bluuebunny/crossref_metadata_2025 huggingface dataset:
Perpendicular propagation of magnetosonic waves
Authors: Chen Lunjin , Thorne Richard M.
Published: June 2012

Azimuthal and radial propagation characteristics of perpendicularly propagating fast magnetosonic waves in an axis‐symmetrical medium with the presence of plasmapause is investigated analytically based on Snell's law. We find thatQ = nr sin ψ is conserved during the propagation, where n is wave refractive index, ris geo‐centered distance, andψ is wave azimuthal angle with zero pointing radially outward. The radial range of wave propagation can be determined by comparing Q and the radial profile of nr.Trapped waves, which are bounded in a narrow radial range but migrate azimuthally and even circularly, are identified inside the plasmapause over a broad range of wave azimuthal angles and over a broad range of wave frequencies from the proton gyrofrequency to the lower hybrid resonance frequency. In contrast untrapped waves launched inside and outside the plasmasphere can travel azimuthally 0–4 hrs and 0–7 hrs in local times respectively. The substantial radial and azimuthal propagation may account for the presence of magnetosonic waves away from the source region.
The orientation and current density of the magnetotail current sheet: A statistical study of the effect of geomagnetic conditions
Authors: Davey E. A. , Lester M. , Milan S. E. , Fear R. C. , Forsyth C.
Published: June 2012

We examine the orientation and current density of the current sheet during current sheet crossings from Cluster's 2001–2007 tail seasons. The curlometer technique is used to estimate the current density and is combined with Minimum Variance Analysis (MVA) to calculate the direction of the current sheet normal. The SYM‐H and AE indices at the time of each crossing are employed to assess how the tilt angle (the angle the normal makes with the Z axis in the GSM YZ plane) and current density depend on geomagnetic conditions. Our results indicate a larger current sheet tilt in the YZ plane during intervals of stronger and/or more prolonged substorm activity, as indicated by the AE index. There is also evidence that when the ring current is enhanced during magnetic storms, the current sheet is less tilted even though the AE index is also disturbed. In addition larger current densities are seen during times of both magnetic storms and substorms, compared to crossings during only substorms and a quiet ring current. We conclude that increased substorm activity disrupts the current sheet structure resulting in greater motion of the current sheet (as found by Davey et al. (2012)) and a greater local tilt to the current sheet. We propose that the increased open flux in the tail during magnetic storms stabilizes the current sheet such that the tilt angle of the current sheet is reduced. The increased amount of open flux during magnetic storms also results in larger current densities within the current sheet.
Localized adaptive inflation in ensemble data assimilation for a radiation belt model
Authors: Godinez H. C. , Koller J.
Published: June 2012

In this work a one‐dimensional radial diffusion model for phase space density, together with observational satellite data, is used in an ensemble data assimilation with the purpose of accurately estimating Earth's radiation belt particle distribution. A particular concern in data assimilation for radiation belt models are model deficiencies, which can adversely impact the solution of the assimilation. To adequately address these deficiencies, a localized adaptive covariance inflation technique is implemented in the data assimilation to account for model uncertainty. Numerical results from identical‐twin experiments, where data is generated from the same model, as well as the assimilation of real observational data, are presented. The results show improvement in the predictive skill of the model solution due to the proper inclusion of model errors in the data assimilation.
Ionosphere state and parameter estimation using the Ensemble Square Root Filter and the global three‐dimensional first‐principle model
Authors: Solomentsev D. V. , Khattatov B. V. , Codrescu M. V. , Titov A. A. , Yudin V. , Khattatov V. U.
Published: June 2012

In the present paper we discuss the setup and the results of series of numerical experiments aiming to recover the
plasma drift and neutral wind velocities using the Ensemble Square Root Filter together with the ionospheric numerical model. One of the objectives of the current research was assessing the performance of the upper atmosphere state and parameter ensemble estimation technique in the framework of the Observational System Simulation Experiment (OSSE). The other purpose was to improve calculation accuracy for the major driving forces in the ionosphere and to increase modeling reliability in real‐data operational cases. In the current paper we describe the setup of the modeling system used to obtain the presented results. In the first section we introduce the background physics‐based model used in the simulations and discuss its main assumptions along with
drift and the neutral wind velocity calculation algorithms. Further we present the observations simulation system and describe the data used for assimilation and parameter estimation. We also provide a brief description of the Ensemble Square Root Filter and its application in the current study. In the last few sections the results of the numerical experiments are presented and discussed.
Sitting and Supine Esophageal Pressures in Overweight and Obese Subjects
Authors: Owens Robert L. , Campana Lisa M. , Hess Lauren , Eckert Danny J. , Loring Stephen H. , Malhotra Atul
Published: June 2012

Esophageal pressure (PEs) can be used to approximate pleural pressure (Ppl) and might be clinically useful, particularly in the obese e.g., to guide mechanical ventilator settings in critical illness. However, mediastinal artifact (the difference between true Ppl and PEs) may limit acceptance of the measurement, and reproducibility of PEs measurements remains unknown. Therefore, we aimed to assess the effect of body posture on PEs in a cohort of obese, but healthy subjects, some of whom had multiple measurements, to address the clinical robustness of esophageal manometry. Twenty‐five overweight and obese subjects (BMI > 25 kg/m2) and 11 control lean subjects (BMI < 25 kg/m2) underwent esophageal manometry with pressures measured seated and supine. Twenty overweight and obese subjects had measurements repeated after ∼1 to 2 weeks. Anthropometric data and sitting and supine spirometry were recorded. The average end‐expiratory PEs sitting and supine were greater in the overweight and obese group than the lean group (sitting −0.1 ± 2.1 vs. −3.3 ± 1.2 cmH2O, supine 9.3 ± 3.3 vs. 6.9 ± 2.8 cmH2O, respectively). The mean differences between repeated measurements were small (−0.3 ± 1.7 cmH2O sitting and −0.1 ± 1.5 cmH2O supine). PEs correlated with a number of anthropometric and spirometric variables. In conclusion, PEs are slightly greater in overweight and obese subjects than lean subjects; but changes with position are similar in both groups. These data indicate that mediastinal weight and postural effects on PEs are within a clinically acceptable range, and suggest that esophageal manometry can be used to inform clinical decision making across wide range of body types.
Eosinophilic Esophagitis: The New Kid on the Block
Authors: Shastri Nikhil , Hogan Walter
Published: June 2012

Management of esophageal dysfunction must be addressed in order to optimize swallow function and, often, patient perception of function. Increased presentation of eosinophilic esophagitis (EoE) has been identified in patients across the lifespan. This article expounds upon the clinical manifestation, diagnostic indicators, and treatment management of EoE. Researchers are currently exploring treatments commonly found in the pediatric literature and finding them to possibly have positive outcomes with the adult population.
Evaluation of Upper Esophageal Sphincter Function
Authors: Sharma Tarun , Massey Benson T.
Published: June 2012

Under normal circumstances, the upper esophageal sphincter (UES) serves as an important gatekeeper, regulating antegrade and retrograde bolus movement through the upper aerodigestive tract. Dysfunction of the UES can result in multiple distressing symptoms and serious complications; accurate diagnosis is critical to appropriate management. The complex anatomy and physiology of the UES precludes any single available testing modality from completely assessing all aspects of UES dysfunction. This article provides an overview of the basic anatomy and physiology of the UES and reviews the advantages and shortcomings of the current techniques for evaluating its function.
Comment on “Continuum modes in rotating plasmas: General equations and continuous spectra for large aspect ratio tokamaks” [Phys. Plasmas 18, 092103 (2011)]
Authors: Goedbloed J. P.
Published: June 2012

It is shown that some of the main results of the recent paper by Lakhin and Ilgisonis [Phys. Plasmas 18, 092103 (2011)], viz. the derivation of the equations for the continuous spectra of poloidally and toroidally rotating plasmas and their special solution for large aspect ratio tokamaks with large parallel flows were obtained before by Goedbloed, Beliën, van der Holst, and Keppens [Phys. Plasmas 11, 28 (2004)]. A further rearrangement of the system of equations for the coupled Alfvén and slow continuous spectra clearly exhibits: (a) coupling through a single tangential derivative, which is a generalization of the geodesic curvature; (b) the “transonic” transitions of the equilibrium, which need to be carefully examined in order to avoid entering hyperbolic flow regimes where the stability formalism breaks down. A critical discussion is devoted to the implications of this failure, which is generally missed in the tokamak literature, possibly as a result of the wide-spread use of the sonic Mach number of gas dynamics, which is an irrelevant and misleading parameter in “transonic” magnetohydrodynamics. Once this obstacle in understanding is removed, further application of the theory of trans-slow Alfvén continuum instabilities to both tokamaks, with possible implications for the L-H transition, and astrophysical objects like “fat” accretion disks, with a possible new route to magnetohydrodynamic turbulence, becomes feasible.
Supersonic helium beam diagnostic for fluctuation measurements of electron temperature and density at the Tokamak TEXTOR
Authors: Kruezi U. , Stoschus H. , Schweer B. , Sergienko G. , Samm U.
Published: June 2012

A supersonic helium beam diagnostic, based on the line-ratio technique for high resolution electron density and temperature measurements in the plasma edge (r/a > 0.9) was designed, built, and optimised at TEXTOR (Torus Experiment for Technology Oriented Research). The supersonic injection system, based on the Campargue skimmer-nozzle concept, was developed and optimised in order to provide both a high neutral helium beam density of n0 = 1.5 × 1018 m−3 and a low beam divergence of ±1° simultaneously, achieving a poloidal resolution of Δpoloidal = 9 mm. The setup utilises a newly developed dead volume free piezo valve for operation in a high magnetic field environment of up to 2 T with a maximum repetition rate of 80 Hz. Gas injections are realised for a duration of 120 ms at a repetition rate of 2 Hz (duty cycle 1/3). In combination with a high sensitivity detection system, consisting of three 32 multi-channel photomultipliers (PMTs), measurements of edge electron temperature and density with a radial resolution of Δradial = 2 mm and a maximum temporal resolution of Δt ≃ 2 μs (470 kHz) are possible for the first time. The diagnostic setup at TEXTOR is presented. The newly developed injection system and its theoretical bases are discussed. The applicability of the stationary collisional-radiative model as basis of the line-ratio technique is shown. Finally, an example of a fluctuation analysis demonstrating the unique high temporal and spatial resolution capabilities of this new diagnostic is presented.
Effects of adiabatic, relativistic, and quantum electrodynamics interactions on the pair potential and thermophysical properties of helium
Authors: Cencek Wojciech , Przybytek Michał , Komasa Jacek , Mehl James B. , Jeziorski Bogumił , Szalewicz Krzysztof
Published: June 2012

The adiabatic, relativistic, and quantum electrodynamics (QED) contributions to the pair potential of helium were computed, fitted separately, and applied, together with the nonrelativistic Born-Oppenheimer (BO) potential, in calculations of thermophysical properties of helium and of the properties of the helium dimer. An analysis of the convergence patterns of the calculations with increasing basis set sizes allowed us to estimate the uncertainties of the total interaction energy to be below 50 ppm for interatomic separations R smaller than 4 bohrs and for the distance R = 5.6 bohrs. For other separations, the relative uncertainties are up to an order of magnitude larger (and obviously still larger near R = 4.8 bohrs where the potential crosses zero) and are dominated by the uncertainties of the nonrelativistic BO component. These estimates also include the contributions from the neglected relativistic and QED terms proportional to the fourth and higher powers of the fine-structure constant α. To obtain such high accuracy, it was necessary to employ explicitly correlated Gaussian expansions containing up to 2400 terms for smaller R (all R in the case of a QED component) and optimized orbital bases up to the cardinal number X = 7 for larger R. Near-exact asymptotic constants were used to describe the large-R behavior of all components. The fitted potential, exhibiting the minimum of −10.996 ± 0.004 K at R = 5.608 0 ± 0.000 1 bohr, was used to determine properties of the very weakly bound 4He2 dimer and thermophysical properties of gaseous helium. It is shown that the Casimir-Polder retardation effect, increasing the dimer size by about 2 Å relative to the nonrelativistic BO value, is almost completely accounted for by the inclusion of the Breit-interaction and the Araki-Sucher contributions to the potential, of the order α2 and α3, respectively. The remaining retardation effect, of the order of α4 and higher, is practically negligible for the bound state, but is important for the thermophysical properties of helium. Such properties computed from our potential have uncertainties that are generally significantly smaller (sometimes by nearly two orders of magnitude) than those of the most accurate measurements and can be used to establish new metrology standards based on properties of low-density helium.
The effect of viscosity on the kinetics of redox reactions in highly viscous silicate liquids
Authors: Kido Ladislav , Müller Matthias , Rüssel Christian
Published: June 2012

The kinetics of the temperature dependent redox reaction between chromium and manganese (Cr6+ + 3Mn2+⇌Cr3+ + 3Mn3+) in highly viscous silicate melts were studied by UV-vis-NIR spectroscopy at temperatures in the range from 25 to 800 °C. At high temperatures, the reaction is in equilibrium. During cooling, it is continuously shifted to the right. During cooling from Tg+50 K to Tg (Tg = glass transition temperature), a further decrease in the Cr6+ concentration was obtained which, however, was less pronounced if larger cooling rates were applied. In this temperature range, the kinetics plays an important part. Finally, at a certain temperatures below Tg, the equilibrium was frozen. The temperature, the equilibrium is frozen in decreases with decreasing cooling rate. It also decreases with the glass transition temperature of the respective composition. The activation energies increase with the activation energies of the viscosity of the respective melt. The redox reaction is controlled by the viscosity, i.e., the rearrangement of the glass network and not by diffusion. The reason is a drastic change in the coordination spheres during the reaction which leads to a high inner reorganization energy according to Marcus’ Theory
Oxidation of CO by NO on planar and faceted Ir(210)
Authors: Chen Wenhua , Bartynski Robert A. , Kaghazchi Payam , Jacob Timo
Published: June 2012

Oxidation of CO by pre-adsorbed NO has been studied on planar Ir(210) and nanofaceted Ir(210) with average facet sizes of 5 nm and 14 nm by temperature programmed desorption (TPD). Both surfaces favor oxidation of CO to CO2, which is accompanied by simultaneous reduction of NO with high selectivity to N2. At low NO pre-coverage, the temperature (Ti) for the onset of CO2 desorption as well as CO2 desorption peak temperature (Tp) decreases with increasing CO exposure, and NO dissociation is affected by co-adsorbed CO. At high NO pre-coverage, Ti and Tp are independent of CO exposure, and co-adsorbed CO has no influence on dissociation of NO. Moreover, at low NO pre-coverage, planar Ir(210) is more active than faceted Ir(210) for oxidation of CO to CO2: Ti and Tp are much lower on planar Ir(210) than that on faceted Ir(210). In addition, faceted Ir(210) with an average facet size of 5 nm is more active for oxidation of CO to CO2 than faceted Ir(210) with an average facet size of 14 nm, i.e., oxidation of CO by pre-adsorbed NO on faceted Ir(210) exhibits size effects on the nanometer scale. In comparison, at low O pre-coverage planar Ir(210) is more active than faceted Ir(210) for oxidation of CO to CO2 but no evidence has been found for size effects in oxidation of CO by pre-adsorbed oxygen on faceted Ir(210) for average facet sizes of 5 nm and 14 nm. The TPD data indicate the same reaction pathway for CO2 formation from CO + NO and CO + O reactions on planar Ir(210). The adsorption sites of CO, NO, O, CO + O, and CO + NO on Ir are characterized by density functional theory.
Potential energy and dipole moment surfaces of HCO− for the search of H− in the interstellar medium
Authors: Ayouz M. , Mikhailov I. , Babikov D. , Raoult M. , Galtier S. , Dulieu O. , Kokoouline V.
Published: June 2012

Potential energy and permanent dipole moment surfaces of the electronic ground state of formyl negative ion HCO− are determined for a large number of geometries using the coupled-cluster theory with single and double and perturbative treatment of triple excitations ab initio method with a large basis set. The obtained data are used to construct interpolated surfaces, which are extended analytically to the region of large separations between CO and H− with the multipole expansion approach. We have calculated the energy of the lowest rovibrational levels of HCO− that should guide the spectroscopic characterization of HCO− in laboratory experiments. The study can also help to detect HCO− in the cold and dense regions of the interstellar medium where the anion could be formed through the association of abundant CO with still unobserved H−.
Water dynamics in silica nanopores: The self-intermediate scattering functions
Authors: Milischuk Anatoli A. , Krewald Vera , Ladanyi Branka M.
Published: June 2012

The dynamics of water molecules confined in approximately cylindrical silica nanopores is investigated using molecular simulation. The model systems are pores of diameter varying between 20 and 40 Å containing water at room temperature and at full hydration, prepared using grand canonical Monte Carlo simulation. Water dynamics in these systems is studied via molecular dynamics simulation. The results of the basic characterization of these systems have been reported in A. A. Milischuk and B. M. Ladanyi [J. Chem. Phys. 135, 174709 (2011)]. The main focus of the present study is the self-intermediate scattering function (ISF), FS(Q, t), of water hydrogens, the observable in quasi-elastic neutron scattering experiments. We investigate how FS(Q, t) depends on the pore diameter, the direction and magnitude of the momentum transfer Q, and the proximity of water molecules to the silica surface. We also study the contributions to FS(Q, t) from rotational and translational motions of water molecules and the extent of rotation-translation coupling present in FS(Q, t). We find that FS(Q, t) depends strongly on the pore diameter and that this dependence is due mainly to the contributions to the ISF from water translational motion and can be attributed to the decreased mobility of water molecules near the silica surface. The relaxation rate depends on the direction of Q and is faster for Q in the axial than in the radial direction. As the magnitude of Q increases, this difference diminishes but does not disappear. We find that its source is mainly the anisotropy in translational diffusion at low Q and in molecular reorientation at higher Q values.
Optimizing hierarchical equations of motion for quantum dissipation and quantifying quantum bath effects on quantum transfer mechanisms
Authors: Ding Jin-Jin , Xu Rui-Xue , Yan YiJing
Published: June 2012

We present an optimized hierarchical equations of motion theory for quantum dissipation in multiple Brownian oscillators bath environment, followed by a mechanistic study on a model donor-bridge-acceptor system. We show that the optimal hierarchy construction, via the memory-frequency decomposition for any specified Brownian oscillators bath, is generally achievable through a universal pre-screening search. The algorithm goes by identifying the candidates for the best be just some selected Padé spectrum decomposition based schemes, together with a priori accuracy control criterions on the sole approximation, the white-noise residue ansatz, involved in the hierarchical construction. Beside the universal screening search, we also analytically identify the best for the case of Drude dissipation and that for the Brownian oscillators environment without strongly underdamped bath vibrations. For the mechanistic study, we quantify the quantum nature of bath influence and further address the issue of localization versus delocalization. Proposed are a reduced system entropy measure and a state-resolved constructive versus destructive interference measure. Their performances on quantifying the correlated system-environment coherence are exemplified in conjunction with the optimized hierarchical equations of motion evaluation of the model system dynamics, at some representing bath parameters and temperatures. Analysis also reveals the localization to delocalization transition as temperature decreases.
Surface hopping simulation of vibrational predissociation of methanol dimer
Authors: Jiang Ruomu , Sibert Edwin L.
Published: June 2012

The mixed quantum-classical surface hopping method is applied to the vibrational predissociation of methanol dimer, and the results are compared to more exact quantum calculations. Utilizing the vibrational SCF basis, the predissociation problem is cast into a curve crossing problem between dissociative and quasibound surfaces with different vibrational character. The varied features of the dissociative surfaces, arising from the large amplitude OH torsion, generate rich predissociation dynamics. The fewest switches surface hopping algorithm of Tully [J. Chem. Phys. 93, 1061 (1990)10.1063/1.459170] is applied to both diabatic and adiabatic representations. The comparison affords new insight into the criterion for selecting the suitable representation. The adiabatic method's difficulty with low energy trajectories is highlighted. In the normal crossing case, the diabatic calculations yield good results, albeit showing its limitation in situations where tunneling is important. The quadratic scaling of the rates on coupling strength is confirmed. An interesting resonance behavior is identified and is dealt with using a simple decoherence scheme. For low lying dissociative surfaces that do not cross the quasibound surface, the diabatic method tends to overestimate the predissociation rate whereas the adiabatic method is qualitatively correct. Analysis reveals the major culprits involve Rabi-like oscillation, treatment of classically forbidden hops, and overcoherence. Improvements of the surface hopping results are achieved by adopting a few changes to the original surface hopping algorithms.
An adaptive potential energy surface generation method using curvilinear valence coordinates
Authors: Richter F. , Carbonniere P. , Dargelos A. , Pouchan C.
Published: June 2012

An automatic Born-Oppenheimer potential energy surface (PES) generation method AGAPES is presented designed for the calculation of vibrational spectra of large rigid and semi-rigid polyatomic molecules within the mid-infrared energy range. An adaptive approach guided by information from intermediate vibrational calculations in connection with a multi-mode expansion of the PES in internal valence coordinates is used and its versatility is tested for a selection of molecules: HNO, HClCO, and formaldoxime. Significant computational savings are reported. The possibility of linear scaling of the sampling grid size with the molecular size due to decrease of correlation of remote coordinates in large molecules is examined and finally, possible improvements are suggested.
Reaction rate calculation with time-dependent invariant manifolds
Authors: Bartsch Thomas , Revuelta F. , Benito R. M. , Borondo F.
Published: June 2012

The identification of trajectories that contribute to the reaction rate is the crucial dynamical ingredient in any classical chemical reactivity calculation. This problem often requires a full scale numerical simulation of the dynamics, in particular if the reactive system is exposed to the influence of a heat bath. As an efficient alternative, we propose here to compute invariant surfaces in the phase space of the reactive system that separate reactive from nonreactive trajectories. The location of these invariant manifolds depends both on time and on the realization of the driving force exerted by the bath. These manifolds allow the identification of reactive trajectories simply from their initial conditions, without the need of any further simulation. In this paper, we show how these invariant manifolds can be calculated, and used in a formally exact reaction rate calculation based on perturbation theory for any multidimensional potential coupled to a noisy environment.
Quasi-equilibria in reduced Liouville spaces
Authors: Halse Meghan E. , Dumez Jean-Nicolas , Emsley Lyndon
Published: June 2012

The quasi-equilibrium behaviour of isolated nuclear spin systems in full and reduced Liouville spaces is discussed. We focus in particular on the reduced Liouville spaces used in the low-order correlations in Liouville space (LCL) simulation method, a restricted-spin-space approach to efficiently modelling the dynamics of large networks of strongly coupled spins. General numerical methods for the calculation of quasi-equilibrium expectation values of observables in Liouville space are presented. In particular, we treat the cases of a time-independent Hamiltonian, a time-periodic Hamiltonian (with and without stroboscopic sampling) and powder averaging. These quasi-equilibrium calculation methods are applied to the example case of spin diffusion in solid-state nuclear magnetic resonance. We show that there are marked differences between the quasi-equilibrium behaviour of spin systems in the full and reduced spaces. These differences are particularly interesting in the time-periodic-Hamiltonian case, where simulations carried out in the reduced space demonstrate ergodic behaviour even for small spins systems (as few as five homonuclei). The implications of this ergodic property on the success of the LCL method in modelling the dynamics of spin diffusion in magic-angle spinning experiments of powders is discussed.
The effective conductivity of a periodic lattice of circular inclusions
Authors: Godin Yuri A.
Published: June 2012

We determine the effective conductivity of a two-dimensional composite consisting of a doubly periodic array of identical circular cylinders within a homogeneous matrix. The problem is reduced to the solution of an infinite system of linear equations. The effective conductivity tensor is obtained in the form of the series expansion in terms of the volume fraction of the cylinders whose coefficients are determined exactly. Results are illustrated by examples.
Azide–water intermolecular coupling measured by two-color two-dimensional infrared spectroscopy
Authors: Borek Joanna , Perakis Fivos , Kläsi Felix , Garrett-Roe Sean , Hamm Peter
Published: June 2012

We utilize two-color two-dimensional infrared spectroscopy to measure the intermolecular coupling between azide ions and their surrounding water molecules in order to gain information about the nature of hydrogen bonding of water to ions. Our findings indicate that the main spectral contribution to the intermolecular cross-peak comes from population transfer between the asymmetric stretch vibration of azide and the OD-stretch vibration of D2O. The azide-bound D2O bleach/stimulated emission signal, which is spectrally much narrower than its linear absorption spectrum, shows that the experiment is selective to solvation shell water molecules for population times up to ∼500 fs. The waters around the ion are present in an electrostatically better defined environment. Afterwards, ∼1 ps, the sample thermalizes and selectivity is lost. On the other hand, the excited state absorption signal of the azide-bound D2O is much broader. The asymmetry in spectral width between bleach/stimulated emission versus excited absorption has been observed in very much the same way for isotope-diluted ice Ih, where it has been attributed to the anharmonicity of the OD potential.
Bulk and shear relaxation in glasses and highly viscous liquids
Authors: Buchenau U.
Published: June 2012

The ratio δB/δG between the couplings of a relaxational process to compression and shear, respectively, is calculated in the Eshelby picture of structural rearrangements within a surrounding elastic matrix, assuming a constant density of stable structures in distortion space. The result is compared to experimental data for the low-temperature tunneling states in glasses and to Prigogine-Defay data at the glass transition, both from the literature.
A path-integral Langevin equation treatment of low-temperature doped helium clusters
Authors: Ing Christopher , Hinsen Konrad , Yang Jing , Zeng Toby , Li Hui , Roy Pierre-Nicholas
Published: June 2012

We present an implementation of path integral molecular dynamics for sampling low temperature properties of doped helium clusters using Langevin dynamics. The robustness of the path integral Langevin equation and white-noise Langevin equation [M. Ceriotti, M. Parrinello, T. E. Markland, and D. E. Manolopoulos, J. Chem. Phys. 133, 124104 (2010)]10.1063/1.3489925 sampling methods are considered for those weakly bound systems with comparison to path integral Monte Carlo (PIMC) in terms of efficiency and accuracy. Using these techniques, convergence studies are performed to confirm the systematic error reduction introduced by increasing the number of discretization steps of the path integral. We comment on the structural and energetic evolution of HeN−CO2 clusters from N = 1 to 20. To quantify the importance of both rotations and exchange in our simulations, we present a chemical potential and calculated band origin shifts as a function of cluster size utilizing PIMC sampling that includes these effects. This work also serves to showcase the implementation of path integral simulation techniques within the molecular modelling toolkit [K. Hinsen, J. Comp. Chem. 21, 79 (2000)]10.1002/(SICI)1096-987X(20000130)21:2<79::AID-JCC1>3.0.CO;2-B, an open-source molecular simulation package.
A simplified implementation of van der Waals density functionals for first-principles molecular dynamics applications
Authors: Wu Jun , Gygi François
Published: June 2012

We present a simplified implementation of the non-local van der Waals correlation functional introduced by Dion et al. [Phys. Rev. Lett. 92, 246401 (2004)] and reformulated by Román-Pérez et al. [Phys. Rev. Lett. 103, 096102 (2009)]. The proposed numerical approach removes the logarithmic singularity of the kernel function. Complete expressions of the self-consistent correlation potential and of the stress tensor are given. Combined with various choices of exchange functionals, five versions of van der Waals density functionals are implemented. Applications to the computation of the interaction energy of the benzene-water complex and to the computation of the equilibrium cell parameters of the benzene crystal are presented. As an example of crystal structure calculation involving a mixture of hydrogen bonding and dispersion interactions, we compute the equilibrium structure of two polymorphs of aspirin (2-acetoxybenzoic acid, C9H8O4) in the P21/c monoclinic structure.
Transcorrelated calculations of homogeneous electron gases
Authors: Luo Hongjun
Published: June 2012

We have constructed the complete transcorrelated equation for homogeneous electron gases and investigated this equation on two- and three-dimensional systems. Correct asymptotic behaviours of the correlation factors can be easily obtained from the transcorrelated equation, both the long-range RPA type decay and the short-range spin dependent cusp conditions. The complete transcorrelated equation is solved numerically and the outcome correlation energies agree very well with variational quantum Monte Carlo results. Possible simplifications of the transcorrelated calculations are discussed, where we find that the RPA equation for the correlation factor can be considerably improved by adding one more term in the equation.
Rapid anharmonic vibrational corrections derived from partial Hessian analysis
Authors: Hanson-Heine Magnus W. D. , George Michael W. , Besley Nicholas A.
Published: June 2012

Vibrational analysis within a partial Hessian framework can successfully describe the vibrational properties of a variety of systems where the vibrational modes of interest are localized within a speciﬁc region of the system. We have developed a new approach to calculating anharmonic frequencies based on vibrational frequencies and normal modes obtained from a partial Hessian analysis using second-order vibrational perturbation theory and the transition optimized shifted Hermite method. This allows anharmonic frequencies for vibrational modes that are spatially localized to be determined at a significantly reduced computational cost. Several molecular systems are examined in order to demonstrate the effectiveness of this method including organic molecules adsorbed on the Si(100)-2×1 surface, model peptides in solution, and the C–H stretching region of polycyclic aromatic hydrocarbons. Overall, for a range of systems, anharmonic frequencies calculated using the partial Hessian approach are found to be in close agreement with the results obtained using full anharmonic calculations while providing a significant reduction in computational cost.
Interpretation of the photoelectron spectra of superalkali species: Na3O and Na3O−
Authors: Zein S. , Ortiz J. V.
Published: June 2012

Recently measured photoelectron spectra of the Na3O− anion have been interpreted with the aid of ab initio electron propagator calculations. As in the case of the Li3O−, we propose that the photoionization of ground and excited neutral states, in a sequential two photon absorption mechanism, plays a role in the interpretation of the observed spectrum. The lowest vertical electron detachment energy of Na3O− corresponds to a Dyson orbital that is composed chiefly of diffuse Na s functions and connects a D3h singlet anion to an uncharged species with the same point group. Electron binding energies of isomers of the anion with different point groups or multiplicities have been considered. The relative magnitudes of the ionization energies of the neutral Li3O and Na3O species are also discussed. Whereas the most recent experimental data hold that Na3O has the higher ionization energy, this work asserts the opposite trend.
Application of a three-dimensional model for a study of the energy transfer of a high-pressure mercury horizontal lamp
Authors: Ben Hamida M. B. , Charrada K.
Published: June 2012

This paper is devoted to study the dynamics of a discharge lamp with high intensity in a horizontal position. As an example of application, we chose the high-pressure mercury lamp. For this, we realized a three-dimensional model, a stable and powered DC. After the validation of this model, we used it to reproduce the influence of some parameters that have appeared on major transport phenomena of mass and energy in studying the lamp operating in a horizontal position. Indeed, the mass of mercury and the electric current are modified and the effect of convective transport is studied.
Interfacial friction between semiflexible polymers and crystalline surfaces
Authors: Priezjev Nikolai V.
Published: June 2012

The results obtained from molecular dynamics simulations of the friction at an interface between polymer melts and weakly attractive crystalline surfaces are reported. We consider a coarse-grained bead-spring model of linear chains with adjustable intrinsic stiffness. The structure and relaxation dynamics of polymer chains near interfaces are quantified by the radius of gyration and decay of the time autocorrelation function of the first normal mode. We found that the friction coefficient at small slip velocities exhibits a distinct maximum which appears due to shear-induced alignment of semiflexible chain segments in contact with solid walls. At large slip velocities, the friction coefficient is independent of the chain stiffness. The data for the friction coefficient and shear viscosity are used to elucidate main trends in the nonlinear shear rate dependence of the slip length. The influence of chain stiffness on the relationship between the friction coefficient and the structure factor in the first fluid layer is discussed.
Excitation wavelength dependent photoluminescence in structurally non-uniform Si/SiGe-island heteroepitxial multilayers
Authors: Modi N. , Lockwood D. J. , Wu X. , Baribeau J.-M. , Tsybeskov L.
Published: June 2012

In nanometer-size Si/SiGe-island heteroepitxial multilayers grown on Si(001), low temperature photoluminescence spectra are observed that strongly depend on the excitation wavelength and show a strong correlation with structural properties revealed by transmission electron microscopy. These experimental results can be explained by assuming that the optically created carriers are strongly localized at Si/SiGe island heterointerfaces. We show that electron-hole pairs are generated and recombine within spatial regions mainly defined by the photoexcitation penetration depth, and that the estimated exciton diffusion length is notably short and comparable with the SiGe-island average size.
Characterizing and modeling an electoral campaign in the context of Twitter: 2011 Spanish Presidential election as a case study
Authors: Borondo J. , Morales A. J. , Losada J. C. , Benito R. M.
Published: June 2012

Transmitting messages in the most efficient way as possible has always been one of politicians’ main concerns during electoral processes. Due to the rapidly growing number of users, online social networks have become ideal platforms for politicians to interact with their potential voters. Exploiting the available potential of these tools to maximize their influence over voters is one of politicians’ actual challenges. To step in this direction, we have analyzed the user activity in the online social network Twitter, during the 2011 Spanish Presidential electoral process, and found that such activity is correlated with the election results. We introduce a new measure to study political sentiment in Twitter, which we call the relative support. We have also characterized user behavior by analyzing the structural and dynamical patterns of the complex networks emergent from the mention and retweet networks. Our results suggest that the collective attention is driven by a very small fraction of users. Furthermore, we have analyzed the interactions taking place among politicians, observing a lack of debate. Finally, we develop a network growth model to reproduce the interactions taking place among politicians.
Spectroscopic observation of nitrous oxide pentamers
Authors: Rezaei Mojtaba , Oliaee J. Norooz , Moazzen-Ahmadi N. , McKellar A. R. W.
Published: June 2012

Two new infrared bands in the ν1 fundamental region of N2O are observed in a supersonic jet expansion and assigned to nitrous oxide pentamers. Each band is measured using both 14N216O and 15N216O. Although they are similar in appearance, the bands have slightly different lower state rotational parameters, and are thus assigned to distinct structural isomers of the pentamer. Cluster calculations using two N2O intermolecular potentials give results in good agreement with the observed spectra, and indicate that the two isomers probably have the same basic structure (which is unsymmetrical), but differ in the alignment (N-N-O or O-N-N) of one or two of the constituent monomers. Calculations using a resonant dipole interaction model also support the proposed assignment and structure. These are the first reported high-resolution spectra for N2O pentamers.
Size-dependent infrared properties of MgO nanoparticles with evidence of screening effect
Authors: Chalopin Yann , Dammak Hichem , Hayoun Marc , Besbes Mondher , Greffet Jean-Jacques
Published: June 2012

We have investigated the infrared (IR) absorption properties of MgO nanoparticles (NPs) with the means of molecular dynamics simulations. Several size effects have been observed. We show in particular that the absorption of IR radiation does not occur predominantly through the polariton mode but preferentially through surface modes. This enhanced surface absorption is found to result from the absence of dielectric screening of the first atomic layer of the NPs. We demonstrate concomitantly that a macroscopic description of electrodynamics is inadequate to capture these unusual IR properties.
Afterpulse-like noise limits dynamic range in time-gated applications of thin-junction silicon single-photon avalanche diode
Authors: Dalla Mora A. , Contini D. , Pifferi A. , Cubeddu R. , Tosi A. , Zappa F.
Published: June 2012

We describe a source of noise in thin-junction silicon single-photon avalanche diode arising after strong illumination either during the ON (voltage above breakdown) or the OFF (voltage below breakdown) time. It increases the background noise with respect to primary dark count rate similarly to the afterpulsing process, but it is not related to a previous detector ignition. The amount of noise is linearly dependent on the power of light impinging on the detector and time constants are independent of the electric field. This phenomenon is the main limiting factor for the dynamic-range during time-gated measurements in condition of strong illumination.
Long lifetime, high density single-crystal erbium compound nanowires as a high optical gain material
Authors: Yin Leijun , Ning Hao , Turkdogan Sunay , Liu Zhicheng , Nichols Patricia L. , Ning C. Z.
Published: June 2012

Erbium-containing materials of long lifetime and high Er density are important for achieving strong luminescence and high optical gain in compact integrated photonics devices. We have systematically studied the lifetime and crystal quality as a function of growth conditions for an erbium compound that we recently reported, erbium chloride silicate (ECS). The lifetime for the best quality ECS nanowires can be as long as 540 μs, the longest for high-density Er-materials, representing a lifetime-density product as high as 8.7 × 1018 s cm−3. Such high density, long lifetime erbium materials can find many interesting applications such as compact lasers or amplifiers.
Structural and optical properties of InAs/AlAsSb quantum dots with GaAs(Sb) cladding layers
Authors: Simmonds Paul J. , Babu Laghumavarapu Ramesh , Sun Meng , Lin Andrew , Reyner Charles J. , Liang Baolai , Huffaker Diana L.
Published: June 2012

We investigate the effect of GaAs1−xSbx cladding layer composition on the growth and properties of InAs self-assembled quantum dots surrounded by AlAs0.56Sb0.44 barriers. Lowering Sb-content in the GaAs1−xSbx improves the morphology of the InAs quantum dots and reduces cladding layer alloy fluctuations. The result is a dramatic increase in photoluminescence intensity from the InAs quantum dots, with a peak at 0.87 eV. The emission energy exhibits a cube root dependence on excitation power, consistent with the type-II band alignment of the quantum dots. The characteristics of this quantum dot system show promise for applications such as intermediate band solar cells.
A compact lab-on-a-chip nanosensor for glycerol detection
Authors: Zhao Jiheng , Hashmi Ali , Xu Jie , Xue Wei
Published: June 2012

Real-time monitoring of glycerol concentration has significant meaning in many lab-on-a-chip applications. The existing sensors for glycerol detection have complicated fabrication and testing procedures and are not truly compatible with microfluidic systems for on-site detection. In this study, we explore the possibility of using an integrated single-walled carbon nanotubes nanosensor for glycerol detection. Our device enables real-time, in-channel detection of the concentration of static or flowing aqueous glycerol solutions. Sensor resistance is found to increase with an increasing glycerol-to-water weight ratio and is sensitive to flow velocity. We also highlight the sensing mechanisms for both conditions.
Sheath width effect on the determination of plasma frequency in the cutoff probe
Authors: Kim D. W. , You S. J. , Kim J. H. , Chang H. Y. , Oh W. Y.
Published: June 2012

To analyze the sheath width effect on the determination of the plasma frequency in the cutoff probe, two simulations (a simplified circuit simulation and an E/M simulation) were conducted. Both two simulations showed a consistent result: the cutoff frequency ωc, which is selected as a frequency of minimum peak in a transmission microwave frequency (TMF) spectrum, deviates from the plasma frequency ωp while increasing the sheath width on the two probe tips. The physics behind the movement of cutoff frequency in the TMF spectrum deviating from the plasma frequency was analyzed and discussed in this paper. This study can provide not only the basis for the precise measurement of the cutoff probe but also the discernible boundary conditions for the cutoff probe application.
High-temperature ferroic phase transitions and paraelectric cubic phase in multiferroic Bi0.95+δFe0.9Zr0.1O3
Authors: Wei Jie , Haumont Raphael , Jarrier Romain , Berthet Patrick , Dkhil Brahim
Published: June 2012

The temperature-dependent phase transitions of Bi0.95+δFe0.9Zr0.1O3 have been studied using high-temperature x-ray powder diffraction together with differential scanning calorimetry measurements. The results show that Bi0.95+δFe0.9Zr0.1O3 undergoes two phase transitions at 815 °C and 905 °C before decomposition at 920 °C. It appears that Zr-doping seems to stabilize the high temperature phases in such compound. Both the sharp contraction in the unit-cell volume and enthalpy thermal hysteresis demonstrate that the ferroelectric phase transition (α←→β) at 815 °C is of first order nature. In contrast, the highest β←→γ phase transition at 905 °C appears to be a second-order-like one. The analysis of the diffraction pattern in addition to Rietveld refinement strongly suggests a cubic symmetry for γ-phase.
Control of the azimuthal orientation of grains in polycrystalline Si films
Authors: Asakawa T.
Published: June 2012

Multi-directional ion beam irradiation is applied to control the crystal orientation of Si thin films during growth via plasma-enhanced chemical vapor deposition. The primary ion beam is generated by electron cyclotron resonance using an SiH4/H2 mixture, and the auxiliary beam is obtained by redirecting a portion of the primary beam with a metal reflector. These two beams simultaneously hit the surface of the growing film from different directions. With this arrangement, we succeeded in controlling the orientation of the growth of deposited Si grains in virtually the same direction.