceanic wave propagation links iceberg calving margins of Antarctica with storms in tropics and Northern Hemisphere
Authors: MacAyeal Douglas R. , Okal Emile A. , Aster Richard C. , Bassis Jeremy N. , Brunt Kelly M. , Cathles L. Mac. , Drucker Robert , Fricker Helen A. , Kim Young‐Jin , Martin Seelye , Okal Marianne H. , Sergienko Olga V. , Sponsler Mark P. , Thom Jonathan E.
Published: September 2006

We deployed seismometers on the Ross Ice Shelf and on various icebergs adrift in the Ross Sea (including B15A, a large 100 km by 30 km fragment of B15, which calved from the Ross Ice Shelf in March, 2000). The data reveal that the dominant energy of these floating ice masses is in the 0.01 to 0.1 Hz band, and is associated with sea swell generated in the tropical and extra‐tropical Pacific Ocean. In one example, a strong storm in the Gulf of Alaska on 21 October 2005, approximately 13,500 km from the Ross Sea, generated swell that arrived at B15A immediately prior to, and during, its break‐up off Cape Adare on 27 October 2005. If sea swell influences iceberg calving and break‐up, a teleconnection exists between the Antarctic ice sheet mass balance and weather systems worldwide.
A new method to produce categorical streamflow forecasts
Authors: Regonda Satish Kumar , Rajagopalan Balaji , Clark Martyn
Published: September 2006

Categorical forecasts of streamflow are important for effective water resources management. Typically, these are obtained by generating ensemble forecasts of streamflow and counting the proportion of ensembles in the desired category. Here we develop a simple and direct method to produce categorical streamflow forecasts at multiple sites. The method involves predicting the probability of the leading mode (or principal component) of the basin streamflows above a given threshold and subsequently translating the predicted probabilities to all the sites in the basin. The categorical probabilistic forecasts are obtained via logistic regression using a set of large‐scale climate predictors. Application to categorical forecasts of the spring (April–June) streamflows at six locations in the Gunnison River Basin exhibited significant long‐lead forecast skill.
Effects of pyrrophenone, an inhibitor of group IVA phospholipase A2, on eicosanoid and PAF biosynthesis in human neutrophils
Authors: Flamand N , Picard S , Lemieux L , Pouliot M , Bourgoin S G , Borgeat P
Published: September 2006

Background and Purpose:

The biosynthesis of leukotrienes (LT) and platelet‐activating factor (PAF) involves the release of their respective precursors, arachidonic acid (AA) and lyso‐PAF by the group IVA PLA2 (cPLA2α). This paper aims at characterizing the inhibitory properties of the cPLA2α inhibitor pyrrophenone on eicosanoids and PAF in human neutrophils (PMN).

Experimental Approach:

Freshly isolated human PMN were activated with physiological and pharmacological agents (fMLP, PAF, exogenous AA, A23187 and thapsigargin) in presence and absence of the cPLA2α inhibitor pyrrophenone and biosynthesis of LT, PAF, and PGE2 was measured.

Key Results:

Pyrrophenone potently inhibited LT, PGE2 and PAF biosynthesis in PMN with IC50s in the range of 1–20 nM. These inhibitory effects of pyrrophenone were specific (the consequence of substrate deprivation), as shown by the reversal of inhibition by exogenous AA and lyso‐PAF. Comparative assessment of pyrrophenone, methyl‐arachidonoyl‐fluoro‐phosphonate (MAFP) and arachidonoyl‐trifluoromethylketone (AACOCF3) demonstrated that pyrrophenone was more specific and 100‐fold more potent than MAFP and AACOCF3 for the inhibition of LT biosynthesis in A23187‐activated PMN. The inhibitory effect of pyrrophenone on LT biosynthesis was reversible as LT biosynthesis was recovered when pyrrophenone‐treated PMN were washed with autologous plasma. No alteration of phospholipase D (PLD) activity in fMLP‐activated PMN was observed with up to 10 μM pyrrophenone, suggesting that the cPLA2α inhibitor does not directly inhibit PLD.

Conclusions and Implications:

Pyrrophenone is a more potent and specific cPLA2α inhibitor than MAFP and AACOCF3 and represents an excellent pharmacological tool to investigate the biosynthesis and the biological roles of eicosanoids and PAF.

British Journal of Pharmacology (2006) 149, 385–392. doi:10.1038/sj.bjp.0706879
Sustained contraction and loss of NO production in TGFβ1‐treated endothelial cells
Authors: Watanabe M , Oike M , Ohta Y , Nawata H , Ito Y
Published: September 2006

Background and purpose:

Transforming growth factor β1 (TGFβ1) is generated in atherosclerotic and injured vessel walls. We examined whether the endothelial‐to‐mesenchymal transdifferentiation induced by TGFβ1 affects endothelial functions.

Experimental approach:

Bovine aortic endothelial cells (BAECs) were treated with 3 ng ml−1 TGFβ1 for 7 days. Contraction of TGFβ1‐treated BAECs was assessed by collagen gel contraction assay. Protein expression and phosphorylation were assessed by Western blotting. Intracellular Ca2+ concentration and NO production were measured using fura2 and DAF‐2, respectively.

Key results:

TGFβ1‐treated BAECs showed dense actin fibers and expressed smooth muscle marker proteins; they also changed into smooth muscle‐like, spindle‐shaped cells in collagen gel cultures. ATP (10 μM) induced a gradual contraction of collagen gels containing TGFβ1‐treated BAECs but not of gels containing control BAECs. ATP‐induced contraction of TGFβ1‐treated BAECs was not reversed by the removal of ATP but was partially suppressed by a high concentration of sodium nitroprusside (1 μM). TGFβ1‐treated BAECs showed sustained phosphorylation of myosin light chain in response to ATP and low levels of basal MYPT1 expression. ATP‐induced Ca2+ transients as well as eNOS protein expression were not affected by TGFβ1 in BAECs. However, ATP‐induced NO production was significantly reduced in TGFβ1‐treated BAECs. Anti‐TGFβ1 antibody abolished all of these TGFβ1‐induced changes in BAECs.

Conclusions and Implications:

Mesenchymal transdifferentiation induced by TGFβ1 leads to sustained contraction and reduced NO production in endothelial cells. Such effects, therefore, would not be beneficial for vascular integrity.

British Journal of Pharmacology (2006) 149, 355–364. doi:10.1038/sj.bjp.0706883
Combination of Ca2+‐activated K+ channel blockers inhibits acetylcholine‐evoked nitric oxide release in rat superior mesenteric artery
Authors: Stankevičius E , Lopez‐Valverde V , Rivera L , Hughes A D , Mulvany M J , Simonsen Ulf
Published: September 2006

Background and purpose:

The present study investigated whether calcium‐activated K+ channels are involved in acetylcholine‐evoked nitric oxide (NO) release and relaxation.

Experimental approach:

Simultaneous measurements of NO concentration and relaxation were performed in rat superior mesenteric artery and endothelial cell membrane potential and intracellular calcium ([Ca2+]i) were measured.

Key results.

A combination of apamin plus charybotoxin, which are, respectively, blockers of small‐conductance and of intermediate‐ and large‐conductance Ca2+‐activated K channels abolished acetylcholine (10 μM)‐evoked hyperpolarization of endothelial cell membrane potential. Acetylcholine‐evoked NO release was reduced by 68% in high K+ (80 mM) and by 85% in the presence of apamin plus charybdotoxin. In noradrenaline‐contracted arteries, asymmetric dimethylarginine (ADMA), an inhibitor of NO synthase inhibited acetylcholine‐evoked NO release and relaxation. However, only further addition of oxyhaemoglobin or apamin plus charybdotoxin eliminated the residual acetylcholine‐evoked NO release and relaxation. Removal of extracellular calcium or an inhibitor of calcium influx channels, SKF96365, abolished acetylcholine‐evoked increase in NO concentration and [Ca2+]i. Cyclopiazonic acid (CPA, 30 μM), an inhibitor of sarcoplasmic Ca2+‐ATPase, caused a sustained NO release in the presence, but only a transient increase in the absence, of extracellular calcium. Incubation with apamin and charybdotoxin did not change acetylcholine or CPA‐induced increases in [Ca2+]i, but inhibited the sustained NO release induced by CPA.

Conclusions and Implications:

Acetylcholine increases endothelial cell [Ca2+]i by release of stored calcium and calcium influx resulting in activation of apamin and charybdotoxin‐sensitive K channels, hyperpolarization and release of NO in the rat superior mesenteric artery.

British Journal of Pharmacology (2006) 149, 560–572. doi:10.1038/sj.bjp.0706886
Crystal and magnetic structures and their temperature dependence of Co2Z-type hexaferrite (Ba,Sr)3Co2Fe24O41 by high-temperature neutron diffraction
Authors: Takada Yukio , Nakagawa Takashi , Tokunaga Masatoshi , Fukuta Yasunari , Tanaka Takayoshi , Yamamoto Takao A. , Tachibana Takeshi , Kawano Shinji , Ishii Yoshinobu , Igawa Naoki
Published: September 2006

We have prepared nonoriented and magnetically oriented specimens of Co2Z-type Ba ferrite Ba3Co2Fe24O41 (Ba3Z) and those with Sr2+ substitution for Ba2+, i.e., Ba1.5Sr1.5Co2Fe24O41 (Ba1.5Sr1.5Z) and Sr3Co2Fe24O41 (Sr3Z) with the conventional solid-state reaction method. Permeability measurements of nonoriented specimens have shown that this substitution improves the frequency characteristic of permeability, though the permeability in Sr3Z significantly decreases. X-ray diffraction (XRD) and magnetization measurements of magnetically oriented specimens have shown that the magnetic moments of iron and cobalt ions in Ba3Z and Ba1.5Sr1.5Z lie in the c plane, but that those in Sr3Z deviate from the c plane. We have studied the substitution effect of Sr2+ for Ba2+ on the crystal structures and the effective sizes and directions of magnetic moments and their temperature dependences with high-temperature neutron diffraction technique. This substitution induces the change in the distribution of cobalt ions and moment directions of magnetic ions. Magnetic moments in Ba3Z and Ba1.5Sr1.5Z turn to the c axis when temperature rises from 523to573K. However, the moments in Sr3Z turn to the c axis at the lower temperature of 50K. These results were consistent with the present measurements on permeability, XRD, and magnetization. The change in moment direction caused by temperature rise must be attributed to the disappearance of effective moments of cobalt ions in these temperature regions.
Study of the molecular structure, ionization spectrum, and electronic wave function of 1,3-butadiene using electron momentum spectroscopy and benchmark Dyson orbital theories
Authors: Deleuze M. S. , Knippenberg S.
Published: September 2006

The scope of the present work is to reconcile electron momentum spectroscopy with elementary thermodynamics, and refute conclusions drawn by Saha et al. in J. Chem. Phys. 123, 124315 (2005) regarding fingerprints of the gauche conformational isomer of 1,3-butadiene in electron momentum distributions that were experimentally inferred from gas phase (e,2e) measurements on this compound [M. J. Brunger et al., J. Chem. Phys. 108, 1859 (1998)]. Our analysis is based on thorough calculations of one-electron and shake-up ionization spectra employing one-particle Green’s function theory along with the benchmark third-order algebraic diagrammatic construction [ADC(3)] scheme. Accurate spherically averaged electron momentum distributions are correspondingly computed from the related Dyson orbitals. The ionization spectra and Dyson orbital momentum distributions that were computed for the trans-conformer of 1,3-butadiene alone are amply sufficient to quantitatively unravel the shape of all available experimental (e,2e) electron momentum distributions. A comparison of theoretical ADC(3) spectra for the s-trans and gauche energy minima with inner- and outer-valence high-resolution photoelectron measurements employing a synchrotron radiation beam [D. M. P. Holland et al., J. Phys. B 29, 3091 (1996)] demonstrates that the gauche structure is incompatible with ionization experiments in high-vacuum conditions and at standard temperatures. On the other hand, outer-valence Green’s function calculations on the s-trans energy minimum form and approaching basis set completeness provide highly quantitative insights, within ∼0.2eV accuracy, into the available experimental one-electron ionization energies. At last, analysis of the angular dependence of relative (e,2e) ionization intensities nicely confirms the presence of one rather intense π−2 π*+1 satellite at ∼13.1eV in the ionization spectrum of the s-trans conformer.
Monte Carlo study of surface roughness scattering in Si inversion layer with improved matrix element
Authors: Cha Won-Jun , Shim Tae-Hun , Park Jea-Gun , Yoo Sang-Dong
Published: September 2006

The electron mobility in the inversion layer of metal oxide semiconductor field-effect transistors was calculated by using a Monte Carlo method. We utilized an improved matrix element for accurately calculating the surface roughness scattering among the many factors determining the electron mobility. The improved matrix element employed different effective electric fields (Eeff) for each subband energy level. From the simulation results, we demonstrated that the relative proportion of surface roughness scattering was about three times greater than that of acoustic phonon scattering at an electric field of 1MV∕cm. In particular, the electron mobility curve calculated with the improved matrix element showed better consistency with universal experimental data as compared to a curve calculated with a conventional matrix element.
Magnetic domains in a metamagnetic La(Fe0.90Si0.10)13 refrigerant
Authors: Kawamoto N. , Murakami Y. , Shindo D. , Fukamichi K. , Fujieda S. , Fujita A.
Published: September 2006

A thermally induced first-order magnetic phase transition in La(Fe0.90Si0.10)13 has been observed in situ by using Lorentz microscopy in the Fresnel mode. On cooling the specimen, the magnetic domains instantly are formed at the Curie temperature TC as in a fashion of the first-order phase transition. The coexistence of the ferromagnetic and paramagnetic phases was observed at TC, and the phase boundary was confirmed to be located at the grain boundaries. In addition, the observed magnetic domain structure is reproduced without any change even after repeating thermal cycles in the vicinity of TC. This fact strongly implies that the phase transition during thermal cycles does not induce lattice defects which cause serious deterioration.
Influence of photon recycling on semiconductor luminescence refrigeration
Authors: Wang J.-B. , Johnson S. R. , Ding D. , Yu S.-Q. , Zhang Y.-H.
Published: September 2006

Luminescence refrigeration in semiconductors is studied using a model that includes the rate equations for carriers and photons as well as the influence of spectral dependent photon recycling. Expressions are derived for cooling efficiency, cooling power density, and the minimum external quantum efficiency required for cooling. These results show that net cooling is accessible and that photon recycling significantly contributes to luminescence refrigeration when the luminescence extraction is small.
Hopping conduction in the columnar liquid crystal phase of a dipolar discogen
Authors: Iino Hiroaki , Hanna Jun-ichi , Bushby Richard J. , Movaghar Bijan , Whitaker Benjamin J.
Published: September 2006

Hole transport has been investigated by the time-of-flight method for the hexagonal columnar phase of a discogen containing a strongly dipolar nitro substituent; 1-nitro-2,3,6,7,10,11-hexakis(hexyloxy)triphenylene (HAT6-NO2). Unlike the corresponding unsubstituted system, 2,3,6,7,10,11-hexakis(hexyloxy)triphenylene (HAT6), and unlike most other liquid crystals, the mobility in HAT-NO2 depends strongly on both electric field and temperature. As in amorphous solids there is disorder dominated hopping and the mobility can be modeled using an energy disorder parameter, σ of just over 100meV. Compared to HAT6, the disorder in HAT6-NO2 is enhanced because of the strong dipole of the nitro group.
Effect of growth interruption on optical properties of In-rich InGaN∕GaN single quantum well structures
Authors: Sun Yuanping , Cho Yong-Hoon , Kim H. M. , Kang T. W. , Kwon S. Y. , Yoon E.
Published: September 2006

In-rich InGaN∕GaN single quantum well (SQW) structures with and without growth interruption (GI) were successfully grown on sapphire substrates by metal-organic chemical vapor deposition. The optical properties were systematically investigated by photoluminescence (PL), selectively excited PL, PL excitation (PLE), and cathodoluminescence (CL) techniques. The integrated PL intensity of the main In-rich InGaN emissions for the sample grown without GI decreased only by a factor of 15.5 when the temperature increased from 11to300K, while that of the sample with GI decreased by about 1040, showing very good quantum efficiency for the sample without GI. The In-rich InGaN SQW emissions have been verified by selectively excited PL spectra and by the different PLE absorption edges. CL observations showed that the epilayer of the sample without GI agglomerated together to form clusters due to the large lattice and thermal mismatches with GaN, which confine the carriers in the clusters and ensure the relatively high quantum efficiency of the sample. The sample with GI showed relatively smooth surface with cluster structures jointed together, which gives two-dimensional QW environment in its energy band structure, and its optical emission is more sensitive to temperatures than that of the sample grown without GI.
Damage accumulation in neon implanted silicon
Authors: Oliviero E. , Peripolli S. , Amaral L. , Fichtner P. F. P. , Beaufort M. F. , Barbot J. F. , Donnelly S. E.
Published: September 2006

Damage accumulation in neon-implanted silicon with fluences ranging from 5×1014to5×1016Necm−2 has been studied in detail. As-implanted and annealed samples were investigated by Rutherford backscattering spectrometry under channeling conditions and by transmission electron microscopy in order to quantify and characterize the lattice damage. Wavelength dispersive spectrometry was used to obtain the relative neon content stored in the matrix. Implantation at room temperature leads to the amorphization of the silicon while a high density of nanosized bubbles is observed all along the ion distribution, forming a uniform and continuous layer for implantation temperatures higher than 250°C. Clusters of interstitial defects are also present in the deeper part of the layer corresponding to the end of range of ions. After annealing, the samples implanted at temperatures below 250°C present a polycrystalline structure with blisters at the surface while in the other samples coarsening of bubbles occurs and nanocavities are formed together with extended defects identified as {311} defects. The results are discussed in comparison to the case of helium-implanted silicon and in the light of radiation-enhanced diffusion.
Room temperature formation of silicon oxynitride/silicon structure by use of electrochemical method
Authors: Im Sung-Soon , Takahashi Masao , Kobayashi Hikaru
Published: September 2006

We have developed a room temperature formation method of silicon oxynitride (SiON) films by use of an electrochemical technique. Immersion of Si in potassium cyanide methanol solutions and application of 1–3V to Si with respect to a reference platinum electrode form SiON films. At 1V, decomposition of cyanide ions occurs, leading to Si nitridation, and subsequent air exposure forms SiON films with 4.2% concentration. At 2 and 3V, on the other hand, oxidation and nitridation simultaneously proceed, resulting in the direct formation of SiON films with 0.8%–2.2% nitrogen concentration. X-ray photoelectron spectroscopy measurements show that nitrogen is present in the form of N-(Si)3 and Si-CN species. Relative dielectric constant of the SiON layers formed at 3V is 5.1 with no heat treatment, and it decreases to 4.9 by a heat treatment at 700°C. The leakage current density of the SiON films is relatively low (e.g., 1×10−6A/cm2 at the gate bias of 1V) in spite of the room temperature formation.
Multidentate functionalized lubricant for ultralow head/disk spacing in a disk drive
Authors: Guo X.-C. , Knigge B. , Marchon B. , Waltman R. J. , Carter M. , Burns J.
Published: September 2006

As areal density in disk drive technology is marching towards the Tbit∕in.2 mark, the slider-to-disk spacing will be required to be within only a few nanometers. Coupled with very high slider velocities (>40m∕s), this leads to extremely high shear stresses on the nanometer-thin lubricant film at the disk surface. As a result, the lubricant film tends to exhibit local redistribution on a micrometer lateral level, decreasing the overall clearance and impacting the integrity of the interface. This paper describes a different approach to functionalized lubricant, where in addition to functional groups placed at the end of the chain, additional attachment moieties are introduced within the main polymer chain. The benefits of this approach is twofold: it increases adhesive interaction with the disk surface, therefore increasing the effective surface viscosity, while at the same time “tying” down the free backbone length for increased clearance. Fabrication, characterization, and performance data obtained on actual magnetic disks will be described.
Thermal modeling of GaInAs∕AlInAs quantum cascade lasers
Authors: Lops Antonia , Spagnolo Vincenzo , Scamarcio Gaetano
Published: September 2006

We measured the facet temperature profiles of GaInAs∕AlInAs quantum cascade lasers (QCLs) operating in continuous wave mode by means of microprobe photoluminescence. These results were used to evaluate the in-plane (k‖) and the cross-plane (k⊥) thermal conductivities of the active region and to validate a two-dimensional model for the anisotropic heat diffusion in QCLs. In the temperature range of 80–250K, k⊥ monotonically increases with temperature and remains one order of magnitude smaller than the thermal conductivities of bulk constituent materials. We found an excellent agreement between the calculated and experimental values of the thermal resistance of GaInAs∕AlInAs QCLs operating in continuous wave up to 400K. Comparison between the calculated thermal performances of QCLs sharing the same active region structure, but having either a buried or a ridge waveguide, shows that devices with Au contact layers thicker than 4μm have better thermal properties than the buried structures.
Thermally induced self-hardening of nanocrystalline Ti–B–N thin films
Authors: Mayrhofer P. H. , Mitterer C. , Wen J. G. , Petrov I. , Greene J. E.
Published: September 2006

Nanocrystalline films with high hardness have attracted increasing interest for wear resistant applications. Specifically, nanocrystalline Ti–B–N layers have been demonstrated to exhibit enhanced hardness and thermal stability. Here, we show that Ti–B–N films grown at 300°C and consisting of a high volume fraction, ∼50%, of a fully percolated disordered phase encapsulating 2–3nm wide TiN and TiB2 grains, have a hardness of 37GPa and an elastic modulus of 332GPa which increase with annealing to 43 and 362GPa, respectively, at Ta=800°C. The structural rearrangement which occurs during annealing results in the formation of compact interface boundaries which lead, in turn, to the observed hardness increase. Annealing at Ta>900°C decreases the hardness, although the elastic modulus continues to increase, due to the combination of grain growth and B loss via the formation of volatile boron oxides and hydroxides. These conclusions, obtained based upon a combination of x-ray diffraction, nanoindentation, electron probe microanalysis, and transmission electron microscopy, are corroborated by calorimetric investigations. The overall results provide insight toward developing “design rules” for high-temperature superhard nanoscale based coatings.
Satellite droplet formation by unstable binary drop collisions
Authors: Brenn G. , Kolobaric V.
Published: September 2006

The formation of satellite droplets by stretching separation in unstable binary collisions of equal-sized drops is investigated. Based on data from detailed experiments on the formation and breakup of ligaments in off-center collisions with appreciable impact parameter, the process of satellite droplet formation is modeled. The experiments are carried out using two streams of monodisperse drops of various liquids. Drop sizes and velocities, and dimensions of ligaments and sizes of satellites are formed after unstable off-center collisions are measured with an image-processing technique. The model represents the physics of the collisions by functions of nondimensional characteristic numbers. The functions are determined using the experimental data. The experiments are well reproduced by the model, which reveals the importance of both the drop size and the liquid viscosity for the formation of the satellites.
Phase formation, texture, and magnetic properties of epitaxial Pr–Co films grown on MgO(100)
Authors: Patra A. K. , Neu V. , Fähler S. , Wendrock H. , Schultz L.
Published: September 2006

Series of Pr–Co thin films have been prepared by pulsed laser deposition on Cr buffered MgO(100) substrate. The effect of deposition temperature for both buffer and Pr–Co layers on the phase formation, texture, and magnetic properties has been investigated. Unlike the buffer deposition temperature (TB), the Pr–Co deposition temperature (TD) has a large influence on the structure and magnetic properties of the thin films. For the extreme temperatures, i.e., 300 and 700°C, no Pr–Co phase formation is observed. However, films prepared between 300 and 700°C develop a hexagonal Pr–Co phase, identified by means of the (110) and (220) peaks observed in x-ray diffractometry. From pole figure measurements, the orientation relationship was determined to be Pr–Co(110)[001]‖Cr(100)[001]‖MgO(100)[001] and Pr–Co(110)[001]‖Cr(100)[011¯]‖MgO(100)[010]. Due to the fourfold symmetry of the MgO(100) substrate, the c axis is therefore found to be along both the MgO[010] and MgO[001] directions, which is also confirmed by magnetic measurements. Optimum magnetic properties, coercivity μ0Hc≈2.24T, remanent polarization JR≈1.01T, squareness (JR∕J9T)≈0.84, and magnetic texture (JRhardaxis∕JReasyaxis)≈0.04 have been obtained for films deposited at 600°C. The (BH)max of 173kJ∕m3 observed in these Pr–Co films is superior over that of comparably textured single phase Sm–Co films.
Absorption enhancement due to scattering by dipoles into silicon waveguides
Authors: Catchpole K. R. , Pillai S.
Published: September 2006

We develop an optical model for absorption enhancement and diffuse reflectance by metal nanoparticles on a silicon waveguide. A point dipole treatment is used, including the effects of the waveguide on both the angular emission spectrum and scattering cross section of the dipoles. The model agrees very well with our experimental results of greatly enhanced electroluminescence and photocurrent from silicon-on-insulator light-emitting diodes and also gives very good agreement with previously reported diffuse reflectance measurements. The results suggest that the main mechanism in the enhancement of diffuse reflectance in this system is a dramatic enhancement in the scattering cross section of waveguided light, rather than a waveguide-mediated dipole-dipole interaction. We also put lower bounds on the radiative efficiency of scattering by the nanoparticles.
Hysteretic mode exchange in the wake of two circular cylinders in tandem
Authors: Tasaka Yuji , Kon Seiji , Schouveiler Lionel , Le Gal Patrice
Published: September 2006

Our experimental study is devoted to the analysis of the flow past two tandem circular cylinders near the vortex shedding threshold. A recent bidimensional numerical analysis of this flow [Mizushima and Suehiro, Phys. Fluids 17, 104107 (2005)] has predicted that the bifurcation diagram should become complex in the vicinity of the instability threshold. Subcritical and saddle node bifurcations that lead to hysteretic exchanges between two different modes of vortex shedding were detected for particular distances of separation of the cylinders. We present here visualizations and velocity measurements of this flow in a water channel that prove the robustness of the complexity of the bifurcation diagram in real flows.
Coulomb correlation effects in zinc monochalcogenides
Authors: Karazhanov S. Zh. , Ravindran P. , Kjekshus A. , Fjellvåg H. , Grossner U. , Svensson B. G.
Published: September 2006

Electronic structure and band characteristics for zinc monochalcogenides with zinc-blende- and wurtzite-type structures are studied by first-principles density-functional-theory calculations with different approximations. It is shown that the local-density approximation underestimates the band gap and energy splitting between the states at the top of the valence band, misplaces the energy levels of the Zn-3d states, and overestimates the crystal-field-splitting energy. The spin-orbit-coupling energy is found to be overestimated for both variants of ZnO, underestimated for ZnS with wurtzite-type structure, and more or less correct for ZnSe and ZnTe with zinc-blende-type structure. The order of the states at the top of the valence band is found to be anomalous for both variants of ZnO, but is normal for the other zinc monochalcogenides considered. It is shown that the Zn-3d electrons and their interference with the O-2p electrons are responsible for the anomalous order. The effective masses of the electrons at the conduction-band minimum and of the holes at the valence-band maximum have been calculated and show that the holes are much heavier than the conduction-band electrons in agreement with experimental findings. The calculations, moreover, indicate that the effective masses of the holes are much more anisotropic than the electrons. The typical errors in the calculated band gaps and related parameters for ZnO originate from strong Coulomb correlations, which are found to be highly significant for this compound. The local-density approximation with multiorbital mean-field Hubbard potential approach is found to correct the strong correlation of the Zn-3d electrons, and thus to improve the agreement between the experimentally established location of the Zn-3d levels and that derived from pure LDA calculations.
Structural characterization of metal organic vapor phase epitaxy grown GaInNAs quantum well with InGaAs and GaNAs barriers
Authors: Kim K. S. , Kim T. , Park Y. J. , Baek S. I. , Kim Y. W.
Published: September 2006

The effects of InGaAs∕GaNAs barrier combinations were investigated in the metal organic vapor phase epitaxy grown GaInNAs quantum wells (QWs). Significant improvement was made in the optical performance of the 1.3μm range with the proposed structures. Structural investigation linked with the optical properties reveals that a thin InGaAs layer, placed between GaInNAs QWs and GaNAs layers, plays a role in redistributing the nitrogen within the QWs while maintaining a sharp interface. The amount of diffused nitrogen is proportional to the strain residing in the InGaAs layer, which explains the tendency of blueshift with the increase in GaNAs thickness.
Electrical properties of unintentionally doped semi-insulating and conducting 6H-SiC
Authors: Mitchel W. C. , Mitchell W. D. , Fang Z. Q. , Look D. C. , Smith S. R. , Smith H. E. , Khlebnikov Igor , Khlebnikov Y. I. , Basceri C. , Balkas C.
Published: September 2006

Temperature dependent Hall effect (TDH), low temperature photoluminescence (LTPL), secondary ion mass spectrometry (SIMS), optical admittance spectroscopy (OAS), and thermally stimulated current (TSC) measurements have been made on 6H-SiC grown by the physical vapor transport technique without intentional doping. n- and p-type as well semi-insulating samples were studied to explore the compensation mechanism in semi-insulating high purity SiC. Nitrogen and boron were found from TDH and SIMS measurements to be the dominant impurities that must be compensated to produce semi-insulating properties. The electrical activation energy of the semi-insulating sample determined from the dependence of the resistivity was 1.0eV. LTPL lines near 1.00 and 1.34eV, identified with the defects designated as UD-1 and UD-3, were observed in all three samples but the intensity of the UD-1 line was almost a factor of 10 more in the n-type sample than in the the p-type sample with that in the semi-insulating sample being intermediate between those two. OAS and TSC experiments confirmed the high purity of this material. The results suggest that the relative concentrations of a dominant deep level and nitrogen and boron impurities can explain the electrical properties in this material.
On a thermally induced readout mechanism in super-resolution optical disks
Authors: Kuwahara Masashi , Shima Takayuki , Fons Paul , Fukaya Toshio , Tominaga Junji
Published: September 2006

We have simultaneously measured the carrier-to-noise ratio (CNR) as well as the transmitted and reflected light intensities from platinum oxide based super-resolution near-field structure (PtOx super-RENS) disks. The the reflected and transmitted light intensities were found to decrease and increase, respectively, as the CNR value increased. The phase-change material AgInSbTe (AIST) used in PtOx super-RENS disks was found to exhibit a strong optical nonlinearity with respect to readout laser power. AIST becomes transparent at higher laser powers. To ascertain whether the presence of Pt nanoparticles is important to the readout mechanism, a super-RENS disk was fabricated in which the PtOx layer was replaced with a metal-free phthalocyanine (H2Pc) layer and the CNR of the H2Pc disk was measured. From the observation that the CNR value was equivalent to that of a disk made using PtOx, we conclude that the presence of nanoparticles does not play an important role in the super-RENS readout mechanism. Finally, we also investigated the use of Si and the alloy Ge2Sb2Te5 in lieu of AIST in a super-RENS disk and simple three layer structure disks. The super-resolution effect was observed for all disk types. Based upon these observations, we discuss the possibility of a thermal origin for the super-resolution effect in all super-resolution disks.
Magnetic and transport properties of nanostructured ferric oxide produced by mechanical attrition
Authors: Brahma P. , Dutta S. , Pal M. , Chakravorty D.
Published: September 2006

Nanosized particles of α-Fe2O3 in the range of 7–11nm were produced by subjecting micron-sized α-Fe2O3 powder to ball milling for durations extending to 10h. The magnetization of the powder was found to decrease as the particle size was reduced. Detailed electrical measurements of the compacted powder of different ball milled samples showed a drastic change in dc resistivity variation as a function of temperature. Two activated processes were observed with activation energies of ∼0.06 and ∼0.7eV, respectively. These are ascribed to a polaron hopping conduction between Fe2+ and Fe3+ sites of neighboring nanoparticles. Large and small particle sizes are responsible for the two activation energies.
Electronic transport characteristics in a one-dimensional constriction defined by a triple-gate structure
Authors: Lee Huang-Ming , Muraki Koji , Chang Edward Yi , Hirayama Yoshiro
Published: September 2006

We investigate the electronic transport characteristics of a one-dimensional (1D) narrow constriction defined in a GaAs∕AlxGa1−xAs heterostructure by a simple triple-gate structure consisting of a pair of split gates and an additional surface Schottky gate (center gate) between them. Comparison between devices with and without a center gate reveals that the center gate, even when zero biased (VCG=0V), significantly modifies the surface potential and facilitates the 1D confinement in a deep two-dimensional electron system. The pinch-off voltages at VCG=0V for various channel widths W (=0.4–0.8μm) and lengths L (=0.2–2μm) are well described by the analytical formula based on the pinned-surface model [J. H. Davies et al., J. Appl. Phys. 77, 4504 (1995)]. Nonlinear transport spectroscopy with an additional dc bias shows that the lowest 1D subband energy separation (ΔE1,2) changes linearly with VCG and can be enhanced by 70% for VCG=0.8V. A simple model assuming an infinitely long channel and no self-consistent potential well reproduces the overall behavior of the measured ΔE1,2. In addition, effects of impurities, occasionally found for long-channel devices (L⩾1μm), are found to be greatly reduced by applying positive VCG and thereby enhancing ΔE1,2. Data are also presented for the transport anomaly below the first conductance plateau, the so-called “0.7 anomaly,” demonstrating that the triple-gate structure is useful for the study of density-dependent phenomena in a 1D system.
Crystallization behavior and magnetic properties of Cu-containing Fe–Cr–Mo–Ga–P–C–B alloys
Authors: Borrego J. M. , Conde C. F. , Conde A. , Stoica M. , Roth S. , Greneche J. M.
Published: September 2006

Multicomponent Fe65.5Cr4Mo4Ga4P12C5B5.5 amorphous alloy is known to have good soft magnetic properties combined with large glass-forming ability. Searching for a nanocrystalline structure that could improve its magnetic properties, Cr and Mo were partially substituted by Cu. The devitrification process of Cu-containing alloys shows an additional broad crystallization event that can be described by an Avrami law, with an exponent close to unity, typical for nanocrystallization phenomena. The microstructure of the nanocrystalline alloys consists of Fe(Ga) nanocrystals with a mean grain size of 5–10nm embedded in an amorphous matrix. The maximum crystalline volume fraction achieved at the end of the nanocrystallization process was found to be of around 20% and for alloys with high Cu content a number of crystals of larger size (∼40nm) were observed. The combined use of x-ray diffraction and Mössbauer spectrometry allowed to estimate the compositions of both the nanocrystalline grains and the amorphous residual phase of nanocrystalline alloys. The complex hyperfine structure of the studied alloys at the end of the nanocrystallization process was characterized.
Acceleration of electrons by a circularly polarized laser pulse in the presence of an intense axial magnetic field in vacuum
Authors: Singh K. P.
Published: September 2006

Acceleration of electrons by a circularly polarized laser pulse in the presence of a short duration intense axial magnetic field has been studied. Resonance occurs between the electrons and the laser field for an optimum magnetic field leading to effective energy transfer from laser to electrons. The value of optimum magnetic field is independent of the laser intensity and decreases with initial electron energy. The electrons rotate around the axis of the laser pulse with small angle of emittance and small energy spread. Acceleration gradient increases with laser intensity and decreases with initial electron energy.
Decay of the electron number density in the nitrogen afterglow using a hairpin resonator probe
Authors: Siefert Nicholas S. , Ganguly Biswa N. , Sands Brian L. , Hebner Greg A.
Published: September 2006

A hairpin resonator was used to measure the electron number density in the afterglow of a nitrogen glow discharge (p=0.25–0.75Torr). Electron number densities were measured using a time-dependent approach similar to the approach used by Spencer et al. [J. Phys. D 20, 923 (1987)]. The decay time of the electron number density was used to determine the electron temperature in the afterglow, assuming a loss of electrons via ambipolar diffusion to the walls. The electron temperature in the near afterglow remained between 0.4 and 0.6eV, depending on pressure. This confirms the work by Guerra et al. [IEEE Trans. Plasma. Sci. 31, 542 (2003)], who demonstrated experimentally and numerically that the electron temperature stays significantly above room temperature via superelastic collisions with highly vibrationally excited ground state molecules and metastables, such as AΣu+3.
Elastic behavior and strength of Al2O3 fiber/Al matrix composite and implications for equation of state measurements in the diamond anvil cell
Authors: Conil N. , Kavner A.
Published: September 2006

To examine pressure relationships in a mixed phase assemblage, we performed room temperature/high pressure radial x-ray diffraction measurements on a controlled-geometry bimaterial composite consisting of oriented Al2O3 fibers embedded in an aluminum matrix. Lattice strains of each material were measured as a function of orientation with respect to the fiber alignment, as a function of orientation with respect to the major principal stress axis of the diamond cell, and as a function of pressure of up to 15GPa. The results show that Al and Al2O3 both support differential stresses, with Al supporting between −0.06(45) and 0.32(65)GPa and Al2O3 supporting between 1.4(3) and 4.9(9)GPa. The hydrostatic pressures determined from the average lattice strains of Al and Al2O3 are not in general equal, with the pressure of Al2O3 higher than that of Al by an average of 0.5(4)GPa throughout the measured range. The geometric relationship between the composite material and the principal stress axis of the diamond cell plays a role in establishing both the absolute and relative strain responses of the composite sample. A comparison of the two composite geometries under the same diamond cell compression shows that when the fibers are oriented vertically along the diamond cell axis, the differential stress supported by Al2O3 is 3.1(5)GPa, at a pressure of 9.35(42)GPa. The corresponding values for Al are much lower: 0.09(18)GPa (differential stress) and 8.67(04)GPa (hydrostatic pressure). When the fibers are oriented horizontally along the radial direction, the pressure supported by Al and Al2O3 is more similar: 9.63(15) vs 9.48(35)GPa. The differential stress supported by both materials is higher: 0.32(65) for Al and 4.9(9) for Al2O3. Understanding the strength and elastic behavior of an intermixed phase assemblage is vital for the interpretation of mineral behavior at high pressures and temperatures. Many in situ measurements of high pressure mineral phase stability and elasticity are performed using intermixed phases—the unknown and a reference marker. Measurement of properties relies on the assumptions that the reference material has an accurate and well-calibrated equation of state and that the pressures of the two materials are identical in the high pressure sample chamber. This latter assumption is clearly violated in our experiments. Therefore, it is important to account for potential pressure effects due to sample geometry when making in situ x-ray measurements of equations of state and phase transformations.
Effect of tip geometry on contrast and spatial resolution of the near-field microwave microscope
Authors: Imtiaz Atif , Anlage Steven M.
Published: September 2006

The near-field scanning microwave microscope (NSMM) can quantitatively image materials properties at length scales far shorter than the free space wavelength (λ). Here we report a study of the effect of tip geometry on the NSMM signals. This particular NSMM utilizes scanning tunneling microscopy (STM) for distance-following control. We systematically examined many commercially available STM tips and found them to have a conical structure on the macroscopic scale, with an embedded sphere (of radius rsphere) at the apex of the tip. The rsphere values used in the study ranged from 0.1to12.6μm. Tips with larger rsphere show good signal contrast [as measured by the frequency shift (Δf) signal between tunneling height and 2μm away from the sample] with NSMM. For example, the tips with rsphere=8μm give signal contrast of 1000kHz compared to 85kHz with a tip of rsphere=0.55μm. However, large rsphere tips distort the topographic features acquired through STM. A theoretical model is used to understand the tip-to-sample interaction. The model quantitatively explains the measured change in quality factor (Q) as a function of height over bulk copper and silicon samples.
Time-domain simulation of Schrödinger equation to determine the effective potential induced by an oscillating standing wave
Authors: Na Y.C. Neil , Yamamoto Yoshihisa
Published: September 2006

The effective potential for a charged particle induced by a rapidly oscillating field is in general difficult to determine. An analytical solution for such a problem is available for the case of Paul trap, i.e., when the sinusoidally oscillating potential is a quadratic function of position. In this paper, we describe time-domain simulation of the time-dependent Schrödinger equation to determine the effective two-dimensional lattice potential induced by an oscillating standing wave. Such a scheme can be generalized to determine the effective potential induced by any rapidly oscillating field.
Doping in quantum cascade lasers. II. GaAs∕Al0.15Ga0.85As terahertz devices
Authors: Ajili Lassaad , Scalari Giacomo , Giovannini Marcella , Hoyler Nicolas , Faist Jérôme
Published: September 2006

The performances of GaAs∕Al0.15Ga0.85As terahertz quantum cascade lasers based on a bound-to-continuum transition are investigated as a function of injector doping. A linear dependence between threshold current and doping is found and agrees with the expected increase of waveguide losses due to free carrier absorption.
Electronic structure and x-ray magnetic circular dichroism in the Heusler alloy Co2MnGe
Authors: Antonov V. N. , Jepsen O. , Yaresko A. N. , Shpak A. P.
Published: September 2006

The electronic structure and x-ray magnetic circular dichroism (XMCD) spectra of the Heusler alloy Co2MnGe were investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band structure method. Densities of valence states, orbital and spin magnetic moments, as well as polarization of the electronic states at the Fermi level are analyzed and discussed. The origin of the XMCD spectra in the Co2MnGe compound is examined. The calculated results are compared with available experimental data.