bluuebunny/crossref_metadata_2025 huggingface dataset:
Relationship between erythemal irradiance and total solar irradiance in South‐Western Spain
Authors: Antón M. , Serrano A. , Cancillo M. L. , García J. A.
Published: July 2008

In the last years, many studies have focused on the study of the ultraviolet radiation or the total solar radiation in Southwestern Europe. This area presents a particular interest due to the intense solar radiation which reaches the Earth's surface at these latitudes. Despite of this high interest, the influence of the atmospheric factors on the relationship between both variables is not well documented for that region. In this work, the ultraviolet erythemal irradiance (UVER) and the total solar irradiance (310–2800 nm) at three locations in Southwestern Spain were measured between January 2001 and December 2005. The main goal of this paper is to analyze the relationship between ultraviolet and total solar irradiance as described by their ratio. This ratio represents the percentage of UV erythemal irradiance over the total solar irradiance at Earth's surface. The results show that this ratio varies in time. It shows a day‐to‐day and seasonal variability mainly due to changes in sky conditions (regarding cloudiness and aerosols) and in the slant column of ozone, respectively. Differences in ozone may cause a relative increase in the ratio higher than 100% between winter and summer. This fact stresses the importance of reporting information about the UV radiation at ground level to the people mainly during summer. Clouds play also an important role, since highly different conditions between consecutive days are responsible for up to 60% of the ratio variability. The results show that cloudiness attenuates more the total solar radiation than the UV solar radiation. Additionally, high changes in aerosol load can produce a short‐term decrease in the ratio of 13–15%, attenuating differently UVER than total solar irradiance.
Climatology of tropopause folds over a European Arctic station (Esrange)
Authors: Rao T. Narayana , Arvelius J. , Kirkwood S.
Published: July 2008

Eleven years (September 1996 to August 2007) of continuous measurements of three‐dimensional wind and backscattered signal strength observed with Esrange Radar (ESRAD) have been utilized to study the annual and interannual variation of tropopause folds over an Arctic station. Two typical tropopause fold events (one is associated with a streamer type of system and the other with a cutoff low) are selected and are characterized with the help of synoptic charts and potential vorticity (PV) analysis. Typical characteristics of radar parameters during the passage of folds are identified, such as the sudden rise in the tropopause altitude, high‐reflectivity layer sloping downward from the tropopause beneath the jet stream, and intensification of the jet stream. These characteristics are utilized to discern the tropopause fold in the radar data. The climatology of tropopause folds exhibits a pronounced annual cycle with a large number of folds in winter and fewer in summer. The annual cycle of folds is more or less similar in all the years; however, significant interannual variation is observed with winter periods exhibiting maximum interannual variability. The climatology of folds and its annual cycle are compared and contrasted with similar climatological studies available in the literature. The differences in the climatologies are discussed in light of differences in the algorithms and the spatial variability of fold frequency.
Methane and nitrous oxide emissions from subtropical rice agriculture in China
Authors: Khalil M. A. K. , Shearer M. J. , Rasmussen R. A. , Xu Li , Liu Jin‐Luan
Published: July 2008

Emissions of methane and nitrous oxide, two significant greenhouse gases, were measured from rice fields at Qingyuan in Guangdong Province, China. The region has a subtropical climate which allows two crops of rice to be grown every year. The prevailing agricultural practices create a complex interaction between factors known to have a major effect on methane and nitrous oxide emissions from rice fields, namely, intermittent flooding and use of organic fertilizers. In this region, the farmers depend on nitrogen fertilizers and, at least in recent years, have only intermittently flooded their fields during the growing seasons. These factors tend to reduce methane emissions. But the rice straw and crop residues from the first crop of the year are plowed into the fields, providing a large addition of organic material under hot weather conditions favorable to quick decomposition during the second crop period. This, and the addition of farmyard manure, increases emissions of methane emissions from these fields. The results of the present study show that the effect of these competing factors and their timing lead to an average rate of emissions of 5 ± 2 and 6 ± 2 mg/m2/h from the first crops for the 2 years when measurements were taken (2003 and 2004), and 12 ± 8 and 13 ± 8 mg/m2/h from the second crop. Further, production measurements showed that during the 2 years of these experiments, the average production rates were about 27 mg/m2/h for the first crop and 22–34 mg/m2/h for the second crop, resulting in estimated oxidation rates of about 80% for the first crop and 50–60% for the second crop. The higher fluxes in the second crop therefore appear to be caused more by reduced oxidation than higher production. Nitrous oxide emissions, when they were detected, usually occurred within a few days after the application of nitrogen fertilizers. The seasonally averaged emissions were between 0.01 and 0.02 mg/m2/h except in the first year when large emissions over one short period pushed the average upward.
Simulated climate near steep topography: Sensitivity to numerical methods for atmospheric transport
Authors: Bala G. , Rood R. B. , Bader D. , Mirin A. , Ivanova D. , Drui Cedric
Published: July 2008

We present the sensitivity of the simulated climate near steep topographical regions when the numerical method for atmospheric transport in the Community Climate System Model (CCSM3) is changed from spectral to a finite volume (FV) transport. Our analysis of the circulation and precipitation shows significant local improvement in three aspects: 1) The Gibbs oscillations present in the cloudiness and shortwave radiative forcing fields in the spectral simulation are absent in the FV simulation. 2) The along‐shore component of wind stress in the western coastal regions of North and South America increases in the FV simulation. This tends to reduce the persistent biases in sea surface temperature through enhanced oceanic upwelling. 3) The FV simulation shows improvement in the wet‐dry contrast of orographically forced precipitation. These local improvements have impact on continental and larger scales and are critical to the confident use of information from climate predictions in adaptation to climate change.
Conditional knockout of nucleolin in DT40 cells reveals the functional redundancy of its RNA‐binding domains
Authors: Storck Sébastien , Thiry Marc , Bouvet Philippe
Published: July 2008

Background information. Nucleolin is a major nucleolar protein which is highly expressed in rapidly dividing cells and cancer cell lines. This protein is claimed to be multifunctional and could play a role in rRNA (ribosomal RNA) synthesis, as well as in cell division or response to cellular stresses. Therefore, how nucleolin influences cell proliferation remained elusive so far.

Results. We have generated conditional nucleolin‐knockout cells using the chicken B lymphocyte cell line DT40. Our results indicate that nucleolin is absolutely required for the proliferation and for the survival of these cells. Depletion of nucleolin drastically inhibits rDNA (ribosomal DNA) transcription while only slightly affecting pre‐rRNA processing. This inhibition is accompanied by modifications of the shape and the structure of the nucleolus. The analysis of mutants of nucleolin, which lack two or three RNA‐binding domains, shows that these domains harbour redundant functions and that nucleolin's roles in transcription, rRNA maturation and nucleolar shape can be partially uncoupled.

Conclusions. The function of nucleolin in ribosomal synthesis could account for its effect on cell division and survival, but this vital role does not seem to be linked to sequence‐specific RNA binding.
Variance reduction in computations of neoclassical transport in stellarators using a δf method
Authors: Allmaier K. , Kasilov S. V. , Kernbichler W. , Leitold G. O.
Published: July 2008

An improved δf Monte Carlo method for the computation of neoclassical transport coefficients in stellarators is presented. Compared to the standard δf method without filtering, the computing time needed for the same statistical error decreases by a factor proportional to the mean free path to the power 3∕2.
Electromechanical response of 1-3 piezoelectric composites with hollow fibers
Authors: Marcheselli C. , Venkatesh T. A.
Published: July 2008

A finite element model is developed to characterize the complete electromechanical response of piezoelectric composite materials with hollow fibers and to obtain a quantitative assessment of the relative effects of the introduction of porosity and a piezoelectric second phase in a piezoelectric matrix material. Fifteen characteristic composites are identified for each of the two model ceramic-based and polymer-based matrix systems (i.e., barium titanate and polyvinylidine difluoride) by systematically varying the volume fractions of porosity and the second phase (i.e., lead zirconate titanate). It is demonstrated that the fundamental properties and the piezoelectric figures of merit of the piezoelectric composites generally exhibit greater sensitivity to the presence of porosity in the electroelastically stiffer ceramic-based systems while the piezoelectrically active second phase has a dominant influence in the electroelastically compliant polymer-based systems.
Dynamic light scattering in turbid nonergodic media
Authors: Medebach Martin , Freiberger Norbert , Glatter Otto
Published: July 2008

We here present a new device based on dynamic light scattering (DLS) for measuring kinetics in turbid and nonergodic systems. This flat cell light scattering instrument has been developed in our laboratory and is based on an original flat cell instrument employing cells of varying thickness in order to measure the static structure and dynamics of a system. The smallest cell thickness is 10μm. To this original instrument, we have integrated the three-dimensional (3D)-DLS technology as well as the echo method, and in comparison with other 3D-DLS instruments, ours show the best performance; the maximum intercept was 0.6 as opposed to 0.15 for regular 3D-DLS devices (recently we reached β=0.75). This was made possible by using crossed polarization filters for the two laser beams, thereby allowing the scattered light from both laser beams to be decoupled and the intercept to no longer be limited at the theoretical value of 0.25. The maximum weight fraction of the sample that is measurable with such a setup is more than ten times higher than with a standard 3D-DLS setup or with the flat cell instrument without the 3D technology. Consequently, with the 3D-DLS flat cell instrument presented here, it truly becomes possible to investigate turbid systems. Moreover, the echo method was integrated to enable measurements of nonergodic systems. Here, a new mechanical design of the echo-DLS component was necessary due to the different geometries of the flat cell in comparison with that of a standard cylindrical cell. The performance of our echo device was compared to that of our multispeckle instrument, and the results were in good agreement for correlation times up to 30000s and more. The main limitation of this instrument in its current version is the maximum scattering angle of about 50° (or 30° if echo is used).
Integration of amyloid nanowires in organic solar cells
Authors: Barrau S. , Zhang F. , Herland A. , Mammo W. , Andersson M. R. , Inganäs O.
Published: July 2008

Amyloid nanowires were incorporated in organic photovoltaic devices in order to enhance the transport properties. Amyloid fibrils act as a template for donor-acceptor materials. The current-voltage characteristics under illumination and in the dark display a maximum for the fill factor and the space charge limit current, respectively, at an amyloid nanowire-donor-acceptor mass ratio of 0.014:1:1, associated to a better charge transport in the donor-acceptor domains. The absorption experiments display a redshift associated to a more planar polymer backbone with increasing concentration of amyloid fibrils. Amyloid nanowires present a significant effect on the donor-acceptor materials organization.
Crystallization and shear modulus of a forming biopolymer film determined by in situ x-ray diffraction and ultrasound reflection methods
Authors: Peura Marko , Karppinen Timo , Soovre Annemai , Salmi Ari , Tenkanen Maija , Hæggström Edward , Serimaa Ritva
Published: July 2008

The structure and the rigidity of a forming biopolymer film were determined using concurrent x-ray diffraction and ultrasonic reflection measurements. Film formation of a xylan solution (de-ionized water, 10g∕l xylan, 4g∕l glycerol) was studied during water evaporation at 24(±2)°C, 37(±5)%RH. X-ray diffraction (XRD) data showed the crystallization and ultrasonic data the increase of the shear modulus (G) during water evaporation. Xylan crystallized into small xylan dihydrate crystallites, the number of which increased as water evaporated. Crystallization began earlier than the increase in G during film formation. The increase in G also continued after the crystallites were fully formed, indicating still ongoing structural changes in the amorphous parts. The maximum value of G was 0.1–0.5GPa. XRD measurements performed ex situ showed a crystallinity of 16%–19% (±5%) and a fairly isotropic crystallite orientation in the surface plane of the films.
An atomistic study of nanosprings
Authors: Chang I-Ling , Yeh Min-Shao
Published: July 2008

Molecular statics method incorporating minimum energy concept was employed to study the one-dimensional copper nanospring with faced-center-cubic crystal structure. Various geometric sizes (wire diameter, radius, pitch), numbers of turns and crystal orientations of nanosprings were systematically modeled to investigate the size dependence of elastic properties. It was observed that as the wire diameter increases and the radius and number of turns decrease, the nanospring stiffness would increase irrespective of the crystal orientations. Moreover, the elastic constants of nanosprings would become larger while the pitches become smaller for almost all the crystal orientations. Also the simulation results were compared to the predictions based on continuum theory in order to clarify whether the classical theory could apply to nanosprings.
Structural, mechanical, and electronic properties of ultrathin ZnO nanowires
Authors: Wang Baolin , Zhao Jijun , Jia Jianming , Shi Daning , Wan Jianguo , Wang Guanghou
Published: July 2008

We report a structural transformation between the regular wurtzite and the unbuckled wurtzite (hexagonal) structure for ultrathin single-crystalline [0001] ZnO nanowires under uniaxial elongation and compression. Our density functional calculations show that hexagonal structure corresponds to a distinct minimum on the transformation path. Young’s moduli of the ZnO nanowires with the hexagonal structures are larger than those with the wurtzite structures at the same size. Within the nanowire size range considered, Young’s moduli of the ZnO nanowires decrease with increasing wire diameter. The electronic properties of these two types of ZnO nanowires exhibit distinctly different behaviors.
Formation of self-affine nanostructures on ZnO surfaces by swift heavy ions
Authors: Agarwal D. C. , Chauhan R. S. , Avasthi D. K. , Khan S. A. , Kabiraj D. , Sulania I.
Published: July 2008

The topography evolution using the high energy heavy ion irradiation revealed the fact that ion bombardment produces self-affine nanostructures, creating peculiar surface morphologies and regular structures on the surface of the ZnO thin film at certain fluences. The self-affine nanopatterns produced on the surface of ZnO thin film under swift heavy ion irradiation are different types of nanostructures such as nanodimensional grains aligned like a linear array. Scanning probe microscopy is used for investigating the ZnO surfaces and UV-visible spectroscopy for studying the effect of surface modification on optical properties. The two-dimensional power spectral density of the irradiated ZnO thin films have been evaluated for each image to extract the value of growth factor (β) and roughness exponent (α). The exponent n increases from 2.2 to 4.0 up to a critical value of fluence and beyond which it decreases. These values suggest that ion assisted∕induced diffusion process plays a crucial role in the evolution of self-affine nanostructures on ZnO surface.
The microscopic structure of an exactly solvable model binary solution that exhibits two closed loops in the phase diagram
Authors: Lungu Radu P. , Huckaby Dale A.
Published: July 2008

An exactly solvable lattice model describing a binary solution is considered where rodlike molecules of types AA and BB cover the links of a honeycomb lattice, the neighboring molecular ends having three-body and orientation-dependent bonding interactions. At phase coexistence of AA-rich and BB-rich phases, the average fraction of each type of triangle of neighboring molecular ends is calculated exactly. The fractions of the different types of triangles are then used to deduce the local microscopic structure of the coexisting phases for a case of the model that contains two closed loops in the phase diagram.
A hybrid recursion method to robustly ensure convergence efficiencies in the simulated scaling based free energy simulations
Authors: Zheng Lianqing , Carbone Irina O. , Lugovskoy Alexey , Berg Bernd A. , Yang Wei
Published: July 2008

Recently, we developed an efficient free energy simulation technique, the simulated scaling (SS) method [H. Li et al., J. Chem. Phys. 126, 024106 (2007)], in the framework of generalized ensemble simulations. In the SS simulations, random walks in the scaling parameter space are realized so that both phase space overlap sampling and conformational space sampling can be simultaneously enhanced. To flatten the distribution in the scaling parameter space, in the original SS implementation, the Wang–Landau recursion was employed due to its well-known recursion capability. In the Wang–Landau recursion based SS free energy simulation scheme, at the early stage, recursion efficiencies are high and free energy regions are quickly located, although at this stage, the errors of estimated free energy values are large; at the later stage, the errors of estimated free energy values become smaller, however, recursions become increasingly slow and free energy refinements require very long simulation time. In order to robustly resolve this efficiency problem during free energy refinements, a hybrid recursion strategy is presented in this paper. Specifically, we let the Wang–Landau update method take care of the early stage recursion: the location of target free energy regions, and let the adaptive reweighting method take care of the late stage recursion: the refinements of free energy values. As comparably studied in the model systems, among three possible recursion procedures, the adaptive reweighting recursion approach is the least favorable one because of its low recursion efficiency during free energy region locations; and compared to the original Wang–Landau recursion approach, the proposed hybrid recursion technique can be more robust to guarantee free energy simulation efficiencies.
Modeling arrested cluster growth in quenched nanoparticle solutions
Authors: Podariu I. , Yan Hao , Sorensen C. M. , Chakrabarti A.
Published: July 2008

We have carried out numerical simulations on a model of phase separation in a binary mixture in the presence of a third, surface-active component. Based on this model we study phase separation in a quenched solution of gold nanoparticles with dodecane thiol ligands in a mixed solvent of butanone and toluene. In our model gold nanoparticles–butanone correspond to the generic binary mixture, while toluene is modeled as a surface-active third component. Our results show that the surface-active component acts as an inhibitor for the phase separation process but its effect decreases with the increase in quench temperature. Simulation results are consistent with two major findings in recent experiments: (a) growth of gold nanoparticle clusters gets arrested at late times and (b) the saturation value of the cluster size is smaller at lower temperatures of quench.
Fundamental measure density functional theory studies on the freezing of binary hard-sphere and Lennard-Jones mixtures
Authors: Warshavsky Vadim B. , Song Xueyu
Published: July 2008

Free energies and correlation functions of liquid and solid hard-sphere (HS) mixtures are calculated using the fundamental measure density functional theory. Using the thermodynamic perturbation theory the free energies of solid and liquid Lennard-Jones (LJ) mixtures are obtained from correlation functions of HS systems within a single theoretical approach. The resulting azeotrope- and spindle-type solid-liquid phase diagrams of HS and LJ binary mixtures are in good agreement with the corresponding ones from computer simulations.
On the minima and convexity of Epstein zeta function
Authors: Lim S. C. , Teo L. P.
Published: July 2008

Let Zn(s;a1,…,an) be the Epstein zeta function defined as the meromorphic continuation of the function ∑k∊Zn\{0}(∑i=1n[aiki]2)−s,  Re s>n/2 to the complex plane. We show that for fixed s≠n/2, the function Zn(s;a1,…,an) as a function of (a1,…,an)∊(R+)n with fixed ∏i=1nai has a unique minimum at the point a1=⋯=an. When ∑i=1nci is fixed, the function (c1,…,cn)↦Zn(s;ec1,…,ecn) can be shown to be a convex function of any (n−1) of the variables {c1,…,cn}. These results are then applied to the study of the sign of Zn(s;a1,…,an) when s is in the critical range (0,n/2). It is shown that when 1≤n≤9, Zn(s;a1,…,an) as a function of (a1,…,an)∊(R+)n can be both positive and negative for every s∊(0,n/2). When n≥10, there are some open subsets In,+ of s∊(0,n/2), where Zn(s;a1,…,an) is positive for all (a1,…,an)∊(R+)n. By regarding Zn(s;a1,…,an) as a function of s, we find that when n≥10, the generalized Riemann hypothesis is false for all (a1,…,an).
Length dependent behavior of a carbon nanotube interacting at liquid-air interface
Authors: de Asis Edward D. , Li You , Ohta Riichiro , Austin Alex , Leung Joseph , Nguyen Cattien V.
Published: July 2008

We report the irreversible structural failure of individual multiwalled carbon nanotube (MWNT) tips after water submersion. We used 11 individual MWNTs with varying geometries and showed length dependent MWNT failure with scanning electron microscopy. Shorter MWNTs are more likely to survive penetration of the water-air interface. We observed the bending of MWNT probes on the water surface using optical microscopy. Surface tension force acting on MWNTs at the water-air interface was calculated. Compared to shorter MWNTs, the calculations suggest that longer MWNTs exert a smaller bending restoring force with respect to surface tension force, leading to MWNT bending until failure.
Change in the interfacial reaction of Hf-silicate film as a function of thickness and stoichiometry
Authors: Cho M.-H. , Kim C. Y. , Moon K. , Chung K. B. , Yim C. J. , Ko D.-H. , Sohn H. C. , Jeon Hyeongtag
Published: July 2008

Medium energy ion scattering and high-resolution transmission electron microscopy are used to investigate the depth of the interfacial reaction of Hf-silicate film. The interfacial reaction is critically affected by the film thickness and the mole fraction of HfO2 in silicate film. The interfacial compressive strain generated at the surface of the Si substrate is dependent on the film thickness during the postannealing process in film with a thickness of ∼4nm. Finally, the phase separation phenomenon demonstrates critically different behaviors at different film thicknesses and stoichiometries because the diffusion of Si from interface to surface is dependent on these factors. Moreover, the oxidation by oxygen impurity in the inert ambient causes SiO2 top formation.
Theoretical investigation of electron transport modulation through benzenedithiol by substituent groups
Authors: Smeu Manuel , Wolkow Robert A. , DiLabio Gino A.
Published: July 2008

Density functional theory combined with nonequilibrium Green’s function techniques was used to model the conduction through disubstituted benzenedithiol molecules bonded to leads composed of 3×3, 5×5 gold and 3×3 aluminum. For the disubstituted 3×3 Au-benzenedithiol-Au systems, the small lead cross section results in a region of nearly zero transmission from −0.4to−0.2eV, relative to EF, due to the absence of lead states. This feature results in negative differential resistance in the current-voltage curves and also causes the main peaks in the transmission spectra, which are dominated by the highest occupied molecular orbitals, to be centered near EF. The zero-bias transmissions for the disubstituted benzenedithiol, as well as currents at applied biases, correlate very well with the Hammett parameter σp, a quantity that relates the electron donating or withdrawing strength of a substituent. Calculations on disubstituted benzenedithiol connected to 5×5 Au leads produced transmission spectra that showed no gaps over the energy range considered and no negative differential resistance. The transmission in these cases also predominately involves the highest occupied molecular orbitals, and electron donating and withdrawing groups are able to increase and decrease current, respectively. However, there is no strong correlation between current and σp for this system. This suggests that the correlation observed in the 3×3 Au systems arises from the abrupt cutoff of the main transmission peaks near EF. The disubstituted 3×3 Al-benzenedithiol-Al systems displayed markedly different behavior from the Au analogs. Electron donating groups and H benzenedithiol-substituted systems display almost no transmission over the energy range considered. However, electron withdrawing group disubstituted benzenedithiol systems had significant peaks in the transmission spectra near EF, which are associated with the lowest-energy, unoccupied π-type molecular orbitals. Higher currents are calculated for cases where the substituents have π-type orbitals that are conjugated with the ring moiety of benzenedithiol. In all cases, the current through the 3×3 Al-benzenedithiol-Al systems is about a factor of 2 less than that through the analogous Au systems. These simulations reveal that the electrical conductance behavior through nanosystems of the type investigated in this work depends on the nature of the molecule as well as the size and composition of the leads to which it is connected. The results suggest that rational design of nanoelectronic systems might be possible under certain conditions but that structure-function relationships cannot be transferred from one system to another.
High cluster formation tendency in Co implanted ZnO
Authors: Potzger K. , Kuepper K. , Xu Q. , Zhou S. , Schmidt H. , Helm M. , Fassbender J.
Published: July 2008

ZnO(0001) single crystals have been implant doped with a maximum of 5 at. % of Co at low temperatures. While as-implanted crystals do not show ferromagnetic properties, postimplantation annealing leads to the transformation of the implanted Co ions into small metallic clusters giving rise to a pronounced hysteresis upon magnetization reversal. The dispersed Co ions are in 2+ oxidation state. Positive magnetoresistance could be observed at low temperatures.
Epitaxial La0.5Sr0.5CoO3 thin films: Structure, magnetism, and transport
Authors: Torija M. A. , Sharma M. , Fitzsimmons M. R. , Varela M. , Leighton C.
Published: July 2008

La 1 − x Sr x CoO 3 has received considerable attention in bulk form. This is due to interest in the fundamental magnetic properties (spin-state transitions and magnetic phase separation) as well as potential applications in ferroelectric memory and solid-oxide fuel cells. The structure and properties in thin film form are not well understood, and the influence of dimensional confinement on effects such as magnetic phase separation is unknown. Here, we report a comprehensive investigation of structure, magnetism, and transport in strained epitaxial La0.5Sr0.5CoO3 (001) films deposited on SrTiO3 (001) substrates by reactive dc magnetron sputtering. The crystalline quality, phase purity, strain state, oxygen stoichiometry, morphology, and magnetic and electronic properties of the epilayers are all probed and are found to be particularly sensitive to the total sputtering gas pressure and the ratio of reactive to inert gas (PO2/PAr). The various structure-property relationships are discussed in detail, particularly with respect to the degree of oxygenation and oxygen-induced resputtering. The films are strained and tetragonally distorted due to the 1.9% lattice mismatch with SrTiO3. Significant strain relaxation occurs at thicknesses around 200 Å, resulting in a crossover from two-dimensional-like to three-dimensional growth. Polarized neutron reflectometry was combined with x-ray reflectometry to obtain chemical and magnetic depth profiles, which are compared with cross-sectional scanning transmission electron microscopy. The results indicate a thin (∼10 Å) layer at the film/substrate interface with significantly different structural properties to the bulk of the film, as well as a strongly graded magnetic and chemical profile at the film surface due to the significant roughness. The Curie temperature was found to decrease very slowly as the thickness is reduced down to ∼50 Å, at which point a rapid decrease occurs, almost coincident with a sharp decrease in saturation magnetization. At this point, the temperature dependence of the resistivity shows a crossover from metallic to insulating, accompanied by dramatic changes in the magnetoresistance. The magnetoresistance has a negative contribution peaking around the Curie point (similar to that seen in bulk), a second negative contribution occurring at low temperature (only for the thinnest samples), as well as a large anisotropic magnetoresistance, which vanishes at the Curie point. Remarkably, the low temperature contribution in the thinnest x=0.5 films bears a striking resemblance to that seen in the insulating phase (x<0.17) in bulk, suggesting the formation of a nonmetallic phase at low thickness that is similar to the low doping bulk phase, i.e., magnetic phase separation near the interface with SrTiO3.
Analytic derivatives for the Cholesky representation of the two-electron integrals
Authors: Aquilante Francesco , Lindh Roland , Pedersen Thomas Bondo
Published: July 2008

We propose a formalism for calculating analytic derivatives of the electronic energy with respect to nuclear coordinates using Cholesky decomposition of the two-electron integrals. The formalism is derived by exploiting the equivalence of Cholesky decomposition and density fitting when a suitable auxiliary basis set is used for expanding atomic orbital product densities in the latter. An implementation of gradients at the nonhybrid density functional theory level is presented, and sample calculations demonstrate that the errors in equilibrium geometries due to the Cholesky representation of the integrals can be controlled by adjusting the decomposition threshold.
Square-shaped metal screens in the infrared to terahertz spectral region: Resonance frequency, band gap, and bandpass filter characteristics
Authors: Sternberg O. , Stewart K. P. , Hor Y. , Bandyopadhyay A. , Federici J. F. , Bornefeld M. , Mathis Y.-L. , Sliwinski D. , Möller K. D. , Grebel H.
Published: July 2008

Resonance frequency of freestanding, square-shaped thick metal screens have been studied here in the wavelength range of infrared (IR) to mm (20 to 0.2 THz). It was found that their peak transmission has a linear relationship to the screen’s pitch. An experimental spectral feature, unaccounted for in typical simulations with plane parallel incident beams, was observed in the transmittance envelope for measurements in focused beams. In the past, this spectral feature was assigned to Wood’s anomaly. Yet, unlike the latter, the observed spectral feature appears here in the long wavelength regime as well. We investigated this phenomenon for a large frequency range and assigned the spectral feature to the formation of a photonic band gap at oblique incidence. Many IR Fourier transform spectrometers use a noncollimated incident beam and such spectral features will appear whenever the local state of polarization includes components which are parallel to the plane of incidence.
Preparation and characterization of Al2O3/Y3Al5O12/ZrO2 ternary hypoeutecticin situcomposites by laser rapid solidification
Authors: Su Haijun , Zhang Jun , Tian Junjie , Liu Lin , Fu Hengzhi
Published: July 2008

The directionally solidified oxide eutectic in situ composite is one of the most promising high-temperature structural materials in oxidizing environments. Pore-free rods and plates of ternary Al2O3/Y3Al5O12 (YAG)/ZrO2 hypoeutectic in situ composites with hypoeutectic composition (71 mol % Al2O3, 17 mol % Y2O3, and 12 mol % ZrO2) are prepared rapidly by the laser zone remelting technique. The hypoeutectic growth and microstructure characteristic of grown crystals are studied using the scanning electron microscopy, x-ray diffraction, and energy dispersive spectroscopy, aiming to understand well the rapid solidification behavior of the ternary oxide system of Al2O3–Y2O3–ZrO2. The rapidly solidified Al2O3/YAG/ZrO2 hypoeutectic shows a refined, interpenetrating, and irregular lamellar structure with a reticular distribution of coarse Al2O3 and yttrium aluminum garnet (YAG) and smaller ZrO2 phases. The fine ZrO2 phases are partially embedded at the Al2O3/YAG interfaces. Moreover, the typical dendrite microstructure similar to the metallic solidification is also observed. The formation of two kinds of microstructures is mainly attributed to the rapid growth during the laser zone remelting and constitutional supercooling. The Al2O3 and YAG phases grow in a typical faceted manner, and the ZrO2 phase presents a weak-faceted growth. The size of the interphase spacing obtained decreases rapidly in increasing the laser scanning rate. The fundamental investigations on the irregular growth mechanisms of the hypoeutectic are suggestive to the comprehension of the complex solidification behavior of the ternary oxide systems.
Electron paramagnetic resonance studies on multiferroic DyMnO3 and Dy0.5Sr0.5MnO3
Authors: Harikrishnan S. , Naveen Kumar C. M. , Rao S. S. , Bhat H. L. , Bhat S. V. , Elizabeth Suja
Published: July 2008

Electron paramagnetic resonance (EPR) studies and magnetic measurements were carried out on single crystals of multiferroic DyMnO3 in hexagonal as well as orthorhombic structures. The interesting effect of strontium dilution on the frustrated antiferromagnetism of DyMnO3 is also probed using EPR. The line shapes are fitted to broad Lorentzian in the case of pure DyMnO3 and to modified Dysonian in the case of Dy0.5Sr0.5MnO3. The linewidth, integrated intensity, and geff derived from the signals are analyzed as a function of temperature. The results of magnetization measurements corroborate with EPR results. Our study clearly reveals the signature of frustrated magnetism in pure DyMnO3 systems. It is found that antiferromagnetic correlations in these systems persist even above the transition. Moreover, a spin-glass-like behavior in Dy0.5Sr0.5MnO3 is indicated by a steplike feature in the EPR signals at low fields.
Direct synthesis and properties of K2SiF6:Mn4+ phosphor by wet chemical etching of Si wafer
Authors: Adachi Sadao , Takahashi Toru
Published: July 2008

A new method of synthesizing Mn-activated phosphor is presented. The method uses only chemical etching of Si wafer in HF/H2O solution with the addition of an oxidizing agent KMnO4. The luminescence centers of red emission are ascribed to Mn4+ ions in the octahedral sites of potassium hexafluorosilicate (K2SiF6). The luminescence intensity becomes much stronger at higher temperatures, without largely changing its spectral feature.
Sheet resistance determination of electrically symmetric planar four-terminal devices with extended contacts
Authors: Cornils Martin , Paul Oliver
Published: July 2008

This paper reports an analytic method to determine the sheet resistance Rsq of symmetric planar four-terminal devices based on resistance measurements. Using the technique of conformal mapping it is first shown that any such device is electrically equivalent to a corresponding symmetric unit disk with the same Rsq and invariant under rotations by 90°. Two independent resistances measurable on these devices are expressed analytically as a function of Rsq and of the contact opening angle α. These two resistances fully characterize the electrical properties of such planar conductive devices. A simple procedure to extract both α and Rsq from the resistance values is then presented. These findings are corroborated by the experimental characterization of four-contact devices of ten different geometries fabricated using a commercial complementary metal oxide semiconductor process. From these widely different devices, the sheet resistance of a n-well is extracted to be 1042Ω with a relative uncertainty of only 0.45%.
Phase structure of the quartic-cubic generalized two dimensional Yang-Mills U(N) on the sphere
Authors: Lavaei-Yanesi L. , Khorrami M.
Published: July 2008

The large-N behavior of the quartic-cubic generalized two dimensional Yang-Mills U(N) on the sphere is investigated for small cubic couplings. It is shown that single transition at the critical area, which is present for the quartic model, is split into two transitions; both of them are third order. The phase diagram of the system for small cubic couplings is obtained.
Structural transformation in antiferroelectric PbZrO3-relaxor ferroelectric Pb(Ni1∕3Nb2∕3)O3 solid solution system
Authors: Wirunchit S. , Vittayakorn N.
Published: July 2008

The solid solution between the antiferroelectric (AFE) PbZrO3 (PZ) and the relaxor ferroelectric (FE) Pb(Ni1∕3Nb2∕3)O3 (PNN) was synthesized by the columbite precursor method. The crystal structure, phase transformations, and dielectric and thermal properties of (1−x)PZ-xPNN where x=0.00–0.30 were investigated. With these data, the FE phase diagram between PZ and PNN has been established. The crystal structure data obtained from X-ray diffraction indicate that the solid solution PZ-PNN, where x=0.00–0.30, successively transforms from orthorhombic to rhombohedral symmetry with an increase in the PNN concentration. The AFE phase→FE phase transition occurs in compositions of 0.00⩽x⩽0.08. The AFE→FE phase transition shifts to lower temperatures with higher compositions of x. The FE phase temperature range width increases with increased PNN. Apparently the replacement of the Zr4+ ion by Ni2+∕Nb5+ ions decreases the driving force for an antiparallel shift of Pb2+ ions because they interrupt the translational symmetry and facilitates the appearance of a rhombohedral FE phase when the amount of PNN is higher than 8mol%.
Remanent polarization reduction with enhanced temperature in ferroelectric thin films
Authors: Jiang A. Q. , Tang T. A.
Published: July 2008

The remanent polarization reduces monotonically with the rising temperature from 295to430K in Pt∕IrO2∕Pb(Zr0.4Ti0.6)O3∕IrO2∕Pt thin-film capacitors, although the saturation polarization of the films is nearly constant in the temperature range. Fast-pulse measurements of polarization retention shortened on the order of domain switching time indicate a rapid increase in the remanent polarization, which suggests backswitching of partial domains after the applied field with a long relaxation time. The qualitative modeling of domain switching currents predicts the presence of interfacial passive layers near top and bottom electrodes with damaged ferroelectricity, and the density of backswitched domains enhances with the elevated temperature along with reductions in both coercive voltage and interfacial capacitance, whereas the whole dielectric capacitance of the films increases abnormally, in agreement with the argument of the reversible domain contribution to the large dielectric permittivity in ferroelectrics. It is believed that the presence of interfacial passive layers can build in an opposite internal field to backswitch partial domains after the field. Therefore, the ultimate remanent polarization depending on the product of coercive voltage and interfacial capacitance reduces with the elevated temperature.
Arbitrary amplitude double layers in warm dust kinetic Alfvén wave plasmas
Authors: Gogoi Runmoni , Devi Nirupama
Published: July 2008

Large amplitude electrostatic structures associated with low-frequency dust kinetic Alfvénic waves are investigated under the pressure (temperature) gradient indicative of dust dynamics. The set of equations governing the dust dynamics, Boltzmann electrons, ions and Maxwell’s equation have been reduced to a single equation known as the Sagdeev potential equation. Parameter ranges for the existence of arbitrary amplitude double layers are observed. Exact analytical expressions for the energy integral is obtained and computed numerically through which sub-Alfvénic arbitrary amplitude rarefactive double layers are found to exist.
Equation of state of a dense boron plasma by Thomas–Fermi–Dirac–Weizsäcker molecular dynamics
Authors: Danel J.-F. , Kazandjian L. , Zérah G.
Published: July 2008

Simulations consistently combining classical molecular dynamics applied to nuclei and Thomas–Fermi–Dirac–Weizsäcker theory applied to electrons are used to calculate both pressure and internal energy of a boron plasma along the isotherms T=1 and 4eV. A regularized pseudopotential must be used to represent the electron-nucleus interaction; a correction of the internal energy computed allowing for the use of this pseudopotential is proposed. Comparison with results obtained by quantum molecular dynamics shows that including the gradient correction of order 1 in the gradient expansion of the electronic free energy allows to increase the domain of validity of simulations by about 30g∕cm3 toward low densities.
Ballistic hot electron transport in graphene
Authors: Tse Wang-Kong , Hwang E. H. , Das Sarma S.
Published: July 2008

We theoretically study the inelastic scattering rate and the carrier mean free path for energetic hot electrons in graphene, including both electron-electron and electron-phonon interactions. Taking account of optical phonon emission and electron-electron scattering, we find that the inelastic scattering time τ∼10−2–10−1ps and the mean free path l∼10–102nm for electron densities n=1012–1013cm−2. In particular, we find that the mean free path exhibits a finite jump at the phonon energy 200meV due to electron-phonon interaction. Our results are directly applicable to device structures where ballistic transport is relevant with inelastic scattering dominating over elastic scattering.
Frank dislocation loops in HgTe∕CdTe superlattices on CdTe∕Si(211)B substrates
Authors: Fu L. F. , Okamoto N. L. , Chi M. F. , Browning N. D. , Jung H. S. , Grein C. H.
Published: July 2008

The defect structures in HgTe∕CdTe superlattices (SLs) on CdTe∕Si(211)B substrates grown by molecular-beam epitaxy have been investigated using (scanning) transmission electron microscopy and electron energy loss spectroscopy. Straight Hg-rich defects perpendicular to the SLs have been observed near the substrate while Frank dislocation loops (FDLs) are seen far from the substrate. The Hg-rich defects exhibit only a compositional variation with no significant atomic shift, and can be considered to be a remnant of a FDL which has climbed by thermal diffusion during the growth.
A system for supplying constant electrical power for postprocessing tin-doped indium oxide films
Authors: Bertinshaw J. , Kirkup L. , Phillips M. , Placido F.
Published: July 2008

Annealing tin doped indium oxide (ITO) thin films by self-heating shows potential for reducing the crystallization temperature required to optimize the optical and electrical properties of the films. It also shows promise as a cost effective method of studying the heat treatment process in situ. A computer based solution was developed to allow for a precise control over the annealing process. To anneal at a fixed temperature, a feedback loop senses changes in the resistance of the sample and adjusts the current across the load accordingly to ensure constant delivery of power to an ITO film.
Trapped particles by large-amplitude waves in two-dimensional Yukawa liquids
Authors: Hou Lu-Jing , Piel Alexander
Published: July 2008

In recent experiments of strongly coupled complex plasmas (SCCPs), trapping-like phenomena of micro-sized dust particles were observed during propagation of large-amplitude dust-density waves. A small number of particles were observed being trapped and carried along with the waves. In the present paper, we study the similar trapping phenomena in two-dimensional SCCPs by using Brownian dynamics simulation. The trapping process and its new features arising from strong-coupling effects are investigated. In particular, similarities and differences between trapping in weakly coupled plasmas and SCCPs are discussed in detail.
The influence of stoichiometry on the structural stability and on the optical emission of erbium silicate thin films
Authors: Lo Savio R. , Miritello M. , Piro A. M. , Priolo F. , Iacona F.
Published: July 2008

We report the effects of thermal annealing performed in N2 or O2 ambient at 1200°C on the structural and optical properties of Er silicate films having different compositions (Er2SiO5, Er2Si2O7, and their mixture). We demonstrate that the chemical composition of the stoichiometric films is preserved after the thermal treatments. All different crystalline structures formed after the thermal annealing are identified. Thermal treatments in O2 lead to a strong enhancement of the photoluminescence intensity, owing to the efficient reduction of defect density. In particular the highest optical efficiency is associated to Er ions in the α phase of Er2Si2O7.
Examining the terahertz signal from a photoexcited biased semiconductor superlattice for evidence of gain
Authors: Lisauskas A. , Dignam M. M. , Demarina N. V. , Mohler E. , Roskos H. G.
Published: July 2008

We present a careful analysis of the coherent terahertz emission from an undoped biased semiconductor superlattice excited via an ultrashort optical pulse. We use both a semiclassical model and a fully quantum-mechanical model that includes the excitation process and excitonic effects to analyze emission. We conclude that, in contrast to what has recently been claimed, it is not possible to deduce whether there is terahertz gain from the analysis of the emitted coherent terahertz pulse.
Development of an ion beam alignment system for real-time scanning tunneling microscope observation of dopant-ion irradiation
Authors: Kamioka Takefumi , Sato Kou , Kazama Yutaka , Watanabe Takanobu , Ohdomari Iwao
Published: July 2008

An ion beam alignment system has been developed in order to realize real-time scanning tunneling microscope (STM) observation of “dopant-ion” irradiation that has been difficult due to the low emission intensity of the liquid-metal-ion-source (LMIS) containing dopant atoms. The alignment system is installed in our original ion gun and STM combined system (IG/STM) which is used for in situ STM observation during ion irradiation. By using an absorbed electron image unit and a dummy sample, ion beam alignment operation is drastically simplified and accurized. We demonstrate that sequential STM images during phosphorus-ion irradiation are successfully obtained for sample surfaces of Si(111)-7×7 at room temperature and a high temperature of 500°C. The LMIS-IG/STM equipped with the developed ion beam alignment system would be a powerful tool for microscopic investigation of the dynamic processes of ion irradiation.