bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of rainfall interannual variability on the stability and resilience of dryland plant ecosystems
Authors: Borgogno F. , D'Odorico P. , Laio F. , Ridolfi L.
Published: June 2007

Positive feedbacks between vegetation and soil moisture may induce, in arid ecosystems, the emergence of two alternative stable states, corresponding to bare and completely vegetated soil. A new analytical model is developed to investigate this behavior and to study the effects of interannual rainfall variability on the dynamics of bistable ecosystems. When dichotomic Markov noise is used to account for the effect of random rainfall fluctuations, the long‐term dynamics of the system can be investigated through an analytical solution of the model. It is found that in a broad class of bistable ecosystems, random rainfall fluctuations may induce an ordered state in the dynamics, i.e., by turning the bistable deterministic system into a stochastic system with only one statistically stable state. This effect is enhanced by increases in noise intensity, whereas the stochastic dynamics become bistable (i.e., the noise‐induced ordered state disappears) as the noise intensity decreases below a critical (nonnull) value. This effect of noise‐induced stability is found in association with an enhancement of ecosystem resilience, indicating that the likelihood of catastrophic shifts to the desert state decreases as the noise intensity increases.
Open Notebook Science Using Blogs and Wikis
Authors: Bradley Jean-Claude
Published: June 2007

The first half is a summary of how the Bradley group at Drexel University is doing Open Notebook Science with the UsefulChem project to synthesize and test novel anti-malarial compounds. Graduate student Dave Strumfels' code to compute kinetics from JCAMP NMR reaction profiles is then highlighted. Finally screenshots are shown of a building on Nature Island in Second Life where Beth Ritter-Guth and Eloise Pasteur have helped to set up a poster room with NMR spectra, molecules and an organic chemistry quiz that can be activated by clicking on an obelisk.
Use of CD107‐based cell sorting ex vivo to enrich subdominant CD8+ T cells in culture
Authors: Moor Rebecca J , Morrison Leanne E , Moss Denis J , Tscharke David C
Published: June 2007

CD8+ T lymphocytes are key effectors in the control of viral diseases and some tumours. In general, the majority of CD8+ T cells recognize a few immunodominant epitopes, but in some circumstances, subdominant specificities may be more relevant as targets for vaccines or immunotherapy. Epstein–Barr virus (EBV)‐associated cancers are an example where knowledge of subdominant‐specific CD8+ T cells is important because the immunodominant EBV proteins are not expressed in these cancers. We have developed a live‐cell sorting method based on CD107 detection to remove CD8+ T cells recognising dominant EBV epitopes and show that this allows enrichment of subdominant‐specific CD8+ T cells in subsequent cultures. This work shows that immunodomination in vitro suppresses the outgrowth of subdominant‐specific CD8+ T cells in culture. The method may have broad applications for finding subdominant targets for immunotherapy and vaccines, and the principle suggests a means of improving subdominant CD8+ T‐cell cultures grown for immunotherapy.
Development of protective immunity in a murine model of melioidosis is influenced by the source of Burkholderia pseudomallei antigens
Authors: Barnes Jodie L , Ketheesan Natkunam
Published: June 2007

Melioidosis is a potentially fatal disease caused by the bacterium, Burkholderia pseudomallei. The current study was carried out to determine the mechanisms involved in the development of protective immunity in a murine model of melioidosis. Following intravenous infection with B. pseudomallei, both C57BL/6 and BALB/c mice demonstrated delayed‐type hypersensitivity responses and lymphocyte proliferation towards B. pseudomallei antigens, indicating the generation of B. pseudomallei‐specific lymphocytes. Adoptive transfer of these lymphocytes to naïve C57BL/6 mice was demonstrated by a delayed‐type hypersensitivity response. Mice were not protected from a subsequent lethal challenge with a highly virulent strain of B. pseudomallei, suggesting that a single intravenous dose of the bacterium is insufficient to induce a protective adaptive immune response. Attempts to induce resistance in susceptible BALB/c mice used repetitive low‐dose exposure to live B. pseudomallei. Immune responses and resistance following subcutaneous immunization with live B. pseudomallei were compared with exposure to heat‐killed, culture filtrate and sonicated B. pseudomallei antigens. Compared to heat‐killed B. pseudomallei, significant protection was generated in BALB/c mice following immunization with live bacteria. Our studies also demonstrate that the type of immune response generated in vivo is influenced by the antigenic preparation of B. pseudomallei used for immunization.
Use of both benthic and drift sampling techniques to assess tropical stream invertebrate communities along an altitudinal gradient, Costa Rica
Authors: Pringle Catherine M. , Ramírez Alonso
Published: June 2007

1. Two sampling techniques were used to characterize invertebrate communities in eight, low‐order streams along an altitudinal gradient in Costa Rica that represents the last continuous tract of primary forest spanning such extremes in elevation (i.e. near sea level to 2900 m a.s.l.) along the Caribbean Slope of Central America. A standard Surber sampler was used to sample invertebrates on the stream bottom, and drift sampling nets were used to sample invertebrates drifting in the stream flow.

2. Sites were established at 30, 50, 700 1800 and 2700 m a.s.l. In one to two streams per site, six Surber samples were collected, and drift was sampled every 3 h over one 24‐h period between April and August 1994. All sites were in primary forest, with the exception of the lowest elevation site (30 m) which was located in banana plantations.

3. Both sampling techniques indicated that Diptera (Chironomidae) and Ephemeroptera were the dominant insect groups at all sites. Disturbed streams draining banana plantations were dominated by Chironomidae and had lower taxon richness and diversity than other sites.

4. While data from benthic samples indicated that insects were the major faunal component (> 90%) at all sites, drift samples were dominated by larval shrimps (> 50%) at the 30 m and 50 m sites.

5. Drift periodicity of invertebrates was observed at those sites characterized by predaceous fishes: nocturnal drift densities were higher than diurnal densities at 30, 50 and 700 m a.s.l., however, no periodicity was observed at 1800 and 2700 m a.s.l. where fish were absent.

6. This study shows the importance of measuring invertebrate drift, in addition to directly sampling the benthos. Drift sampling provided data on a major community component (shrimps) of lowland tropical streams, that would have been overlooked using traditional benthic sampling techniques, and in some cases provided additional information on taxon richness.

7. Based on results of the present study, it is recommended that drift sampling be included as a standard complementary tool to benthic sampling in biological assessments (e.g. bioassessment protocols) of tropical streams, which are often characterized by migratory invertebrate species such as shrimps. Drift samples provide critical information on the presence or absence of shrimps and also on the timing and magnitude of their migration which is an important link between many tropical rivers and their estuaries.
Point vortex dynamics: A classical mathematics playground
Authors: Aref Hassan
Published: June 2007

The idealization of a two-dimensional, ideal flow as a collection of point vortices embedded in otherwise irrotational flow yields a surprisingly large number of mathematical insights and connects to a large number of areas of classical mathematics. Several examples are given including the integrability of the three-vortex problem, the interplay of relative equilibria of identical vortices and the roots of certain polynomials, addition formulas for the cotangent and the Weierstraß ζ function, projective geometry, and other topics. The hope and intent of the article is to garner further participation in the exploration of this intriguing dynamical system from the mathematical physics community.
Studies of perturbed three vortex dynamics
Authors: Blackmore Denis , Ting Lu , Knio Omar
Published: June 2007

It is well known that the dynamics of three point vortices moving in an ideal fluid in the plane can be expressed in Hamiltonian form, where the resulting equations of motion are completely integrable in the sense of Liouville and Arnold. The focus of this investigation is on the persistence of regular behavior (especially periodic motion) associated with completely integrable systems for certain (admissible) kinds of Hamiltonian perturbations of the three vortex system in a plane. After a brief survey of the dynamics of the integrable planar three vortex system, it is shown that the admissible class of perturbed systems is broad enough to include three vortices in a half plane, three coaxial slender vortex rings in three space, and “restricted” four vortex dynamics in a plane. Included are two basic categories of results for admissible perturbations: (i) general theorems for the persistence of invariant tori and periodic orbits using Kolmogorov-Arnold-Moser- and Poincaré-Birkhoff-type arguments and (ii) more specific and quantitative conclusions of a classical perturbation theory nature guaranteeing the existence of periodic orbits of the perturbed system close to cycles of the unperturbed system, which occur in abundance near centers. In addition, several numerical simulations are provided to illustrate the validity of the theorems as well as indicating their limitations as manifested by transitions to chaotic dynamics.
Nonlinear dynamics in combinatorial games: Renormalizing Chomp
Authors: Friedman Eric J. , Landsberg Adam Scott
Published: June 2007

We develop a new approach to combinatorial games that reveals connections between such games and some of the central ideas of nonlinear dynamics: scaling behaviors, complex dynamics and chaos, universality, and aggregation processes. We take as our model system the combinatorial game Chomp, which is one of the simplest in a class of “unsolved” combinatorial games that includes Chess, Checkers, and Go. We discover that the game possesses an underlying geometric structure that “grows” (reminiscent of crystal growth), and show how this growth can be analyzed using a renormalization procedure adapted from physics. In effect, this methodology allows one to transform a combinatorial game like Chomp into a type of dynamical system. Not only does this provide powerful insights into the game of Chomp (yielding a complete probabilistic description of optimal play in Chomp and an answer to a longstanding question about the nature of the winning opening move), but more generally, it offers a mathematical framework for exploring this unexpected relationship between combinatorial games and modern dynamical systems theory.
Electromagnetic wave radiation by an electron beam spiraling in a magnetized plasma column
Authors: Zaboronkova T. M. , Krafft C.
Published: June 2007

The paper studies the electromagnetic wave radiation by a density modulated and thin electron beam of finite length injected obliquely with respect to the constant external magnetic field into a cylindrical plasma column embedded in a homogeneous medium (plasma, dielectric, or free space) and aligned along the magnetic field lines. The time-averaged power radiated at the modulation frequency is determined as a function of the beam, the plasma column, and the medium parameters. Particular attention is devoted to the case when the beam modulation frequency belongs to the whistler frequency band. The paper shows what significant differences exist between the physical features of the emissions when the beam radiates in a plasma column embedded in a homogeneous medium or in a uniform and unbounded magnetized plasma. Based on the results of numerical calculations, the time-averaged power radiated by pulsed and modulated beams has been estimated for typical laboratory plasma experiments. In particular, it is shown that a beam propagating in a plasma column can efficiently enhance its wave emission due to Cherenkov and normal cyclotron excitation of guided whistler modes.
Control of the gas phase and the surface reactions during the high rate synthesis of high quality microcrystalline silicon films: Effects of the source gas supply method and the substrate bias
Authors: Jia Haijun , Shirai Hajime , Kondo Michio
Published: June 2007

In this article, to realize precise control of gas phase and surface reactions for high rate synthesis of high quality microcrystalline silicon (μc-Si) films, influences of source gas supply method and direct current (dc) substrate bias during film deposition process are investigated utilizing a microwave plasma source. It is found that the supply of SiH4 source gas near the substrate holder significantly enhances the deposition rate and mass density of deposited film and reduces film defect density while maintaining high crystallinity. On the other hand, a negative dc substrate bias induces film crystallization from the initial growth stage. An appropriate negative dc bias also leads to higher film cystallinity and density. Kinetics of the controlling gas phase and surface reactions through the optimizing source gas supply method and dc substrate bias is demonstrated. We claim that the high flux film precursor SiH3 and H along with less contribution of short-lifetime radicals on the film growing surface and ion-assist structural relaxation benefit the high-rate synthesis of high quality μc-Si films.
Collisional bulk ion transport and poloidal rotation driven by neutral beam injection
Authors: Newton Sarah L. , Helander Per , Catto Peter J.
Published: June 2007

Neutral beam injection (NBI) is known to significantly affect radial transport in a tokamak plasma. Furthermore, recent observations have shown poloidal velocities, in the presence of NBI, significantly in excess of the standard neoclassical value. Motivated by this, the additional collisional radial bulk ion fluxes of particles, heat and toroidal angular momentum, and the poloidal velocity, driven by fast ions from NBI have been evaluated for a low-collisionality, pure plasma, with strong toroidal rotation and arbitrary aspect ratio. Higher order velocity space structure of the fast ion distribution function can be significant, whilst the effects of toroidal acceleration caused by strong NBI dominate at large aspect ratio. The driven poloidal velocity depends strongly on system parameters, becoming larger at higher beam density and lower beam energy.
Kinetic Alfvén waves and electron physics. II. Oblique slow shocks
Authors: Yin L. , Winske D. , Daughton W.
Published: June 2007

One-dimensional (1D) particle-in-cell (PIC; kinetic ions and electrons) and hybrid (kinetic ions; adiabatic and massless fluid electrons) simulations of highly oblique slow shocks (θBn=84° and β=0.1) [Yin et al., J. Geophys. Res., 110, A09217 (2005)] have shown that the dissipation from the ions is too weak to form a shock and that kinetic electron physics is required. The PIC simulations also showed that the downstream electron temperature becomes anisotropic (Te‖>Te⊥), as observed in slow shocks in space. The electron anisotropy results, in part, from the electron acceleration/heating by parallel electric fields of obliquely propagating kinetic Alfvén waves (KAWs) excited by ion-ion streaming, which cannot be modeled accurately in hybrid simulations. In the shock ramp, spiky structures occur in density and electron parallel temperature, where the ion parallel temperature decreases due to the reduction of the ion backstreaming speed. In this paper, KAW and electron physics in oblique slow shocks are further examined under lower electron beta conditions. It is found that as the electron beta is reduced, the resonant interaction between electrons and the wave parallel electric fields shifts to the tail of the electron velocity distribution, providing more efficient parallel heating. As a consequence, for βe=0.02, the electron physics is shown to influence the formation of a θBn=75° shock. Electron effects are further enhanced at a more oblique shock angle (θBn=84°) when both the growth rate and the range of unstable modes on the KAW branch increase. Small-scale electron and ion phase-space vortices in the shock ramp formed by electron-KAW interactions and the reduction of the ion backstreaming speed, respectively, are observed in the simulations and confirmed in homogeneous geometries in one and two spatial dimensions in the accompanying paper [Yin et al., Phys. Plasmas 14, 062104 (2007)]. Results from this study conclude that the inability of the hybrid method to model the Landau resonance of KAW with electrons and the resulting time-evolving electron parallel heating lead to differences between the PIC and hybrid simulations.
Self-induced transparency scenario revisited via beat-wave heating induced by Doppler shift in overdense plasma layer
Authors: Ghizzo A. , DelSarto D. , Réveillé T. , Besse N. , Klein R.
Published: June 2007

Maxwell-fluid simulations on a flat-topped moderately overdense plasma slab (typically n0∕nc=1–2) by Berezhiani et al. [Phys. Plasmas 66, 062308 (2005)] {see also the previous work of Tushentsov et al. [Phys. Rev. Lett. 87, 275002 (2001)]} were seen to lead to dynamic penetration of an ultrahigh intensity laser pulse into an overdense plasma. Two qualitatively different scenarios for the penetration of laser pulse into the overdense plasma were presented depending on the background density. In the first one, the penetration of laser energy occurs by soliton-like structures moving into the plasma. In the last one, electron cavitation occurs and the penetration is possible over a finite length only. A kinetic extension is made in this paper using Vlasov-Maxwell simulations. Vlasov simulations revealed a rich variety of new phenomena associated with the trapped particle dynamics, which cannot be described in fluid models. Most notably is the observation, during the penetration phase of the pump electromagnetic wave, of a beat-wave heating scenario induced by the Doppler shift on the reflected wave at the (moving) wave front. This beat-wave generates low-frequency acoustic-like electron modes characterized by coherent trapping-type structures in phase space leading to an efficient (nonstochastic) heating process.
Moisture content characterization in composite materials based on ultrasonic transmission measurements
Authors: Fortineau J. , Le Clézio E. , Vander Meulen F. , Feuillard G.
Published: June 2007

Moisture absorption is known to alter many of the mechanical properties of polymer-based composites. This study presents ultrasonic characterization of moisture content based on the transmission of acoustic waves through composite plates. The elastic constants of 14 plates with different controlled water contents are obtained. The redundancy of the results allows to validate the characterization process and the assumption made on the sample symmetry. The sensitivity of the process to moisture content is then discussed.
Biopolymer structure simulation and optimization via fragment regrowth Monte Carlo
Authors: Zhang Jinfeng , Kou S. C. , Liu Jun S.
Published: June 2007

An efficient exploration of the configuration space of a biopolymer is essential for its structure modeling and prediction. In this study, the authors propose a new Monte Carlo method, fragment regrowth via energy-guided sequential sampling (FRESS), which incorporates the idea of multigrid Monte Carlo into the framework of configurational-bias Monte Carlo and is suitable for chain polymer simulations. As a by-product, the authors also found a novel extension of the Metropolis Monte Carlo framework applicable to all Monte Carlo computations. They tested FRESS on hydrophobic-hydrophilic (HP) protein folding models in both two and three dimensions. For the benchmark sequences, FRESS not only found all the minimum energies obtained by previous studies with substantially less computation time but also found new lower energies for all the three-dimensional HP models with sequence length longer than 80 residues.
Why do ultrasoft repulsive particles cluster and crystallize? Analytical results from density-functional theory
Authors: Likos Christos N. , Mladek Bianca M. , Gottwald Dieter , Kahl Gerhard
Published: June 2007

We demonstrate the accuracy of the hypernetted chain closure and of the mean-field approximation for the calculation of the fluid-state properties of systems interacting by means of bounded and positive pair potentials with oscillating Fourier transforms. Subsequently, we prove the validity of a bilinear, random-phase density functional for arbitrary inhomogeneous phases of the same systems. On the basis of this functional, we calculate analytically the freezing parameters of the latter. We demonstrate explicitly that the stable crystals feature a lattice constant that is independent of density and whose value is dictated by the position of the negative minimum of the Fourier transform of the pair potential. This property is equivalent with the existence of clusters, whose population scales proportionally to the density. We establish that regardless of the form of the interaction potential and of the location on the freezing line, all cluster crystals have a universal Lindemann ratio Lf=0.189 at freezing. We further make an explicit link between the aforementioned density functional and the harmonic theory of crystals. This allows us to establish an equivalence between the emergence of clusters and the existence of negative Fourier components of the interaction potential. Finally, we make a connection between the class of models at hand and the system of infinite-dimensional hard spheres, when the limits of interaction steepness and space dimension are both taken to infinity in a particularly described fashion.
Geometrical properties of a “snowflake” divertor
Authors: Ryutov D. D.
Published: June 2007

Using a simple set of poloidal field coils, one can reach the situation in which the null of the poloidal magnetic field in the divertor region is of second order, not of first order as in the usual X-point divertor. Then, the separatrix in the vicinity of the null point splits the poloidal plane not into four sectors, but into six sectors, making the whole structure look like a snowflake (hence the name). This arrangement allows one to spread the heat load over a much broader area than in the case of a standard divertor. A disadvantage of this configuration is that it is topologically unstable, and, with the current in the plasma varying with time, it would switch either to the standard X-point mode, or to the mode with two X-points close to each other. To avoid this problem, it is suggested to have a current in the divertor coils that is roughly 5% higher than in an “optimum” regime (the one in which a snowflake separatrix is formed). In this mode, the configuration becomes stable and can be controlled by varying the current in the divertor coils in concert with the plasma current; on the other hand, a strong flaring of the scrape-off layer still remains in force. Geometrical properties of this configuration are analyzed. Potential advantages and disadvantages of this scheme are discussed.
Ab initio design of picosecond infrared laser pulses for controlling vibrational-rotational excitation of CO molecules
Authors: Herrmann Thomas , Ren Qinghua , Balint-Kurti Gabriel G. , Manby Frederick R.
Published: June 2007

Optimal control of rovibrational excitations of the CO molecule using picosecond infrared laser pulses is described in the framework of the electric-nuclear Born-Oppenheimer approximation [G. G. Balint-Kurti et al., J. Chem. Phys. 122, 084110 (2005)]. The potential energy surface of the CO molecule in the presence of an electric field is calculated using coupled cluster theory with a large orbital basis set. The quantum dynamics of the process is treated using a full three dimensional treatment of the molecule in the laser field. The detailed mechanisms leading to efficient control of the selected excitation processes are discussed.
The laboratory glass transition
Authors: Gupta Prabhat K. , Mauro John C.
Published: June 2007

The phenomenology of the laboratory glass transition is examined in the enthalpy landscape framework. It is shown that a generic description of the glassy state based on partitioning of the phase space caused by the finiteness of the time of observation explains all universal features of glass transition. Using this description of glass, which is referred to as the extrinsically constrained liquid, expressions are derived for properties of glass and for property changes at the laboratory glass transition. A model enthalpy landscape is used to illustrate the basic concepts of this description. Additional new consequences of this description, such as the role of complexity in glass transition and the zero residual entropy of a glass, are discussed.
Translational diffusion in sucrose benzoate near the glass transition: Probe size dependence in the breakdown of the Stokes-Einstein equation
Authors: Rajian Justin Rajesh , Quitevis Edward L.
Published: June 2007

The translational diffusion coefficient Dtrans for rubrene, 9,10-bis(phenylethynyl)anthracene (BPEA), and tetracene in the fragile molecular glass-former sucrose benzoate (SB) (Tg=337K) was studied as a function of temperature from Tg+3KtoTg+71K by use of the holographic fluorescence recovery after photobleaching technique. The values of Dtrans vary by five to six orders of magnitude in this temperature range. Contrary to the predictions of the Stokes-Einstein equation, the temperature dependence of probe diffusion in SB over the temperature range of the measurements is weaker than that of T∕η, where η is the shear viscosity. In going from the crossover temperature Tx≈1.2Tg to Tg, Dtransη∕T increases by factors of 2.4±0.2 decades for rubrene, 3.4±0.2 decades for BPEA, and 3.8±0.4 decades for tetracene. The decoupling between probe diffusion in SB and viscosity is characterized by the scaling law Dtrans∼T∕ηξ, with ξ=0.621 for tetracene, 0.654 for BPEA, and 0.722 for rubrene. Data for probe diffusion in SB are combined with data from the literature for probe diffusion in ortho-terphenyl and ααβ-tris(naphthyl)benzene in a plot of enhancement versus the relative probe size parameter ρm=(mp∕mh)1∕3, where mp and mh are, respectively, the molecular weights of the probe and host solvent. The plot clearly shows a sharp increase in enhancement of translational diffusion at ρm≈1. By applying temperature shifts, Dtrans for probe diffusion in SB and the dielectric relaxation time τD can be superimposed on a single master curve based on the Williams-Landel-Ferry equation. This suggests that the dynamics of probe diffusion in SB is described by the scaling relationship Dtrans∼1∕τD(T+ΔT), where τD(T+ΔT) is the temperature-shifted dielectric relaxation time. The results from this study are discussed within the context of dynamic heterogeneity in glass-forming liquids.
Effect of softness of the potential on the stress anisotropy in liquids
Authors: Alejandre José , Bresme Fernando , González-Melchor Minerva , del Río Fernando
Published: June 2007

We have performed molecular dynamics simulations of dense liquids using nonconformal and Gaussian potential models. We investigate the effect of the softness of the potential on the pressure tensor of liquids and dense fluids when the simulations are carried out using parallelepiped cells. The combination of periodic boundary conditions and small cross sectional areas induces an anisotropy in the diagonal components of the pressure tensor. This anisotropy results in an artificial stress in the system that has to be taken into account in simulations of explicit interfaces, where the artificial stress introduces errors in the computation of the surface tension. At high liquid densities the stress anisotropy exhibits an oscillatory dependence with the cross sectional area of the simulation box. We find that the softness of the potential has a dramatic effect on the amplitude of the oscillations, which can be significantly reduced in soft potentials, such as those used in the modeling of hydrocarbon liquids or polymers.
Numerical simulation of shock wave structure in nitrogen
Authors: Elizarova Tatiana G. , Khokhlov Anton A. , Montero Salvador
Published: June 2007

The one-dimensional problem of the structure of a stationary shock wave in nitrogen is solved in the frame of the Navier-Stokes (NS) equations. Proper interpretation of the bulk viscosity coefficient included in the shear stress tensor leads to a numerical solution close to the experiment, showing that the NS equations provide more accurate solutions to the problem than supposed previously.
Mixing properties of steady flow in thermocapillary driven droplets
Authors: Vainchtein Dmitri L. , Widloski John , Grigoriev Roman O.
Published: June 2007

We consider mixing via chaotic advection in microdroplets suspended at the free surface of a liquid substrate and driven along a straight line using the thermocapillary effect. With the help of a model derived by Grigoriev [Phys. Fluids 17, 033601 (2005)] we show that the mixing properties of the flow inside the droplet can vary dramatically as a function of the physical properties of the fluids and the imposed temperature profile. Proper characterization of the mixing quality requires introduction of two different metrics. The first metric determines the relative volumes of the domain of chaotic streamlines and the domain of regular streamlines. The second metric describes the time for homogenization inside the chaotic domain. We compute both metrics using perturbation theory in the limit of weak temperature dependence of the surface tension coefficient at the free surface of the substrate.
Nonlinear electrostatic structures in the presence of correlations
Authors: Bose Anirban , Janaki M. S.
Published: June 2007

The nature of nonlinear electrostatic potential distribution in a dusty plasma is investigated in the presence of dust-dust correlations by developing an equilibrium kinetic equation that contains the effects of pair correlations. For a plasma in equilibrium, the role of pair correlations is to give rise to a force in the kinetic equation that is proportional to the dust density gradient. The solutions of such a kinetic equation with pair correlations and Poisson’s equation in the presence of a trapped particle population are obtained in the small-amplitude limit. The electrostatic potential represents a localized solitary wave-like structure with the amplitude and width varying with the correlation parameter.
Dispersion of linear waves in quantum plasmas
Authors: Ren Haijun , Wu Zhengwei , Chu Paul K.
Published: June 2007

The dispersion of linear waves in a uniform cold quantum plasma is derived using the quantum hydrodynamic equations with the magnetic field of the Wigner-Poisson system. The dispersion of the Langmuir wave becomes whistler-like due to quantum effects and, therefore, the Langmuir wave can propagate in a cold plasma. It is also found that quantum effects do not affect the dispersion of the left-handed, right-handed, and ordinary waves.
ZnO-based p-i-n and n-i-p heterostructure ultraviolet sensors: a comparative study
Authors: Wang Kai , Vygranenko Yuriy , Nathan Arokia
Published: June 2007

A comparative study is reported on p-NiO∕i-ZnO∕n-ITO and n-ITO∕i-ZnO∕p-NiO heterostructure ultraviolet sensors. In comparing reverse-bias current-voltage characteristics, dark current transients, and dark-current field dependence of both diodes, we observed that thermal and Poole–Frenkel generation currents dominate in the leakage. The difference in the leakage level and field dependence essentially depends on the p-i interface. Analysis of forward-bias current-voltage characteristics identifies the difference in Schottky barrier height between two diodes, which causes the difference in the contact leakage level as well. Furthermore, the p-i-n diode has better performance in photosensitivity and responsivity due to less optical loss in the top contact, and both diodes are promising for low levels of ultraviolet detection.
Comprehensive analysis and optimal design of top-emitting organic light-emitting devices
Authors: Chen Xue-Wen , Choy Wallace C. H. , He Sailing , Chui P. C.
Published: June 2007

We present an accurate analysis of light emission in top-emitting organic light-emitting devices (TOLEDs) by explicitly considering the Purcell effect. TOLEDs are optimized separately for maximum zero-degree luminance, maximum electroluminescence (EL) efficiency, and wide viewing angle with high EL efficiency. For fluorescent material with an internal quantum efficiency (ηint0) of 0.25, the maximum zero-degree luminance and EL efficiency can be achieved by locating the emitters around the first antinode of the microcavity while for phosphorescent material with ηint0=1.0, the maximum zero-degree luminance and EL efficiency are around the second antinode. Through relaxing the efficiency by 10%–20%, the angular intensity distribution can be even better than the Lambertian distribution; meanwhile, the color shows only a small variation over an angle range of 150°. Our results, which are in good agreement with experiments, show that the Purcell effect on TOLED performances is significant and should be carefully examined in studying TOLEDs.
Comparison of efficiencies of GaAs-based pulsed terahertz emitters
Authors: Reklaitis A.
Published: June 2007

Pulsed terahertz (THz) radiation from the electron-hole plasma excited by a femtosecond optical pulse in GaAs-based emitters is studied by Monte Carlo simulations. The THz energy radiated from the n- and p-doped surface emitters, from the contactless p-i-n emitter, and from the photoconductive emitter is evaluated. It is found that the THz energy radiated by photoconductive emitter exceeds the energy radiated by the surface and p-i-n THz emitters by more than one order. The results of the simulations show that the efficiencies of the surface emitters are below the efficiency of the p-i-n emitter.
Model for solid-liquid and solid-solid friction of rough surfaces with adhesion hysteresis
Authors: Nosonovsky Michael
Published: June 2007

Mechanisms of energy dissipation during solid-solid and solid-liquid friction are discussed. A conservative van der Waals adhesion force, when combined with surface imperfectness, such as deformation, leads to adhesion hysteresis (AH). When an asperity slides upon a substrate, the substrate is subjected to a loading-unloading cycle, and energy is dissipated due to the AH. Another mechanism, which leads to energy dissipation, involves energy barriers between metastable states due to surface roughness. Both mechanisms are fundamental for sliding and result in both solid-liquid and solid-solid friction.
Self-similarity of coflowing jets: The virtual origin
Authors: Uddin Mesbah , Pollard Andrew
Published: June 2007

From round and plane jet experimental data, it can be inferred that the existence of the virtual origin can be attributed to variations in thickness of the internal boundary layer at the orifice. Based on this, and further support from large eddy simulations (LES) of axisymmetric coflowing round jets with various inlet conditions, we propose that the local value of the variance in the thickness of the boundary layer and thus the velocity distribution at the jet inlet can be used to predict the location of the virtual origin. This, in turn, indicates the inadequacy of the traditionally used length scale, the jet half-width/radius, and suggests the local variance is a length scale with better physical significance. It appears unlikely that the turbulence intensity distribution at the orifice plays a significant role in the determination of the virtual origin or the axial spread of the effective jet width.
Design of a scanning probe microscope with advanced sample treatment capabilities: An atomic force microscope combined with a miniaturized inductively coupled plasma source
Authors: Hund Markus , Herold Hans
Published: June 2007

We describe the design and performance of an atomic force microscope (AFM) combined with a miniaturized inductively coupled plasma source working at a radio frequency of 27.12MHz. State-of-the-art scanning probe microscopes (SPMs) have limited in situ sample treatment capabilities. Aggressive treatments such as plasma etching or harsh treatments such as etching in aggressive liquids typically require the removal of the sample from the microscope. Consequently, time consuming procedures are required if the same sample spot has to be imaged after successive processing steps. We have developed a first prototype of a SPM which features a quasi in situ sample treatment using a modified commercial atomic force microscope. A sample holder is positioned in a special reactor chamber; the AFM tip can be retracted by several millimeters so that the chamber can be closed for a treatment procedure. Most importantly, after the treatment, the tip is moved back to the sample with a lateral drift per process step in the 20nm regime. The performance of the prototype is characterized by consecutive plasma etching of a nanostructured polymer film.
Spectroscopic constants and potential energy curves of yttrium carbide (YC)
Authors: Suo Bingbing , Balasubramanian Krishnan
Published: June 2007

The potential energy curves of the low-lying electronic states of yttrium carbide (YC) and its cation are calculated at the complete active space self-consistent field and the multireference single and double excitation configuration interaction (MRSDCI) levels of theory. Fifteen low-lying electronic states of YC with different spin and spatial symmetries were identified. The XΣ−4 state prevails as the ground state of YC, and a low-lying excited AΠ4 state is found to be 1661cm−1 higher at the MRSDCI level. The computations of the authors support the assignment of the observed spectra to a BΔ4(Ω=7∕2)←AΠ4(Ω=5∕2) transition with a reinterpretation that the AΠ4 state is appreciably populated under the experimental conditions as it is less than 2000cm−1 of the XΣ−4 ground state, and the previously suggested Π4 ground state is reassigned to the first low-lying excited state of YC. The potential energy curves of YC+ confirm a previous prediction by Seivers et al. [J. Chem. Phys. 105, 6322 (1996)] that the ground state of YC+ is formed through a second pathway at higher energies. The calculated ionization energy of YC is 6.00eV, while the adiabatic electron affinity is 0.95eV at the MRSDCI level. The computed ionization energy of YC and dissociation energy of YC+ confirm the revised experimental estimates provided by Seivers et al. although direct experimental measurements yielded results with greater errors due to uncertainty in collisional cross sections for YC+ formation.
Structure factor for hard hyperspheres in higher dimensions
Authors: Whitlock Paula A. , Bishop Marvin , Tiglias John L.
Published: June 2007

The structure factor for hard hyperspheres in two to eight dimensions is computed by Fourier transforming the pair correlation function obtained by computer simulation at a variety of densities. The resulting structure factors are compared to the known Percus-Yevick equations for odd dimensions and to the model proposed by Leutheusser [J. Chem. Phys. 84, 1050 (1986)] and Rosenfeld [J. Chem. Phys. 87, 4865 (1987)] in even dimensions. It is found that there is fine agreement among all these approaches at low to moderate densities but that the accuracy of the analytical models breaks down as the freezing transition is approached. The structure factor gives another insight into the decrease in the ordering of the hyperspheres as the dimension is increased.
Realization of biferroic properties in La0.6Sr0.4MnO3∕0.7Pb(Mg1∕3Nb2∕3)O3–0.3(PbTiO3) epitaxial superlattices
Authors: Roy Chaudhuri Ayan , Ranjith R. , Krupanidhi S. B. , Mangalam R. V. K. , Sundaresan A. , Majumdar S. , Ray S. K.
Published: June 2007

A set of symmetric and asymmetric superlattices with ferromagnetic La0.6Sr0.4MnO3 (LSMO) and ferroelectric 0.7Pb(Mg1∕3Nb2∕3)O3–0.3(PbTiO3) as the constituting layers was fabricated on LaNiO3 coated (100) oriented LaAlO3 substrates using pulsed laser ablation. The crystallinity, and magnetic and ferroelectric properties were studied for all the superlattices. All the superlattice structures exhibited a ferromagnetic behavior over a wide range of temperatures between 10 and 300K, whereas only the asymmetric superlattices exhibited a reasonably good ferroelectric behavior. Strong influence of an applied magnetic field was observed on the ferroelectric properties of the asymmetric superlattices. Studies were conducted toward understanding the influence of conducting LSMO layers on the electrical responses of the heterostructures. The absence of ferroelectricity in the symmetric superlattice structures has been attributed to their high leakage characteristics. The effect of an applied magnetic field on the ferroelectric properties of the asymmetric superlattices indicated strong influence of the interfaces on the properties. The dominance of the interface on the dielectric response was confirmed by the observed Maxwell-Wagner-type dielectric relaxation in these heterostructures.
Operation of silicon single photon avalanche diodes at cryogenic temperature
Authors: Rech Ivan , Labanca Ivan , Armellini Giacomo , Gulinatti Angelo , Ghioni Massimo , Cova Sergio
Published: June 2007

This article reports a complete characterization of single photon avalanche diodes (SPADs) at temperatures down to 120K. We show that deep cooling of the device by means of a compact liquid-nitrogen Dewar brings several advantages, such as extremely low dark counting rates (down to 1counts∕s), better time resolution, and higher quantum efficiency in the visible range. By using a special current pick-off circuit, we achieved a time resolution of 20ps full width at half maximum at 120K for a 50μm diameter SPAD. Afterpulsing effects are avoided by using a sufficiently long hold-off time (microseconds).
Phase diagram and surface tension of the hard-core attractive Yukawa model of variable range: Monte Carlo simulations
Authors: Duda Yurko , Romero-Martínez Ascención , Orea Pedro
Published: June 2007

The liquid-vapor phase diagram and surface tension for hard-core Yukawa potential with 4⩽κ⩽7 are calculated by applying canonical Monte Carlo simulation. The authors’ new simulation results are more precise than those reported before, which allows them to affirm the accuracy of the previously reported self-consistent Ornstein-Zernike approximation calculations for κ=5 and 7.
A Fokker-Planck study of the eigenmodes in an unmagnetized pair plasma
Authors: Zhao Bin , Zheng Jian
Published: June 2007

Linearized Fokker-Planck equations for unmagnetized pair plasmas are solved as an eigenvalue problem to investigate the sound waves and Langmuir waves. The frequencies and damping rates of sound waves and Langmuir waves as a function of kλ and kλD are presented, where k is the wave number, λ is the mean-free path, and λD is the Debye length. It is found that no electrostatic field is evolved in the process of sound wave. As a consequence, Landau damping is not relevant to the sound waves in a pair plasma. The damping mechanics of sound waves is fully governed by the Coulomb collisions. The valid regimes of fluid descriptions for the waves are also discussed by comparing with the computational results.
Three-dimensional gold nanoparticle superlattices: Structures and optical absorption characteristics
Authors: Oonishi Tsuguo , Sato Seiichi , Yao Hiroshi , Kimura Keisaku
Published: June 2007

We synthesized micrometer-sized three-dimensional superlattices consisting of gold (Au) nanocrystals modified with three types of water-soluble thiolates in different dimensions: mercaptosuccinic acid (MSA), N-(2-mercaptopropionyl)-glysine (MPG), and glutathione in its reduced form (GTR). The nanoparticles were assembled at an air/water interface by decreasing the repulsive interactions between the surface modifiers. This was accomplished by adjusting acid concentration. Using x ray and transmission electron diffractometry, MSA- and MPG-modified Au nanoparticle superlattices were identified as hcp single-crystal structures, while the GTR-modified nanoparticle superlattices were determined to be a fcc single-crystal structure. Optical absorption studies show that the surface plasmon peaks redshifted as the nanoparticle density of the assemblages increased and the peak widths decreased when the nanoparticles formed lattice arrangements. In contrast, optical absorption studies of randomly assembled nanoparticles demonstrated increased peak widths as nanoparticle density increased. To explain these observed characteristic features in the surface plasmons, we introduced a qualitative model for the electronic polarizability of these superlattices.
Optical characterization of Si1−xGex nanodots grown on Si substrates via ultrathin SiO2 buffer layers
Authors: Poweleit C. D. , Hu C.-W. , Tsong I. S. T. , Tolle J. , Kouvetakis J.
Published: June 2007

Growth of Si1−xGex nanodots with x=0, 0.33, 0.67, and 1.0 was accomplished on ultrathin SiO2 buffer layers of 1–2 ML on Si(001) and Si(111) substrates using single-source gaseous precursors at 550°C. The Si1−xGex dots have diameters of ∼10nm and an areal density of ∼1011cm−2. Raman spectroscopy conducted on the nanodots shows that they are relaxed and their compositions correlate closely with the molecular ratios in the precursors used in their fabrication. Photoluminescence (PL) spectra were taken with reduced laser power density which enhanced the PL contribution from the nanodots while suppressing the PL contribution from the Si substrate. Two groups of PL peaks were observed, in the ranges of 0.8–1.0 and 1.0–1.1eV. The first group in the 0.8–1.0eV range shows peaks similar to those observed in Si with dislocations. The second group of peaks at the 1.0–1.1eV range shows an increase in intensity with increasing Ge concentration in the dots. However, both groups of peaks appear to be reproducible in Si substrates after conventional flash cleaning at 1150°C with no nanodots present. Since there is no evidence that the defect density in Si has increased after flash cleaning, the reason for their appearance is as yet undetermined.
On the definition of effective permittivity and permeability for thin composite layers
Authors: Saenz Elena , Ikonen Pekka M. T. , Gonzalo Ramon , Tretyakov Sergei A.
Published: June 2007

The definition of effective material parameters (permittivity and permeability) for composite layers containing only one-two parallel arrays of complex-shaped inclusions is discussed. Such structures are of high importance for the design of novel metamaterials, where the realizable layers quite often have only one or two layers of particles across the sample thickness. Effective parameters which describe the averaged induced polarizations are introduced. As an explicit example, we develop an analytical model suitable for calculation of the effective material parameters εeff and μeff for double arrays of electrically small, electrically polarizable scatterers. Electric and magnetic dipole moments induced in the structure and the corresponding reflection and transmission coefficients are calculated using the local field approach for the normal plane-wave incidence, and effective parameters are introduced through the averaged fields and polarizations. In the absence of losses both material parameters are purely real and satisfy the Kramers-Kronig relations and the second law of thermodynamics. We compare the analytical results to the simulated and experimental results available in the literature. The physical meaning of the introduced parameters is discussed in detail.