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  • American Institute of Physics (AIP)  (61,554)
  • 2010-2014  (61,554)
  • 1
    Publication Date: 2014-01-31
    Description: A new tandem mass spectrometry (MS/MS) method based on time of flight measurements performed on an event-by-event detection technique is presented. This “ correlated ion and neutral time of flight ” method allows to explore Collision Induced Dissociation (CID) fragmentation processes by directly identifying not only all ions and neutral fragments produced but also their arrival time correlations within each single fragmentation event from a dissociating molecular ion. This constitutes a new step in the characterization of molecular ions. The method will be illustrated here for a prototypical case involving CID of protonated water clusters H + (H 2 O) n = 1–5 upon collisions with argon atoms.
    Print ISSN: 0034-6748
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  • 2
    Publication Date: 2014-01-31
    Description: Non-contact scanning probe microscopy (SPM) has developed into a powerful technique to image many different properties of samples. The conventional method involves monitoring the amplitude, phase, or frequency of a cantilever oscillating at or near its resonant frequency as it is scanned across the surface of a sample. For high Q factor cantilevers, monitoring the resonant frequency is the preferred method in order to obtain reasonable scan times. This can be done by using a phase-locked-loop (PLL). PLLs can be obtained as commercial integrated circuits, but these do not have the frequency resolution required for SPM. To increase the resolution, all-digital PLLs requiring sophisticated digital signal processors or field programmable gate arrays have also been implemented. We describe here a hybrid analog/digital PLL where most of the components are implemented using discrete analog integrated circuits, but the frequency resolution is provided by a direct digital synthesis chip controlled by a simple peripheral interface controller (PIC) microcontroller. The PLL has excellent frequency resolution and noise, and can be controlled and read by a computer via a universal serial bus connection.
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  • 3
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    American Institute of Physics (AIP)
    Publication Date: 2014-02-01
    Description: A magnetic loading technique was used to study the strength of polycrystalline tantalum ramp compressed to peak stresses between 60 and 250 GPa. Velocimetry was used to monitor the planar ramp compression and release of various tantalum samples. A wave profile analysis was then employed to determine the pressure-dependence of the average shear stress upon unloading at strain rates on the order of 10 5  s −1 . Experimental uncertainties were quantified using a Monte Carlo approach, where values of 5% in the estimated pressure and 9–17% in the shear stress were calculated. The measured deviatoric response was found to be in good agreement with existing lower pressure strength data as well as several strength models. Significant deviations between the experiments and models, however, were observed at higher pressures where shear stresses of up to 5 GPa were measured. Additionally, these data suggest a significant effect of the initial material processing on the high pressure strength. Heavily worked or sputtered samples were found to support up to a 30% higher shear stress upon release than an annealed material.
    Print ISSN: 0021-8979
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  • 4
    Publication Date: 2014-02-01
    Description: We report bulk SmCo 3 /α-Fe nanocomposite magnets prepared via high energy ball milling and warm compaction. The evolution of structure and magnetic properties with soft phase fraction have been systematically studied. Microstructural studies revealed that grain size of the nanocomposite magnets can be controlled below 20 nm with a homogeneous distribution of α-Fe phase in the matrix of hard magnetic SmCo 3 phase after severe plastic deformation. The refinement of the hard and soft phases morphology in nanoscale leads to effective inter-phase exchange coupling that gives rise to single-phase-like demagnetization behavior with enhanced remanence and maximum energy product (BH) max . The (BH) max up to 13.5 MGOe in the isotropic SmCo 3 /α-Fe nanocomposites with 25 wt. % of the soft phase has been obtained. Magnetic characterization at elevated temperatures shows that the nanocomposite SmCo 3 /α-Fe magnets have improved energy product compared to the single-phase SmCo 3 magnets.
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  • 5
    Publication Date: 2014-02-01
    Description: In order to clarify the effects of dislocation scattering on electron transport properties, temperature dependent (15-300 K) Hall-effect measurements were applied to the dislocated coalescent and dislocation-free non-coalescent ELO layers of InP prepared by the liquid phase epitaxy. The coalescent ELO layers contain a large number of dislocations, and the non-coalescent ELO layers are dislocation-free. Taking into account, the various electron scattering mechanisms in compound semiconductors, the temperature dependences of electron mobility were analyzed. It is shown that the dislocation scattering based on the charged dislocation line model is dominant transport mechanism in the dislocated coalescent ELO layers at low temperature.
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  • 6
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    American Institute of Physics (AIP)
    Publication Date: 2014-02-01
    Description: Carbohydrates (2-deoxyribose, ribose, and xylose) and nucleotides (adenosine-, cytidine-, guanosine-, and uridine-5 ′ -monophosphate) are generated in the gas phase, and ionized with vacuum ultraviolet photons (VUV, 118.2 nm). The observed time of flight mass spectra of the carbohydrate fragmentation are similar to those observed [J.-W. Shin, F. Dong, M. Grisham, J. J. Rocca, and E. R. Bernstein, Chem. Phys. Lett. 506 , 161 (2011)] for 46.9 nm photon ionization, but with more intensity in higher mass fragment ions. The tendency of carbohydrate ions to fragment extensively following ionization seemingly suggests that nucleic acids might undergo radiation damage as a result of carbohydrate, rather than nucleobase fragmentation. VUV photoionization of nucleotides (monophosphate-carbohydrate-nucleobase), however, shows that the carbohydrate-nucleobase bond is the primary fragmentation site for these species. Density functional theory (DFT) calculations indicate that the removed carbohydrate electrons by the 118.2 nm photons are associated with endocyclic C–C and C–O ring centered orbitals: loss of electron density in the ring bonds of the nascent ion can thus account for the observed fragmentation patterns following carbohydrate ionization. DFT calculations also indicate that electrons removed from nucleotides under these same conditions are associated with orbitals involved with the nucleobase-saccharide linkage electron density. The calculations give a general mechanism and explanation of the experimental results.
    Print ISSN: 0021-9606
    Electronic ISSN: 1089-7690
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  • 7
    Publication Date: 2014-02-01
    Description: A permutationally invariant global potential energy surface for the HOCO system is reported by fitting a larger number of high-level ab initio points using the newly proposed permutation invariant polynomial-neural network method. The small fitting error (∼5 meV) indicates a faithful representation of the potential energy surface over a large configuration space. Full-dimensional quantum and quasi-classical trajectory studies of the title reaction were performed on this potential energy surface. While the results suggest that the differences between this and an earlier neural network fits are small, discrepancies with state-to-state experimental data remain significant.
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  • 8
    Publication Date: 2014-02-01
    Description: The principal challenge in using explicitly correlated wavefunctions for molecules is the evaluation of nonfactorizable integrals over the coordinates of three or more electrons. Immense progress was made in tackling this problem through the introduction of a single-particle resolution of the identity. Decompositions of sufficient accuracy can be achieved, but only with large auxiliary basis sets. Density fitting is an alternative integral approximation scheme, which has proven to be very reliable for two-electron integrals. Here, we extend density fitting to the treatment of all three-electron integrals that appear at the MP2-F12/3*A level of theory. We demonstrate that the convergence of energies with respect to auxiliary basis size is much more rapid with density fitting than with the traditional resolution-of-the-identity approach.
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  • 9
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    American Institute of Physics (AIP)
    Publication Date: 2014-02-01
    Description: The kinetics of order-disorder transition of FePt nanoparticles during high temperature annealing is theoretically investigated. A model is developed to address the influence of large surface to volume ratio of nanoparticles on both the thermodynamic and kinetic aspect of the ordering process; specifically, the nucleation and growth of L1 0 ordered domain within disordered nanoparticles. The size- and shape-dependence of transition kinetics are quantitatively addressed by a revised Johnson-Mehl-Avrami equation that included corrections for deviations caused by the domination of surface nucleation in nanoscale systems and the non-negligible size of the ordered nuclei. Calculation results based on the model suggested that smaller nanoparticles are kinetically more active but thermodynamically less transformable. The major obstacle in obtaining completely ordered nanoparticles is the elimination of antiphase boundaries. The results also quantitatively confirmed the existence of a size-limit in ordering, beyond which, inducing order-disorder transitions through annealing is impossible. A good agreement is observed between theory, experiment, and computer simulation results.
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  • 10
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    American Institute of Physics (AIP)
    Publication Date: 2014-02-01
    Description: Cyclotrimethylene trinitramine (RDX) crystalizes in the orthorhombic α -phase at the ambient pressure and temperature. In principle, the point defects commonly found in monatomic crystals, such as vacancies and interstitials, may exist in RDX as well. However, in molecular crystals one encounters additional point defects associated with the distortion of the molecules. A set of rotational defects are described in this article. These are molecules which are located in the proper positions in the crystal but are rotated relative to the molecules in the perfect crystal, and their ring is slightly puckered. The energetic barriers for defect formation and for their annealing back to the perfect crystal configuration are computed using an atomistic model. It is shown that the formation energy of rotational defects is smaller than the vacancy formation energy. Such defects are identified in the cores of dislocations in RDX and hence their concentration in the crystal is expected to increase during plastic deformation. The importance of such point defects is related to their role in phonon scattering and in dislocation-mediated plastic deformation.
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  • 11
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    American Institute of Physics (AIP)
    Publication Date: 2014-02-01
    Description: Density functional theory (DFT) calculations are typically based on approximate functionals that link the free energy of a multi-body system of interest with the underlying one-body density distributions. Whereas good performance is often proclaimed for new developments, it is difficult to vindicate the theoretical merits relative to alternative versions without extensive comparison with the numerical results from molecular simulations. Besides, approximate functionals may defy statistical-mechanical sum rules and result in thermodynamic inconsistency. Here we compare systematically several versions of density functionals for ionic distributions near a charged surface using the primitive model of electric double layers. We find that the theoretical performance is sensitive not only to the specific forms of the density functional but also to the range of parameter space and the precise properties under consideration. In general, incorporation of the thermodynamic sum rule into the DFT calculations shows significant improvements for both electrochemical properties and ionic distributions.
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  • 12
    Publication Date: 2014-02-01
    Description: Global warming and pressing concern about CO 2 emission along with increasing fuel and oil cost have brought about great challenges for energy companies and homeowners. In this regard, a potential candidate solution is widely used for Distributed Energy Resources, which are capable of providing high quality, low-cost heat and power to off-grid or remote facilities. To appropriately manage thermal and electrical energy, a Smart Energy Management System (SEMS) with hierarchical control scheme has been presented. The developed SEMS model results in mixed integer non-linear programming optimization problem with the objective function of minimizing the operation cost as well as considering emissions. Moreover, the optimization problem has been solved for deterministic and stochastic scheduling algorithms. The novelty of this work is basically reliant on using data mining approach to reduce forecasting error. Several case studies have been carried out to evaluate the performance of proposed data mining method on both energy cost and expected cost.
    Electronic ISSN: 1941-7012
    Topics: Energy, Environment Protection, Nuclear Power Engineering
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  • 13
    Publication Date: 2014-02-01
    Description: Ferrofluid flow in cylindrical and annular geometries under the influence of a uniform rotating magnetic field was studied experimentally using aqueous ferrofluids consisting of low concentrations (
    Print ISSN: 1070-6631
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  • 14
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    American Institute of Physics (AIP)
    Publication Date: 2014-02-01
    Description: A self-consistent kinetic theory of a laser-accelerated plasma target with distributed electron/ion densities is developed. The simplified model assumes that after an initial transition period the bulk of cold ions are uniformly accelerated by the self-consistent electric field generated by hot electrons trapped in combined ponderomotive and electrostatic potentials. Several distinct target regions (non-neutral ion tail, non-neutral electron sheath, and neutral plasma bulk) are identified and analytically described. It is shown analytically that such laser-accelerated finite-thickness target is susceptible to Rayleigh-Taylor (RT) instability. Particle-in-cell simulations of the seeded perturbations of the plasma target reveal that, for ultra-relativistic laser intensities, the growth rate of the RT instability is depressed from the analytic estimates.
    Print ISSN: 1070-664X
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  • 15
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    American Institute of Physics (AIP)
    Publication Date: 2014-02-01
    Description: A gyrokinetic system of equations for turbulent toroidal plasmas in time-dependent axisymmetric background magnetic fields is derived from the variational principle. Besides governing equations for gyrocenter distribution functions and turbulent electromagnetic fields, the conditions which self-consistently determine the background magnetic fields varying on a transport time scale are obtained by using the Lagrangian, which includes the constraint on the background fields. Conservation laws for energy and toroidal angular momentum of the whole system in the time-dependent background magnetic fields are naturally derived by applying Noether's theorem. It is shown that the ensemble-averaged transport equations of particles, energy, and toroidal momentum given in the present work agree with the results from the conventional recursive formulation with the WKB representation except that collisional effects are disregarded here.
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  • 16
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    American Institute of Physics (AIP)
    Publication Date: 2014-02-04
    Description: Uniform Cu-doped Bi 2 Te 3 hexagonal nanoplates with widths of ∼200 nm and thicknesses of ∼20 nm were synthesized using a solvothermal method. According to the structural characterization and compositional analysis, the Cu 2+ ions were found to substitute Bi 3+ ions in the lattice. High-level Cu doping induces a lattice distortion and decreases the crystal lattice by 1.17% in the a axis and 2.38% in the c axis. A paramagnetic state is observed in these nanoplates from 2 to 295 K, which is a significant difference from their diamagnetic un-doped Bi 2 Te 3 .
    Print ISSN: 0003-6951
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  • 17
    Publication Date: 2014-02-04
    Description: We demonstrate a multi-wavelength distributed feedback (DFB) quantum cascade laser (QCL) operating in a lensless external micro-cavity and achieve switchable single-mode emission at three distinct wavelengths selected by the DFB grating, each with a side-mode suppression ratio larger than 30 dB. Discrete wavelength tuning is achieved by modulating the feedback experienced by each mode of the multi-wavelength DFB QCL, resulting from a variation of the external cavity length. This method also provides a post-fabrication control of the lasing modes to correct for fabrication inhomogeneities, in particular, related to the cleaved facets position.
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  • 18
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    American Institute of Physics (AIP)
    Publication Date: 2014-02-04
    Description: High efficiency hybrid solar cells are fabricated based on silicon with a nanohole (SiNH) structure and poly (3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS). The SiNH structure is fabricated using electroless chemical etching with silver catalyst, and the heterojunction is formed by spin coating of PEDOT on the SiNH. The hybrid cells are optimized by varying the hole depth, and a maximum power conversion efficiency of 8.3% is achieved with a hole depth of 1  μ m. The SiNH hybrid solar cell exhibits a strong antireflection and light trapping property attributed to the sub-wavelength dimension of the SiNH structure.
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  • 19
    Publication Date: 2014-02-04
    Description: A hydrophobic front surface in Janus catalytic motors could introduce two effects to the motion kinetics: a partially slippery surface and a change in catalytic reaction rate. Experimentally, the hydrophobic Janus motors have been observed to move appreciably faster than the hydrophilic ones for large size motors with high fuel concentrations. Numerical investigation and experimental data reveal that the slippery surface has an insignificant effect on motor kinetics compared to an enhanced catalytic reaction rate, which could result from the water depletion layer around the hydrophobic surface and the strong hydrophobic interaction between the generated O 2 and the hydrophobic surface.
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  • 20
    Publication Date: 2014-02-04
    Description: We report in situ manipulation of the in-plane strain ε x x ( B F M O ) and coercive field E C ( B F M O ) of BiFe 0.95 Mn 0.05 O 3 (BFMO) films epitaxially grown on La 0.7 Sr 0.3 MnO 3 film buffered 0.71Pb(Mg 1/3 Nb 2/3 )O 3 -0.29PbTiO 3 (PMN-PT) substrates. PMN-PT poling-induced strain is effectively transferred to BiFe 0.95 Mn 0.05 O 3 films and enhances ε x x ( B F M O ) and E C ( B F M O ) , with a gauge factor ( Δ E C ( B F M O ) / E C ( B F M O ) ) / ( δ ε x x ) ∼−25 and −326 for the BFMO(001) and BFMO(111) films, respectively. Based on the strain dependence of E C ( B F M O ) , we established a quantitative relationship between E C ( B F M O ) and ε x x ( B F M O ) . Using ferroelastic strain of PMN-PT, we achieved reversible and non-volatile modulation of strain and E C ( B F M O ) of BFMO films, providing an approach for non-volatile and reversible turning of strain and physical properties of ferroelectric films.
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  • 21
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    American Institute of Physics (AIP)
    Publication Date: 2014-02-04
    Description: Electric fields are known to favor a ferroelectric phase with parallel electric dipoles over an antiferroelectric phase. We demonstrate in this Letter that electric fields can induce an antiferroelectric phase out of a ferroelectric phase in a NaNbO 3 -based lead-free polycrystalline ceramic. Such an unlikely ferroelectric-to-antiferroelectric phase transition occurs at fields with a reversed polarity and competes with the ferroelectric polarization reversal process.
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  • 22
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    American Institute of Physics (AIP)
    Publication Date: 2014-02-04
    Description: This paper reports a wideband multi-mass multi-spring piezoelectric vibration energy harvester (VEH) based on a folded asymmetric gapped cantilever, which enables multiple resonant modes formed by pure bending of every stage. Moreover, the heaviest proof mass is placed at the last stage of the cantilever to increase the harvested power. The VEH's energy conversion efficiency is further increased using the asymmetric gapped structure. A prototype has been developed and characterized. The experimental results match with finite element simulation well. The prototype was tested on an air conditioning unit to demonstrate its energy harvesting capability with a realistic broadband vibration source.
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  • 23
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    American Institute of Physics (AIP)
    Publication Date: 2014-02-04
    Description: This paper presents a systematic study on the charge transfer and ionicity in various metallic-glass forming systems, as well as its relationship with other atomic-level structure indicators, using the Bader analysis method and molecular dynamics simulation. It is shown that in a binary or multicomponent system, the chemical effects (when more than one elements present) appear to play a more important role in setting the absolute level of the atomic-level pressure, compared to the topological fluctuation.
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  • 24
    Publication Date: 2014-02-04
    Description: There has been considerable interest in chalcogenide alloys with high concentrations of native vacancies that lead to properties desirable for thermoelectric and phase-change materials. Recently, vacancy ordering has been identified as the mechanism for metal-insulator transitions observed in GeSb 2 Te 4 and an unexpectedly low thermal conductivity in Ga 2 Te 3 . Here, we report the direct observation of vacancy ordering in Ga 2 SeTe 2 utilizing aberration-corrected electron microscopy. Images reveal a cation-anion dumbbell inversion associated with the accommodation of vacancy ordering across the entire crystal. The result is a striking example of the interplay between native defects and local structure.
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  • 25
    Publication Date: 2014-02-04
    Description: Favorable temperature- and size-dependent device characteristics in mechanically flexible, thin (∼6.45  μ m thick), microscale inorganic InGaN/GaN-based light-emitting diodes enable their use as highly efficient, robust devices that are capable of integration on diverse classes of unconventional substrates, including sheets of plastic. Finite element analysis and systematic studies of the operational properties establish important thermal, electrical, and optical considerations for this type of device.
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  • 26
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    American Institute of Physics (AIP)
    Publication Date: 2014-02-04
    Description: We report here our results on the formation of a plasma configuration with the generic name of compact toroid (CT). A method of compact toroid formation to confine, heat and compress a plasma is investigated. Formation of a compact torus using an additional toroidal magnetic field helps to increase the plasma current to a maintainable level of the original magnetic field. We design the Compact Toroid Challenge (CTC) experiment in order to improve the magnetic flux trapping during field reversal in the formation of a compact toroid. The level of the magnetic field immersed in the plasma about 70% of the primary field is achieved. The CTC device and scheme of high level capturing of magnetic flux are presented.
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  • 27
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    American Institute of Physics (AIP)
    Publication Date: 2014-02-04
    Description: In this study, an improvement in energy conversion efficiency has been reported, which is realized by using a double-clamped piezoelectric beam, based on uniaxial stretching strain. The buckling mechanism is applied to maximize axial stress in the double-clamped beam. The voltage generated by using the double-clamped piezoelectric beam is higher than that generated by using other conventional structures, such as bending cantilevers coated/sandwiched with piezoelectric film, which is proven both theoretically and experimentally. The power generation efficiency is enhanced by further optimizing the double-clamped structure. The optimized high-efficiency energy harvester utilizing double-clamped piezoelectric beams generates a peak output power of 80 μW, under an acceleration of 0.1 g .
    Print ISSN: 0034-6748
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  • 28
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    American Institute of Physics (AIP)
    Publication Date: 2014-02-05
    Description: We investigated the effects of Al doping on ferromagnetism in Co-doped ZnO and the mechanisms that give rise to ferromagnetism in hydrogen-injected ZnCoO. The aim of this study was to determine whether the occurrence of ferromagnetism or the strength of the magnetization is related to the charge carrier mobility, charge carrier density, or the presence of defects in the crystal lattice. Al doping increased the carrier density, as well as the density of oxygen vacancies and the lattice strain; however, these physical properties were not related to the changes in magnetism. Al-doped and undoped ZnCoO showed an increase in ferromagnetism as a function of the hydrogen plasma treatment time. Al doping suppressed the hydrogen-mediated ferromagnetism in ZnCoO:H by trapping hydrogen via oxygen vacancies created by Al doping.
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  • 29
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    American Institute of Physics (AIP)
    Publication Date: 2014-02-05
    Description: The coefficient of thermal expansion (CTE) of peapod-derived double-walled carbon nanotubes and their host empty single-walled carbon nanotubes (SWCNTs) was determined using Raman spectroscopy. This was performed by measuring the dependence of Raman band frequency of the nanotubes in epoxy resin matrix composites and considering the effects of both the strain and temperature on the Raman bands. Both types of nanotubes show positive thermal expansion at room temperature of around +2 × 10 −5 K −1 , and the CTE of the SWCNTs was unaffected by the introduction of the inner wall nanotubes. It was also demonstrated that the temperature-induced Raman band shifts can be used to determine both the CTE and glass transition temperature of the matrix polymers.
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  • 30
    Publication Date: 2014-02-05
    Description: High quality surface passivation (S eff   ±8 × 10 12  cm −2 ) into a dual layer stack of Plasma Enhanced Chemical Vapor Deposition (PECVD) Silicon Nitride (SiN x )/PECVD Silicon Oxide (SiO 2 ) films using a corona charging tool. We demonstrate long term stability and uniform charge distribution in the SiN x film by manipulating the charge on K center defects while negating the requirement of a high temperature thermal oxide step.
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  • 31
    Publication Date: 2014-02-05
    Description: The dielectric properties and microwave attenuation performance of N-doped SiC have been evaluated in 8.2–12.4 GHz in the temperature range of 293–673 K. The N doping dramatically improves the microwave absorption capability of SiC. The minimum reflection loss of N-doped SiC is enhanced to nearly −30 dB with the effective absorption bandwidth [ RL (dB) ≤ −10 dB] up to 3 GHz at 673 K. The excellent high-temperature dielectric properties are attributed to multi-relaxations, originated from the polarization relaxations of dipoles induced by the N doping and vacancy defects.
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  • 32
    Publication Date: 2014-02-05
    Description: The carbon nanotube network decorated with Pt nanoparticles (PtCNT) irradiated by excimer laser as counter electrode (CE) of dye-sensitized solar cells (DSSCs) has been systematically demonstrated. The conversion efficiency would be improved from 7.12% to 9.28% with respect to conventional Pt-film one. It was attributed to the enhanced catalytic surface from Pt nanoparticles and the improved conductivity due to the adjoining phenomenon of PtCNTs irradiated by laser. Moreover, the laser annealing could also promote the interface contact between CE and conductive glass. Therefore, such a simple laser-irradiated PtCNT network is promising for the future flexible DSSCs applications.
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  • 33
    Publication Date: 2014-02-05
    Description: The spatial distribution of luminescence in Si-doped AlGaN epitaxial layers that differ in Al content and Si concentration has been studied by cathodoluminescence (CL) mapping in combination with scanning electron microscopy. The density of surface hillocks increased with decreasing Al content and with increasing Si concentration. The mechanisms giving rise to those hillocks are likely different. The hillocks induced surface roughening, and the compositional fluctuation and local donor-acceptor-pair (DAP) emission at hillock edges in AlGaN epitaxial layers were enhanced irrespective of the origin of the hillocks. The intensity of local DAP emission was related to Si concentration, as well as to hillock density. CL observation revealed that DAP emission areas were present inside the samples and were likely related to dislocations concentrated at hillock edges. Possible candidates for acceptors in the observed DAP emission that are closely related in terms of both Si concentration and hillock edges with large deformations are a V III -Si III complex and Si N , which are unfavorable in ordinary III-nitrides.
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  • 34
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    American Institute of Physics (AIP)
    Publication Date: 2014-02-05
    Description: We analyzed the dipolar energy of Nd-Fe-B nanocrystalline magnets in magnetization reversal process through visualizing magnetic dipolar interaction. We obtained magnetization distribution images experimentally by using scanning transmission X-ray microscopy (STXM). The magnetic dipolar interaction was calculated by the interaction between the magnetization at each point and those at the other points on the STXM image. We showed the dipolar energy in the nanocrystalline Nd-Fe-B magnets and compared it with the exchange energy at various applied fields. Our results indicated the significance of the dipolar energy in magnetization reversal process.
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  • 35
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    Unknown
    American Institute of Physics (AIP)
    Publication Date: 2014-02-05
    Description: We report a theoretical study of nanobubble stabilization on a substrate by contact line pinning. In particular, we predict the magnitude of the pinning force required to stabilize a nanobubble and the threshold values of the pinning force that the substrate can provide. We show that the substrate chemistry and the local structures of substrate heterogeneity together determine whether or not surface nanobubbles are stable. We find that for stable nanobubbles, the contact angles are independent of substrate chemistry as its effects are cancelled out by the pinning effect. This prediction is in agreement with available experimental data.
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  • 36
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    Unknown
    American Institute of Physics (AIP)
    Publication Date: 2014-02-05
    Description: Structural relaxation of As-Se glasses through the glass-to-supercooled liquid transition interval is studied with temperature-modulated differential scanning calorimetry. It is shown that connectivity of glass network and long-term physical ageing change not only the full width at half maximum of the out-of-phase component of complex heat capacity, which is conventionally used for analysis, but also its asymmetry value. The latter is shown to carry very important information on the dynamic heterogeneity in glasses. Raman spectroscopy is used as complementary technique to reveal possible structural rearrangements in the investigated glass network.
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  • 37
    Publication Date: 2014-02-05
    Description: Herein, the multi-dimensional nature of the conical intersection seam has been experimentally revealed in the photodissociation reaction of thioanisole-d 3 (C 6 H 5 SCD 3 ) excited on S 1 , giving C 6 H 5 S·( A ̃ or X ̃ ) +·CD 3 products. The translational energy distribution of the nascent·CD 3 fragment, reflecting the relative yields of the C 6 H 5 S·( A ̃ ) and C 6 H 5 S·( X ̃ ) products, was measured at each S 1 vibronic band using the velocity map ion imaging technique. Direct access of the reactant flux to the conical intersection seam leads to the increase of the nonadiabatic transition probability resulting in sharp resonances in the X ̃ / A ̃ C 6 H 5 S ·product branching ratio at several distinct S 1 vibronic bands. The nature of the S 1 vibronic bands associated with such dynamic resonances was clarified by the mass-analyzed threshold ionization spectroscopy. The bound state embedded in continuum generated by the conical intersection is observed as a distinct dynamic resonance, revealing the nature of the nuclear motion responsible for the nonadiabatic coupling of two potential energy surfaces at the conical intersection. The multi-dimensional facets of the conical intersection seam in terms of its detailed structure and dynamic role are discussed with the aid of theoretical calculations.
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  • 38
    facet.materialart.
    Unknown
    American Institute of Physics (AIP)
    Publication Date: 2014-02-05
    Description: In a previous study, we used diffusing wave spectroscopy (DWS) to investigate the aging signatures of a thermo-sensitive colloidal glass and compared them with those of molecular glasses from the perspective of the Kovacs temperature-jump, volume recovery experiments [X. Di, K. Z. Win, G. B. McKenna, T. Narita, F. Lequeux, S. R. Pullela, and Z. Cheng, Phys. Rev. Lett.106, 095701 (2011)]. In order to further look into the glassy behavior of colloidal systems, we have synthesized a new core/shell particle with lower temperature sensitivity and studied the aging signatures of concentrated systems, again following Kovacs’ protocol. Similar signatures of aging to those observed previously were seen in this new system. Moreover, a systematic study of the temperature dependence of the dynamics of the new system for different weight concentrations was performed and the dynamic fragility index m was determined. We have also explored the use of the properties determined from the DWS measurements to obtain macroscopic rheological parameters – storage modulus G ′ ( ω ) and loss modulus G ″ ( ω ) – using a generalized Stokes-Einstein approach. The micro-rheological and macro-rheological values are in reasonable agreement.
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  • 39
    Publication Date: 2014-02-05
    Description: For several molecular glass formers, the nonlinear dielectric effects (NDE’s) are investigated for the so-called excess wing regime, i.e., for the relatively high frequencies between 10 2 and 10 7 times the peak loss frequency. It is found that significant nonlinear behavior persists across the entire frequency window of this study, and that its magnitude traces the temperature dependence of the activation energy. A time resolved measurement of the dielectric loss at fields up to 480 kV/cm across tens of thousands of periods reveals that it takes an unexpectedly long time for the steady state NDE to develop. For various materials and at different temperatures and frequencies, it is found that the average structural relaxation with time scale τ α governs the equilibration of these fast modes that are associated with time constants τ which are up to 10 7 times shorter than τ α . It is argued that true indicators of structural relaxation (such as rate exchange and aging) of these fast modes are slaved to macroscopic softening on the time scale of τ α , and thus many orders of magnitude slower than the time constant of the mode itself.
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  • 40
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    Unknown
    American Institute of Physics (AIP)
    Publication Date: 2014-02-05
    Description: Various formulas for the local pressure tensor based on a spherical subvolume of radius, R , are considered. An extension of the Method of Planes (MOP) formula of Todd et al. [Phys. Rev. E52, 1627 (1995)] for a spherical geometry is derived using the recently proposed Control Volume formulation [E. R. Smith, D. M. Heyes, D. Dini, and T. A. Zaki, Phys. Rev. E85, 056705 (2012)]. The MOP formula for the purely radial component of the pressure tensor is shown to be mathematically identical to the Radial Irving-Kirkwood formula. Novel offdiagonal elements which are important for momentum conservation emerge naturally from this treatment. The local pressure tensor formulas for a plane are shown to be the large radius limits of those for spherical surfaces. The radial-dependence of the pressure tensor computed by Molecular Dynamics simulation is reported for virtual spheres in a model bulk liquid where the sphere is positioned randomly or whose center is also that of a molecule in the liquid. The probability distributions of angles relating to pairs of atoms which cross the surface of the sphere, and the center of the sphere, are presented as a function of R . The variance in the shear stress calculated from the spherical Volume Averaging method is shown to converge slowly to the limiting values with increasing radius, and to be a strong function of the number of molecules in the simulation cell.
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  • 41
    Publication Date: 2014-02-05
    Description: Synchrotron X-ray diffraction measurements have been conducted on liquid phosphorus trichloride, tribromide, and triiodide. Molecular Dynamics simulations for these molecular liquids were performed with a dual purpose: (1) to establish whether existing intermolecular potential functions can provide a picture that is consistent with diffraction data and (2) to generate reliable starting configurations for subsequent Reverse Monte Carlo modelling. Structural models (i.e., sets of coordinates of thousands of atoms) that were fully consistent with experimental diffraction information, within errors, have been prepared by means of the Reverse Monte Carlo method. Comparison with reference systems, generated by hard sphere-like Monte Carlo simulations, was also carried out to demonstrate the extent to which simple space filling effects determine the structure of the liquids (and thus, also estimating the information content of measured data). Total scattering structure factors, partial radial distribution functions and orientational correlations as a function of distances between the molecular centres have been calculated from the models. In general, more or less antiparallel arrangements of the primary molecular axes that are found to be the most favourable orientation of two neighbouring molecules. In liquid PBr 3 electrostatic interactions seem to play a more important role in determining intermolecular correlations than in the other two liquids; molecular arrangements in both PCl 3 and PI 3 are largely driven by steric effects.
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  • 42
    facet.materialart.
    Unknown
    American Institute of Physics (AIP)
    Publication Date: 2014-02-05
    Description: Quantum beat spectroscopy has been used to measure rate coefficients at 300 K for collisional depolarization for NO(A  2 Σ + ) and OH(A  2 Σ + ) with krypton. Elastic depolarization rate coefficients have also been determined for OH(A) + Kr, and shown to make a much more significant contribution to the total depolarization rate than for NO(A) + Kr. While the experimental data for NO(A) + Kr are in excellent agreement with single surface quasiclassical trajectory (QCT) calculations carried out on the upper 2 A ′ potential energy surface, the equivalent QCT and quantum mechanical calculations cannot account for the experimental results for OH(A) + Kr collisions, particularly at low N . This disagreement is due to the presence of competing electronic quenching at low N , which requires a multi-surface, non-adiabatic treatment. Somewhat improved agreement with experiment is obtained by means of trajectory surface hopping calculations that include non-adiabatic coupling between the ground 1 A ′ and excited 2 A ′ states of OH(X/A) + Kr, although the theoretical depolarization cross sections still significantly overestimate those obtained experimentally.
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  • 43
    facet.materialart.
    Unknown
    American Institute of Physics (AIP)
    Publication Date: 2014-02-05
    Description: The application of resonant magnetic perturbations results in a non-axisymmetric striation pattern of magnetic field lines from the plasma interior which intersect the divertor targets. The impact on related particle and heat fluxes is investigated by three dimensional computer simulations for two different recycling conditions (controlled via neutral gas pumping). It is demonstrated that a mismatch between the particle and heat flux striation pattern (splitting vs. no splitting), as is repeatedly observed in ITER similar shape H-mode plasmas at DIII-D, can be reproduced by the simulations for high recycling conditions at the onset of partial detachment. These results indicate that a detailed knowledge of the particle and energy balance is at least as important for realistic simulations as the consideration of a change in the magnetic field structure by plasma response effects.
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  • 44
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    Unknown
    American Institute of Physics (AIP)
    Publication Date: 2014-02-05
    Description: Supercoiling is the large-scale secondary coiling or buckling of a structure that is already coiled at a smaller scale. Here, we show experimentally that a fluid-mechanical analog of supercoiling can occur when a thin “rope” of viscous fluid falls vertically from a great height onto a surface. For appropriate values of the viscosity ν, the flow rate Q , and the fall height H , a primary coiling instability of the rope forms a hollow coiled cylinder that then experiences a secondary buckling instability in the form of periodic folding accompanied by slow rotation of the folding plane. To delineate the conditions under which this supercoiling state appears, we carry out systematic laboratory experiments over wide ranges of Q and H using several fluids with different viscosities. We find that five different states of the rope are possible: supercoiling (SC), periodic collapse of the fluid cylinder formed by a primary coiling instability (PC), periodic folding (F), and steady coiling (C) of the rope itself, and axisymmetric stagnation flow (S). Up to three of these states can be realized for a given set of experimental conditions, and we determine detailed state diagrams showing which combinations are observed as a function of ν, Q , and H . The selection of the states is controlled by the dimensionless parameter gHQ 2 /ν 4 ( g is the gravitational acceleration), which is directly related to the ratio of the rope radius a to the coil radius R in steady primary coiling with the parameters ν, Q , and H .
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  • 45
    Publication Date: 2014-02-05
    Description: Minor ion (such as He 2+ ) heating via nonresonant interaction with spectra of linearly and circularly polarized Alfvén waves (LPAWs and CPAWs hereafter) is studied. The obtained analytic solutions are in good agreement with the simulation results, indicating that newborn ions are heated by low-frequency Alfvén waves with finite amplitude in low-beta plasmas such as the solar corona. The analytic solutions also reproduce the preferential heating of heavy ions in the solar wind. In the presence of parallel propagating Alfvén waves, turbulence-induced particle motion is clearly observed in the wave (magnetic field) polarized directions. After the waves diminish, the newborn ions are heated, which is caused by the phase difference (randomization) between ions due to their different parallel thermal motions. The heating is dominant in the direction perpendicular to the ambient magnetic field. The perpendicular heating, η = ( T i ⊥ R − T i 0 ⊥ R ) / T i 0 ⊥ R (where T i 0 ⊥ R and T i ⊥ R are the perpendicular temperature of species i before and after genuine heating, respectively), in the spectrum of CPAWs is a factor of two stronger than that of LPAWs. Moreover, we also study the effect of field-aligned differential flow speed of species i relative to H + , δ v i p = ( v i − v p ) · B / | B | (where v i and v p denote vector velocities of the H + and species i , respectively), on the perpendicular heating. It reveals that large drift speed, v d = δ v i p , has an effect on reducing the efficiency of perpendicular heating, which is consistent with observations.
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  • 46
    Publication Date: 2014-02-05
    Description: We evaluate the modulation of cosmic microwave background polarization using a rapidly rotating, half-wave plate (HWP) on the Atacama B-Mode Search. After demodulating the time-ordered-data (TOD), we find a significant reduction of atmospheric fluctuations. The demodulated TOD is stable on time scales of 500–1000 s, corresponding to frequencies of 1–2 mHz. This facilitates recovery of cosmological information at large angular scales, which are typically available only from balloon-borne or satellite experiments. This technique also achieves a sensitive measurement of celestial polarization without differencing the TOD of paired detectors sensitive to two orthogonal linear polarizations. This is the first demonstration of the ability to remove atmospheric contamination at these levels from a ground-based platform using a rapidly rotating HWP.
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  • 47
    facet.materialart.
    Unknown
    American Institute of Physics (AIP)
    Publication Date: 2014-02-06
    Description: In tunnel junctions with ferroelectric barriers, switching the polarization direction modifies the electrostatic potential profile and the associated average tunnel barrier height. This results in strong changes of the tunnel transmission and associated resistance. The information readout in ferroelectric tunnel junctions (FTJs) is thus resistive and non-destructive, which is an advantage compared to the case of conventional ferroelectric memories (FeRAMs). Initially, endurance limitation (i.e., fatigue) was the main factor hampering the industrialization of FeRAMs. Systematic investigations of switching dynamics for various ferroelectric and electrode materials have resolved this issue, with endurance now reaching 10 14  cycles. Here we investigate data retention and endurance in fully patterned submicron Co/BiFeO 3 /Ca 0.96 Ce 0.04 MnO 3 FTJs. We report good reproducibility with high resistance contrasts and extend the maximum reported endurance of FTJs by three orders of magnitude (4 × 10 6  cycles). Our results indicate that here fatigue is not limited by a decrease of the polarization or an increase of the leakage but rather by domain wall pinning. We propose directions to access extreme and intermediate resistance states more reliably and further strengthen the potential of FTJs for non-volatile memory applications.
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  • 48
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    American Institute of Physics (AIP)
    Publication Date: 2014-02-06
    Description: We used MARTINI coarse-grained force field to study poration of a lipid bilayer by a shock wave induced nanobubble collapse. Different systems containing different sized nanobubbles that were exposed to shock waves propagating with different velocities were simulated. We observed creation of pores and damage to bilayers and also subsequent pore closing and the bilayer recovery after shock wave passed the bilayer. In all our systems where bilayers were damaged, they recovered; nevertheless we observed that a large amount of water crossed the pore that was temporarily created. We also observed that not every lipid molecule remained in the bilayer after recovery, some lipids moved out into water and created micelles.
    Electronic ISSN: 1931-9223
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  • 49
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    American Institute of Physics (AIP)
    Publication Date: 2014-02-06
    Description: The propagation characteristics of electromagnetic waves in waveguides implemented using the “Fakir's bed of nails” are investigated both analytically and numerically. The classical metal walls of a parallel-plate waveguide are replaced by a Fakir's bed of nails metamaterial having arbitrary pin lengths on both walls; treated as a homogenized effective spatially dispersive dielectric. A modal analysis of the electromagnetic fields is presented for the first time, and dispersion expressions for the propagating modes are derived analytically and independently validated with full-wave numerical simulations. An equivalent transmission line model is also given, and similarities with the classical metal-dielectric-metal structure commonly used in optics are discussed.
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  • 50
    Publication Date: 2014-02-06
    Description: We study the anisotropic magnetoresistance and planar Hall effect of La 0.8 Sr 0.2 MnO 3 when the field is rotated out of the film plane. We fit the data with a model, which consists of two independent contributions related to (a) the orientation of the magnetization relative to the crystal axes and (b) the orientation of the magnetization relative to the current. We find that the first contribution exhibits a two-fold symmetry, which cannot be explained by the angular dependence of the magnetization magnitude, suggesting an intrinsic source which reflects a deviation from cubic symmetry.
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  • 51
    Publication Date: 2014-02-06
    Description: A resonant engine in which the piston-cylinder assembly is replaced by a flexible cavity is realized at the mesoscale using flexible metal bellows to demonstrate the feasibility of the concept. A four stroke motoring technique is developed and measurements are performed to determine parasitic losses. A non-linear lumped parameter model is developed to evaluate the engine performance. Experimentally, the heat transfer and friction effects are separated by varying the engine speed and operating frequency. The engine energy flow diagram showing the energy distribution among various parasitic elements reveals that the friction loss in the bellows is smaller than the sliding friction loss in a typical piston-cylinder assembly.
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  • 52
    Publication Date: 2014-02-06
    Description: The origins of bias-induced metastabilities in Cu(In,Ga)Se 2 thin film samples were investigated using isothermal capacitance transient spectroscopy. We confirmed that these metastabilities are associated with deep hole trap levels, and examined these levels in detail. The trap has multiple charge states, tentatively assigned to a double acceptor, where the first (−1/0) acceptor level has an apparent activation energy of 0.72 eV and the second acceptor level (−2/−1) has an apparent activation energy of 0.51 eV, indicating negative-U-like properties. The metastable behavior becomes prominent on forward biasing of the junction, which was attributed to the slow hole-capture rate of the first (−1/0) acceptor level. This slow rate is probably because of a large lattice relaxation associated with the hole-capture process, as well as depletion of free holes because of the excess concentration of the deep level compared with the net concentration of shallow acceptors.
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  • 53
    Publication Date: 2014-02-06
    Description: The nucleophilic attack of a chloride ion on methyl chloride is an important prototype S N 2 reaction in organic chemistry that is known to be sensitive to the effects of the surrounding solvent. Herein, we develop a highly accurate Specific Reaction Parameter (SRP) model based on the Austin Model 1 Hamiltonian for chlorine to study the effects of solvation into an aqueous environment on the reaction mechanism. To accomplish this task, we apply high-level quantum mechanical calculations to study the reaction in the gas phase and combined quantum mechanical/molecular mechanical simulations with TIP3P and TIP4P-ew water models and the resulting free energy profiles are compared with those determined from simulations using other fast semi-empirical quantum models. Both gas phase and solution results with the SRP model agree very well with experiment and provide insight into the specific role of solvent on the reaction coordinate. Overall, the newly parameterized SRP Hamiltonian is able to reproduce both the gas phase and solution phase barriers, suggesting it is an accurate and robust model for simulations in the aqueous phase at greatly reduced computational cost relative to comparably accurate ab initio and density functional models.
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  • 54
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    American Institute of Physics (AIP)
    Publication Date: 2014-02-06
    Description: The rate of escape of polymers from a two-dimensionally confining potential well has been evaluated using self-avoiding as well as ideal chain representations of varying length, up to 80 beads. Long timescale Langevin trajectories were calculated using the path integral hyperdynamics method to evaluate the escape rate. A minimum is found in the rate for self-avoiding polymers of intermediate length while the escape rate decreases monotonically with polymer length for ideal polymers. The increase in the rate for long, self-avoiding polymers is ascribed to crowding in the potential well which reduces the free energy escape barrier. An effective potential curve obtained using the centroid as an independent variable was evaluated by thermodynamic averaging and Kramers rate theory then applied to estimate the escape rate. While the qualitative features are well reproduced by this approach, it significantly overestimates the rate, especially for the longer polymers. The reason for this is illustrated by constructing a two-dimensional effective energy surface using the radius of gyration as well as the centroid as controlled variables. This shows that the description of a transition state dividing surface using only the centroid fails to confine the system to the region corresponding to the free energy barrier and this problem becomes more pronounced the longer the polymer is. A proper definition of a transition state for polymer escape needs to take into account the shape as well as the location of the polymer.
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  • 55
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    American Institute of Physics (AIP)
    Publication Date: 2014-02-06
    Description: The effect of physical aging on surface diffusion has been determined for two organic glasses, Indomethacin and Nifedipine. The two systems exhibit similar aging kinetics typical of organic glasses. Surface diffusivity remains unchanged despite significant bulk aging that nearly equilibrates the systems and increases the bulk relaxation time by orders of magnitude. The finding is relevant for understanding the stability of amorphous materials and the formation of low-energy glasses by vapor deposition.
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  • 56
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    American Institute of Physics (AIP)
    Publication Date: 2014-02-06
    Description: To identify families of stable planar anchor groups for use in single molecule electronics, we report detailed results for the binding energies of two families of anthracene and pyrene derivatives adsorbed onto graphene. We find that all the selected derivatives functionalized with either electron donating or electron accepting substituents bind more strongly to graphene than the parent non-functionalized anthracene or pyrene. The binding energy is sensitive to the detailed atomic alignment of substituent groups over the graphene substrate leading to larger than expected binding energies for –OH and –CN derivatives. Furthermore, the ordering of the binding energies within the anthracene and pyrene series does not simply follow the electron affinities of the substituents. Energy barriers to rotation or displacement on the graphene surface are much lower than binding energies for adsorption and therefore at room temperature, although the molecules are bound to the graphene, they are almost free to move along the graphene surface. Binding energies can be increased by incorporating electrically inert side chains and are sensitive to the conformation of such chains.
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  • 57
    Publication Date: 2014-02-06
    Description: A new and efficient hydrogen-producing strain, Clostridium perfringens ATCC 13124, was isolated from activated sewage sludge, and this enriched strain was employed to produce hydrogen by fermentation from glucose-based medium. Bio-hydrogen (bio-H 2 ) production by Clostridium perfringens ATCC 13124 was examined using the batch cultivation mode in culture medium containing glucose or agricultural waste. Various carbon and inorganic nitrogen sources could be utilized to produce hydrogen, which indicates that it has the potential of converting renewable wastes into hydrogen. A series of experiments were designed to discuss the effects of different initial pH, operational pH, and inorganic nitrogen and carbon sources on bio-H 2 production by the isolated strain. Furthermore, culture processes were investigated by controlling pH with 1 mol·L −1 of sodium hydroxide or aqua ammonia in the fermentative process of the initial pH 7.0. It was obvious that controlling process of the pH 4.7–5.0 was the most suitable for H 2 production from glucose fermentation. The maximum cumulative hydrogen yield of 1094.3 ml·L −1 -solution and 490.6 ml·g −1 -Glu were observed at 5 and 1.0 g-Glu·L −1 , respectively (with controlling pH by aqua ammonia).
    Electronic ISSN: 1941-7012
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  • 58
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    American Institute of Physics (AIP)
    Publication Date: 2014-02-06
    Description: The hydroelastic response of mat-type Very Large Floating Structures (VLFSs) to severe sea conditions, such as tsunamis and hurricanes, must be assessed for safety and survivability. An efficient and robust nonlinear hydroelastic model is required to predict accurately the motion of and the dynamic loads on a VLFS due to such large waves. We develop a nonlinear theory to predict the hydroelastic response of a VLFS in the presence of cnoidal waves and compare the predictions with the linear theory that is also developed here. This hydroelastic problem is formulated by directly coupling the structure with the fluid, by use of the Level I Green-Naghdi theory for the fluid motion and the Kirchhoff thin plate theory for the runway. The coupled fluid structure system, together with the appropriate jump conditions are solved in two-dimensions by the finite-difference method. The numerical model is used to study the nonlinear response of a VLFS to storm waves which are modeled by use of the cnoidal-wave theory. Parametric studies show that the nonlinearity of the waves is very important in accurately predicting the dynamic bending moment and wave run-up on a VLFS in high seas.
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  • 59
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    American Institute of Physics (AIP)
    Publication Date: 2014-02-06
    Description: The careful filtering of microwave electromagnetic radiation is critical for controlling the electromagnetic environment for experiments in solid-state quantum information processing and quantum metrology at millikelvin temperatures. We describe the design and fabrication of a coaxial filter assembly and demonstrate that its performance is in excellent agreement with theoretical modelling. We further perform an indicative test of the operation of the filters by making current-voltage measurements of small, underdamped Josephson junctions at 15 mK.
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  • 60
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    American Institute of Physics (AIP)
    Publication Date: 2014-02-07
    Description: We present a detailed description of full-field optical micro-angiography on the basis of frequency-domain laser speckle imaging with intensity fluctuation modulation (LSI-IFM). The imaging approach works based on the instantaneous local intensity fluctuation realized via the combination of short exposure and low sampling rate of a camera and appropriate magnification of a microscope. In vivo experiments on mouse ear verify the theoretical description we made for the imaging mechanism and demonstrate the ability of LSI-IFM as optical micro-angiography. By introducing a fundus camera into LSI-IFM system, our approach has a potential application in label-free retina optical micro-angiography.
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  • 61
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    American Institute of Physics (AIP)
    Publication Date: 2014-02-07
    Description: A nonlinear microwave phase shifter based on a planar multiferroic composite has been studied. The multiferroic structure is fabricated in the form of a bilayer consisting of yttrium iron garnet and barium strontium titanate. The principle of operation of the device is based on the linear and nonlinear control of the phase shift of the hybrid spin-electromagnetic waves propagating in the bilayer. The linear control is realized with magnetic and electric fields. The nonlinear control is provided by the input power of microwave signal. The device showed a nonlinear phase shift up to 250°, electric field induced phase shift up to 330°, and magnetic field induced phase shift of more than 180°.
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  • 62
    Publication Date: 2014-02-07
    Description: A long wavelength infrared minority electron unipolar photodetector based on InAs/GaSb type-II superlattices is demonstrated. At 77 K, a dark current of 3 × 10 −5  A/cm 2 and a differential resistance-area of 3700 Ω.cm 2 are achieved at the turn-on bias, with a 50%-cutoff of 10.0  μ m and a specific detectivity of 6.2 × 10 11  Jones. The dark current is fitted as a function of bias and temperature using a model combining generation-recombination and trap-assisted tunneling. Good agreement was observed between the theory and the experimental dark current.
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  • 63
    Publication Date: 2014-02-07
    Description: Negative differential resistance (NDR) behavior was observed in low Al-composition p-GaN/Mg-doped-Al 0.15 Ga 0.85 N/n + -GaN hetero-junction grown by metal-organic chemical vapor deposition on sapphire substrate. The energy band and free carrier concentration of hetero-junction were studied by the model of the self-consistent solution of Schrödinger-Poisson equations combined with polarization engineering theory. At the forward bias of 0.95 V, the NDR effect has a high peak-to-valley current ratio of ∼9 with a peak current of 22.4 mA (∼current density of 11.4 A/cm 2 ). An interesting phenomenon of NDR disappearance after consecutive scans and recurrence after electrical treatment was observed, which was associated with Poole-Frenkel effect.
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  • 64
    Publication Date: 2014-02-07
    Description: P-type doping of GaN by pulsed sputtering deposition (PSD) at a low growth temperature of 480 °C and dramatic reduction in the growth process temperature for InGaN-based light-emitting diodes (LEDs) were achieved. Mg-doped GaN layers grown on semi-insulating GaN at 480 °C exhibited clear p-type conductivity with a hole concentration and mobility of 3.0 × 10 17  cm −3 and 3.1 cm 2 V −1 s −1 , respectively. GaN/In 0.33 Ga 0.67 N/GaN LEDs fabricated at 480 °C showed clear rectifying characteristics and a bright electroluminescence emission near 640 nm. These results indicate that this low temperature PSD growth technique is quite promising for the production of nitride-based light-emitting devices on large-area glass substrates.
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  • 65
    facet.materialart.
    Unknown
    American Institute of Physics (AIP)
    Publication Date: 2014-02-07
    Description: Ultrathin CoFeB/MgO/CoFeB system with perpendicular magnetic anisotropy is a promising candidate for the high density magnetic random access memory. However, a dipolar interaction between the CoFeB layers may introduce a minor loop shift (H s ) and causes uncertainty during the operation. In this report, we systematically studied the dipolar effect in these structures and found that the coupling may be either ferromagnetic or antiferromagnetic (15 Oe 〉 H s  〉 −15 Oe) depending upon the CoFeB thickness (0.9–1.4 nm). A modified Fabry-Perot model, which accounts the Bloch wave interference, may explain the present observations of the dipolar effect in the perpendicular junctions of CoFeB/MgO/CoFeB.
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  • 66
    Publication Date: 2014-02-07
    Description: This paper presents a batch-fabrication approach to imprint complex magnetic pole patterns (perpendicular north/south poles) into hard magnetic films using fine-scale, laser-machined iron foil “masks.” In this approach, pulsed magnetic fields varying between 0.1 and 1.0 T are used in conjunction with the field-concentrating effect of the iron foil masks to selectively reverse the magnetization of selected regions of 10- μ m-thick Co 80 Pt 20 films (Br = 1 T, Hci = 340 kA/m). Simple stripe array patterns are systematically studied both experimentally and via simulation, achieving patterned feature sizes (poles) down to 30  μ m. Double exposures are also explored to create checkerboard patterns from simple stripe-like masks. Finally, masks with more complex patterns are used to demonstrate the viability of the approach to imprint arbitrary features.
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  • 67
    Publication Date: 2014-02-07
    Description: Hexagonal ferrites, such as BaFe 12 O 19 and SrFe 12 O 19 , have strong uniaxial anisotropic magnetic field and remanent magnetism. By employing these properties, magnetic devices, such as phase shifter, isolator and circulator, can work up to tens of GHz frequency range without strong external magnetic field or even self-biasing. As the monolithic microwave integrated circuit extends to higher millimeter wave frequencies, the demand for high performance integrated passive magnetic components is more and more eminent. The micro- and nano-sized hexagonal ferrite can be conveniently utilized to fabricate magnetic components integrated in CMOS circuits via post processing. A nano-ferrite circulator working at 60 GHz is designed, fabricated, and integrated into the CMOS front end for the first time.
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  • 68
    facet.materialart.
    Unknown
    American Institute of Physics (AIP)
    Publication Date: 2014-02-07
    Description: Modern, high frequency, microwave devices for communications technologies can be made with thin ferromagnetic films with narrow microwave resonance linewidths. Recently, there has been interest in magnetostrictive materials where the material constants can change substantially with stresses and applied magnetic fields. We report the development of single crystal thin (20 nm thick) magnetostrictive films of Fe 1−x Ga x (x = 0.20 FeGa(A), 0.23 FeGa(B), 0.28 FeGa(C) on GaAs(001) substrates and on their use in prototype microwave devices. These Galfenol films have a narrower linewidth than any previously reported similar thin films. We fabricate and characterize novel microstrip-based monolithic microwave devices using Galfenol thin films as an active element. We find a number of important features: (1) There is a large absorption (up to 30 dB/cm) at the resonance frequency. (2) The linewidth of the device is narrow ∼1.5 GHz. (3) The saturation magnetization of the samples decreases with the increase in Ga contents. (4) The cubic anisotropy is close to zero (∼0.06 kOe for FeGa(A)) and becomes negative for higher concentration of Ga content in the samples, and (5) the damping increases with increase in Ga concentration.
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  • 69
    facet.materialart.
    Unknown
    American Institute of Physics (AIP)
    Publication Date: 2014-02-07
    Description: The Bender-Dunne basis operators, T − m , n = 2 − n ∑ k = 0 n n k q k p − m q n − k where q and p are the position and momentum operators, respectively, are formal integral operators in position representation in the entire real line R for positive integers n and m . We show, by explicit construction of a dense domain, that the operators T − m , n 's are densely defined operators in the Hilbert space L 2 ( R ) .
    Print ISSN: 0022-2488
    Electronic ISSN: 1089-7658
    Topics: Mathematics , Physics
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  • 70
    facet.materialart.
    Unknown
    American Institute of Physics (AIP)
    Publication Date: 2014-02-07
    Description: Low-density foams irradiated by a 20 kJ laser at the Omega laser facility (Laboratory for Laser Energetics, Rochester, NY, USA) are shown to convert more than 5% of the laser energy into 4.6 to 6.0 keV x rays. This record efficiency with foam targets is due to novel fabrication techniques based on atomic-layer-deposition of Ti atoms on an aerogel scaffold. A Ti concentration of 33 at. % was obtained in a foam with a total density of 5 mg/cm 3 . The dynamics of the ionization front through these foams were investigated at the 1 kJ laser of the Gekko XII facility (Institute for Laser Engineering, Osaka, Japan). Hydrodynamic simulations can reproduce the average electron temperature but fail to predict accurately the heat front velocity in the foam. This discrepancy is shown to be unrelated to the possible water adsorbed in the foam but could be attributed to effects of the foam micro-structure.
    Print ISSN: 1070-664X
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  • 71
    Publication Date: 2014-02-07
    Description: Large-scale two-dimensional (2D) full particle-in-cell (PIC) simulations are carried out for studying the relationship between the dynamics of a perpendicular shock and microinstabilities generated at the shock foot. The structure and dynamics of collisionless shocks are generally determined by Alfven Mach number and plasma beta, while microinstabilities at the shock foot are controlled by the ratio of the upstream bulk velocity to the electron thermal velocity and the ratio of the plasma-to-cyclotron frequency. With a fixed Alfven Mach number and plasma beta, the ratio of the upstream bulk velocity to the electron thermal velocity is given as a function of the ion-to-electron mass ratio. The present 2D full PIC simulations with a relatively low Alfven Mach number ( M A ∼ 6) show that the modified two-stream instability is dominant with higher ion-to-electron mass ratios. It is also confirmed that waves propagating downstream are more enhanced at the shock foot near the shock ramp as the mass ratio becomes higher. The result suggests that these waves play a role in the modification of the dynamics of collisionless shocks through the interaction with shock front ripples.
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  • 72
    Publication Date: 2014-02-07
    Description: We discuss an innovative new high-performance apparatus for performing low-field Nuclear Magnetic Resonance (NMR) relaxation times and diffusion measurements on fluids at very high pressures and high temperatures. The apparatus sensor design and electronics specifications allow for dual deployment either in a fluid sampling well logging tool or in a laboratory. The sensor and electronics were designed to function in both environments. This paper discusses the use of the apparatus in a laboratory environment. The operating temperature and pressure limits, and the signal-to-noise ratio (SNR) of the new system exceed by a very wide margin what is currently possible. This major breakthrough was made possible by a revolutionary new sensor design that breaks many of the rules of conventional high pressure NMR sensor design. A metallic sample holder capable of operating at high pressures and temperatures is provided to contain the fluid under study. The sample holder has been successfully tested for operation up to 36 Kpsi. A solenoid coil wound on a slotted titanium frame sits inside the metallic sample holder and serves as an antenna to transmit RF pulses and receive NMR signals. The metal sample holder is sandwiched between a pair of gradient coils which provide a linear field gradient for pulsed field gradient diffusion measurements. The assembly sits in the bore of a low-gradient permanent magnet. The system can operate over a wide frequency range without the need for tuning the antenna to the Larmor frequency. The SNR measured on a water sample at room temperature is more than 15 times greater than that of the commercial low-field system in our laboratory. Thus, the new system provides for data acquisition more than 200 times faster than was previously possible. Laboratory NMR measurements of relaxations times and diffusion coefficients performed at pressures up to 25 Kpsi and at temperatures up to 175 °C with crude oils enlivened with dissolved hydrocarbon gases (referred to as “live oils”) are shown. This is the first time low-field NMR measurements have been performed at such high temperatures and pressures on live crude oil samples. We discuss the details of the apparatus design, tuning, calibration, and operation. NMR data acquired at multiple temperatures and pressures on a live oil sample are discussed.
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  • 73
    facet.materialart.
    Unknown
    American Institute of Physics (AIP)
    Publication Date: 2014-02-07
    Description: The emergence of high position resolution (∼10 μm) silicon detectors in recent times have highlighted the urgent need for the development of new automated optical scanners of micron level resolution suited for characterizing microscopic features of these detectors. More specifically, for the newly developed silicon photo-multipliers (SiPM) that are compact, possessing excellent photon detection efficiency with gain comparable to photo-multiplier tube. In a short time, since their invention the SiPMs are already being widely used in several high-energy physics and astrophysics experiments as the photon readout element. The SiPM is a high quantum efficiency, multi-pixel photon counting detector with fast timing and high gain. The presence of a wide variety of photo sensitive silicon detectors with high spatial resolution requires their performance evaluation to be carried out by photon beams of very compact spot size. We have designed a high resolution optical scanner that provides a monochromatic focused beam on a target plane. The transverse size of the beam was measured by the knife-edge method to be 1.7 μm at 1 − σ level. Since the beam size was an order of magnitude smaller than the typical feature size of silicon detectors, this optical scanner can be used for selective excitation of these detectors. The design and operational details of the optical scanner, high precision programmed movement of target plane (0.1 μm) integrated with general purpose data acquisition system developed for recording static and transient response photo sensitive silicon detector are reported in this paper. Entire functionality of scanner is validated by using it for selective excitation of individual pixels in a SiPM and identifying response of active and dead regions within SiPM. Results from these studies are presented in this paper.
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  • 74
    Publication Date: 2014-02-08
    Description: We present a theoretical study of the polarization engineering in semi-polar III-nitrides heterostructures. As a case study, we investigate the influence of GaN, AlGaN, and AlInN barrier material on the performance of semi-polar ( 11 2 ¯ 2 ) InGaN-based quantum wells (QWs) for blue (450 nm) and yellow (560 nm) emission. We show that the magnitude of the total built-in electric field across the QW can be controlled by the barrier material. Our results indicate that AlInN is a promising candidate to achieve (i) reduced wavelength shifts with increasing currents and (ii) strongly increased electron-hole wave function overlap, important for reduced optical recombination times.
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  • 75
    Publication Date: 2014-02-08
    Description: La 0.85 (Na 1−x K x ) 0.15 MnO 3 (0 ≤ x ≤ 1) ceramics were synthesized from the raw La(OH) 3 , NaOH, KOH, and MnO 2 powders using Reactive Spark Plasma Sintering. All the compounds were obtained as pure, dense, and ultrafine grained pellets. The Rietveld refinement of the X-Ray powder diffraction shows that all our synthesized samples are single phase and crystallize in the distorted rhombohedral system with R-3c space group. The thermal variation of their magnetization under a magnetic applied field of 50 mT shows a paramagnetic to ferromagnetic transition at a Curie temperature very close to room temperature. The magnetic entropy change, deduced from magnetization measurements versus magnetic applied field up to 5 T at several temperatures exhibits a maximum |ΔS M | max which slightly increases with increasing K content. The relative cooling power values, inferred from the |ΔS M | vs T peak broadening, vary slightly with the potassium content, reaching, values between 316 and 289 Jkg −1 , in an applied magnetic field of 5 T, when x increases from 0 to 1. Technically, these large values make the prepared materials very promising for domestic magnetic refrigeration.
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  • 76
    Publication Date: 2014-02-08
    Description: A computational model for the optimization of the second order optical nonlinearities in GaInAs/AlInAs quantum cascade laser structures is presented. The set of structure parameters that lead to improved device performance was obtained through the implementation of the Genetic Algorithm. In the following step, the linear and second harmonic generation power were calculated by self-consistently solving the system of rate equations for carriers and photons. This rate equation system included both stimulated and simultaneous double photon absorption processes that occur between the levels relevant for second harmonic generation, and material-dependent effective mass, as well as band nonparabolicity, were taken into account. The developed method is general, in the sense that it can be applied to any higher order effect, which requires the photon density equation to be included. Specifically, we have addressed the optimization of the active region of a double quantum well In 0.53 Ga 0.47 As/Al 0.48 In 0.52 As structure and presented its output characteristics.
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  • 77
    facet.materialart.
    Unknown
    American Institute of Physics (AIP)
    Publication Date: 2014-02-08
    Description: Due to the high intrinsic thermal conductivity, carbon nanotubes are very promising to serve as effective thermal interface materials for microelectronics or other cooling applications. However, the performance of carbon nanotube based thermal interface material is strongly limited by the small effective contact area and weak bonding at carbon nanotube and material interface. Here, we propose a junction structure that the carbon nanotube is bonded with a monolayer graphene, which could potentially enhance the interface thermal conductance. Molecular dynamics simulations show that the interface thermal conductance can be enhanced by at least 40% compared to direct carbon nanotube and silicon interface with strong covalent bonding, while for weak van der Waals bonding the conductance can be enhanced by almost one order of magnitude. The enhancement of thermal conductance is attributed to the efficient thermal transport between carbon nanotube and graphene, as well as the good contact between graphene and silicon surface.
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  • 78
    facet.materialart.
    Unknown
    American Institute of Physics (AIP)
    Publication Date: 2014-02-08
    Description: CoPt L 1 1 thin films with different thicknesses of 2-50 nm were epitaxially grown on Pt(111) underlayer on amorphous glass substrates by magnetron sputtering. The results show that high out-of-plane coercivity ( H c⊥ ), good perpendicular magnetic anisotropy (PMA), large magnetocrystalline anisotropy constant ( K u ), and negative nucleation field ( H n ) were achieved simultaneously in the thinner CoPt films, indicating a high impedance in the domain wall motion. TEM cross-section images displayed a well developed epitaxy of CoPt L 1 1 (111) orientation starting from Pt(111) plane and extending into magnetic CoPt(111) film, inducing the PMA of CoPt films. Surface morphologies of CoPt/Pt/glass films confirmed that surface CoPt grain sizes are similar to their magnetic domain sizes found in magnetic force microscopy results, evidencing the grain boundary pinning effect on the enhancement of H c⊥ . When the thickness of CoPt film was reduced to 3 nm, higher PMA with H c⊥ ∼ 1.8 kOe and K u ∼ 1.9 × 10 7  erg/cm 3 suggests that L 1 1 CoPt thin film is a promising candidate in the design of advanced spintronic devices, magnetic sensors, and magnetic recording media.
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  • 79
    facet.materialart.
    Unknown
    American Institute of Physics (AIP)
    Publication Date: 2014-02-08
    Description: We report the efficiency of genomic DNA extraction as a function of particle size and quantity. For DNA extraction, we synthesized magnetic nanoclusters of various sizes and coated the surface of these magnetic nanoclusters with meso-2,3-dimercaptosuccinic acid. We showed that the nanoclusters had a tight particle size distribution and high crystallinity. Furthermore, we observed that the three types of magnetic nanoclusters studied exhibited ferrimagnetic behavior and that larger nanoclusters showed larger saturation magnetization values. The resultant efficiency of DNA extraction is inversely proportional to particle size in the range of nanoclusters tested, due to the fact that the surface-to-volume ratio decreases as particle size increases.
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  • 80
    facet.materialart.
    Unknown
    American Institute of Physics (AIP)
    Publication Date: 2014-02-08
    Description: We study the properties of water molecules adjacent to a hydrophobic molecular layer with vibrational sum-frequency generation spectroscopy. We find that the water molecules at D 2 O/hexane, D 2 O/heptane, and D 2 O/polydimethylsiloxane interfaces show an enhanced ordering and stronger hydrogen-bond interactions than the water molecules at a D 2 O/air interface. With increasing temperature (up to 80 °C) the water structure becomes significantly less ordered and the hydrogen bonds become weaker.
    Print ISSN: 0021-9606
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    Topics: Chemistry and Pharmacology , Physics
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  • 81
    Publication Date: 2014-02-08
    Description: With shear interest in nanoporous materials, the ultraconfining interlayer spacing of calcium–silicate–hydrate (C–S–H) provides an excellent medium to study reactivity, structure, and dynamic properties of water. In this paper, we present how substrate composition affects chemo-physical properties of water in ultraconfined hydrophilic media. This is achieved by performing molecular dynamics simulation on a set of 150 realistic models with different compositions of calcium and silicon contents. It is demonstrated that the substrate chemistry directly affects the structural properties of water molecules. The motion of confined water shows a multi-stage dynamics which is characteristic of supercooled liquids and glassy phases. Inhomogeneity in that dynamics is used to differentiate between mobile and immobile water molecules. Furthermore, it is shown that the mobility of water molecules is composition-dependent. Similar to the pressure-driven self-diffusivity anomaly observed in bulk water, we report the first study on composition-driven diffusion anomaly, the self diffusivity increases with increasing confined water density in C–S–H. Such anomalous behavior is explained by the decrease in the typical activation energy required for a water molecule to escape its dynamical cage.
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  • 82
    facet.materialart.
    Unknown
    American Institute of Physics (AIP)
    Publication Date: 2014-02-08
    Description: Exchange coupling in magnetic heterostructures can be modified via introduction of additional magnetic spacer layers at the interfaces. The magnetic characteristics and the spacer layer thickness determine the functional properties of the whole system. We show that the hysteresis loop area of trilayer spring magnets with two different soft magnetic layers (s1, s2) and one hard magnetic layer (h) with the sequence s1/s2/h can be increased as compared to both bilayer structures s1/h and s2/h with the same total thickness of the soft layers and for definite thickness ratios of the soft layers and their sequences. For ferrimagnetic spin valves, the perpendicular exchange bias effect can be tuned via the thickness of non-magnetic spacer layers at the interface, which determine the exchange coupling between ferrimagnets. A simple quasi one-dimensional phenomenological model is able to describe the magnetic hysteresis of even complex layered structures and to predict optimal geometrical and magnetic parameters of such heterostructures.
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  • 83
    Publication Date: 2014-02-08
    Description: Phase change from ferromagnetic to nonmagnetic phase by ion implantation was investigated for bit patterning. An antiferromagnetic L 1 0 FePt 0.64 Rh 0.36 film was implanted with Pt ions at 6.0 × 10 15 ions/cm 2 in order to control its magnetic properties. The film changed to a ferromagnetic one, with the (001) crystalline texture being normal to the film plane and a magnetocrystalline anisotropy of 2.3 × 10 7  erg/cm 3 in the perpendicular direction. Using this magnetic phase change, a planar dot pattern was fabricated. The average height between the dots and the spacing was 0.35 nm, and ferromagnetic dots 50 nm in diameter were observed.
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  • 84
    Publication Date: 2014-02-08
    Description: We report on the modification of the thermoelectric properties of poly(3,4-ethylenedioxylthiophene):poly(styrenesulfonate) (PEDOT:PSS) thin films by means of a simple post treatment of the solid thin films realized by drop-coating. We show that the organic polar solvents, dimethyl sulfoxide and ethylene glycol as secondary dopants for PEDOT:PSS, only affect the film morphology for which a high electrical conductivity is observed. In contrast, ethanolamine (MEA) and ammonia solutions are reduction agents that improve the density of PEDOT chains in the reduced forms (polaron and neutral states), resulting in the trade-off between Seebeck coefficient and electrical conductivity. Furthermore, we show that the nature of amines determines the reduction degree: the nitrogen lone pair electrons in MEA are easier to be donated than those in ammonia solution and will therefore neutralize the PEDOT chains.
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  • 85