bluuebunny/crossref_metadata_2025 huggingface dataset:
Solar activity parameters and their interrelationship: Continuous decrease in flare activity from solar cycles 20 to 23
Authors: Gupta Meera , Mishra V. K. , Mishra A. P.
Published: May 2007

The paper gives an insight regarding the interrelationship among a variety of solar activity (SA) parameters. A detailed correlative study has been performed using the monthly data of various solar activity parameters, for example, sunspot numbers (SSN), solar flux (SF, 10.7 cm), grouped solar flares (GSF), solar flare index (SFI), coronal index (CI), and tilt angle (TA) for the solar cycles 19 to 23, the present cycle. Though in general SSN shows a high degree of correlation with other SA parameters, the relationship between SSN and flare indices (GSF and SFI) is not straightforward. The slope of the regression lines between SSN and GSF (for cycles 20 to 23) and those between SSN and SFI (for cycles 21 to 23) is found to decrease continuously with the progression of cycles, without any associated unidirectional change in the correlation coefficient between these indices. In conformity, the similar feature is also observed from the peak values of SSN and flare indices. It is not seen, however, in the case of other SA parameters. The different trend of regression lines between SSN‐GSF and SSN‐SFI has been explained on the basis of the duration of the major solar flares. The present analysis also includes the statistical/numerical investigations of the occurrence of flares (imp ≥ 1B) over the span of solar cycles 20 to 23. These findings shed new light on the underlying physical processes responsible for solar activity and their generic relationship.
Evolution of the outer radiation belt during the November 1993 storms driven by corotating interaction regions
Authors: Miyoshi Y. , Morioka A. , Kataoka R. , Kasahara Y. , Mukai T.
Published: May 2007

Evolution of energetic electron fluxes, related solar wind conditions, and relevant plasma waves in the inner magnetosphere are examined during the two corotating interaction region (CIR)‐driven magnetic storms in November 1993. In this paper we focus on the fact that the flux of the outer radiation belt electrons increased significantly during the 3 November storm, while it did not increase above the prestorm level during the 18 November storm. The recovery phase of the 3 November storm is associated with the prolonged substorm activity; continuous injections of hot and subrelativistic electrons, and enhanced chorus wave activity which can accelerate subrelativistic electrons to MeV energies by means of wave‐particle interactions. In contrast, the recovery phase of the 18 November storm is associated with reduced substorm activity, weak injections of hot and subrelativistic electrons, and low chorus wave activity. These differences in the recovery phase can be related to the southward offset of interplanetary magnetic field (IMF) in the high‐speed coronal hole stream, which is influenced by the IMF sector polarity via the Russell‐McPherron effect (dipole tilt effect associated with the IMF polarity).
Small 660‐km seismic discontinuity beneath Tibet implies resting ground for detached lithosphere
Authors: Chen Wang‐Ping , Tseng Tai‐Lin
Published: May 2007

Using seismic profiles comprising of high‐resolution, triplicate waveforms across apertures of over 1,000 km, we show that because of high P wave speed (VP) near the bottom of the mantle transition zone, the contrast in VP across the 660‐km discontinuity beneath central Tibet is small: only about 70% of that beneath the northern Indian shield. This subhorizontal anomaly of high VP is most likely a remnant of detached mantle lithosphere that recently sank to depth, thus providing key evidence for a direct connection between continental collision near the surface and deep‐seated dynamics in the mantle.
Comparative analysis of SBUV/2 and HALOE ozone profiles and trends
Authors: Nazaryan Hovakim , McCormick M. Patrick , Russell James M.
Published: May 2007

The Halogen Occultation Experiment (HALOE) (version 19) ozone vertical profiles are compared with the data obtained by the second generation Solar Backscatter Ultraviolet (SBUV)/2 instruments from the National Oceanic and Atmospheric Administration’s NOAA‐11 and NOAA‐16 satellites using its version 8 algorithm. We examine the inverse noise‐weighted mean percent differences between the HALOE and SBUV/2 ozone profiles at near coincident points and obtain a bias of less than 9% from 40 to 1.5 mb for the NOAA‐11 and NOAA‐16 satellite data records, with few exceptions. The weighted root mean square (RMS) differences between the HALOE and SBUV/2 observations are generally between 4 and 15% for pressure levels 40–1.5 mb. The RMS differences are larger for the NOAA‐16 SBUV/2 data for higher latitudes. We also analyze the time‐dependent differences between the two instruments in order to establish whether there is a significant relative drift. The slopes of time series of differences between the HALOE ozone profiles and the NOAA‐11 and NOAA‐16 SBUV/2 retrievals are, by and large, less than 1% per year and 2% per year, respectively. These results suggest a changing calibration in the NOAA‐11 SBUV/2 instrument. Then we investigate the differences between ozone trends determined by the HALOE and SBUV/2 instruments. Trend differences are less than 0.5% per year in almost all latitude bands at pressure levels 10 and 3 mb. There are statistically significant biases of about 0.7% per year at 5 mb from 20°S to 50°N, and the trend differences are less than 1.3% per year at 60°–30°S.
SHARAD sounding radar on the Mars Reconnaissance Orbiter
Authors: Seu Roberto , Phillips Roger J. , Biccari Daniela , Orosei Roberto , Masdea Arturo , Picardi Giovanni , Safaeinili Ali , Campbell Bruce A. , Plaut Jeffrey J. , Marinangeli Lucia , Smrekar Suzanne E. , Nunes Daniel C.
Published: May 2007

SHARAD (SHAllow RADar) is a sounding radar provided by Agenzia Spaziale Italiana (ASI) as a Facility Instrument on the Mars Reconnaissance Orbiter mission. Its 20‐MHz center frequency and 10‐MHz bandwidth complement the lower‐frequency, relatively narrower bandwidth capability of the MARSIS sounding radar. A joint Italian‐U.S. team has guided the experiment development and is responsible for data analysis and interpretation. The radar transmits signals at a 700 Hz pulse repetition frequency (PRF) and collects reflections from both the surface and near subsurface of Mars. Vertical and horizontal resolutions are, respectively, 15 m (free‐space) and 3–6 km (cross‐track) by 0.3–1 km (along‐track). The scientific objective of SHARAD is to map, in selected locales, dielectric interfaces to at least several hundred meters depth in the Martian subsurface and to interpret these results in terms of the occurrence and distribution of expected materials, including competent rock, soil, water, and ice. A signal‐to‐noise ratio of ∼50 dB (for a specular surface return) is achieved with 10 W of radiated power by using range and azimuth focusing in ground data processing. Preprocessed data as well as range‐ and azimuth‐focused data will be formatted according to Planetary Data System (PDS) standards and be made available from the ASI Science Data Center (ASDC) and from the Geosciences Node of the Planetary Data System (PDS). Important targets for SHARAD include the polar layered deposits, sedimentary stacks (especially in Terra Meridiani), buried channel systems, buried impact craters, volcanic complexes, and shallow ice deposits in equilibrium with the atmosphere.
Suspended‐sediment rating curve response to urbanization and wildfire, Santa Ana River, California
Authors: Warrick Jonathan A. , Rubin David M.
Published: May 2007

River suspended‐sediment concentrations provide insights to the erosion and transport of materials from a landscape, and changes in concentrations with time may result from landscape processes or human disturbance. Here we show that suspended‐sediment concentrations in the Santa Ana River, California, decreased 20‐fold with respect to discharge during a 34‐year period (1968−2001). These decreases cannot be attributed to changes in sampling technique or timing, nor to event or seasonal hysteresis. Annual peak and total discharge, however, reveal sixfold increases over the 34‐year record, which largely explain the decreases in sediment concentration by a nonlinear dilution process. The hydrological changes were related to the widespread urbanization of the watershed, which resulted in increases in storm water discharge without detectable alteration of sediment discharge, thus reducing suspended‐sediment concentrations. Periodic upland wildfire significantly increased water discharge, sediment discharge, and suspended‐sediment concentrations and thus further altered the rating curve with time. Our results suggest that previous inventories of southern California sediment flux, which assume time‐constant rating curves and extend these curves beyond the sampling history, may have substantially overestimated loads during the most recent decades.
Evolution of biogenic gases in peat blocks inferred from noninvasive dielectric permittivity measurements
Authors: Comas Xavier , Slater Lee
Published: May 2007

Peatlands contain methanogenic archea responsible for generating significant amounts of free‐phase biogenic gases (for example, methane and carbon dioxide), but considerable uncertainty still exists regarding the mechanisms of formation and spatial distribution of these gases within the soil matrix. We demonstrate the effectiveness of a new method to record noninvasively the evolution, spatial distribution, and emission patterns of biogenic gases in a peat soil. A peat block (0.022 m3) was extracted from a large freshwater peatland in Maine. The bulk dielectric permittivity (at 1.2 GHz) for multiple slices of the block was measured noninvasively (1) as temperature was increased 2°C d−1 from 5°C to 21°C, and (2) for a subsequent 2‐month period during which temperature was held constant at 21 ± 1°C. Methane emissions at the surface and peat surface deformation were monitored concurrently using a portable methane detector and a grid of surface elevation rods, respectively. Our results demonstrate that (1) the measurement of electromagnetic wave traveltimes across a peat block offers a unique (and more accurate when compared to surface deformation measurements) way to monitor gasdynamics and spatial gas distribution within a peat block without any disturbance to the natural gas regime; (2) the ebullition under our experimental conditions seems to preferentially occur from the near‐surface peat and shows some correspondence with changes in atmospheric pressure; and (3) the ebullition flux exhibits periodicity, suggesting that it may be predictable and quantifiable, which could assist climate modeling efforts. Our findings are consistent with previous studies based on gasdynamics in peat soils (including gas volumes and fluxes associated with biogenic gas ebullition).
Uptake of NO3 on soot and pyrene surfaces
Authors: Mak Jackson , Gross Simone , Bertram Allan K.
Published: May 2007

The reaction of NO3 with methane soot, hexane soot, and solid pyrene was investigated using a flow tube reactor. The uptake of NO3 on fresh soot was fast (uptake coefficient >0.1). Based on this result and an assumed density of reactive sites on soot, the time to process or oxidize 90% of a soot surface in the atmosphere would take only approximately five minutes. This suggests that NO3 chemistry can rapidly oxidize soot surfaces under atmospheric conditions. After exposing soot films to NO3 for approximately 180 minutes in the laboratory, the uptake reaches a steady‐state value. The steady state uptake coefficients (assuming a geometric surface area) were 0.0054 ± 0.0027 and 0.0025 ± 0.0018 for methane and hexane soot, respectively. These numbers are used to show that heterogeneous reactions between NO3 and soot are not likely a significant sink of gas‐phase NO3 under most atmospheric conditions. The uptake of NO3 on fresh pyrene surfaces was also fast (uptake coefficient >0.1), and much faster than previously suggested. We argue that under certain atmospheric conditions reactions between NO3 and surface‐bound polycyclic aromatic hydrocarbons (PAHs) may be an important loss process of PAHs in the atmosphere.
On the reliability of simulated Arctic sea ice in global climate models
Authors: Eisenman I. , Untersteiner N. , Wettlaufer J. S.
Published: May 2007

While most of the global climate models (GCMs) currently being evaluated for the IPCC Fourth Assessment Report simulate present‐day Arctic sea ice in reasonably good agreement with observations, the intermodel differences in simulated Arctic cloud cover are large and produce significant differences in downwelling longwave radiation. Using the standard thermodynamic models of sea ice, we find that the GCM‐generated spread in longwave radiation produces equilibrium ice thicknesses that range from 1 to more than 10 meters. However, equilibrium ice thickness is an extremely sensitive function of the ice albedo, allowing errors in simulated cloud cover to be compensated by tuning of the ice albedo. This analysis suggests that the results of current GCMs cannot be relied upon at face value for credible predictions of future Arctic sea ice.
Error modeling of simulated reflectivity observations for ensemble Kalman filter assimilation of convective storms
Authors: Xue Ming , Jung Youngsun , Zhang Guifu
Published: May 2007

The impact of two different ways of modeling errors in simulated radar reflectivity data for observing system simulation experiments (OSSEs) with an ensemble Kalman filter is investigated. An error model different from the one used in earlier studies is introduced, and it specifies relative Gaussian‐distributed errors in the linear domain of the equivalent radar reflectivity factor. This model is consistent with the processes of error propagation in real radar data. When the error variances specified in the filter and in the data are consistently smaller or larger, the analysis is more accurate, but when these values do not match, poorer analyses result. Such behaviors agree with expectation but are not observed when errors are directly added to the reflectivity in the log domain. These results point to the importance of properly modeling observation errors in OSSEs when the observation operator is nonlinear.
Cross‐validation of geo‐electrical and hydrogeological models to evaluate seawater intrusion in coastal aquifers
Authors: Comte Jean‐Christophe , Banton Olivier
Published: May 2007

The validation of variable‐density flow models simulating seawater intrusion in coastal aquifers requires information about concentration distribution in groundwater. Electrical resistivity tomography (ERT) provides relevant data for this purpose. However, inverse modeling is not accurate because of the non‐uniqueness of solutions. Such difficulties in evaluating seawater intrusion can be overcome by coupling geophysical data and groundwater modeling. First, the resistivity distribution obtained by inverse geo‐electrical modeling is established. Second, a 3‐D variable‐density flow hydrogeological model is developed. Third, using Archie's Law, the electrical resistivity model deduced from salt concentration is compared to the formerly interpreted electrical model. Finally, aside from that usual comparison‐validation, the theoretical geophysical response of concentrations simulated with the groundwater model can be compared to field‐measured resistivity data. This constitutes a cross‐validation of both the inverse geo‐electrical model and the groundwater model.
Implosion and stagnation of wire array Z pinches
Authors: Bland S. N. , Lebedev S. V. , Chittenden J. P. , Hall G. N. , Suzuki-Vidal F. , Ampleford D. J. , Bott S. C. , Palmer J. B. A. , Pikuz S. A. , Shelkovenko T. A.
Published: May 2007

Detailed measurements of the dynamics of aluminum wire array Z pinches from immediately prior to implosion until stagnation and dissipation on axis are presented. Before implosion, the ∼0.5mm axial modulation seen in earlier laser probing images is observed as ablation on the surface of the wire cores facing away from the array axis. This results in the complete ablation of sections of the wire cores and a redistribution of current at the start of implosion. The dynamics of implosion are then strongly influenced by the number of wires in the array. With only eight wires, discrete snowplough bubbles expand from each wire toward the precursor. There is little, if any, correlation between the bubbles from adjacent wires, and a large temporal spread over which the bubbles arrive at the precursor is observed, along with a long rise time, low power soft x-ray pulse. With 32 or more wires, bubbles from adjacent wires merge close to the array edge to form an imploding sheath. The front edge of the sheath is well defined with a small spatial spread, and upon reaching the precursor, the start of a fast rising high power soft-x-ray pulse is seen. As x-ray emission increases, the stagnating column on axis starts to decrease in diameter, reaching a minimum at peak x-ray emission, which also coincides with the time when the rear edge of the snowplough reaches the column. Thereafter, the stagnated column is seen to go unstable, and trailing mass left behind during the implosion is accelerated toward the axis. Intense x-ray emission ends as this mass becomes cleared out.
Observation of phonon modes in epitaxial PbTe films grown by molecular beam epitaxy
Authors: Wu Huizhen , Cao Chunfang , Si Jianxiao , Xu Tianning , Zhang Hanjie , Wu Haifei , Chen Jing , Shen Wenzhong , Dai Ning
Published: May 2007

Phonon modes of PbTe films grown by molecular beam epitaxy have been studied by micro-Raman scattering. On the as-grown PbTe surface, strong TeO2 phonon vibrational modes were detected, which obscured the observation of the longitudinal optical (LO) phonons of PbTe in early conventional Raman scattering experiments. Existence of a TeO2 layer on the PbTe surface is confirmed by observation with x-ray photoemission spectroscopy. After removal of TeO2 by chemical etching, the LO phonons for PbTe films were unambiguously observed. Misfit strain accommodated in the epitaxial films makes the lattice distorted from cubic structure, which lowers the crystal symmetry and leads to observation of what would normally be Raman inactive LO phonon modes for PbTe.
Improvement of thermoelectric properties of icosahedral AlPdRe quasicrystals by Fe substitution for Re
Authors: Okada Junpei T. , Hamamatsu Takehito , Hosoi Shizuka , Nagata Tomohiro , Kimura Kaoru , Kirihara Kazuhiro
Published: May 2007

The thermoelectric properties of quaternary icosahedral AlPdReFe quasicrystals obtained by replacing Re with Fe in icosahedral AlPdRe quasicrystals have been studied according to the guiding principle of weakly bonded rigid heavy clusters (WBRHCs). Anisotropy in the microstructures of arc-melted and annealed samples was found, and the electrical and thermal conductivities are larger in the direction perpendicular to the bottom of the arc-melted button than in the parallel case. The dimensionless figure of merit (ZT) in the perpendicular direction was increased 2.6 times from 0.08 to 0.21 by Fe substitution for Re in the icosahedral AlPdRe quasicrystals, which was caused by the increase of the Seebeck coefficient and the decrease of the phonon thermal conductivity, and the increase of the electrical conductivity, expected and not for the WBRHCs, respectively.
Evenly distributed unitaries: On the structure of unitary designs
Authors: Gross D. , Audenaert K. , Eisert J.
Published: May 2007

We clarify the mathematical structure underlying unitary t-designs. These are sets of unitary matrices, evenly distributed in the sense that the average of any tth order polynomial over the design equals the average over the entire unitary group. We present a simple necessary and sufficient criterion for deciding if a set of matrices constitutes a design. Lower bounds for the number of elements of 2-designs are derived. We show how to turn mutually unbiased bases into approximate 2-designs whose cardinality is optimal in leading order. Designs of higher order are discussed and an example of a unitary 5-design is presented. We comment on the relation between unitary and spherical designs and outline methods for finding designs numerically or by searching character tables of finite groups. Further, we sketch connections to problems in linear optics and questions regarding typical entanglement.
Absolute time derivatives
Authors: Matolcsi T. , Ván P.
Published: May 2007

A four dimensional treatment of nonrelativistic spacetime gives a natural frame to deal with objective time derivatives. In this framework some well known objective time derivatives of continuum mechanics appear as Lie derivatives. Their coordinatized forms depend on the tensorial properties of the relevant physical quantities. We calculate the particular forms of objective time derivatives for scalars, vectors, covectors, and different second order tensors from the point of view of a rotating observer. The relation of substantial, material, and objective time derivatives is treated.
Comparative spectra and efficiencies of ions laser-accelerated forward from the front and rear surfaces of thin solid foils
Authors: Fuchs J. , Sentoku Y. , d’Humières E. , Cowan T. E. , Cobble J. , Audebert P. , Kemp A. , Nikroo A. , Antici P. , Brambrink E. , Blazevic A. , Campbell E. M. , Fernández J. C. , Gauthier J.-C. , Geissel M. , Hegelich M. , Karsch S. , Popescu H. , Renard-LeGalloudec N. , Roth M. , Schreiber J. , Stephens R. , Pépin H.
Published: May 2007

The maximum energy of protons that are accelerated forward by high-intensity, short-pulse lasers from either the front or rear surfaces of thin metal foils is compared for a large range of laser intensities and pulse durations. In the regime of moderately long laser pulse durations (300–850fs), and for high laser intensities [(1−6)×1019W∕cm2], rear-surface acceleration is shown experimentally to produce higher energy particles with smaller divergence and a higher efficiency than front-surface acceleration. For similar laser pulse durations but for lower laser intensities (2×1018Wcm−2), the same conclusion is reached from direct proton radiography of the electric fields associated with proton acceleration from the rear surface. For shorter (30–100fs) or longer (1–10ps) laser pulses, the same predominance of rear-surface acceleration in producing the highest energy protons is suggested by simulations and by comparison of analytical models with measured values. For this purpose, we have revised our previous analytical model of rear-surface acceleration [J. Fuchs et al., Nat. Phys. 2, 48 (2006)] to adapt it to the very short pulse durations. Finally, it appears, for the explored parameters, that rear-surface acceleration is the dominant mechanism.
Viscous resuspension in a tube: The impact of secondary flows resulting from second normal stress differences
Authors: Ramachandran Arun , Leighton David T.
Published: May 2007

The viscous resuspension of non-neutrally buoyant particles has been modeled as a competition between the rate of shear-induced diffusion and sedimentation or as a balance between gradients in the particle stress and gravity. Typically, however, the rheology of the suspension has been modeled using a simple concentration-dependent Newtonian viscosity. In this paper, we demonstrate through theory and comparison with existing experimental results that the anisotropy of the total stress tensor must be included to accurately describe the resuspension process in a tube. At steady state, the isotropic model predicts a secondary current within the tube cross section that flows downwards at the center and upwards near the sidewalls, resulting in a concave upward interface between the clear suspending fluid and the particles [K. Zhang and A. Acrivos, Int. J. Multiphase Flow 20, 579 (1994)]. In contrast, with the inclusion of the known non-Newtonian suspension rheology, the secondary current profile is reversed: upwards near the center and downwards near the walls. This leads to a concave downward shape of the interface between the suspending fluid and the suspension and is in quantitative agreement with the experimental measurements of Altobelli et al. [J. Rheol. 35, 721 (1991)].
Diffusion in tight confinement: A lattice-gas cellular automaton approach. I. Structural equilibrium properties
Authors: Demontis Pierfranco , Pazzona Federico G. , Suffritti Giuseppe B.
Published: May 2007

The thermodynamic and transport properties of diffusing species in microporous materials are strongly influenced by their interactions with the confining framework, which provide the energy landscape for the transport process. The simple topology and the cellular nature of the α cages of a ZK4 zeolite suggest that it is appropriate to apply to the study of the problem of diffusion in tight confinement a time-space discrete model such as a lattice-gas cellular automaton (LGCA). In this paper we investigate the properties of an equilibrium LGCA constituted by a constant number of noninteracting identical particles, distributed among a fixed number of identical cells arranged in a three-dimensional cubic network and performing a synchronous random walk at constant temperature. Each cell of this network is characterized by a finite number of two types of adsorption sites: the exit sites available to particle transfer and the inner sites not available to such transfers. We represent the particle-framework interactions by assuming a differentiation in binding energy of the two types of sites. This leads to a strong dependence of equilibrium and transport properties on loading and temperature. The evolution rule of our LGCA model is constituted by two operations (randomization, in which the number of particles which will be able to try a jump to neighboring cells is determined, and propagation, in which the allowed jumps are performed), each one applied synchronously to all of the cells. The authors study the equilibrium distribution of states and the adsorption isotherm of the model under various conditions of loading and temperature. In connection with the differentiation in energy between exit and inner sites, the adsorption isotherm is described by a conventional Langmuir isotherm at high temperature and by a dual-site Langmuir isotherm at low temperature, while a first order diffuse phase transition takes place at very low temperature.
Electrical bias stressing and radiation induced charge trapping in HfO2/SiO2 dielectric stacks
Authors: Devine R. A. B , Busani T. , Quevedo-Lopez Manuel , Alshareef H. N.
Published: May 2007

Electrical bias stressing and X irradiation experiments have been carried out on HfO2/SiO2 dielectric stacks. Evidence is found for a negative electric field induced positive charge injection and trapping and radiation induced positive charge trapping. Positive electric fields associated with radiation result in negative charge trapping. Potential-induced cyclic injection/removal of positive charge is also observed. It is suggested that these structures are radiation soft.
Deposition of ultraviolet photoconductive films of amorphous hydrogenated carbon
Authors: Dutta Gupta N. , Longeaud C. , Bazin C. , Vignoli S. , Paillard V. , Bandyopadhyay A. , Bhaduri A. , Chaudhuri P.
Published: May 2007

The structural, optical, and transport properties of hydrogenated amorphous carbon films deposited on the grounded electrode under different dilutions (pure CH4, CH4+H2, and CH4+Ar) in radio frequency powered plasma enhanced chemical vapor deposition process have been studied. Though all the samples deposited were observed to be polymer-like carbon, they have different optical and transport properties. While some of the samples were highly conductive under dark (∼10−1Scm−1 at room temperature), others exhibited very low (<10−12Scm−1) dark conductivity and were photoconductive (∼10−9–10−10Scm−1) only under ultraviolet (UV) light (wavelength λ<250nm). There appears to be a close link between the transport properties and optical absorption. Based on optical absorption, we propose two possible models for the distribution of the density of states involving the π and σ states. The evolution of these states and of the deep defect absorption has a profound impact on the transport properties and particularly on the UV photoconductivity. Comparing the two models, it seems more probable that the UV photoconductivity (for photon energy >4.96eV) is rather linked to transitions involving the σ-σ* states.
Deep level transient spectroscopy on as-grown and electron-irradiated p-type 4H-SiC epilayers
Authors: Danno Katsunori , Kimoto Tsunenobu
Published: May 2007

The authors have investigated deep levels in as-grown and electron-irradiated p-type 4H-SiC epilayers by deep level transient spectroscopy. In as-grown epilayers, the D center and four deep levels are observed. In p-type 4H-SiC, reactive ion etching followed by thermal treatment (at 1150 °C) induces the HK0 (EV+0.79 eV) and HK2 (EV+0.84 eV) centers. By the electron irradiation, two deep levels at 0.98 eV (EP1) and 1.44 eV (EP2) are observed in all the samples irradiated at 116–400 keV, while two additional deep levels (EP3 and EP4) are observed only in the samples irradiated at 400 keV. After annealing at 950 °C, these centers are annealed out, and the HK4 (EV+1.44 eV) concentration is increased. By the electron irradiation at more than 160 keV followed by annealing at 950 °C, three deep levels are always observed at 0.30 eV (UK1), 0.58 eV (UK2), and 1.44 eV (HK4). These centers may be defect complexes including carbondisplacement-related defects. All the centers except for the D center are reduced to below the detection limit (1−3×1011 cm−3) by annealing at 1550 °C for 30 min.
Self-assembly of patchy particles into polymer chains: A parameter-free comparison between Wertheim theory and Monte Carlo simulation
Authors: Sciortino Francesco , Bianchi Emanuela , Douglas Jack F. , Tartaglia Piero
Published: May 2007

The authors numerically study a simple fluid composed of particles having a hard-core repulsion, complemented by two short-ranged attractive (sticky) spots at the particle poles, which provides a simple model for equilibrium polymerization of linear chains. The simplicity of the model allows for a close comparison, with no fitting parameters, between simulations and theoretical predictions based on the Wertheim perturbation theory. This comparison offers a unique framework for the analytic prediction of the properties of self-assembling particle systems in terms of molecular parameters and liquid state correlation functions. The Wertheim theory has not been previously subjected to stringent tests against simulation data for ordering across the polymerization transition. The authors numerically determine many of the thermodynamic properties governing this basic form of self-assembly (energy per particle, order parameter or average fraction of particles in the associated state, average chain length, chain length distribution, average end-to-end distance of the chains, and the static structure factor) and find that predictions of the Wertheim theory accord remarkably well with the simulation results.
Exciton energy transfer between optically forbidden states of molecular aggregates
Authors: Kobayashi Takayoshi , Taneichi Takashi , Takasaka Shigehiro
Published: May 2007

3-ethyl-2-[3-(3-ethyl-2(3H)-benzoxazolylidene)-1-propenyl]benzoxazolium iodide (dye I) and pseudoisocyanine bromide are employed to form H aggregates as donors and J aggregates as acceptors. The energy of an H band of the H aggregates is higher than that of a J band of the J aggregates. It was confirmed that excitation of the H band does not emit fluorescence by comparison of excitation spectra of dye I H aggregates with that of dye I monomer. Absorption, fluorescence, and excitation spectra of spin-coated films of H aggregates mixed with various quantities of J aggregates have been observed. Excitation spectra probed at the J band are found to have a component of the H band. Fluorescence spectra originated from excitation of the H band are extracted and qualitatively analyzed. It is confirmed that excitation of the H band causes to emit fluorescence of a J band of the J aggregates. These phenomena show that exciton energy can transfer from the lowest energy in electronic states of the H aggregate, which state is optically forbidden, to electronic state of the J aggregate.
Unstable coupled-mode structures in a one-dimensional Raman free-electron laser
Authors: Mohsenpour T. , Maraghechi B. , Mirzanejhad S.
Published: May 2007

The dispersion relation of a free-electron laser (FEL) with a one-dimensional helical wiggler and an axial guide magnetic field, in the collective regime, is used to study interactions among all possible waves. In group II orbits, with relatively large wiggler induced velocities, new couplings between the negative and positive-energy space-charge waves as well as between the right and left circularly polarized electromagnetic waves are found. These instabilities are found to be distinct from the usual FEL resonance. The space-charge wave was not found to be intrinsically unstable when it couples to the electromagnetic wave and the coupling to the radiation was found to be the only way that makes it unstable.
Noncoincidence of separatrices in two-layer modons
Authors: Khvoles R. , McWilliams J. C. , Kizner Z.
Published: May 2007

Modons, i.e., localized steady-state solutions to the nonlinear equations of potential vorticity conservation, represent a paradigm for coherent structures in geophysical flows. A characteristic property of the baroclinic modons suggested until the present time is that, in such solutions, the boundary of the trapped-fluid area, the separatrix, is essentially independent of depth. In this paper, the existence of translating two-layer modons with noncoincident separatrices in the layers is demonstrated. The solutions are constructed by a combination of analytical and numerical methods. The exterior fields (outside the separatrices), governed by linear equations, are given analytically by explicit formulas; implicitly, these formulas also determine the separatrices. The interior fields are then determined using an iterative numerical algorithm for solving nonlinear partial differential equations. Applying this semianalytical approach, a variety of modon solutions are produced. Among them, the smooth modons (i.e., those with continuous vorticity) are of special geophysical significance. The existence of solutions of even higher smoothness, the so-called supersmooth modons marked by continuity of both the vorticity and its derivatives at the separatrices, is shown. The supersmooth modon possesses a number of features typical of the baroclinic vortical pairs observed earlier in numerical simulations.
Gyrotactic bioconvection in three dimensions
Authors: Ghorai S. , Hill N. A.
Published: May 2007

The bioconvection equations, based on the continuum model of Pedley et al. [J. Fluid Mech. 195, 223 (1988)], consist of the Navier-Stokes equations for an incompressible fluid coupled with a micro-organism conservation equation. These equations are solved efficiently using a semi-implicit second-order accurate conservative finite-difference method. The structure and stability of a three-dimensional plume in deep rectangular boxes with stress-free sidewalls are investigated. Comparisons are made with the two-dimensional and axisymmetric bioconvection. In deep chambers, the three-dimensional plume that forms initially along the central axis of the chamber typically breaks down via a meandering instability.
Structure stability of epitaxial MgO-CaO solid-solution films: effect of diffusion
Authors: Li H. D. , Zhang X. N. , Zhang Z. , Mei Z. X. , Du X. L. , Xue Q. K.
Published: May 2007

The nonequilibrium epitaxial growth process of MgxCa1−xO solid-solution films at 600°C is carefully investigated. No obvious phase separation is observed until annealing at 800°C despite a large miscibility gap. The solid-solution film is featured with disorder alloy as confirmed by transmission electron microscopy (TEM). Spinodal decomposition caused by uprising diffusion happens after short-time annealing which explicitly indicates the structure stability of the metastable solid-solution films does result from diffusion quenching.
High magnetoelectric effect in laminated composites of giant magnetostrictive alloy and lead-free piezoelectric ceramic
Authors: Jia Yanmin , Or Siu Wing , Wang Jie , Chan Helen Lai Wa , Zhao Xiangyong , Luo Haosu
Published: May 2007

Magnetoelectric (ME) laminated composites with all phases environmentally friendly were prepared by sandwiching one layer of thickness-polarized (Bi1∕2Na1∕2)TiO3-(Bi1∕2K1∕2)TiO3-BaTiO3 lead-free piezoelectric ceramic disk between two layers of thickness-magnetized Tb0.3Dy0.7Fe1.92 giant magnetostrictive alloy disk along the thickness direction. The composites exhibited the maximum ME voltage coefficient of 40.7 mV/Oe with a flat response in the measured frequency range of 0.1−20 kHz under a dc magnetic bias of 5 kOe. The induced ME voltage showed an extremely linear relationship to the applied ac magnetic field with amplitude varying from 3×10−5 to 10 Oe over a broad range of dc magnetic bias of 0−5.5 kOe. The high ME effect was analyzed and found to be comparable to most major lead-based ME composites. The present study opens up possibilities for developing green ME devices.
Nano-Raman spectroscopy with metallized atomic force microscopy tips on strained silicon structures
Authors: Zhu Liang , Georgi Carsten , Hecker Michael , Rinderknecht Jochen , Mai Andreas , Ritz Yvonne , Zschech Ehrenfried
Published: May 2007

In this paper, intensity enhancements of the Raman signal from strained silicon films utilizing the tip enhanced Raman spectroscopy (TERS) effect are reported. Specially shaped metallized atomic force microscopy tips have been prepared by sputter deposition of thin silver films onto sharpened quartz tips and subsequent focused ion beam (FIB) modification. Raman signal enhancements of more than 20%, which are attributed to the strained silicon film of 70nm thickness only, have been obtained due to approaching the TERS tips the laser spot. On samples with patterned trench structures prepared by FIB milling, lateral sample scans have been performed. These scans revealed a resolution of strained silicon lines with center-to-center distances below 250nm, well below the classical optical diffraction limit. Based on an analysis of the stress state in the strained silicon structures, relaxation effects close to the trench edges have been investigated. The described approach of nano-Raman spectroscopy is promising for strain characterization in devices, e.g., in field-effect transistor structures.
Low frequency modulated photoconductivity in semiconductors having multiple species of traps
Authors: Schmidt J. A. , Longeaud C. , Koropecki R. R. , Arce R.
Published: May 2007

Modulated photoconductivity has proved to be an excellent tool to probe the density of states of semiconductors. However, though a great deal of attention has been devoted to the high frequency regime for its simplicity of application, the modulated photoconductivity in the low frequency (LF-MPC) regime has been the object of researches only recently. In the case of a dominant species of states present in the forbidden gap, it was shown that LF-MPC is a good complementary technique to the high frequency method to extract different transport parameters. In this paper, we present a complete theoretical analysis of the LF-MPC experiment for the case when different species of traps are present. We solve the complete system of equations that describe the experiment, and we apply simplifying assumptions to deduce a simple formula relating the photocurrent phase shift to the density of states (DOS) at the majority carriers’ quasi-Fermi level. By means of numerical calculations, we discuss the accuracy of our development, and of previous approximate formulas, to reconstruct a given DOS. Finally, we present experimental and theoretical evidence that, under sensitization conditions, the sign of the phase shift of the modulated photocurrent reverses and the steady-state photoconductivity depends superlinearly on the generation rate.
Effect of alkyl substituents on excited state intramolecular proton transfer dynamics of jet-cooled bis(benzoxazolyl)phenoles
Authors: Luzina Elena , Sepioł Jerzy , Svartsov Yuriy N. , Grabowska Anna
Published: May 2007

Structural factors affecting the dynamics of the excited state intramolecular proton transfer (ESIPT) are studied for alkyl derivatives of 2,5-bis(2-benzoxazolyl)phenol. Two fluorescence bands with equal decay times are observed in solution, while only one—emitted by the phototautomer—in supersonic jet. All evidence indicates the existence of a potential barrier in the S1 state. Upon deuteration of the OH group the laser induced fluorescence (LIF) excitation spectra become much sharper as a result of slowing down the proton transfer reaction. Two conformers (rotamers) of each compound in the ground state were detected using hole burning technique. With a help of theoretical calculations three vibrations were identified as the most active ones in reducing the distance between two heavy atoms, N and O, involved in H-bond formation. The widths of (0,0) transitions in LIF excitation spectra decrease with increasing size or number of alkyl substituents at terminal aromatic rings. The corresponding calculated rate constants of ESIPT reaction (∼1012s−1) decrease approximately three times upon the substituent effect. In contrast, model compound 2,5-bis(2-benzoxazolyl)-4-methoxyphenol (BBMP) with OCH3 parasubstituent in central ring slows down the ESIPT reaction to such an extent that double, primary and phototautomeric, fluorescences coexist.
Numerical simulation of tunneling current in GaN Schottky diodes
Authors: Osvald J.
Published: May 2007

We have calculated and compared the current components that are expected to flow in GaN Schottky diodes. Depending on the doping concentration, the ratio between the thermionic emission (drift- diffusion) and the tunneling currents changes. The thermionic current is in absolute value lower than the tunneling current for every studied doping concentration for both forward and reverse directions and its influence decreases with increasing doping concentration. The current calculated from the drift-diffusion approximation is higher in magnitude than the thermionic current and for lower doping concentrations is even higher than the tunneling current. Also the temperature dependence of the tunneling current as a function of the doping concentration is studied and analyzed.
Trends in C–O and C–N bond formations over transition metal surfaces: An insight into kinetic sensitivity in catalytic reactions
Authors: Crawford Paul , Hu P.
Published: May 2007

Transition metal catalyzed bond formation is a fundamental process in catalysis and is of general interest throughout chemistry. To date, however, the knowledge of association reactions is rather limited, relative to what is known about dissociative processes. For example, surprisingly little is known about how the bond-forming ability of a metal, in general, varies across the Periodic Table. In particular, the effect of reactant valency on such trends is poorly understood. Herein, the authors examine these key issues by using density functional theory calculations to study CO and CN formations over the 4d metals. The calculations reveal that the chemistries differ in a fundamental way. In the case of CO formation, the reaction enthalpies span a much greater range than those of CN formation. Moreover, CO formation is found to be kinetically sensitive to the metal; here the reaction barriers (Ea) are found to be influenced by the reaction enthalpy. CN formation, conversely, is found to be relatively kinetically insensitive to the metal, and there is no correlation found between the reaction barriers and the reaction enthalpy. Analysis has shown that at the final adsorbed state, the interaction between N and the surface is relatively greater than that of O. Furthermore, in comparison with O, relatively less bonding between the surface and N is observed to be lost during transition state formation. These greater interactions between N and the surface, which can be related to the larger valency of N, are found to be responsible for the relatively smaller enthalpy range and limited variation in Ea for CN formation.
Interface dynamics of microscopic cavities in water
Authors: Dzubiella Joachim
Published: May 2007

An analytical description of the interface motion of a collapsing nanometer-sized spherical cavity in water is presented by a modification of the Rayleigh-Plesset equation in conjunction with explicit solvent molecular dynamics simulations. Quantitative agreement is found between the two approaches for the time-dependent cavity radius R(t) at different solvent conditions while in the continuum picture the solvent viscosity has to be corrected for curvature effects. The typical magnitude of the interface or collapse velocity is found to be given by the ratio of interfacial tension and solvent viscosity, v≃γ∕η, while the curvature correction accelerates collapse dynamics on length scales below the equilibrium crossover scales (∼1 nm) of hydrophobic solvation. The study offers a starting point for an efficient implicit modeling of water dynamics in aqueous nanoassembly and protein systems in nonequilibrium.
Electronic spectrum of UO22+ and [UO2Cl4]2− calculated with time-dependent density functional theory
Authors: Pierloot Kristine , van Besien Els , van Lenthe Erik , Baerends Evert Jan
Published: May 2007

The electronic spectra of UO22+ and [UO2Cl4]2− are calculated with a recently proposed relativistic time-dependent density functional theory method based on the two-component zeroth-order regular approximation for the inclusion of spin-orbit coupling and a noncollinear exchange-correlation functional. All excitations out of the bonding σu+ orbital into the nonbonding δu or ϕu orbitals for UO22+ and the corresponding excitations for [UO2Cl4]2− are considered. Scalar relativistic vertical excitation energies are compared to values from previous calculations with the CASPT2 method. Two-component adiabatic excitation energies, U–O equilibrium distances, and symmetric stretching frequencies are compared to CASPT2 and combined configuration-interaction and spin-orbit coupling results, as well as to experimental data. The composition of the excited states in terms of the spin-orbit free states is analyzed. The results point to a significant effect of the chlorine ligands on the electronic spectrum, thereby confirming the CASPT2 results: The excitation energies are shifted and a different luminescent state is found.