bluuebunny/crossref_metadata_2025 huggingface dataset:
Kilimanjaro Glaciers: Recent areal extent from satellite data and new interpretation of observed 20th century retreat rates
Authors: Cullen Nicolas J. , Mölg Thomas , Kaser Georg , Hussein Khalid , Steffen Konrad , Hardy Douglas R.
Published: August 2006

Recent and long term variations in ice extent on Kilimanjaro are investigated in the context of 20th century climate change in East Africa. Quickbird satellite data show that the areal extent of glaciers on Kilimanjaro is 2.51 km2 in February 2003. To assess glacier retreat on Kilimanjaro two glacier systems are identified: (1) plateau (≥5700 m) and (2) slope (<5700 m). Vertical wall retreat that governs the retreat of plateau glaciers is irreversible, and changes in 20th century climate have not altered their continuous demise. Rapid retreat of slope glaciers at the beginning of the 20th century implies a strong departure from steady state conditions during this time. This strong imbalance can only be explained by a sudden shift in climate, which is not observed in the early 20th century. Results suggest glaciers on Kilimanjaro are merely remnants of a past climate rather than sensitive indicators of 20th century climate change.
Spatial variability and trends in observed snow depth over North America
Authors: Dyer Jamie L. , Mote Thomas L.
Published: August 2006

This study uses a gridded dataset of daily U.S. and Canadian surface observations from 1960–2000 to study regional spatial and temporal variability and trends in snow depth across North America. Analysis shows minimal change in North American snow depth through January, with regions of decreasing snow depths beginning in late January. These regional decreases grow in intensity and extent through March and into April, implying an earlier onset of spring melt. The region showing the greatest decreases in snow depth occurs in central Canada, along a line from the Yukon Territory in northwestern Canada to the Great Lakes region. The regional decreases in spring snow depth across central Canada are likely a result of more rapid melt of shallower winter snowpacks, evident through shallower snow cover (2–10 cm) during May and October and a decrease in extent of deeper snowpacks (>40cm) through March and April.
Solar activity variations of the ionospheric peak electron density
Authors: Liu Libo , Wan Weixing , Ning Baiqi , Pirog O. M. , Kurkin V. I.
Published: August 2006

The daily averaged Solar EUV Monitor (SEM)/Solar Heliospheric Observatory (SOHO) EUV measurements, solar proxies, and foF2 data at 20 ionosonde stations in the east Asia/Australia sector are collected to investigate the solar activity dependences of the ionospheric peak electron density (NmF2). The intensities of solar EUV from the SEM/SOHO measurements from 1996 to 2005 show a nonlinear relationship with F107, and the SEM/SOHO EUV can be better represented by a solar activity factor P = (F107 + F107A)/2. Seasonal and latitudinal dependences are found in the solar activity variation of NmF2 in the east Asia/Australian sector. The slope of NmF2 with P in the linear segment further shows similar annual variations as the background electron densities at moderate solar activity. Observations show a nonlinear dependence of NmF2 on solar EUV (the saturation effect of NmF2 for high solar EUV). On the basis of a simple model of photochemistry, taking the neutral atmospheric consequences into account, calculations at fixed height simulate the saturation effect of NmF2, but the observed change rate of NmF2 with P is inadequately reproduced. Calculations taking into account the influence of dynamics (via a simple model of the solar EUV dependence of the ionospheric height) tend to reproduce the observed change rate of NmF2. Results indicate that besides solar EUV changes, the influence of dynamics and the atmospheric consequences should substantially contribute to the solar activity variations of NmF2.
Directional spectra of hurricane wind waves
Authors: Young I. R.
Published: August 2006

A comprehensive directional wave buoy data set showing the directional wave spectrum during the passage of a number of hurricanes is presented. The data confirm remote sensing measurements, showing that waves in the forward quadrants of the storm are dominated by swell radiating out from the intense wind regions to the right of the storm centre. The data show that for almost all quadrants of the storm, the dominant waves are remotely generated swell. The directional spectra are composed of swell at low frequency (the dominant waves) and locally generated waves above approximately three times the spectral peak frequency. There is, however, no tendency for the spectrum to become bi‐modal in either frequency or direction. Rather, the spectra are directionally skewed, with a smooth directional transition from low frequency to high frequency. As for uni‐directional wind field cases, the spectra are narrowest at the spectral peak frequency and broaden at frequencies above and below the peak. Despite the fact that much of the wave field is dominated by swell, the spectral width, as a function of non‐dimensional frequency is very similar to that reported for uni‐directional wind fields. The one‐dimensional spectrum can be approximated by the parametric form proposed by Donelan et al. (1985). The parameters defining the spectrum also follow the same functional dependence as that reported for uni‐directional winds. The fact that both the one‐dimensional and directional spectra are very similar to spectra reported under simple uni‐directional winds is interpreted as being a result of the shape stabilization effects of non‐linear interactions. The data exhibit these same functional forms at low frequencies where they can be receiving no significant local input from the wind. This result indicates that the spectral shape is being controlled almost completely by the non‐linear interactions with input and dissipation terms of lesser importance. This result indicates that input and dissipation are important in determining the total quantity of energy in the wave field, but appear to play only a minor role in determining the spectral shape.
Single‐particle measurements of midlatitude black carbon and light‐scattering aerosols from the boundary layer to the lower stratosphere
Authors: Schwarz J. P. , Gao R. S. , Fahey D. W. , Thomson D. S. , Watts L. A. , Wilson J. C. , Reeves J. M. , Darbeheshti M. , Baumgardner D. G. , Kok G. L. , Chung S. H. , Schulz M. , Hendricks J. , Lauer A. , Kärcher B. , Slowik J. G. , Rosenlof K. H. , Thompson T. L. , Langford A. O. , Loewenstein M. , Aikin K. C.
Published: August 2006

A single‐particle soot photometer (SP2) was flown on a NASA WB‐57F high‐altitude research aircraft in November 2004 from Houston, Texas. The SP2 uses laser‐induced incandescence to detect individual black carbon (BC) particles in an air sample in the mass range of ∼3–300 fg (∼0.15–0.7 μm volume equivalent diameter). Scattered light is used to size the remaining non‐BC aerosols in the range of ∼0.17–0.7 μm diameter. We present profiles of both aerosol types from the boundary layer to the lower stratosphere from two midlatitude flights. Results for total aerosol amounts in the size range detected by the SP2 are in good agreement with typical particle spectrometer measurements in the same region. All ambient incandescing particles were identified as BC because their incandescence properties matched those of laboratory‐generated BC aerosol. Approximately 40% of these BC particles showed evidence of internal mixing (e.g., coating). Throughout profiles between 5 and 18.7 km, BC particles were less than a few percent of total aerosol number, and black carbon aerosol (BCA) mass mixing ratio showed a constant gradient with altitude above 5 km. SP2 data was compared to results from the ECHAM4/MADE and LmDzT‐INCA global aerosol models. The comparison will help resolve the important systematic differences in model aerosol processes that determine BCA loadings. Further intercomparisons of models and measurements as presented here will improve the accuracy of the radiative forcing contribution from BCA.
Estimation of geomagnetically induced current levels from different input data
Authors: Pulkkinen Antti , Viljanen Ari , Pirjola Risto
Published: August 2006

Pragmatic schemes for estimating geomagnetically induced current (GIC) values using different levels of knowledge about the physical quantities associated with the geomagnetic induction process are studied. The fundamental idea behind the proposed schemes is that as the knowledge about the detailed behavior of the relevant physical quantities decreases, the lack of knowledge is compensated by statistical characteristics of the geomagnetic induction process and GIC. It is shown that reasonable GIC estimates can be obtained if some information about the state of the geomagnetic environment is available. For example, single values of the time derivative of the magnetic field at the surface of the Earth or even a mere single value of the field deflection from the baseline level can sometimes be sufficient to give a reasonable statistical GIC estimate. It is also shown that the sampling rate of the magnetic field variations used to make the GIC estimates can be lowered down to about 60 s without crucially degrading the accuracy of the estimation.
Measured dependence of nuclear burn region size on implosion parameters in inertial confinement fusion experiments
Authors: Séguin F. H. , DeCiantis J. L. , Frenje J. A. , Li C. K. , Rygg J. R. , Chen C. D. , Petrasso R. D. , Delettrez J. A. , Regan S. P. , Smalyuk V. A. , Glebov V. Yu. , Knauer J. P. , Marshall F. J. , Meyerhofer D. D. , Roberts S. , Sangster T. C. , Stoeckl C. , Mikaelian K. , Park H. S. , Robey H. F. , Tipton R. E.
Published: August 2006

Radial profiles of nuclear burn in directly driven, inertial-confinement-fusion implosions have been systematically studied for the first time using a proton emission imaging system sensitive to energetic 14.7MeV protons from the fusion of deuterium (D) and 3-helium (He3) at the OMEGA laser facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. Experimental parameters that were varied include capsule size, shell composition and thickness, gas fill pressure, and laser energy. Clear relationships have been identified between changes in a number of these parameters and changes in the size of the burn region, which we characterize here by the median “burn radius” Rburn containing half of the total DHe3 reactions. Different laser and capsule parameters resulted in burn radii varying from 20to80μm. For example, reducing the DHe3 fill pressure from 18to3.6atm in capsules with 20μm thick CH shells resulted in Rburn changing from 31to25μm; this reduction is attributed to increased fuel-shell mix for the more unstable 3.6atm implosions rather than to increased convergence, because total areal density did not change very much. Fuel-shell-interface radii estimated from hard (4–5keV) x-ray images of some of the same implosions were observed to closely track the burn radii. Burn radii from one-dimensional (1D) simulations agree fairly well with measurements for glass-shell capsules, but are systematically smaller than measurements for CH-shell capsules. A search for possible sources of systematic measurement error that could account for this discrepancy has been unsuccessful. Possible physical sources of discrepancies are mix, hydrodynamic instabilities, and/or preheat not included in the 1D code. Since measured burn-region sizes indicate where fusion actually occurs as a consequence of all the complicated processes that affect capsule implosion dynamics, it provides exacting tests of simulations.
Magnetic field effects on secondary electron emission during ion implantation in a nitrogen plasma
Authors: Tan Ing Hwie , Ueda Mario , Dallaqua Renato S. , de Moraes Oliveira Rogerio , Rossi José O.
Published: August 2006

In this work, the magnetic suppression of secondary electrons emitted during nitrogen plasma immersion ion implantation is investigated. Secondary electrons were measured by two Faraday cups with and without the presence of a magnetic field parallel to the target surface. One Faraday cup detects the electrons emerging perpendicularly to the target surface and magnetic field lines, while another cup detects electrons flowing along the field lines. Increase of magnetic field intensity resulted in a decrease of the amount of electrons detected by the perpendicular Faraday cup and in an increase of the electrons detected by the longitudinal one. This shows that secondary electrons were transversally confined by the magnetic field but diffused away from the target ends along the field lines. The secondary electron emission coefficient (γ) was estimated and the results showed that partial suppression (decrease in γ) was achieved when the plasma density was increased by an order of magnitude. We propose an explanation based on the formation of an electron layer (virtual cathode) near the target surface. Better suppression in denser plasmas would be expected because the virtual cathode would be maintained if the electron layer is formed faster than it is longitudinally lost.
Current and optical noise of GaN∕AlGaN light emitting diodes
Authors: Sawyer S. , Rumyantsev S. L. , Shur M. S. , Pala N. , Bilenko Yu. , Zhang J. P. , Hu X. , Lunev A. , Deng J. , Gaska R.
Published: August 2006

Low frequency noise of current and light intensity of ultraviolet light emitting diodes (LED) with wavelength from 265to340nm are the superposition of the 1∕f and generation-recombination noise. The dependence of generation-recombination noise on the LED current has a maximum caused by a relatively shallow trap level in the quantum well. The upper bound of this trap level concentration is estimated to be Nt=7×1015cm−3. The relative spectral noise density of the light intensity fluctuations decreased with an increase of the LED forward current. At high currents, the difference in the noise level for LEDs with different wavelength is small and is of the same order of magnitude or even smaller than for visible LEDs.
Alignment and self-assembly of elongated micronsize rods in several flow fields
Authors: Katz E. , Yarin A. L. , Salalha W. , Zussman E.
Published: August 2006

Microfluidic alignment has attracted attention as a possible tool for the orientation of micronsize rods suspended in a solvent and the deposition of these rods in ordered arrays on solid surfaces. In the present work the following situations are realized experimentally: (i) a flow through an abruptly converging (or diverging) joint between wide and narrow straight channels which entrains and aligns (or misaligns) microrods and (ii) a tiny droplet of the order of a hundred microns that contains microrods is displaced due to Marangoni convection forces in a channel subjected to a temperature gradient. The flow near the advancing contact line orients the random suspension of rods and guides them towards the channel bottom where they are deposited in an ordered array. In parallel with this experimental study, the processes of microrod reorientation and alignment in the above-mentioned flows and in a similar sinklike flow into an issuing jet were studied using solutions of the Fokker-Planck equation for the orientation probability density function.
Low temperature transport properties of Ni-doped CoGe1.5Se1.5
Authors: Nolas G. S. , Beekman M. , Ertenberg R. W. , Yang J.
Published: August 2006

We have synthesized and characterized the electrical resistivity [ρ(T)], Seebeck coefficient [S(T)], and thermal conductivity of Ni-doped polycrystalline CoGe1.5Se1.5 skutterudite. Nickel acts as an electron donor and has a significant effect on the transport properties of these ternary skutterudites. These compounds show semiconducting transport above 50K. Below 50K the Seebeck coefficient changes sign and rapidly increases by several thousand μVK−1 in magnitude. Both ρ(T) and S(T) are activated at low temperatures, but with different activation energies. We interpret this as evidence of impurity band conduction via variable range hopping. We also discuss these results in terms of the potential for thermoelectric applications.
Mathematical modeling of heat transfer, fluid flow, and solidification during linear welding with a pulsed laser beam
Authors: Roy G. G. , Elmer J. W. , DebRoy T.
Published: August 2006

The evolution of temperature and velocity fields during welding of 304 stainless steel with a pulsed laser beam was simulated using a three dimensional numerical heat transfer and fluid flow model. The weld pool solidified between pulses and regions of the weld bead melted and solidified several times during welding. Short laser pulses restricted the width of the weld track and velocities in the weld pool. However, convection still remained an important mechanism of heat transfer in the weld pool. The computed high cooling rates during linear welding with neodymium-doped yttrium aluminum garnet pulsed laser operated at 140W average power, 20Hz frequency, and 5ms pulse duration were consistent with those observed in typical laser welding. After the laser beam was switched off, the mushy zone expanded, reaching its maximum size when no pure liquid region remained. Calculations of solidification parameters indicated that the criterion for plane front solidification was not satisfied. The results demonstrate that the application of numerical transport phenomena can significantly improve the current understanding of linear welding with a pulsed laser beam.
Time-resolved dynamic tensile spall of pure aluminum under laser irradiation
Authors: Wang Yonggang , He Hongliang , Wang Lili , Jing Fuqian , Boustie Michel , Sekine Toshimori
Published: August 2006

A diagnostic technique with high time resolution in the velocity-history measurement is presented by coupling an electronic streak camera with a push-pull type velocity interferometer system for any reflector. This technique has been applied to investigate the dynamic tensile spall induced by laser irradiation in pure aluminum by measuring the free surface velocity profiles of samples. Laser irradiation intensities are in the range of 1010–1012W∕cm2. Spall strength in pure aluminum is calculated from the measured free surface velocity profile as a function of the tensile strain rate. The results show a rapid increase with the tensile strain rate more than 106s−1. The damage influence on the tensile spall of laser-irradiated pure aluminum is analyzed by experimental and numerical studies. In the frame of percolation theory, a physical model is proposed to describe the rapid stress release due to void coalescence. A good agreement between the calculation and the experiment is obtained.
Transmission electron microscopy assessment of the Si enhancement of Ti∕Al∕Ni∕Au Ohmic contacts to undoped AlGaN∕GaN heterostructures
Authors: Desmaris Vincent , Shiu Jin-Yu , Lu Chung-Yu , Rorsman Niklas , Zirath Herbert , Chang Edward-Yi
Published: August 2006

The microstructure of Si∕Ti∕Al∕Ni∕Au was investigated using transmission electron microscopy and energy dispersive x-ray spectroscopy. The dependence of the contact resistance on the silicon layer thickness and the temperature was correlated to the microstructure of the alloyed contacts. The enhancement of the contact resistance by inserting a 30Å thick Si layer under the Ti∕Al∕Ni∕Au metallization was attributed to diffusion of the contact into the AlGaN layer. Increasing the Si thickness and or the temperature resulted in the formation of Gold (Au)-based silicides, which prevent the formation of low interfacial TiN or AlN layers.
Strontium-modified lead zirconate titanate thin films for electrically tunable device applications
Authors: Shao Qi-Yue , Li Ai-Dong , Xia Yi-Dong , Wu Di , Liu Zhi-Guo , Ming Nai-Ben
Published: August 2006

Strontium-modified lead zirconate titanate Pb1−xSrxZr0.52Ti0.48O3 [(PSZT) x=0.2–0.8] thin films were prepared on Pt∕TiO2∕SiO2∕Si substrates by the sol-gel method. The Curie temperature of PSZT films decreases with the increase of Sr contents and paraelectric PSZT films at room temperature are demonstrated as x approaches 0.4. The increase of Sr contents also leads to the simultaneous decrease of dielectric constant, tunability, and dielectric loss. The composition dependence of Curie temperature and tunability can be attributed to the shrinkage of crystal lattice due to a Sr addition. PSZT films with x=0.6 show the largest figure of merit of 24 with a moderate tunability of 48% and a dielectric loss of 0.02. This suggests that Sr-modified PZT is a potential candidate for voltage tunable applications.
Electrical properties of transparent and conducting Ga doped ZnO
Authors: Bhosle V. , Tiwari A. , Narayan J.
Published: August 2006

In this paper, we report on the metal-semiconductor transition behavior observed in transparent and conducting ZnO:Ga films grown by pulsed-laser deposition. The electrical resistivity measurements were carried out on ZnO films with varying Ga concentration in the temperature range of 14to300K. The electrical properties were correlated with film structure, and detailed structural characterization was performed using x-ray diffraction, transmission electron microscopy, and x-ray photoelectron spectroscopy. The room-temperature resistivity of these films was found to decrease with Ga concentration up to 5% Ga, and then increase. The lowest value of resistivity (1.4×10−4Ωcm) was found at 5% Ga. Temperature dependent resistivity measurements showed a metal-semiconductor transition, which is rationalized by localization of degenerate electrons. A linear variation of conductivity with T below the transition temperature suggests that the degenerate electrons are in a weak-localization regime. It was also found that the transition temperature is dependent on the Ga concentration and is related to the increase in disorder induced by dopant addition. The results of this research help to understand the additional effects of dopant addition on transport characteristics of transparent conducting oxides (TCOs) and are critical to further improvement and optimization of TCO properties.
Optical properties of epitaxial and polycrystalline Sr1.8Ca0.2NaNb5O15 thin-film waveguides grown by pulsed laser deposition
Authors: Liu W. C. , Mak C. L. , Wong K. H. , Wang D. Y. , Chan H. L. W.
Published: August 2006

Epitaxial and polycrystalline ferroelectric Sr1.8Ca0.2NaNb5O15 thin films have been prepared by pulsed laser deposition on (100)MgO and fused quartz substrates, respectively. These films exhibit excellent surface flatness and high optical transmittance of about 80% over the whole visible region. Their good optical waveguide characteristics are demonstrated using prism coupling technique. Our results show that the internal scattering of the film is more influential in governing the propagation loss than the surface scattering, and that a heteroepitaxial growth of ferroelectric oxide films is essential for waveguide devices.
Gauge theories of Dirac type
Authors: Tolksdorf Jürgen , Thumstädter Torsten
Published: August 2006

A specific class of gauge theories is geometrically described in terms of fermions. In particular, it is shown how the geometrical frame presented naturally includes spontaneous symmetry breaking of Yang-Mills gauge theories without making use of a Higgs potential. In more physical terms, it is shown that the Yukawa coupling of fermions, together with gravity, necessarily yields a symmetry reduction provided the fermionic mass is considered as a globally well-defined concept. The structure of this symmetry breaking is shown to be compatible with the symmetry breaking that is induced by the Higgs potential of the minimal Standard Model. As a consequence, it is shown that the fermionic mass has a simple geometrical interpretation in terms of curvature and that the (semiclassical) “fermionic vacuum” determines the intrinsic geometry of space-time. We also discuss the issue of “fermion doubling” in some detail and introduce a specific projection onto the “physical subspace” that is motivated by the Standard Model.
Effects of SiN passivation by catalytic chemical vapor deposition on electrical properties of AlGaN∕GaN heterostructure field-effect transistors
Authors: Higashiwaki Masataka , Onojima Norio , Matsui Toshiaki , Mimura Takashi
Published: August 2006

We investigated the effects of SiN passivation by catalytic chemical vapor deposition (Cat-CVD) on the electrical properties of AlGaN∕GaN heterostructure field-effect transistors. The two-dimensional electron density (Ns) greatly increased after the Cat-CVD SiN deposition, and the tendency of the increase was enhanced with decreasing AlGaN barrier thickness. As a result of the large increase in Ns, the sheet resistance (Rsh) significantly decreased after the deposition, and it had low values of 320–460Ω∕◻ for extremely thin AlGaN barriers of 4–10nm. The increase in Ns showed little dependence on SiN thickness, indicating that the stress applied to the AlGaN barrier by SiN cannot be the origin of the increase. Cat-CVD SiN also improved the in-plane uniformity of mobility for extremely thin-barrier structures, which in turn improved the uniformity of Rsh. Moreover, we found that Cat-CVD was more effective than plasma-enhanced chemical vapor deposition in increasing Ns. A comparison of theoretical calculations and experimental results indicated that these behaviors can be explained by a decrease in the AlGaN surface barrier height due to the SiN deposition.
Effect of nitridation on the growth of GaN on ZrB2(0001)∕Si(111) by molecular-beam epitaxy
Authors: Wang Zhi-Tao , Yamada-Takamura Y. , Fujikawa Y. , Sakurai T. , Xue Q. K. , Tolle J. , Kouvetakis J. , Tsong I. S. T.
Published: August 2006

The effect of nitridation on the epitaxial growth of GaN on lattice-matched ZrB2(0001) films prepared ex situ and in situ was studied using an ultrahigh-vacuum molecular-beam epitaxy (MBE)-scanning probe microscopy system. The growth of GaN was carried out by rf-plasma-assisted MBE, and epitaxy of wurtzite GaN was observed on both ex situ and in situ prepared ZrB2 samples. The polarity was found to be consistently N-polar regardless of the samples, based on the observation of a series of N-polar Ga-rich reconstructions: (3×3), (6×6), and c(6×12). The nitridation of ZrB2 film was conducted by exposing it to active nitrogen and well-ordered hexagonal-BN (h-BN) formation was observed when the annealing temperature was above 900°C. The partially formed BN layer affected neither the epitaxy nor the polarity of GaN, but when the surface was fully covered with well-ordered h-BN, GaN growth did not occur.
String equations in Whitham hierarchies: τ-functions and Virasoro constraints
Authors: Alonso Luis Martínez , Medina Elena , Mañas Manuel
Published: August 2006

A scheme for solving Whitham hierarchies satisfying a special class of string equations is presented. The τ-function of the corresponding solutions is obtained and the differential expressions of the underlying Virasoro constraints are characterized. Illustrative examples of exact solutions of Whitham hierarchies are derived and applications to conformal maps dynamics are indicated.
Optical investigation of femtosecond laser induced microstress in neodymium doped lithium niobate crystals
Authors: Ródenas A. , Sanz García J. A. , Jaque D. , Torchia G. A. , Mendez C. , Arias I. , Roso L. , Agulló-Rueda F.
Published: August 2006

The depth-resolved micromodification of single-crystalline femtosecond laser irradiated Nd3+ doped MgO:LiNbO3 crystals is investigated by means of micro-Raman and microluminescence experiments. We have found that a permanent tensile stress of the order of 2GPa is induced in the vicinity of ablated volume as a consequence of the pressure-wave propagation due to the thermoelastic relaxation of the laser irradiated material. Microluminescence experiments have revealed that, as a consequence of the permanent laser induced microstress, a localized redshift of the F3∕24→I9∕24 luminescence band of Nd3+ ions also takes place due to a crystal field modification. The analysis of Raman and fluorescence bandwidths indicates that a slight lattice disorder and densification is induced by femtosecond laser irradiation.
Behavior of the plasma in the anchor cell of the GAMMA 10 tandem mirror
Authors: Islam Md. Khairul , Nakashima Yousuke , Higashizono Yuta , Cho Teruji
Published: August 2006

Anchor cells of the GAMMA 10 tandem mirror are composed of minimum-B magnetic field configuration. Behavior of the plasma in the anchor cell is studied using puffing gas flow rate (PGFR) modulation and by covering some parts of the flux tube of each anchor cell by conducting plates (APs). Ambient neutral pressure is decreased by PGFR modulation during axial/potential confinement in both cases without and with APs and it is found that both line density (nlEA) and diamagnetism (dmEA) of the anchor cell plasma increase. Lower neutral pressure in the minimum-B region is investigated during the floated APs and significant improvement of the GAMMA 10 plasma parameters is observed. Possible explanations of the experimental results are given from the viewpoint of ambient neutral pressure oriented anchor plasma behavior. Probable reasons of enhanced ambient neutral pressure in the anchor cell and adverse effects of these neutrals on GAMMA 10 plasma parameters are also pointed out.
Anomalous photoconductivity in gamma In2Se3
Authors: Sreekumar R. , Jayakrishnan R. , Sudha Kartha C. , Vijayakumar K. P.
Published: August 2006

Negative photoconductivity in indium selenide (γ-In2Se3) thin films was observed at room temperature and was attributed to trapping of electrons and destruction of minority carriers during illumination through recombination. Photoconductivity of the films exhibited a strong dependence on the concentration of indium in the films. Photoconductivity decreased gradually and became negative as indium concentration increased. But there was no considerable variation in the optical band gap (1.84eV) of the films, on varying indium concentration. Increase of indium concentration introduced defect levels at 1.46 and 1.32eV above the valance band. Photoluminescence study revealed the emission to a recombination center, which is situated at 290meV above valance band for all the samples. Levels at 1.46 and 1.32eV prevented photogenerated carriers from reaching conduction band, during illumination. Thus the capture of conduction band electrons and destruction of minority carries via recombination, resulted in negative photoconductivity.
Equation of state and mechanical response of NiTi during one-dimensional shock loading
Authors: Meziere Y. J. E. , Millett J. C. F. , Bourne N. K.
Published: August 2006

The equation of state and the mechanical response (dynamic tensile strength and dynamic shear strength) of the shape memory alloy NiTi have been investigated using plate impact. The Hugoniot has been extended with additional data and a nonlinear behavior of the Hugoniot (shock velocity-particle velocity) has been noted. A bilinear representation has been proposed for the trend. These two behaviors were attributed to the shock-induced phase transformation from B2 to monoclinic. However, this phase transformation seems to have no influence on the dynamic tensile strength. A minimum impact stress value was found necessary to create the spallation in NiTi but the pull back stress remains near constant above this value. A negative strain-rate dependency was also noted on the spallation. The shear strength (τ) of NiTi appears to increase continuously with the impact stress. The evolution of τ behind the shock front seems to be linked to the phase transformation observed in determination of the Hugoniot. Indeed, It was observed that there was no change of τ until an impact stress value corresponding to the inflection point on the Hugoniot, followed by increase behind the shock front above this value.
Microwave and infrared dielectric response of monoclinic bismuth zinc niobate based pyrochlore ceramics with ion substitution in A site
Authors: Wang Hong , Kamba Stanislav , Zhang Meiling , Yao Xi , Denisov Sergey , Kadlec Filip , Petzelt Jan
Published: August 2006

It is well known that the cubic pyrochlore Bi1.5ZnNb1.5O7 exhibits higher permittivity and dielectric loss than monoclinic Bi2Zn2∕3Nb4∕3O7 due to structural disorder in the A sites of Bi1.5ZnNb1.5O7. We have studied systematically the impact of the ion substitution in the A site of monoclinic Bi2Zn2∕3Nb4∕3O7 on the structure and microwave dielectric properties. It is shown that the structure and permittivity of (Bi1.92M0.08)(Zn0.64Nb1.36)O7 (M=Zn,Ca,Cd,Sr,Ba) ceramics remain almost the same as in Bi2Zn2∕3Nb4∕3O7; only the Ba substituted ceramics have higher permittivity due to multiphase structure. Microwave dielectric properties were compared with complex dielectric response in terahertz and infrared frequency range of 0.1–100THz, which allows us to estimate intrinsic and extrinsic contributions to microwave dielectric losses. The best microwave properties were obtained in (Bi1.92Ca0.08)(Zn0.64Nb1.36)O7 with ε=76, Qf⩾5000 (sintered below 950°C), which is promising for microwave low temperature cofiring ceramic application.
Thermal conductivity of composites of aligned nanoscale and microscale wires and pores
Authors: Prasher Ravi
Published: August 2006

Two-dimensional composite materials made from aligned nano- and microwires hold great promise for various applications such as thermoelectric device. Similarly, two-dimensional composites made from aligned nanoscale pores are also very important for various technologies. Phonon transport along such composites primarily involves three nondimensional parameters based on the phonon mean paths in the host medium and the wire. The first of these is the ratio of interwire distance to the phonon mean free path in the host medium, the second is the ratio of the diameter of the wire to the phonon mean free path in the host medium, and the third is the ratio of the diameter of the wire to the phonon mean free path in the wire. We develop an analytical model for the thermal conductivity in the longitudinal direction of these composites by solving the phonon Boltzmann transport equation. The analytical model includes the dependences of all three parameters and is in excellent agreement with a recently reported numerical model [Yang et al., Phys. Rev. B 72, 125418 (2005)]. Our solution shows that the scattering of phonons in the host medium at the wire interface reduces the thermal conductivity of the host medium.
Energy distributions of positive and negative ions during magnetron sputtering of an Al target in Ar∕O2 mixtures
Authors: Andersson Jon M. , Wallin E. , Münger E. P. , Helmersson U.
Published: August 2006

The ion flux obtained during reactive magnetron sputtering of an Al target in Ar∕O2 gas mixtures was studied by energy-resolved mass spectrometry, as a function of the total and O2 partial pressures. The positive ions of film-forming species exhibited bimodal energy distributions, both for direct current and radio frequency discharges, with the higher energy ions most likely originating from sputtered neutrals. For the negative oxygen ions a high-energy peak was observed, corresponding to ions formed at the target surface and accelerated towards the substrate over the sheath potential. As the total pressure was increased the high-energy peaks diminished due to gas-phase scattering. Based on these results, the role of energetic bombardment for the phase constituent of alumina thin films are discussed.
Ablative modification of graphite surfaces by single intense femtosecond pulses
Authors: Kudryashov Sergey I.
Published: August 2006

Several types of center-symmetrical (elliptical) microstructures of multinanometer heights are fabricated on a surface of quasicrystalline graphite ablated by single femtosecond laser pulses with peak intensities in the range of 1–102TW∕cm2. Potential underlying physical mechanisms for these high-intensity ablative micromodification phenomena are discussed.
Superior material properties of AlN on vicinal 4H-SiC
Authors: Kakanakova-Georgieva A. , Persson P. O. Å , Kasic A. , Hultman L. , Janzén E.
Published: August 2006

The crystal structure and optical properties of thick (>100nm) AlN layers grown by hot-wall metalorganic chemical vapor deposition are characterized by infrared spectroscopic ellipsometry, cathodoluminescence, and transmission electron microscopy. The choice of substrates among the available SiC wafer polytype modifications (4H∕6H) and misorientations (on-/off-axis cut) is found to determine the AlN defect interaction, stress homogeneity, and luminescence. The growth of thick AlN layers benefits by performing the epitaxy on off-axis substrates because, due to stacking faults, the propagation of threading defects in AlN layers is stopped in a narrow interface region.
Magnetic and carrier transport properties of Mn-doped p-type semiconductor LaCuOSe: An investigation of the origin of ferromagnetism
Authors: Yanagi Hiroshi , Ohno Shuichi , Kamiya Toshio , Hiramatsu Hidenori , Hirano Masahiro , Hosono Hideo
Published: August 2006

LaCuOSe is a wide band gap p-type semiconductor in which high density positive holes can be doped to exhibit degenerate conduction. These features should allow room-temperature ferromagnetism in a dilute magnetic semiconductor (DMS), which follows a theoretical prediction [T. Dietl et al., Science 287, 1019 (2000)], yet to be realized. A weak ferromagnetic response is observed in both bulk and thin epitaxial film samples of Mn-doped LaCuOSe with hole concentrations of ∼1020cm−3. However, the observed small magnetization (μB per Mn ion of less than unity) is reasonably explained by the inclusion of 0.2mol% of LaMnO3 and 0.1mol% of Mn3O4, which were quantitatively analyzed in the bulk sample using highly sensitive x-ray diffractometry, i.e., Mn-doped LaCuOSe did not exhibit ferromagnetism due to the low solubility limit of Mn in LaCuOSe (<0.5mol%). This result demonstrates the importance of complementary characterization using both thin film and bulk samples for studying DMSs.
Band offsets and chemical bonding states in N-plasma-treated HfSiON gate stacks studied by photoelectron spectroscopy and x-ray absorption spectroscopy
Authors: Oshima M. , Takahashi H. , Okabayashi J. , Toyoda S. , Kumigashira H. , Inoue M. , Mizutani M. , Yugami J.
Published: August 2006

We have investigated valence band and conduction band electronic structures and interfacial chemical bonding states of nitrogen-plasma-treated HfSiON∕SiON gate stacks on Si substrates with and without rapid thermal annealing by photoelectron spectroscopy and x-ray absorption spectroscopy to correlate them with electrical properties. We have found that the N3 component in N 1s can be correlated with electron trapping. Photoelectron spectra and O K-edge absorption spectra confirm the existence of the SiO2-rich phase-separated area in HfSiON films. Band offsets for the HfSiON∕Si and the SiO2-rich HfSiON∕Si are determined to be 2.5 and 4.2eV for valence band and 1.7 and 3.6eV for conduction band, respectively.
Raman stress maps from finite-element models of silicon structures
Authors: Bonera Emiliano , Fanciulli Marco , Carnevale Gianpietro
Published: August 2006

Raman spectroscopy finds applications in many research fields to monitor stress on a micrometric scale. Unfortunately, the interpretation of Raman maps is always complicated by the tensorial nature of stress and by averaging effects due to the finite spatial resolution of the technique. Usually, the interpretation is either purely qualitative or based on thorough simplifications. In this paper we present an approach that compares the experimental data with a virtual experiment using a finite-element model. The method is applied to the study of the stress generated in a microelectronic device during the manufacturing process. The results are used both as a feedback and a validation for the modeling and also for the interpretation of the Raman data.
Faraday ellipticity and Faraday rotation of a doped-silicon wafer studied by terahertz time-domain spectroscopy
Authors: Morikawa O. , Quema A. , Nashima S. , Sumikura H. , Nagashima T. , Hangyo M.
Published: August 2006

Free-carrier Faraday ellipticity and Faraday rotation are measured for a moderately doped n-type silicon wafer with the resistivity of 1.1Ωcm under magnetic fields of ±3T using the terahertz time-domain spectroscopy. From the experimental data, we obtain the time evolution of the electric-field vector of the terahertz radiation pulses. When the magnetic field is applied to the sample, the transmitted radiation has an elliptic polarization with its major axis rotated from the polarization direction of the incident radiation (Faraday effect). The Faraday ellipticity and Faraday rotation angle are obtained for the directly transmitted pulse (first terahertz pulse) and the pulse reflected twice at the sample surfaces (second terahertz pulse) separately. They are compared with the calculations using the Drude model. A slight deviation is observed between the experimental and calculated Faraday ellipticities and Faraday rotation angles probably due to the energy dependence of the carrier scattering time.
Nonlinear full two-fluid study of m= sausage instabilities in an axisymmetric Z pinch
Authors: Loverich J. , Shumlak U.
Published: August 2006

A nonlinear full five-moment two-fluid model is used to study axisymmetric instabilities in a Z pinch. When the electron velocity due to the current J is greater than the ion acoustic speed, high wave-number sausage instabilities develop that initiate shock waves in the ion fluid. This condition corresponds to a pinch radius on the order of a few ion Larmor radii.
Critical size for defects in nanostructured materials
Authors: Narayan Jagdish
Published: August 2006

This paper addresses some of the fundamental issues and critical advantages in reducing the grain size/feature size to the nanoscale regime. We find that as the grain size or feature size is reduced, there is a critical size below which the defect content can be reduced virtually to zero. This critical size for most defects in solid state materials falls in the nanoscale regime. Thus, nanostructured materials offer a unique opportunity to realize the property of a perfect material. However, with this opportunity comes a great challenge in terms of engineering a large fraction of atoms near the surfaces/interfaces. Since the fraction of atoms near the surfaces/interfaces increases inversely with size, this fraction can get closer to unity and determine efficacy of nanostructured materials in terms of efficiency, stability, and reliability of nanostructures. We discuss briefly ways to enhance the stability and reliability of nanostructured systems by controlling the properties of interfaces.
Vibrational properties of mixed (Y3Al5O12)x–(Y3Sc2Ga3O12)1−x crystals
Authors: Chiriu D. , Ricci P. C. , Carbonaro C. M. , Anedda A. , Aburish-Hmidat M. , Grosu A. , Lorrai P. G. , Fortin E.
Published: August 2006

The vibrational properties of yttrium aluminum garnet (YAG), yttrium scandium gallium garnet (YSGG), and mixed (YAG)x–(YSGG)1−x structures (0<x<1) were investigated by means of Raman spectroscopy. By comparison with the vibrational spectrum of the YAG structure, the attribution of the Raman modes of the YSGG is proposed. The spectra were analyzed with a model of isolated polyhedra and the formation of mixed structures was verified. The Raman modes in the mixed structures lie between those of the pure garnets indicating the presence of truly mixed and not separated phases. An analytical model based on the free tetrahedral approximation to study the position of the vibration frequencies as a function of the relative concentration of the garnets is proposed and applied to the study of the A1g breathing mode at 785cm−1 in the pure YAG structure and downshifted at 752cm−1 in the pure YSGG structure. The proposed analysis allows to verify both the quality of the grown crystal and to assess the relative concentration of the garnets.
Complex dynamics in simple Hopfield neural networks
Authors: Yang Xiao-Song , Huang Yan
Published: August 2006

A class of simple Hopfield neural networks with a parameter is investigated. Numerical simulations show that the simple Hopfield neural networks can display chaotic attractors and limit cycles for different parameters. The Lyapunov exponents are calculated; the bifurcation plot and several important phase portraits are presented as well. By virtue of a recent result of horseshoe theory in dynamical systems, we present rigorous computer-assisted verifications for chaotic behavior in the simple Hopfield neural networks for certain parameters and give a brief discussion on the robustness of the chaotic behavior. Quantitative descriptions of the complexity of these neural networks are also given in terms of topological entropy.
Effects of InxGa1−xAs matrix layer on InAs quantum dot formation and their emission wavelength
Authors: Yin Zongyou , Tang Xiaohong , Liu Wei , Zhang Daohua , Du Anyan
Published: August 2006

Self-assembled InAs quantum dots (QDs) grown on different InxGa1−xAs∕InP matrixes by metal-organic chemical vapor deposition with fast nucleation rate and atomic layer epitaxy in pure nitrogen ambient have been studied. It has been found that the InAs QD formation efficiency is enhanced when the dots are grown on the strained InxGa1−xAs∕InP matrix compared with the dots grown on an unstrained InxGa1−xAs∕InP matrix. By changing the indium composition x of the InxGa1−xAs barrier from 0.53 to 0.72, the 77K photoluminescence (PL) emission wavelength of the InAs QD structures is redshifted by 220–300nm. The transition energy of the InAs∕InGaAs∕InP QD structure has been calculated by using the eight-band k∙p theory with consideration of the strain effect from QD’s upper InxGa1−xAs barrier layer. The calculated E1-HH1 transition energies agree with the measured PL emission peaks quite well.
Load-displacement relations for nanoindentation of viscoelastic materials
Authors: Liu Chien-Kuo , Lee Sanboh , Sung Li-Piin , Nguyen Tinh
Published: August 2006

A model based on the Burgers viscoelastic concept has been developed to describe the nanoindentation behaviors of polymeric materials. An analytical solution of displacement at the indenter tip has been derived based on the analog of the governing equation of elasticity in the time coordinate system to the governing equation of the viscoelastic model in Laplace transform coordinate system. The solution consists of the elastic, viscous, and plastic displacements during loading and unloading. Nanoindentation experiments have been conducted for poly(methyl methacrylate), polycarbonate, hydroxyethyl methacrylate copolymer, amorphous syndiotactic polystyrene, and fast-cure acrylic polymers to provide data for validating the model. The results show excellent agreement between experimental load-displacement data and model prediction for both the loading and unloading before the contact area decreases for all five polymers. The viscosity decreases but the hardness increases with increasing loading rate. Young’s moduli of the interior material obtained by the present study are close to those reported in the literature. Additional analysis showed that the prediction by both the Kelvin and Maxwell viscoelastic models does not agree with the experimental data generated in this study.