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  • American Institute of Physics (AIP)  (241,950)
  • 1
    Publication Date: 2016-04-19
    Description: The two dimensional electron gas (2DEG) characteristics of gated metal-face wurtzite AlInGaN/GaN hetero-junctions including positions of subband energy levels, fermi energy level, and the 2DEG concentration as functions of physical and compositional properties of the hetero-junction (i.e., barrier thickness and metal mole-fractions) are theoretically evaluated using the variational method. The calculated values of the 2DEG concentration are in good agreement with the sparsely available experimental data reported in the literature. According to our simulation results, a considerable shift in the positive direction of threshold voltage of AlInGaN/GaN hetero-junction field-effect transistors can be achieved by engineering both the spontaneous and the piezoelectric polarizations using a quaternary AlInGaN barrier-layer of appropriate mole-fractions.
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  • 2
    Publication Date: 2016-04-19
    Description: A mechanism has been established for pressure diffusion waves in dual-porosity media. Pressure diffusion waves are heavily damped with relatively low velocities and short wavelengths. The characteristic frequency dominates the attenuation behavior of pressure diffusions and separates wave fields into two asymptotic regimes: relaxed and unrelaxed. Characteristic delay times control the pressure diffusion between the matrix and the fractures. The transition zones in wavelength and attenuation peak shift toward high frequencies when the characteristic delay time decreases. In contrast, the transition zones in both phase and group velocity shift toward low frequencies as the characteristic time of the delay increases. In a spatially dependent diffusivity field, the pressure diffusion waves in dual-porosity media obey an accumulation-depletion law.
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  • 3
    Publication Date: 2016-04-19
    Description: The influence of field emission of electrons from surfaces on the fast ionization wave (FIW) propagation in high-voltage nanosecond pulse discharge in the atmospheric-pressure nitrogen is studied by a one-dimensional Particle-in-Cell Monte Carlo Collisions model. A strong influence of field emission on the FIW dynamics and plasma parameters is obtained. Namely, the accounting for the field emission makes possible the bridging of the cathode–anode gap by rather dense plasma (∼10 13  cm −3 ) in less than 1 ns. This is explained by the generation of runaway electrons from the field emitted electrons. These electrons are able to cross the entire gap pre-ionizing it and promoting the ionization wave propagation. We have found that the propagation of runaway electrons through the gap cannot be accompanied by the streamer propagation, because the runaway electrons align the plasma density gradients. In addition, we have obtained that the field enhancement factor allows controlling the speed of ionization wave propagation.
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  • 4
    Publication Date: 2016-04-19
    Description: As a promising charge carrier transfer scheme, optical coupling could potentially improve the performance of an optoelectronic device for energy harvesting based on well developed nanotechnology. By extracting carriers optically, the functional features of the nano-structured material could be better used by minimizing the concerns about its electrical properties. In this paper, we present a rigorous electromagnetic model to analyze the optical carrier transfer problem. The flow of the energy is analyzed carefully by the photon transfer spectrum, and the photon emitters (electron-hole pairs) are assumed in a thermal equilibrium described by Bose-Einstein distribution. The result shows that an energy selective carrier transfer can be optically achieved at the device level by integrating the emitter and receiver into a nano-optical resonator, where both the photon emission and absorption are significantly amplified by a near-field coupling around the resonant frequency. General design and optimization schemes in practice are addressed by examining the influence of the photonic design and an energy dependent emissivity of the emitter, which can be used to develop the optical contacting concept further.
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  • 5
    Publication Date: 2016-04-19
    Description: We investigated numerically the specific spatially localized intense optical structure, a photonic nanojet (PNJ), formed in the near-field scattering of optical radiation at phase diffraction gratings. The finite-difference time-domain technique was employed to study the PNJ key parameters (length, width, focal distance, and intensity) produced by diffraction gratings with the saw-tooth, rectangle, and hemispheric line profiles. Our analysis showed that each type of diffraction gratings produces a photonic jet with unique characteristics. Based on the numerical calculations, we demonstrate that the PNJ could be manipulated in a wide range through the variation of period, duty cycle, and shape of diffraction grating rulings.
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  • 6
    Publication Date: 2016-04-19
    Description: Few-layer MoS 2 field-effect transistors often show an n- type conduction behavior due to the presence of high-density sulfur vacancies. Here, we investigated the possibility of surface defect passivation of MoS 2 by sulfur treatment in (NH 4 ) 2 S solution or coating with an ultrathin layer of selenium or tellurium. It was found that all three elements investigated are able to induce a p- doping effect through suppressing the residual electron concentration by an amount exceeding 0.5 × 10 12  cm −2 in few-layer MoS 2 . Among them, the sulfur-treatment exhibits the most superior thermal stability that survives thermal annealing at temperatures ≥120 °C for at least 10 h. Tellurium exhibits the strongest p -doping effect due to electron trapping by physisorption-induced gap states near the valence band edge. On the other hand, selenium is highly volatile on MoS 2 ; it evaporates and desorbs easily due to Joule heating during electrical measurements in vacuum. The results of first-principles calculations support the experimental observations.
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  • 7
    Publication Date: 2016-04-19
    Description: We have combined X-ray diffraction, neutron diffraction with polarization analysis, small angle neutron scattering, differential scanning calorimetry, and broad band dielectric spectroscopy to investigate the structure and dynamics of binary mixtures of poly(2-(dimethylamino)ethyl methacrylate) with either water or tetrahydrofuran (THF) at different concentrations. Aqueous mixtures are characterized by a highly heterogeneous structure where water clusters coexist with an underlying nano-segregation of main chains and side groups of the polymeric matrix. THF molecules are homogeneously distributed among the polymeric nano-domains for concentrations of one THF molecule/monomer or lower. A more heterogeneous situation is found for higher THF amounts, but without evidences for solvent clusters. In THF-mixtures, we observe a remarkable reduction of the glass-transition temperature which is enhanced with increasing amount of solvent but seems to reach saturation at high THF concentrations. Adding THF markedly reduces the activation energy of the polymer β -relaxation. The presence of THF molecules seemingly hinders a slow component of this process which is active in the dry state. The aqueous mixtures present a strikingly broad glass-transition feature, revealing a highly heterogeneous behavior in agreement with the structural study. Regarding the solvent dynamics, deep in the glassy state all data can be described by an Arrhenius temperature dependence with a rather similar activation energy. However, the values of the characteristic times are about three orders of magnitude smaller for THF than for water. Water dynamics display a crossover toward increasingly higher apparent activation energies in the region of the onset of the glass transition, supporting its interpretation as a consequence of the freezing of the structural relaxation of the surrounding matrix. The absence of such a crossover (at least in the wide dynamic window here accessed) in THF is attributed to the lack of cooperativity effects in the relaxation of these molecules within the polymeric matrix.
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  • 8
    Publication Date: 2016-04-19
    Description: We have fabricated Si-on-insulator (SOI) layers with a thickness h 1 of a few nanometers and examined them by Raman spectroscopy with 363.8 nm excitation. We have found that phonon and electron confinement play important roles in SOI with h 1  
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  • 9
    Publication Date: 2016-04-19
    Description: The predissociation dynamics of H 2 + XUV → H 2 * → H ( 1 s ) + H ( 2 s , 2 p ) has been studied by measuring the fragment branching ratios between the H(2 s ) and H(2 p ) states and the fragment angular distributions using the XUV (extreme ultraviolet) laser pump and UV (ultraviolet) laser probe method. The fragment angular distributions for the predissociation of the 3 p π D 1 Π u + υ = 3 state show parallel transitions, demonstrating that the main components of the dissociating state have 1 Σ u + symmetry. The measured fragment branching ratios, H(2 s )/(H(2 s ) + H(2 p )), for the transitions R(0), R(1), and P(2) in 3 p π D 1 Π u + υ = 3 ← X 1 Σ g + υ ″ = 0 are in good agreement with one of the previous theoretical predictions. The predissociations of the 3 p π D 1 Π u − ( υ = 3 ) state arising from the Q(1), Q(2), and Q(3) lines have also been observed. The angular distributions and the state distributions of the excited fragments (all found from the H(2 p ) state) illustrate that the dissociating states for the Q lines have the expected Π u − symmetry. The predissociation dynamics of the transition 4 p σ B ″ 1 Σ u + υ = 1 ← X 1 Σ g + υ ″ = 0 was also studied. Their fragment angular distributions show the expected parallel transitions, and most of the fragments are in the H(2 s ) states. The Beutler-Fano profiles and the associated spectroscopic parameters for the predissociations have also been obtained by measuring the fragment yield of H(2 s , 2 p ) as a function of excitation photon energies.
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  • 10
    Publication Date: 2016-04-19
    Description: Photoinduced ultrafast non-adiabatic decay of 9-methylhypoxanthine (9MHPX) in aqueous solution was investigated by ab initio surface-hopping dynamics calculations using a combined quantum mechanical/molecular mechanical approach. The absorption spectra of 9MHPX in aqueous solution were also explored by the hybrid cluster-continuum model at the level of time-dependent density functional theory along with the polarizable continuum model (PCM). The static electronic-structure calculations indicate that the absorption spectra of 9MHPX simulated by TD-B3LYP/PCM and TD-X3LYP/PCM can reproduce very well the experimental findings, with the accuracy of about 0.20 eV. According to dynamics simulations, irradiation of 9MHPX populates the bright excited singlet S 1 state, which may undergo an ultrafast non-radiative deactivation to the S 0 state. The lifetime of the S 1 state of 9MHPX in aqueous solution is predicted to be 115.6 fs, slightly longer than that in the gas phase (88.8 fs), suggesting that the solvent water has no significant influence on the excited-state lifetime of 9MHPX. Such a behavior in 9MHPX is distinctly different from its parent hypoxanthine keto-N9H tautomer in which the excited-state lifetime of the latter in water solution was remarkably enhanced as compared to the gas phase. The significant difference of the photodynamical behaviors between 9MHPX and keto-N9H can be ascribed to their different hydrogen bond environment in aqueous solution.
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  • 11
    Publication Date: 2016-04-19
    Description: The predissociation spectrum of the cold, argon-tagged, 9-methylanthracenium radical cation is reported from 8000 cm −1 to 44 500 cm −1 . The reported spectrum contains bands corresponding to at least eight electronic transitions ranging from the near infrared to the ultraviolet. These electronic transitions are assigned through comparison with ab initio energies and intensities. The infrared D 1 ← D 0 transitions exhibit significant vibronic activity, which is assigned through comparison with TD-B3LYP excited state frequencies and intensities, as well as modelled vibronic interactions. Dissociation of 9-methylanthracenium is also observed at high visible-photon energies, resulting in the loss of either CH 2 or CH 3 . The relevance of these spectra, and the spectra of other polycyclic aromatic hydrocarbon radical cations, to the largely unassigned diffuse interstellar bands, is discussed.
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  • 12
    Publication Date: 2016-04-19
    Description: Well-defined criteria are proposed for assessing the accuracy of quantum master equations whose memory functions are approximated by Padé resummation of the first two moments in the electronic coupling. These criteria partition the parameter space into distinct levels of expected accuracy, ranging from quantitatively accurate regimes to regions of parameter space where the approach is not expected to be applicable. Extensive comparison of Padé-resummed master equations with numerically exact results in the context of the spin–boson model demonstrates that the proposed criteria correctly demarcate the regions of parameter space where the Padé approximation is reliable. The applicability analysis we present is not confined to the specifics of the Hamiltonian under consideration and should provide guidelines for other classes of resummation techniques.
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  • 13
    Publication Date: 2016-04-19
    Description: Equilibrated systems of entangled polymer melts cannot be produced using direct brute force equilibration due to the slow reptation dynamics exhibited by high molecular weight chains. Instead, these dense systems are produced using computational techniques such as Monte Carlo-Molecular Dynamics hybrid algorithms, though the use of soft potentials has also shown promise mainly for coarse-grained polymeric systems. Through the use of soft-potentials, the melt can be equilibrated via molecular dynamics at intermediate and long length scales prior to switching to a Lennard-Jones potential. We will outline two different equilibration protocols, which use various degrees of information to produce the starting configurations. In one protocol, we use only the equilibrium bond angle, bond length, and target density during the construction of the simulation cell, where the information is obtained from available experimental data and extracted from the force field without performing any prior simulation. In the second protocol, we moreover utilize the equilibrium radial distribution function and dihedral angle distribution. This information can be obtained from experimental data or from a simulation of short unentangled chains. Both methods can be used to prepare equilibrated and highly entangled systems, but the second protocol is much more computationally efficient. These systems can be strictly monodisperse or optionally polydisperse depending on the starting chain distribution. Our protocols, which utilize a soft-core harmonic potential, will be applied for the first time to equilibrate a million particle system of polyethylene chains consisting of 1000 united atoms at various temperatures. Calculations of structural and entanglement properties demonstrate that this method can be used as an alternative towards the generation of entangled equilibrium structures.
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  • 14
    Publication Date: 2016-04-19
    Description: The effect of fluorine substitution on the molecular structure, crystal packing, and n -type semiconducting properties of a set of poly(arylene-ethynylene) polymers based on alternating thiadiazole and phenyl units linked through ethynylene groups has been studied by means of Density Functional Theory. As a result, an enlargement in the interplanar distance between cofacial polymer chains, as well as a decrease of the electronic coupling and electron mobility is predicted. On the other hand, fluorination could facilitate electron injection into the material. A polymer containing both alkoxy pendant chains and fluorine atoms is proposed as a compromise solution between efficiency of electron injection and charge transport within the material.
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  • 15
    Publication Date: 2016-04-19
    Description: Electronic structures and coherent quantum transport properties are explored for spin-crossover molecule iron-benzene Fe(Bz) 2 using density functional theory combined with non-equilibrium Green’s function. High- and low-spin states are investigated for two different lead-molecule junctions. It is found that the asymmetrical T-shaped contact junction in the high-spin state behaves as an efficient spin filter while it has a smaller conductivity than that in the low-spin state. Large spin Seebeck effect is also observed in asymmetrical T-shaped junction. Spin-polarized properties are absent in the symmetrical H-shaped junction. These findings strongly suggest that both the electronic and contact configurations play significant roles in molecular devices and metal-benzene complexes are promising materials for spintronics and thermo-spintronics.
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  • 16
    Publication Date: 2016-04-19
    Description: Infrared spectra of CO 2 physisorbed on titania nanotubes (TiNTs), predominantly in the anatase polymorph, were measured at 81 K. Asymmetric and symmetric absorption peaks due to the antisymmetric stretch vibration ( ν 3 ) of CO 2 were observed at 2340 cm −1 and 2350 cm −1 , respectively. On the basis of the exposure- and time-dependence of the spectrum, the 2340 cm −1 peak was attributed to CO 2 at the defective sites related to subsurface O vacancies (V o s) while the 2350 cm −1 peak was assigned to that at the fivefold coordinated Ti 4+ sites. It was found that the generalized Fano line shape was well fitted to the 2340 cm −1 peak. We also observed an absorption peak at 2372 cm −1 , which was attributed to the combination band of ν 3 and the external mode of CO 2 at Ti 4+ .
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  • 17
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    American Institute of Physics (AIP)
    Publication Date: 2016-04-19
    Description: It is a well known fact that the Black-Scholes equation admits an alternative representation as a Schrödinger equation expressed in terms of a non-self-adjoint Hamiltonian. We show how pseudo-bosons , linear or not, naturally arise in this context, and how they can be used in the computation of the pricing kernel.
    Print ISSN: 0022-2488
    Electronic ISSN: 1089-7658
    Topics: Mathematics , Physics
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  • 18
    Publication Date: 2016-04-19
    Description: In this work, we consider the q -deformation of the Virasoro algebra [M. Chaichian and P. Presnajder, Phys. Lett. B 277 , 109 (1992)] expressed in terms of free fermions, and we then realize this algebra, when the deformation parameter is a root of unity, on the lattice in a truncated form in terms of the Clifford algebra of Γ matrices. For this finite size truncation, the commutation relations of the Deformed algebra hold exactly albeit without central extension term. We then study the relations existing between this lattice truncation of the deformed Virasoro algebra at roots of unity and the tower of commuting Temperley-Lieb hamiltonians introduced in a previous work.
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  • 19
    Publication Date: 2016-04-19
    Description: In this paper we investigate Bose-Einstein condensation into the one-particle ground state in interacting quantum many-particle systems on graphs. We extend previous results obtained for particles on an interval and show that even arbitrarily small repulsive two-particle interactions destroy the condensate in the one-particle ground state present in the non-interacting Bose gas. Our results also cover singular two-particle interactions, such as the well-known Lieb-Liniger model, in the thermodynamic limit.
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  • 20
    Publication Date: 2016-04-19
    Description: A 3D numerical model is established to simulate a wave-induced seabed dynamic response around composite bucket foundations and to consider the sinking processes that are due to negative pressures. The negative pressure has an influence on the pore-pressure of the seabed close to the foundation. The sinking velocity of the foundation has little influence on the wave-induced pore-water-pressure of the seabed. The liquefaction depth around the composite bucket wind turbine foundation decreased as a result of the negative pressure. The effects of the sinking process on seabed response weaken as the negative pressure dissipates.
    Electronic ISSN: 1941-7012
    Topics: Energy, Environment Protection, Nuclear Power Engineering
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  • 21
    Publication Date: 2016-04-20
    Description: The effects of the fabrication process conditions on the microstructure of silicon dioxide thin films of
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  • 22
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    American Institute of Physics (AIP)
    Publication Date: 2016-04-20
    Description: The thermoelectric transport properties of a rectangular Aharonov-Bohm ring at low temperature are investigated using a theoretical approach based on Green's functions. The oscillations in the transmission coefficient as the field is varied can be used to tune the thermoelectric response of the ring. Large magnitude thermopowers are obtainable which, in conjunction with low conductance, can result in a high thermoelectric figure of merit. The effects of single site impurities and more general Anderson disorder are considered explicitly in the context of evaluating their effect on the Fano-type resonances in the transmission coefficient. Importantly, it is shown that even for moderate levels of disorder, the thermoelectric figure of merit can remain significant, increasing the appeal of such structures from the perspective of specialist thermoelectric applications.
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  • 23
    Publication Date: 2016-04-20
    Description: We examined the crystal structure of the new thermoelectric material LaOBiS 2− x Se x , whose thermoelectric performance is enhanced by Se substitution, by using powder synchrotron X-ray diffraction and Rietveld refinement. The emergence of metallic conductivity and enhancement of the thermoelectric power factor of LaOBiS 2− x Se x can be explained with the higher in-plane chemical pressure caused by the increase of Se concentration at the in-plane Ch1 site (Ch = S, Se). High-temperature X-ray diffraction measurements for optimally substituted LaOBiSSe revealed anomalously large atomic displacement parameters ( U iso ) for Bi and Ch atoms in the BiCh 2 conduction layers. The anisotropic analysis of the atomic displacement parameters ( U 11 and U 33 ) for the in-plane Bi and Ch1 sites suggested that Bi atoms exhibit large atomic displacement along the c -axis direction above 300 K, which could be the origin of the low thermal conductivity in LaOBiSSe. The large Bi vibration along the c -axis direction could be related to in-plane rattling, which is a new strategy for attaining low thermal conductivity and phonon-glass-electron-crystal states.
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  • 24
    Publication Date: 2016-04-20
    Description: Peeling of one-dimensional (1D) nanostructures from flat substrates is an essential technique in studying their adhesion properties. The mechanical deformation of the nanostructure in the peeling experiment is critical to the understanding of the peeling process and the interpretation of the peeling measurements, but it is challenging to measure directly and quantitatively at the nanoscale. Here, we investigate the peeling deformation of a bundled carbon nanotube (CNT) fiber by using an in situ scanning electron microscopy nanomechanical peeling technique. A pre-calibrated atomic force microscopy cantilever is utilized as the peeling force sensor, and its back surface acts as the peeling contact substrate. The nanomechanical peeling scheme enables a quantitative characterization of the deformational behaviors of the CNT fiber in both positive and negative peeling configurations with sub-10 nm spatial and sub-nN force resolutions. Nonlinear continuum mechanics models and finite element simulations are employed to interpret the peeling measurements. The measurements and analysis reveal that the structural imperfections in the CNT fiber may have a substantial influence on its peeling deformations and the corresponding peeling forces. The research findings reported in this work are useful to the study of mechanical and adhesion properties of 1D nanostructures by using nanomechanical peeling techniques.
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  • 25
    Publication Date: 2016-04-20
    Description: Control cocatalyst location on a metal-free semiconductor to promote surface charge transfer for decreasing the electron-hole recombination is crucial for enhancing solar energy conversion. Based on the findings that some metals have an affinity for bonding with the specific atoms of polar semiconductors at a heterostructure interface, we herein control Pt deposition selectively on the Si sites of a micro-SiC photocatalyst surface via in-situ photo-depositing. The Pt-Si bond forming on the interface constructs an excellent channel, which is responsible for accelerating photo-electron transfer from SiC to Pt and then reducing water under visible-light. The hydrogen production is enhanced by two orders of magnitude higher than that of bare SiC, and 2.5 times higher than that of random-depositing nano-Pt with the same loading amount.
    Print ISSN: 0003-6951
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  • 26
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    American Institute of Physics (AIP)
    Publication Date: 2016-04-20
    Description: This paper reports on the study of the dynamics of 1D magneto-granular phononic crystals composed of a chain of spherical steel beads inside a properly designed magnetic field. This field is induced by an array of permanent magnets, located in a holder at a given distance from the chain. The theoretical and experimental results of the band gap structure are displayed, including all six degrees of freedom for the beads, i.e., three translations and three rotations. Experimental evidence of transverse-rotational modes of propagation is presented; moreover, by changing the strength of the magnetic field, the dynamic response of the granular chain is tuned. The combination of non-contact tunability with the potentially strong nonlinear behavior of granular systems ensures the suitability of magneto-granular phononic crystals as nonlinear, tunable mechanical metamaterials for use in controlling elastic wave propagation.
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  • 27
    Publication Date: 2016-04-20
    Description: We measure basic network parameters of silver nanowire (AgNW) networks commonly used as transparent conducting electrodes in organic optoelectronic devices. By means of four point probing with nanoprobes, the wire-to-wire junction resistance and the resistance of single nanowires are measured. The resistance R NW of a single nanowire shows a value of R NW = ( 4.96 ± 0.18 )   Ω / μ m . The junction resistance R J differs for annealed and non-annealed NW networks, exhibiting values of R J = ( 25.2 ± 1.9 )   Ω (annealed) and R J = ( 529 ± 239 )   Ω (non-annealed), respectively. Our simulation achieves a good agreement between the measured network parameters and the sheet resistance R S of the entire network. Extrapolating R J to zero, our study show that we are close to the electrical limit of the conductivity of our AgNW system: We obtain a possible R S reduction by only ≈ 20 % (common R S ≈ 10   Ω / sq ). Therefore, we expect further performance improvements in AgNW systems mainly by increasing NW length or by utilizing novel network geometries.
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  • 28
    Publication Date: 2016-04-20
    Description: Determination of the full elastic constants ( c ij ) of methane hydrates (MHs) at extreme pressure-temperature environments is essential to our understanding of the elastic, thermodynamic, and mechanical properties of methane in MH reservoirs on Earth and icy satellites in the solar system. Here, we have investigated the elastic properties of singe-crystal cubic MH-sI, hexagonal MH-II, and orthorhombic MH-III phases at high pressures in a diamond anvil cell. Brillouin light scattering measurements, together with complimentary equation of state (pressure-density) results from X-ray diffraction and methane site occupancies in MH from Raman spectroscopy, were used to derive elastic constants of MH-sI, MH-II, and MH-III phases at high pressures. Analysis of the elastic constants for MH-sI and MH-II showed intriguing similarities and differences between the phases′ compressional wave velocity anisotropy and shear wave velocity anisotropy. Our results show that these high-pressure MH phases can exhibit distinct elastic, thermodynamic, and mechanical properties at relevant environments of their respective natural reservoirs. These results provide new insight into the determination of how much methane exists in MH reservoirs on Earth and on icy satellites elsewhere in the solar system and put constraints on the pressure and temperature conditions of their environment.
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  • 29
    Publication Date: 2016-04-20
    Description: Supported lipid bilayers (SLBs) offer an excellent model system for investigating the physico-chemical properties of the cell membrane. In this work, dynamic and mechanical properties of SLBs are explored by dissipative particle dynamics simulations for lipids with different architectures (chain length, kink, and asymmetry associated with lipid tails). It is found that the lateral diffusivity (D x ) and flip-flop rate (FF) grow with increasing temperature in both gel and liquid phases and can be described by an Arrhenius-like expression. Three regimes can be clearly identified for symmetric and asymmetric saturated lipids but only two regimes are observed for kinked lipids. Both D x and FF grow with decreasing tail length and increasing number of kinks. The stretching (K A ) and apparent bending (K B ) moduli exhibit concave upward curves with temperature and the minima are attained at T m . In general, the minima of K A and K B decrease with the chain length and increase with number of kinks. The typical relation among the bending modulus, area stretching modulus, and bilayer thickness is still followed, K B = β K A h 2 and β is much smaller in the gel phase. The dynamic and mechanical properties of lipids with asymmetric tails are found to situate between their symmetric counterparts.
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  • 30
    Publication Date: 2016-04-20
    Description: We use density functional theory to describe a novel way for metal free catalytic reduction of nitric oxide NO utilizing boron doped graphene. The present study is based on the observation that boron doped graphene and O—N=N—O − act as Lewis acid-base pair allowing the graphene surface to act as a catalyst. The process implies electron assisted N=N bond formation prior to N—O dissociation. Two N 2 + O 2 product channels, one of which favoring N 2 O formation, are envisaged as outcome of the catalytic process. Besides, we show also that the N 2 + O 2 formation pathways are contrasted by a side reaction that brings to N 3 O 3 − formation and decomposition into N 2 O + NO 2 − .
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  • 31
    Publication Date: 2016-04-20
    Description: Simulated tempering (ST) is a widely used enhancing sampling method for Molecular Dynamics simulations. As one expanded ensemble method, ST is a combination of canonical ensembles at different temperatures and the acceptance probability of cross-temperature transitions is determined by both the temperature difference and the weights of each temperature. One popular way to obtain the weights is to adopt the free energy of each canonical ensemble, which achieves uniform sampling among temperature space. However, this uniform distribution in temperature space may not be optimal since high temperatures do not always speed up the conformational transitions of interest, as anti-Arrhenius kinetics are prevalent in protein and RNA folding. Here, we propose a new method: Enhancing Pairwise State-transition Weights (EPSW), to obtain the optimal weights by minimizing the round-trip time for transitions among different metastable states at the temperature of interest in ST. The novelty of the EPSW algorithm lies in explicitly considering the kinetics of conformation transitions when optimizing the weights of different temperatures. We further demonstrate the power of EPSW in three different systems: a simple two-temperature model, a two-dimensional model for protein folding with anti-Arrhenius kinetics, and the alanine dipeptide. The results from these three systems showed that the new algorithm can substantially accelerate the transitions between conformational states of interest in the ST expanded ensemble and further facilitate the convergence of thermodynamics compared to the widely used free energy weights. We anticipate that this algorithm is particularly useful for studying functional conformational changes of biological systems where the initial and final states are often known from structural biology experiments.
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  • 32
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    American Institute of Physics (AIP)
    Publication Date: 2016-04-20
    Description: The reductive conversion of CO 2 into industrial products (e.g., oxalic acid, formic acid, methanol) can occur via aqueous CO 2 − as a transient intermediate. While the formation, structure, and reaction pathways of this radical anion have been modelled for decades using various spectroscopic and theoretical approaches, we present here, for the first time, a vibrational spectroscopic investigation in liquid water, using pulse radiolysis time-resolved resonance Raman spectroscopy for its preparation and observation. Excitation of the radical in resonance with its 235 nm absorption displays a transient Raman band at 1298 cm −1 , attributed to the symmetric CO stretch, which is at ∼45 cm −1 higher frequency than in inert matrices. Isotopic substitution at C ( 13 CO 2 − ) shifts the frequency downwards by 22 cm −1 , which confirms its origin and the assignment. A Raman band of moderate intensity compared to the stronger 1298 cm −1 band also appears at 742 cm −1 and is assignable to the OCO bending mode. A reasonable resonance enhancement of this mode is possible only in a bent CO 2 − (C 2v /C s ) geometry. These resonance Raman features suggest a strong solute-solvent interaction, the water molecules acting as constituents of the radical structure, rather than exerting a minor solvent perturbation. However, there is no evidence of the non-equivalence (C s ) of the two CO bonds. A surprising resonance Raman feature is the lack of overtones of the symmetric CO stretch, which we interpret due to the detachment of the electron from the CO 2 − moiety towards the solvation shell. Electron detachment occurs at the energies of 0.28 ± 0.03 eV or higher with respect to the zero point energy of the ground electronic state. The issue of acid-base equilibrium of the radical, which has been in contention for decades, as reflected in a wide variation in the reported pK a (−0.2 to 3.9), has been resolved. A value of 3.4 ± 0.2 measured in this work is consistent with the vibrational properties, bond structure, and charge distribution in aqueous CO 2 − .
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  • 33
    Publication Date: 2016-04-20
    Description: We introduce a system-independent method to derive effective atomic C 6 coefficients and polarizabilities in molecules and materials purely from charge population analysis. This enables the use of dispersion-correction schemes in electronic structure calculations without recourse to electron-density partitioning schemes and expands their applicability to semi-empirical methods and tight-binding Hamiltonians. We show that the accuracy of our method is en par with established electron-density partitioning based approaches in describing intermolecular C 6 coefficients as well as dispersion energies of weakly bound molecular dimers, organic crystals, and supramolecular complexes. We showcase the utility of our approach by incorporation of the recently developed many-body dispersion method [Tkatchenko et al. , Phys. Rev. Lett. 108 , 236402 (2012)] into the semi-empirical density functional tight-binding method and propose the latter as a viable technique to study hybrid organic-inorganic interfaces.
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  • 34
    Publication Date: 2016-04-20
    Description: Various topics such as CO 2 emissions, industry, human activities, and electricity distribution grids have attracted considerable attention because of the current state of crude oil production. Furthermore, estimations of solar radiation levels and of the efficiencies of photovoltaics (PVs), concentrated solar power (CSP), and solar chimney towers, as well as other renewable energy sources such as wind, tidal, and geothermal, have all been investigated. Here, an overview is presented of the potential future demands and possible supply of solar energy in relation to Iraq. Solar and wind energy sources, which are clean, inexhaustible, and environmentally friendly, are presented as renewable energy resources. Those systems that combine various sources of energy are called hybrids and they have received considerable attention in recent decades. The fundamental characteristics of solar radiation in Iraq are summarized, and the selection of those sites with potential for development of solar plants is based on the local maximum solar radiation. Furthermore, longitudinal and latitudinal orientation, wind, solar intensity, dust, temperature, rain, humidity, and pollution factors are all considered in the calculation of PV/CSP efficiencies. A study of the variation of annual radiation levels was conducted. The average UV radiation was found to comprise 3.25% of the global radiation. Therefore, most of the 47% reduction in received incoming global solar UV radiation is due to scattering and absorption in the atmosphere. Details regarding the desalination of underground or polluted water resources to support solar energy systems and plants and to preserve a clean low-dust environment are presented.
    Electronic ISSN: 1941-7012
    Topics: Energy, Environment Protection, Nuclear Power Engineering
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  • 35
    Publication Date: 2016-04-20
    Description: High-frequency (50 Hz) observational data from the 200-m tower data (Reese Technology Center, Texas) have been prescribed as inflow conditions into the NREL FAST code in order to evaluate the structural impacts of Low Level Jets (LLJs) on a typical commercial wind turbine. A vertical region of interest for the analysis of interaction LLJ–wind turbine has been delimited, and the LLJ length scales have been calculated. The analysis of power spectra exhibited a deviation within the inertial subrange from the classical −5/3 slope in a log-log representation towards a lower slope, which indicated a lower rate of energy transfer when the LLJ was present. It has been observed that during a LLJ event the turbulence intensity and turbulence kinetic energy were significantly lower than those during unstable conditions; and cyclical aerodynamic loads on the turbine blades produced a negative impact on the wind turbine, mainly due to the enhanced wind shear. Dominant frequencies present in the power spectra of the incoming wind were also observed in frequencies related to the dynamic loads of the turbines. It was found that the wind turbine can mimic the signals from the approaching inlet flow, although some of the replication can be altered or annulled in a wind farm.
    Electronic ISSN: 1941-7012
    Topics: Energy, Environment Protection, Nuclear Power Engineering
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  • 36
    Publication Date: 2016-04-20
    Description: E-waste is certainly increasing to become a global issue. Fast evolution of mobile electronic devices leads to their premature replacement for always newer, faster, and nicer design and therefore may be a source of environmental pollution and contributes to global warming. Fortunately, due to the properties of the lithium ion (Li-ion) battery that powers them, there is in average 3 years of life left for the battery when the device is recycled or no longer in use after 2 years. This wastage can be valued by giving to the batteries a second life as energy storage for lighting. In fact, it can be a real opportunity for access to electricity in remote rural areas of developing countries for low cost and quality lighting. We present that used mobile phone batteries associated with a solar panel and a light emitting diode lamp can be a good replacement for candles or kerosene lamps that generate pollution, are hazardous, and only give poor lighting. Such a replacement can be done for much lower cost than current expenses, better quality of light, and contributes to poverty alleviation and job creation. This reduces wastage in developed countries and overcomes the challenges of cost and durability in small off-grid photovoltaic systems in non-electrified rural areas of developing countries. The creation of social incentive can have a real effect on the reduction of e-waste.
    Electronic ISSN: 1941-7012
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  • 37
    Publication Date: 2016-04-21
    Description: Hysteresis phenomenon in the transfer characteristics of carbon nanotube field effect transistor (CNT FET) is being considered as the main obstacle for successful realization of electronic devices based on CNTs. In this study, we prepare four kinds of CNTFETs and explore their hysteretic behavior. Two kinds of devices comprise on-surface CNTs (type I) and suspended CNTs (type II) with thin insulating layer underneath and a single global gate which modulates the CNT conductance. The third and fourth types (types III and IV) consist of suspended CNT over a metallic local gate underneath, where for type IV the local gate was patterned self aligned with the source and drain electrodes. The first two types of devices, i.e., type I and II, exhibit substantial hysteresis which increases with scanning range and sweeping time. Under high vacuum conditions and moderate electric fields ( | E | 〉 4 × 10 6   V / cm ), the hysteresis for on-surface devices cannot be eliminated, as opposed to suspended devices. Interestingly, type IV devices exhibit no hysteresis at all at ambient conditions, and from the different roles which the global and local gates play for the four types of devices, we could learn about the hysteresis mechanism of this system. We believe that these self aligned hysteresis free FETs will enable the realization of different electronic devices and sensors based on CNTs.
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  • 38
    Publication Date: 2016-04-21
    Description: Two-dimensional transition-metal dichalcogenide (TMDs) MoTe 2 has attracted much attention due to its predicted Weyl semimetal state and a quantum spin Hall insulator in bulk and monolayer form, respectively. We find that the superconductivity in MoTe 2 single crystal can be greatly enhanced by the partial substitution of the Te ions by the S ones. The maximum superconducting temperature T C of MoTe 1.8 S 0.2 single crystal is about 1.3 K. Compared with the parent MoTe 2 single crystal ( T C  = 0.1 K), nearly 13-fold in T C is improved in the MoTe 1.8 S 0.2 one. The superconductivity has been investigated through the resistivity and magnetization measurements. MoTe 2−x S x single crystals belong to weak coupling superconductors and the improvement of the superconductivity may be related to the enhanced electron-phonon coupling induced by the S-ion substitution. A dome-shaped superconducting phase diagram is obtained in the S-doped MoTe 2 single crystals. MoTe 2−x S x materials may provide a new platform for our understanding of superconductivity phenomena and topological physics in TMDs.
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  • 39
    Publication Date: 2016-04-21
    Description: We propose, fabricate, and characterize the on-chip integration of suspended p-n junction InGaN/GaN multiple quantum wells (MQWs) device and multiple waveguides on the same GaN-on-silicon platform. The integrated devices are fabricated via a wafer-level process and exhibit selectable functionalities for diverse applications. As the suspended p-n junction InGaN/GaN MQWs device operates under a light emitting diode (LED) mode, part of the light emission is confined and guided by the suspended waveguides. The in-plane propagation along the suspended waveguides is measured by a micro-transmittance setup. The on-chip data transmission is demonstrated for the proof-of-concept photonic integration. As the suspended p-n junction InGaN/GaN MQWs device operates under photodiode mode, the light is illuminated on the suspended waveguides with the aid of the micro-transmittance setup and, thus, coupled into the suspended waveguides. The guided light is finally sensed by the photodiode, and the induced photocurrent trace shows a distinct on/off switching performance. These experimental results indicate that the on-chip photonic integration is promising for the development of sophisticated integrated photonic circuits in the visible wavelength region.
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  • 40
    Publication Date: 2016-04-21
    Description: We report the fabrication of a Si(111) crystalline thin film on graphene by the aluminum-induced crystallization (AIC) process. The AIC process of Si(111) on graphene is shown to be enhanced compared to that on an amorphous SiO 2 substrate, resulting in a more homogeneous Si(111) thin film structure as revealed by X-ray diffraction and atomic force microscopy measurements. Raman measurements confirm that the graphene is intact throughout the process, retaining its characteristic phonon spectrum without any appearance of the D peak. A red-shift of Raman peaks, which is more pronounced for the 2D peak, is observed in graphene after the crystallization process. It is found to correlate with the red-shift of the Si Raman peak, suggesting an epitaxial relationship between graphene and the adsorbed AIC Si(111) film with both the graphene and Si under tensile strain.
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  • 41
    Publication Date: 2016-04-21
    Description: The band alignment between AlN and Atomic-Layer-Deposited (ALD) HfO 2 was determined by X-ray photoelectron spectroscopy (XPS). The shift of Al 2 p core-levels to lower binding energies with the decrease of take-off angles θ indicated upward band bending occurred at the AlN surface. Based on the angle-resolved XPS measurements combined with numerical calculations, valence band discontinuity Δ E V of 0.4 ± 0.2 eV at HfO 2 /AlN interface was determined by taking AlN surface band bending into account. By taking the band gap of HfO 2 and AlN as 5.8 eV and 6.2 eV, respectively, a type-II band line-up was found between HfO 2 and AlN.
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  • 42
    Publication Date: 2016-04-21
    Description: The Helium Ion Microscope (HIM) has the capability to image small features with a resolution down to 0.35 nm due to its highly focused gas field ionization source and its small beam-sample interaction volume. In this work, the focused helium ion beam of a HIM is utilized to create nanopores with diameters down to 1.3 nm. It will be demonstrated that nanopores can be milled into silicon nitride, carbon nanomembranes, and graphene with well-defined aspect ratio. To image and characterize the produced nanopores, helium ion microscopy and high resolution scanning transmission electron microscopy were used. The analysis of the nanopores' growth behavior allows inferring on the profile of the helium ion beam.
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  • 43
    Publication Date: 2016-04-21
    Description: Samples of the magneto-active material—Tb 3+ :Y 2 O 3 ceramics with Tb 3+ ion concentrations of 10%, 20%, 30%, and 100% (Tb 2 O 3 )—were prepared and studied. The wavelength dependence of Verdet constant in the 380 nm–1750 nm range was approximated for all investigated ceramic samples and was predicted for a pure Tb 2 O 3 material. Tb 2 O 3 ceramics demonstrates a more than three times higher Verdet constant in comparison with terbium gallium garnet crystal or ceramics. The linear dependence of the Verdet constant on Tb 3+ ion concentration in the Tb 3+ :Y 2 O 3 ceramics was demonstrated. The obtained data will be useful for fabricating magneto-optical elements of Faraday devices based on Tb 3+ :Y 2 O 3 with arbitrary Tb 3+ ion concentration operating at room temperature in the wavelength range of 380 nm–1750 nm.
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  • 44
    Publication Date: 2016-04-21
    Description: Using an aspect ratio trapping technique, we demonstrate molecular beam epitaxy of GaAs nanostubs on Si(001) substrates. Nanoholes in a SiO 2 mask act as a template for GaAs-on-Si selective-area growth (SAG) of nanostubs 120 nm tall and ≤100 nm in diameter. We investigate the influence of growth parameters including substrate temperature and growth rate on SAG. Optimizing these parameters results in complete selectivity with GaAs growth only on the exposed Si(001). Due to the confined-geometry, strain and defects in the GaAs nanostubs are restricted in lateral dimensions, and surface energy is further minimized. We assess the electrical properties of the selectively grown GaAs nanostubs by fabricating heterogeneous p + –Si/n + –GaAs p–n diodes.
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  • 45
    Publication Date: 2016-04-21
    Description: In this paper, intrinsic and extrinsic factors dependent switching process in P(VDF-TrFE) thin films is investigated through time domain polarization measurements. The thinning-induced increase of switching time is observed for samples below 80 nm due to the surface oxide layer. For thinner samples the switchable polarization decreases with decreasing temperature since domain pinning prevails. Switching is faster using metal electrode with higher work function, which can be attributed to the lower depolarization field. Furthermore, the switching time increases with increasing the waiting time and increasing the pulse width of the prepolarization pulse caused by imprint effect.
    Electronic ISSN: 2166-532X
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
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  • 46
    Publication Date: 2016-04-21
    Description: We report the design and experimental realization of a type of miniaturized device for efficient unidirectional launching and shaping of surface plasmon polaritons (SPPs). Each device consists of an array of evenly spaced gap plasmon resonators with varying dimensions. Particle swarm optimization is used to achieve a theoretical two-dimensional launching efficiency of about 51%, under the normal illumination of a 5- μ m waist Gaussian beam at 780 nm. By modifying the wavefront of the SPPs, unidirectional SPPs with focused, Bessel, and Airy profiles are launched and imaged with leakage radiation microscopy.
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  • 47
    Publication Date: 2016-04-21
    Description: The steady-state and time-resolved photoluminescence (PL) are used to study the electron injection from graphene quantum dots (GQDs) to poly(amido amine) (PAMAM) dendrimers. The PL is enhanced by depositing GQDs on the surfaces of the PAMAM dendrimers. The maximum enhancement of PL with a factor of 10.9 is achieved at a GQD concentration of 0.9 mg/ml. The dynamics of PL in the GQD/PAMAM composite are analyzed, evidencing the existence of electron injection. On the basis of Kelvin probe measurements, the electron injection from the GQDs to the PAMAM dendrimers is accounted for by the work function difference between them.
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  • 48
    Publication Date: 2016-04-21
    Description: Two-component second- and third-order Burgers type systems with nondiagonal constant matrix of leading order terms are classified for higher symmetries. New integrable systems are obtained. Master symmetries of the obtained symmetry integrable systems, and bi-Poisson structures of those that also possess conservation laws, are given.
    Print ISSN: 0022-2488
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    Topics: Mathematics , Physics
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  • 49
    Publication Date: 2016-04-21
    Description: We report a metasurface for focusing reflected ultrasonic waves over a wide frequency band of 0.45–0.55 MHz. The broadband focusing effect of the reflective metasurface is studied numerically and then confirmed experimentally using near-field scanning techniques. The focusing mechanism can be attributed to the hyperboloidal reflection phase profile imposed by different depths of concentric grooves on the metasurface. In particular, the focal lengths of the reflective metasurface are extracted from simulations and experiments, and both exhibit good linear dependence on frequency over the considered frequency band. The proposed broadband reflective metasurface with tunable focal length has potential applications in the broad field of ultrasonics, such as ultrasonic tomographic imaging, high intensity focused ultrasound treatment, etc.
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  • 50
    Publication Date: 2016-04-21
    Description: Here, we investigate the directional control of Galinstan liquid metal droplets when transferring from the high-viscosity glycerol core into the parallel low-viscosity NaOH sheath streams within a flow focusing microfluidic system. In the presence of sufficient flow mismatch between the sheath streams, the droplets are driven toward the higher velocity interface and cross the interface under the influence of surface tension gradient. A minimum flow mismatch of 125  μ l/min is required to enable the continuous transfer of droplets toward the desired sheath stream. The response time of droplets, the time required to change the direction of droplet transfer, is governed by the response time of the syringe pump driven microfluidic system and is found to be 3.3 and 8.8 s when increasing and decreasing the flow rate of sheath stream, respectively.
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  • 51
    Publication Date: 2016-04-21
    Description: We investigate the confined states and transport of three-dimensional Weyl electrons around a one-dimensional external rectangular electrostatic potential. The confined states with finite transverse wave vector exist at energies higher than the half well depth or lower than the half barrier height. The rectangular potential appears completely transparent to the normal incident electrons but not otherwise. The tunneling transmission coefficient is sensitive to their incident angle and shows resonant peaks when their energy coincides with the confined spectra. In addition, for the electrons in the conduction (valence) band through a potential barrier (well), the transmission spectrum has a gap of width increasing with the incident angle. Interestingly, the electron linear zero-temperature conductance over the potential can approach zero when the Fermi energy is aligned to the top and bottom energies of the potential, when only electron beams normal to the potential interfaces can pass through. The considered structure can be used to collimate the Weyl electron beams.
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  • 52
    Publication Date: 2016-04-21
    Description: We report the rapid prototyping of passive electrical components (resistors and capacitors) on plain paper by an additive and parallel technology consisting of supersonic cluster beam deposition (SCBD) coupled with shadow mask printing. Cluster-assembled films have a growth mechanism substantially different from that of atom-assembled ones providing the possibility of a fine tuning of their electrical conduction properties around the percolative conduction threshold. Exploiting the precise control on cluster beam intensity and shape typical of SCBD, we produced, in a one-step process, batches of resistors with resistance values spanning a range of two orders of magnitude. Parallel plate capacitors with paper as the dielectric medium were also produced with capacitance in the range of tens of picofarads. Compared to standard deposition technologies, SCBD allows for a very efficient use of raw materials and the rapid production of components with different shape and dimensions while controlling independently the electrical characteristics. Discrete electrical components produced by SCBD are very robust against deformation and bending, and they can be easily assembled to build circuits with desired characteristics. The availability of large batches of these components enables the rapid and cheap prototyping and integration of electrical components on paper as building blocks of more complex systems.
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  • 53
    Publication Date: 2016-04-21
    Description: We introduce a new type of cluster beam source based on the assembly of (metal) clusters within a condensed (rare gas) matrix. The “Matrix Assembly Cluster Source” employs an ion beam to enhance collisions between metal atoms in the matrix and to sputter out clusters to form a beam. We demonstrate the formation and deposition of gold and silver nanoclusters with mean size tunable from a few atoms to a few thousand atoms. The cluster flux is equivalent to a current nanoAmp regime but potentially scalable to milliAmps, which would open up a number of interesting experiments and applications.
    Print ISSN: 0034-6748
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    Topics: Electrical Engineering, Measurement and Control Technology , Physics
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  • 54
    Publication Date: 2016-04-22
    Description: We report on the microstructural features associated with the formation of incipient spall and damage in a fully recrystallized, high purity copper sample. Before and after ballistic shock loading, approximately 0.8 mm 3 of the sample's crystal lattice orientation field is mapped using non-destructive near-field High Energy Diffraction Microscopy. Absorption contrast tomography is used to image voids after loading. This non-destructive interrogation of damage initiation allows for novel characterization of spall points vis-a-vis microstructural features and a fully 3D examination of microstructural topology and its influence on incipient damage. The spalled region is registered with and mapped back onto the pre-shock orientation field. As expected, the great majority of voids occur at grain boundaries and higher order microstructural features; however, we find no statistical preference for particular grain boundary types. The damaged region contains a large volume of Σ – 3   ( 60 ° 〈 111 〉 ) connected domains with a large area fraction of incoherent Σ-3 boundaries.
    Print ISSN: 0021-8979
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  • 55
    Publication Date: 2016-04-22
    Description: High-power impulse magnetron sputtering (HiPIMS) is used to deposit amorphous carbon thin films with sp 3 fractions of 13% to 82%. Increasing the pulse length results in a transition from conventional HiPIMS deposition to a “mixed-mode” in which an arc triggers on the target surface, resulting in a large flux of carbon ions. The films are characterized using X-ray photoelectron spectroscopy, Raman spectroscopy, ellipsometry, nanoindentation, elastic recoil detection analysis, and measurements of stress and contact angle. All properties vary in a consistent manner, showing a high tetrahedral character only for long pulses, demonstrating that mixed-mode deposition is the source of the high carbon ion flux. Varying the substrate bias reveals an “energy window” effect, where the sp 3 fraction of the films is greatest for a substrate bias around −100 V and decreases for higher or lower bias values. In the absence of bias, the films' properties show little dependence on the pulse length, showing that energetic ions are the origin of the highly tetrahedral character.
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  • 56
    Publication Date: 2016-04-22
    Description: The correlation between phosphorus concentration in phospho-silicate glass (PSG) gate dielectrics and electrical properties of 4H-SiC MOS devices has been investigated. Varying P uptake in PSG is achieved by changing the POCl 3 post-oxidation annealing temperature. The density of interface traps (D it ) at the PSG/4H-SiC interface decreases as the amount of interfacial P increases. Most significantly, the MOSFET channel mobility does not correlate with D it for all samples, which is highly unusual for SiC MOSFETs. Further analysis reveals two types of field-effect mobility ( μ fe ) behavior, depending on the annealing temperature. Annealing at 1000 °C improves the channel mobility most effectively, with a peak value ∼105 cm 2 V −1 s −1 , and results in a surface phonon scattering limited mobility at high oxide field. On the other hand, PSG annealed at other temperatures results in a surface roughness scattering limited mobility at similar field.
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  • 57
    Publication Date: 2016-04-22
    Description: We evaluate the applicability of a hierarchy of quantum models in characterizing the binding energy of excess electrons to water clusters. In particular, we calculate the vertical detachment energy of an excess electron from water cluster anions with methods that include one-electron pseudopotential calculations, density functional theory (DFT) based calculations, and ab initio quantum chemistry using MP2 and eom-EA-CCSD levels of theory. The examined clusters range from the smallest cluster size ( n = 2) up to nearly nanosize clusters with n = 1000 molecules. The examined cluster configurations are extracted from mixed quantum-classical molecular dynamics trajectories of cluster anions with n = 1000 water molecules using two different one-electron pseudopotenial models. We find that while MP2 calculations with large diffuse basis set provide a reasonable description for the hydrated electron system, DFT methods should be used with precaution and only after careful benchmarking. Strictly tested one-electron psudopotentials can still be considered as reasonable alternatives to DFT methods, especially in large systems. The results of quantum chemistry calculations performed on configurations, that represent possible excess electron binding motifs in the clusters, appear to be consistent with the results using a cavity structure preferring one-electron pseudopotential for the hydrated electron, while they are in sharp disagreement with the structural predictions of a non-cavity model.
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    Topics: Chemistry and Pharmacology , Physics
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  • 58
    Publication Date: 2016-04-22
    Description: The crystallization mechanism in deeply undercooled ZrCu metallic glass-forming liquids was investigated via molecular dynamics simulations. It was found that the crystallization process is mainly controlled by the growth of crystal nuclei formed by the BCC-like atomic clusters, consistent with experimental speculations. The crystallization rate is found to relate to the number of growing crystal nuclei in the crystallization process. The crystallization rate in systems with more crystal nuclei is significantly hindered by the larger surface fractions of crystal nuclei and their different crystalline orientations. It is further revealed that in the crystallization in deeply undercooled regions, the BCC-like crystal nuclei are formed from the inside of the precursors formed by the FCC-like atomic clusters, and growing at the expense of the precursors. Meanwhile, the precursors are expanding at the expense of the outside atomic clusters. This process is consistent with the so-called Ostwald step rule. The atomic structures of metallic glasses are found to have significant impact on the subsequent crystallization process. In the Zr 85 Cu 15 system, the stronger spatial correlation of Cu atoms could hinder the crystallization processes in deeply undercooled regions.
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  • 59
    Publication Date: 2016-04-22
    Description: We report the observation of a manganese-centered tubular boron cluster (MnB 16 − ), which is characterized by photoelectron spectroscopy and ab initio calculations. The relatively simple pattern of the photoelectron spectrum indicates the cluster to be highly symmetric. Ab initio calculations show that MnB 16 − has a Mn-centered tubular structure with C 4v symmetry due to first-order Jahn-Teller effect, while neutral MnB 16 reduces to C 2v symmetry due to second-order Jahn-Teller effect. In MnB 16 − , two unpaired electrons are observed, one on the Mn 3d z 2 orbital and another on the B 16 tube, making it an unusual biradical. Strong covalent bonding is found between the Mn 3d orbitals and the B 16 tube, which helps to stabilize the tubular structure. The current result suggests that there may exist a whole class of metal-stabilized tubular boron clusters. These metal-doped boron clusters provide a new bonding modality for transition metals, as well as a new avenue to design boron-based nanomaterials.
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  • 60
    Publication Date: 2016-04-22
    Description: To fast and accurately compute rate coefficients of the H/D + CH 4 → H 2 /HD + CH 3 reactions, we propose a segmented strategy for fitting suitable potential energy surface (PES), on which ring-polymer molecular dynamics (RPMD) simulations are performed. On the basis of recently developed permutation invariant polynomial neural-network approach [J. Li et al. , J. Chem. Phys. 142 , 204302 (2015)], PESs in local configuration spaces are constructed. In this strategy, global PES is divided into three parts, including asymptotic, intermediate, and interaction parts, along the reaction coordinate. Since less fitting parameters are involved in the local PESs, the computational efficiency for operating the PES routine is largely enhanced by a factor of ∼20, comparing with that for global PES. On interaction part, the RPMD computational time for the transmission coefficient can be further efficiently reduced by cutting off the redundant part of the child trajectories. For H + CH 4 , good agreements among the present RPMD rates and those from previous simulations as well as experimental results are found. For D + CH 4 , on the other hand, qualitative agreement between present RPMD and experimental results is predicted.
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  • 61
    Publication Date: 2016-04-22
    Description: An approximate implementation of the multiconfiguration time-dependent Hartree-Fock method is proposed, in which the matrix of configuration-interaction coefficients is decomposed into a product of matrices of smaller dimension. The applicability of this method in which all the configurations are kept in the expansion of the wave function, while the configuration-interaction coefficients are approximately calculated, is discussed by showing the results on three model systems: a one-dimensional model of a beryllium atom, a one-dimensional model of a carbon atom, and a one-dimensional model of a chain of four hydrogen atoms. The time-dependent electronic dynamics induced by a few-cycle, long-wavelength laser pulse is found to be well described at a lower computational cost compared to the standard multiconfiguration time-dependent Hartree-Fock treatment. Drawbacks of the method are also discussed.
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  • 62
    Publication Date: 2016-04-22
    Description: We present a semi-quantitative model of condensation of short nucleic acid (NA) duplexes induced by trivalent cobalt( iii ) hexammine (CoHex) ions. The model is based on partitioning of bound counterion distribution around single NA duplex into “external” and “internal” ion binding shells distinguished by the proximity to duplex helical axis. In the aggregated phase the shells overlap, which leads to significantly increased attraction of CoHex ions in these overlaps with the neighboring duplexes. The duplex aggregation free energy is decomposed into attractive and repulsive components in such a way that they can be represented by simple analytical expressions with parameters derived from molecular dynamic simulations and numerical solutions of Poisson equation. The attractive term depends on the fractions of bound ions in the overlapping shells and affinity of CoHex to the “external” shell of nearly neutralized duplex. The repulsive components of the free energy are duplex configurational entropy loss upon the aggregation and the electrostatic repulsion of the duplexes that remains after neutralization by bound CoHex ions. The estimates of the aggregation free energy are consistent with the experimental range of NA duplex condensation propensities, including the unusually poor condensation of RNA structures and subtle sequence effects upon DNA condensation. The model predicts that, in contrast to DNA, RNA duplexes may condense into tighter packed aggregates with a higher degree of duplex neutralization. An appreciable CoHex mediated RNA-RNA attraction requires closer inter-duplex separation to engage CoHex ions (bound mostly in the “internal” shell of RNA) into short-range attractive interactions. The model also predicts that longer NA fragments will condense more readily than shorter ones. The ability of this model to explain experimentally observed trends in NA condensation lends support to proposed NA condensation picture based on the multivalent “ion binding shells.”
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  • 63
    Publication Date: 2016-04-22
    Description: We determine the interfacial properties of mixtures of spherical square-well molecules from direct simulation of the vapor-liquid interface. We consider mixtures with the same molecular size and intermolecular potential range but different dispersive energy parameter values. We perform Monte Carlo simulations in the canonical ensemble to obtain the interfacial properties of mixtures of square-well molecules. In particular, we determine the pressure tensor using the mechanical (virial) route and the vapor-liquid interfacial tension evaluated using the Irving-Kirkwood method. In addition to the pressure tensor and the surface tension, we also obtain density profiles, coexistence densities, and interfacial thickness as functions of pressure, at a given temperature. This work can be considered as the extension of our previous work [F. J. Martínez-Ruiz and F. J. Blas, Mol. Phys. 113 , 1217 (2015)] to deal with mixtures of spherical molecules that interact through a discontinuous intermolecular potential. According to our results, the main effect of increasing the ratio between the dispersive energy parameters of the mixture, ϵ 22 / ϵ 11 , is to sharpen the vapor-liquid interface and to increase the width of the biphasic coexistence region. Particularly interesting is the presence of a relative maximum in the density profiles of the more volatile component at the interface. This maximum is related with adsorption or accumulation of these molecules at the interface, since there are stronger attractive interactions between these molecules in comparison with the rest of intermolecular interactions. Also, the interfacial thickness decreases and the surface tension increases as ϵ 22 / ϵ 11 is larger, a direct consequence of the increasing of the cohesive energy of the system.
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  • 64
    Publication Date: 2016-04-22
    Description: Novel uses for 2-dimensional materials like graphene and hexagonal boron nitride (h-BN) are being frequently discovered especially for membrane and catalysis applications. Still however, a great deal remains to be understood about the interaction of environmentally and industrially relevant molecules such as water with these materials. Taking inspiration from advances in hybridising graphene and h-BN, we explore using density functional theory, the dissociation of water, hydrogen, methane, and methanol on graphene, h-BN, and their isoelectronic doped counterparts: BN doped graphene and C doped h-BN. We find that doped surfaces are considerably more reactive than their pristine counterparts and by comparing the reactivity of several small molecules, we develop a general framework for dissociative adsorption. From this a particularly attractive consequence of isoelectronic doping emerges: substrates can be doped to enhance their reactivity specifically towards either polar or non-polar adsorbates. As such, these substrates are potentially viable candidates for selective catalysts and membranes, with the implication that a range of tuneable materials can be designed.
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  • 65
    Publication Date: 2016-04-23
    Description: The enhancement of the electrocaloric effect (ECE) was achieved by the amplifying effect of applied electric field through devising the thin films' structure. The PbZr 0.95 Ti 0.05 O 3 /PbZr 0.52 Ti 0.48 O 3 bilayer structured thin films were fabricated on a Pt(111)/Ti/SiO 2 /Si substrate using sol-gel method. The Δ S  = 20.5 J K −1  kg −1 and Δ T  = 24.8 K for bilayer thin films is achieved around 125 °C much below T c , which is caused by the amplifying electric field induced phase transition of O AFE /R FE in PZr 0.95 Ti 0.05 O 3 layer. It is also worth mentioning that the films exhibit outstanding ECE at room temperature; Δ S  = 11.9 J K −1  kg −1 and Δ T  = 10.7 K are observed, which are attributed to the effect of amplifying electric field in PbZr 0.95 Ti 0.05 O 3 layer and R FE /T FE at morphotropic phase boundaries (MPB) in PbZr 0.52 Ti 0.48 O 3 layer. This result indicates that to amplify the local electric field engineering and to maximize the number of coexisting phases in heterostructures or multilayer thin films may be an effective way for cooling applications.
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  • 66
    Publication Date: 2016-04-23
    Description: The current-perpendicular-to-plane magnetoresistivity (CPP-MR) / ρ c ( B ) is investigated in single crystal ruthenates Ca 3 (Ru 1− x Ti x ) 2 O 7 ( x  = 0.02). This material is naturally composed of ferromagnetic metallic bilayers (Ru,Ti)O 2 separated by nonmagnetic insulating layers of Ca 2 O 2 , resulting in tunneling magnetoresistivity. Non-monotonic ρ c ( B ) curves as well as the inverse spin valve effect are observed around the magnetic phase transition associating with the metal-to-insulator transition. A spin dependent tunneling model with alternate distribution of hard and soft magnetic layers [(Ru,Ti)O 2 ] is proposed to explain the exotic CPP-MR behavior. This eccentric CPP-MR behavior highlights the strong spin-charge coupling in double-layered ruthenates and provides a potential material for spintronic devices.
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  • 67
    Publication Date: 2016-04-23
    Description: We report the fabrication and device characteristics of exfoliated, few-layer, ReSe 2 field effect transistors (FET) and a method to improve contact resistance by up to three orders of magnitude using ultra-high-vacuum annealing (UHV). Many devices were studied in the absence of light and we found an average contact of 750 Ω · cm after UHV treatment. The median FET metrics were similar to other transition metal dichalcogenides: field effect mobility ∼6.7 cm 2 /V · s, subthreshold swing ∼1.2 V/decade, and I on /I off  ∼ 10 5 . In devices with low R c current saturation was observed and is attributed to injection limited transport.
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  • 68
    Publication Date: 2016-04-23
    Description: We investigate the photovoltaic performance of pseudomorphic In 0.1 Ga 0.9 N/GaN multiple-quantum well (MQW) solar cells as a function of the total active region thickness. An increase in the number of wells from 5 to 40 improves the short-circuit current and the open-circuit voltage, resulting in a 10-fold enhancement of the overall conversion efficiency. Further increasing the number of wells leads to carrier collection losses due to an incomplete depletion of the active region. Capacitance-voltage measurements point to a hole diffusion length of 48 nm in the MQW region.
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  • 69
    Publication Date: 2016-04-23
    Description: This paper presents a characterization method for extracting the reflection coefficient of materials and the real part of their permittivity. The characterization is performed in a real environment, as opposed to the classical measurement methods that require an anechoic chamber. In order to reduce the effects of the multipath propagation, a free space bistatic measurement was performed at different distances material-antennas in far field. By using a Teflon sample and a commercial absorbing material sample, measurements have been performed in order to validate the characterization technique.
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  • 70
    Publication Date: 2016-04-23
    Description: Interacting nano- and micro-particles provide opportunities to create a wide range of useful colloidal and soft matter constructs. In this letter, we examine interacting superparamagnetic polymeric particles residing on designed permalloy (Ni 0.8 Fe 0.2 ) shapes that are subject to weak time-orbiting magnetic fields. The precessing field and magnetic barriers that ensue along the outer perimeter of the shapes allow for containment concurrent with independent field-tunable ordering of the dipole-coupled particles. These remotely activated arrays with inter-particle spacing comparable to the wavelength of light yield microscopic on-chip surface gratings for beam steering and magnetically regulated light diffraction applications.
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  • 71
    Publication Date: 2016-04-22
    Description: This paper describes an experimental study that used a fast-response temperature-sensitive paint (TSP) to investigate the unsteady aerothermodynamic phenomena occurring on a shock-tube wall. To understand these phenomena in detail, a fast-response TSP with high temperature sensitivity developed for transient temperature measurement was applied to the wall. The shock-tube experiment was carried out under the over-tailored condition, with a pressure ratio of 110 for test gases of air in driver/driven tubes. The following aspects were clarified using the TSP: (a) the TSP could be used to visualize the unsteady aerothermodynamic phenomena and estimate the quantitative heat flux on the shock-tube wall; (b) an x - t diagram based on the TSP response showed shock-tube wall characteristics that included the incident/reflected shocks, laminar-to-turbulent boundary-layer transition, streaks in the turbulent boundary layer, reflected shock/turbulent boundary layer interaction, and waves reflected from a contact surface; (c) the TSP graphically showed that a transition front from the plate’s leading edge and turbulent spots moved with 80% of the free-stream velocity behind the incident shock. In addition, the TSP could track the growth of the turbulent spots on the wall.
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  • 72
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    American Institute of Physics (AIP)
    Publication Date: 2016-04-22
    Description: In this paper, some properties of relativistically degenerate quantum plasmas, such as static ion screening, structure factor, and Thomson scattering cross-section, are studied in the framework of linearized quantum hydrodynamic theory with the newly proposed kinetic γ -correction to Bohm term in low frequency limit. It is found that the correction has a significant effect on the properties of quantum plasmas in all density regimes, ranging from solid-density up to that of white dwarf stars. It is also found that Shukla-Eliasson attractive force exists up to a few times the density of metals, and the ionic correlations are seemingly apparent in the radial distribution function signature. Simplified statically screened attractive and repulsive potentials are presented for zero-temperature Fermi-Dirac plasmas, valid for a wide range of quantum plasma number-density and atomic number values. Moreover, it is observed that crystallization of white dwarfs beyond a critical core number-density persists with this new kinetic correction, but it is shifted to a much higher number-density value of n 0  ≃ 1.94 × 10 37  cm −3 (1.77 × 10 10  gr cm −3 ), which is nearly four orders of magnitude less than the nuclear density. It is found that the maximal Thomson scattering with the γ -corrected structure factor is a remarkable property of white dwarf stars. However, with the new γ -correction, the maximal scattering shifts to the spectrum region between hard X-ray and low-energy gamma-rays. White dwarfs composed of higher atomic-number ions are observed to maximally Thomson-scatter at slightly higher wavelengths, i.e., they maximally scatter slightly low-energy photons in the presence of correction.
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  • 73
    facet.materialart.
    facet.materialart.
    American Institute of Physics (AIP)
    Publication Date: 2016-04-22
    Description: An electrically isolated particle on a substrate surface will be electrically charged when a plasma is applied above it. We show that the magnitude of this charge is determined by a balance of the impeding ion and electron fluxes that are strongly influenced by the nearby substrate. By simulating this process, we find that the charge density of the particle can be much higher than that of the substrate. This is due to the height of the particle, which causes additional electron collection.
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  • 74
    Publication Date: 2016-04-22
    Description: Neoclassical and turbulent transport in tokamaks has been studied extensively over the past decades, but their possible interaction remains largely an open question. The two are only truly independent if the length scales governing each of them are sufficiently separate, i.e., if the ratio ρ * between ion gyroradius and the pressure gradient scale length is small. This is not the case in particularly interesting regions such as transport barriers. Global simulations of a collisional ion-temperature-gradient-driven microturbulence performed with the nonlinear global gyrokinetic code G ene are presented. In particular, comparisons are made between systems with and without neoclassical effects. In fixed-gradient simulations, the modified radial electric field is shown to alter the zonal flow pattern such that a significant increase in turbulent transport is observed for ρ * ≳ 1 / 300 . Furthermore, the dependency of the flux on the collisionality changes. In simulations with fixed power input, we find that the presence of neoclassical effects decreases the frequency and amplitude of intermittent turbulent transport bursts (avalanches) and thus plays an important role for the self-organisation behaviour.
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  • 75
    Publication Date: 2016-04-22
    Description: An experimental investigation of the laser produced plasma induced shock wave in the presence of confining walls placed along the axial as well as the lateral direction has been performed. A time resolved Mach Zehnder interferometer is set up to track the primary as well as the reflected shock waves and its effect on the evolving plasma plume has been studied. An attempt has been made to discriminate the electronic and medium density contributions towards the changes in the refractive index of the medium. Two dimensional spatial distributions for both ambient medium density and plasma density (electron density) have been obtained by employing customised inversion technique and algorithm on the recorded interferograms. The observed density pattern of the surrounding medium in the presence of confining walls is correlated with the reflected shock wave propagation in the medium. Further, the shock wave plasma interaction and the subsequent changes in the shape and density of the plasma plume in confined geometry are briefly described.
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  • 76
    Publication Date: 2016-04-22
    Description: The collisionless trapped electron modes in the short wavelength region k ⊥ ρ s 〉 1 (SWTEMs) are studied with the gyrokinetic integral eigenmode equation in tokamak plasmas. Here, we present a systematic study of the correlation between the SWTEMs and short wavelength ion temperature gradient (SWITG) modes. The k θ ρ s spectra of TEM have double humps in the short wavelength and long wavelength regions, respectively. The SWITG modes with trapped electron effects taking into account have broader k θ ρ s spectra. Dependences of growth rate and real frequency of SWTEMs on the various parameters, such as ion temperature gradient (η i ), the temperature gradient of trapped electrons (η e ), toroidicity (ε n ), magnetic shear ( s ̂ ), safety factor (q), and the ratio of temperature (T e /T i ), are investigated in detail. It is found that the SWTEMs propagate in the electron diamagnetic drift direction and require temperature gradient of trapped electrons η e exceeding thresholds. Moreover, the ion temperature gradient has a strong stabilizing effect on the SWTEMs. The SWTEMs become stable in both regimes of toroidicity ε n  〉 0.1 and magnetic shear s ̂ 〉 0.5 regardless of the fraction of trapped electrons. In addition, the properties of short wavelength ITG (SWITG) modes are discussed with different ratio of trapped electrons. It is found that trapped electrons of greater fraction have a stronger destabilizing effect on the SWTEM and SWITG modes. These results are significant for the electrons anomalous transport experiments in the future.
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  • 77
    Publication Date: 2016-04-23
    Description: High-frequency surface acoustic waves (SAWs) in the gigahertz range can be generated using absorption from an ultrafast laser in a patterned metallic grating on a substrate. Reducing the attenuation at these frequencies can yield better sensors as well as enable them to better probe phonon and electron-phonon interactions near surfaces. It is not clear from existing experiments which mechanisms dominate damping at high frequencies. We calculate damping times of SAWs due to various mechanisms in the 1–100 GHz range to find that mechanical loading of the grating on the substrate dominates dissipation by radiating energy from the surface into the bulk. To overcome this and enable future measurements to probe intrinsic damping, we propose incorporating distributed acoustic Bragg reflectors in the experimental structure. Layers of alternating materials with contrasting acoustic impedances embedded a wavelength away from the surface serve to reflect energy back to the surface. Using numerical simulations, we show that a single Bragg reflector is sufficient to increase the energy density at the surface by more than five times. We quantify the resulting damping time to find that it is longer than the intrinsic damping time. The proposed structure can enable future measurements of intrinsic damping in SAWs at ∼100 GHz.
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  • 78
    Publication Date: 2016-04-23
    Description: We determine the size of argon clusters generated with a planar nozzle, based on the optical measurements in conjunction with theoretical modelling. Using a quasi-one dimensional model for the moments of the cluster size distribution, we determine the influence of critical physical assumptions. These refer to the surface tension depending on the presence of thermal equilibrium, the mass density of clusters, and different methods to model the growth rate of the cluster radius. We show that, despite strong variation in the predicted cluster size, 〈 N 〉 , the liquid mass ratio, g , can be determined with high trustworthiness, because g is predicted as being almost independent of the specific model assumptions. Exploiting this observation, we use the calculated value for g to retrieve the cluster size from optical measurements, i.e., calibrated Rayleigh scattering and interferometry. Based on the measurements of the cluster size vs . the nozzle stagnation pressure, we provide a new power law for the prediction of the cluster size in experiments with higher values of the Hagena parameter ( Γ * 〉 10 4 ) . This range is of relevance for experiments on high-intensity laser matter interactions.
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  • 79