bluuebunny/crossref_metadata_2025 huggingface dataset:
On the advantage over a random assignment
Authors: Håstad Johan , Venkatesh S.
Published: June 2004

We initiate the study of a new measure of approximation. This measure compares the performance of an approximation algorithm to the random assignment algorithm. This is a useful measure for optimization problems where the random assignment algorithm is known to give essentially the best possible polynomial time approximation. In this paper, we focus on this measure for the optimization problems Max‐Lin‐2 in which we need to maximize the number of satisfied linear equations in a system of linear equations modulo 2, and Max‐k‐Lin‐2, a special case of the above problem in which each equation has at most k variables. The main techniques we use, in our approximation algorithms and inapproximability results for this measure, are from Fourier analysis and derandomization. © 2004 Wiley Periodicals, Inc. Random Struct. Alg., 2004
Integrating diverse paradigms in evolution and maintenance by an XML‐based unified model
Authors: Lu Chih‐Wei , Chu William C. , Chang Chih‐Hung , Yang Don‐Lin , Lian Wen‐Da
Published: June 2004

Many disparate software analysis/design methods and tools (called paradigms in this paper) individually offer faster and more efficient software design/evolution, yet they are generally partially incompatible. Therefore, when they are used together, they suffer from a lack of communication and integration. Unanticipated and inadvertent inconsistencies arising from the implicit incompatibilities result in significantly increased costs and errors during software maintenance. To improve both software evolution and maintenance, we propose an XML‐based model that effectively unifies and integrates current software paradigms while helping reduce ripple effects and related software maintenance difficulties. Copyright © 2003 John Wiley & Sons, Ltd.
Architecture‐based semantic evolution of embedded remotely controlled systems
Authors: Chung Lawrence , Subramanian Nary
Published: June 2004

Evolution of a software system is a natural process. In most systems, evolution takes place during the maintenance phase of their life cycles. Those systems that have reached their limit in evolution have usually reached their end of useful life and may have to be replaced. However, there are systems in which evolution occurs during the operational phase of their life cycles. Such systems are designed to evolve while in use or, in other words, be adaptable. Semantically adaptable systems are of particular interest to industry as such systems often times adapt themselves to environment change with little or no intervention from their developing or maintaining organization. Since embedded systems usually have a restricted hardware configuration, it is difficult to apply the techniques developed for non‐embedded systems directly to embedded systems. This paper focuses on evolution through adaptation and develops the concepts and techniques for semantic evolution in embedded systems. As the first step in the development of a software solution, architectures of software systems themselves have to be made semantically evolvable. In this paper we explore various architectural alternatives for the semantic evolution of embedded systems—these architectures are based on four different techniques that we have identified for semantic evolution in embedded systems. The development of these architectures follows the systematic process provided by the non‐functional requirement (NFR) framework, which also permits the architectures to be rated in terms of their evolvability. As the field of embedded systems is vast, this paper concentrates on those embedded systems that can be remotely controlled. In this application domain the embedded system is connected to an external controller by a communication link such as ethernet, serial, radio frequency, etc., and receives commands from and sends responses to the external controller via the communication link. The architectures developed in this paper have been partly validated by applying them in a real embedded system—a test instrument used for testing cell phones. These architectures and techniques for semantic evolution in this application domain give a glimpse of what can be done in achieving semantic evolution in software‐implemented systems. Copyright © 2003 John Wiley & Sons, Ltd.
Decreased plasma membrane targeting of NMDA‐NR1 receptor subunit in dendrites of medial nucleus tractus solitarius neurons in rats self‐administering morphine
Authors: Glass Michael J. , Kruzich Paul J. , Kreek Mary Jeanne , Pickel Virginia M.
Published: June 2004

Opioid abuse is associated with repeated administration and escalation of dose that can result in profound adaptations in homeostatic processes. Potential cellular mechanisms and neural sites mediating opiate‐dependent adaptations may involve NMDA‐dependent synaptic plasticity within brain areas participating in behaviors related to consumption of natural reinforcers, as well as affective‐autonomic integration, notably the medial nucleus tractus solitarius (mNTS). NMDA‐dependent synaptic plasticity may be mediated by changes in the intracellular and surface targeting of NMDA receptors, particularly in postsynaptic sites including spines or small distal dendrites. High‐resolution immunogold electron microscopic immunocytochemistry combined with morphometry were used to measure changes in targeting of the NMDA‐NR1 (NR1) receptor subunit between intracellular and plasmalemmal sites in dendrites of neurons of the intermediate mNTS of rats self‐administering escalating doses of morphine (EMSA). In control and EMSA rats, the density of plasmalemmal and cytosolic gold particles was inversely related to profile size. Collapsed across all NR1‐labeled dendrites, rats self‐administering morphine had a lower number of plasmalemmal gold particles per unit surface area (7.1 ± 0.8 vs. 14.4 ± 1 per 100 μm), but had a higher number of intracellular gold particles per unit cross‐sectional area (169 ± 6.1 vs. 148 ± 5.1 per 100 μm2) compared to saline self‐administering rats. Morphometric analysis showed that the decrease in plasma membrane labeling of NR1 was most robust in small dendritic profiles (<1 μm), where there was a reciprocal increase in the density of intracellular particles. These results indicate that the plasmalemmal distribution of the essential NR1 subunits in distal sites may prominently contribute to NMDA receptor‐dependent modulation of neural circuitry regulating homeostatic processes, and targeting of these proteins can be prominently affected by morphine self‐administration. Synapse 53:191–201, 2004. © 2004 Wiley‐Liss, Inc.
Microearthquake seismicity of the Mid‐Atlantic Ridge at 5°S: A view of tectonic extension
Authors: Tilmann Frederik , Flueh Ernst , Planert Lars , Reston Tim , Weinrebe Wilhelm
Published: June 2004

We report measurements made with an ocean bottom array which was operated for 10 days on the Mid‐Atlantic Ridge just south of the 5°S transform fault/fracture zone. A total of 148 locatable earthquakes with magnitudes ∼0.5–2.8 were recorded; seismic activity appears to be concentrated within the western half of the median valley. The median valley seismic zone is bounded in along‐axis direction by the transform fault to the north and the tip of the axial volcanic ridge to the south. A few scattered events occurred within the inside corner high, on the transform fault, and in the western sidewall close to the segment center. Earthquakes reach a maximum depth of 8 km below the median valley floor and appear to be predominantly in the mantle, although a few crustal earthquakes also occurred. The presence of earthquakes in the mantle indicates that it is not strongly serpentinized. We infer the median valley seismic activity to primarily arise from normal faulting.
Optimal dynamic pricing of water in the presence of waterlogging and spatial heterogeneity of land
Authors: Xabadia A. , Goetz R. , Zilberman D.
Published: June 2004

Agricultural production is a major contributor to numerous environmental problems. Most of these problems have a dynamic aspect, and moreover, the magnitude of these environmental problems depends on the distribution of the characteristics of the land. Therefore policies designed to establish the social outcome need to be simultaneously targeted site specifically and time specifically. This paper integrates both aspects and presents a theoretical model that allows us to determine the socially optimal outcome over time and space. Moreover, the applicability of our approach, defined as optimal control in two stages, is demonstrated by reformulating the mathematical model such that it can be solved with standard mathematical software. For this purpose, we present an empirical study based on the cotton produced in the San Joaquin Valley in California and determine the socially optimal water price in the presence of waterlogging.
A concept of maximum stream depletion rate for leaky aquifers in alluvial valleys
Authors: Zlotnik Vitaly A.
Published: June 2004

Existing analytical models for evaluating stream depletion by wells in alluvial aquifers are based on the assumption that stream depletion supplies 100% of groundwater withdrawals. Analysis of specific hydrostratigraphic conditions in leaky aquifers indicates that stream depletion may range from 0 to 100%. A new concept of maximum stream depletion rate (MSDR) is introduced and defined as a maximum fraction of the pumping rate contributed by the stream depletion. Several new analytical solutions indicate that the MSDR is determined by aquifer hydrostratigraphic conditions, geometry of recharge and discharge zones, and locations of pumping wells.
How important is nucleation in regional/global modelling?
Authors: Pirjola L. , Lehtinen K. E. J. , Hansson H.‐C. , Kulmala M.
Published: June 2004

Modelling nucleation and subsequent growth is a difficult task especially in global and regional models. Therefore it is important to know under which conditions the freshly nucleated particles are able to survive the coagulation barrier and grow to the Aitken mode sizes and further to cloud condensation nuclei thus affecting climate. By using a sectional aerosol dynamics model AEROFOR at various conditions, the regions of parameter space in which nucleation plays an important role are identified and the contour plots are presented. Sulphuric acid is assumed to participate in both nucleation and condensation processes, additionally there is present some other condensable vapour. Our simulations show that nucleation is almost always an important process in the atmosphere excluding the cases when condensable vapour concentration is not high enough so that the nucleated particles have time to coagulate away before reaching the Aitken mode sizes. Thus it is well established to include nucleation in regional/global models.
Titan's haze structure in 1999 from spatially‐resolved narrowband imaging surrounding the 0.94 μm methane window
Authors: Anderson C. M. , Chanover N. J. , McKay C. P. , Rannou P. , Glenar D. A. , Hillman J. J.
Published: June 2004

We use narrowband images of Titan obtained in November 1999 to explore the haze vertical structure in Titan's lower atmosphere. The images were taken with the Mount Wilson 2.54 m telescope using the NASA/GSFC‐built Acousto‐Optic Imaging Spectrometer. These images were recorded at five wavelengths surrounding the 0.94 μm methane window and are sensitive to a range of altitudes in Titan's lower atmosphere. We characterize Titan's limb darkening by fitting the Minnaert function limb darkening coefficient k to our data at 0° latitude and compare our observed limb darkening to that predicted from three radiative transfer models. The vertical haze profiles suggest a gap in haze below 75 km, with an increase in haze abundance near the surface.
Evolution of the Lofoten‐Vesterålen margin inferred from gravity and crustal modeling
Authors: Braun Alexander , Marquart Gabriele
Published: June 2004

The Lofoten‐Vesterålen continental margin in Norway shows pronounced elongated positive and negative gravity anomalies. The source of these anomalies cannot be explained by the flat bathymetry, but can be explained by the crustal density anomalies that are observed. We propose a boudinage mechanism to explain the generation of these density anomalies and respective crustal layer undulations. Free‐air gravity anomalies have been derived using ERS‐1 and Geosat altimetry data and have been validated by comparison with shipborne data. Structural models of the margin are available from extensive seismic surveys, which have been used to carry out a gravity inversion to cross‐validate the observed gravity anomalies with seismic data. The crustal structure data reveal two wavelengths of crustal undulations. The first is related to crustal blocks separated by listric faults, and the second wavelength is related to an undulated Moho. The undulations were likely generated during the opening of the North Atlantic, 56 Myr ago, and were particularly formed during the transition phase between rifting and spreading. The dominating wavelengths of the undulations are 46 and 109 km, respectively. To explain the evolution of crustal undulations in an extensional stress regime, we use a two‐dimensional finite element model. A viscoelastoplastic power law rheology is used to study the growth of instabilities developing in a three‐layer lithosphere. Prescribed random vertical undulations show wavelength‐dependent growth under extension, and the dominating wavelengths are 18–65 and 129 km, respectively. These results are in good agreement with the gravity observations and provide a strong indication that the proposed boudinage mechanism influences the evolution of the passive margin segment.
Effect of upper airway obstruction on blood pressure variability after stroke
Authors: TURKINGTON Peter M. , BAMFORD John , WANKLYN Peter , ELLIOTT Mark W.
Published: June 2004

Approx. 60% of acute stroke patients have periods of significant UAO (upper airway obstruction) and this is associated with a worse outcome. UAO is associated with repeated fluctuation in BP (blood pressure) and increased BP variability is also associated with a poor outcome in patients with acute stroke. UAO-induced changes in BP, at a time when regional cerebral perfusion is pressure-dependent in areas of critically ischaemic brain, could explain the detrimental effect of UAO on outcome in these patients. The aim of the present study was to examine the relationship between UAO and BP variability in patients with acute stroke. Twelve acute stroke patients and 12 age-, sex- and BMI (body mass index)-matched controls underwent a sleep study with non-invasive continuous monitoring of BP to assess the impact of UAO on BP control after stroke. Stroke patients had significantly more 15 mmHg dips in BP/h than the controls (51 compared with 6.7 respectively; P<0.004). Stroke patients also demonstrated significantly higher BP variability than the controls (26.8 compared with 14.4 mmHg; P<0.001). There were significantly more 15 mmHg dips in BP/h in stroke patients who had significant UAO than those who did not (85.7 compared with 29.5 respectively; P<0.032). Furthermore, stroke patients without UAO (RDI <10, where RDI is respiratory disturbance index) had significantly more 15 mmHg dips in BP/h than the controls (29.5 compared with 6.7 respectively; P<0.037). There was a positive correlation between the severity of UAO (RDI) and 15 mmHg dips in BP/h (r=0.574, P<0.005) in stroke patients. Our results suggest that UAO alone does not explain BP variation post-stroke, but it does play an important role, particularly in determining the severity of the BP fluctuation.
Monte Carlo simulations of critical cluster sizes and nucleation rates of water
Authors: Merikanto Joonas , Vehkamäki Hanna , Zapadinsky Evgeni
Published: June 2004

We have calculated the critical cluster sizes and homogeneous nucleation rates of water at temperatures and vapor densities corresponding to experiments by Wölk and Strey [J. Phys. Chem B 105, 11683 (2001)]. The calculations have been done with an expanded version of a Monte Carlo method originally developed by Vehkamäki and Ford [J. Chem. Phys. 112, 4193 (2000)]. Their method calculates the statistical growth and decay probabilities of molecular clusters. We have derived a connection between these probabilities and kinetic condensation and evaporation rates, and introduce a new way for the calculation of the work of formation of clusters. Three different interaction potential models of water have been used in the simulations. These include the unpolarizable SPC/E [J. Phys. Chem. 91, 6269 (1987)] and TIP4P [J. Chem. Phys. 79, 926 (1983)] models and a polarizable model by Guillot and Guissani [J. Chem. Phys. 114, 6720 (2001)]. We show that TIP4P produces critical cluster sizes and a temperature and vapor density dependence for the nucleation rate that agree well with the experimental data, although the magnitude of nucleation rate is constantly overestimated by a factor of 2×104. Guissani and Guillot’s model is somewhat less successful, but both the TIP4P and Guillot and Guissani models are able to reproduce a much better experimental temperature dependency of the nucleation rate than the classical nucleation theory. Using SPC/E results in dramatically too small critical clusters and high nucleation rates. The water models give different average binding energies for clusters. We show that stronger binding between cluster molecules suppresses the decay probability of a cluster, while the growth probability is not affected. This explains the differences in results from different water models.
Exact solution of the excited-state geminate A*+B⇄C*+D reaction with two different lifetimes and quenching
Authors: Park Soohyung , Shin Kook Joe , Agmon Noam
Published: June 2004

The authors obtain, in the Laplace transform space, the exact analytic solution for the Green function and survival probabilities for the excited-state diffusion-influenced reversible geminate reaction, A*+B⇄C*+D, with two different lifetimes and in the presence of an added quenching process. This extends a previous investigation by Popov and Agmon [J. Chem. Phys. 117, 5770 (2002)] of the ground-state reaction without quenching. The long-time asymptotic behavior of the survival probabilities is obtained in the time domain. It is found to be different from the equal-lifetime case. This paper also provides a useful short-time approximation for the kinetics.
Internal structure of dendrimers in the melt under shear: A molecular dynamics study
Authors: Bosko Jaroslaw T. , Todd B. D. , Sadus Richard J.
Published: June 2004

The molecular structure of fluids composed of dendrimers of different generations is studied using nonequilibrium molecular dynamics (NEMD). NEMD results for dendrimer melts undergoing planar Couette flow are reported and analyzed with particular attention paid to the shear-induced changes in the internal structure of dendrimers. The radii of gyration, pair distribution functions and the fractal dimensionality of the dendrimers are determined at different strain rates. The location of the terminal groups is analyzed and found to be uniformly distributed throughout the space occupied by the molecules. The fractal dimension as a function of strain rate displays crossover behavior analogous to the Newtonian/non-Newtonian transition of shear viscosity.
The structure of calcium–ammonia solutions by neutron diffraction
Authors: Wasse Jonathan C. , Howard Chris A. , Thompson Helen , Skipper Neal T. , Delaplane Robert G. , Wannberg Anders
Published: June 2004

The microscopic structures of calcium–ammonia solutions have been established by using neutron diffraction. Total structure factors measured at 230 K reveal immediately the evolution of an uncommonly intense diffraction prepeak in the metallic solutions. As concentration is increased from 4 mole percent metal to 10 mole percent metal (i.e., saturation), this feature intensifies and shifts from 0.6 to 0.9 Å−1. It is therefore evidence of well developed intermediate-range ordering among the solvated cations, and is a microstructural signature of the observed strong phase separation of metallic (concentrated) and nonmetallic (dilute) solutions. The technique of isotopic labelling of *N by N15 was then used in conjunction with difference analysis to focus on the solvent structure in metallic solutions at 4 and 10 mole percent metal. These nitrogen-centered functions are analyzed in conjunction with classical Monte Carlo computer simulation techniques, to provide us with detailed insight into the calcium solvation and the extent of hydrogen bonding. We find that calcium is solvated by approximately 6–7 ammonia molecules, with a Ca–N distance of around 2.45 Å. There is evidence of hydrogen bonding among the solvent molecules, even in the saturated 10 mole percent metal solution.
Theory of polarization dependent intersubband transitions in p-type SiGe/Si self-assembled quantum dots
Authors: Lin Yih-Yin , Singh Jasprit
Published: June 2004

Electronic and optoelectronic properties of SiGe/Si self-assembled quantum dots are calculated by the eight-band k⋅p method with a revised set of parameters. The model confirms that the Si1−xGex transforms to a type-II structure when x is greater than 0.25 and given accurate effective masses for Si and Ge. The polarization dependent absorption spectra show a behavior quite different from what is seen in conduction band intersubband transitions in self-assembled InGaAs/GaAs dots. In-plane or x-polarized absorption increases as germanium content is increased but z-polarized absorption is highest for low germanium content. It is also shown that the z-polarized absorption can be of the same magnitude as in the x-direction by adjusting the dot composition. We also clarify how the envelope functions and the Block parts of the electronic states contribute to the absorption spectra.
A simple model for NMR relaxation in the presence of internal motions with dynamical coupling
Authors: Abergel Daniel , Bodenhausen Geoffrey
Published: June 2004

In this paper some effects of dynamical coupling between two interaction-carrying vectors on the internal auto- and cross-correlation functions are investigated in the limit of small amplitude motions. A linearized Langevin approach allows the derivation of explicit expressions for these correlation functions and for the corresponding order parameters.
Equilibrium crystal shape of GaAs in nanoscale patterned growth
Authors: Lee S. C. , Brueck S. R. J.
Published: June 2004

The equilibrium crystal shape (ECS) of GaAs homoepitaxially grown on a nanoscale SiO2-patterned (001) plane by molecular beam epitaxy is investigated. A GaAs epilayer selectively grown on a nanoscale area bounded by a circular SiO2 mask undergoes faceting, resulting in a pyramidal shape with {110} sidewalls. Growth is slowed or terminated with the generation of these {110} facets even with a continuing supply of Ga atoms. This implies that the pyramidal shape is energetically very stable. Based on experimental results and the Wulff construction, a {110}-type sidewall pyramid is proposed as an ECS of GaAs on (001) in nanoscale patterned growth.
Comment on “The incomplete beta function law for parallel tempering sampling of classical canonical systems” [J. Chem. Phys. 120, 4119 (2004)]
Authors: Kofke David A.
Published: June 2004

We explain the apparent discrepancy between the analysis of the indicated article and that of earlier work cited by it, and present an asymptotic formula that encompasses limiting behaviors described in both.
Quantum-classical dynamics of scattering processes in adiabatic and diabatic representations
Authors: Puzari Panchanan , Sarkar Biplab , Adhikari Satrajit
Published: June 2004

We demonstrate the workability of a TDDVR based [J. Chem. Phys. 118, 5302 (2003)], novel quantum-classical approach, for simulating scattering processes on a quasi-Jahn–Teller model [J. Chem. Phys. 105, 9141 (1996)] surface. The formulation introduces a set of DVR grid points defined by the Hermite part of the basis set in each dimension and allows the movement of grid points around the central trajectory. With enough trajectories (grid points), the method converges to the exact quantum formulation whereas with only one grid point, we recover the conventional molecular dynamics approach. The time-dependent Schrödinger equation and classical equations of motion are solved self-consistently and electronic transitions are allowed anywhere in the configuration space among any number of coupled states. Quantum-classical calculations are performed on diabatic surfaces (two and three) to reveal the effects of symmetry on inelastic and reactive state-to-state transition probabilities, along with calculations on an adiabatic surface with ordinary Born–Oppenheimer approximation. Excellent agreement between TDDVR and DVR results is obtained in both the representations.
Potential energy surface for an electron transfer reaction mediated by a metal adlayer
Authors: Mishra A. K. , Schmickler W.
Published: June 2004

A model Hamiltonian for electron transfer from a metal electrode to a solvated reactant via a metallic adsorbate is proposed. The adsorbates are distributed randomly over the electrode surface, and a coherent-potential approximation has been employed to treat this randomness. Both the adsorbates and the reactant are assumed to interact with the solvent, which is modeled as a bath of phonons with frequencies in the classical regime. Both the adiabatic and the nonadiabatic potential energy surfaces are calculated, and their dependence on the adsorbate coverage is highlighted. In the low coverage regime the potential-energy surfaces exhibit features similar to a bridge-assisted electron transfer reaction, whereas for higher coverages the surfaces resemble those for direct heterogeneous transfer. This change of shape is caused by the metallization of the adsorbate layer at higher coverages.
Presence of Stark ladders in scattering of shear horizontal piezoelectric waves
Authors: Rodrı́guez-Ramos R. , Monsivais G. , Otero J. A. , Calás H. , Guerra V. , Stern C.
Published: June 2004

The global matrix techniques are used in the analysis of the Stark-Ladder resonances for transverse horizontal and surface waves in piezoelectric multilayers. Stark-Ladder resonances are observed when these waves propagate inside composites consisting of N piezoelectric layers whose piezoelectric properties obey a special linear relation. Each layer is made of materials with hexagonal 6 mm symmetry. The resonances are studied through the transmission coefficient. The transmission coefficient shows the well known band structure for a periodic piezocomposite, and when a linear term in the values of the piezoelectric parameters of the layers (which breaks the periodicity) is added, the band structure is destroyed and, in certain cases, resonances of the Stark-Ladder type are shown. The transmission coefficient is studied as a function of the properties of the materials and angle of incidence of the waves. Numerical results for two different configurations of piezocomposites are presented, one of them showing the existence of Stark-Ladders resonances. An analysis of the resonance spectrum is given.
A combined experimental and computational investigation of the microscopic external heavy atom effect in van der Waals clusters
Authors: Doyle Richard J. , Da Campo Raffaello , Taylor Peter R. , Mackenzie Stuart R.
Published: June 2004

We present a combined experimental and computational study of the external heavy atom effect in van der Waals clusters of para-difluorobenzene (pDFB) with rare-gas atoms. Experimentally, clustering with rare-gas atoms is observed to shorten significantly the S1 fluorescence lifetime compared with that of the pDFB monomer, an effect we interpret in terms of an enhancement of the S1-T1 intersystem crossing rate. In order to test the validity of this widely held assumption, we have calculated the S1-T1 spin–orbit coupling matrix elements in the X-pDFB complexes (X=Ne,Ar,Kr) using a multiconfigurational linear response approach.
Folding of the GB1 hairpin peptide from discrete path sampling
Authors: Evans David A. , Wales David J.
Published: June 2004

The discrete path sampling technique is used to calculate folding pathways of the 16-amino acid β hairpin-forming sequence from residues 41–56 of the B1 domain of protein G. The folding time is obtained using master equation dynamics and kinetic Monte Carlo simulations, and the time evolution of different order parameters and occupation probabilities of groups of minima are calculated and used to characterize intermediates on the folding pathway.
Characterization of ablated species in laser-induced plasma plume
Authors: Furusawa Hideki , Sakka Tetsuo , Ogata Yukio H.
Published: June 2004

Plasma electron density and atomic population densities in the plasma plume produced by a laser ablation of aluminum metal were determined in various ambient gases at relatively high pressures. The method is based on the fit of a spectral line profile of Al(I) 2P∘-2S emission to the theoretical spectrum obtained by one-dimensional radiative transfer calculation. The electron density was higher for a higher ambient gas pressure, suggesting the confinement of the plume by an ambient gas. The electron density also depends on the type of ambient gases, i.e., it increased in the order He<CH4<N2<CF4, while the atomic population density is almost independent of the type of ambient species and pressure. The population densities of the upper and lower levels of the transition were compared, and the ratio between their spatial distribution widths was calculated. These results provide valuable information regarding the confinement of the plume by the ambient gas and give insight into the time evolution of the plume.
Application of Lagrangian theorem-based density-functional approximation free of adjustable parameters to nonhard-sphere fluid
Authors: Zhou Shiqi
Published: June 2004

A recently proposed parameter free version of a Lagrangian theorem-based density functional approximation (LTDFA) [S. Zhou, Phys. Lett. A 319, 279 (2003)] for hard-sphere fluid is applied to hard-core attractive Yukawa model fluid by dividing bulk second-order direct correlation function (DCF) of fluid under consideration into hard-core part and tail part. The former is treated by the parameter free version of the LTDFA, while the tail part is treated by second-order functional perturbation expansion approximation as done in a recent partitioned DFA [S. Zhou, Phys. Rev. E 68, 061201 (2003)]. Two versions of mean spherical approximation (MSA) for the bulk second-order DCF are employed as input, one is the less accurate plain MSA whose tail part of the second-order DCF is strictly independent of a density argument, the other is the more accurate inverse temperature expansion version of the MSA whose tail part is not strictly independent of the density argument. Calculational results indicate that prediction based on the plain MSA is far more accurate than that based on the inverse temperature expansion version of the MSA. The reason is considered to be that the partitioned DFA requires that the tail part is highly or completely independent of the density argument, the plain MSA, by assuming that the tail part is exactly the potential itself, embodies all of the nonlinearities into the hard-core part which can be treated satisfactorily by the parameter free version of the LTDFA. The present investigation results in a universal method for constructing DFA for nonuniform any nonhard-sphere interaction potential fluids.
“Cooperativity blockage” in the mixed alkali effect as revealed by molecular-dynamics simulations of alkali metasilicate glass
Authors: Habasaki Junko , Ngai K. L. , Hiwatari Yasuaki
Published: June 2004

The relaxation dynamics of a complex interacting system can be drastically changed when mixing with another component having different dynamics. In this work, we elucidate the effect of the less mobile guest ions on the dynamics of the more mobile host ions in mixed alkali glasses by molecular-dynamics (MD) simulations. One MD simulation was carried out on lithium metasilicate glass with the guest ions created by freezing some randomly chosen lithium ions at their initial locations at 700 K. A remarkable slowing down of the dynamics of the majority mobile Li ions was observed both in the self-part of the density–density correlation function, Fs(k,t), and in the mean-squared displacements. On the other hand, there is no significant change in the structure. The motion of the Li ions in the unadulterated Li metasilicate glass is dynamically heterogeneous. In the present work, the fast and slow ions were divided into two groups. The number of fast ions, which shows faster dynamics (Lévy flight) facilitated by cooperative jumps, decreases considerably when small amount of Li ions are frozen. Consequently there is a large overall reduction of the mobility of the Li ions. The result is also in accordance with the experimental finding in mixed alkali silicate glasses that the most dramatic reduction of ionic conductivity occurs in the dilute foreign alkali limit. Similar suppression of the cooperative jumps is observed in the MD simulation data of mixed alkali system, LiKSiO3. Naturally, the effect found here is appropriately described as “cooperativity blockage.” Slowing down of the motion of Li ions also was observed when a small number of oxygen atoms chosen at random were frozen. The effect is smaller than the case of freezing some the Li ions, but it is not negligible. The cooperativity blockage is also implemented by confining the Li metasilicate glass inside two parallel walls formed by freezing Li ions in the same metasilicate glass. Molecular-dynamics simulations were performed on the dynamics of the Li ions in the confined glass. Slowing down of the dynamics is largest near the wall and decreases monotonically with distance away from the wall.
A density-functional study for the liquid–vapor coexistence curve of nitrogen fluid
Authors: Sumi Tomonari , Shirahama Hideaki , Sekino Hideo
Published: June 2004

We have used a density-functional theory based on the interaction site model to predict the liquid–vapor coexistence curve of nitrogen fluid. The pressure and chemical potential were calculated from thermodynamic integrations. The different paths of thermodynamic integration provide slightly different predictions for the liquid–vapor coexistence curve. However, these critical points and coexistence curves evaluated by the theory are in qualitative agreement with the experimental data. The theoretical coexistence curves scaled to critical constants agree with the experimental data quantitatively.
Combining molecular dynamics and ab initio quantum-chemistry to describe electron transfer reactions in electrochemical environments
Authors: Domı́nguez-Ariza David , Hartnig Christoph , Sousa Carmen , Illas Francesc
Published: June 2004

A theoretical model is presented aimed to provide a detailed microscopic description of the electron transfer reaction in an electrochemical environment. The present approach is based on the well-known two state model extended by the novelty that the energy of the two states involved in the electron transfer reaction is computed quantum mechanically as a function of the solvent coordinate, as defined in the Marcus theory, and of the intensity of an external electric field. The solvent conformations defining the reaction coordinate are obtained from classical molecular dynamics and then transferred to the quantum mechanical model. The overall approach has been applied to the electron transfer between a chloride anion and a single crystal Cu(100) electrode. It is found that the solvent exerts a strong influence on the equilibrium geometry of the halide and hence on the relative energy of the two states involved in the electron transfer reaction. Finally, both solvent fluctuations and external field facilitate the electron transfer although solvent effects have a stronger influence.
Size induced variations in structural and magnetic properties of double exchange La0.8Sr0.2MnO3−δ nano-ferromagnet
Authors: Roy Sujoy , Dubenko Igor , Edorh Dossah D. , Ali Naushad
Published: June 2004

A detailed study on the influence of particle size varied from 8 nm to 53 nm on the structural and magnetic properties of La0.8Sr0.2MnO3−δ has been done. The unit cell volume increases and the microstrain in the compound shows peak formation as the particle size decreases. Nano particles of La0.8Sr0.2MnO3−δ exhibit superparamagnetism whose blocking temperature has a nonlinear and logarithmic decreasing tendency as function of particle size and applied magnetic field, respectively. Evidence of formation of a magnetically dead layer at the surface has been found and the ratio of the thickness of the dead layer to the particle size increases exponentially with particle size. The coercivity of the nanoparicles increases manifold as particle size varies from 53 nm to 21 nm. In the single domain region the coercivity exhibits a d−1.125 behavior. The temperature dependence of the saturation magnetization shows strong collective excitation due to the spin wave that varies as Tα with α>αbulk of 3/2. Thus the spin wave does not follow the Bloch law in the case of nano particles of La0.8Sr0.2MnO3−δ.
Dispersion relations for the convective instability of an acidity front in Hele-Shaw cells
Authors: Vasquez Desiderio A. , De Wit A.
Published: June 2004

Autocatalytic chemical fronts of the chlorite-tetrathionate (CT) reaction become buoyantly unstable when they travel downwards in the gravity field because they imply an unfavorable density stratification of heavier products on top of lighter reactants. When such a density fingering instability occurs in extended Hele-Shaw cells, several fingers appear at onset which can be characterized by dispersion relations giving the growth rate of the perturbations as a function of their wave number. We analyze here theoretically such dispersion curves comparing the results for various models obtained by coupling Darcy’s law or Brinkman’s equation to either a one-variable reaction-diffusion model for the CT reaction or an eikonal equation. Our theoretical results are compared to recent experimental data.
Charge transfer efficiency in hybrid bulk heterojunction composites
Authors: Kucur Erol , Riegler Jürgen , Urban Gerald , Nann Thomas
Published: June 2004

On the basis of recently published electrochemical measurements, the charge transfer efficiency within CdSe nanocrystal/conducting polymer heterojunction composites was investigated by means of luminescence interaction strength. It was found that poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] and poly-9-vinylcarbazole luminescence was not totally quenched by nanocrystals, whereas poly-3-octylthiophene and polyvinylpyrrolidone was completely quenched. In case of poly-3-hexylthiophene, the nanocrystal luminescence was quenched. The results are in complete agreement with the electrochemical findings and thus, the CdSe nanocrystal/Polyvinylpyrrolidone composite should be a promising material for electroluminescent devices.
Discrete solution of the electrokinetic equations
Authors: Capuani Fabrizio , Pagonabarraga Ignacio , Frenkel Daan
Published: June 2004

We present a robust scheme for solving the electrokinetic equations. This goal is achieved by combining the lattice-Boltzmann method with a discrete solution of the convection-diffusion equation for the different charged and neutral species that compose the fluid. The method is based on identifying the elementary fluxes between nodes, which ensures the absence of spurious fluxes in equilibrium. We show how the model is suitable to study electro-osmotic flows. As an illustration, we show that, by introducing appropriate dynamic rules in the presence of solid interfaces, we can compute the sedimentation velocity (and hence the sedimentation potential) of a charged sphere. Our approach does not assume linearization of the Poisson–Boltzmann equation and allows us for a wide variation of the Peclet number.
Improved theoretical description of protein folding kinetics from rotations in the phase space of relevant order parameters
Authors: Baumketner A. , Shea J.-E. , Hiwatari Y.
Published: June 2004

A method is introduced to construct a better approximation for the reaction coordinate for protein folding from known order parameters. The folding of a two-state off-lattice α helical Gō-type protein is studied using molecular dynamics simulations. Folding times are computed directly from simulation, as well as theoretically using an equation derived by considering Brownian-type dynamics for the putative reaction coordinate. Theoretical estimates of the folding time using the number of native contacts (Qn) as a reaction coordinate were seen to differ quite significantly from the true folding time of the protein. By considering the properties of the bimodal free energy surface of this protein as a function of Qn and another relevant coordinate for folding Q (the total number of contacts), we show that by introducing a rotation in the phase space of the order parameters Q and Qn, we can construct a new reaction coordinate q that leads to a fivefold improvement in the estimate of the folding rate. This new coordinate q, resulting from the rotation, lies along the line connecting the unfolded and folded ensemble minima of the free energy map plotted as a function of the original order parameters Q and Qn. Possible reasons for the remaining discrepancy between the folding time computed theoretically and from folding simulations are discussed.
p-benzoquinone-benzene clusters as potential nanomechanical devices: A theoretical study
Authors: Manojkumar T. K. , Choi H. S. , Hong B. H. , Tarakeshwar P. , Kim K. S.
Published: June 2004

The equilibrium structures and binding energies of the benzene complexes of p-benzoquinones (PBQ) and its negatively charged anionic species (PBQ− and PBQ2−) have been investigated theoretically using second-order Møller-Plesset calculations. While neutral p-benzoquinone-benzene clusters (PBQ-Bz) prefer to have a parallel displaced geometry (P-c), CH⋯π interactions (T-shaped geometries) prevail in the di-anionic PBQ-benzene (PBQ2−-Bz) complexes (T-e2−). Studies on dianionic p-benzoquinone-benzene clusters showed that two nonbonded intermolecular interactions compete in the most stable conformation. One is H-bonding interaction (C–H⋯O type) between carbonyl oxygen of p-benzoquinone and one of the hydrogen atoms of benzene, and the other is a π–H interaction between π-electron cloud of PBQ2− and another hydrogen atom of benzene. Blueshifted H-bonds were observed in T-shaped clusters. The changes in the geometrical preference of PBQ-Bz complex upon addition of electrons would be useful in designing optimized molecular mechanical devices based on the edge-to-face and face-to-face aromatic interactions.
Phonon scattering of excitons and biexcitons in ZnO
Authors: Hazu K. , Sota T. , Adachi S. , Chichibu Sf. , Cantwell G. , Reynolds D. C. , Litton C. W.
Published: June 2004

The phase relaxation time of biexcitons T2bi and that of excitons T2ex in a bulk ZnO have been measured by the use of femtosecond four-wave mixing as functions of excitation wavelength, excitation power, and temperature. The biexciton-acoustic phonon interaction coefficient has been determined to be βacbi≈53 μeV/K as an average value. For A excitons, the interaction coefficients with acoustic and optical phonons have been determined to be βacex≈8.4 μeV/K and βoptex≈14 meV, respectively.
Dependence of incommensurate phase formation on vacancy type in La-doped (Na1/2Ba1/2)TiO3
Authors: Lee Jung-Kun , Yi Jae Yun , Hong Kug Sun
Published: June 2004

Changes in structure and phase transition behavior are investigated for two types of La-doped sodium bismuth titanate ceramics. When A-site vacancies are formed by La incorporation, the phase transition near 200 °C becomes pronounced. Hysteresis loop and ε(T) demonstrate that the structure above 200 °C is the incommensurate antiferroelectric phase. In contrast, B-site vacancies produced by La doping do not contribute to the incommensurate phase. The origin for the formation of the incommensurate antiferroelectric state between the rhombohedral ferroelectric phase and tetragonal paraelectric phase is discussed in a view of decoupling effects due to the A-site vacancies.
Effects of substrate treatment and growth conditions on structure, morphology, and luminescence of homoepitaxial ZnTe deposited by metalorganic vapor phase epitaxy
Authors: Traversa M. , Lovergine N. , Prete P. , Yoshino K. , Di Luccio T. , Scalia G. , Pentimalli M. , Tapfer L. , Morales P. , Mancini A. M.
Published: June 2004

The structure, morphology, and luminescence of homoepitaxial ZnTe layers grown by metalorganic vapor phase epitaxy on (100)ZnTe:P wafers are reported as function of substrate surface treatment and growth conditions. Epilayers grown on substrates in situ H2 treated at temperatures above 240 °C exhibit long range crystalline perfection comparable to the substrate, but their structure rapidly degrades at lower temperatures, an effect ascribed to the incomplete removal of native oxides from the wafer surface. Instead, the material microscopic structure improves monotonically by annealing the wafer up to 350 °C. A nearly featureless surface morphology is obtained for epilayers grown within a narrow temperature interval around 350 °C, corresponding to the transition between surface kinetics and mass transport limited growth. Surface ridging along a 〈110〉 in-plane direction is observed at lower temperatures, while large pyramidlike hillocks occur randomly on the surface of samples grown above 350 °C. Besides the epilayer band-edge (BE) emission, 4.2 K photoluminescence (PL) of samples show recombination bands characteristic of the ZnTe:P wafer, their excitation being due to diffusion into the substrate of excitons photogenerated in the epilayer. Within the BE emission, both 1s- and 2s- state free exciton (at 2.3809 eV and 2.3904 eV), as well as a neutral donor (at 2.3776 eV) and acceptor bound (at 2.3748 eV, 2.3739 eV, and 2.3706 eV) exciton lines occur. Intense and narrow (1.7 meV) free-exciton emission is observed for samples grown at 350 °C and Te:Zn precursors molar flow ratio around 1, while bound-exciton lines dominate the PL spectra below 350 °C. A weak donor-acceptor pair (DAP) band, ascribed to a transition involving a donor substitutional on Zn site (DZn) and a CTe-DZn acceptor-type complex, appears between 2.16 and 2.27 eV in the PL spectra of epilayers grown below 350 °C or at low Zn precursor supply rates. The DAP band intensity well correlates with that of the 2.3776 eV donor and the 2.3706 eV acceptor bound exciton lines. At higher growth temperatures the epilayer crystallinity and radiative efficiency decrease.
Dot-by-dot analysis of magnetization reversal in perpendicular patterned CoCrPt medium by using magnetic force microscopy
Authors: Bai J. , Takahoshi H. , Ito H. , Saito H. , Ishio S.
Published: June 2004

A dot-by-dot analysis to investigate the magnetization switching behavior of the perpendicular patterned CoCrPt medium is reported. The medium is investigated by in situ and ex situ magnetic force microscopy (MFM) techniques. The topographical and magnetic characteristics of each magnetic dot were studied in given areas. The perpendicular dc demagnetization (DCD) remanence data evaluated by counting the volume of reversed magnetic dots according to the ex situ MFM measurements were in good agreement with alternating gradient magnetometry measurements. The influence of the magnetostatic energy associated with nanodots to the distribution of the intrinsic coercivity of individual magnetic dots and the intrinsic DCD curve of the medium are estimated. Further, the mechanism of magnetization reversal of individual magnetic dot is discussed.