bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison between Weibull and gamma distributions to derive synthetic unit hydrograph using Horton ratios
Authors: Bhunya P. K. , Berndtsson R. , Singh P. K. , Hubert P.
Published: April 2008

The two‐parameter Weibull distribution (2PWD), similar to an instantaneous unit hydrograph (IUH), is parameterized in terms of Horton catchment ratios on the basis of a geomorphologic model of catchment response. For this the shape and scale parameters of the Weibull distribution are expressed analytically in terms of Horton's catchment ratios. The two parameters of the IUH derived using Nash's model, which is a two‐parameter gamma distribution (2PGD), are also expressed analytically in terms of Horton ratios. The performance of the proposed methods is tested for describing a synthetic unit hydrograph (SUH) under limited data conditions. A comparison is made with the unit hydrographs derived from the event data of two real catchments, and with the existing geomorphological based 2PGD for developing SUH given by Rosso (1984). The sensitivity analysis of the 2PWD to the nondimensional parameter β of the UH (a product of peak discharge and time to peak) shows β to be more sensitive to the shape parameter a than the scale parameter b. Further examination to find any similarity between the behavior of 2PWD and 2PGD showed that a in 2PWD corresponds to the shape parameter n in the 2PGD, and b behaves similar to the scale parameter k in the 2PGD. Finally, practical applicability of the proposed approach to ungauged catchments is tested using field data.
Role of atmospheric adjustments in the tropical Indian Ocean warming during the 20th century in climate models
Authors: Du Yan , Xie Shang‐Ping
Published: April 2008

The tropical Indian Ocean has been warming steadily since 1950s, a trend simulated by a large ensemble of climate models. In models, changes in net surface heat flux are small and the warming is trapped in the top 125 m depth. Analysis of the model output suggests the following quasi‐equilibrium adjustments among various surface heat flux components. The warming is triggered by the greenhouse gas‐induced increase in downward longwave radiation, amplified by the water vapor feedback and atmospheric adjustments such as weakened winds that act to suppress turbulent heat flux from the ocean. The sea surface temperature dependency of evaporation is the major damping mechanism. The simulated changes in surface solar radiation vary considerably among models and are highly correlated with inter‐model variability in SST trend, illustrating the need to reduce uncertainties in cloud simulation.
Numbers, biomass and cultivable diversity of microbial populations relate to depth and borehole-specific conditions in groundwater from depths of 4–450 m in Olkiluoto, Finland
Authors: Pedersen Karsten , Arlinger Johanna , Eriksson Sara , Hallbeck Anna , Hallbeck Lotta , Johansson Jessica
Published: April 2008

Microbiology, chemistry and dissolved gas in groundwater from Olkiluoto, Finland, were analysed over 3 years; samples came from 16 shallow observation tubes and boreholes from depths of 3.9–16.2 m and 14 deep boreholes from depths of 35–742 m. The average total number of cells (TNC) was 3.9 × 105 cells per ml in the shallow groundwater and 5.7 × 104 cells per ml in the deep groundwater. There was a significant correlation between the amount of biomass, analysed as ATP concentration, and TNC. ATP concentration also correlated with the stacked output of anaerobic most probable number cultivations of nitrate-, iron-, manganese- and sulphate-reducing bacteria, and acetogenic bacteria and methanogens. The numbers and biomass varied at most by approximately three orders of magnitude between boreholes, and TNC and ATP were positively related to the concentration of dissolved organic carbon. Two depth zones were found where the numbers, biomass and diversity of the microbial populations peaked. Shallow groundwater down to a depth of 16.2 m on average contained more biomass and cultivable microorganisms than did deep groundwater, except in a zone at a depth of approximately 300 m where the average biomass and number of cultivable microorganisms approached those of shallow groundwater. Starting at a depth of approximately 300 m, there were steep gradients of decreasing sulphate and increasing methane concentrations with depth; together with the peaks in biomass and sulphide concentration at this depth, these suggest that anaerobic methane oxidation may be a significant process at depth in Olkiluoto.
Cigarette Smoking and Fat Distribution in 21, 828 British Men and Women: A Population‐based Study
Authors: Canoy Dexter , Wareham Nicholas , Luben Robert , Welch Ailsa , Bingham Sheila , Day Nicholas , Khaw Kay‐Tee
Published: April 2008

Objective: To examine the relationship between cigarette smoking habits and fat distribution in a population‐based cohort of men and women.

Research Methods and Procedures: We analyzed cross‐sectional data from 21, 828 men and women who were 45 to 79 years of age, residents in Norfolk, United Kingdom, and were recruited between 1993 and 1997. Cigarette smoking habits and other lifestyle factors were assessed using self‐reported questionnaires. Anthropometric measures were obtained during a health examination.

Results: Waist‐hip ratio was highest among current smokers and least among never smokers after adjusting for age, BMI, alcohol intake, total energy intake, physical activity, and education. Higher waist‐hip ratio was directly associated with higher smoking pack‐years in current and former smokers and inversely with duration since quitting smoking in former smokers. Adjusting for age, BMI, and other covariates, current smokers had higher waist circumference but lower hip circumference compared with former or never smokers.

Discussion: Cigarette smoking habits seem to influence fat distribution patterns. Although smokers have lower mean BMI compared with nonsmokers, they have a more metabolically adverse fat distribution profile, with higher central adiposity. The explanation for this association may help elucidate the mechanisms underlying the adverse health consequences of cigarette smoking and abdominal obesity.
Etat des maladies virales de la tomate et stratégie de lutte en Algérie*
Authors: Nechadi S. , Benddine F. , Moumen A. , Kheddam M.
Published: April 2008

La culture de la tomate occupe une place prépondérante dans l’économie agricole algérienne. Sur une superficie globale de primeurs évaluée à plus de 292 000 ha, la culture de tomate représente 51% de la production totale en produits maraîchers. La particularitéécologique de cette culture l’expose à diverses nuisances, notamment les insectes déprédateurs et les maladies. Parmi les espèces nuisibles rencontrées, les aleurodes constituent en tant qu’agents vecteurs de maladies virales une contrainte phytosanitaire majeure. Cinq viroses ont été recensés à ce jour. Il s’agit de l’AMV, CMV, PVX, PVY et ToMV. La stratégie de lutte mise en oeuvre est orientée vers la lutte intégrée, basée particulièrement sur les moyens biologiques. En égard à l’importance de l’incidence économique des maladies virales sur la culture de tomate, les services officiels phytosanitaires lui consacrent une attention particulière grâce notamment au programme de dépistage, orienté vers les principales prohibitions.
Methods of controlling Tomato yellow leaf curl virus (TYLCV) and its vector Bemisia tabaci in the Maltese Islands*
Authors: Attard D.
Published: April 2008

Bemisia tabaci and Tomato yellow leaf curl virus (TYLCV) were first observed in Malta in the early 1990s and caused serious damage to glasshouse and outdoor tomato crops. Chemical, physical and biological control methods have been developed, but the effective method has been the use of virus‐tolerant cultivars.
Performance of direct-drive cryogenic targets on OMEGA
Authors: Goncharov V. N. , Sangster T. C. , Radha P. B. , Betti R. , Boehly T. R. , Collins T. J. B. , Craxton R. S. , Delettrez J. A. , Epstein R. , Glebov V. Yu. , Hu S. X. , Igumenshchev I. V. , Knauer J. P. , Loucks S. J. , Marozas J. A. , Marshall F. J. , McCrory R. L. , McKenty P. W. , Meyerhofer D. D. , Regan S. P. , Seka W. , Skupsky S. , Smalyuk V. A. , Soures J. M. , Stoeckl C. , Shvarts D. , Frenje J. A. , Petrasso R. D. , Li C. K. , Seguin F. , Manheimer W. , Colombant D. G.
Published: April 2008

The success of direct-drive-ignition target designs depends on two issues: the ability to maintain the main fuel adiabat at a low level and the control of the nonuniformity growth during the implosion. A series of experiments was performed on the OMEGA Laser System [T. R. Boehly, D. L. Brown, R. S. Craxton et al., Opt. Commun. 133, 495 (1997)] to study the physics of low-adiabat, high-compression cryogenic fuel assembly. Modeling these experiments requires an accurate account for all sources of shell heating, including shock heating and suprathermal electron preheat. To increase calculation accuracy, a nonlocal heat-transport model was implemented in the 1D hydrocode. High-areal-density cryogenic fuel assembly with ρR>200mg∕cm2 [T. C. Sangster, V. N. Goncharov, P. B. Radha et al., “High-areal-density fuel assembly in direct-drive cryogenic implosions,” Phys. Rev. Lett. (submitted)] has been achieved on OMEGA in designs where the shock timing was optimized using the nonlocal treatment of the heat conduction and the suprathermal-electron preheat generated by the two-plasmon-decay instability was mitigated.
Metadynamics simulations of ice nucleation and growth
Authors: Quigley D. , Rodger P. M.
Published: April 2008

The metadynamics method for accelerating rate events in molecular simulations is applied to the problem of ice freezing. We demonstrate homogeneous nucleation and growth of ice at 180 K in the isothermal-isobaric ensemble without the presence of external fields or surfaces. This result represents the first report of continuous and dynamic ice nucleation in a system of freely evolving density. Simulations are conducted using a variety of periodic simulation domains. In all cases the cubic polymorph ice Ic is grown. The influence of boundary effects on estimates of the nucleation free energy barrier are discussed in relation to differences between this and earlier work.
Measurements of core electron temperature and density fluctuations in DIII-D and comparison to nonlinear gyrokinetic simulations
Authors: White A. E. , Schmitz L. , McKee G. R. , Holland C. , Peebles W. A. , Carter T. A. , Shafer M. W. , Austin M. E. , Burrell K. H. , Candy J. , DeBoo J. C. , Doyle E. J. , Makowski M. A. , Prater R. , Rhodes T. L. , Staebler G. M. , Tynan G. R. , Waltz R. E. , Wang G.
Published: April 2008

For the first time, profiles (0.3<ρ<0.9) of electron temperature and density fluctuations in a tokamak have been measured simultaneously and the results compared to nonlinear gyrokinetic simulations. Electron temperature and density fluctuations measured in neutral beam-heated, sawtooth-free low confinement mode (L-mode) plasmas in DIII-D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] are found to be similar in frequency and normalized amplitude, with amplitude increasing with radius. The measured radial profile of two fluctuation fields allows for a new and rigorous comparison with gyrokinetic results. Nonlinear gyrokinetic flux-tube simulations predict that electron temperature and density fluctuations have similar normalized amplitudes in L-mode. At ρ=0.5, simulation results match experimental heat diffusivities and density fluctuation amplitude, but overestimate electron temperature fluctuation amplitude and particle diffusivity. In contrast, simulations at ρ=0.75 do not match either the experimentally derived transport properties or the measured fluctuation levels.
Structural relaxation in the hydrogen-bonding liquids N-methylacetamide and water studied by optical Kerr effect spectroscopy
Authors: Turton David A. , Wynne Klaas
Published: April 2008

Structural relaxation in the peptide model N-methylacetamide (NMA) is studied experimentally by ultrafast optical Kerr effect spectroscopy over the normal-liquid temperature range and compared to the relaxation measured in water at room temperature. It is seen that in both hydrogen-bonding liquids, β relaxation is present, and in each case, it is found that this can be described by the Cole–Cole function. For NMA in this temperature range, the α and β relaxations are each found to have an Arrhenius temperature dependence with indistinguishable activation energies. It is known that the variations on the Debye function, including the Cole–Cole function, are unphysical, and we introduce two general modifications: One allows for the initial rise of the function, determined by the librational frequencies, and the second allows the function to be terminated in the α relaxation.
Search for lowest-energy structure of Zintl dianion Si122−, Ge122−, and Sn122−
Authors: Shao Nan , Bulusu Satya , Zeng X. C.
Published: April 2008

We perform an unbiased search for the lowest-energy structures of Zintl dianions (Si122−, Ge122−, and Sn122−), by using the basin-hopping (BH) global optimization method combined with density functional theory geometric optimization. High-level ab initio calculation at the coupled-cluster level is used to determine relative stabilities and energy ranking among competitive low-lying isomers of the dianions obtained from the BH search. For Si122−, all BH searches (based on independent initial structures) lead to the same lowest-energy structure Si12a2−, a tricapped trigonal prism (TTP) with Cs group symmetry. Coupled-cluster calculation, however, suggests that another TTP isomer of Si12c2− is nearly isoenergetic with Si12a2−. For Sn122−, all BH searches lead to the icosahedral structure Ih-Sn12a2−, i.e., the stannaspherene. For Ge122−, however, most BH searches lead to the TTP-containing Ge12b2−, while a few BH searches lead to the empty-cage icosahedral structure Ih-Ge12a2− (named as germaniaspherene). High-level ab initio calculation indicates that Ih-Ge12a2− and TTP-containing Ge12b2− are almost isoenergetic and, thus, both may be considered as candidates for the lowest-energy structure at 0K. Ge12a2− has a much larger energy gap (2.04eV) between highest occupied molecular orbital and lowest unoccupied molecular orbital than Ge12b2− (1.29eV), while Ge12b2− has a lower free energy than Ih-Ge12a2− at elevated temperature (>980K). The TTP-containing Si12a2− and Ge12b2− exhibit large negative nuclear independent chemical shift (NICS) value (∼−44) at the center of TTP, indicating aromatic character. In contrast, germaniaspherene Ih-Ge12a2− and stannaspherene Ih-Sn12a2− exhibit modest positive NICS values, ∼12 and 3, respectively, at the center of the empty cage, indicating weakly antiaromatic character.
Time-resolved absorption in cryogenic and room-temperature direct-drive implosions
Authors: Seka W. , Edgell D. H. , Knauer J. P. , Myatt J. F. , Maximov A. V. , Short R. W. , Sangster T. C. , Stoeckl C. , Bahr R. E. , Craxton R. S. , Delettrez J. A. , Goncharov V. N. , Igumenshchev I. V. , Shvarts D.
Published: April 2008

Time-dependent and time-integrated absorption fractions are inferred from scattered-light measurements in room-temperature and cryogenic direct-drive-implosion experiments on OMEGA. The measurements agree reasonably well with hydrodynamic simulations that include nonlocal electron-heat transport. Discrepancies in the time-resolved scattered-light spectra between simulations and experiments remain for complex laser pulse shapes, indicating beam-to-beam energy transfer and commensurate coupling losses. Time-resolved scattered-light spectra near ω∕2 and 3ω∕2 as well as time-resolved hard-x-ray measurements indicate the presence of a strongly driven two-plasmon-decay (TPD) instability at high intensities that may influence the observed laser light absorption. Experiments indicate that energetic electron production due to the TPD instability can be mitigated with high-Z-doped plastic shells.
Symmetry breaking effect in the ferrocene electronic structure by hydrocarbon-monosubstitution: An experimental and theoretical study
Authors: Boccia A. , Marrani A. G. , Stranges S. , Zanoni R. , Alagia M. , Cossi M. , Iozzi M. F.
Published: April 2008

We present here the results of a synchrotron radiation-excited UV-photoemission investigation and density functional theory calculations on a structurally related series of organometallic free molecules: ethylferrocene (EtFC), vinylferrocene (VFC), and ethynylferrocene (EFC). This series exemplifies the electronic interactions operating when the CC substituent group of an aromatic ring is bound to the substrate surface atoms, from a single CC bond to the double and triple CC bond π systems which are still able to preserve substrate-molecule conjugation. A detailed assignment of the gas phase valence photoelectron spectra is discussed, providing new data on the electronic structure of EtFC and EFC and offering a partial reinterpretation of previous assignments on VFC. The broken symmetry of ferrocene caused by the monosubstitution has notable effects on the removal of the molecular orbital (MO) degeneracy which is found to be especially remarkable for the ferrocenelike e1′ MOs. This effect is ascribed to the interaction between the aromatic cyclopentadyenyl ring and the substituent through σ∕π hyperconjugation and π-conjugation mechanisms depending on the nature of the hydrocarbon moiety and its conformational geometry. The vertical ionization energy values of the highest occupied MO for the alkylferrocene and ferrocene free molecules linearly correlate with the redox potential in acetonitrile for ferrocene and the corresponding hybrids obtained by covalently anchoring the free molecule on silicon.
Study of percolation and clustering in supercritical water-CO2 mixtures
Authors: Botti A. , Bruni F. , Mancinelli R. , Ricci M. A. , Lo Celso F. , Triolo R. , Ferrante F. , Soper A. K.
Published: April 2008

The microscopic structure of supercritical water-CO2 mixture is investigated by neutron diffraction experiments exploiting the isotopic H∕D substitution. The investigated water reach mixtures are in the liquidlike region of the phase diagram, according to the behavior of the radial distribution functions, yet a reduction of the average number of hydrogen bonds, compared to equivalent states of pure water, is found. As a consequence, the average dimension of water clusters is reduced and the system stays below the percolation threshold. These results, along with the shift of the main peaks of the site-site radial distribution functions, suggest that the excess volume in these supercritical mixtures is likely associated with the CO2 solvation shell.
Importance sampling for quantum Monte Carlo in manifolds: Addressing the time scale problem in simulations of molecular aggregates
Authors: Luan T. , Curotto E. , Mella Massimo
Published: April 2008

Several importance sampling strategies are developed and tested for stereographic projection diffusion Monte Carlo in manifolds. We test a family of one parameter trial wavefunctions for variational Monte Carlo in stereographically projected manifolds which can be used to produce importance sampling. We use the double well potential in one dimensional Euclidean space to study systematically sampling issues for diffusion Monte Carlo. We find that diffusion Monte Carlo with importance sampling in manifolds is orders of magnitude more efficient compared to unguided diffusion Monte Carlo. Additionally, diffusion Monte Carlo with importance sampling in manifolds can overcome problems with nonconfining potentials and can suppress quasiergodicity effectively. We obtain the ground state energy and the wavefunction for the Stokmayer trimer.
Laser spectroscopy of iridium monoboride
Authors: Ye Jianjun , Pang H. F. , Wong A. M.-Y. , Leung J. W.-H. , Cheung A. S.-C.
Published: April 2008

High resolution laser induced fluorescence spectrum of IrB in the spectral region between 545 and 610nm has been recorded and analyzed. Reacting laser-ablated iridium atoms with 1% B2H6 seeded in argon produced the IrB molecule. This is the first experimental observation of the IrB molecule. Four vibronic transition bands, (v,0) with v=0–3 of an electronic transition system, have been observed. Spectra of all four isotopic molecules, Ir191B10, Ir193B10, Ir191B11, and Ir193B11, were recorded. Isotopic relationships confirmed the carrier of the spectra and the vibrational quantum number assignment. Preliminary analysis of rotational lines showed that these vibronic bands are with Ω′=2 and Ω″=3. The electronic transition identified is assigned as the [16.5]Π23−XΔ33 system. Partially resolved hyperfine structure which conforms to the Hund’s case aβ coupling scheme has been observed and analyzed. The bond length r0 of the lower XΔ33 state of IrB was determined to be 1.7675Å.
Formation mechanism of ZnSiO3 nanoparticles embedded in an amorphous interfacial layer between a ZnO thin film and an n-Si (001) substrate due to thermal treatment
Authors: Yuk J. M. , Lee J. Y. , Jung J. H. , Lee D. U. , Kim T. W. , Son D. I. , Choi W. K.
Published: April 2008

The x-ray diffraction patterns, transmission electron microscopy images, and selected-area electron diffraction patterns for the ZnO∕Si heterostructures annealed at 900°C showed that orthorhombic ZnSiO3 nanoparticles were formed in the amorphous layer between the ZnO film and the Si substrate, resulting from the interdiffusion between the ZnO film and the Si substrate due to thermal treatment. Auger electron spectroscopy depth profiles for the ZnO∕Si heterostructures annealed at 900°C demonstrated the formation of amorphous Zn2xSi1−xO2, an interfacial layer. A formation mechanism for the orthorhombic ZnSiO3 nanoparticles embedded in the amorphous Zn2xSi1−xO2 layer is described on the basis of the experimental results.
Realization, characterization, and optical modeling of inverted bulk-heterojunction organic solar cells
Authors: Ameri Tayebeh , Dennler Gilles , Waldauf Christoph , Denk Patrick , Forberich Karen , Scharber Markus C. , Brabec Christoph J. , Hingerl Kurt
Published: April 2008

Inverted bulk-heterojunction organic solar cells (OSCs) using solution-processed layers possess significant advantages compared to the usual noninverted devices. To investigate the full potential of this type of OSC, we have carried out some optical modeling by rigorous coupled wave analysis. The influence of the thickness of several different layers in the device has been quantified, as well as the maximum possible number of photons absorbed in the poly(3-hexyltiophene):[6,6]-phenyl-C61-butyric acid methyl ester active layer for both conventional and inverted structures. It appears that the thickness of the hole injecting layer placed in front of the metallic mirror can influence the electromagnetic field distribution in the OSC, but no additional beneficial optical spacer effect is observed. The thickness of the electron injecting layer deposited on the semitransparent electrode also has a negligible influence on the photons absorbed in the active layer for the inverted structure.
Effect of initial properties on the flow strength of aluminum during quasi-isentropic compression
Authors: Asay J. R. , Ao T. , Davis J.-P. , Hall C. , Vogler T. J. , Gray G. T.
Published: April 2008

A magnetic loading technique was used to ramp load pure aluminum and 6061 aluminum alloy to peak stresses of approximately 29GPa. The peak loading rate was approximately 106∕s, followed by unloading from peak stress at a rate of about 105∕s. The pure aluminum samples had impurity levels ranging from about 10ppmto0.5wt% and average grain sizes in the range of 144–454μm. The 6061 alloy was prepared in either the T6 condition with grain sizes of 5–50μm, or in the T0 or T6 heat treatment condition with a grain size of about 40μm. A wave profile technique was used to estimate the compressive strength during unloading. It was found that the compressive strength estimated during unloading increased with peak stress for all materials and that the change in strength was insensitive to initial material properties. This observation is in agreement with previous results obtained from shock loading of the same materials [H. Huang and J. R. Asay, J. Appl. Phys. 98, 033524 (2005)] and suggests that the deformation mechanisms, which govern strength properties at high strain rates, are similar for the two cases. It was also found that the quasielastic strain occurring during unloading saturated at about 2% in 6061-T6 alloy for peak stresses in the range of 10–50GPa. The change in quasielastic strain was found to be similar for pure aluminum and 6061 alloys.
Thickness dependent transport properties of compressively strained La0.88Sr0.12MnO3 ultrathin films
Authors: Prasad Ravikant , Singh H. K. , Singh M. P. , Prellier W. , Siwach P. K. , Kaur Amarjeet
Published: April 2008

Thickness dependent magnetic and transport properties of compressively strained La0.88Sr0.12MnO3 thin films grown on single crystalline SrTiO3 (100) substrates have been studied. All films exhibit a large enhancement of ∼130 K in TC/TIM as compared to that of the bulk target (TC∼175 K). This has been explained in terms of suppression of the cooperative Jahn–Teller distortion due to in-plane compressive strain. The TC/TIM of the 5 nm film is 315 K/318 K and slightly increases for film thicknesses ≤25 nm. At higher film thicknesses, ∼60 nm, the TC/TIM starts decreasing. At T>TIM, the electrical transport is due to thermally activated hopping of small polarons. The activation energy is found to be sensitive to film thickness and shows a minimum at around L∼15–25 nm, which corresponds to the maximum of TC/TIM. All films possess large magnetoresistances (MRs) in the vicinity of room temperature. Low as well as high field MRs are observed to nearly double as the film thickness increases from 5 nm (MR∼28%/3 T) to 60 nm (MR%∼50%/3 T). Thicker films (L≥25 nm) are found to exhibit sufficiently large temperature coefficients of resistivities, ∼4%–5%/K, which could be suitable for bolometric applications.
Crystal structure, Raman spectroscopy, far-infrared, and microwave dielectric properties of (1−x)La(MgSn)0.5O3–xNd(MgSn)0.5O3 system
Authors: Santosh Babu G. , Subramanian V. , Murthy V. R. K. , Moreira R. L. , Lobo R. P. S. M.
Published: April 2008

The complex perovskite system (1−x)La(MgSn)0.5O3–xNd(MgSn)0.5O3 with the composition (x=0–1) was prepared by the solid state reaction method. Structural and spectroscopic studies were carried out to understand the variation of dielectric properties with x. Rietveld refinement was carried out with the initial model obtained by using the structure prediction and diagnostic software. The symmetry of the compositions was determined to be monoclinic with space group P21/n, which corresponds to the a−a−c+ tilting system, and the long-range order parameter was found to decrease with an increase in neodymium concentration. Raman spectra were analyzed by fitting the A1g-like mode to a Lorentzian peak shape. Intrinsic dielectric parameters were estimated by fitting infrared reflectance spectra with the four-parameter semiquantum model. Transverse optic phonon mode strengths and average phonon damping were calculated. The origin of increase in the intrinsic loss with the composition variation is discussed. Microwave measurements were carried out in the frequency range of 9–11 GHz. The dielectric constant decreases and the temperature coefficient of resonant frequency becomes less negative with the increase in neodymium concentration.
Input reconstruction of chaos sensors
Authors: Yu Dongchuan , Liu Fang , Lai Pik-Yin
Published: April 2008

Although the sensitivity of sensors can be significantly enhanced using chaotic dynamics due to its extremely sensitive dependence on initial conditions and parameters, how to reconstruct the measured signal from the distorted sensor response becomes challenging. In this paper we suggest an effective method to reconstruct the measured signal from the distorted (chaotic) response of chaos sensors. This measurement signal reconstruction method applies the neural network techniques for system structure identification and therefore does not require the precise information of the sensor’s dynamics. We discuss also how to improve the robustness of reconstruction. Some examples are presented to illustrate the measurement signal reconstruction method suggested.
Effect of size polydispersity in polymer-dispersed liquid-crystal films
Authors: Ferrari José A. , Dalchiele Enrique A. , Frins Erna M. , Gentilini Juan A. , Perciante César D. , Scherschener Elizabeth
Published: April 2008

The electro-optical effect of size polydispersity in polymer-dispersed liquid-crystal (PDLC) films is investigated. Liquid crystal (LC) droplets of different sizes require different electric field amplitudes for director reorientation, which will produce a mismatch of effective refractive indices. This, in turn, will produce an increase in interdroplet scattering (i.e., decrease in the direct transmitted light) within a specific range of electric field amplitudes. We propose a phenomenological “two-size” model describing this enhanced “pseudo-off-state” achieved under application of an electrical field and present validation experiments. Our approach may be used to get more insight into the transmittance behavior of PDLCs affecting the contrast ratio.
Nonlinear electron magnetohydrodynamics physics. I. Whistler spheromaks, mirrors, and field reversed configurations
Authors: Stenzel R. L. , Urrutia J. M. , Strohmaier K. D.
Published: April 2008

The nonlinear interactions of time-varying magnetic fields with plasmas is investigated in the regime of electron magnetohydrodynamics. Simple magnetic field geometries are excited in a large laboratory plasma with a loop antenna driven with large oscillatory currents. When the axial loop field opposes the ambient field, the net field can be reversed to create a field-reversed configuration (FRC). In the opposite polarity, a strong field enhancement is produced. The time-varying antenna field excites whistler modes with wave magnetic fields exceeding the ambient magnetic field. The resulting magnetic field topologies have been measured. As the magnetic topology is changed from FRC to strong enhancement, two propagating field configurations resembling spheromaks are excited, one with positive and the other with negative helicity. Such “whistler spheromaks” propagate with their null points along the weaker ambient magnetic field, with the current density localized around its O-line. In contrast, “whistler mirrors” which have topologies similar to linear whistlers, except with Bwave>B0, have no null regions and, therefore, broad current layers. This paper describes the basic field topologies of whistler spheromaks and mirrors, while companion papers discuss the associated nonlinear phenomena as well as the interaction between them.
Large dielectric permittivity in the paraelectric RMn2O5 with R=Tb, Dy, and Er
Authors: Han T. C. , Lin J. G. , Kuo K. M. , Chern G.
Published: April 2008

The complex dielectric permittivities (ε′,ε″) of RMn2O5 (R=Tb, Dy, and Er) are measured in the temperature range of 78–328 K with frequencies ranging from 100 Hz to 1 MHz. A large real part of the dielectric constant ε′ (40–140) is observed at room temperature in all three samples studied. These values are higher than the peak value found at the ferroelectric transition of RMn2O5 single crystals. Our results reveal that high dielectric permittivities are related to thermally activated dipolar motions with activation energies of ∼300 meV and a relaxation frequency of ∼3×1012 Hz. The possible relation between the high dielectric permittivity and the size effect of R ions based on the spin-phonon interaction is suggested.
Nonlinear electron magnetohydrodynamics physics. II. Wave propagation and wave-wave interactions
Authors: Urrutia J. M. , Stenzel R. L. , Strohmaier K. D.
Published: April 2008

The propagation of low-frequency whistler modes with wave magnetic field exceeding the ambient field is investigated experimentally. Such nonlinear waves are excited with magnetic loop antennas whose axial field is aligned with the background magnetic field and greatly exceeds its strength. The oscillatory antenna field excites propagating wave packets with field topologies alternating between whistler spheromaks and mirrors. The propagation speed of spheromaks is observed to decrease with amplitude while that of mirrors increases with amplitude. The field distribution varies with amplitude: Spheromaks contract axially while mirrors spread out compared to linear whistlers. Consequently, the peak magnetic field and current densities in spheromaks exceed that of mirrors. Wave-wave interactions of nonlinear whistler modes is also studied. Counterpropagating spheromaks collide inelastically and form a stationary field-reversed configuration. The radius of the toroidal current ring depends on current and can be larger than that of the loop antenna. A tilted field-reversed configuration precesses in the direction of the electron drift. The free magnetic energy is dissipated in the plasma volume and converted into electron heat.
Nonlinear electron magnetohydrodynamics physics. III. Electron energization
Authors: Strohmaier K. D. , Urrutia J. M. , Stenzel R. L.
Published: April 2008

Wave-particle interactions of low-frequency whistler modes with wave magnetic fields exceeding the ambient field are investigated experimentally. These highly nonlinear modes are excited with magnetic loop antennas in a large magnetized afterglow plasma. While the nonlinear wave properties are described elsewhere, the present paper focuses on the modification of the electron distribution function by the whistler waves. When the electron current flows in regions of magnetic nulls, such as in spheromak and field-reversed configurations (FRCs), strong electron energization is observed. When the whistler modes are created by electron Hall currents, such as in whistler mirrors, no significant energization occurs. The electron temperature can be raised locally by an order of magnitude. Non-Maxwellian distributions with energetic tail electrons are observed. Electron energization to ≳10eV produces visible light emission whose time and space dependence is mapped. The light source travels with the subthermal speed of whistler spheromaks. When counterpropagating spheromaks collide, the resultant FRC produces strong local heating and light which dissipates its free magnetic energy.
Dynamics of domain wall depinning driven by a combination of direct and pulsed currents
Authors: Hayashi Masamitsu , Thomas Luc , Rettner Charles , Moriya Rai , Parkin Stuart S. P.
Published: April 2008

The current-induced depinning of a domain wall from an artificial pinning site is investigated in Permalloy nanowires. The depinning probability exhibits an oscillatory dependence on the current pulse length, which is, surprisingly, reduced when a dc current of the same polarity is added to the current pulse. By contrast, the depinning probability increases when dc and pulsed currents have opposite polarities even though the total current flowing though the device is smaller. These results are understood within an analytical one-dimensional model of the domain wall dynamics.
Systematic investigation on anomalous positive Vfb shift in Al-incorporated high-k gate stacks
Authors: Wang Wenwu , Mizubayashi Wataru , Akiyama Koji , Nabatame Toshihide , Toriumi Akira
Published: April 2008

An anomalous positive shift in flatband voltage (Vfb) with decreasing the equivalent oxide thickness (EOT) of high-k gate stacks is reported in this paper. As the SiO2 interfacial layer is aggressively thinned in Al-incorporated HfxAl1−xOy gate stacks, the Vfb first lies on the well known linear Vfb-EOT plot and deviates toward positive voltage direction (Vfb roll up), followed by shifting toward negative voltage (Vfb roll off). The Vfb roll-up behavior was ascribed to variations in fixed charges near the SiO2∕Si interface which are caused by Al diffusion from HfxAl1−xOy through SiO2 to the SiO2∕Si interface.
One dimensional plasmons in pyroelectric-semiconductor composites
Authors: Dmitriev A. P. , Shur M. S.
Published: April 2008

We show that the spontaneous and piezoelectric polarization fields inside semiconductor wires embedded into a pyroelectric matrix induce thin mobile electron and/or hole stripes at the wire surfaces. These charged stripes of mobile carriers support longitudinal and transverse plasma waves with typical frequencies in the terahertz range. The longitudinal waves are conventional one-dimensional plasma waves with the frequency approximately proportional to the wave vector. The transverse waves are optical plasma waves with the frequency being nearly independent of the wave vector for the wave vectors approaching zero. The optical plasma branch is expected to strongly interact with the terahertz radiation.
From pure C60 to silicon carbon fullerene-based nanotube: An ab initio study
Authors: Li Jiling , Xia Yueyuan , Zhao Mingwen , Liu Xiangdong , Song Chen , Li Lijuan , Li Feng
Published: April 2008

The energetics, geometrical, and electronic properties of the silicon carbon fullerene-based materials, obtained from C60 by replacing 12 carbon atoms of the C60 cage with silicon atoms, are studied based on ab initio calculations. We have found that, of the two C48Si12 isomers obtained, the one with the carbon atoms and the silicon atoms located in separated region, i.e., with a phase-separated structure is more stable. Fullerene-based C36Si24 cluster, C36Si24C36Si24 dimer, and the nanotube constructed from the clusters are then studied. The calculations on the electronic properties of these silicon carbon fullerene-based nanomaterials demonstrate that the energy gaps are greatly modified and show a decreasing trend with increasing the size of the clusters. The silicon carbon fullerene-based nanotube has a narrow and direct energy band gap, implying that it is a narrow gap semiconductor and may be a promising candidate for optoelectronic devices.
A multilayer structured acoustic cloak with homogeneous isotropic materials
Authors: Cheng Ying , Yang Fan , Xu Jian Yi , Liu Xiao Jun
Published: April 2008

An acoustic cloak with concentric alternating layered structure is designed with homogeneous isotropic materials on the basis of effective medium theory. The cloak possesses low-reflection and power-flow bending properties for different shapes of wavefront both in far- and near-fields, which could perfectly approximate the ideal cloak with inhomogeneous and anisotropic distribution of material parameters.
Effect of ionic substitutions on the structure and dielectric properties of hafnia: A first principles study
Authors: Cockayne Eric
Published: April 2008

First principles calculations were used to study the effects of Si, Ti, Zr, and Ta (+N) substitutional impurities on the structure and dielectric properties of crystalline HfO2. The dielectric constant of monoclinic HfO2 can be enhanced by substituting more polarizable ions for Hf, but the band gap is decreased. Enhancing the permittivity without decreasing the band gap requires forming the tetragonal or cubic phase of HfO2. Among the ions studied, Si alone is found to stabilize a nonmonoclinic phase of HfO2 relative to the monoclinic phase, but only at an atomic concentration above about 20%. Various experiments have reported the formation of nonmonoclinic phases of HfO2 with increased permittivity when other ions are substituted for Hf. It is concluded that these structures are, in general, either metastable or are stabilized by extrinsic factors or by a layered arrangement of the substitutional cations.
Wide band gap semiconductor alloy: x(LiGaO2)1∕2–(1−x)ZnO
Authors: Omata Takahisa , Tanaka Keizo , Tazuke Atsushi , Nose Katsuhiro , Otsuka-Yao-Matsuo Shinya
Published: April 2008

Oxide semiconductor alloys of x(LiGaO2)1∕2–(1−x)ZnO were synthesized by a standard solid state reaction. The wurtzite-type single phases were obtained in the wide composition range of x⩽0.38 because the β-LiGaO2 possesses a wurtzite-derived structure and approximately the same lattice constants with ZnO. The resulting alloys showed characteristics of wide band gap oxide n-type semiconductor; the optical transparency reached over to the ultraviolet (UV) light. The electrical conductivity and energy band gap for 0.38(LiGaO2)1∕2–0.62ZnO were 8.2Ω−1cm−1 and 3.7eV, respectively, at room temperature. The alloy is a good candidate for the barrier material of the ZnO-based heterostructures and UV-transparent electrodes.
Surface tension of water and acid gases from Monte Carlo simulations
Authors: Ghoufi A. , Goujon F. , Lachet V. , Malfreyt P.
Published: April 2008

We report direct Monte Carlo (MC) simulations on the liquid-vapor interfaces of pure water, carbon dioxide, and hydrogen sulfide. In the case of water, the recent TIP4P/2005 potential model used with the MC method is shown to reproduce the experimental surface tension and to accurately describe the coexistence curves. The agreement with experiments is also excellent for CO2 and H2S with standard nonpolarizable models. The surface tensions are calculated by using the mechanical and the thermodynamic definitions via profiles along the direction normal to the surface. We also discuss the different contributions to the surface tension due to the repulsion-dispersion and electrostatic interactions. The different profiles of these contributions are proposed in the case of water.
Extrapolating potential energy surfaces by scaling electron correlation: Isomerization of bicyclobutane to butadiene
Authors: Lutz Jesse J. , Piecuch Piotr
Published: April 2008

The recently proposed potential energy surface (PES) extrapolation scheme, which predicts smooth molecular PESs corresponding to larger basis sets from the relatively inexpensive calculations using smaller basis sets by scaling electron correlation energies [A. J. C. Varandas and P. Piecuch, Chem. Phys. Lett. 430, 448 (2006)], is applied to the PESs associated with the conrotatory and disrotatory isomerization pathways of bicyclo[1.1.0]butane to buta-1,3-diene. The relevant electronic structure calculations are performed using the completely renormalized coupled-cluster method with singly and doubly excited clusters and a noniterative treatment of connected triply excited clusters, termed CR-CC(2,3), which is known to provide a highly accurate description of chemical reaction profiles involving biradical transition states and intermediates. A comparison with the explicit CR-CC(2,3) calculations using the large correlation-consistent basis set of the cc-pVQZ quality shows that the cc-pVQZ PESs obtained by the extrapolation from the smaller basis set calculations employing the cc-pVDZ and cc-pVTZ basis sets are practically identical, to within fractions of a millihartree, to the true cc-pVQZ PESs. It is also demonstrated that one can use a similar extrapolation procedure to accurately predict the complete basis set (CBS) limits of the calculated PESs from the results of smaller basis set calculations at a fraction of the effort required by the conventional pointwise CBS extrapolations.
Experimental bifurcations and homoclinic chaos in a laser with a saturable absorber
Authors: Cavalcante Hugo L. D. de S. , Rios Leite J. R.
Published: April 2008

The shape and the peak values of the pulses from a passive Q-switching CO2 laser with SF6 as saturable absorber were detected while the laser was tuned in frequency across a longitudinal mode. A succession of stability windows, typical for bifurcation diagrams in the homoclinic scenario, was observed and the widths of those windows were measured. The expansion rate of the undulations in individual pulses was also obtained and compared to Floquet’s multipliers given by the ratio of widths in consecutive windows. The dynamics is consistent with a homoclinic tangency to a periodic orbit.
Redox entropy of plastocyanin: Developing a microscopic view of mesoscopic polar solvation
Authors: LeBard David N. , Matyushov Dmitry V.
Published: April 2008

We report applications of analytical formalisms and molecular dynamics (MD) simulations to the calculation of redox entropy of plastocyanin metalloprotein in aqueous solution. The goal of our analysis is to establish critical components of the theory required to describe polar solvation at the mesoscopic scale. The analytical techniques include a microscopic formalism based on structure factors of the solvent dipolar orientations and density and continuum dielectric theories. The microscopic theory employs the atomistic structure of the protein with force-field atomic charges and solvent structure factors obtained from separate MD simulations of the homogeneous solvent. The MD simulations provide linear response solvation free energies and reorganization energies of electron transfer in the temperature range of 280–310K. We found that continuum models universally underestimate solvation entropies, and a more favorable agreement is reported between the microscopic calculations and MD simulations. The analysis of simulations also suggests that difficulties of extending standard formalisms to protein solvation are related to the inhomogeneous structure of the solvation shell at the protein-water interface combining islands of highly structured water around ionized residues along with partial dewetting of hydrophobic patches. Quantitative theories of electrostatic protein hydration need to incorporate realistic density profile of water at the protein-water interface.
Molecular dynamics simulations of carbon nanotube/silicon interfacial thermal conductance
Authors: Diao Jiankuai , Srivastava Deepak , Menon Madhu
Published: April 2008

Using molecular dynamics simulations with Tersoff reactive many-body potential for Si–Si, Si–C, and C–C interactions, we have calculated the thermal conductance at the interfaces between carbon nanotube (CNT) and silicon at different applied pressures. The interfaces are formed by axially compressing and indenting capped or uncapped CNTs against 2×1 reconstructed Si surfaces. The results show an increase in the interfacial thermal conductance with applied pressure for interfaces with both capped and uncapped CNTs. At low applied pressure, the thermal conductance at interface with uncapped CNTs is found to be much higher than that at interface with capped CNTs. Our results demonstrate that the contact area or the number of bonds formed between the CNT and Si substrate is key to the interfacial thermal conductance, which can be increased by either applying pressure or by opening the CNT caps that usually form in the synthesis process. The temperature and size dependences of interfacial thermal conductance are also simulated. These findings have important technological implications for the application of vertically aligned CNTs as thermal interface materials.
Bias-driven local density of states alterations and transport in ballistic molecular devices
Authors: Deretzis Ioannis , La Magna Antonino
Published: April 2008

We study dynamic nonequilibrium electron charging phenomena in ballistic molecular devices at room temperature that compromise their response to bias and whose nature is evidently distinguishable from static Schottky-type potential barriers. Using various metallic/semiconducting carbon nanotubes and alkane dithiol molecules as active parts of a molecular bridge, we perform self-consistent quantum transport calculations under the nonequilibrium Green’s function formalism coupled to a three-dimensional Poisson solver for a mutual description of chemistry and electrostatics. Our results sketch a particular tracking relationship between the device’s local density of states and the contact electrochemical potentials that can effectively condition the conduction process by altering the electronic structure of the molecular system. Such change is unassociated to electronic/phononic scattering effects while its extent is highly correlated to the conducting character of the system, giving rise to an increase of the intrinsic resistance of molecules with a semiconducting character and a symmetric mass-center disposition.