bluuebunny/crossref_metadata_2025 huggingface dataset:
Estimating the greenhouse gas fluxes of European grasslands with a process‐based model: 1. Model evaluation from in situ measurements
Authors: Vuichard Nicolas , Soussana Jean‐François , Ciais Philippe , Viovy Nicolas , Ammann Christof , Calanca Pierluigi , Clifton‐Brown John , Fuhrer Jürg , Jones Mike , Martin Cécile
Published: January 2007

We improved a process‐oriented biogeochemical model of carbon and nitrogen cycling in grasslands and tested it against in situ measurements of biomass and CO2 and CH4 fluxes at five European grassland sites. The new version of the model (PASIM) calculates the growth and senescence of aboveground vegetation biomass accounting for sporadic removals when the grassland is cut and for continuous removals when it is grazed. Limitations induced by high leaf area index (LAI), soil water deficits and aging of leaves are also included. We added to this a simple empirical formulation to account for the detrimental impact on vegetation of trampling and excreta by grazing animals. Finally, a more realistic methane emission module than is currently used was introduced on the basis of the quality of the animals' diet. Evaluation of this improved version of PASIM is performed at (1) Laqueuille, France, on grassland continuously grazed by cattle with two plots of intensive and extensive grazing intensities, (2) Oensingen, Switzerland, on cut grassland with two fertilized and nonfertilized plots, and (3) Carlow, Ireland, on grassland that is both cut and grazed by cattle during the growing season. In addition, we compared the modeled animal CH4 emissions with in situ measurements on cattle for two grazing intensities at the grazed grassland site of Laqueuille. Altogether, when all improvements to the PASIM model are included, we found that the new parameterizations resulted into a better fit to the observed seasonal cycle of biomass and of measured CO2 and CH4 fluxes. However, the large uncertainties in measurements of biomass and LAI make simulation of biomass dynamics difficult to make. Also simulations for cut grassland are better than for grazed swards. This work paves the way for simulating greenhouse gas fluxes over grasslands in a spatially explicit manner, in order to quantify and understand the past, present and future role of grasslands in the greenhouse gas budget of the European continent.
Moderate Resolution Imaging Spectroradiometer bidirectional reflectance distribution function–based albedo parameterization for weather and climate models
Authors: Wang Zhuo , Zeng Xubin , Barlage Michael
Published: January 2007

The albedo of land surfaces is an important parameter for surface energy balance and is dependent on the solar zenith angle and the three‐dimensional structure of the canopies. The Moderate Resolution Imaging Spectroradiometer (MODIS) bidirectional reflectance distribution function (BRDF) algorithm is reformulated to derive a new two‐parameter scheme for the computation of land surface albedo and its solar zenith angle (SZA) dependence for use in weather and climate models as well as in the remote sensing retrieval of surface solar fluxes. In this formulation the season‐ and pixel‐dependent black‐sky albedo at 60° SZA can be directly prescribed using the MODIS BRDF data while the two parameters are taken as a function of vegetation type only. Comparison of this formulation with those used in weather, climate, and data assimilation models (at NCAR, NCEP, and NASA) reveals the deficiencies in the treatment of SZA dependence and white‐sky albedo in these models. Similarly, comparison of this formulation with those computed using the remotely sensed solar flux data from three groups (International Satellite Cloud Climatology Project (ISCCP)‐FD, NASA's Clouds and the Earth's Radiant Energy System Tropical Rainfall Measuring Mission (CERES/TRMM), and University of Maryland) shows that surface albedos from these data sets are at times unrealistic, and suggestions are made regarding how to better treat the surface albedo and its SZA dependence in the retrieval of surface and atmospheric solar fluxes.
Surface currents in the western North Atlantic during the Last Glacial Maximum
Authors: LeGrande Allegra N. , Lynch‐Stieglitz Jean
Published: January 2007

During the last ice age, the density gradient across the Florida Current was reduced, implying a reduction in the flow of the Gulf Stream through the Florida Straits. Here we investigate the possibility that a significant portion of this wind‐driven western boundary flow bypassed the Florida Straits during glacial times due to either changes in bathymetry induced by the sea level drop or changes in wind patterns. Down core records of the oxygen isotope ratios of the planktonic foraminifer Globorotalia truncatulinoides are used to locate the density gradients and thus the locations of upper ocean currents in the western North Atlantic. We find that western boundary flow was largely confined within the Florida Straits during the Last Glacial Maximum as it is today. This finding supports the idea that the reduced density gradient across the Florida Current represents a reduction in the surface branch of the surface to deep meridional overturning circulation in the Atlantic rather than a reduction in the proportion of the wind‐driven flow carried by the Florida Current.
Satellite‐based assessment of cloud‐free net radiative effect of dust aerosols over the Atlantic Ocean
Authors: Christopher Sundar A. , Jones Thomas
Published: January 2007

Using eighteen months (June–August, 2000–2005) of spatially and temporally collocated Moderate Resolution Imaging Spectroradiometer (MODIS) and the Clouds and the Earth's Radiant Energy System (CERES) data from the Terra satellite over the Atlantic Ocean [10W–60W, 0–30N], we first separate the dust aerosol optical thickness at 0.55 μm (AOT) from the total column MODIS AOT. We then calculate the cloud‐free TOA net radiative effect (NRE) of dust aerosols by accounting for diurnal effects and sample biases. The cloud‐free NRE is −6.31 ± 1.16 Wm−2 and nearly twenty percent of the shortwave radiative effect (−7.75 ± 0.86 Wm−2) is cancelled by the longwave radiative effect (+1.44 ± 0.57 Wm−2) indicating the importance of the dust aerosols in the thermal portion of the electromagnetic spectrum. This is the first multi‐year satellite‐based assessment of the NRE of dust aerosols indicating the importance of both the shortwave and longwave radiative effects of dust aerosols over the oceans unlike anthropogenic aerosols that have negligible TOA longwave radiative forcing effects.
Indirect radiative forcing of the ozone layer during the 21st century
Authors: Portmann Robert W. , Solomon Susan
Published: January 2007

The response of a coupled two‐dimensional radiative‐chemical‐dynamical model to possible 21st century changes of the greenhouse gasses (GHGs) carbon dioxide, nitrous oxide and methane are explored using a range of IPCC marker scenarios of GHG emissions. The changes to the ozone layer caused by these GHGs are found to be relatively large (e.g., up to 5% global mean column ozone changes and 30% local changes for CO2 using the IPCC A2 scenario between 2000 and 2100) and the mechanisms for these changes are discussed. The ozone changes are compared to the recovery of ozone due to expected decreases in chlorine containing compounds. Since carbon dioxide, nitrous oxide, and methane affect ozone they induce an indirect radiative forcing in addition to their direct radiative forcing. These indirect radiative forcings are computed using a combination of accurate line‐by‐line and band radiative transfer models and are compared to the radiative forcing of ozone during the 1979–2000 time period. Although the changes in ozone are large at some altitudes over the 2000–2100 time horizon, the range of associated future indirect radiative forcings from ozone over the range of IPCC scenarios are found to be −0.1 to 0.1 W m−2, which is small compared with the corresponding range of total direct radiative forcing of 2.2 to 6.2 W m−2 for these GHGs over this time horizon.
Changes of North Pacific Intermediate Water properties in the subtropical gyre
Authors: Kouketsu Shinya , Kaneko Ikuo , Kawano Takeshi , Uchida Hiroshi , Doi Toshimasa , Fukasawa Masao
Published: January 2007

A comparison of trans‐Pacific hydrographic observations revealed changes in properties of North Pacific Intermediate Water (NPIW) in the subtropical gyre of the North Pacific from 1980s and 1990s to 2000s. Salinities decreased on density surfaces above the NPIW salinity minimum and increased below the salinity minimum. The potential temperature of the NPIW core increased by 0.10 – 0.17°C, although the salinity of NPIW core did not change substantially. The pattern of salinity changes on neutral surfaces was consistent with a density decrease of the NPIW core. It is suggested that the property changes of the NPIW core can be explained by NPIW warming.
Use of generalized cross validation for identification of global lightning distribution by using Schumann resonances
Authors: Ando Yoshiaki , Hayakawa Masashi
Published: January 2007

In this paper, generalized cross validation is implemented to solve an inverse problem to identify global lightning distribution from measured Schumann resonance. The inverse problem to identify the lightning distribution as a function of the distance from the observer is formulated in terms of Schumann resonance spectra calculated by the finite difference method. Tikhonov's regularization is introduced for proper inversion, and the optimal parameter for the regularization is evaluated by using the generalized cross validation as an objective estimation. We compared the results obtained in the present paper with the data collected by the Lightning Image Sensor during December 1999, and this comparison shows a better agreement than the previous results.
Long memory of rivers from spatial aggregation
Authors: Mudelsee Manfred
Published: January 2007

Long memory is a hydrological property that can lead to prolonged droughts or the temporal clustering of extreme floods in a river. Analyses of 28 long (up to 145 years), continuous instrumental runoff series from six European, American, and African rivers reveal that this effect increases downstream. Simulations reproduce the increase qualitatively and show that a river network aggregates short‐memory precipitation and converts it into long‐memory runoff. In view of projected changes in climate and the hydrological cycle, these findings show that decadal‐scale variations in drought or flood risk can be predicted for individual rivers, with higher predictability downstream. Spatial aggregation may also explain the emergence of long memory in other networks, such as the brain or those formed by computers.
Dialectics of Resilience: A Multi–Level Analysis of a Telehealth Innovation
Authors: Cho Sunyoung , Mathiassen Lars , Robey Daniel
Published: January 2007

Resilience is commonly portrayed as a positive capability that allows individuals, groups, and organizations to thrive in dynamic contexts. This paper questions this oversimplified view based on a dialectical analysis of a telehealth innovation within a network of collaborating hospitals. We analyze the major contradictions that characterize the adoption of the innovation. First, we analyze contradictions between individuals and groups within each adopting organization. Second, we analyze contradictions between the adopting organizations. This multi-level analysis leads to a deeper understanding of resilience as a dialectical process. The analysis of the case shows that, although the participating individuals, groups, and organizations demonstrated apparent resilience in adopting the telehealth innovation, the innovation remained in a fragile state, where it was unclear whether it would continue to diffuse, stabilize as-is, or slowly deteriorate. Hence, while resilience facilitated swift and successful adoption, it also created tensions that endangered further diffusion and the long-term sustainability of the telehealth innovation. We suggest that understanding the future success of the innovation would be facilitated to a large extent by a dialectical analysis of the involved contradictions.
Exploring Knowledge Exchange in Electronic Networks of Practice
Authors: Whelan Eoin
Published: January 2007

The recently developed knowledge-based view of the firm argues that knowledge is the firm's most valuable resource. Within this field of study, informal social networks are rapidly gaining attention as mechanisms that facilitate knowledge flows. Electronic networks of practice (NoPs) are a special case of informal network where the sharing of practice-related knowledge occurs primarily through computer-based communication technologies. However, we know relatively little about the dynamics of knowledge exchange that occur in these electronic networks. This conceptual paper posits that there is a relationship between the structural properties of electronic NoPs and successful knowledge exchange. The theoretical positions of social network theory and the knowledge-based view of the firm are used to support this claim. A review of the extant literature reveals that a clear conceptual framework for NoPs is lacking. The objective of this research is to contribute to the development of such a framework by addressing a largely neglected area within the growing network of practice literature – namely, the relationship between electronic network communication structure and knowledge flows.
Dawn of E-Government – an Institutional Analysis of Seven Initiatives and Their Impact
Authors: Henriksen Helle Zinner , Damsgaard Jan
Published: January 2007

Most countries have defined strategies for e-government. The objectives for implementing e-government are often defined but the means for fuelling the adoption and diffusion of e-government are typically less well clear in the policy statements. The present study assesses the impact of latest Danish initiative implemented to stimulate e-government adoption. The e-Day initiative simply yet powerfully states that ‘one governmental authority has the right to demand that its communication with another authority must be in electronic format’ which is expected to create ripple effects both internally and externally. The e-Day initiative represents a drastic change in the former policy statements concerning IT adoption and diffusion in Danish government. The policy statements had previously been based on voluntary adoption focusing on visions and pedagogical intervention in governmental agencies, but the e-Day initiative marked a departure from that strategy and the carrot has been exchanged by the whip, and the voice is imperative.
Mode-locking optimization with a real-time feedback system in a Nd:yttrium lithium fluoride laser cavity
Authors: Marengoni C. , Canova F. , Batani D. , Benocci R. , Librizzi M. , Narayanan V. , Gomareschi M. , Lucchini G. , Kilpio A. , Shashkov E. , Stuchebrukhov I. , Vovchenko V. , Chernomyrdin V. , Krasuyk I. , Hall T. , Bittanti S.
Published: January 2007

We present a control system, which allows an automatic optimization of the pulse train stability in a mode-locked laser cavity. In order to obtain real-time corrections, we chose a closed loop approach. The control variable is the cavity length, mechanically adjusted by gear system acting on the rear cavity mirror, and the controlled variable is the envelope modulation of the mode-locked pulse train. Such automatic control system maintains the amplitude of the mode-locking pulse train stable within a few percent rms during the working time of the laser. Full implementation of the system on an Nd:yttrium lithium fluoride actively mode-locked laser is presented.
Discrete retardance second harmonic generation ellipsometry
Authors: Dehen Christopher J. , Everly R. Michael , Plocinik Ryan M. , Hedderich Hartmut G. , Simpson Garth J.
Published: January 2007

A new instrument was constructed to perform discrete retardance nonlinear optical ellipsometry (DR-NOE). The focus of the design was to perform second harmonic generation NOE while maximizing sample and application flexibility and minimizing data acquisition time. The discrete retardance configuration results in relatively simple computational algorithms for performing nonlinear optical ellipsometric analysis. NOE analysis of a disperse red 19 monolayer yielded results that were consistent with previously reported values for the same surface system, but with significantly reduced acquisition times.
Measurement of the thermal conductance of silicon nanowires at low temperature
Authors: Bourgeois Olivier , Fournier Thierry , Chaussy Jacques
Published: January 2007

We have performed thermal conductance measurements on individual single-crystalline silicon suspended nanowires. The nanowires (130nm thick and 200nm wide) are fabricated by e-beam lithography and suspended between two separated pads on silicon on insulator substrate. We measure the thermal conductance of the phonon waveguide by the 3ω method. The cross section of the nanowire approaches the dominant phonon wavelength in silicon which is of the order of 100nm at 1K. Above 1.3K the conductance behaves as T3, but a deviation is measured at the lowest temperature which can be attributed to the reduced geometry.
Relaxed graded SiGe donor substrates incorporating hydrogen-gettering and buried etch stop layers for strained silicon layer transfer applications
Authors: Isaacson David M. , Pitera Arthur J. , Fitzgerald Eugene A.
Published: January 2007

We report on the use of buried, tensilely strained Si (i.e., ε-Si) layers for improving the fabrication of strained silicon-on-insulator (SSOI) substrate via the hydrogen-induced layer exfoliation process. Previous work involving tensile ε-Si0.4Ge0.6 layers in relaxed Ge∕∇x(Si1−xGex)∕Si demonstrated significant hydrogen gettering via the formation of strain-relieving platelets within the tensile ε-Si0.4Ge0.6 layers, leading to an overall increase in layer transfer efficiency for germanium-on-insulator substrate fabrication. The use of buried ε-Si layers in relaxed Si1−xGex for SSOI fabrication, however, exhibits more complex exfoliation behavior that is dependent on a combination of both the degree of tensile strain as well the amount of damage present in the adjacent Si1−xGex alloy. It was determined that a tensile strain level of approximately 1.6% in Si (corresponding to a Si0.6Ge0.4-based donor structure) was needed to overcome the intrinsic gettering potential of implantation damage in the adjacent Si0.6Ge0.4 layers to result in enhanced layer exfoliation. The primary mechanism responsible for improving layer exfoliation was found to be the formation of strain-relieving {100}-type platelets in the 1.6% strained-Si epilayer. Lastly, a Si0.6Ge0.4-based donor structure which incorporated ε-Si layers as transfer, hydrogen gettering, and etch stop layers was demonstrated. Such a structure may prove useful for the reuse of a significant portion of the original SSOI donor structure.
Accuracy limits and window corrections for photon Doppler velocimetry
Authors: Jensen B. J. , Holtkamp D. B. , Rigg P. A. , Dolan D. H.
Published: January 2007

Symmetric, plate-impact experiments were performed to validate photon Doppler velocimetry (PDV) with established shock wave diagnostics. Impact velocity measurements using shorting pins demonstrated that the velocity accuracy of PDV can be 0.1% or better. Shock velocities and refractive indices were also measured with PDV (at 1550 nm) and velocity interferometer system for any reflector (VISAR) (at 532 nm) to obtain window corrections for single crystal LiF (100), c-cut sapphire, and z-cut quartz. Time-dependent, free-surface velocity histories for shocked LiF(100) provide a direct comparison between PDV and VISAR diagnostics and illustrate the benefits and shortcomings of the new diagnostic. Further implications of these results are presented.
Electrical properties of Er-doped CdS thin films
Authors: Dávila-Pintle J. A. , Lozada-Morales R. , Palomino-Merino M. R. , Rivera-Márquez J. A. , Portillo-Moreno O. , Zelaya-Angel O.
Published: January 2007

Cadmium sulfide thin films were prepared by chemical bath on glass substrates at 80°C. CdS was Er-doped during the growth process by adding water-diluted Er(NO3)33∙H2O to the CdS aqueous growing solution. The relative volume of the doping solution was varied in order to obtain different doping levels. The crystalline structure of CdS:Er films was cubic zinc blende for all the doped layers prepared. The (111) interplanar distance has an irregular variation with the Er doping level. Consequently, the band gap energy (Eg) firstly increases and afterward diminishes becoming, at last, approximately constant at around Eg=2.37eV. For higher doping levels, in the as-grown films, dark electrical conductivity (σ) values reach 1.8×10−2Ω−1cm−1 at room temperature. The logarithm of σ vs 1∕kT plot, where k is Boltzmann’s constant and T the absolute temperature, indicates an effective doping of CdS as a result of the Er introduction into the lattice of the material. Hall effect measurements reveal a n-type doping with 2.8×1019cm−3 as maximum carrier density.
Surface layer of SrRuO3 epitaxial thin films under oxidizing and reducing conditions
Authors: Mlynarczyk M. , Szot K. , Petraru A. , Poppe U. , Breuer U. , Waser R. , Tomala K.
Published: January 2007

Imperfect stoichiometry and heterogeneity of a surface layer of SrRuO3 epitaxial thin films, grown on SrTiO3 substrates, are presented with the help of various methods. Rutherford backscattering spectroscopy, x-ray photoemission spectroscopy (XPS), and time of flight secondary ion mass spectrometry are used to obtain information about the stoichiometry and uniformity of the SrRuO3 structure. The temperature of chemical decomposition is first determined for polycrystalline samples under different conditions using thermogravimetry analysis. Then the determined values are used for thin film annealings in high and low oxygen pressure ambients, namely, air, vacuum, and hydrogen. The surface deterioration of the thin film together with changes in its electronic structure is investigated. O1s and Sr3d core lines measured by XPS for as-made samples obviously consist of multiple components indicating different chemical surroundings of atoms. Thanks to different incident beam angle measurements it is possible to distinguish between interior and surface components. Valence band spectra of the interior of the film are consistent with theoretical calculations. After annealing, the ratio of the different components changes drastically. Stoichiometry near the surface changes, mostly due to ruthenium loss (RuOX) or a segregation process. The width and position of the Ru3p line for as-made samples suggest a mixed oxidation state from metallic to fully oxidized. Long annealing in hydrogen or vacuum ambient leads to a complete reduction of ruthenium to the metallic state. Local conductivity atomic force microscopy scans reveal the presence of nonconductive adsorbates incorporated in the surface region of the film. Charge transport in these measurements shows a tunneling character. Scanning tunneling microscopy scans show some loose and mobile adsorbates on the surface, likely containing hydroxyls. These results suggest that an adequate description of a SrRuO3 thin film should take into account imperfections and high reactivity of its surface region.
Reduction of the residual compressive stress of tetrahedral amorphous carbon film by Ar background gas during the filtered vacuum arc process
Authors: Kim Tae-Young , Lee Churl Seung , Lee Young Jin , Lee Kwang-Ryeol , Chae Keun-Hwa , Oh Kyu Hwan
Published: January 2007

Tetrahedral amorphous carbon (ta-C) film was prepared by the filtered vacuum arc process with Ar background gas. The residual compressive stress of the film decreased significantly as the Ar flow rate increased, whereas negligible change in the mechanical properties was observed. Structure analysis by Raman spectroscopy, near edge x-ray absorption fine structure, and electron spin resonance spectra revealed a close relationship between the residual compressive stress and the bond distortion that invokes paramagnetic defects or unpaired π electrons. These results demonstrate that the stress reduction can occur by relaxation of the distorted or twisted atomic bonds at the same fraction of sp3 hybridized bonds, which enhances the stability of the ta-C coating without deterioration in the advantageous properties. The effect of Ar background gas is discussed in terms of the increased ion population of low kinetic energy in the plasma beam.
Large N behavior of two dimensional supersymmetric Yang-Mills quantum mechanics
Authors: Trzetrzelewski Maciej
Published: January 2007

We analyze the N→∞ limit of supersymmetric Yang-Mills quantum mechanics (SYMQM) in two space time dimensions. To do so we introduce a particular class of SU(N) invariant polynomials and give the solutions of two-dimensional SYMQM in terms of them. We conclude that in this limit the system is not fully described by the single trace operators Tr(a†n) since there are other, bilinear operators Trn(a†a†) that play a crucial role when the Hamiltonian is free.
Charge storage in self-aligned doubly stacked Si nanocrystals in SiNx dielectric
Authors: Wang Jiumin , Wu Liangcai , Chen Kunji , Yu Linwei , Wang Xiang , Song Jie , Huang Xinfan
Published: January 2007

Doubly stacked layers of silicon nanocrystals (nc-Si) in amorphous silicon nitride (α-SiNx) dielectric have been fabricated by plasma enhanced chemical vapor deposition and subsequent high-temperature thermal annealing. Capacitance-voltage (C-V) and current-voltage (I-V) measurements were used to investigate its charge trapping phenomena and confirmed the memory effects of the nc-Si, which exhibited capacitance hysteresis and asymmetric current peaks, respectively. This structure has realized a two-level charge storage by Fowler-Nordheim tunneling in high-field regime. While under low field, the resonant tunneling of holes into stacked nc-Si was responsible for current peaks. The transport property of stacked nc-Si was discussed and a model was put forward to explain the retention mechanism.
Gas temperature and electron temperature measurements by emission spectroscopy for an atmospheric microplasma
Authors: Mariotti Davide , Shimizu Yoshiki , Sasaki Takeshi , Koshizaki Naoto
Published: January 2007

A microplasma suitable for material processing at atmospheric pressure in argon and argon-oxygen mixtures is being studied here. The microplasma is ignited by a high voltage dc pulse and sustained by low power (1–5W) at 450MHz. the mechanisms responsible for sustaining the microplasma require a more detailed analysis, which will be the subject of further study. Here it is shown that the microplasma is in nonequilibrium and appears to be in glow mode. The effect of power and oxygen content is also analyzed in terms of gas temperature and electron temperature. Both the gas temperature and the electron temperature have been determined by spectral emission and for the latter a very simple method has been used based on a collisional-radiative model. It is observed that power coupling is affected by a combination of factors and that prediction and control of the energy flow are not always straightforward even for simple argon plasmas. Varying gas content concentration has shown that oxygen creates a preferential energy channel towards increasing the gas temperature. Overall the results have shown that combined multiple diagnostics are necessary to understand plasma characteristics and that spectral emission can represent a valuable tool for tailoring microplasma to specific processing requirements.
Monte Carlo modeling of the dc saddle field plasma: Discharge characteristics of N2 and SiH4
Authors: Wong Johnson , Kherani Nazir P. , Zukotynski Stefan
Published: January 2007

Enhancements in rates of ionization, dissociation, and current in the dc saddle field (DCSF) glow discharge are studied using the direct Monte Carlo method (DMCM). The DCSF consists of a planar semitransparent anode positioned between two planar cathodes, producing a symmetric electric field that serves to lengthen the path of electrons. Simulations of current versus anode transparency of a N2 discharge agree very well with experimental results reported previously. Numerical results of the DCSF SiH4 discharge are also presented. At typical operating conditions (600V, 5cm cathode-anode spacing, anode transparency of 0.8), DMCM predicts a significant increase in the dissociation rate at pressures below 100mTorr in comparison to the dc diode. In consideration of its use for plasma enhanced chemical vapor deposition, the efficiency with which the DCSF operates at low pressures makes it attractive for the production of thin films whose qualities are sensitive to gas phase reactions.
Modeling the optical constants of Cu2In4Se7 and CuGa3Se5 crystals
Authors: León M. , Serna R. , Levcenko S. , Nateprov A. , Nicorici A. , Merino J. M. , Arushanov E.
Published: January 2007

Spectral dependence of the pseudodielectric function ε(E)=ε1(E)+iε2(E) as well as of the complex refractive index, extinction coefficient, absorption coefficient, and normal-incidence reflectivity of Cu2In4Se7 and CuGa3Se5 ordered vacancy compound crystals are modeled in the 0.8–4.4eV photon energy range using a modification of Adachi’s model [T. Kawashima et al. J. Appl. Phys. 84, 5202 (1998)] for optical properties of semiconductors. Model parameters are determined using the acceptance-probability-controlled simulated annealing method. Excellent agreement with experimental data is obtained; the relative errors for the real ε1 and for imaginary ε2 part of the dielectric function are equal to 0.9%–1.5% and 3.2%–4.1% for the studied compounds.
Modeling and characterization of InAs∕GaAs quantum dot lasers grown using metal organic chemical vapor deposition
Authors: Sears K. , Buda M. , Tan H. H. , Jagadish C.
Published: January 2007

We report on the lasing characteristics of three- and five-stack InAs∕GaAs quantum dot (QD) lasers grown by metal organic chemical vapor deposition. By increasing the number of stacked dot layers to 5, lasing was achieved from the ground state at 1135nm for device lengths as short as 1.5mm (no reflectivity coatings). The unamplified spontaneous emission and Z ratio as a function of injection current were also investigated. While the five-stack QD lasers behaved as expected with Z ratios of ≈2 prior to lasing, the three-stack QD lasers, which lased from the excited state, exhibited Z-ratio values as high as 4. A simple model was developed and indicated that high Z ratios can be generated by three nonradiative recombination pathways: (i) high monomolecular recombination within the wetting layer, (ii) Auger recombination involving carriers within the QDs (“unmixed” Auger), and (iii) Auger recombination involving both the QD and wetting layer states (“mixed” Auger), which dominate once the excited and wetting layer states become populated.
Size dependence on the ordering process in colloidal FePt nanoparticles
Authors: Vargas J. M. , Zysler R. D. , Socolovsky L. M. , Knobel M. , Zanchet D.
Published: January 2007

An alternative method to study the effects of annealing process on colloidal FePt nanoparticles (2–4 nm) has been achieved. Annealing experiments at temperatures between 773 and 1073 K under inert atmosphere flux were performed in powder samples with excess of surfactant molecules on nanoparticle surface. Transmission electron microscopy, x-ray diffraction and magnetic measurements were performed to evidence the evolution of the chemically disordered fcc to chemically ordered face-centered tetragonal phase transformation. Magnetization measurements under zero-field-cooling and field-cooling (MZFC−MFC) conditions, and hysteresis loops are extremely sensitive to the particle size distribution and were strongly affected by the annealing treatment.
Phase transitions in two-dimensional colloidal particles at oil/water interfaces
Authors: Lin Bo-Jiun , Chen Li-Jen
Published: January 2007

Enhanced digital video microscopy is applied to study the equilibrium structure of a two-dimensional charged sulfate-polystyrene particle (2μm in diameter) monolayer at decane/water interfaces. When the surface density is decreased, a sequential phase transition, pure solid phase→pure hexatic phase→liquid-hexatic-coexisting phase→pure liquid phase, is observed. In addition, the transition between liquid and hexatic phases is first order, while the solid-hexatic phase transition is second order. The temperature effect on this two-dimensional melting transition is discussed by performing the experiments at three different temperatures. The Voronoi [J. Reine Angew. Math. 134, 198 (1908)] construction is applied to analyze the defect structure in the two-dimensional particle monolayer. The pair interaction potential of the two-dimensional colloidal particles is found to be a very long range repulsion and to decay with distance to the power of −3.
Photothermal, Photocarrier and Raman Characterization of Te-doped GaSb
Authors: Velázquez-Hernández R. , García-Rivera J. , Rodriguez Garcia M. E. , Jimenez Sandoval S. , Mendoza-Alvarez J. G. , Garcia J. A.
Published: January 2007

Noncontact, nonintrusive photocarrier (PCR) and photothermal radiometry (PTR) as well as Raman spectroscopy were used to characterize GaSb and Te-doped GaSb wafers used to grow GaSb-based heterostructures for infrared applications. The results indicated excellent sensitivity of PTR and PCR to the Te distribution on the wafer surface. The results from the three methodologies were consistent and indicated that the Te was segregated toward the edge of the GaSb wafer. The PTR and PCR laser-based techniques show great potential advantages to characterize the homogeneity of the Te doping in the GaSb wafers and for other steps in the fabrication of semiconductor heterostructures.
Exploring Renner-Teller induced quenching in the reaction H(S2)+NH(aΔ1): A combined experimental and theoretical study
Authors: Adam L. , Hack W. , McBane G. C. , Zhu H. , Qu Z.-W. , Schinke R.
Published: January 2007

Experimental rate coefficients for the removal of NH(aΔ1) and ND(aΔ1) in collisions with H and D atoms are presented; all four isotope combinations are considered: NH+H, NH+D, ND+H, and ND+D. The experiments were performed in a quasistatic laser-flash photolysis/laser-induced fluorescence system at low pressures. NH(aΔ1) and ND(aΔ1) were generated by photolysis of HN3 and DN3, respectively. The total removal rate coefficients at room temperature are in the range of (3–5)×1013cm3mol−1s−1. For two isotope combinations, NH+H and NH+D, quenching rate coefficients for the production of NH(XΣ−3) or ND(XΣ−3) were also determined; they are in the range of 1×1013cm3mol−1s−1. The quenching rate coefficients directly reflect the strength of the Renner-Teller coupling between the A″2 and A′2 electronic states near linearity and so can be used to test theoretical models for describing this nonadiabatic process. The title reaction was modeled with a simple surface-hopping approach including a single parameter, which was adjusted to reproduce the quenching rate for NH+H; the same parameter value was used for all isotope combinations. The agreement with the measured total removal rate is good for all but one isotope combination. However, the quenching rates for the NH+D combination are only in fair (factor of 2) agreement with the corresponding measured data.
Anomalous excitation dependence of electroluminescence in InGaN∕GaN light-emitting diodes
Authors: Nee Tzer-En , Shen Hui-Tang , Wang Jen-Cheng , Wu Ya-Fen
Published: January 2007

We have systematically investigated the anomalous excitation dependence of the electroluminescence (EL) in InGaN∕GaN multiple-quantum-well light-emitting diodes over a temperature range from 300to20K. Initially, an increase in the emission intensity occurred upon decreasing the temperature, until a maximum was reached at the temperature Tm. A blueshift in the position of the EL peak was followed by a redshift that occurred at the crossover temperature Tc. Both of these characteristic temperatures correlate with the presence of statistic microbarriers arising from potential inhomogeneity. The higher the In content incorporated into the heterobarriers, named multiple quantum barriers, the lower the values of Tm and Tc obtained from the spectral observations; this phenomenon implies an augmentation in the microscopic nonradiative transport through the microbarriers. An increase in the injection current also led to decreases in both of these characteristic temperatures. In addition, a functional correlation exists between the values of Tc and Tm. All of these experimental results suggest that InGaN∕GaN microstructures can be viewed as disordered collections of InGaN nanocrystallites. Further experimental verification will be necessary if this phenomenological model is to be used to account for the properties of any other disorderlike heteromaterials.
Dielectric property of particles at interface in random sequential adsorption and its application to whispering gallery mode resonance-shift sensors
Authors: Teraoka Iwao , Arnold Stephen
Published: January 2007

Dielectric properties of particles adsorbed onto a planar interface between two dielectric media are considered, taking into account the dipolar fields created by polarization induced in nearby particles as well as by their image dipoles. The use of a dipolar approximation allowed us to derive a self-consistent expression for the effective field experienced by each particle. The expression uses the pair correlation function of the particle distribution and is presented in a close form for normal and tangential directions of the applied field. To obtain the correlation function for the distribution of irreversibly adsorbed particles and calculate the effective field, a computer simulation of random sequential adsorption was carried out at prefixed particle densities and until placing the next particle became extremely difficult. The effective field was also calculated for hexagonal closest packing. Results of these calculations were then used to estimate resonance wavelength shifts of whispering gallery modes in a dielectric microsphere when it adsorbs dielectric particles. A gradual change was seen from the shifts for isolated particles at low densities to those for a packed layer.
Adhesion-delamination mechanics of a prestressed rectangular film adhered onto a rigid substrate
Authors: Wong Ming-Fung , Duan Gang , Wan Kai-Tak
Published: January 2007

A prestressed rectangular film clamped at both ends delaminates from a rigid punch. Based on a thermodynamic energy balance, the delamination mechanics is derived to relate the simultaneous external tensile force applied to the punch, punch displacement, and contact area. Effects of the coupled tensile residual membrane stress and adhesion energy at the punch-film interface are investigated. A “pinch off” (stable shrinking of the contact area to a line) is predicted, contrasting the nonzero “pull-off” radius in a clamped circular film. The model is useful in understanding the behavior of various adhesion-delamination phenomena, especially in one dimensional capacitive microelectromechanical systems radio frequency switches, microstructure network, and nanostructures.
Light transmitting electromagnetic wave shielding composite materials using electromagnetic wave polarizing effect
Authors: Matumura K. , Kagawa Y. , Baba K.
Published: January 2007

Light transmitting and electromagnetic wave shielding material using a cross-polarizing effect of unidirectionally aligned electrically conductive fiber-dielectric composite has been developed. Two-electrically isolated unidirectionally aligned continuous electrically conductive fiber layers were incorporated into an optically transparent polymethylmethacrylate (PMMA) matrix. Electromagnetic wave shielding by the polarizing effect of the composite was examined by the free-space method in a frequency range from 18to24GHz. The composite shields electromagnetic wave with a small expense of light transmittance of the PMMA matrix. Effect of fiber spacing, angle of fiber axis direction, and polarizing mode of incident electromagnetic wave are discussed based on experimental results. It is concluded that the use of the polarizing effect is an effective way to obtain an optically transparent electromagnetic shielding material.
Magnetic and optical properties of (Ga1−xMnx)As diluted magnetic semiconductor quantum wires with above room ferromagnetic transition temperature
Authors: Jeon H. C. , Kang T. W. , Kim T. W. , Yu Y.-J. , Jhe W. , Song S. A.
Published: January 2007

Scanning electron microscopy and high-resolution transmission electron microscopy measurements showed that the self-assembled (Ga0.8Mn0.2)As quantum wires (QWRs) grown on GaAs (100) substrates by using molecular beam epitaxy were straight crystals. The magnetization curve as functions of the magnetic field at 5K indicated that the (Ga0.8Mn0.2)As QWRs were ferromagnetic, and the magnetization curve as a function of the temperature showed that the ferromagnetic transition temperature of the (Ga0.8Mn0.2)As diluted magnetic semiconductor (DMS) QWRs was as high as 350K. Near-field scanning optical microscopy spectra showed the interband transitions of the (Ga0.8Mn0.2)As QWRs, indicative of the Mn atoms acting as substituents. These results indicate that the (Ga1−xMnx)As DMS QWRs with a high Mn concentration hold promise for potential application in spin optoelectric devices operating at room temperature.
Anomalous hysteresis loops measured by the magneto-optical Kerr effect in a Co∕NiO∕Cu∕Co∕Cu structure
Authors: Zhao Z. C. , Wang H. , Xiao S. Q. , Xia Y. X. , McGuire John A. , Ren Y. , Jin Q. Y. , Gao T. R.
Published: January 2007

Anomalous hysteresis loops measured by longitudinal and polar magneto-optical Kerr effects (MOKE) were observed in a Co∕NiO∕Cu∕Co∕Cu structure. After excluding other possible mechanisms, we explain the anomalous hysteresis loops in terms of the opposite rotation angles for the top- and bottom-Co layers. Ultimately, the opposite rotation angles are likely caused by the different incidence angles for the top- and bottom-Co layers.
Quantitative analysis of capillary condensation in fractal-like porous silicon nanostructures
Authors: Moretti Luigi , De Stefano Luca , Rendina Ivo
Published: January 2007

The condensation of vapor chemical substances inside the nanometric structures of porous silicon is experimentally and theoretically investigated. The liquid phase covers the pore surface as a thin film and fills a volume fraction in the spongelike structure depending on the physical and chemical properties of each compound. The filling factor of different substances has been measured as a function of the porous silicon film porosity by means of an interferometric technique. We have merged a classical effective medium approximation, such as the Bruggemann theory, with a fractal model of the pore in order to find out how the wetting liquid thickness depends on the porosity. Our results demonstrate that the capillary condensation exhibits a nonlinear behavior at high porosities (greater than 0.8), due to the strong decrease of the vapor confinement degree inside the coalescent nanometric pores.
Inverse estimation of viscoelastic material properties for solids immersed in fluids using vibroacoustic techniques
Authors: Brigham J. C. , Aquino W. , Mitri F. G. , Greenleaf J. F. , Fatemi M.
Published: January 2007

This work presents an approach to inversely determine material properties for solids immersed in fluids through the use of steady-state dynamic response. The methodology uses measured acoustic pressure amplitudes in the fluid surrounding a structure being vibrated with a harmonic force to determine the parameters for elastic and viscoelastic material models. Steady-state dynamic finite element analysis is used to compute the frequency response function of homogeneous and heterogeneous solids. The frequency response is then used to inversely estimate material parameters. In order to solve the inverse problem, an optimization method is presented which combines the global search capabilities of the random search method with the reduced computational time of a surrogate model approach. Through numerical and laboratory experiments, this work shows that acoustic emissions hold sufficient information for quantifying both elastic and viscoelastic material behaviors. Furthermore, the examples show that the surrogate model accelerated random search algorithm is an efficient and accurate method for solving these types of inverse problems. This research has direct and important implications in nondestructive evaluation of material properties in general. However, the examples shown in this paper are based on applications related to noninvasive characterization of biological materials through methodologies currently applied to medical imaging.
Reduced ferroelectric coercivity in multiferroic Bi0.825Nd0.175FeO3 thin film
Authors: Yuan G. L. , Or Siu Wing , Chan Helen Lai Wa , Liu Z. G.
Published: January 2007

Bi 0.825 Nd 0.175 Fe O 3 (BNFO) thin film is grown on Pt∕TiO2∕SiO2∕Si substrate by pulsed laser deposition, and its multiferroic properties are compared with those of BiFeO3 (BFO) thin film. With limited Fe2+ ions and its twinborn oxygen vacancies, both samples show low dielectric losses of <0.026 at 100Hz and high maximum ferroelectric polarizations of ∼34μC∕cm2. The ferroelectric coercive field of BNFO is reduced by ∼40%, reaching a low value of 235kV∕cm, compared to that of BFO due to the increased ratio of 180° and curved ferroelectric domains to total ferroelectric domains and better nucleation of the ferroelectric domains at the BNFO∕Pt interface. The Raman scattering spectra confirm that the ferroelectric polarizations of both samples originate in the stereochemical activity of the Bi lone electron pair. Weak ferromagnetism is observed in both samples as a result of the limited amount of Fe2+ ions and γ-Fe2O3 impurity.
Low-resistance and thermally stable indium tin oxide Ohmic contacts on strained p-In0.15Ga0.85N∕p-GaN layer
Authors: Jang Ja-Soon , Seong Tae-Yeon
Published: January 2007

We report on the formation of low-resistance and thermally stable indium tin oxide (ITO) Ohmic contacts on p-GaN using a strained p-InGaN (5nm) layer. Unlike as-deposited ITO contacts on p-GaN, the as-deposited contacts to a strained p-InGaN∕p-GaN layer exhibit Ohmic behavior. The current-voltage characteristics of the ITO∕InGaN∕GaN contacts are further improved upon annealing at 550°C for 1min and the specific contact resistance is 3.2(±0.8)×10−5Ωcm2. In addition, the contact resistivity and surface morphology of the ITO∕InGaN∕GaN contacts annealed for 30min are only slightly increased, confirming the thermal stability of this scheme. Based on the electrical and Auger spectroscopic data, the low contact resistivity and thermal stability of the ITO∕InGaN∕GaN contacts are described in terms of the polarization effect, increased acceptor concentration, and formation of diffusion barrier layer at the interface.
Interface effect on metal-insulator transition of strained vanadium dioxide ultrathin films
Authors: Nagashima Kazuki , Yanagida Takeshi , Tanaka Hidekazu , Kawai Tomoji
Published: January 2007

The interface effects on the metal-insulator transition (MIT) of strained VO2 ultrathin films grown epitaxially on TiO2 (001) single crystal substrate were investigated. Varying the surface conditions of TiO2 substrate, such as the roughness and the surface reconstructions, produced the remarkable changes in the MIT events of VO2 thin films, including the transition temperature and the abruptness. The presence of the surface reconstructions was found to be detrimental for applying effectively strain effects due to the strain relaxation in the c axis of VO2 thin films. The abrupt MIT in strained VO2 thin films, deposited on the substrate without such detrimental surface reconstructions, was successfully maintained down to around 5nm film thickness.