bluuebunny/crossref_metadata_2025 huggingface dataset:
Are eddies nature's trigger to enhance biological productivity in the Bay of Bengal?
Authors: Prasanna Kumar S. , Nuncio M. , Narvekar Jayu , Kumar Ajoy , Sardesai S. , de Souza S. N. , Gauns Mangesh , Ramaiah N. , Madhupratap M.
Published: April 2004

The Bay of Bengal is traditionally considered to be a less productive basin compared to the Arabian Sea. Despite the contrasting chlorophyll and primary productivity pattern, sediment trap data shows that annual fluxes of organic carbon reach comparable rates in both the basins. The traditional mechanisms of nutrient supply to the upper ocean waters cannot account for this. We propose eddy pumping as a possible mechanism of vertical transfer of nutrients across the halocline to the oligotrophic euphotic zone during summer monsoon when upper ocean is highly stratified. This would induce rapid biological uptake and in turn significantly increase biological production. In the northern Bay, riverine input acts as an additional source of nutrients and augments the subsurface nutrient injection to the euphotic zone by eddy pumping. Notwithstanding this, the lower than expected primary production in the north suggests the possible role of riverine sediment in limiting the sunlight for photosynthesis.
Mass transport in gravity waves revisited
Authors: Ng Chiu‐On
Published: April 2004

On re‐examining the problem of mass transport due to partially standing waves in a domain that can be closed or open ended, it is pointed out that several aspects of the existing theories need to be clarified. Particular attention is paid to the free surface setup, whose expression is shown to be different when described by the Eulerian and Lagrangian approaches. In the Lagrangian system, it is the horizontal gradient of the setup, rather than the mean pressure gradient alone, that is implied by the condition of no net flux in a closed domain. In connection with this, one may determine for an unbounded domain the streamline value along the free surface, which cannot be fixed in the stream function formulation alone, by requiring the setup due to the unidirectional part of the mass transport to be zero. Three components of the free surface setup, which are of different orders of magnitude and arise owing to various mechanisms, are obtained in the process of deriving the solutions for the mass transport velocity in the boundary layers and the fluid core.
A new explanation for long‐duration meteor radar echoes: Persistent charged dust trains
Authors: Kelley Michael C.
Published: April 2004

Meteor radar echoes persist for timescales from 3 ms to several minutes. At the low end, head echoes provide the target while underdense, diffusing charged meteor trains are associated with timescales of several tenths of a second to several tens of seconds. Longer duration trains are less well understood, and a variety of explanations have been put forth. Indeed, a variety of mechanisms probably contribute. In this paper the author shows that a dust‐filled, underdense turbulent meteor train yielded a VHF meteor which lasted over 10 min. Remarkably, the train was also penetrated by a sounding rocket, which verifies the high Schmidt‐number nature of the train. Although the turbulence is not yet explained, it is possible that the gravitational sedimentation of the structure provided the free energy via a shear instability of the dusty plasma. These observations further support the notion that meteor trains are associated with dust production and that the double train phenomenon is related to sedimentation of a dusty train.
Estimating soil electric properties from monostatic ground‐penetrating radar signal inversion in the frequency domain
Authors: Lambot S. , Slob E. C. , van den Bosch I. , Stockbroeckx B. , Scheers B. , Vanclooster M.
Published: April 2004

A new integrated approach for identifying the shallow subsurface electric properties from ground‐penetrating radar (GPR) signal is proposed. It is based on an ultrawide band (UWB) stepped frequency continuous wave (SFCW) radar combined with a dielectric filled transverse electric and magnetic (TEM) horn antenna to be used off the ground in monostatic mode; that is, a single antenna is used as emitter and receiver. This radar configuration is appropriate for subsurface mapping and allows for an efficient and more realistic modeling of the radar‐antenna‐subsurface system. Forward modeling is based on linear system response functions and on the exact solution of the three‐dimensional Maxwell equations for wave propagation in a horizontally multilayered medium representing the subsurface. Subsurface electric properties, i.e., dielectric permittivity and electric conductivity, are estimated by model inversion using the global multilevel coordinate search optimization algorithm combined sequentially with the local Nelder‐Mead simplex algorithm (GMCS‐NMS). Inversion of synthetic data and analysis of the corresponding response surfaces proved the uniqueness of the inverse solution. Laboratory experiments on a tank filled with a homogeneous sand subject to different water content levels further demonstrated the stability and accuracy of the solution toward measurement and modeling errors, particularly those associated with the dielectric permittivity. Inversion for the electric conductivity led to less satisfactory results. This was mainly attributed to the characterization of the frequency response of the antenna and to the high frequency dependence of the electric conductivity.
Anabranching and maximum flow efficiency in Magela Creek, northern Australia
Authors: Jansen John D. , Nanson Gerald C.
Published: April 2004

Anabranching is the prevailing river pattern found along alluvial tracts of the world's largest rivers. Hydraulic geometry and bed material discharge are compared between single channel and anabranching reaches up to 4 times bank‐full discharge in Magela Creek, northern Australia. The anabranching channels exhibit greater sediment transporting capacity per unit available stream power, i.e., maximum flow efficiency (MFE). Simple flume experiments corroborate our field results showing the flow efficiency gains associated with anabranching, and highlight the prospect of a dominant anabranch, which is found in many anabranching rivers. These results demonstrate that anabranching can constitute a stable river pattern in dynamic equilibrium under circumstances in which a continuous single channel would be unable to maintain sediment conveyance. We propose the existence of a flow efficiency continuum that embraces dynamic equilibrium and disequilibrium (vertically accreting) anabranching rivers.
The dependence of Langmuir wave amplitudes on position in Earth's foreshock
Authors: Sigsbee K. , Kletzing C. A. , Gurnett D. A. , Pickett J. S. , Balogh A. , Lucek E.
Published: April 2004

We present the first results of Langmuir wave observations in the foreshock from the Cluster WBD Plasma Wave Receiver. When the data were binned by distance to the foreshock boundary, the Langmuir wave amplitude probability distributions followed the log‐normal statistics predicted by stochastic growth theory for all regions of the foreshock. The Cluster data show for the first time that the centers of the probability distributions shift to lower amplitudes with increasing distance to the boundary, and that a spatially averaged power law distribution results from summing these distributions.
On the generation and structure of the quadrupole magnetic field in the reconnection process: Comparative simulation study
Authors: Karimabadi H. , Huba J. D. , Krauss‐Varban D. , Omidi N.
Published: April 2004

We investigate magnetic reconnection dynamics using three simulation codes to isolate ion kinetic effects from Hall effects: a full hybrid code, a Hall MHD code, and a new Hall‐less hybrid code. The structure of the quadrupole magnetic field (also known as the core field or out‐of‐plane field) is used as the physical quantity for this comparison because it is a good proxy of fast magnetic reconnection. Examination of the core field time evolution equations for the Hall‐less hybrid and Hall MHD models reveal inherent differences between these models. In particular, it demonstrates that core fields can be generated in the kinetic regime even in the absence of the Hall term. We attribute this result to ion kinetic effects, e.g., ion diamagnetic drifts driven by an anisotropic ion stress tensor. This result is borne out by our simulations. Additionally, kinetic ion dynamics leads to core fields with widths that can extend to several ion inertial lengths in the vicinity of X‐line and tens of ion inertial lengths further away. In contrast, in Hall MHD, the typical case is about an ion inertial length in the vicinity of the X‐line and a few ion inertial lengths further away. These results pose challenges for improving fluid models of reconnection.
Thermomechanical behavior of coated tapered optical fibers for scanning force microscopy
Authors: Finot Eric , Lacroute Yvon , Bourillot Eric , Rouessac Vincent , Durand Jean
Published: April 2004

Measuring the shift in resonant frequency of a coated tapered optical fiber can be used to estimate the dramatic increase in the temperature at the end of the optical fiber when the light is confined at the fiber apex. The resonance frequency of the tapered fiber was measured as a function of the fiber length and the coating profile. The good agreement between the experimental values and the mechanical model based on a finite element method allows one to propose a simple empirical approximation of the frequency shift. Varying the injected power in the fiber was found to shift the fiber frequency due to the thermal expansion and temperature dependence of Young’s modulus of both the metal coating and the fiber core. A thermal model for the temperature distribution along the fiber is proposed and was found to agree with the experimental values when the temperature does not exceed 200 °C. Above 200 °C, the nonlinearity between the frequency and the injected power is correlated to the coating damage observed by electron microscopy. This behavior has been applied to investigate the phase transition in a thermoplastic polymer as a function of the temperature of the fiber apex.
Quasi real-time Raman studies on the growth of Cu–In–S thin films
Authors: Rudigier Eveline , Barcones Beatriz , Luck Ilka , Jawhari-Colin T. , Pérez-Rodrı́guez Alejandro , Scheer Roland
Published: April 2004

In this work annealing and growth of CuInS2 thin films is investigated with quasireal-time in situ Raman spectroscopy. During the annealing a shift of the Raman A1 mode towards lower wave numbers with increasing temperature is observed. A linear temperature dependence of the phonon branch of −2 cm−1/100 K is evaluated. The investigation of the growth process (sulfurization of metallic precursors) with high surface sensitivity reveals the occurrence of phases which are not detected with bulk sensitive methods. This allows a detailed insight in the formation of the CuInS2 phases. Independent from stoichiometry and doping of the starting precursors the CuAu ordering of CuInS2 initially forms as the dominating ordering. The transformation of the CuAu ordering into the chalcopyrite one is, in contrast, strongly dependent on the precursor composition and requires high temperatures.
Magnetic and transport properties of (La0.7−2xEux)(Ca0.3Srx)MnO3: Effect of simultaneous size disorder and carrier density
Authors: Rana D. S. , Thaker C. M. , Mavani K. R. , Kuberkar D. G. , Kundaliya Darshan C. , Malik S. K.
Published: April 2004

Structural, magnetic and transport studies have been carried out on (La0.7−2xEux)(Ca0.3Srx)MnO3 (0.05⩽x⩽0.25) compounds forming in a distorted orthorhombic structure (space group Pnma, No. 62). The Eu and Sr substitutions avoid any average A-site cation size disparity throughout the series. However, increasing both the cation size mismatch at the A-site and carrier concentration induces interesting changes in structural, transport and magnetic properties. Both insulator–metal transition temperature (Tp) and Curie temperature (TC) decrease with increasing x. The resistivity of all the samples in the semiconducting regime fits to the Variable Range Hopping (VRH) of Mott type model. Carrier localization length, L, obtained from VRH plots, decreases from 4.6 Å for x=0.05 to 3.9 Å for the x=0.20 sample. In the metallic region, the n term in the resistivity fits to the Zener-Double exchange polynomial law (ρ=ρ0+ρ2T2+ρnTn) increases from n=5.5 for x=0.05 to n=7.5 for x=0.15. From magnetic susceptiblity measurements, it is observed that there is an increase in the deviation in susceptibility from Curie–Weiss behavior with increasing size disorder and Mn4+ ion concentration. Further, complex magnetic behavior appears in paramagnetic and ferromagnetic regimes for half-doped sample indicative of phase separation. A disparity between Tp and TC is also observed and is as a result of phase segregation. At low temperatures, a large CMR effect occurs with a decrease in Tp.
Structural engineering of ferromagnetism in III–V digital ferromagnetic heterostructures
Authors: Schuller J. A. , Johnston-Halperin E. , Gallinat C. S. , Knotz H. , Gossard A. C. , Awschalom D. D.
Published: April 2004

We investigate the possibility of modulating the magnetic properties of (Ga,Mn)As digital ferromagnetic heterostructures (DFHs) via strain engineering. We p-dope DFHs below the compensation threshold of residual As antisites to achieve variations in strain without introducing free carriers and with relatively modest concentrations of impurity atoms. X-ray diffraction and superconducting quantum interference device measurements reveal a trend toward higher TC as the out-of-plane strain is increased. Additionally, we demonstrate a second method for strain engineering wherein DFHs are grown on anisotropically relaxed (Ga,In)As stressor layers. We show that the ferromagnetic properties are independent of strain in this regime and conclude that the structure-dependent modulation of magnetic properties in DFHs cannot be explained by simple strain effects alone.
Mode mixing in asymmetric double-trench photonic crystal waveguides
Authors: Vlasov Yu. A. , Moll N. , McNab S. J.
Published: April 2004

We investigate, both experimentally and theoretically, the waveguiding properties of a double-trench waveguide in which a conventional single-mode strip waveguide is embedded in a two-dimensional photonic crystal (PhC) slab formed in silicon-on-insulator wafers. We demonstrate that the bandwidth for relatively low-loss (50 dB/cm) waveguiding is significantly expanded to 250 nm, covering almost all the photonic bandgap owing to nearly linear dispersion of the TE-like waveguiding mode. The flat transmission spectrum, however, is interrupted by numerous narrow stop bands. We found that these stop bands can be attributed to anticrossing between TE-like (positive parity) and TM-like (negative parity) modes. This effect is a direct result of the strong asymmetry of the waveguides that have an upper cladding of air and lower cladding of oxide.
Spectral and spatial narrowing of the emission of silicon nanocrystals in a microcavity
Authors: Amans D. , Callard S. , Gagnaire A. , Joseph J. , Huisken F. , Ledoux G.
Published: April 2004

We report on the fabrication and optical characterization of different silicon nanocrystal layers embedded in various structures: deposited on a fused quartz plate, on top of a half cavity, and embedded in a full cavity. The half and full cavity are, respectively, a mirror and a Fabry–Pérot resonator, both being composed of dielectric distributed Bragg reflectors. Spectral and angular dependencies of the photoluminescence (PL) behavior have been measured. It is shown that the fabrication process does not affect the PL properties of the nanocrystals. Efficient spectral and angular narrowing is observed for the full cavity, leading to a spectral width of 13 nm and an aperture cone of 15°. Moreover, the cavity enhances the intensity of the vertically emitted light by a factor of approximately 20.
Monte Carlo simulations of electron transport properties of diamond in high electric fields using full band structure
Authors: Watanabe Tomokatsu , Teraji Tokuyuki , Ito Toshimichi , Kamakura Yoshinari , Taniguchi Kenji
Published: April 2004

Electron transport properties in diamond under high electric fields (⩽5×106 V/cm) have been investigated by means of Monte Carlo simulations which include a full band structure, a wave-vector- and frequency-dependent dielectric function, phonon scattering rates with phonon dispersion relations, and impact ionization rates. The full band structure of diamond was calculated using an empirical pseudopotential method with an expansion of 113 plane waves, and was utilized to evaluate the dielectric function using the Lindhard method while suitable deformation potential coefficients were chosen in an adiabatic bond-charge model. Calculated results such as transition energies at the main points of symmetry and lines in the Brillouin zone as well as phonon dispersions were in good agreement with corresponding experimental data previously reported. The impact ionization rates of electrons in diamond were then evaluated from Fermi’s golden rule using the full band structure and dielectric function. The electric field (F) dependence of the electron drift velocity obtained reproduced experimental results previously reported on hot electron effects well and was fitted well by an analytical equation. It is found that the impact ionization probability rapidly increases with F⩾1×106 V/cm so that an electron in the diamond conduction band can yield electron-hole pair production for 1 μm travel on average at ≈1.5×106 V/cm. The F dependence of the impact ionization probabilities obtained is discussed in relation to energy (E) distributions of hot electrons created in high F on the order of 106 V/cm which are represented well by a Gaussian function, exp[−(E–Egap)2/wF2], for E≈7 eV, where Egap is the band gap energy and wF is a F-dependent constant corresponding to energy spread of ≈4 eV.
Monitoring sheath voltages and ion energies in high-density plasmas using noninvasive radio-frequency current and voltage measurements
Authors: Sobolewski Mark A.
Published: April 2004

To obtain optimal results from plasma processing, the energy of ions incident on substrate wafers must be carefully controlled. Such control has been difficult to achieve, however, because no practical method exists for monitoring the energy distributions of ions at a wafer surface during processing. To solve this problem, we have developed a noninvasive, model-based method for determining ion energy distributions that is suitable for use during actual processing in commercial plasma reactors. The method relies solely on measurements of the rf current and voltage applied to the reactor. The method was validated by tests performed in argon and CF4 discharges at 1.3 Pa (10 mTorr) in an inductively coupled, high-density plasma reactor, with rf substrate bias at frequencies of 100 kHz to 20 MHz. Plasma potential and sheath voltage wave forms obtained from the noninvasive rf technique agreed well with independent measurements made using a capacitive probe. Ion energy distributions from the rf technique were also in good agreement with distributions measured by ion energy analyzers. To further test the technique a sensitivity analysis was performed which quantifies the sensitivity of the noninvasive results to any uncertainties in the electrical measurements or the other input parameters required by the technique.
Theoretical analysis of interaction between electron beam and electromagnetic wave for unidirectional optical amplifier
Authors: Asada Masahiro , Yamada Minoru
Published: April 2004

Interaction between an electron beam and an electromagnetic wave is analyzed on the basis of semiclassical quantum mechanics that includes the influence of electron scattering and thermal distribution. The result is applied to determine the gain coefficient of the unidirectional optical amplifier utilizing this interaction. A general formula of the gain is obtained, which is applicable not only to an electron beam in vacuum but also to a semiconductor. The gain in the optical region due to an electron beam in vacuum is calculated, and it is shown that the gain coefficient is strongly reduced by the energy broadening due to scattering. This reduction is more significant for lower frequencies. The thermal distribution of the electron energy also reduces the gain, although the influence is less than that of electron scattering. The gain obtained in the quantum mechanical analysis is compared with that of the classical analysis including electron scattering. The difference between the classical and quantum mechanical analyses approaches zero in the low-frequency limit, while it is large in the optical frequency range.
Giant exchange bias and the vertical shifts of hysteresis loops in γ-Fe2O3-coated Fe nanoparticles
Authors: Zheng R. K. , Wen G. H. , Fung K. K. , Zhang X. X.
Published: April 2004

We fabricated core/shell-structured Fe nanoparticles, in which the α-Fe core is about 5 nm in diameter and the γ-Fe2O3 shell is about 3 nm thick, and systematically studied their structural and magnetic properties. The magnetic hysteresis (M–H) loops, measured at low temperatures, after the particles were cooled from 350 K in a 50 kOe field, show significant shifts in both horizontal and vertical directions. It has been found that the exchange-bias field can be as large as 6.3 kOe at 2 K, and that the coercive field is also enhanced greatly in the field-cooled (FC) loops. The large exchange bias and vertical shifts of the FC loops at low temperatures may be ascribed to the frozen spins in the shells. A simple model is proposed to interpret the observations.
Effect of nanorope waviness on the effective moduli of nanotube sheets
Authors: Berhan L. , Yi Y. B. , Sastry A. M.
Published: April 2004

Using a micromechanics approach, we recently investigated the theoretical limits on achievable moduli in nanotube mats by stiffening of bonds. However, the waviness intrinsic to many manufacturing processes also clearly plays an important role in stiffness of these materials. To study the effect of waviness on mechanical properties, we modeled fiber segments as sinusoids, generated networks comprised of these fibers, and performed simulations of deformations of the networks. In contradiction of classical work by Kallmes and Corte [Tappi J. 43, 737 (1960)], we found the number of fiber crossings in these networks to be independent of fiber waviness, leading to identification of the number of fiber crossings as a necessary and sufficient parameter to specify network geometry, for either wavy or straight fibers. Our mechanical modeling results suggest that reducing the waviness of nanotube ropes would significantly improve Young’s moduli in these materials. However, reduction of waviness would not produce the improvements achievable with higher bond density; for random sheets, assuring connections among all intersecting ropes appears to be the most direct route toward improving the overall sheet properties. There remains a persistent discrepancy between statistically predicted bond densities and physical bond densities, based on moduli of these materials.
Optical second-harmonic spectroscopy of Au nanowires
Authors: Kitahara Takeshi , Sugawara Akira , Sano Haruyuki , Mizutani Goro
Published: April 2004

We have measured the optical second-harmonic (SH) intensity spectra from an array of Au nanowires of average width 60 nm on a faceted NaCl(110) template. For the s-polarized input and p-polarized output polarization combination, the SH intensity with the incident field parallel to the nanowire axes was much stronger than that with the incident field perpendicular to the axes. The anisotropy is attributed to the anisotropic depolarization field in the Au nanowires.
Revised Shockley–Read–Hall lifetimes for quantum transport modeling
Authors: Höhr Timm , Schenk Andreas , Fichtner Wolfgang
Published: April 2004

The inclusion of quantization effects on the carrier densities is now the state of the art in modern semiconductor device simulators and yields, for example, quantum-corrected threshold voltages and quantum-mechanical models of the channel mobility. However, the effect of charge quantization on nonradiative thermal generation–recombination has not received much attention. In this article, Shockley–Read–Hall recombination is examined for situations in which electrons and/or holes are confined in semiconductor devices. For the transitions between band states and a single deep level, a previously developed multiphonon description is adopted. It is found that the lifetimes have to be altered due to the same quantized local density of states that also accounts for the carrier distribution. Numerical evaluation of this model for one-dimensional potentials and small phonon energies results in spatially varying lifetime profiles that exhibit two opposite regimes. The additional nonclassical offset of the subband eigenenergies causes an increased lifetime in the limit of strong quantum confinement. For nondegenerate statistics, an analytical high-temperature approximation is presented for this limit, where the activation energy of the lifetime is increased by the lowest-subband offset. In the absence of confinement, however, high electric fields reduce the lifetime due to carrier tunneling into the bandgap.
Hydrogen behavior in SiO2 with high density of defects and locally concentrated silicon
Authors: Ikeda M. , Nakagawa M. , Mitsusue R. , Kondo S. , Imanishi N.
Published: April 2004

Using ion implantation techniques, we have studied the trapping, detrapping, and diffusion of H in SiO2 containing a high density of defects and a high concentration of excess Si. In SiO2 sample implanted with Si and H, the implanted H moves toward the surface after stopping at the projected range, and is trapped by vacancies and silicon dangling bonds. If the Si-implanted SiO2 is annealed to 1423 K prior to H implantation, the trap sites changes from the Si–H bonds in the unannealed sample to the dangling bonds at the nanocrystalline interfaces. The dependence of retention and trapping site of hydrogen on the Si implantation showed that, irrespective of the preannealing, the retention of hydrogen rapidly increases above a dose of 1×1017 Si/cm2 and then levels off, and the trapping site for 6×1017 Si/cm2 is distinctly different from that for 3×1017 Si/cm2. A possible reason for the latter result is that, in the case of the unannealed sample, the implanted silicon atoms coagulate to form clusters at 6×1017 Si/cm2 and hydrogen is trapped by the defects inside the silicon clusters. In the case of the annealed sample, the volume fraction of the silicon nanocrystals becomes large for the 6×1017 Si/cm2 implantation and, as a result, the fraction of the V–H complex inside the nanocrystals increases.
X-ray structure factors for Si nanoparticles
Authors: Belomoin G. , Alsalhi M. , Al Aql A. , Nayfeh M. H.
Published: April 2004

We examine the structural change of Si nanocrystals as a function of size. We present calculated x-ray structure factors over the range of 1–6 nm. We examined how the scattering evolves from broad to sharp features as the particle size increases. The transition from molecular-like broad unresolved features to polycrystalline-like resolved peaks can be set at ∼2.15 nm. The scattering change due to surface dimer relaxation, which has been proposed as the basic mechanism of the acquired optical characteristics, is examined.
Modeling of the near field plume of a Hall thruster
Authors: Boyd Iain D. , Yim John T.
Published: April 2004

In this study, a detailed numerical model is developed to simulate the xenon plasma near-field plume from a Hall thruster. The model uses a detailed fluid model to describe the electrons and a particle-based kinetic approach is used to model the heavy xenon ions and atoms. The detailed model is applied to compute the near field plume of a small, 200 W Hall thruster. Results from the detailed model are compared with the standard modeling approach that employs the Boltzmann model. The usefulness of the model detailed is assessed through direct comparisons with a number of different measured data sets. The comparisons illustrate that the detailed model accurately predicts a number of features of the measured data not captured by the simpler Boltzmann approach.
Elastic constants of body-centered-cubic titanium monocrystals
Authors: Ledbetter Hassel , Ogi Hirotsugu , Kai Satoshi , Kim Sudook , Hirao Masahiko
Published: April 2004

We report estimates of body-centered-cubic titanium’s monocrystal elastic constants C11, C12, and C44. Two constants resulted from measuring a pure-titanium polycrystal at high temperatures using resonant-ultrasound spectroscopy. The third constant resulted from assuming a Zener elastic anisotropy and using inversely Kröner’s monocrystal–polycrystal elastic-constant relationship. Our values are C11=97.7, C12=82.7, and C44=37.5 GPa at 1000 °C.
Microstructure and magnetic properties of melt-spun Sm(Co0.58Fe0.31Cu0.04Zr0.05B0.02)z ribbons
Authors: Gopalan R. , Ping D. H. , Hono K. , Huang M. Q. , Smith B. R. , Chen Z. , Ma B. M.
Published: April 2004

We have investigated the microstructures of melt-spun Sm(Co0.58Fe0.31Cu0.04Zr0.05B0.02)z (z=7.5 and 12) ribbons by transmission electron microscopy (TEM) and three dimensional atom probe (3DAP) to correlate them with the hard magnetic properties. The ribbon with z=7.5 has a high coercivity (iHc∼12 kOe) while the ribbon with z=12 possesses a low coercivity (iHc∼4.9 kOe). The high Hci ribbon comprises of nanocrystals (∼10 nm) having 1:7H (TbCu7) type structure as main phase. However, in the low Hci ribbon, the microstructure consists of both 1:7H and 2:17H (Th2Ni17) phases with crystal size ranging from 50 to 80 nm. The low coercivity ribbon also contains (CoFeZr)23B6 (M23B6 type) soft magnetic phase. The low coercivity obtained in the melt-spun ribbon with z=12 is due to: (i) the formation of Th2Ni17 type low anisotropy phase, (ii) the presence of boron-rich soft magnetic phase, and (iii) larger crystal size. Realization of high coercivity in the ribbon with z=7.5 is due to the formation of 1:7H, a high anisotropy phase, as well as reduced grain size.
Sheath structure in electronegative plasmas with finite positive ion temperature
Authors: Palop J. I. Fernández , Ballesteros J. , Hernández M. A. , Crespo R. Morales , del Pino S. Borrego
Published: April 2004

An earlier theoretical work, concerning the sheath structure in electronegative plasmas, is extended to include the effect of the positive ion thermal motion. A significant change is observed in the quantities characterizing the sheath with respect to the cold ion assumption. The sheath is contracted when the positive ion thermal motion is considered causing a decrease in the sheath thickness. The ion saturation current and the floating potential are shown to be distinguished quantities in plasma diagnosis of electronegative plasmas by using plane Langmuir probes.
Dielectric behaviors of Pb1−3x/2LaxTiO3 derived from mechanical activation
Authors: Soon H. P. , Xue J. M. , Wang J.
Published: April 2004

To investigate the origin of ultrahigh relative permittivity that has been observed for lanthanum-doped lead titanate, Pb1−3x/2LaxTiO3 (PLT-A) with x ranging from 0.10 to 0.25 were synthesized by mechanical activation of constituent oxides. Their sintered density, grain size and relative permittivity demonstrated a steady increase with increasing of La doping. Upon thermal annealing in oxygen, the relative permittivity of Pb0.70La0.2TiO3 (PLT-A20) at Tc showed an initial rise and a peak at 4h of annealing, and then a steady fall with further increase in annealing time. In contrast, when annealed in nitrogen for 4 h, a significant rise in relative permittivity was observed, although the increase rate falls with prolonged annealing. The observed dependence of relative permittivity and dielectric loss for PLT-A20 on the initial annealing in both oxygen and nitrogen demonstrated the domination of space charge polarization as a result of PbO loss through evaporation from the surface region. While the high activation energy for Pb2+ and O2− diffusion through the surface scale slows down the rate of PbO loss through evaporation, excess loss of PbO adversely affect space charge polarization, leading to a fall in relative permittivity of PLT-A20, upon prolonged annealing in oxygen. In addition to PbO loss, prolonged annealing in nitrogen generated oxygen vacancies, which played an important role in affecting the relative permittivity.
Thermoelastic and texture behavior of aluminum at high pressure and high temperature investigated by in situ neutron diffraction
Authors: He Duanwei , Zhao Yusheng , Daemen L. L. , Qian J. , Lokshin K. , Shen T. D. , Zhang J. , Lawson A. C.
Published: April 2004

The behavior of aluminum under high pressures and temperatures was investigated by in situ time-of-flight neutron diffraction with a developed Toroidal Anvil Press (TAP-98). The effect of the displacement of the center of diffraction, which is caused by sample movement during compression, is corrected by an additional calibration. Unit-cell dimensions, measured up to P=5.7 GPa and T=900 K, were derived from the refinement results and fitted to the high-temperature Birch–Murnaghan equation of state. With (∂KT/∂P)T fixed at 4, we obtained K0=72.8(±2.4) GPa, (∂KT/∂T)P=−0.04(±0.01) GPa K−1, and αT (K−1)=3.7(±1.6)×10−5+9.7(±3.5)×10−8T. Our data are compared with previous experimental data involving shock wave, static compression, ultrasonic, and thermal-expansion measurements and with theoretical predictions. The results demonstrate that the newly developed high-pressure high-temperature system for in situ neutron diffraction is reliable. It was also found that the crystalline orientation of Al grains became highly preferred when the sample was heated to 900 K at ∼4 GPa.
Segregation inhibited grain coarsening in nanocrystalline alloys
Authors: Beke D. L. , Cserháti C. , Szabó I. A.
Published: April 2004

It will be shown, in the framework of a general statistical (atomistic) treatment, that segregation inhibited grain coarsening in binary nanocrystalline alloys can be described by analytical expressions, if the effect can be attained with highly saturated grain boundaries. Relatively simple relations describing the temperature dependence of the stabilized grain-boundary fraction will be derived. The validity conditions of the relation proposed very recently by R. Kirchheim [Acta Mater. 50, 413 (2002)] are also analyzed. Our conclusions are compared with experimental results and the reliability of the few experimental data, known at present, will be discussed as well.
Study of bulk grown silicon–germanium radiation detectors
Authors: Ruzin A. , Marunko S. , Gusakov Y.
Published: April 2004

Results of p+−i−n+-like (PIN) devices fabricated with a single crystal, Czochralski grown silicon germanium are reported. It is shown that the material has a high potential for x-ray detector applications. The devices were characterized under dark conditions as well as under infrared laser illumination and gamma-ray photons. The Si1−xGex semiconductor contains ∼5% (atomic concentration) of germanium and has a measured resistivity of ∼100 Ω cm. The devices show diode behavior with frequency and temperature independent capacitance–voltage characteristics. The deep level transient spectroscopy results reveal a substantial concentration of trapping centers, some possibly related to the boron-vacancy complex. The devices were operated with x- and gamma-ray photons in spectroscopy mode at low temperatures and the results clearly show the beneficial effect of the added germanium on the absorption coefficient, which increased by a factor 2–3 compared to silicon. The results of the transient current characterization technique with a 1060 nm laser indicate a fast photoresponse with no afterglow effects. The main challenges seem to be the relatively low resistivity of the currently available material, limiting the active volume, and the high generation rate at room temperature. It should be emphasized that the SiGe used in this study was not particularly tailored for detector application during the growth.
Resonant magneto-tunneling through shallow impurity states in double barrier heterostructures
Authors: Paredes Gutierrez H. , Porras-Montenegro N. , Arce J. C. , Latgé A.
Published: April 2004

A theoretical study is presented of the effects of in-plane magnetic fields on the I–V characteristic curves associated with resonant electron tunneling through shallow impurity states in GaAs/(Al,Ga)As double-barrier heterostructures. A simple one-band tight-binding Hamiltonian is used and the current is obtained by adopting Keldysh diagrammatic techniques for nonequilibrium processes. As the field strength increases the peak associated with the donor-assisted tunneling shifts to lower voltages whereas the resonance associated with tunneling through the first subband shifts to higher ones. Also, the intensities are both reduced, in qualitatively good agreement with the experimental reports of Sakai et al. [Phys. Rev. B. 48, 5664 (1993)].
Electrically controllable polarization-dependent phase grating from photocurable liquid crystals
Authors: Ghosh Anup K. , Takanishi Yoichi , Ishikawa Ken , Takezoe Hideo , Ono Yoshiyuki , Kawamura Joji
Published: April 2004

Electrically controllable Raman–Nath phase diffraction grating has been fabricated in a very simple way using photopolymerizable liquid crystals. The efficiency of the first-order diffraction peak has been measured with the variation of the polarization angle of the incident light as well as with the variation of an applied electric field. Dynamic response and the relaxation dynamics of the first-order peak were measured by applying a dc electric field on and off, respectively, and were found to be very fast.
Effect of microgeometry on switching and transport in lead zirconate titanate capacitors: Implications for etching of nano-ferroelectrics
Authors: Jung D. J. , Morrison F. D. , Dawber M. , Kim H. H. , Kim Kinam , Scott J. F.
Published: April 2004

We evaluate different switching behaviors of lead zirconate titanate (PZT) thin film capacitors with two different geometries: one, a square 100×100 μm; the second, a ribbon (1.6 μm width but with approximately the same total area), as a function of temperature T, and applied electric field E. The ribbon capacitor shows a stronger dependence (by ca. 70%) of activation field on T and E. This is interpreted as a chemical reduction of edge material in the long-perimeter ribbons due to plasma etching. In order to understand and model the different domain switching of these two types of PZT capacitor, we also investigate impedance spectra at various temperatures from 27 to 470 °C. From ac conductance spectra, both square- and ribbon-type capacitors have similar trap levels, 0.38±0.02 eV. From analysis of conductivity spectra, we find that the ribbon capacitor contains an additional 0.19±0.02 eV trap level attributed to H–O dipoles due to hydrogen reduction; the latter trap results in domain pinning, which is interpreted as the origin of the relatively strong dependence of the activation field on T and E in the ribbons compared to the square capacitors. The additional loss peak in the ribbon capacitors is also observed in real cell-type capacitors, but only when their edge/area ratio becomes large, as in submicron cells for megabit scale.
Frequency selective surfaces as near-infrared electromagnetic filters for thermophotovoltaic spectral control
Authors: Kristensen Ryan T. , Beausang John F. , DePoy David M.
Published: April 2004

Frequency selective surfaces (FSSs) effectively filter electromagnetic radiation in the microwave band (1 mm to 100 mm). Interest exists in extending this technology to the near infrared (1 μm to 10 μm) for use as a filter of thermal radiation in thermophotovoltaic (TPV) direct energy conversion. This article assesses the ability of FSSs to meet the strict spectral performance requirements of a TPV system. Inherent parasitic absorption, which is the result of the induced currents in the FSS metallization, is identified as a significant obstacle to achieving high spectral performance.
Mechanisms in fs-laser ablation in fused silica
Authors: Jia T. Q. , Xu Z. Z. , Li R. X. , Feng D. H. , Li X. X. , Cheng C. F. , Sun H. Y. , Xu N. S. , Wang H. Z.
Published: April 2004

A theoretical model is proposed to describe the microscopic processes involved in the ablation in fused silica induced by femtosecond-laser pulse. Conduction-band electron (CBE) can absorb laser energy, the rate is calculated by quantum mechanical method and classical method. CBE is produced via photoionization (PI) and impact ionization (II). The PI and II rates are calculated by using the Keldysh theory and double-flux model, respectively. Besides the CBE production, we investigate laser energy deposition and its distribution. The equation of energy diffusion in physical space is resolved numerically. Taking energy density Edep=54 kJ/cm3 as the criterion, we calculate damage threshold, ablation depth, and ablation volumes. It is found that if energy diffusion is considered, energy density near sample surface is reduced to 1/10, damage threshold is enhanced more than 30%, ablation depth is increased by a factor of 10. Our theoretical results agree well with experimental measurements. Several ultrafast phenomena in fused silica are also discussed.
Evolution of surface roughness of AlN and GaN induced by inductively coupled Cl2/Ar plasma etching
Authors: Zhu K. , Kuryatkov V. , Borisov B. , Yun J. , Kipshidze G. , Nikishin S. A. , Temkin H. , Aurongzeb D. , Holtz M.
Published: April 2004

We study the effects of plasma etching on the evolution of surface roughness of GaN and AlN. The etch-induced roughness is investigated using atomic force microscopy by systematically varying plasma power, chamber pressure, and Cl2/Ar mixture gas composition. GaN etches three to four times more rapidly than AlN for identical plasma conditions. For both GaN and AlN, we find that the surface roughness is correlated to etch rate. Induced roughness remains comparable to the as-grown value provided etching is carried out below rates 400 (GaN) and 90 nm/min (AlN). Above these cutoff etch rates, the roughness increases in proportion to etch rate. This result is independent of plasma parameters varied to produce the higher etching rates. By analyzing the surface properties through the power spectral density (PSD), we correlate roughness with the formation of fine-scale features present as a consequence of more aggressive etching. The cutoff etch rates and spatial-frequency dependence of the PSD are interpreted using the theory of kinetic roughening. A low-frequency range exhibits saturation corresponding to large-scale feature sizes, and a high-frequency regime exhibits scaling properties.
Structure of epitaxial Ca2Fe2O5 films deposited on different perovskite-type substrates
Authors: Rossell M. D. , Lebedev O. I. , Van Tendeloo G. , Hayashi N. , Terashima T. , Takano M.
Published: April 2004

The influence of the substrate on the structure and the microstructure of Ca2Fe2O5 (CFO) thin films has been investigated by transmission electron microscopy. Ca2Fe2O5 films have been deposited by pulsed laser deposition on (001)-oriented SrTiO3 (STO), LaAlO3 (LAO), and (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (LSAT) substrates. Minor variations in the lattice mismatch of the CFO film with the three different substrates strongly determine the growth direction of the film. The misfit stress also promotes the development of strain accommodation mechanisms. (101) “orthogonal” twins are formed to accommodate the misfit in epitaxial films of CFO on STO(001) and LSAT(001), while CFO films on LAO(001) develop 90° rotation twins leading to film fragmentation into grains. The growth direction of the CFO film on LAO not only depends on the lattice mismatch but also on the lattice energy involved at the initial stages of the growth process. Our results also evidence the formation of an unexpected phase with composition Ca3Fe2O7 in CFO thin films on STO and LSAT. High-resolution microscopy and electron diffraction suggest that this structure belongs to the Ruddlesden–Popper family.
Electrically active defects in irradiated 4H-SiC
Authors: David M. L. , Alfieri G. , Monakhov E. M. , Hallén A. , Blanchard C. , Svensson B. G. , Barbot J. F.
Published: April 2004

4H-SiC epilayers were irradiated with either protons or electrons and electrically active defects were studied by means of deep level transient spectroscopy. Motion of defects has been found to occur at temperature as low as 350–400 K. Indeed, the application of an electric field has been found to enhance modifications in defect concentrations that can also occur during long time annealing at elevated temperature. Two levels have been revealed and labeled B and M. Two other levels, referred to as S1 and S2 and located at 0.40 and 0.71 eV below the conduction band edge have been studied in detail (capture cross sections, profiling, formation energy, activation energy during annealing). The S1 and S2 levels have been found to exhibit a one to one relation and are proposed to be two charge states of the same acceptor center, labeled the S center.
Chemically guided epitaxy of Rb-irradiated α-quartz
Authors: Ga̧siorek S. , Dhar S. , Lieb K. P. , Sajavaara T. , Keinonen J.
Published: April 2004

The solid-phase epitaxial regrowth of 175 keV Rb+-implanted α-quartz during thermal annealing in air or 18O2 was studied as a function of the temperature (⩽1170 K) and the implanted Rb fluence [(0.1–6)×1016 ions/cm2]. Rutherford backscattering channeling spectrometry was used to characterize the damage profiles. The role of the oxygen exchange between the annealing gas and the SiO2 matrix was highlighted by measuring the 16O and 18O profiles by means of time-of-flight elastic recoil detection analysis. Complete epitaxial recrystallization of the amorphized layers was observed after a 1 h annealing in air (at 1170 K) or in 18O2 (at 1130 K). The recrystallization rate in air follows a two-step Arrhenius process, with activation energies of 2.7±0.4 and 0.6±0.2 eV above and below an annealing temperature of 1070 K. The three processes, namely, planar recrystallization of the amorphized a-SiO2 layer, alkali ion out-diffusion, and 16O⇔18O exchange, are highly correlated. This correlation is discussed with the help of the concept of the SiO2 network topology. Finally, the surface topography was measured using an atomic force microscope, which gave evidence of swelling of the SiO2 matrix during implantation and recompaction during epitaxy.
Symmetry-related growth of carbon nanocoils from Ni–P based alloy particles
Authors: Ding D. Y. , Wang J. N. , Dozier A.
Published: April 2004

Nanocrystalline Ni–P and Ni–X–P (X=W,Co,Cu) alloys deposited on the surfaces of SiC whiskers were evaluated on their roles in the catalysis growth of carbon nanocoils during thermal pyrolysis of acetylene. Experimental results indicated that the Ni–P and Ni–W–P systems were effective in catalyzing the growth of carbon nanocoils with varying chirality. Single-particle related growth of the single- or multihelixed carbon nanocoils was found, which suggests that these nanocoils have specific growth characteristics closely related to the symmetroid or surface symmetry of the catalyst particle.