bluuebunny/crossref_metadata_2025 huggingface dataset:
Infrasonic propagation from the 2010 Eyjafjallajökull eruption: Investigating the influence of stratospheric solar tides
Authors: Green D. N. , Matoza R. S. , Vergoz J. , Le Pichon A.
Published: October 2012

The stratospheric infrasound duct, formed between Earth's surface and altitudes of ∼50 km, is sensitive to spatiotemporal variations in stratospheric wind speed and temperature. Infrasound recorded at long range from the 2010 summit eruption of Eyjafjallajökull volcano, Iceland, exhibits temporal variability correlated with diurnal stratospheric solar tidal wind speed variations. Between 18 and 28 April 2010, signal observations at stations BKNI, U.K. (range, 1745 km), and IS18, Greenland (range, 2285 km), exhibit prominent diurnal variations in infrasonic amplitude, bandwidth, back azimuth, and apparent speed, which we identify using the CLEAN spectral analysis algorithm for unevenly sampled time series. Results of 3‐D acoustic ray tracing through operational atmospheric specifications indicate that tidal wind speed variations (with amplitudes of ∼20 m/s) can generate diurnal variations in the proportion of the acoustic wavefield propagating within the stratospheric acoustic duct. Range‐dependent meteorology is required; propagation modeling using averaged meteorological profiles fails to predict the leakage of acoustic energy out of the stratospheric acoustic duct at times of low observed signal amplitudes. Ray tracing correctly predicts the phase of the observed signal amplitude and apparent speed variations. Diurnal variability in ducting, combined with diurnal variations in ambient noise at the sensors, can explain the observed signal bandwidth variations. Back azimuth variations (observed only along the Eyjafjallajökull to BKNI path) are not predicted by 3‐D ray tracing. Tidal variations have implications for models of infrasound array network detection capability and for studies that utilize amplitude and bandwidth measurements to make inferences about the acoustic source.
Hygroscopic growth and CCN activity of HULIS from different environments
Authors: Kristensen Thomas B. , Wex Heike , Nekat Bettina , Nøjgaard Jacob K. , van Pinxteren Dominik , Lowenthal Douglas H. , Mazzoleni Lynn R. , Dieckmann Katrin , Bender Koch Christian , Mentel Thomas F. , Herrmann Hartmut , Gannet Hallar A. , Stratmann Frank , Bilde Merete
Published: October 2012

Humic‐like substances (HULIS) constitute a significant fraction of aerosol particles in different environments. Studies of the role of HULIS in hygroscopic growth and cloud condensation nuclei (CCN) activity of aerosol particles are scarce, and results differ significantly. In this work the hygroscopic growth and CCN activity of water extracts (WE) and HULIS extracted from particulate matter (PM) collected at a polluted urban site (Copenhagen, Denmark), a rural site (Melpitz, Germany) and the remote site Storm Peak Laboratory (Colorado, USA) were investigated. Measurements of inorganic ions, elemental carbon, organic carbon and water soluble organic carbon (WSOC) within the PM confirmed that the sources of aerosol particles most likely differed for the three samples. The hygroscopic properties of the filtered WE were characterized by hygroscopicity parameters for subsaturated conditions (κGF) of 0.25, 0.41 and 0.22, and for supersaturated conditions κCCN were 0.23, 0.29 and 0.22 respectively for the urban, rural and remote WE samples. The measured hygroscopic growth and CCN activity were almost identical for the three HULIS samples and could be well represented by κGF = 0.07 and κCCN = 0.08–0.10 respectively. Small amounts of inorganic ions were present in the HULIS samples so the actual values for pure HULIS are expected to be slightly lower (κGF* = 0.04–0.06 and κCCN* = 0.07–0.08). The HULIS samples are thus less hygroscopic compared to most previous studies. To aid direct comparison of hygroscopic properties of HULIS from different studies, we recommend that the fraction of inorganic species in the HULIS samples always is measured and reported.
Characterization of the morphometry of impact craters hosting polar deposits in Mercury's north polar region
Authors: Talpe Matthieu J. , Zuber Maria T. , Yang Di , Neumann Gregory A. , Solomon Sean C. , Mazarico Erwan , Vilas Faith
Published: October 2012

Earth‐based radar images of Mercury show radar‐bright material inside impact craters near the planet's poles. A previous study indicated that the polar‐deposit‐hosting craters (PDCs) at Mercury's north pole are shallower than craters that lack such deposits. We use data acquired by the Mercury Laser Altimeter on the MESSENGER spacecraft during 11 months of orbital observations to revisit the depths of craters at high northern latitudes on Mercury. We measured the depth and diameter of 537 craters located poleward of 45°N, evaluated the slopes of the northern and southern walls of 30 PDCs, and assessed the floor roughness of 94 craters, including nine PDCs. We find that the PDCs appear to have a fresher crater morphology than the non‐PDCs and that the radar‐bright material has no detectable influence on crater depths, wall slopes, or floor roughness. The statistical similarity of crater depth‐diameter relations for the PDC and non‐PDC populations places an upper limit on the thickness of the radar‐bright material (<170 m for a crater 11 km in diameter) that can be refined by future detailed analysis. Results of the current study are consistent with the view that the radar‐bright material constitutes a relatively thin layer emplaced preferentially in comparatively young craters.
Hydration dynamics promote bacterial coexistence on rough surfaces
Authors: Wang Gang , Or Dani
Published: October 2012

Identification of mechanisms that promote and maintain the immense microbial diversity found in soil is a central challenge for contemporary microbial ecology. Quantitative tools for systematic integration of complex biophysical and trophic processes at spatial scales, relevant for individual cell interactions, are essential for making progress. We report a modeling study of competing bacterial populations cohabiting soil surfaces subjected to highly dynamic hydration conditions. The model explicitly tracks growth, motion and life histories of individual bacterial cells on surfaces spanning dynamic aqueous networks that shape heterogeneous nutrient fields. The range of hydration conditions that confer physical advantages for rapidly growing species and support competitive exclusion is surprisingly narrow. The rapid fragmentation of soil aqueous phase under most natural conditions suppresses bacterial growth and cell dispersion, thereby balancing conditions experienced by competing populations with diverse physiological traits. In addition, hydration fluctuations intensify localized interactions that promote coexistence through disproportional effects within densely populated regions during dry periods. Consequently, bacterial population dynamics is affected well beyond responses predicted from equivalent and uniform hydration conditions. New insights on hydration dynamics could be considered in future designs of soil bioremediation activities, affect longevity of dry food products, and advance basic understanding of bacterial diversity dynamics and its role in global biogeochemical cycles.
A scanning tunneling microscope with a scanning range from hundreds of micrometers down to nanometer resolution
Authors: Kalkan Fatih , Zaum Christopher , Morgenstern Karina
Published: October 2012

A beetle type stage and a flexure scanning stage are combined to form a two stages scanning tunneling microscope (STM). It operates at room temperature in ultrahigh vacuum and is capable of scanning areas up to 300 μm × 450 μm down to resolution on the nanometer scale. This multi-scale STM has been designed and constructed in order to investigate prestructured metallic or semiconducting micro- and nano-structures in real space from atomic-sized structures up to the large-scale environment. The principle of the instrument is demonstrated on two different systems. Gallium nitride based micropillars demonstrate scan areas up to hundreds of micrometers; a Au(111) surface demonstrates nanometer resolution.
Observation of proximities between spin-1/2 and quadrupolar nuclei: Which heteronuclear dipolar recoupling method is preferable?
Authors: Lu X. , Lafon O. , Trébosc J. , Tricot G. , Delevoye L. , Méar F. , Montagne L. , Amoureux J. P.
Published: October 2012

We have recently shown that the dipolar-mediated heteronuclear multiple-quantum coherence (D-HMQC) method allows observing through-space proximities between spin-1/2 (1H, 13C, 31P…) and quadrupolar (23Na, 27Al…) nuclei. However, the D-HMQC effectiveness depends on the choice of the heteronuclear dipolar recoupling sequence. Here, we compare the efficiency and the robustness of four rotor-synchronized sequences: the symmetry-based ones, ${\rm R}4_1^2 {\rm R}4_1^{ - 2}$R412R41−2 and its super-cycled version, ${\rm SR}4_1^2$ SR 412, and two schemes based on simultaneous amplitude and frequency modulations, denoted SFAM-1 and SFAM-2. For the SFAM methods, we point out efficient recoupling conditions that facilitate their experimental optimization and we introduce analytical expressions for the buildup of D-HMQC signal in the case of an isolated spin pair. We show that the main differences between these four sequences lie in the number of adjustable parameters and in their robustness with respect to chemical shift and homonuclear dipolar interactions. The relative performances of these four recoupling sequences are analyzed using average Hamiltonian theory, numerical simulations, and 27Al-{31P} D-HMQC experiments on crystalline aluminophosphate.
Pseudo hard-sphere potential for use in continuous molecular-dynamics simulation of spherical and chain molecules
Authors: Jover J. , Haslam A. J. , Galindo A. , Jackson G. , Müller E. A.
Published: October 2012

We present a continuous pseudo-hard-sphere potential based on a cut-and-shifted Mie (generalized Lennard-Jones) potential with exponents (50, 49). Using this potential one can mimic the volumetric, structural, and dynamic properties of the discontinuous hard-sphere potential over the whole fluid range. The continuous pseudo potential has the advantage that it may be incorporated directly into off-the-shelf molecular-dynamics code, allowing the user to capitalise on existing hardware and software advances. Simulation results for the compressibility factor of the fluid and solid phases of our pseudo hard spheres are presented and compared both to the Carnahan-Starling equation of state of the fluid and published data, the differences being indistinguishable within simulation uncertainty. The specific form of the potential is employed to simulate flexible chains formed from these pseudo hard spheres at contact (pearl-necklace model) for mc = 4, 5, 7, 8, 16, 20, 100, 201, and 500 monomer segments. The compressibility factor of the chains per unit of monomer, mc, approaches a limiting value at reasonably small values, mc < 50, as predicted by Wertheim's first order thermodynamic perturbation theory. Simulation results are also presented for highly asymmetric mixtures of pseudo hard spheres, with diameter ratios of 3:1, 5:1, 20:1 over the whole composition range.
Experimental and molecular dynamics study of the growth of crystalline TiO2
Authors: Houska J. , Mraz S. , Schneider J. M.
Published: October 2012

Thin films of crystalline TiO2 are of high interest due to their photoactivity and photoinduced hydrophilicity. Previously, preparation of TiO2 has been described in terms of extrinsic process parameters, such as total pressure, oxygen partial pressure, or substrate bias potential. We study the growth of TiO2 phases, rutile and anatase, by atom-by-atom molecular dynamics simulations. We focus on the effect of intrinsic process parameters including particle energy, growth temperature, and growth template. While experiments indicate that the deposition of rutile requires higher temperatures and/or energies compared to anatase, we show that the growth of previously nucleated rutile can take place in a wider range of temperatures and energies compared to anatase. In parallel, we show relationships between crystal growth and the lateral size of a crystal. The results facilitate defining new synthesis pathways for TiO2, and constitute phenomena which may be relevant for other ceramics.
Resistive and ferritic-wall plasma dynamos in a sphere
Authors: Khalzov I. V. , Brown B. P. , Kaplan E. J. , Katz N. , Paz-Soldan C. , Rahbarnia K. , Spence E. J. , Forest C. B.
Published: October 2012

We numerically study the effects of varying electric conductivity and magnetic permeability of the bounding wall on a kinematic dynamo in a sphere for parameters relevant to Madison plasma dynamo experiment. The dynamo is excited by a laminar, axisymmetric flow of von Kármán type. The flow is obtained as a solution to the Navier-Stokes equation for an isothermal fluid with a velocity profile specified at the sphere's boundary. The properties of the wall are taken into account as thin-wall boundary conditions imposed on the magnetic field. It is found that an increase in the permeability of the wall reduces the critical magnetic Reynolds number Rmcr. An increase in the conductivity of the wall leaves Rmcr unaffected but reduces the dynamo growth rate.
Electromagnetic weak turbulence theory revisited
Authors: Yoon P. H. , Ziebell L. F. , Gaelzer R. , Pavan J.
Published: October 2012

The statistical mechanical reformulation of weak turbulence theory for unmagnetized plasmas including fully electromagnetic effects was carried out by Yoon [Phys. Plasmas 13, 022302 (2006)]. However, the wave kinetic equation for the transverse wave ignores the nonlinear three-wave interaction that involves two transverse waves and a Langmuir wave, the incoherent analogue of the so-called Raman scattering process, which may account for the third and higher-harmonic plasma emissions. The present paper extends the previous formalism by including such a term.
Low-dimensional model of turbulent mixed convection in a complex domain
Authors: Bailon-Cuba Jorge , Shishkina Olga , Wagner Claus , Schumacher Jörg
Published: October 2012

We construct a low-dimensional model (LDM) of turbulent mixed convection in a Cartesian cell with in- and outlets and local sources of heat which is narrow in one of the two horizontal space directions. The basis is a high-resolution three-dimensional direct numerical simulation (DNS) record. The model is derived with basis functions, which have been obtained by a proper orthogonal decomposition (POD) using the snapshot method. The POD analysis is applied for a sequence of three-dimensional snapshots as well as for data which are bulk-averaged in the direction of narrow extension. This step is taken since the flow is found to have no significant dependence along this direction in the cell. We compare the three-dimensional and two-dimensional POD modes. This simplification reduces the complexity of the problem significantly and allows us to construct and run a two-dimensional LDM with a small number of degrees of freedom. We study the long-time dynamical behavior of this system using a closure of the LDM based on a mode-dependent viscosity and diffusivity. The LDM has been optimized in terms of the standard deviation of the energy spectrum and the transient energy for different numbers of degrees of freedom by comparison with the original DNS data. We find that the evolution of the coherent structures of flow and temperature agrees well with the two-dimensional original data and determine their contribution to the global transfer of heat. Root-mean-square profiles of the fluctuations of the turbulent fields agree qualitatively well with the original simulation data, but deviate slightly in amplitude. We conclude that the reduction in the dimensionality and the number of degrees of freedom can reproduce the gross features of the mixed convection flow in this particular setup well.
Theoretical study of vapor-liquid homogeneous nucleation using stability analysis of a macroscopic phase
Authors: Carreón-Calderón Bernardo
Published: October 2012

Stability analysis is generally used to verify that the solution to phase equilibrium calculations corresponds to a stable state (minimum of the free energy). In this work, tangent plane distance analysis for stability of macroscopic mixtures is also used for analyzing the nucleation process, reconciling thus this analysis with classical nucleation theories. In the context of the revised nucleation theory, the driving force and the nucleation work are expressed as a function of the Lagrange multiplier corresponding to the mole fraction constraint from the minimization problem of stability analysis. Using a van der Waals fluid applied to a ternary mixture, Lagrange multiplier properties are illustrated. In particular, it is shown how the Lagrange multiplier value is equal to one on the binodal and spinodal curves at the same time as the driving force of nucleation vanishes on these curves. Finally, it is shown that, on the spinodal curve, the nucleation work from the revised and generalized nucleation theories are characterized by two different local minima from stability analysis, irrespective of any interfacial tension models.
Phospholipid bilayer relaxation dynamics as revealed by the pulsed electron-electron double resonance of spin labels
Authors: Syryamina V. N. , Dzuba S. A.
Published: October 2012

Electron paramagnetic resonance (EPR) spectroscopy in the form of pulsed electron-electron double resonance (ELDOR) was applied to 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) phospholipid bilayers containing lipids that were spin-labeled at different carbon positions along the lipid acyl chain. Pulsed ELDOR detects motionally induced spin flips of nitrogen nuclei in the nitroxide spin labels, which manifests itself as magnetization transfer (MT) in the nitroxide EPR spectrum. The MT effect was observed over a wide temperature range (100–225 K) on a microsecond time scale. In line with a previous study on molecular glasses [N. P. Isaev and S. A. Dzuba, J. Chem. Phys. 135, 094508 (2011)10.1063/1.3633241], the motions that induce MT effect were suggested to have the same nature as those in dielectric secondary (β) Johari-Goldstein fast relaxation. The results were compared with literature dielectric relaxation data for POPC bilayers, revealing some common features. Molecular motions resulting in MT are faster for deeper spin labels in the membrane interior. The addition of cholesterol to the bilayer suppresses the lipid motions near the steroid nucleus and accelerates the lipid motions beyond the steroid nucleus, in the bilayer interior. This finding was attributed to the lipid acyl chains being more ordered near the steroid nucleus and less ordered in the bilayer interior. The motions are absent in dry lipids, indicating that the motions are determined by intermolecular interactions in the bilayer.
Imprinting substrate structures onto a nematic liquid crystal
Authors: Greschek Manuel , Gubbins Keith E. , Schoen Martin
Published: October 2012

By means of Monte Carlo simulations in the grand canonical ensemble we study the morphology of the nematic phase of a simple model liquid crystal interacting with an alternating sequence of chemically different stripes. The stripes anchor molecules such that their orientation is either parallel or perpendicular with the substrate plane. The different molecular orientations are realized through anchoring functions that cause an energetic penalty for molecules oriented in an undesired fashion. We consider combinations of monostable and degenerate anchoring fields. The nature of the nematic phase is characterized through both the local nematic order parameter and the associated local director field. We observe states of uniaxial or biaxial symmetry depending on the ratio of stripe widths and the range of fluid-substrate attraction. In some cases the specific substrate pattern causes regions of biaxial symmetry to coexist with a bulk-like regime sufficiently far away from the substrates in which the local director field indicates a (homogeneous) bent state of the nematic liquid crystal.
Effect of paramagnetic Mn ion on the superconductivity of Cu0.5Tl0.5Ba2Ca2−xMnxCu3O10−y
Authors: Khurram A. A. , Qurat-ul-Ain  , Khan Nawazish A.
Published: October 2012

The Mn-doped Cu0.5Tl0.5Ba2(Ca2−xMnx)Cu3O10−y(x = 0, 0.15, 0.25, 0.35, 0.5, 0.75 1.0, and 1.25) superconductor samples were prepared at atmospheric pressure using solid state reaction method. The objectives of Mn doping at the Ca site were to improve the inter plane coupling and to see the effect of paramagnetic Mn atom on copper spins in the CuO2 planes and in turn on superconductivity in the light of charge stripe theory. The Mn doped samples have orthorhombic structure and the cell parameters were decreased after the increase of Mn concentration. The onset critical temperature and zero resistance critical temperature have been decreased, which were further decreased after oxygen annealing.
Vacuum-integrated electrospray deposition for highly reliable polymer thin film
Authors: Park Soohyung , Lee Younjoo , Yi Yeonjin
Published: October 2012

Vacuum electrospray deposition (ESD) equipment was designed to prepare polymer thin films. The polymer solution can be injected directly into vacuum system through multi-stage pumping line, so that the solvent residues and ambient contaminants are highly reduced. To test the performance of ESD system, we fabricated organic photovoltaic cells (OPVCs) by injecting polymer solution directly onto the substrate inside a high vacuum chamber. The OPVC fabricated has the structure of Al/P3HT:PCBM/PEDOT:PSS/ITO and was optimized by varying the speed of solution injection and concentration of the solution. The power conversion efficiency (PCE) of the optimized OPVC is 3.14% under AM 1.5G irradiation without any buffer layer at the cathode side. To test the advantages of the vacuum ESD, we exposed the device to atmosphere between the deposition steps of the active layer and cathode. This showed that the PCE of the vacuum processed device is 24% higher than that of the air exposed device and confirms the advantages of the vacuum prepared polymer film for high performance devices.
Effects of excess tellurium and growth parameters on the band gap defect levels in CdxZn1−xTe
Authors: Soundararajan Raji , Lynn Kelvin G.
Published: October 2012

This research summarizes an effective way to understand compensation for use of CdZnTe as ambient temperature radiation detector. The indium doped CdZnTe passivates certain detrimental intrinsic defects and defect complexes in the band gap. This was achieved by using a combination of excess tellurium in the starting material (0% to 7.5% by weight) and the process variables during growth, including the imposed temperature gradient, growth rate, and cool-down process. These studies have shown that a combination of slight excess tellurium as well as the cool-down scheme could control certain intrinsic defect levels and defect level complexes in the band gap of CdZnTe by causing favorable carrier compensation. At a macroscopic level, these manipulations help to minimize thermal instabilities during growth and determine the final grain structure, integrity, and yield of the ingot. Also, these manipulations help to control the formation of certain intrinsic defect levels and defect level complexes in the band gap, which have a direct bearing on the ability of the CdZnTe crystals to function as room temperature radiation detectors. The band-gap defects in CdZnTe were studied using the thermally stimulated current (TSC) technique. The thermal ionization energy and capture cross-section for 8 prominent defect levels (current peaks in the TSC spectrum) were calculated using the variable heating rate method. These fitted values were compared to transition energy levels of possible defects in the band gap of CdTe and purity data of CdZnTe samples used in this study. The theoretical values of the transition energy levels of defects in the band gap of CdTe were determined by the first principle band gap structure studies as well as our earlier studies on defects and defect levels in the band gap of CdTe. We have tentatively associated some prominent current peaks to certain defect levels and defect level complexes in Cd1−xZnxTe. The improvement in the detector properties was correlated to the reduction of a proposed deep level defect complex (TeCd + VCd) (thermal ionization energy >0.8 eV and capture cross-section of 10−13 to 10−14 cm2), and the reduction of the ionized species corresponding to an acceptor defect level (thermal ionization energy ∼0.2[03] eV), associated with dislocations/dislocation complexes with Te clusters. The best crystals tested had an average μτe (electrons) of 1.8 × 10−3 cm2/V, a peak-to-valley ratio of 2.0 for the 122 keV x-ray peak using a Co-57 source and bulk resistivity the order of 3 × 1010 Ω cm. The best radiation detector crystals corresponded to those grown with an excess tellurium of 0.5% (by weight in the starting CdZnTe charge) grown at a rate of 0.5 to 0.86 mm/h with an imposed temperature gradient of > 50 °C/in. There was a high yield without any post-processing of the ingots.
First principle molecular dynamic simulation of the rapid solidification process of Ca50Mg20Cu30 alloy
Authors: Chang H. J. , Chen L. F. , Zhu X. F.
Published: October 2012

The rapid solidification processes of Ca50Mg20Cu30 liquid alloy have been simulated by first principle molecular dynamics simulation based on the density functional theory. The local structural evolution of the alloy is analysed using Honeycutt Andersen (HA) bond-type index and bond-angle distribution methods. The electronic properties of this amorphous solid are also studied. The simulated coordination numbers are very close to the theoretical values according to the efficient cluster packing model (ECP) model. The interaction between Ca-Cu atomic pairs is strongest in this alloy. The HA bond-type result shows that a large quantity of pentagonal bipyramids are formed in undercooled alloy liquid and become most common polyhedral local structures of the amorphous solid, which suppress the crystallization and increase the glass forming ability of Ca-Mg-Cu alloy. The bond-angle distribution indicates that the close packing of the three neighbor atoms and the pentagon configurations become the primary short range order (SRO) as temperature decreases. The electronic density demonstrates that the covalent bond of Ca-Mg, Ca-Cu, Mg-Cu, and Cu-Cu pairs is existed in this alloy. This chemical SRO also benefits the glass transition.
Monte Carlo simulation of thermal conductivity of Si nanowire: An investigation on the phonon confinement effect on the thermal transport
Authors: Bera Chandan
Published: October 2012

Thermal conductivity of Si nanowire is calculated by applying Monte Carlo (MC) simulation of 110 nm, 37 nm, and 22 nm wire diameter. To study the thermal conductivity of both thick and thin nanowires different phonon group velocity is used in the simulation. This change in the phonon velocity for small diameter nanowire is due to the phonon confinement effect, which decreases the slope of phonon acoustic modes. Very good agreement with previously reported experimental value is obtained for all nanowire diameters. Another investigation by using average relaxation time approximation is also discussed along with the Monte Carlo simulation.
Co-existence of whistler waves with kinetic Alfven wave turbulence for the high-beta solar wind plasma
Authors: Mithaiwala Manish , Rudakov Leonid , Crabtree Chris , Ganguli Gurudas
Published: October 2012

It is shown that the dispersion relation for whistler waves is identical for a high or low beta plasma. Furthermore, in the high-beta solar wind plasma, whistler waves meet the Landau resonance with electrons for velocities less than the thermal speed, and consequently, the electric force is small compared to the mirror force. As whistlers propagate through the inhomogeneous solar wind, the perpendicular wave number increases through refraction, increasing the Landau damping rate. However, the whistlers can survive because the background kinetic Alfven wave (KAW) turbulence creates a plateau by quasilinear (QL) diffusion in the solar wind electron distribution at small velocities. It is found that for whistler energy density of only ∼10−3 that of the kinetic Alfven waves, the quasilinear diffusion rate due to whistlers is comparable to KAW. Thus, very small amplitude whistler turbulence can have a significant consequence on the evolution of the solar wind electron distribution function.
Bidirectionally tuning Kapitza conductance through the inclusion of substitutional impurities
Authors: Duda John C. , English Timothy S. , Piekos Edward S. , Beechem Thomas E. , Kenny Thomas W. , Hopkins Patrick E.
Published: October 2012

We investigate the influence of substitutional impurities on Kapitza conductance at coherent interfaces via non-equilibrium molecular dynamics simulations. The reference interface is comprised of two mass-mismatched Lennard-Jones solids with atomic masses of 40 and 120 amu. Substitutional impurity atoms with varying characteristics, e.g., mass or bond, are arranged about the interface in Gaussian distributions. When the masses of impurities fall outside the atomic masses of the reference materials, substitutional impurities impede interfacial thermal transport; on the other hand, when the impurity masses fall within this range, impurities enhance transport. Local phonon density of states calculations indicate that this observed enhancement can be attributed to a spatial grading of vibrational properties near the interface. Finally, for the range of parameters investigated, we find that the mass of the impurity atoms plays a dominant role as compared to the impurity bond characteristics.
Local structure origin of higher glass forming ability in Ta doped Co65B35 amorphous alloy
Authors: Wang H. , Hu T. , Qin J. Y. , Zhang T.
Published: October 2012

By using first principles molecular dynamics, the correlation between composition, local structure, and dynamics was investigated to reveal the origin of higher glass forming ability in Ta doped Co65B35 amorphous alloy. The types and quantity of dominant polyhedra are quite similar in Co55Ta10B35 and Co65B35 supercooled liquid; however, in the former these polyhedra exhibit larger average lifetime. Further components analysis indicates polyhedra containing Ta atom play a critical role in slowing down the cluster mobility. The relative slower atomic diffusivity in Co55Ta10B35 supercooled liquid indicates Ta addition results in a higher viscosity and difficulty in atomic rearrangement. Moreover, the dominant cluster Co7Ta1B2 in Co55Ta10B35 amorphous alloys has larger combined energy than the dominant cluster Co8B2 in Co65B35 amorphous alloy. Compared with Co, Ta donates more electrons to B, which leads to the strengthening of B–B bond and thus stabilizes the local structure. It is proposed that by enhancing the stability of local structure, Ta addition greatly lowers the cluster mobility, slows down the dynamics, and thus promotes the glass formation.
Dielectric and polarized Raman spectroscopic studies on 0.85Pb(Zn1/3Nb2/3)O3-0.15PbTiO3 single crystal
Authors: Mishra K. K. , Arora A. K. , Tripathy S. N. , Pradhan Dillip
Published: October 2012

Tetragonal-cubic phase transition has been investigated in relaxor-ferroelectric 0.85Pb(Zn1/3Nb2/3)O3-0.15PbTiO3 single crystal using dielectric and Raman spectroscopy. A detailed analysis of the dielectric data suggests that the transition is of second order. In the tetragonal phase (P4mm), all the modes predicted by group theory were found in the Raman spectra and were assigned based on the symmetry and polarization configuration. Frequencies of several modes were found to disappear while a few modes exhibited discontinuous change across the phase transition temperature TC ∼473 K. While in the high temperature cubic phase (Pm3¯m) no first order Raman spectrum is expected, the presence of several Raman peaks at elevated temperature suggests substitutional disorder causing the appearance of symmetry-forbidden Raman bands. The line-width of A1(TO) mode at 273 cm−1 shows anomalies across TC and the intermediate temperature T*. Furthermore, based on the temperature dependence of total integrated intensities of all the modes in the polarized and depolarized spectra, the Burns temperature TB, T*, and TC are identified at 650, 525, and 473 K, respectively.
Tunable symmetric and asymmetric resonances in an asymmetrical split-ring metamaterial
Authors: Shi J. H. , Zhu Z. , Ma H. F. , Jiang W. X. , Cui T. J.
Published: October 2012

We demonstrate the coexistence of two tunable symmetric and asymmetric resonances in a metamaterial composed of asymmetrical split-rings (ASRs) patterned on a dielectric layer numerically and experimentally. The full-wave simulation and measurement results demonstrate that the metamaterial reveals a symmetric cross-polarization transmission band with a ripple-free peak and asymmetric co-polarization transmission bands characterized by trapped-mode resonances. Both symmetric and asymmetric resonances can be easily tuned via the incident angle of electromagnetic waves. The resonant excitation and coupling of the electric and magnetic dipole moments contribute to the conversion of two orthogonal linear polarizations. The ASR metamaterial shows a directionally asymmetric transmission for both linearly and circularly polarized waves for large angles of incidence. The proposed ASR metamaterial is of importance to develop novel metamaterial-based devices.
Ferromagnetic fluctuation in doped armchair graphene nanoribbons
Authors: Ma Tianxing , Liu Suhang , Gao Pan , Huang Zhongbing , Lin Hai-Qing
Published: October 2012

Within determinant quantum Monte Carlo simulation, it is found that the armchair graphene nanoribbons show carrier mediated magnetic correlation. The antiferromagnetic correlation dominates around half filling, while the ferromagnetic correlation dominates as electron filling is lower than 0.8. As the ferromagnetic properties sensitively depend on electron fillings, it may be manipulated by electric gates. Moreover, the ferromagnetic correlation is strengthened markedly as the next-nearest-neighbor hopping energy increases. The predicted manipulation of ferromagnetic correlation may be realized in armchair graphene nanoribbons and ultracold atoms system.
Ionization effects in the generation of wake-fields by ultra-high contrast femtosecond laser pulses in argon gas
Authors: Makito K. , Zhidkov A. , Hosokai T. , Shin J.-H. , Masuda S. , Kodama R.
Published: October 2012

Difference in mechanisms of wake-field generation and electron self-injection by high contrast femtosecond laser pulses in an initially neutral Argon gas and in pre-ionized plasma without ionization is studied via 2D particle-in-cell simulations including optical ionization of the media. For shorter laser pulses, 40 fs, ionization results only in an increase of the charge of accelerated electrons by factor of ∼3 with qualitatively the same energy distribution. For longer pulses, 80 fs, a more stable wake field structure is observed in the neutral gas with the maximal energy of the accelerated electrons exceeding that in the fixed density plasma. In higher density Argon, an ionizing laser pulse converts itself to a complex system of solitons at a self-induced, critical density ramp.
Ion acceleration from thin foil and extended plasma targets by slow electromagnetic wave and related ion-ion beam instability
Authors: Bulanov S. V. , Esirkepov T. Zh. , Kando M. , Pegoraro F. , Bulanov S. S. , Geddes C. G. R. , Schroeder C. B. , Esarey E. , Leemans W. P.
Published: October 2012

When ions are accelerated by the radiation pressure of a laser pulse, their velocity cannot exceed the pulse group velocity which can be considerably smaller than the speed of light in vacuum. This is demonstrated in two cases corresponding to a thin foil target irradiated by high intensity laser light and to the hole boring produced in an extended plasma by the laser pulse. It is found that the beams of accelerated ions are unstable against Buneman-like and Weibel-like instabilities which results in the broadening of the ion energy spectrum.
Observation of a strong correlation between electromagnetic soliton formation and relativistic self-focusing for ultra-short laser pulses propagating through an under-dense plasma
Authors: Zhu Bin , Wu Yu-Chi , Dong Ke-Gong , Hong Wei , Teng Jian , Zhou Wei-Min , Cao Lei-Feng , Gu Yu-Qiu
Published: October 2012

A strong correlation is observed between the formation of electromagnetic solitons, generated during the interaction of a short intense laser pulse (30 fs, ∼1018 W/cm2) with a rarefied (<0.1nc) plasma, and pulse self-focusing. Pulse defocusing, which occurs after soliton generation, results in laser-pulse energy depletion. The role of stimulated Raman scattering in soliton generation is analyzed from 2D particle-in-cell simulations. An observed relationship between initial plasma density and soliton generation is presented that might have relevance to wake-field accelerators.
Thermoelectric properties of highly doped n-type polysilicon inverse opals
Authors: Ma Jun , Sinha Sanjiv
Published: October 2012

Nanostructured single-crystal silicon exhibits a remarkable increase in the figure of merit for thermoelectric energy conversion. Here we theoretically investigate a similar enhancement for polycrystalline silicon inverse opals. An inverse opal provides nanoscale grains and a thin-film like geometry to scatter phonons preferentially over electrons. Using solutions to the Boltzmann transport equation for electrons and phonons, we show that the figure of merit at 300 K is fifteen times that of bulk single-crystal silicon. Our models predict that grain boundaries are more effective than surfaces in enhancing the figure of merit. We provide insight into this effect and show that preserving a grain size smaller than the shell thickness of the inverse opal increases the figure of merit by as much as 50% when the ratio between the two features is a third. At 600 K, the figure of merit is as high as 0.6 for a shell thickness of 10 nm. This work advances the fundamental understanding of charge and heat transport in nanostructured inverse opals.
Enhanced carrier transport by defect passivation in Si/SiO2 nanostructure-based solar cells
Authors: Watanabe Keiji , Tsuchiya Ryuta , Mine Toshiyuki , Yonamoto Yoshiki , Akamatsu Naotoshi , Hatano Mutsuko
Published: October 2012

We investigate the relationship between the defect states and the carrier transport property of Si nanostructure-based solar cells. The solar cell consists of a Schottky junction including Si/SiO2 multiple quantum wells. The carrier transport is significantly enhanced by forming gas annealing of Si/SiO2 multiple quantum wells, which is well correlated with the decrease in the Pb and E′ center densities evaluated by electron spin resonance. In particular, we find that high temperature (>600 °C) annealing is necessary to passivate E′ center. Our results demonstrate the significance of defect passivation for the realization of high efficiency Si nanostructure-based solar cells.
Magnetic field-dependent effective microwave properties of microwire-epoxy composites
Authors: Qin F. X. , Peng H. X. , Fuller J. , Brosseau C.
Published: October 2012

In this work, we report on the observation of the electromagnetic parameters of glass-covered amorphous microwires/epoxy composites with various concentrations of microwires over a broad magnetic field range up to 1 kOe. The effective permittivity presents a strong tunability with respect to a dc magnetic field excitation. An inherent crossover field due to the microwires is further observed at 300 Oe suggesting that the giant magnetoimpedance effect dominates the features of dipolar absorption at low magnetic field bias (<300 Oe), while the opposite behavior is expected at larger dc magnetic field excitations. This crossover field is found to be insensitive to changes in the microwire content.
Transition from planar to island growth mode in SiGe structures fabricated on SiGe/Si(001) strain-relaxed buffers
Authors: Shaleev M. V. , Novikov A. V. , Yurasov D. V. , Hartmann J. M. , Kuznetsov O. A. , Lobanov D. N. , Krasilnik Z. F.
Published: October 2012

The specifics of the two-to-three dimensional growth mode transition of pure Ge on SiGe/Si(001) strain-relaxed buffers (SRBs) were investigated. It was shown that the critical thickness for elastic relaxation is influenced not only by the lattice mismatch between the substrate and the film but also by Ge segregation and surface roughness dependent on parameters of strained layers. Critical thickness was found to be smaller for Ge grown on SiGe strain-relaxed buffers than on pure Si(001) substrates, in spite of the lesser lattice mismatch. Insertion of thin tensile-strained Si layers between Ge and SiGe strain-relaxed buffers increased the critical thickness.
Capillary filling dynamics of water in nanopores
Authors: Bakli Chirodeep , Chakraborty Suman
Published: October 2012

We portray a universal description of dynamic slip-stick behavior of water flowing through nanoscale pores. Based on fundamental molecular transport considerations, we derive a generalized constitutive model for describing resistive forces acting on the water column in a capillary that is being dynamically filled, as a combined function of the meniscus height, surface wettability, and roughness. This effectively acts like a unique signature of nanopore imbibition characteristics of water, which, when substituted in a simple one-dimensional force balance model agrees quantitatively with results from molecular dynamics simulations for a general class of problems, without necessitating the employment of any artificially tunable fitting parameters.
Reduced temperature sensitivity of the polarization properties of hydrogenated InGaAsN V-groove quantum wires
Authors: Felici Marco , Polimeni Antonio , Tartaglini Elena , Notargiacomo Andrea , De Luca Marta , Carron Romain , Fekete Dan , Dwir Benjamin , Rudra Alok , Capizzi Mario , Kapon Eli
Published: October 2012

We investigated the effects of hydrogen irradiation on the degree of linear polarization, ρ, of the light emitted by site-controlled, dilute-nitride InGaAsN V-groove quantum wires (QWRs). While in the as-grown sample the polarization of the QWR emission is highly sensitive to the increasing temperature (T), after sample hydrogenation the value of ρ remains nearly unchanged (and ∼25%) for T ≤ 220 K. This observation—potentially important for the development of devices based on the QWR polarization—points to a larger energy separation between hole subbands in the hydrogenated QWRs, due to the strain increase associated with the H-induced passivation of nitrogen.
Three-port beam splitter for slow neutrons using holographic nanoparticle-polymer composite diffraction gratings
Authors: Klepp J. , Tomita Y. , Pruner C. , Kohlbrecher J. , Fally M.
Published: October 2012

Diffraction of slow neutrons by nanoparticle-polymer composite gratings has been observed. By carefully choosing grating parameters such as grating thickness and spacing, a three-port beam splitter operation for slow neutrons—splitting the incident neutron intensity equally into the ±1st and the 0th diffraction orders—has been realized. As a possible application, a Zernike three-path interferometer is briefly discussed.
High power femtosecond Bessel-X pulses directly from a compact fiber laser system
Authors: Xie Chen , Hu Minglie , Xu Zongwei , Wu Wei , Gao Haifeng , Qin Peng , Zhang Dapeng , Liu Bowen , Wang Ching-Yue
Published: October 2012

We report on high power ultrashort Bessel-X pulses directly from a compact fiber laser system. For spatial profiles, non-diffracting Bessel-like wavepackets were generated with the combination of a collimating lens and a silica-based inverse micro-axicon, which is specially designed and fabricated on the facet of an amplifying fiber doped by ytterbium. For temporal profiles, the seed pulses from a mode-locked dissipative soliton fiber laser at a repetition rate of 56.9 MHz were pre-shaped by a grating pair and a spectral filter. Amplified 18.7 W average power chirped ps pulses were obtained and then dechirped to a duration of 38 fs.
Determination of 16O and 18O sensitivity factors and charge-exchange processes in low-energy ion scattering
Authors: Téllez H. , Chater R. J. , Fearn S. , Symianakis E. , Brongersma H. H. , Kilner J. A.
Published: October 2012

Quantitative analysis in low-energy ion scattering (LEIS) requires an understanding of the charge-exchange processes to estimate the elemental sensitivity factors. In this work, the neutralization of He+ scattered by 18O-exchanged silica at energies between 0.6 and 7 keV was studied. The process is dominated by Auger neutralization for Ei < 0.8 keV. An additional mechanism starts above the reionization threshold. This collision-induced neutralization becomes the dominant mechanism for Ei > 2 keV. The ion fractions P+ were determined for Si and O using the characteristic velocity method to quantify the surface density. The 18O/16O sensitivity ratio indicates an 18% higher sensitivity for the heavier O isotope.
Note: Free energy calculations for atomic solids through the Einstein crystal/molecule methodology using GROMACS and LAMMPS
Authors: Aragones J. L. , Valeriani C. , Vega C.
Published: October 2012

In this work the free energy of solid phases is computed for the Lennard-Jones potential and for a model of NaCl. The free energy is evaluated through the Einstein crystal/molecule methodologies using the molecular dynamics programs: GROMACS and LAMMPS. The obtained results are compared with the results obtained from Monte Carlo. Good agreement between the different programs and methodologies was found. The procedure to perform the free energy calculations for the solid phase in the molecular dynamic programs is described. Since these programs allow to study any continuous intermolecular potential (when given in a tabulated form) this work shows that for isotropic potentials (describing for instance atomic solids or colloidal particles) free energy calculations can be performed on a routinely basis using GROMACS and/or LAMMPS.
Electrical and microstructural analyses of 200 MeV Ag14+ ion irradiated Ni/GaN Schottky barrier diode
Authors: Kumar Ashish , Hähnel A. , Kanjilal D. , Singh R.
Published: October 2012

Ni/GaN Schottky barrier diodes were irradiated with 200 MeV Ag ions up to fluence of 1 × 1011 ions/cm2. The current-voltage measurements showed that the ideality factor, n, increased and the reverse leakage current, IR, decreased with increase in fluence. But, Schottky barrier height increased only marginally with increase in fluence. Cross-sectional transmission electron microscopy images revealed the presence of defect clusters in bulk GaN after irradiation. However, the Ni/GaN interface did not show any intermixing or degradation after the irradiation.
Assembled Fe3O4 nanoparticles on graphene for enhanced electromagnetic wave losses
Authors: Guan P. F. , Zhang X. F. , Guo J. J.
Published: October 2012

We present an one-step procedure for the synthesis of hybrid Fe3O4 nanoparticles/graphene composite with self-assemble superlattices of Fe3O4 nanoparticles. We find that the Fe3O4/graphene nanocomposite exhibits both enhanced dielectric losses and magnetic losses at 2–18 GHz, resulting from the interfacial polarizations between the Fe3O4 nanoparticles and the graphene, as well as the coupling between the neighbouring Fe3O4 nanoparticles. Theoretical simulations uncover the origins of the electromagnetic losses of hybrid Fe3O4/graphene nanocomposite and provide additional insight into the microscopic dielectric polarization mechanism.