bluuebunny/crossref_metadata_2025 huggingface dataset:
Biglobal linear stability analysis for the flow in eccentric annular channels and a related geometry
Authors: Merzari Elia , Wang Sheng , Ninokata Hisashi , Theofilis Vassilis
Published: November 2008

Recently, it has been observed that simple geometry characterized by a low level of symmetry present interesting peculiarities in the process of transition from laminar Poiseuille flow to turbulent flow. Examples of this type of geometry are eccentric channels and, more generally, parallel channels containing a narrow gap. In the present work, a global linear stability analysis for the flow in this class of geometry has been performed. The problem is discretized through spectral collocation and the eigenvalue problem has been solved with the Arnoldi-method based algorithms and the QZ algorithm. Since no numerical studies of this type have yet been performed to address the issue of transition in this geometry, the codes have been validated toward results obtained in simplified geometries (e.g., concentric annular channel and square channel). The eigenvalue spectra of the Poiseuille flow in eccentric channels and a U-shaped channel have then been computed and analyzed for a wide range of geometric parameters. After comparison with spectra typical of channel flow and pipe flow it is shown that an additional linear mechanism of instability is present, related to the spanwise variation of the laminar velocity profile.
Transitions of epitaxially lifted-off bulk GaAs and GaAs/AlGaAs quantum well under thermal-induced compressive and tensile strain
Authors: Mateo C. M. N. , Ibañez J. J. , Fernando J. G. , Garcia J. C. , Omambac K. , Jaculbia R. B. , Defensor M. , Salvador A. A.
Published: November 2008

Low temperature photoluminescence and reflectance measurements on epitaxially lifted-off (ELO) bulk GaAs and GaAs/AlGaAs multiple quantum wells (MQWs) bonded to Si and MgO substrates are reported. Photoluminescence measurements indicate no strain at room temperature for the ELO bulk GaAs film but show biaxial strain at 10 K. Si-bonded films undergo tensile strain, while films with MgO host substrates experience compressive strain. Reflectance measurements at 10 K show that light hole band is closer to the conduction band for the tensile strained film. In GaAs MQW ELO films, the separation of the heavy hole and light hole band is reduced in tensile strained films by 4.7 meV, corresponding to a strain ε=−0.7±0.05×10−3 and stress X=0.9±0.05 kbar (90±5 MPa). For compressively strained films, this separation is enhanced by 3.9 meV, equivalent to a strain ε=0.6±0.05×10−3 and X=0.8±0.05×10−3 kbar (80±5 MPa). The findings demonstrate that ELO is an effective technique to introduce tensile and compressive strain in GaAs heterostructures and is appropriate for strain-related spectroscopy.
Stretching chimeric DNA: A test for the putative S-form
Authors: Whitelam Stephen , Pronk Sander , Geissler Phillip L.
Published: November 2008

Double-stranded DNA “overstretches” at a pulling force of about 65 pN, increasing in length by a factor of 1.7. The nature of the overstretched state is unknown, despite its considerable importance for DNA’s biological function and technological application. Overstretching is thought by some to be a force-induced denaturation and by others to consist of a transition to an elongated, hybridized state called S-DNA. Within a statistical mechanical model, we consider the effect upon overstretching of extreme sequence heterogeneity. “Chimeric” sequences possessing halves of markedly different AT composition elongate under fixed external conditions via distinct, spatially segregated transitions. The corresponding force-extension data vary with pulling rate in a manner that depends qualitatively and strikingly upon whether the hybridized S-form is accessible. This observation implies a test for S-DNA that could be performed in experiment.
Rotationally inelastic collisions of methinoposphide (HCP) with para-H2 at low temperature
Authors: Hammami K. , Nkem C. , Owono Owono L. C. , Jaidane N. , Ben Lakhdar Z.
Published: November 2008

The interaction potential energy surface of the methinoposphide (HCP)-H2 complex is calculated at the ab initio coupled-cluster level of theory with an aug-cc-pVTZ Gaussian basis set. The [H-C] and [C-P] bond lengths of HCP are set to their values at the linear equilibrium ground vibrational level of the molecule. The calculated interaction energy presents two minima located 106.3 and 67.6 cm−1 below the HCP+H2 dissociation limit. Using the interaction potential obtained, we have computed collision excitation cross sections in the close-coupling approach and downward rate coefficients at low temperature, i.e., T≤70 K. These quantities are significantly magnified in comparison with their counterparts for the HCP-He collisions. It is shown that there is a propensity toward ΔJ=1 transitions.
An extremely high dielectric constant in bismuth-based pyrochlore multilayer film capacitors combined with percolative structure
Authors: Cuong Nguyen Duy , Ahn Jun-Ku , Park Kyung-Woo , Seong Nak-Jin , Yoon Soon-Gil
Published: November 2008

The percolative film capacitor structure of Pt/Bi2Mg2/3Nb4/3O7 [BMN(Ar) films deposited at pure argon atmosphere]/Pt was addressed for achievement of a high dielectric constant in the films deposited at room temperature by radio frequency magnetron sputtering. The deterioration of the leakage current characteristics in the percolative capacitor was mitigated using the multilayer films of BMN(O)/BMN(Ar)/BMN(O), where ultrathin BMN(O) films were deposited at a mixed atmosphere of argon and oxygen. An extremely high dielectric constant of 120 and a low leakage current density of 6×10−6 A/cm2 at 3 V were observed in percolative BMN multilayer films as-deposited at room temperature.
Single ion hydration free energies: A consistent comparison between experiment and classical molecular simulation
Authors: Ashbaugh Henry S. , Asthagiri D.
Published: November 2008

The determination of single ion hydration free energies is troubled by the thermodynamic constraint that only the properties of neutral pairs can be uniquely determined. As such, single ion properties depend on extrathermodynamic information, which can differ between experimental and molecular simulation measurements. This comparison is hampered by the quantum mechanical nature of the proton, the reference ion of choice for developing standard tables, and uncertainty in the experimental reference potential to which properties are measured. We revisit the methodology of Latimer et al. [J. Chem. Phys. 7, 108 (1939)], which extracts single ion properties from neutral pair transfer free energies under the assumption that the Born equation provides an accurate description of the charging of monovalent ions. This methodology permits us to make a consistent comparison between experimental and theoretical values for single ion hydration free energies and gives insight into nonpolar contributions to the ion hydration free energy as well as the potential at the center of a hypothetical uncharged ion.
Modeling the doubly excited state with time-dependent Hartree–Fock and density functional theories
Authors: Isborn Christine M. , Li Xiaosong
Published: November 2008

Multielectron excited states have become a hot topic in many cutting-edge research fields, such as the photophysics of polyenes and in the possibility of multiexciton generation in quantum dots for the purpose of increasing solar cell efficiency. However, obtaining multielectron excited states has been a major obstacle as it is often done with multiconfigurational methods, which involve formidable computational cost for large systems. Although they are computationally much cheaper than multiconfigurational wave function based methods, linear response adiabatic time-dependent Hartree–Fock (TDHF) and density functional theory (TDDFT) are generally considered incapable of obtaining multielectron excited states. We have developed a real-time TDHF and adiabatic TDDFT approach that is beyond the perturbative regime. We show that TDHF/TDDFT is able to simultaneously excite two electrons from the ground state to the doubly excited state and that the real-time TDHF/TDDFT implicitly includes double excitation within a superposition state. We also present a multireference linear response theory to show that the real-time electron density response corresponds to a superposition of perturbative linear responses of the S0 and S2 states. As a result, the energy of the two-electron doubly excited state can be obtained with several different approaches. This is done within the adiabatic approximation of TDDFT, a realm in which the doubly excited state has been deemed missing. We report results on simple two-electron systems, including the energies and dipole moments for the two-electron excited states of H2 and HeH+. These results are compared to those obtained with the full configuration interaction method.
A single molecule rectifier with strong push-pull coupling
Authors: Saraiva-Souza Aldilene , Macedo de Souza Fabricio , Aleixo Vicente F. P. , Girão Eduardo Costa , Filho Josué Mendes , Meunier Vincent , Sumpter Bobby G. , Souza Filho Antônio Gomes , Del Nero Jordan
Published: November 2008

We theoretically investigate the electronic charge transport in a molecular system composed of a donor group (dinitrobenzene) coupled to an acceptor group (dihydrophenazine) via a polyenic chain (unsaturated carbon bridge). Ab initio calculations based on the Hartree–Fock approximations are performed to investigate the distribution of electron states over the molecule in the presence of an external electric field. For small bridge lengths (n=0–3) we find a homogeneous distribution of the frontier molecular orbitals, while for n>3 a strong localization of the lowest unoccupied molecular orbital is found. The localized orbitals in between the donor and acceptor groups act as conduction channels when an external electric field is applied. We also calculate the rectification behavior of this system by evaluating the charge accumulated in the donor and acceptor groups as a function of the external electric field. Finally, we propose a phenomenological model based on nonequilibrium Green’s function to rationalize the ab initio findings.
Redox potentials and screening in ionic liquids: Effects of sizes and shapes of solute ions
Authors: Lynden-Bell R. M.
Published: November 2008

Simulations of a model ionic liquid, [dmim][PF6] (dimethylimidazolium hexafluorophosphate), containing solute ions of different sizes and shapes have been used to investigate the changes in redox potentials of and screening around solute ions of different sizes and shapes. The effective solute size of spherical ions increases with the actual solute size although more slowly than expected. The effective solute size of tetrahedral or square planar ions varies little with actual ligand size. These results are clarified by reference to the charge density in the solvent around the ions, which is also used to calculate the potential within the solvent. Screening is essentially complete within 1 nm of the solute ion although charge density oscillations propagate further into the liquid. The results are compared to theoretical models and the implications for experiments are discussed.
The infrared spectroscopy and photochemistry of NO3 trapped in solid neon
Authors: Jacox Marilyn E. , Thompson Warren E.
Published: November 2008

N O 3 can be stabilized in solid neon either by codeposition at 4.3K of a Ne:O2 mixture with a Ne:NO mixture that has been passed through a microwave discharge or, in higher yield, by codeposition of a Ne:NO mixture with a Ne:O2 mixture, followed by annealing of the deposit at ∼7K and exposure of the solid to near ultraviolet radiation. All of the previously reported bands of NO3 between 700 and 3000cm−1 were observed, most with neon-matrix shifts of less than 2.5cm−1. The infrared spectra of eight isotopic species of NO3 were obtained. The observed isotopic shifts demonstrate the occurrence of extensive mixing of ground-state levels of e′ symmetry and their strong vibronic interaction with the B̃E′2 state. Photodissociation of NO3 by irradiation of the deposit at wavelengths longer than 520nm leads to new absorptions near the fundamentals of NO and O2 and other new absorptions at relatively low frequencies. These absorptions were depleted and NO3 regenerated by subsequent near ultraviolet irradiation of the deposit, suggesting the stabilization of a weakly bound NO(O2) complex in solid neon.
Evaluation of capacitance-voltage characteristic and memory window of metal-ferroelectric-insulator-silicon capacitors
Authors: Zheng X. J. , Sun J. , Zhang J. J. , Tang M. H. , Li W.
Published: November 2008

It is recognized that the conventional model for metal-ferroelectric-insulator-silicon (MFIS) capacitor is always not consistent with the experimental observation very well due to negligence of the history-dependent electric field effect. In this letter, combining the switching physics of ferroelectric with silicon physics, an improved model is proposed to investigate the capacitance-voltage (C-V) characteristic and memory window. For two MFIS capacitors with SrBi2Ta2O9 and Bi3.25La0.75Ti3O12 ferroelectric layers, C-V characteristic and memory window were evaluated, and the results are more consistent with the previous experiments than that of the Lue model. The improved model could be extendedly applied to MFIS structure devices.
Effects of indium segregation on optical properties of nitrogen-doped InAs/GaAs quantum dots
Authors: Inoue Tomoya , Mamizuka Masataka , Mizuno Hiroshi , Kojima Osamu , Kita Takashi , Wada Osamu
Published: November 2008

The emission wavelength of InAs quantum dots (QDs) capped by GaAs is found to be systematically controlled by doping nitrogen on the QDs surface with a thin spacer layer in between QDs and the nitrogen-doped layer. Cross-sectional transmission electron microscope images of the nitrogen-doped QD indicate that the nitrogen-doped layer acts as a blocking layer for In segregation. Furthermore, the in-plane linear polarization of the emission has been demonstrated to be controlled by the spacer layer thickness.
Fast electron-correlation methods for molecular crystals: An application to the α, β1, and β2 modifications of solid formic acid
Authors: Hirata So
Published: November 2008

A method for the routine first-principles determination of energies, structures, and phonons of molecular crystals by high-accuracy electron-correlation theories has been proposed. It approximates the energy per unit cell of a crystal by a sum of monomer and dimer energies in an embedding field of self-consistent (and, therefore, polarizable) atomic charges and dipole moments. First and second energy derivatives with respect to atom positions and lattice constants (useful for characterizing structures and phonons) have also been computed efficiently with a long-range electrostatic correction. The method has been applied to solid formic acid modeled as infinite one-dimensional hydrogen-bonded chains. Accurate energies (with corrections for basis-set superposition errors), structural parameters, and frequencies have been obtained for three polymorphic structures (β1, β2, and α) with second-order perturbation theory or higher. On this basis, reliable assignments of their infrared, Raman, and inelastic neutron scattering spectral bands have been proposed. The diffraction and spectroscopic data are shown to be consistent with the pristine β1 form and the hitherto-inexplicable infrared band splitting can be assigned to the in-phase and out-of-phase vibrations of adjacent hydrogen-bonded molecules rather than speculated polymorphism. Spectral features expected from the β2 and α forms have also been predicted and are found to be incompatible with the observed Raman and inelastic neutron scattering spectra in the low-frequency region.
Tuning of oxidation states in the LaNiO3−δ perovskite around the insulator-metal transition
Authors: Berini Bruno , Keller Niels , Pigeau Benjamin , Dumont Yves , Popova Elena , Franco Nuno , da Silva Rui M. C.
Published: November 2008

The LaNiO3−δ perovskite has been studied during its reoxygenation process at the reversible insulator-metal transition by spectroscopic ellipsometry for different pressures and temperatures conditions. First, it was demonstrated that the reoxygenation dynamics increases with both increasing pressure and temperature. Considering the temperature dependent experiments, two regimes of kinetics have been identified: a slow reoxygenation dynamics at low temperature (below 523 K) and fast dynamics above 623 K. Second, contrary to our expectations, the reoxygenation process of a preliminary reduced sample is completed after a sufficient time delay even for the smallest investigated temperature of 473 K or oxygen pressure of 0.03 μbar, respectively. Modeling the change in extinction coefficient as a function of time during the reoxygenation, it was found that the oxygen diffusion coefficient varies from 4.2×10−14 to 1.1×10−13 cm2/s depending on the temperature (473 to 523 K). This oxygen diffusion constant is similar in magnitude order to those observed in the YBCO superconductor. At the light of these investigations, a pulsed oxygen injection was used, allowing a control of the injected oxygen quantity and by consequence, it was possible to precisely tune the oxidation state for a LaNiO3−δ film between the reversible reduced sample and the stoichiometric compound. In situ ellipsometry measurements are simultaneously performed to follow changes in optical constants.
Bias-dependent rectifying properties of n-n manganite heterojunctions La1−xCaxMnO3/SrTiO3:Nb (x=0.65–1)
Authors: Lü W. M. , Sun J. R. , Wang D. J. , Xie Y. W. , Liang S. , Chen Y. Z. , Shen B. G.
Published: November 2008

The transport property of n-n type manganite heterojunctions, composed of La1−xCaxMnO3 films (x=0.6, 0.75, 0.85, and 1) and 0.05 wt % Nb-doped SrTiO3, has been experimentally studied. Different from p-n junctions, the rectifying behavior of which is either thermionic emission/diffusion-dominated or tunneling-dominated; the electronic process in the n-n junction undergoes a nonthermal to thermal transition as bias voltage increases, which is a feature emerging when Ca content exceeds x=0.75 and developing with the increase in x. The two processes can be well described by the Shockley equation and the Newman equation, respectively. Possible mechanisms for this phenomenon are discussed.
Global particle-in-cell simulations of Alfvénic modes
Authors: Mishchenko Alexey , Hatzky Roman , Könies Axel
Published: November 2008

Global linear gyrokinetic particle-in-cell (PIC) simulations of electromagnetic modes in pinch and tokamak geometries are reported. The global Alfvén eigenmode, the mirror Alfvén eigenmode, the toroidal Alfvén eigenmode, and the kinetic ballooning modes have been simulated. All plasma species have been treated kinetically (i.e., no hybrid fluid-kinetic or reduced-kinetic model has been applied). The main intention of the paper is to demonstrate that the global Alfvén modes can be treated with the gyrokinetic PIC method.
Simple kinetic Monte Carlo models for dissolution pitting induced by crystal defects
Authors: Meakin Paul , Rosso Kevin M.
Published: November 2008

The solid-on-solid kinetic Monte Carlo model of Lasaga and Blum [Geochim. Cosmochim. Acta 50, 2363 (1986)] for dislocation-controlled etch-pit growth has been extended to the growth of etch pits under the control of multiple dislocations and point defects. This required the development of algorithms that are O(103)–O(104) times faster than primitive kinetic Monte Carlo models for surfaces with areas in the range of 1024×1024–4096×4096 lattice sites. Simulations with multiple line defects indicate that the surface morphology coarsens with increasing time and that the coarsening is more pronounced for large bond-breaking activation energies. For small bond breaking activation energies dissolution enhanced by line defects perpendicular to the dissolving surface results in pits with steep sides terminated by deep narrow hollow tubes (nanopipes). Larger bond breaking activation energies lead to shallow pits without deep nanopipes, and if the bond breaking activation energy is large enough, step flow is the primary dissolution mechanism, and pit formation is suppressed. Simplified models that neglect the far field strain energy density but include either a rapidly dissolving core or an initially empty core lead to results that are qualitatively similar to those obtained using models that include the effects of the far field stress and strain. Simulations with a regular array of line defects show that microscopic random thermal fluctuations play an important role in the coarsening process.
Structure and predissociation of the 3pσuD Σ3u+ Rydberg state of N2: First extreme-ultraviolet and new near-infrared observations, with coupled-channels analysis
Authors: Lewis B. R. , Baldwin K. G. H. , Heays A. N. , Gibson S. T. , Sprengers J. P. , Ubachs W. , Fujitake M.
Published: November 2008

The 3pσuD Σ3u+ Rydberg state of N2 is studied experimentally using two high-resolution spectroscopic techniques. First, the forbidden D Σ3u+−X Σ1g+ transition is observed for the first time via the (0,0) band of N142 and the (1,0) band of N152, using 1 extreme-ultraviolet +1 ultraviolet two-photon-ionization laser spectroscopy. Second, the Rydberg–Rydberg transition D Σ3u+−E Σ3g+ is studied using near-infrared diode-laser photoabsorption spectroscopy, thus extending the previous measurements of Kanamori et al. [J. Chem. Phys. 95, 80 (1991)], to higher transition energies, and thereby revealing the (2,2) and (3,3) bands. The combined results show that the D(v=0–3) levels exhibit rapidly increasing rotational predissociation as v increases, spanning nearly four orders of magnitude. The D-state level structure and rotational predissociation signature are explained by means of a coupled-channels model which considers the electrostatically coupled Π3u Rydberg-valence manifold, together with a pure-precession L-uncoupling rotational interaction between the 3pσuD Σ3u+ and 3pπuG Π3u Rydberg p-complex components.
Quantum path-integral study of the phase diagram and isotope effects of neon
Authors: Ramírez R. , Herrero C. P.
Published: November 2008

The phase diagram of natural neon has been calculated for temperatures in the range of 17–50 K and pressures between 10−2 and 2×103 bar. The phase coexistence between solid, liquid, and gas phases has been determined by the calculation of the separate free energy of each phase as a function of temperature. Thus, for a given pressure, the coexistence temperature was obtained by the condition of equal free energy of coexisting phases. The free energy was calculated by using nonequilibrium techniques such as adiabatic switching and reversible scaling. The phase diagram obtained by classical Monte Carlo simulations has been compared to that obtained by quantum path-integral simulations. Quantum effects related to the finite mass of neon cause that coexistence lines are shifted toward lower temperatures when compared to the classical limit. The shift found in the triple point amounts to 1.5 K, i.e., about 6% of the triple-point temperature. The triple-point isotope effect has been determined for N20e, N21e, N22e, and natural neon. The simulation data show satisfactory agreement to previous experimental results, which report a shift of about 0.15 K between triple-point temperatures of N20e and N22e. The vapor pressure isotope effect has been calculated for both solid and liquid phases at triple-point conditions. The quantum simulations predict that this isotope effect is larger in the solid than in the liquid phase, and the calculated values show nearly quantitative agreement to available experimental data.
p
-type conduction in nitrogen-doped ZnS nanoribbons
Authors: Yuan G. D. , Zhang W. J. , Zhang W. F. , Fan X. , Bello I. , Lee C. S. , Lee S. T.
Published: November 2008

We report reproducible p-type transport properties in nitrogen-doped ZnS nanoribbons (NRs) synthesized by applying ammonia gas as the acceptor source. Field-effect transistors fabricated from individual ZnS NRs revealed the p-type behavior of ZnS NRs and significant enhancement in p-type transport properties upon annealing in argon ambient. Annealing-induced conversion of highly insulating to p-type conducting ZnS NRs was attributed to activation of N acceptors from the passivated states of NS–H bonding.
Computation of the free energy change associated with one-electron reduction of coenzyme immersed in water: A novel approach within the framework of the quantum mechanical/molecular mechanical method combined with the theory of energy representation
Authors: Takahashi Hideaki , Ohno Hajime , Kishi Ryohei , Nakano Masayoshi , Matubayasi Nobuyuki
Published: November 2008

The isoalloxazine ring (flavin ring) is a part of the coenzyme flavin adenine dinucleotide and acts as an active site in the oxidation of a substrate. We have computed the free energy change Δμred associated with one-electron reduction of the flavin ring immersed in water by utilizing the quantum mechanical/molecular mechanical method combined with the theory of energy representation (QM/MM-ER method) recently developed. As a novel treatment in implementing the QM/MM-ER method, we have identified the excess charge to be attached on the flavin ring as a solute while the remaining molecules, i.e., flavin ring and surrounding water molecules, are treated as solvent species. Then, the reduction free energy can be decomposed into the contribution Δμred(QM) due to the oxidant described quantum chemically and the free energy Δμred(MM) due to the water molecules represented by a classical model. By the sum of these contributions, the total reduction free energy Δμred has been given as −80.1 kcal/mol. To examine the accuracy and efficiency of this approach, we have also conducted the Δμred calculation using the conventional scheme that Δμred is constructed from the solvation free energies of the flavin rings at the oxidized and reduced states. The conventional scheme has been implemented with the QM/MM-ER method and the calculated Δμred has been estimated as −81.0 kcal/mol, showing excellent agreement with the value given by the new approach. The present approach is efficient, in particular, to compute free energy change for the reaction occurring in a protein since it enables ones to circumvent the numerical problem brought about by subtracting the huge solvation free energies of the proteins in two states before and after the reduction.
Current and fluctuation in a two-state stochastic system under nonadiabatic periodic perturbation
Authors: Ohkubo Jun
Published: November 2008

We calculate a current and its fluctuation in a two-state stochastic system under a periodic perturbation. The system could be interpreted as a channel on a cell surface or a single Michaelis–Menten catalyzing enzyme. It has been shown that the periodic perturbation induces a so-called pump current, and the pump current and its fluctuation are calculated with the aid of the geometrical phase interpretation. We give a simple calculation recipe for the statistics of the current, especially in a nonadiabatic case. The calculation scheme is based on the nonadiabatic geometrical phase interpretation. Using the Floquet theory, the total current and its fluctuation are calculated, and it is revealed that the average of the current shows a stochastic-resonance-like behavior. In contrast, the fluctuation of the current does not show such behavior.
Efficient stochastic sampling of first-passage times with applications to self-assembly simulations
Authors: Misra Navodit , Schwartz Russell
Published: November 2008

Models of reaction chemistry based on the stochastic simulation algorithm (SSA) have become a crucial tool for simulating complicated biological reaction networks due to their ability to handle extremely complicated networks and to represent noise in small-scale chemistry. These methods can, however, become highly inefficient for stiff reaction systems, those in which different reaction channels operate on widely varying time scales. In this paper, we develop two methods for accelerating sampling in SSA models: an exact method and a scheme allowing for sampling accuracy up to any arbitrary error bound. Both methods depend on the analysis of the eigenvalues of continuous time Markov models that define the behavior of the SSA. We show how each can be applied to accelerate sampling within known Markov models or to subgraphs discovered automatically during execution. We demonstrate these methods for two applications of sampling in stiff SSAs that are important for modeling self-assembly reactions: sampling breakage times for multiply connected bond networks and sampling assembly times for multisubunit nucleation reactions. We show theoretically and empirically that our eigenvalue methods provide substantially reduced sampling times for a large class of models used in simulating self-assembly. These techniques are also likely to have broader use in accelerating SSA models so as to apply them to systems and parameter ranges that are currently computationally intractable.
Equilibrium sedimentation profile of dilute, salt-free charged colloids
Authors: Wang Tzu-Yu , Li Hsien-Tsung , Sheng Yu-Jane , Tsao Heng-Kwong
Published: November 2008

The sedimentation profile of a dilute colloidal solution follows the barometric distribution owing to the balance between gravitational force and thermal fluctuation. However, the electrostatic interactions may lead to significant deviation even in the low volume fraction limit (e.g., 10−5). On the basis of Monte Carlo simulations for a dilute, salt-free colloidal dispersion, five regimes can be identified through the resulting colloidal sedimentation profile and the counterion distribution. The electrostatic interactions depends on the Coulomb strength Ec defined as the ratio of the Bjerrum length to the colloid size. At weak colloid-ion attractions (small Ec), counterions tend to distribute uniformly in the container. However, both barometric and inflated profiles of colloids can be observed. On the contrary, at strong colloid-ion attraction (large Ec), counterions accumulate in the vicinity of the colloids. Significant counterion condensation effectively decreases the strength of colloid-colloid repulsion and barometric profile of colloids can be obtained as well. As a result, the sedimentation profile and counterion distribution are indicative of the strength of effective colloid-colloid and colloid-ion interactions. It is also found that local electroneutrality condition is generally not satisfied and charge separation (or internal electric field) is neither a sufficient nor necessary condition for nonbarometric distributions.
Correlation of local structure and electrical activation in arsenic ultrashallow junctions in silicon
Authors: Giubertoni Damiano , Pepponi Giancarlo , Gennaro Salvatore , Bersani Massimo , Sahiner Mehmet Alper , Kelty Stephen P. , Doherty Roisin , Foad Majeed A. , Kah Max , Kirkby Karen J. , Woicik Joseph C. , Pianetta Piero
Published: November 2008

The understanding of the behavior of arsenic in highly doped near surface silicon layers is of crucial importance for the formation of N-type ultrashallow junctions in current and future very large scale integrated technology. This is of particular relevance when studying recently developed implantation and annealing methods. Past theoretical as well as experimental investigations have suggested that the increase in As concentration, and therefore the reciprocal proximity of several As atoms, leads to a drastic increase in electrically inactive defects giving only marginal reduction in sheet resistance. Monoclinic SiAs aggregates as well as various arsenic-vacancy clusters contribute to the deactivation of arsenic. This study aims to correlate between the results of electrical activation measurements and x-ray absorption fine structure measurements. Samples were doped with a nominal fluence of 1×1015–3×1015 atoms/cm2, implanted at 2 keV, and annealed by rapid thermal treatments, laser submelt treatments, and a combination of both. Hall effect and sheet resistance measurements have been performed to obtain the density of charge carriers. Secondary ion mass spectrometry has been employed to measure the depth profile and the total retained fluences. The percentage of substitutional arsenic has been obtained by least-squares fits of the measured x-ray absorption spectra with simulated spectra of relaxed structures of the defects obtained by density functional theory. A good agreement with the Hall effect measured electrically active dose fraction has been obtained and a quantification of the population of the different defects involved has been attempted.
Strain relaxation in InAs self-assembled quantum dots induced by a high N incorporation
Authors: Chen J. F. , Yang C. H. , Wu Y. H. , Chang L. , Chi J. Y.
Published: November 2008

The effect of a high N incorporation in self-assembled InAs quantum dots (QDs) is investigated by analyzing the electronic and structural properties around QD region. Capacitance-voltage profiling and admittance spectroscopy shows that N incorporation into the InAs QD layer leads to drastic carrier depletion in the QD layer and neighboring GaAs layers due to the formation of a deep defect state at 0.34–0.41 eV. The signature of this defect state is similar to those defects observed in strain relaxed QDs or InGaAs/GaAs quantum wells when the InAs deposition thickness exceeds a critical thickness. Accordingly, the N incorporation might result in strain relaxation either by increasing localized strain or by inducing composition inhomogeneities, which provide nucleation sources for strain relaxation. The argument of strain relaxation is supported by transmission electron microscopy that reveals lattice misfits at the QD layer and neighboring GaAs layers.
Magnetic ordering of the Fe/Si interface and its initial formation
Authors: Pronin I. I. , Gomoyunova M. V. , Malygin D. E. , Vyalikh D. V. , Dedkov Yu. S. , Molodtsov S. L.
Published: November 2008

High-resolution photoelectron spectroscopy with synchrotron radiation and magnetic linear dichroism in Fe 3p core-level photoemission has been used to study both the initial stages of Fe/Si(111)7×7 and Fe/Si(100)2×1 interface formation and their ferromagnetic ordering. The correlation between the phase composition, electronic structure, and magnetic behavior of the interfaces has been established. It is shown that in-plane ferromagnetic ordering of the interfaces has a threshold nature and arises after the deposition of ∼7 Å Fe in both systems. However, the remanent magnetization of Fe/Si(111) is greater than that of Fe/Si(100) due to the difference in the chemistry of the phases being formed. In the former case, it was found that at room temperature an ultrathin metastable FeSi film with the CsCl structure grows at the first stage of Fe deposition on Si. At higher Fe coverages, a Si solid solution in iron, and later an Fe film, is found to develop on FeSi. The magnetic properties of the solid solution and the Fe film of ∼15 Å thick are quite similar. At the Fe/Si(100) interface, we observe the formation of iron-rich Fe3Si, in which the average magnetic moment of Fe atoms is markedly lower than that of the Fe film. Solid-phase reactions, starting at ∼100 °C in both systems, lead to a gradual conversion of ferromagnetic phases to the nonmagnetic ε-FeSi phase, which transforms to β-FeSi2 at temperatures above 600 °C.
Analysis and modeling of edge fluctuations and transport mechanism in the Maryland Centrifugal Experiment
Authors: Uzun-Kaymak I. U. , Guzdar P. N. , Clary R. , Ellis R. F. , Hassam A. B. , Teodorescu C.
Published: November 2008

The Maryland Centrifugal Experiment [R. F. Ellis et al., Phys. Plasmas 12, 055704 (2005)] is a mirror machine designed to have a plasma axially confined by supersonic rotation and dominantly interchange stable by the radial shear in the azimuthal velocity. Nevertheless, residual fluctuations still persist. To investigate the presence of such fluctuations, an azimuthal array of 16 magnetic pickup coils at the edge region of the plasma has been employed. A comprehensive analysis of the magnetic fluctuations reveals that, under the imposed shear flow, only m=0 and m=2 modes are dominant; yet, the observed frequency spectrum is broadband. Using higher order spectral analysis, clear evidence of nonlinear mode coupling is detected. It is also observed that the amplification of magnetic fluctuations leads to enhanced transport consistent with the drop of the plasma density and voltage. As a result, the magnetic fluctuations start to decrease in amplitude as the central plasma pressure drops. In return, the anomalous radial particle and momentum transport are reduced; thus, the plasma confinement improves. As the plasma pressure starts to build up, the plasma voltage increases, destabilizing the m=2 interchange mode. The cycle of enhanced transport and intermittent fluctuations repeats itself. A two-dimensional magnetohydrodynamics code in slab geometry is employed to investigate the dynamics of the primary interchange instability and to assess the level of transport. For very low sheared rotation, a broad spatial spectrum of unstable modes is obtained. As the sheared rotation is increased, the high mode numbers become stabilized and low mode numbers dominate the spectrum. Both the experimental data obtained from the azimuthal array probes and the simulations in case of parabolic shear flow show clear evidence of nonlinear mode coupling, explaining the broadband frequency spectrum for low mode numbers. A detailed comparison of spatiotemporal dynamics of simulations with the experimental data is presented.
Electrostatic solitary waves in a magnetized dusty plasma
Authors: Maharaj S. K. , Bharuthram R. , Singh S. V. , Pillay S. R. , Lakhina G. S.
Published: November 2008

The nonlinear evolution of driven low-frequency electrostatic waves is investigated in a three-component magnetized dusty plasma comprised of a warm dust fluid, electrons, and ions. Electrons as well as ions are considered to have Boltzmann distributions. The fluid equations for the dust along with the quasineutrality condition are used to obtain a single nonlinear differential equation for the electric field. Periodic solutions of the nonlinear differential equation yield sinusoidal, sawtooth and bipolar structures which are similar to nonlinear structures supported in electron-ion plasmas. Results of our findings are applied to Saturn’s rings.
Narrow-band deep-ultraviolet light emitting device using Al1−xGdxN
Authors: Kita Takashi , Kitayama Shinya , Kawamura Masashi , Wada Osamu , Chigi Yoshitaka , Kasai Yoshihiro , Nishimoto Tetsuro , Tanaka Hiroyuki , Kobayashi Mikihiro
Published: November 2008

We demonstrated mercury-free narrow-band deep-ultraviolet luminescence from field-emission devices with Al1−xGdxN thin films. The Al1−xGdxN thin films were grown on fused silica substrates by a radio frequency reactive magnetron sputtering method. The deposited film shows a strong c-axis preferential orientation. A resolution limited, narrow intra-4f luminescence line from Gd3+ ions has been observed at 315nm. The luminescence spectrum depends on the growth temperature of the thin film, and the intensity varies as a function of the GdN mole fraction.
Characterization of dilute InPN layers grown by liquid phase epitaxy
Authors: Das T. D. , Dhar S. , Arora B. M.
Published: November 2008

We have grown dilute InPN layers by liquid phase epitaxy and characterized them using high resolution x-ray diffraction, optical absorption, low temperature photoluminescence, and Hall measurement techniques. Our results indicate that a maximum amount of 0.2% nitrogen has been incorporated in the material with a band gap lowering consistent with expectations. The crystalline quality of the material is found to improve upon nitrogen incorporation. Large increase in luminescence from the material is observed after a high temperature annealing.
Strain behavior of lanthanum modified BiFeO3 thin films prepared via soft chemical method
Authors: Simões A. Z. , Aguiar E. C. , Gonzalez A. H. M. , Andrés J. , Longo E. , Varela J. A.
Published: November 2008

Pure and lanthanum modified BFO (LaxBi1−xFeO3, x=0.0,0.08,0.15,0.30) thin films were fabricated on Pt(111)/Ti/SiO2/Si substrates by the soft chemical method. The effect of La substitution on the structural and electrical properties was studied. Scanning electron microscopy, x-ray diffraction, and Raman spectroscopy have been employed to characterize the thin films while the piezoelectric measurements were carried out using a setup based on an atomic force microscope. It was found that La-doped BFO thin films exhibited good ferroelectric properties, such as improved leakage current density and retention-free characteristics. The unipolar strain is strongly reduced by the amount of lanthanum added to the system.
Composition dependence of electronic structure and optical properties of Hf1−xSixOy gate dielectrics
Authors: He G. , Zhang L. D. , Meng G. W. , Li G. H. , Fei G. T. , Wang X. J. , Zhang J. P. , Liu M. , Fang Q. , Boyd Ian W.
Published: November 2008

Composition-dependent electronic structure and optical properties of Hf1−xSixOy (0.1≤x≤0.6) gate dielectrics on Si at 450 °C grown by UV-photo-induced chemical vapor deposition (UV-CVD) have been investigated via x-ray photoemission spectroscopy and spectroscopy ellipsometry (SE). By means of the chemical shifts in the Hf 4f, Si 2p, and O 1s spectra, the Hf–O–Si bondings in the as-deposited films have been confirmed. Analyses of composition-dependent band alignment of Hf1−xSixOy/Si gate stacks have shown that the valence band (VB) offset (ΔEv) demonstrates little change; however, the values of conduction band offset (ΔEc) increase with the increase in the silicon atomic composition, resulting from the increase in the separation between oxygen 2p orbital VB state and antibonding d states intermixed of Hf and Si. Analysis by SE, based on the Tauc–Lorentz model, has indicated that decreases in the optical dielectric constant and increase in band gap have been observed as a function of silicon contents. Changes in the complex dielectric functions and band gap Eg related to the silicon concentration in the films are discussed systematically. From the band offset and band gap viewpoint, these results suggest that Hf1−xSixOy films provide sufficient tunneling barriers for electrons and holes, making them promising candidates as alternative gate dielectrics.
A new fast reconnection model in a collisionless regime
Authors: Tsiklauri David
Published: November 2008

Based on the first principles [i.e., (i) by balancing the magnetic field advection with the term containing electron pressure tensor nongyrotropic components in the generalized Ohm’s law; (ii) using the conservation of mass; and (iii) assuming that the weak magnetic field region width, where electron meandering motion supports electron pressure tensor off-diagonal (nongyrotropic) components, is of the order of electron Larmor radius] a simple model of magnetic reconnection in a collisionless regime is formulated. The model is general, resembling its collisional Sweet–Parker analog in that it is not specific to any initial configuration, e.g., Harris-type tearing unstable current sheet, X-point collapse or otherwise. In addition to its importance from the fundamental point of view, the collisionless reconnection model offers a much faster reconnection rate [Mc′less=(c∕ωpe)2∕(rL,eL)] than Sweet–Parker’s classical one (Msp=S−1∕2). The width of the diffusion region (current sheet) in the collisionless regime is found to be δc′less=(c∕ωpe)2∕rL,e, which is independent of the global reconnection scale L and is only prescribed by microphysics (electron inertial length, c∕ωpe, and electron Larmor radius, rL,e). Amongst other issues, the fastness of the reconnection rate alleviates, e.g., the problem of interpretation of solar flares by means of reconnection, as for the typical solar coronal parameters the obtained collisionless reconnection time can be a few minutes, as opposed to Sweet–Parker’s equivalent value of less than a day. The new theoretical reconnection rate is compared to the Magnetic Reconnection Experiment device experimental data by Yamada et al. [Phys. Plasmas 13, 052119 (2006)] and Ji et al. [Geophys. Res. Lett. 35, 13106 (2008)], and a good agreement is obtained.
Development of a local electromagnetic shielding for an extremity magnetic resonance imaging system
Authors: Handa Shinya , Haishi Tomoyuki , Kose Katsumi
Published: November 2008

A local radio frequency (rf) shielding consisting of a Cu plate and an LC balun circuit has been developed for a compact magnetic resonance imaging (MRI) system with a 0.3 T permanent magnet. Performance of the local rf shielding was evaluated using an artificial external noise source irradiating a human subject whose hand was inserted into the rf coil of the MRI system. Power spectra of the rf signal detected through the rf coil demonstrated that the local rf shield achieved 30.1 dB external noise suppression. With the local rf shielding, a MRI of the subject’s hand was performed using a three-dimensional gradient-echo sequence. Anatomical structures of the subject’s hand were clearly visualized. It was concluded that the local rf shielding could be used for the compact MRI system instead of a rf shielded room.
Study of electric and magnetic properties of (Bi0.9Pb0.1) (Fe0.9Ti0.1)O3 nanomultiferroic system
Authors: Singh K. , Kotnala R. K. , Singh M.
Published: November 2008

Multiferroic compound (Bi0.9Pb0.1)(Fe0.9Ti0.1)O3 (BPFTO), having particle size of 15–25 nm, was synthesized by solution combustion method. BPFTO shows the coexistence of ferroelectricity and magnetism at room temperature. Significant enhancement in magnetic moment with moderate value of the electrical polarization is observed for nano-BPFTO. Remanent polarization and maximum polarizations were 1.056 and 1.866 μC/cm2, respectively (at 20 kV/cm), with remanent magnetization of 285 memu/g. Such materials, possessing good value of dielectric constant and showing ferroelectric and ferromagnetism in same phase, are of great importance from technical point of view.
Solution-processed poly(3-hexylthiophene) vertical organic transistor
Authors: Li Sheng-Han , Xu Zheng , Yang Guanwen , Ma Liping , Yang Yang
Published: November 2008

The fabrication and operation of a solution-processed vertical organic transistor are now demonstrated. The vertical structure provides a large cross section and a short channel length to counter the inherent limitations of the organic materials. The operation of a vertical organic transistor relies on a transition metal oxide layer, V2O5, to lower the carrier injection barrier at the organic/metal interface. The effect of the oxide thickness was examined to verify the role of transition metal oxide in device operation. By studying the device performance at different temperatures and in solvent environments, an operating mechanism that occurs via an ion drift and doping process was proposed. The drift direction of the dissolved Li+ ion can be controlled by altering the gate voltage bias in order to change the carrier injection barrier.
Efficient formation of surface relief grating on azopolymer films by gold nanoparticles
Authors: Na Sung-Kwan , Kim Jung-Sung , Song Seok-Ho , Oh Cha-Hwan , Han Yang-Kyoo , Lee Young-Ho , Oh Seong-Geun
Published: November 2008

This paper reports a method for the efficient formation of surface relief grating (SRG) on azopolymer films in which gold nanoparticles are dispersed. The height and shape of the SRG, which were formed by a two-beam interference technique, were found to be dependent on the content of gold nanoparticles. The optimum concentration of gold nanoparticles for the efficient formation of SRG was confirmed by atomic force microscopy (AFM), and the height of SRG at the content of 0.06 wt % was about 2.2 times higher than that of pure azopolymer film without gold nanoparticles. This is due to the field enhancement effect derived from the localized plasmon excitations of gold nanoparticles dispersed in the azopolymer film.