bluuebunny/crossref_metadata_2025 huggingface dataset:
Climate Change From the User’s Perspective
Authors: Taddicken Monika
Published: March 2013

Scientific information about the global climate and its development is both complex and uncertain. For laypersons, mass media and the Internet constitute the main sources of information about climate change. However, so far, little is known about the impact of mass media and, in particular, of online content about climate change on the audience’s knowledge regarding and attitudes toward climate change. Therefore, a survey with a representative sample of German Internet users (n = 1,523) was conducted. Based on previous research and theoretical considerations, a measurement model was developed that also included the individual variables environmental awareness and perceived involvement. Additionally, the individual’s perspective on media content was taken into account by considering moderator effects of the individual’s evaluation of media content and need for information. In this way, television and radio use was shown to slightly affect the levels of knowledge and problem awareness. The moderators media evaluation and need for information proved to be particularly important for the impact of Internet use. The more media reporting was perceived as exaggerated, the more Internet use negatively affected levels of knowledge, problem awareness, and behavioral intentions. Conversely, the interaction between need for information and Internet use increased the impact on problem awareness and behavioral intentions. Hence, it becomes clear that the Internet plays a crucial role in people’s attitude generation processes, but is utilized in different ways.
High-resolution SAR11 ecotype dynamics at the Bermuda Atlantic Time-series Study site by phylogenetic placement of pyrosequences
Authors: Vergin Kevin L , Beszteri Bánk , Monier Adam , Thrash J Cameron , Temperton Ben , Treusch Alexander H , Kilpert Fabian , Worden Alexandra Z , Giovannoni Stephen J
Published: March 2013

Advances in next-generation sequencing technologies are providing longer nucleotide sequence reads that contain more information about phylogenetic relationships. We sought to use this information to understand the evolution and ecology of bacterioplankton at our long-term study site in the Western Sargasso Sea. A bioinformatics pipeline called PhyloAssigner was developed to align pyrosequencing reads to a reference multiple sequence alignment of 16S ribosomal RNA (rRNA) genes and assign them phylogenetic positions in a reference tree using a maximum likelihood algorithm. Here, we used this pipeline to investigate the ecologically important SAR11 clade of Alphaproteobacteria. A combined set of 2.7 million pyrosequencing reads from the 16S rRNA V1–V2 regions, representing 9 years at the Bermuda Atlantic Time-series Study (BATS) site, was quality checked and parsed into a comprehensive bacterial tree, yielding 929 036 Alphaproteobacteria reads. Phylogenetic structure within the SAR11 clade was linked to seasonally recurring spatiotemporal patterns. This analysis resolved four new SAR11 ecotypes in addition to five others that had been described previously at BATS. The data support a conclusion reached previously that the SAR11 clade diversified by subdivision of niche space in the ocean water column, but the new data reveal a more complex pattern in which deep branches of the clade diversified repeatedly across depth strata and seasonal regimes. The new data also revealed the presence of an unrecognized clade of Alphaproteobacteria, here named SMA-1 (Sargasso Mesopelagic Alphaproteobacteria, group 1), in the upper mesopelagic zone. The high-resolution phylogenetic analyses performed herein highlight significant, previously unknown, patterns of evolutionary diversification, within perhaps the most widely distributed heterotrophic marine bacterial clade, and strongly links to ecosystem regimes.
Immune status, antibiotic medication and pH are associated with changes in the stomach fluid microbiota
Authors: von Rosenvinge Erik C , Song Yang , White James R , Maddox Cynthia , Blanchard Thomas , Fricke W Florian
Published: March 2013

The stomach acts as a barrier to ingested microbes, thereby influencing the microbial ecology of the entire gastrointestinal (GI) tract. The stomach microbiota and the role of human host and environmental factors, such as health status or medications, in shaping its composition remain largely unknown. We sought to characterize the bacterial and fungal microbiota in the stomach fluid in order to gain insights into the role of the stomach in GI homeostasis. Gastric fluid was collected from 25 patients undergoing clinically indicated upper endoscopy. DNA isolates were used for PCR amplification of bacterial 16S ribosomal RNA (rRNA) genes and fungal internal transcribed spacers (ITS). RNA isolates were used for 16S rRNA cDNA generation and subsequent PCR amplification. While all stomach fluid samples are dominated by the phyla Firmicutes, Bacteroidetes, Actinobacteria, Proteobacteria and Fusobacteria (>99% of sequence reads), the transcriptionally active microbiota shows significant reduction in Actinobacteria (34%) and increase in Campylobacter (444%) (P<0.003), specifically the oral commensal and suspected intestinal pathogen Campylobacter concisus. Bacterial but not fungal diversity is reduced by antibiotic treatment (28%; P<0.02), immunosuppression in transplant recipients and HIV/AIDS patients (42%; P<0.001) and gastric fluid pH >4 (70%; P<0.05). Immunosuppression correlates with decreased abundance of Prevotella (24%), Fusobacterium (2%) and Leptotrichia (6%) and increased abundance of Lactobacillus (3844%) (P<0.003). We have generated the first in-depth characterization of the human gastric fluid microbiota, using bacterial 16S rRNA gene and transcript, and fungal ITS amplicon sequencing and provide evidence for a significant impact of the host immune status on its composition with likely consequences for human health.
Characteristics and neural-like differentiation of mesenchymal stem cells derived from foetal porcine bone marrow
Authors: Liu Ying , Liu Limei , Ma Xin , Yin Yupeng , Tang Bo , Li Ziyi
Published: March 2013

MSCs (mesenchymal stem cells) are a stem cell source that can be easily obtained from bone marrow. Despite the increasing importance of the pig as a large animal model, little is known about foetal pMSCs (porcine MSCs). In this study, we observed the gene expression of pluripotent markers in foetal pMSCs and the capacity of pMSCs to differentiate into adipocytes, osteocytes and neural-like cells using quantitative RT–PCR (reverse transcription–PCR), normal histological staining and immunohistochemistry. Foetal pMSCs have either a spindle or a flattened shape, and flow cytometry revealed the expression of the MSC-related proteins CD44 and CD105 (endoglin) but not CD34 and CD45. pMSCs express pluripotent markers such as Oct4 (octamer-binding transcription factor 4) and Nanog at the protein and mRNA levels. qRT-PCR (quantitative real-time PCR) analyses revealed that pMSCs expressed nestin [for NSCs (neural stem cells)]. Immunocytochemical and RT–PCR data showed that 29% and 23% of pMSCs expressed MAP2 (microtubule-associated protein 2) for neurons and β-tubulin III (Tuj1) for immature neurons, respectively, after induction of neural differentiation. These findings demonstrate the plasticity of pMSCs and their potential for use in cellular replacement therapy for neural diseases.
Food for Thought
Authors: Sterling Laurie , Axline Rebecca , Ragland Patrice
Published: March 2013

Speech-language pathologists have little, if any, formal training in counseling during graduate school. Working with neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS) requires more listening than talking and direct therapy. Knowing what to say and how to address patient emotions about loss of speech and swallowing function can be difficult. We share an patient with ALS's perspective of her experiences in life as she lost her ability to go out to lunch with her coworkers and fought to maintain her weight. We introduce the SPIKES protocol (Baile et al., 2000) for delivering “bad news” as a format for speech-language pathology sessions when broaching difficult topics such as percutaneous endoscopic gastrostomy (PEG) tube placement.
Four-component united-atom model of bitumen
Authors: Hansen J. S. , Lemarchand Claire A. , Nielsen Erik , Dyre Jeppe C. , Schrøder Thomas
Published: March 2013

We propose a four-component united-atom molecular model of bitumen. The model includes realistic chemical constituents and introduces a coarse graining level that suppresses the highest frequency modes. Molecular dynamics simulations of the model are carried out using graphic-processor-units based software in time spans in order of microseconds, which enables the study of slow relaxation processes characterizing bitumen. This paper also presents results of the model dynamics as expressed through the mean-square displacement, the stress autocorrelation function, and rotational relaxation. The diffusivity of the individual molecules changes little as a function of temperature and reveals distinct dynamical time scales. Different time scales are also observed for the rotational relaxation. The stress autocorrelation function features a slow non-exponential decay for all temperatures studied. From the stress autocorrelation function, the shear viscosity and shear modulus are evaluated, showing a viscous response at frequencies below 100 MHz. The model predictions of viscosity and diffusivities are compared to experimental data, giving reasonable agreement. The model shows that the asphaltene, resin, and resinous oil tend to form nano-aggregates. The characteristic dynamical relaxation time of these aggregates is larger than that of the homogeneously distributed parts of the system, leading to strong dynamical heterogeneity.
Viscous instability of a sheared liquid-gas interface: Dependence on fluid properties and basic velocity profile
Authors: Otto Thomas , Rossi Maurice , Boeck Thomas
Published: March 2013

In the framework of linear stability theory, we analyze how a liquid-gas mixing layer is affected by several parameters: viscosity ratio, density ratio, and several length scales. These scales reflect the presence of a velocity defect induced by the wake behind the splitter plate and the presence of boundary layers which develop ahead of the plate trailing edge. Incorporating such effects, we compute the various temporal and spatial instability modes and identify their driving instability mechanism based on their Reynolds number dependence, spatial structure, and energy budget. It is examined how the velocity defect modifies the temporal and the spatial stability properties. In addition, the transition from convective to absolute instability occurs at lower velocity contrast between gas and liquid free streams when a defect is present. This transition is also promoted by surface tension. Compared to inviscid stability computations, our spatial stability analysis displays a better agreement with measured growth rates obtained in two recent air-water experiments.
Computational study of detonation wave propagation in narrow channels
Authors: Chinnayya Ashwin , Hadjadj Abdellah , Ngomo Davy
Published: March 2013

A numerical study of the propagation of regular detonation waves is conducted in the context of narrow channels undergoing strong wall confinement. To deal with shock waves, chemical reactions, heat and viscous stresses, a high-order Navier-Stokes solver based on Weighted Essentially Non-Oscillatory (WENO) scheme, coupled with the Strang splitting method, is used in the framework of multi-species reacting mixtures. Results show that the wall dissipative effects decrease the speed of the detonation wave compared to the Chapman-Jouguet (CJ) detonation velocity. In addition, the multidimensional results reveal that the development of the thermo-diffusive boundary layers behind the leading shock wave induces an expansion flow, which then determines the contour of the sonic envelope. From the Master Equation and the generalized CJ condition, which are derived and compared to the results of the current simulations, the main energy withdrawals are found to be related to the streamline divergence as well as to the growth of the boundary layer. Moreover, a fraction of the released energy is trapped in the vicinity of the wall and does not contribute to drive the shock front. The influence of the channel height is also investigated. It was found that the transverse instabilities are damped when the channel is scaled down, which results in an increase of the dissipative effects. Finally, the validity of the Fay model is discussed with regard to the channel height and the curvature of the detonation front.
Standing spin waves in magnonic crystals
Authors: Mruczkiewicz M. , Krawczyk M. , Sakharov V. K. , Khivintsev Yu. V. , Filimonov Yu. A. , Nikitov S. A.
Published: March 2013

The features of standing spin waves (SWs) excited during ferromagnetic resonance in three different one-dimensional magnonic crystals (MC) are intensively studied. The investigated magnonic crystals were: an array of air-spaced cobalt stripes, an array of air-spaced permalloy (Py) stripes, and a bi-component MC composed of alternating Co and Py stripes. All MC structures were made by etching technique from Co and Py thin films deposited onto Si substrates. Two configurations are considered with the in-plane external magnetic field applied parallel or perpendicular to the stripes. The supporting calculations are performed by the finite element method in the frequency domain. A number of intensive SW modes occurred in periodic structures under ferromagnetic resonance conditions as a consequence of standing spin waves excitation. These modes were analyzed theoretically in order to explain the origins of SW excitations. With the support of numerical calculations, we analyze also the possible scenarios for the occurrence of standing SWs in the investigated structures. It is demonstrated that the SW propagation length is an important factor conditioning the standing SW formation in MCs.
On the stability of the cuboid singlet (S2)4 supermolecule: Benchmark ab initio studies
Authors: Ochoa-Calle A. J. , Hernández-Lamoneda R. , Ramírez-Solís A.
Published: March 2013

We report high level ab initio supermolecular calculations for the cuboid structure of the disulfur tetramer, (S2)4. Accurate geometries and interaction energies with respect to 4S2 (\documentclass[12pt]{minimal}\begin{document}$\sideset{^{3}}{^{-}_{{\rm g}}}{\Sigma}$\end{document}Σg−3) were obtained using four different methods, Möller-Plesset perturbation theory (MP2), complete-active-space SCF (CASSCF) + complete active space second-order perturbation (CASPT2), RCCSD(T), and a hybrid CASPT2(singlet-nonet)/RCCSD(T)-nonet approach with systematic sequences of augmented correlation-consistent basis sets extrapolated to the complete basis set limit. Unlike the van der Waals-like (O2)4 cluster, (S2)4 is found to be much more chemically bound. Our best estimate of the dissociation energy to four S2 molecules is 65 kcal/mol including the counterpoise correction and an intermolecular distance of 2.74 Å. The singlet ground state of (S2)4 is much less multiconfigurational than that of (O2)4 van der Waals complex, which allows a reliable CCSD(T) description of the singlet potential energy surface of the supermolecule around its equilibrium geometry. The electron pair localization function clearly reveals electron pairing between the S2 units in the complex at the ROHF and the CASSCF/aug-cc-pVTZ levels. Vibrational analysis at the MP2/cc-pV(D,T,Q)Z,aug-cc-pVTZ levels yield stable cuboid structures; however, at the CCSD/aug-cc-pV(D,T)Z levels this analysis reveals a transition state with one imaginary frequency. Thus, further multireference-based studies with large basis sets are required to reliably settle the stability issue for this supermolecular sulfur species.
Time-dependent quantum wave packet dynamics of the C + OH reaction on the excited electronic state
Authors: Rao T. Rajagopala , Goswami Sugata , Mahapatra S. , Bussery-Honvault B. , Honvault P.
Published: March 2013

Quantum state-selected dynamics of C(3P) + OH (X2Π) → CO(a3Π) + H (2S) reaction on its first excited electronic potential energy surface (12A″) is examined here using a time-dependent wave packet propagation approach. All partial wave contributions for the total angular momentum, J = 0−95, are included to obtain the converged cross sections and initial state-selected rate constants in the temperature range of 10−500 K. The reaction probability, as a function of collision energy, exhibits dense oscillatory structures owing to the formation of resonances during collision. These resonance structures also persist in reaction cross sections. The effect of reagent rotational and vibrational excitation on the dynamical attributes is examined and discussed. Reagent rotational excitation decreases the reactivity whereas, vibrational excitation of the reagent has minor effects on the reactivity. The results presented here are in good accord with those obtained using the time-independent quantum mechanical and quasi-classical trajectory methods.
Electrochemical deposition of platinum within nanopores on silicon: Drastic acceleration originating from surface-induced phase transition
Authors: Fukami Kazuhiro , Koda Ryo , Sakka Tetsuo , Ogata Yukio , Kinoshita Masahiro
Published: March 2013

An electrochemical reaction within nanopores is remarkably decelerated once a diffusion-limited condition is reached due to the difficulty in supply of reactants from the bulk. Here, we report a powerful method of overcoming this problem for electrochemical deposition of platinum within nanopores formed on silicon. We made the pore wall surface of the silicon electrode hydrophobic by covering it with organic molecules and adopted platinum complex ions with sufficiently large sizes. Such ions, which are only weakly hydrated, are excluded from the bulk aqueous electrolyte solution to the surface and rather hydrophobic in this sense. When the ion concentration in the bulk was gradually increased, at a threshold the deposition behavior exhibited a sudden change, leading to drastic acceleration of the electrochemical deposition. Using our statistical-mechanical theory for confined molecular liquids, we show that this change originates from a surface-induced phase transition: The space within nanopores is abruptly filled with the second phase within which the ion concentration is orders of magnitude higher. When the affinity of the surface with water was gradually reduced with fixing the ion concentration, qualitatively the same transition phenomenon was observed, which can also be elucidated by our theory. The utilization of the surface-induced phase transition sheds new light on the design and control of a chemical reaction in nanospace.
Enhancement of sound in chirped sonic crystals
Authors: Romero-García V. , Picó R. , Cebrecos A. , Sánchez-Morcillo V. J. , Staliunas K.
Published: March 2013

We propose and experimentally demonstrate a mechanism of sound wave concentration based on soft reflections in chirped sonic crystals. The reported controlled field enhancement occurs at around particular (bright) planes in the crystal and is related to a progressive slowing down of the sound wave as it propagates along the material. At these bright planes, a substantial concentration of the energy (with a local increase up to 20 times) was obtained for a linear chirp and for frequencies around the first band gap. A simple couple mode theory is proposed that interprets and estimates the observed effects. Wave concentration energy can be applied to increase the efficiency of detectors and absorbers.
Effect of crystallographic alignment on the magnetocaloric effect in alloys near the Ni2MnGa stoichiometry
Authors: Giri Anit K. , Paterson Brigitte A. , McLeod Michael V. , Dennis Cindi L. , Majumdar Bhaskar S. , Cho Kyu C. , Shull Robert D.
Published: March 2013

Prior to the development of commercial applications of magnetic refrigerator technology, a large magnetocaloric effect (MCE) in polycrystalline materials must be realized for relatively low magnetic field changes. To increase the MCE, a crystallographic alignment technique, consisting of thermal cycling about the martensite phase transition temperature under a compressive stress, was applied to Heusler alloys with nominal composition Ni2+xMn1−xGa (x = 0.14, 0.16). Magnetic measurements prior to grain alignment show that the maximum entropy changes of −16 J kg−1K−1 and −24 J kg−1K−1 for samples with x = 0.14 and 0.16, respectively, occurred for a magnetic field change of 7 T. After grain alignment, there was a 56%–79% enhancement of the maximum magnetic entropy change for the same magnetic field change of 7 T. This suggests that thermal cycling under compressive stress may either increase grain alignment (e.g., texture) along the magnetic easy (001) axis, and/or enhance the ease with which a magnetic field is later able to grow favorably oriented twin variants that manifests as an increase of magnetization of the material. Therefore, such an alignment technique may be utilized to enhance the MCE of similar Heusler alloys.
Soft magnetic composites manufactured by warm co-extrusion of bulk metallic glass and steel powders
Authors: Johnson Francis , Raber Thomas R. , Zabala Robert J. , Buresh Steve J. , Tanico Brian
Published: March 2013

Soft magnetic composites of Fe-based bulk metallic glass and low-alloy steel have been manufactured by warm co-extrusion of precursor powders at temperatures within the supercooled liquid region of the glass. Composites were manufactured with amorphous volume fractions of 75%, 67%, and 100%. Full consolidation of the constituent powders was observed with the bulk metallic glass remaining substantially amorphous. The composite electrical resistivity was observed to be anisotropic with a resistivity of 79 μΩ cm measured transverse to the extrusion axis in a sample with 75% amorphous volume fraction. A 0-3 connectivity pattern with the low-resistivity steel phase embedded in a 3-dimensionally connected high-resistivity bulk metallic glass phase was observed with scanning electron microscopy. This confirms that the flow characteristics of the bulk metallic glass and the steel powders were comparable during extrusion at these temperatures. The saturation magnetization of 1.3 T was consistent with the volume weighted average of the saturation magnetization of the two phases. A relatively high quasistatic coercivity of 8 Oe was measured and is likely due to slight crystallization of the bulk metallic glass as well as domain wall pinning at prior particle boundaries. Careful control of the thermal environment during the extrusion process is required to minimize glass crystallization and achieve the desired balance of magnetic and electrical properties.
Molecular dynamics simulations with replica-averaged structural restraints generate structural ensembles according to the maximum entropy principle
Authors: Cavalli Andrea , Camilloni Carlo , Vendruscolo Michele
Published: March 2013

In order to characterise the dynamics of proteins, a well-established method is to incorporate experimental parameters as replica-averaged structural restraints into molecular dynamics simulations. Here, we justify this approach in the case of interproton distance information provided by nuclear Overhauser effects by showing that it generates ensembles of conformations according to the maximum entropy principle. These results indicate that the use of replica-averaged structural restraints in molecular dynamics simulations, given a force field and a set of experimental data, can provide an accurate approximation of the unknown Boltzmann distribution of a system.
Granular suspension avalanches. II. Plastic regime
Authors: Andreini Nicolas , Ancey Christophe , Epely-Chauvin Gaël
Published: March 2013

We present flume experiments showing plastic behavior for perfectly density-matched suspensions of non-Brownian particles within a Newtonian fluid. In contrast with most earlier experimental investigations (carried out using coaxial cylinder rheometers), we obtained our rheological information by studying thin films of suspension flowing down an inclined flume. Using particles with the same refractive index as the interstitial fluid made it possible to measure the velocity field far from the wall using a laser-optical system. At long times, a stick-slip regime occurred as soon as the fluid pressure dropped sufficiently for the particle pressure to become compressive. Our explanation was that the drop in fluid pressure combined with the surface tension caused the flow to come to rest by significantly increasing flow resistance. However, the reason why the fluid pressure diffused through the pores during the stick phases escaped our understanding of suspension rheology.
Scanning retarding field analyzer for plasma profile measurements in the boundary of the Alcator C-Mod tokamak
Authors: Brunner D. , LaBombard B. , Ochoukov R. , Whyte D.
Published: March 2013

A new Retarding Field Analyzer (RFA) head has been created for the outer-midplane scanning probe system on the Alcator C-Mod tokamak. The new probe head contains back-to-back retarding field analyzers aligned with the local magnetic field. One faces “upstream” into the field-aligned plasma flow and the other faces “downstream” away from the flow. The RFA was created primarily to benchmark ion temperature measurements of an ion sensitive probe; it may also be used to interrogate electrons. However, its construction is robust enough to be used to measure ion and electron temperatures up to the last-closed flux surface in C-Mod. A RFA probe of identical design has been attached to the side of a limiter to explore direct changes to the boundary plasma due to lower hybrid heating and current drive. Design of the high heat flux (>100 MW/m2) handling probe and initial results are presented.
Identification of As-vacancy complexes in Zn-diffused GaAs
Authors: Elsayed M. , Krause-Rehberg R. , Korff B. , Richter S. , Leipner H. S.
Published: March 2013

We have used positron annihilation spectroscopy to study the introduction of point defects in Zn-diffused semi-insulating GaAs. The diffusion was performed by annealing the samples for 2 h at 950 °C. The samples were etched in steps of 7 μm. Both Doppler broadening using slow positron beam and lifetime spectroscopy studies were performed after each etching step. Both techniques showed the existence of vacancy-type defects in a layer of about 45 μm. Secondary ion mass spectroscopy measurements illustrated the presence of Zn at high level in the sample almost up to the same depth. Vacancy-like defects as well as shallow positron traps were observed by lifetime measurements. We distinguish two kinds of defects: As vacancy belongs to defect complex, bound to most likely one Zn atom incorporated on Ga sublattice, and negative-ion-type positron traps. Zn acceptors explained the observation of shallow traps. The effect of Zn was evidenced by probing GaAs samples annealed under similar conditions but without Zn treatment. A defect-free bulk lifetime value is detected in this sample. Moreover, our positron annihilation spectroscopy measurements demonstrate that Zn diffusion in GaAs system is governed by kick-out mechanism.
Luminescence properties of defects in nanocrystalline ZnO
Authors: Smith Jeremy , Akbari-Sharbaf Arash , Ward Matthew J. , Murphy Michael W. , Fanchini Giovanni , Kong Sham Tsun
Published: March 2013

Formation of intrinsic acceptor defects in ZnO is rare due to the low formation energy of donors. Understanding this phenomenon is of interest for the fabrication of high quality light emitting diodes. Herein, we examine the temperature dependent formation of defects in nanocrystalline ZnO through a combination of X-ray excited optical luminescence (XEOL) together with X-ray absorption near edge structures (XANES) and electron spin resonance (ESR). Certain defects are shown to form under low temperature and are unstable above 700 °C. These defects have high g-values characteristic of acceptors and short spin-lattice relaxation times. XEOL measurements show that acceptor defects with a characteristic red luminescence are also formed under these conditions. Low g-value (donor) defects forming at temperatures >700 °C are shown to have spin-lattice relaxation characteristic of nonradiative recombination centers.
Analysis of the residual stress distribution in AlGaN/GaN high electron mobility transistors
Authors: Choi Sukwon , Heller Eric , Dorsey Donald , Vetury Ramakrishna , Graham Samuel
Published: March 2013

A comparative analysis of the residual stress distributions across the conductive channel of Ga-face AlGaN/GaN high electron mobility transistors (HEMTs) is presented. Stress was measured by means of micro-Raman spectroscopy and micro-photoluminescence (PL). Raman measurements probed the volume average of the stress through the GaN layer whereas the stress near the GaN surface (AlGaN/GaN heterointerface) was acquired via PL. By combining Raman, PL, and x-ray diffraction, a self-consistent method was developed to accurately determine the variation in magnitude of stress throughout the thickness of the GaN layer. Based on this framework, it is observed in AlGaN/GaN HEMTs that a depth variation in the GaN residual stress occurs near the gate and ohmic electrodes. At these regions, the stress near the AlGaN/GaN interface (or GaN surface) exhibits a tensile shift compared to the stress averaged through the entire thickness of GaN. Across the conductive channel (away from the metal pads), the bulk average stress and the stress near this interface remain nearly identical, showing little evidence of a vertical gradient. It is expected that the induced tensile strain at the drain side gate edge will have an impact on device reliability by contributing to the elastic energy built in the AlGaN barrier in addition to the inverse piezoelectric contribution at operating conditions, which may lead to formation of crystallographic defects.
Structural and electronic properties of perylene from first principles calculations
Authors: Fedorov I. A. , Zhuravlev Y. N. , Berveno V. P.
Published: March 2013

The electronic structure of crystalline perylene has been investigated within the framework of density functional theory including van der Waals interactions. The computations of the lattice parameters and cohesive energy have good agreement with experimental values. We have also calculated the binding distance and energy of perylene dimers, using different schemes, which include van der Waals interactions.
Vibrational study of HgGa2S4 under high pressure
Authors: Vilaplana R. , Robledillo M. , Gomis O. , Sans J. A. , Manjón F. J. , Pérez-González E. , Rodríguez-Hernández P. , Muñoz A. , Tiginyanu I. M. , Ursaki V. V.
Published: March 2013

In this work, we report on high-pressure Raman scattering measurements in mercury digallium sulfide (HgGa2S4) with defect chalcopyrite structure that have been complemented with lattice dynamics ab initio calculations. Our measurements evidence that this semiconductor exhibits a pressure-induced phase transition from the completely ordered defect chalcopyrite structure to a partially disordered defect stannite structure above 18 GPa which is prior to the transition to the completely disordered rocksalt phase above 23 GPa. Furthermore, a completely disordered zincblende phase is observed below 5 GPa after decreasing pressure from 25 GPa. The disordered zincblende phase undergoes a reversible pressure-induced phase transition to the disordered rocksalt phase above 18 GPa. The sequence of phase transitions here reported for HgGa2S4 evidence the existence of an intermediate phase with partial cation-vacancy disorder between the ordered defect chalcopyrite and the disordered rocksalt phases and the irreversibility of the pressure-induced order-disorder processes occurring in ordered-vacancy compounds. The pressure dependence of the Raman modes of all phases, except the Raman-inactive disordered rocksalt phase, have been measured and discussed.
Experimental and simulation study of pinning phenomena in superconductors with regular composite pinning arrays
Authors: Horng Lance , Cao Rong , Wu Tian-Chiuan , Yang Shirley , Wang Sheng-Hao , Wu Jong-Ching , Yang Tzuan-Je
Published: March 2013

Pinning phenomena were investigated in the superconducting Nb thin film with composite hexagonal arrays. The composite pinning arrays were consisting of two kinds of pinning sites with different pinning sizes. The smaller pinning defects were added to the center of every honeycomb pinning array to observe the configuration of the vortices, comparing to the periodic triangular and honeycomb arrays. The film with this kind of composite array can be regarded as a transition between the films with triangular array and honeycomb array. The critical current as a function of magnetic field for four samples were measured. Regular 150 Oe interval of the matching fields for the triangular arrays was found, while for the honeycomb arrays, the interstitial vortices were caged at the center of every honeycomb array, causing 50 Oe interval of the matching fields. For the samples with composite arrays, the 100 Oe or 50 Oe intervals of matching fields correspond to every larger or smaller pinning site capturing one more vortex, respectively. We found that the relative pinning strength of the large pinning sites is greater than that of the small ones. We conducted the simulations based on the time dependent Ginzburg-Landau theory to confirm that the interstitial vortices did exist in the honeycomb arrays which caused the various intervals between the matching fields.
Dependence of efficiency of thin-film CdS/CdTe solar cell on optical and recombination losses
Authors: Mohamed H. A.
Published: March 2013

Thin-film CdS/CdTe solar cells fabricated on glass substrates have been considered as one of the most promising candidates for large-scale applications in the field of photovoltaic energy conversion. The recorded experimental efficiency of these cells is about 16%–17% and the corresponding theoretical values are more than 28%. The main causes of efficiency loss are due to optical and recombination losses. Most of the theoretical literatures either study the effect of recombination or optical losses on the CdS/CdTe solar cell efficiency. The present work studies the effect of both the optical and recombination losses on the current density and hence the solar cell efficiency. Calculations of optical losses have been carried out based on the optical constants (refractive index and extinction coefficient) of materials used and the thickness of ITO and CdS layers. Calculation of recombination losses has been based on the values of width of space-charge region and the absorption coefficient for CdTe. It has been found that the reflection losses of the interfaces air-glass, glass-ITO, ITO-CdS, and CdS-CdTe decrease the short-circuit current (JSC) from 31.24 to 28.2 mA/cm2 (9%). The absorption losses in ITO and CdS layers decrease JSC to 22.2 (20%). The recombination losses decrease JSC to 19.7 mA/cm2 (8%). The optical and recombination losses yield efficiency of CdS/CdTe solar cells in the range of 12%–16% at thickness 100 nm of each layer of ITO and CdS. According to these results, there is a good agreement between experimental and theoretical studies and this is the real start to develop the solar cells efficiency in the future studies.
Physics basis of Multi-Mode anomalous transport module
Authors: Rafiq T. , Kritz A. H. , Weiland J. , Pankin A. Y. , Luo L.
Published: March 2013

The derivation of Multi-Mode anomalous transport module version 8.1 (MMM8.1) is presented. The MMM8.1 module is advanced, relative to MMM7.1, by the inclusion of peeling modes, dependence of turbulence correlation length on flow shear, electromagnetic effects in the toroidal momentum diffusivity, and the option to compute poloidal momentum diffusivity. The MMM8.1 model includes a model for ion temperature gradient, trapped electron, kinetic ballooning, peeling, collisionless and collision dominated magnetohydrodynamics modes as well as model for electron temperature gradient modes, and a model for drift resistive inertial ballooning modes. In the derivation of the MMM8.1 module, effects of collisions, fast ion and impurity dilution, non-circular flux surfaces, finite beta, and Shafranov shift are included. The MMM8.1 is used to compute thermal, particle, toroidal, and poloidal angular momentum transports. The fluid approach which underlies the derivation of MMM8.1 is expected to reliably predict, on an energy transport time scale, the evolution of temperature, density, and momentum profiles in plasma discharges for a wide range of plasma conditions.
Influence of ambient air on the flowing afterglow of an atmospheric pressure Ar/O2 radiofrequency plasma
Authors: Duluard C. Y. , Dufour T. , Hubert J. , Reniers F.
Published: March 2013

The influence of ambient air on the flowing afterglow of an atmospheric pressure Ar/O2 radiofrequency plasma has been investigated experimentally. Spatially resolved mass spectrometry and laser induced fluorescence on OH radicals were used to estimate the intrusion of air in between the plasma torch and the substrate as a function of the torch-to-substrate separation distance. No air is detected, within the limits of measurement uncertainties, for separation distances smaller than 5 mm. For larger distances, the effect of ambient air can no longer be neglected, and radial gradients in the concentrations of species appear. The Ar 4p population, determined through absolute optical emission spectroscopy, is seen to decrease with separation distance, whereas a rise in emission from the N2(C–B) system is measured. The observed decay in Ar 4p and N2(C) populations for separation distances greater than 9 mm is partly assigned to the increasing collisional quenching rate by N2 and O2 molecules from the entrained air. Absorption measurements also point to the formation of ozone at concentrations from 1014 to 1015 cm−3, depending both on the injected O2 flow rate and the torch-to-substrate separation distance.
Magnetic interactions and reversal mechanisms in Co nanowire and nanotube arrays
Authors: Proenca M. P. , Sousa C. T. , Escrig J. , Ventura J. , Vazquez M. , Araujo J. P.
Published: March 2013

Ordered hexagonal arrays of Co nanowires (NWs) and nanotubes (NTs), with diameters between 40 and 65 nm, were prepared by potentiostatic electrodeposition into suitably modified nanoporous alumina templates. The geometrical parameters of the NW/NT arrays were tuned by the pore etching process and deposition conditions. The magnetic interactions between NWs/NTs with different diameters were studied using first-order reversal curves (FORCs). From a quantitative analysis of the FORC measurements, we are able to obtain the profiles of the magnetic interactions and the coercive field distributions. In both NW and NT arrays, the magnetic interactions were found to increase with the diameter of the NWs/NTs, exhibiting higher values for NW arrays. A comparative study of the magnetization reversal processes was also performed by analyzing the angular dependence of the coercivity and correlating the experimental data with theoretical calculations based on a simple analytical model. The magnetization in the NW arrays is found to reverse by the nucleation and propagation of a transverse-like domain wall; on the other hand, for the NT arrays a non-monotonic behavior occurs above a diameter of ∼50 nm, revealing a transition between the vortex and transverse reversal modes.
Optimum experimental design for extended Gaussian disorder modeled organic semiconductor devices
Authors: Weiler C. K. F. , Körkel S.
Published: March 2013

We apply optimum experimental design (OED) to organic semiconductors modeled by the extended Gaussian disorder model (EGDM) which was developed by Pasveer et al. [Phys. Rev. Lett. 94, 206601 (2005)]. We present an extended Gummel method to decouple the corresponding system of equations and use automatic differentiation to get derivatives with the required accuracy for OED. We show in two examples, whose parameters are taken from Pasveer et al. [Phys. Rev. Lett. 94, 206601 (2005)] and Mensfoort and Coehoorn [Phys. Rev. B 78, 085207 (2008)] that the linearized confidence regions of the parameters can be reduced significantly by applying OED resulting in new experiments with a different setup.
Diameter dependence of electron mobility in InGaAs nanowires
Authors: Hou Jared J. , Wang Fengyun , Han Ning , Zhu Haoshen , Fok KitWa , Lam WaiChak , Yip SenPo , Hung TakFu , Lee Joshua E.-Y. , Ho Johnny C.
Published: March 2013

In this work, we present the diameter dependent electron mobility study of InGaAs nanowires (NWs) grown by gold-catalyzed vapor transport method. These single crystalline nanowires have an In-rich stoichiometry (i.e., In0.7Ga0.3As) with dispersed diameters from 15 to 55 nm. The current-voltage behaviors of fabricated nanowire field-effect transistors reveal that the aggressive scaling of nanowire diameter will induce a degradation of electron mobility, while low-temperature measurements further decouple the effects of surface/interface traps and phonon scattering, highlighting the impact of surface roughness scattering on the electron mobility. This work suggests a careful design consideration of nanowire dimension is required for achieving the optimal device performances.
Laser-induced condensation by ultrashort laser pulses at 248 nm
Authors: Joly P. , Petrarca M. , Vogel A. , Pohl T. , Nagy T. , Jusforgues Q. , Simon P. , Kasparian J. , Weber K. , Wolf J.-P.
Published: March 2013

We compare laser-induced condensation by UV laser pulses of femtosecond, sub-picosecond, and nanosecond duration between each other, as well as with respect to near-infrared (NIR) (800 nm) ultrashort laser pulses. Particle nucleation by UV pulses is so efficient that their growth beyond several hundreds of nm is limited by the local concentration of water vapour molecules. Furthermore, we evidence a dual mechanism: While condensation induced by ultrashort UV pulses rely on nitrogen photo-oxidative chemistry like in the NIR, nanosecond laser-induced condensation occurs without NO2 production, evidencing the domination of a mechanism distinct from that previously identified in the femtosecond regime.
Computational study of graphene-based vertical field effect transistor
Authors: Chen Wenchao , Rinzler Andrew , Guo Jing
Published: March 2013

Poisson and drift-diffusion equations are solved in a three-dimensional device structure to simulate graphene-based vertical field effect transistors (GVFETs). Operation mechanisms of the GVFET with and without punched holes in the graphene source contact are presented and compared. The graphene-channel Schottky barrier can be modulated by gate electric field due to graphene's low density of states. For the graphene contact with punched holes, the contact barrier thinning and lowering around punched hole edge allow orders of magnitude higher tunneling current compared to the region away from the punched hole edge, which is responsible for significant performance improvement as already verified by experiments. Small hole size is preferred due to less electrostatic screening from channel inversion layer, which gives large electric field around the punched hole edge, thus, leading to a thinner and lower barrier. Bilayer and trilayer graphenes as the source contact degrade the performance improvement because stronger electrostatic screening leads to smaller contact barrier lowering and thinning. High punched hole area percentage improves current performance by allowing more gate electric field to modulate the graphene-channel barrier. Low effective mass channel material gives better on-off current ratio.
Influence of surface optical phonons on exciton binding energies of a quasi-one-dimensional wurtzite GaN-based nanowire: Quantum size effect
Authors: Zhang Li , Shi J. J.
Published: March 2013

Based on the surface optical phonon states and their electron-phonon coupling functions obtained recently, the surface polaronic exciton states in a quasi-one-dimensional (Q1D) wurtzite nanowire (NW) are investigated by using the variational method and Lee-Low-Pines transform. In order to reflect the different confined features of Q1D wurtzite NWs in axial and radial directions and anisotropy of wurtzite nitride crystals, a two-parameter variational approach is proposed and applied to describe the polaronic exciton states in the NWs. Numerical calculations are performed for GaN NWs. The quantum size effects on the binding energies of polaronic exciton and the contributions of surface phonon modes are analyzed in detail. Our results show that the binding energy of polaronic exciton and surface phonon contribution in the wurtzite GaN NWs reaches 190 and 95 meV, respectively, which are one or two orders of magnitude larger than those in cubic GaAs-based quantum wells and NWs with the same radius. This is mainly ascribed to the strong electron-phonon interaction, the large effective masses of carriers and relatively small dielectric constants in GaN material. The numerical results also show that the two-parameter variational approach is reasonable and necessary for the description of polaronic exciton states in Q1D wurtzite GaN NWs. Moreover, the behaviors of the two variational parameters for the polaronic exciton states are quite different from those for the bound polaron states in Q1D wurtzite NWs.
Simple and low cost incremental conductance maximum power point tracking using buck-boost converter
Authors: Kok Soon Tey , Mekhilef Saad , Safari Azadeh
Published: March 2013

Cost of the photovoltaic (PV) systems and their peripherals has been always an issue when considering them for various applications. Using an inexpensive maximum power point tracking (MPPT) system is a simple but efficient solution to reduce the cost of the PV systems and increase the public acceptance. This paper presents the simulation and hardware implementation of incremental conductance algorithm using buck-boost converter and PIC18F4520 controller. Design and simulation of the proposed system are presented using matlab and simulink tools. The proposed system is also tested on KC85T PV module for constant and changing weather conditions. Experimental results indicated the capability and functionality of the proposed system on tracking maximum power point with the voltage ripple ratio = 0.006, which is near to the ideal mathematically calculated assessment. The proposed MPPT system helps to reduce the complexity and cost of the PV systems and also ensures the largest operating region of the PV module.
Electrical poling below coercive field for large piezoelectricity
Authors: Guo Hanzheng , Ma Cheng , Liu Xiaoming , Tan Xiaoli
Published: March 2013

Isotropic polycrystalline ferroelectric ceramics have to be electrically poled to develop a net macroscopic polarization and hence piezoelectricity. It is well accepted that a sufficient poling can only be realized under an electric field that is much higher than the coercive field. In this study, we observed in (Bi1/2Na1/2)TiO3-BaTiO3 ceramics that large piezoelectricity can develop at poling fields far below the measured coercive field. Using in situ transmission electron microscopy, such an unusual behavior, is interpreted with the polarization alignment of polar nanodomains in the non-ergodic relaxor phase.