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  • American Institute of Physics (AIP)  (241,953)
  • Periodicals Archive Online (PAO)  (190,459)
  • Blackwell Publishing Ltd  (182,011)
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  • 1
    Publication Date: 2016-01-01
    Description: We demonstrate that the magnetic state of a superconducting spin valve, that is normally controlled with an external magnetic field, can also be manipulated by varying the temperature which increases the functionality and flexibility of such structures as switching elements. In this case, switching is driven by changes in the magnetostatic energy due to spontaneous Meissner screening currents forming in the superconductor below the critical temperature. Our scanning Hall probe measurements also reveal vortex-mediated pinning of the ferromagnetic domain structure due to the pinning of quantized stray fields in the adjacent superconductor. The ability to use temperature as well as magnetic field to control the local magnetisation structure raises the prospect of potential applications in magnetic memory devices.
    Print ISSN: 0003-6951
    Electronic ISSN: 1077-3118
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  • 2
    Publication Date: 2016-01-01
    Description: Synaptic plasticity functions play a crucial role in the transmission of neural signals in the brain. Short-term plasticity is required for the transmission, encoding, and filtering of the neural signal, whereas long-term plasticity establishes more permanent changes in neural microcircuitry and thus underlies memory and learning. The realization of bioinspired circuits that can actually mimic signal processing in the brain demands the reproduction of both short- and long-term aspects of synaptic plasticity in a single device. Here, we demonstrate the implementation of neuromorphic functions similar to biological memory, such as short- to long-term memory transition, in non-volatile organic electrochemical transistors (OECTs). Depending on the training of the OECT, the device displays either short- or long-term plasticity, therefore, exhibiting non von Neumann characteristics with merged processing and storing functionalities. These results are a first step towards the implementation of organic-based neuromorphic circuits.
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  • 3
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    American Institute of Physics (AIP)
    In: Chaos
    Publication Date: 2016-01-01
    Description: In the mid 1980s, the development of multifractal concepts and techniques was an important breakthrough for complex system analysis and simulation, in particular, in turbulence and hydrology. Multifractals indeed aimed to track and simulate the scaling singularities of the underlying equations instead of relying on numerical, scale truncated simulations or on simplified conceptual models. However, this development has been rather limited to deal with scalar fields, whereas most of the fields of interest are vector-valued or even manifold-valued. We show in this paper that the combination of stable Lévy processes with Clifford algebra is a good candidate to bridge up the present gap between theory and applications. We show that it indeed defines a convenient framework to generate multifractal vector fields, possibly multifractal manifold-valued fields, based on a few fundamental and complementary properties of Lévy processes and Clifford algebra. In particular, the vector structure of these algebra is much more tractable than the manifold structure of symmetry groups while the Lévy stability grants a given statistical universality.
    Print ISSN: 1054-1500
    Electronic ISSN: 1089-7682
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  • 4
    Publication Date: 2016-01-01
    Description: A detailed experimental investigation of the effects giving rise to the magnetic energy level structure in the vicinity of the level crossing (LC) at low temperature is reported for the open antiferromagnetic molecular ring Cr 8 Zn. The study is conducted by means of thermodynamic techniques (torque magnetometry, magnetization and specific heat measurements) and microscopic techniques (nuclear magnetic resonance line width, nuclear spin lattice, and spin-spin relaxation measurements). The experimental results are shown to be in excellent agreement with theoretical calculations based on a minimal spin model Hamiltonian, which includes a Dzyaloshinskii-Moriya interaction. The first ground state level crossing at μ 0 H c1 = 2.15 T is found to be an almost true LC while the second LC at μ 0 H c2 = 6.95 T has an anti-crossing gap of Δ 12 = 0.19 K. In addition, both NMR and specific heat measurements show the presence of a level anti-crossing between excited states at μ 0 H = 4.5 T as predicted by the theory. In all cases, the fit of the experimental data is improved by introducing a distribution of the isotropic exchange couplings ( J ), i.e., using a J strain model. The peaks at the first and second LCs in the nuclear spin-lattice relaxation rate are dominated by inelastic scattering and a value of Γ ∼ 10 10 rad/s is inferred for the life time broadening of the excited state of the open ring, due to spin phonon interaction. A loss of NMR signal (wipe-out effect) is observed for the first time at LC and is explained by the enhancement of the spin-spin relaxation rate due to the inelastic scattering.
    Print ISSN: 0021-9606
    Electronic ISSN: 1089-7690
    Topics: Chemistry and Pharmacology , Physics
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  • 5
    Publication Date: 2016-01-01
    Description: Atmospheric aerosols play a vital role in affecting climate by influencing the properties and lifetimes of clouds and precipitation. Understanding the underlying microscopic mechanisms involved in the nucleation of aerosol droplets from the vapour phase is therefore of great interest. One key thermodynamic quantity in nucleation is the excess free energy of cluster formation relative to that of the saturated vapour. In our current study, the excess free energy is extracted for clusters of pure water modelled with the TIP4P/2005 intermolecular potential using a method based on nonequilibrium molecular dynamics and the Jarzynski relation. The change in free energy associated with the “mitosis” or division of a cluster of N water molecules into two N /2 sub-clusters is evaluated. This methodology is an extension of the disassembly procedure used recently to calculate the excess free energy of argon clusters [H. Y. Tang and I. J. Ford, Phys. Rev. E 91 , 023308 (2015)]. Our findings are compared to the corresponding excess free energies obtained from classical nucleation theory (CNT) as well as internally consistent classical theory (ICCT). The values of the excess free energy that we obtain with the mitosis method are consistent with CNT for large cluster sizes but for the smallest clusters, the results tend towards ICCT; for intermediate sized clusters, we obtain values between the ICCT and CNT predictions. Furthermore, the curvature-dependent surface tension which can be obtained by regarding the clusters as spherical droplets of bulk density is found to be a monotonically increasing function of cluster size for the studied range. The data are compared to other values reported in the literature, agreeing qualitatively with some but disagreeing with the values determined by Joswiak et al. [J. Phys. Chem. Lett. 4 , 4267 (2013)] using a biased mitosis approach; an assessment of the differences is the main motivation for our current study.
    Print ISSN: 0021-9606
    Electronic ISSN: 1089-7690
    Topics: Chemistry and Pharmacology , Physics
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  • 6
    Publication Date: 2016-01-01
    Description: The structural and optical properties of lattice-matched InAs 0.911 Sb 0.089 bulk layers and strain-balanced InAs/InAs 1 − x Sb x ( x  ∼ 0.1–0.4) superlattices grown on (100)-oriented GaSb substrates by molecular beam epitaxy are examined using X-ray diffraction, spectroscopic ellipsometry, and temperature dependent photoluminescence spectroscopy. The photoluminescence and ellipsometry measurements determine the ground state bandgap energy and the X-ray diffraction measurements determine the layer thickness and mole fraction of the structures studied. Detailed modeling of the X-ray diffraction data is employed to quantify unintentional incorporation of approximately 1% Sb into the InAs layers of the superlattices. A Kronig-Penney model of the superlattice miniband structure is used to analyze the valence band offset between InAs and InAsSb, and hence the InAsSb band edge positions at each mole fraction. The resulting composition dependence of the bandgap energy and band edge positions of InAsSb are described using the bandgap bowing model; the respective low and room temperature bowing parameters for bulk InAsSb are 938 and 750 meV for the bandgap, 558 and 383 meV for the conduction band, and −380 and −367 meV for the valence band.
    Print ISSN: 0021-8979
    Electronic ISSN: 1089-7550
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  • 7
    Publication Date: 2016-01-01
    Description: The construction and operating principle of a pendulum magnetometer for measuring the magnetic susceptibility of solids under high gas pressures are described. This device is distinctive in having the pendulum mounted directly in the high pressure chamber. Experimental plots of the susceptibility of the compound V 4 S 9 Br 4 as a function of pressure up to 2 kbar at temperatures of 60–300 K are presented as an example of the use of the magnetometer.
    Print ISSN: 1063-777X
    Electronic ISSN: 1090-6517
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  • 8
    Publication Date: 2016-01-01
    Description: The crystal structure and magnetic properties of the stoichiometric compounds La 1−2 x Sr 2 x Mn 1− x Sb x O 3 (0.1 ≤ x ≤ 0.2) with manganese ions in the 3+ oxidation state are studied. These compounds exhibit rhombohedral structure at room temperatures, which transforms upon cooling into an orthorhombic structure without any signature of orbital ordering. The compounds with x ≤ 0.2 reveal a ferromagnetic ordering; their magnetization decreases with increasing the concentration of substitution ions. It is suggested that the observed ferromagnetism originates from the breakdown of orbital ordering and significant hybridization between the e g orbitals of Mn 3+ and 2 p orbitals of oxygen.
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  • 9
    Publication Date: 2016-01-01
    Description: Exchange bias phenomenon, evident of antiferromagnetic–ferromagnetic phase segregation state, has been observed in (Nd 1− x Y x ) 2/3 Ca 1/3 MnO 3 ( x = 0, 0.1) compounds at low temperatures. A contribution to the total magnetization of the compounds due to the ferromagnetic phase has been evaluated. It has been found that yttrium doping leads to the growth of the ferromagnetic phase fraction. The ferromagnetic phase in the doped compound has a lower coercivity H c and more rectangular form of the hysteresis loop. The values of the exchange bias field H EB and coercivity are found to be strongly dependent on the cooling magnetic field H cool . In sufficiently high magnetic fields, H cool 〉 5 kOe, H EB in the doped compound is about twice as low as in the parent compound. This difference is attributed to a lower exchange interaction and higher saturation magnetization of the ferromagnetic phase in (Nd 0.9 Y 0.1 ) 2/3 Ca 1/3 MnO 3 .
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  • 10
    Publication Date: 2016-01-01
    Description: We used atomic-layer molecular beam epitaxy to synthesize bilayers of a cuprate metal (La 1.55 Sr 0.45 CuO 4 ) and a cuprate insulator (La 2 CuO 4 ), in which each layer is just one unit cells thick. We have studied the magnetic field and temperature dependence of the complex sheet conductance, σ ( ω ) , of these films. Experiments have been carried out at frequencies between 2 and 50 MHz using the single-spiral coil technique. We found that: (i) the inductive response starts at Δ T = 3 K lower temperatures than Re σ ( T ), which in turn is characterized by a peak close to the transition, (ii) this shift is almost constant with magnetic field up to 14 mT; (iii) Δ T increases sharply up to 4 K at larger fields and becomes constant up to 8 T; (iv) the vortex diffusion constant D ( T ) is not linear with T at low temperatures as in the case of free vortices, but is rather exponential due to pinning of vortex cores, and (v) the dynamic Berezinski–Kosterlitz–Thouless (BKT) transition temperature occurs at the point where Y = ( l ω / ξ + ) 2 = 1 . Our experimental results can be described well by the extended dynamic theory of the BKT transition and dynamics of bound vortex–antivortex pairs with short separation lengths.
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  • 11
    Publication Date: 2016-01-01
    Description: We consider the field dependence of magnetization in the paramagnetic phase of manganese monosilicde, MnSi, which is characterized by an anomalously large effective magnetic moment μ* = 5.3μ B , and a small saturation magnetization M 0 = 0.3μ B /Mn. It follows from the conducted analysis, that neither the theory of band magnetism, nor the cluster approach can explain the experimental data, but an adequate description is possible within the framework of the spin-polaron model, in which the spin-polaron is a quasi-bonded state of the band electron and localized magnetic moment of Mn. It is found that the inclusion of specific interaction for a simple ferrimagnet-like configuration that characterizes the spin-polaron, allows us to explain the experimental data, wherein the observed values of μ* are not associated with large localized magnetic moments in the sample volume. The possible methods of experimental verification of the spin-polaron model are analyzed, including experimentum crucis . The obtained results show that the paramagnetic phase of MnSi should be regarded as a phase in which its physical properties are determined by the magnetic inhomogeneities on a nanometer spatial scale.
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  • 12
    Publication Date: 2016-01-01
    Description: The intermetallic compound YbInCu 4 undergoes an isostructural first-order valence-changing phase transition from a state with localized moments of the Yb into a Fermi liquid state with strong Kondo screening. This article reviews studies of the structure and of the electronic, magnetic, and magnetoelastic properties of compounds based on YbInCu 4 . The effects of alloying and pressure on such parameters of the electronic and magnetic state as the Kondo temperature, Curie paramagnetic temperature, effective magnetic moment, and valence of Yb are determined. The observational data are interpreted in terms of a single-impurity Kondo model. The magnetic anisotropy of single-crystal YbInCu 4 and magnetostriction in high magnetic fields are studied. It is shown that a crystal field model for localized states of Yb 3+ provides a good description of magnetic anisotropy and anisotropic magnetostriction in the high-temperature phase.
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  • 13
    Publication Date: 2016-01-01
    Description: A study of the magnetocaloric effect for Tm 2 Fe 16 , Tm 2 Fe 17 , Tm 2 Fe 18 , Tm 2 Fe 19 , Tm 2 Fe 17− x Mn x ( x ≤ 1.5), Ce 2 Fe 17− x Mn x H y ( x 〈 2, y ≤ 3), Lu 2 Fe 17− x Mn x ( x ≤ 2) systems at magnetic ordering temperatures T C , T N and a ferro-antiferromagnet phase transition Θ T , from magnetization measurements. In alloys with ground ferromagnetic and high-temperature antiferromagnetic states, the change in the magnetic entropy peak −Δ S M increases as the magnetic phase transition temperatures Θ T and T N converge with varying compound composition. Extrapolating the difference T N − Θ T to zero for the initial and doped alloys allows us to determine the maximum possible value of −Δ S M in such systems and the composition of the corresponding alloy. A superposition of the −Δ S M (T) maxima at Θ T and T N for compounds with two magnetic phase transitions creates much higher values of relative cooling power in comparison to compounds with only one magnetic phase transition at T C .
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  • 14
    Publication Date: 2016-01-01
    Description: Nuclear magnetic resonance (NMR) experiments are reported at up to 30.5 GPa of pressure using radiofrequency (RF) micro-coils with anvil cell designs. These are the highest pressures ever reported with NMR, and are made possible through an improved gasket design based on nano-crystalline powders embedded in epoxy resin. Cubic boron-nitride (c-BN), corundum ( α -Al 2 O 3 ), or diamond based composites have been tested, also in NMR experiments. These composite gaskets lose about 1/2 of their initial height up to 30.5 GPa, allowing for larger sample quantities and preventing damages to the RF micro-coils compared to precipitation hardened CuBe gaskets. It is shown that NMR shift and resolution are less affected by the composite gaskets as compared to the more magnetic CuBe. The sensitivity can be as high as at normal pressure. The new, inexpensive, and simple to engineer gaskets are thus superior for NMR experiments at high pressures.
    Print ISSN: 0034-6748
    Electronic ISSN: 1089-7623
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
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  • 15
    Publication Date: 2016-01-01
    Description: The geometry of a single aperture in the extraction grid plays a relevant role for the optimization of negative ion transport and extraction probability in a hybrid negative ion source. For this reason, a three-dimensional particle-in-cell/Monte Carlo collision model of the extraction region around the single aperture including part of the source and part of the acceleration (up to the extraction grid (EG) middle) regions has been developed for the new aperture design prepared for negative ion optimization 1 source. Results have shown that the dimension of the flat and chamfered parts and the slope of the latter in front of the source region maximize the product of production rate and extraction probability (allowing the best EG field penetration) of surface-produced negative ions. The negative ion density in the plane yz has been reported.
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  • 16
    Publication Date: 2016-01-01
    Description: We present a high-flux, broadband gamma-ray spectrometry capable of characterizing the betatron radiation spectrum over the photon energy range from 10 keV to 20 MeV with respect to the peak photon energy, spectral bandwidth, and unique discrimination from background radiations, using a differential filtering spectrometer and the unfolding procedure based on the Monte Carlo code GEANT4. These properties are experimentally verified by measuring betatron radiation from a cm-scale laser wakefield accelerator (LWFA) driven by a 1-PW laser, using a differential filtering spectrometer consisting of a 15-filter and image plate stack. The gamma-ray spectra were derived by unfolding the photostimulated luminescence (PSL) values recorded on the image plates, using the spectrometer response matrix modeled with the Monte Carlo code GEANT4. The accuracy of unfolded betatron radiation spectra was assessed by unfolding the test PSL data simulated with GEANT4, showing an ambiguity of less than 20% and clear discrimination from the background radiation with less than 10%. The spectral analysis of betatron radiation from laser wakefield-accelerated electron beams with energies up to 3 GeV revealed radiation spectra characterized by synchrotron radiation with the critical photon energy up to 7 MeV. The gamma-ray spectrometer and unfolding method presented here facilitate an in-depth understanding of betatron radiation from LWFA process and a novel radiation source of high-quality photon beams in the MeV regime.
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  • 17
    Publication Date: 2016-01-05
    Description: For use in high-magnetic-field coil-based applications, the critical current density ( J c ) of REBa 2 Cu 3 O y (REBCO, where RE = rare earth) coated conductors must be isotropically improved, with respect to the direction of the magnetic field; these improvements must be realized at the operating conditions of these applications. In this study, improvement of the J c for various applied directions of magnetic field was achieved by controlling the morphology of the BaHfO 3 (BHO) nano-rods in a SmBCO film. We fabricated the 3.0 vol. % BHO-doped SmBCO film at a low growth temperature of 720 °C, by using a seed layer technique ( T s = 720 °C film). The low-temperature growth resulted in a morphological change in the BHO nano-rods. In fact, a high number density of (3.1 ± 0.1) × 10 3 μ m −2 of small (diameter: 4 ± 1 nm), discontinuous nano-rods that grew in various directions, was obtained. In J c measurements, the J c of the T s = 720 °C film in all directions of the applied magnetic field was higher than that of the non-doped SmBCO film. The J c min (6.4 MA/cm 2 ) of the former was more than 6 times higher than that (1.0 MA/cm 2 ) of the latter at 40 K, under 3 T. The aforementioned results indicated that the discontinuous BHO nano-rods, which occurred with a high number density, exerted a 3D-like flux pinning at the measurement conditions considered. Moreover, at 4.2 K and under 17 T, a flux pinning force density of 1.6 TN/m 3 was realized; this value was comparable to the highest value recorded, to date.
    Electronic ISSN: 2166-532X
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
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  • 18
    Publication Date: 2016-01-05
    Description: Transport characteristics of TiN/Ta/TaO x /TiN resistive-switching crossbar devices with amorphous TaO x functional layer have been investigated at cryogenic temperatures. Quasi-DC I - V characteristics at 10 K show a negative differential resistance region followed by a rapid transition to the non-volatile formed state. Accounting for Joule heating, the device temperature at the point of switching was estimated at 150 K. Measurements of transient resistance at low stage temperatures revealed an abrupt drop of resistance delayed by a characteristic incubation time after the leading edge of the voltage pulse. The incubation time was a strong function of applied voltage but did not depend on temperature between 10 K and 100 K. This implies a very low activation energy of the threshold switching process at low temperatures. Both of these observations argue against the involvement of oxygen vacancy motion at the onset of the forming process.
    Electronic ISSN: 2166-532X
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
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  • 19
    Publication Date: 2016-01-05
    Description: We report an in situ microbeam grazing incidence X-ray scattering study of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C 8 -BTBT) organic semiconductor thin film deposition by hollow pen writing. Multiple transient phases are observed during the crystallization for substrate temperatures up to ≈93 °C. The layered smectic liquid-crystalline phase of C 8 -BTBT initially forms and preceedes inter-layer ordering, followed by a transient crystalline phase for temperature 〉60 °C, and ultimately the stable phase. Based on these results, we demonstrate a method to produce extremely large grain size and high carrier mobility during high-speed processing. For high writing speed (25 mm/s), mobility up to 3.0 cm 2 /V-s has been observed.
    Electronic ISSN: 2166-532X
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
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  • 20
    Publication Date: 2016-01-05
    Description: The damping of spin waves parametrically excited in the magnetic insulator Yttrium Iron Garnet (YIG) is controlled by a dc current passed through an adjacent normal-metal film. The experiment is performed on a macroscopically sized YIG(100 nm)/Pt(10 nm) bilayer of 4 × 2 mm 2 lateral dimensions. The spin-wave relaxation frequency is determined via the threshold of the parametric instability measured by Brillouin light scattering spectroscopy. The application of a dc current to the Pt film leads to the formation of a spin-polarized electron current normal to the film plane due to the spin Hall effect. This spin current exerts a spin transfer torque in the YIG film and, thus, changes the spin-wave damping. Depending on the polarity of the applied dc current with respect to the magnetization direction, the damping can be increased or decreased. The magnitude of its variation is proportional to the applied current. A variation in the relaxation frequency of ± 7.5 % is achieved for an applied dc current density of 5 × 10 10  A/m 2 .
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  • 21
    Publication Date: 2016-01-05
    Description: Amorphous TbFeCo thin films sputter deposited at room temperature on thermally oxidized Si substrate are found to exhibit strong perpendicular magnetic anisotropy. Atom probe tomography, scanning transmission electron microscopy, and energy dispersive X-ray spectroscopy mapping have revealed two nanoscale amorphous phases with different Tb atomic percentages distributed within the amorphous film. Exchange bias accompanied by bistable magneto-resistance states has been uncovered near room temperature by magnetization and magneto-transport measurements. The exchange anisotropy originates from the exchange interaction between the ferrimagnetic and ferromagnetic components corresponding to the two amorphous phases. This study provides a platform for exchange bias and magneto-resistance switching using single-layer amorphous ferrimagnetic thin films that require no epitaxial growth.
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  • 22
    Publication Date: 2016-01-05
    Description: Both high speed and efficiency of photoelectric conversion are essential for photodetectors. As an emerging layered metal dichalcogenide (LMD), tin disulfide owns intrinsic faster photodetection ability than most other LMDs but poor light absorption and low photoelectric conversion efficiency. We develop an efficient method to enhance its performance by constructing a SnS 2 -copper indium sulfide hybrid structure. As a result, the responsivity reaches 630 A/W, six times stronger than pristine SnS 2 and much higher than most other LMDs photodetectors. Additionally, the photocurrents are enhanced by more than 1 order of magnitude. Our work may open up a pathway to improve the performance of photodetectors based on LMDs.
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  • 23
    Publication Date: 2016-01-05
    Description: We report electric-field-induced switching with write energies down to 6 fJ/bit for switching times of 0.5 ns, in nanoscale perpendicular magnetic tunnel junctions (MTJs) with high resistance-area product and diameters down to 50 nm. The ultra-low switching energy is made possible by a thick MgO barrier that ensures negligible spin-transfer torque contributions, along with a reduction of the Ohmic dissipation. We find that the switching voltage and time are insensitive to the junction diameter for high-resistance MTJs, a result accounted for by a macrospin model of purely voltage-induced switching. The measured performance enables integration with same-size CMOS transistors in compact memory and logic integrated circuits.
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  • 24
    Publication Date: 2016-01-05
    Description: Laser-induced magnetization precessional dynamics was investigated in epitaxial films of Mn 3 Ge, which is a tetragonal Heusler-like nearly compensated ferrimagnet. The ferromagnetic resonance (FMR) mode was observed, the precession frequency for which exceeded 0.5 THz and originated from the large magnetic anisotropy field of approximately 200 kOe for this ferrimagnet. The effective damping constant was approximately 0.03. The corresponding effective Landau-Lifshitz constant is approximately 60 Mrad/s and is comparable with those of the similar Mn-Ga materials. The physical mechanisms for the Gilbert damping and for the laser-induced excitation of the FMR mode were also discussed in terms of the spin-orbit-induced damping and the laser-induced ultrafast modulation of the magnetic anisotropy, respectively.
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  • 25
    Publication Date: 2016-01-05
    Description: Materials that possess low density, low thermal conductivity, and high stiffness are desirable for engineering applications, but most materials cannot realize these properties simultaneously due to the coupling between them. Nanotrusses, which consist of hollow nanoscale beams architected into a periodic truss structure, can potentially break these couplings due to their lattice architecture and nanoscale features. In this work, we study heat conduction in the exact nanotruss geometry by solving the frequency-dependent Boltzmann transport equation using a variance-reduced Monte Carlo algorithm. We show that their thermal conductivity can be described with only two parameters, solid fraction and wall thickness. Our simulations predict that nanotrusses can realize unique combinations of mechanical and thermal properties that are challenging to achieve in typical materials.
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  • 26
    Publication Date: 2016-01-05
    Description: In this study we fabricated lateral superlattices (LSLs) based on the selectively doped GaAs/AlAs heterostructures with a high-mobility two-dimensional (2D) electron gas. The LSLs were formed using the electron-beam lithography and lift-off techniques, which produced a set of metallic strips on top of a heterojunction. The amplitude of the 2D electron gas modulation in the LSL was controlled by the gate voltage applied to the metallic strips. The LSLs with two different periods ( a  = 200 nm and 500 nm) were used to investigate the influence of microwave radiation with the frequency of 110–150 GHz on the 2D electron transport at the temperature T  = 1.6 K in the magnetic field B  
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  • 27
    Publication Date: 2016-01-05
    Description: The averaged toroidal flow of energetic minority ions during ICRF (ion cyclotron range of frequencies) heating is investigated in the Alcator C-Mod plasma by applying the GNET code, which can solve the drift kinetic equation with complicated orbits of accelerated energetic particles. It is found that a co-directional toroidal flow of the minority ions is generated in the region outside of the resonance location, and that the toroidal velocity reaches more than 40% of the central ion thermal velocity ( V tor ∼ 300 km/s with P ICRF ∼ 2 MW). When we shift the resonance location to the outside of | r / a | ∼ 0.5 , the toroidal flow immediately inside of the resonance location is reduced to 0 or changes to the opposite direction, and the toroidal velocity shear is enhanced at r / a ∼ 0.5. A radial diffusion equation for toroidal flow is solved by assuming a torque profile for the minority ion mean flow, and good agreements with experimental radial toroidal flow profiles are obtained. This suggests that the ICRF driven minority ion flow is related to the experimentally observed toroidal rotation during ICRF heating in the Alcator C-Mod plasma.
    Print ISSN: 1070-664X
    Electronic ISSN: 1089-7674
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  • 28
    Publication Date: 2016-01-05
    Description: Acoustic waves in fluids undergoing the transition from sub- to supersonic flow satisfy governing equations similar to those for light waves in the immediate vicinity of a black hole event horizon. This acoustic analogy has been used by Unruh and others as a conceptual model for “Hawking radiation.” Here, we use variational methods, originally introduced by Brizard for the study of linearized MHD, and ray phase space methods, to analyze linearized acoustics in the presence of background flows. The variational formulation endows the evolution equations with natural Hermitian and symplectic structures that prove useful for later analysis. We derive a 2 × 2 normal form governing the wave evolution in the vicinity of the “event horizon.” This shows that the acoustic model can be reduced locally (in ray phase space) to a standard (scalar) tunneling process weakly coupled to a unidirectional non-dispersive wave (the “incoming wave”). Given the normal form, the Hawking “thermal spectrum” can be derived by invoking standard tunneling theory, but only by ignoring the coupling to the incoming wave. Deriving the normal form requires a novel extension of the modular ray-based theory used previously to study tunneling and mode conversion in plasmas. We also discuss how ray phase space methods can be used to change representation, which brings the problem into a form where the wave functions are less singular than in the usual formulation, a fact that might prove useful in numerical studies.
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  • 29
    Publication Date: 2016-01-05
    Description: We present a time-implicit numerical method to solve the relativistic Vlasov–Ampere system of equations on a two dimensional phase space grid. The time-splitting algorithm we use allows the generalization of the work presented here to higher dimensions keeping the linear aspect of the resulting discrete set of equations. The implicit method is benchmarked against linear theory results for the relativistic Landau damping for which analytical expressions using the Maxwell-Jüttner distribution function are derived. We note that, independently from the shape of the distribution function, the relativistic treatment features collective behaviours that do not exist in the nonrelativistic case. The numerical study of the relativistic two-stream instability completes the set of benchmarking tests.
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  • 30
    Publication Date: 2016-01-05
    Description: We present a scheme of electron injection for enhanced electron energy gain by using a radially polarized (RP) laser pulse in vacuum under the influence of magnetic wiggler. The inherent symmetry of an RP laser pulse enforces the trapping and acceleration of electrons in the direction of propagation of laser pulse during laser electron interaction. A magnetic wiggler encircles the trajectory of accelerated electron and improves the strength of v → × B → force which supports the retaining of betatron resonance for longer duration and leads to enhance electron acceleration. Four times higher electron energy is observed with a RP laser pulse of peak intensity 8.5 × 10 20     W / cm 2 in the presence of magnetic wiggler of 10.69   kG than that in the absence of magnetic wiggler. We have also analyzed the electron injection for enhanced energy gain and observe that the electron energy gain is relatively higher with a sideway injection than that of axial injection of electron. Injection angle δ is optimized and found that at δ = 10 ° to the direction of propagation of laser pulse, maximum energy is obtained.
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  • 31
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    American Institute of Physics (AIP)
    Publication Date: 2016-01-05
    Description: The spectral energy distribution in turbulent flows is observed to follow a k −5/3 power scaling, as originally predicted by Kolmogorov’s theory. However, the underlying assumptions in Kolmogorov’s theory appear not to hold with most experimental and numerical data showing evidence of small-scale anisotropy and significant direct energy transfer between the large- and the small-scales. Here, we present a flow structure that reconciles the k −5/3 spectrum with small-scale universality, small-scale anisotropy, and direct scale interactions. The flow structure is a shear layer, which contains the small-scales of motion and is bounded by the large-scales. The anisotropic shear layer reveals the expected scaling of the energy spectrum in nearly all directions.
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  • 32
    Publication Date: 2016-01-05
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  • 33
    Publication Date: 2016-01-05
    Description: Dust particles observed in universe as well as in laboratory and technological plasma devices are still under investigation. At low temperature, these particles are strongly negatively charged and are able to form a 2D or 3D coulomb crystal. In this work, our aim was to check the ideal gas law validity for a 2D single-layer dust crystal recently reported in the literature. For this purpose, we have simulated, using the molecular dynamics method, its thermodynamic properties for different values of dust particles number and confinement parameters. The obtained results have allowed us to invalidate the ideal gas behaviour and to propose an effective equation of state which assumes a near zero dust temperature. Furthermore, the value of the calculated sound velocity was found to be in a good agreement with experimental data published elsewhere.
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  • 34
    Publication Date: 2016-01-06
    Description: We carry out a systematic qualitative analysis of the two quadratic schemes of generalized oscillators recently proposed by Quesne [J. Math. Phys. 56 , 012903 (2015)]. By performing a local analysis of the governing potentials, we demonstrate that while the first potential admits a pair of equilibrium points one of which is typically a center for both signs of the coupling strength λ , the other points to a centre for λ 〈 0 but a saddle λ 〉 0. On the other hand, the second potential reveals only a center for both the signs of λ from a linear stability analysis. We carry out our study by extending Quesne’s scheme to include the effects of a linear dissipative term. An important outcome is that we run into a remarkable transition to chaos in the presence of a periodic force term f cos ωt .
    Print ISSN: 0022-2488
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  • 35
    Publication Date: 2016-01-06
    Description: Tensor generalizations of affine vector fields called symmetric and antisymmetric affine tensor fields are discussed as symmetry of spacetimes. We review the properties of the symmetric ones, which have been studied in earlier works, and investigate the properties of the antisymmetric ones, which are the main theme in this paper. It is shown that antisymmetric affine tensor fields are closely related to one-lower-rank antisymmetric tensor fields which are parallelly transported along geodesics. It is also shown that the number of linear independent rank- p antisymmetric affine tensor fields in n -dimensions is bounded by ( n + 1)!/ p !( n − p )!. We also derive the integrability conditions for antisymmetric affine tensor fields. Using the integrability conditions, we discuss the existence of antisymmetric affine tensor fields on various spacetimes.
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  • 36
    Publication Date: 2016-01-06
    Description: The exponential of an N × N matrix can always be expressed as a matrix polynomial of order N − 1. In particular, a general group element for the fundamental representation of SU ( N ) can be expressed as a matrix polynomial of order N − 1 in a traceless N × N hermitian generating matrix, with polynomial coefficients consisting of elementary trigonometric functions dependent on N − 2 invariants in addition to the group parameter. These invariants are just angles determined by the direction of a real N -vector whose components are the eigenvalues of the hermitian matrix. Equivalently, the eigenvalues are given by projecting the vertices of an N − 1 -simplex onto a particular axis passing through the center of the simplex. The orientation of the simplex relative to this axis determines the angular invariants and hence the real eigenvalues of the matrix.
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  • 37
    Publication Date: 2016-01-06
    Description: In this paper, the energy efficiency of airlines is measured. Number of employees and tons of aviation kerosene are chosen as the inputs. Revenue tonne kilometers, revenue passenger kilometers, and total business income are the outputs. Capital stock is selected as the dynamic factor. A new model, Virtual Frontier Dynamic range adjusted measure (RAM), is proposed to calculate the energy efficiencies of 22 airlines from 2008 to 2012. In Virtual Frontier Dynamic RAM, the reference DMU (decision-making unit) set and the evaluated DMU set are two different sets to distinguish between efficient DMUs. The results demonstrate the following: (1) Air Greenland exhibits the highest energy efficiency, while the efficiency score of Air France-KLM is at the bottom of the 22 airlines. (2) Aggregate airline energy efficiency consistently increased from 2008 to 2012.
    Electronic ISSN: 1941-7012
    Topics: Energy, Environment Protection, Nuclear Power Engineering
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  • 38
    Publication Date: 2016-01-06
    Description: A new type of n-p-n transistor photovoltaic device based on CdS/multi-wall carbon nanotube (MWNT)/n-Si configuration was fabricated in a facile process. CdS quantum dots were deposited on fluorine-doped tin-oxide glass using a chemical bath deposition method, and MWNT film was coated on n-type Si substrate by airbrushing. The materials used for the n-p-n transistor solar cells were characterized by multiple techniques including X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, Raman, Ultraviolet visible (UV-vis) spectrophotometer, and I-V characteristic measurements. The CdS layer acts as a good n-type material for the transistor solar cells. The thickness of the CdS layer can be controlled by the chemical bath deposition time to achieve different photovoltaic responses. I-V characteristic measurements show that the efficiency increases with increasing the thickness of the CdS thin layer. Compared with the tandem solar cells based on (p/n)–(p/n) semiconductor junctions, our n-p-n transistor solar cell has a simple structure without using tunnel junctions or wafer bonding schemes for interconnecting the cells.
    Electronic ISSN: 1941-7012
    Topics: Energy, Environment Protection, Nuclear Power Engineering
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  • 39
    Publication Date: 2016-01-06
    Description: Positive feedback is a common feature in signal transduction networks and can lead to phenomena such as bistability and signal propagation by domain growth. Physical features of the cellular environment, such as spatial confinement and the mobility of proteins, play important but inadequately understood roles in shaping the behavior of signaling networks. Here, we use stochastic, spatially resolved kinetic Monte Carlo simulations to explore a positive feedback network as a function of system size, system shape, and mobility of molecules. We show that these physical properties can markedly alter characteristics of bistability and stochastic switching when compared with well-mixed simulations. Notably, systems of equal volume but different shapes can exhibit qualitatively different behaviors under otherwise identical conditions. We show that stochastic switching to a state maintained by positive feedback occurs by cluster formation and growth. Additionally, the frequency at which switching occurs depends nontrivially on the diffusion coefficient, which can promote or suppress switching relative to the well-mixed limit. Taken together, the results provide a framework for understanding how confinement and protein mobility influence emergent features of the positive feedback network by modulating molecular concentrations, diffusion-influenced rate parameters, and spatiotemporal correlations between molecules.
    Print ISSN: 0021-9606
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  • 40
    Publication Date: 2016-01-06
    Description: In this study, Boltzmann inversion is applied in conjunction with molecular dynamics simulations to derive inter-molecular potential for protein lysozyme in aqueous solution directly from experimental static structure factor. The potential has a soft repulsion at short distances and an attraction well at intermediate distances that give rise to the liquid-liquid phase separation. Moreover, Gibbs ensemble Monte Carlo simulations demonstrate that a non-spherical description of lysozyme is better suited to correctly reproduce the experimentally observed properties of such a phase separation. Our findings shed new light on the common problem in molecular and cell biology: “How to model proteins in their natural aqueous environments?”
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  • 41
    Publication Date: 2016-01-06
    Description: The Sign Learning Kink (SiLK) based Quantum Monte Carlo (QMC) method is used to calculate the ab initio ground state energies for multiple geometries of the H 2 O, N 2 , and F 2 molecules. The method is based on Feynman’s path integral formulation of quantum mechanics and has two stages. The first stage is called the learning stage and reduces the well-known QMC minus sign problem by optimizing the linear combinations of Slater determinants which are used in the second stage, a conventional QMC simulation. The method is tested using different vector spaces and compared to the results of other quantum chemical methods and to exact diagonalization. Our findings demonstrate that the SiLK method is accurate and reduces or eliminates the minus sign problem.
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  • 42
    Publication Date: 2016-01-06
    Description: A new method for quantifying the contributions of local excitation, charge resonance, and multiexciton configurations in correlated wave functions of multichromophoric systems is presented. The approach relies on fragment-localized orbitals and employs spin correlators. Its utility is illustrated by calculations on model clusters of hydrogen, ethylene, and tetracene molecules using adiabatic restricted-active-space configuration interaction wave functions. In addition to the wave function analysis, this approach provides a basis for a simple state-specific energy correction accounting for insufficient description of electron correlation. The decomposition scheme also allows one to compute energies of the diabatic states of the local excitonic, charge-resonance, and multi-excitonic character. The new method provides insight into electronic structure of multichromophoric systems and delivers valuable reference data for validating excitonic models.
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  • 43
    Publication Date: 2016-01-06
    Description: In this paper, we demonstrate how to use a personal computer sound card to develop an experimental platform for evaluating the jettability and jetting behavior of inkjet fluids. The test fluid is driven out of a nozzle acoustically using a loudspeaker, forming a jet. The subsequent jet breakup process is then captured using a stroboscopic light source and a camera. Instead of using a delay generator as in previous work, the current setup uses a computer sound card and audio amplifier to (i) generate actuation waveforms of arbitrary shapes and (ii) synchronize the jet actuation and imaging with a time precision close to 5 μ s. To correct for any signal distortions caused by the built-in high pass filters of the sound card and amplifier, a numerical filter is created and applied before sending the desired signal to the sound card. Such correction method does not require physically modifying the hardware of the sound card or amplifier and is applicable to different waveforms and filters provided that the transfer function is correctly identified. The platform has been tested using 20% (v/v) glycerol in water as a model fluid. Combining this platform with digital image analysis further enables a quantitative assessment of parameters such as the volumes and positions of the jet and drop that are important for quality control and development of new ink formulations.
    Print ISSN: 0034-6748
    Electronic ISSN: 1089-7623
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  • 44
    Publication Date: 2016-01-07
    Description: The physical origin of the ultrahigh piezoelectricity of [001]-oriented ferroelectric single crystals at the morphotropic phase boundary (MPB) is investigated via computer simulations using the phase field method. A wide composition range is explored and the existence of a monoclinic phase at the MPB is considered. The relative contributions from polarization rotation and domain wall motion are discriminated. The domain structures, hysteresis and hysteresis-free strain-electric field curves, and change of d 33 as a function of composition for poled samples near the MPB predicted by the simulations are all in agreement with experimental observations. It is found that under small polarization anisotropy the ultrahigh piezoelectricity of [001]-oriented ferroelectric single crystals at the MPB originates mainly from polarization rotation. Although domain wall motion also takes place for poled samples with monoclinic nanodomain structures and contributes to the hysteresis, its contribution to the ultrahigh piezoelectricity is insignificant.
    Print ISSN: 0003-6951
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  • 45
    Publication Date: 2016-01-07
    Description: Realization of an 18.6% efficient 4H-silicon carbide (4H-SiC) large area betavoltaic power source using the radioisotope tritium is reported. A 200 nm 4H-SiC P + N junction is used to collect high-energy electrons. The electron source is a titanium tritide (TiH 3 x ) foil, or an integrated titanium tritide region formed by the diffusion of tritium into titanium. The specific activity of the source is directly measured. Dark current measured under short circuit conditions was less than 6.1 pA/cm 2 . Samples measured with an external tritium foil produced an open circuit voltage of 2.09 V, short circuit current of 75.47 nA/cm 2 , fill factor of 0.86, and power efficiency of 18.6%. Samples measured with an integrated source produced power efficiencies of 12%. Simulations were done to determine the beta spectrum (modified by self absorption) exiting the source and the electron hole pair generation function in the 4H-SiC. The electron-hole pair generation function in 4H-SiC was modeled as a Gaussian distribution, and a closed form solution of the continuity equation was used to analyze the cell performance. The effective surface recombination velocity in our samples was found to be 10 5 –10 6  cm/s. Our analysis demonstrated that the surface recombination dominates the performance of a tritium betavoltaic device but that using a thin P + N junction structure can mitigate some of the negative effects.
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  • 46
    Publication Date: 2016-01-07
    Description: Tuning low resistance state is crucial for resistance random access memory (RRAM) that aims to achieve optimal read margin and design flexibility. By back-to-back stacking two nanometallic bipolar RRAMs with different thickness into a complementary structure, we have found that its low resistance can be reliably tuned over several orders of magnitude. Such high tunability originates from the exponential thickness dependence of the high resistance state of nanometallic RRAM, in which electron wave localization in a random network gives rise to the unique scaling behavior. The complementary nanometallic RRAM provides electroforming-free, multi-resistance-state, sub-100 ns switching capability with advantageous characteristics for memory arrays.
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  • 47
    Publication Date: 2016-01-07
    Description: Using light-emitting diodes (LEDs) for visible light communication has become an alternative choice of radio source due to channel crowding of the radio-frequency (RF) signal. The modulation bandwidth of LEDs is usually limited by the spontaneous carrier lifetime in multiple quantum wells. Here, sub-GHz modulation of GaN-based LED employing photonic crystal (PhC) nanostructure is demonstrated. The guided photonic modes of the LEDs are modulated by the RF signal. Both carrier lifetime of lower- and higher-order modes are studied in time-resolved photoluminescence (TRPL) at room temperature. The f - 3 dB - J curve of the PhC LED exhibits a higher bandwidth than the typical LED structure. At 11.41 kA/cm 2 , the optical −3-dB bandwidth ( f - 3 dB ) up to 234 MHz of the PhC LED (PhCLED) is achieved. Our studies on TRPL at different wavelengths and frequency response at different injection current densities conclude that the higher operation speed is attributed to faster radiative carrier recombination of extracted guided modes from the PhC nanostructure.
    Print ISSN: 0021-8979
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  • 48
    Publication Date: 2016-01-07
    Description: We report a joint experimental and theoretical study on the ultrafast excited state dynamics of allene and a series of its methylated analogues (1,2-butadiene, 1,1-dimethylallene, and tetramethylallene) in order to elucidate the conical intersection mediated dynamics that give rise to ultrafast relaxation to the ground electronic state. We use femtosecond time-resolved photoelectron spectroscopy (TRPES) to probe the coupled electronic-vibrational dynamics following UV excitation at 200 nm (6.2 eV). Ab initio multiple spawning (AIMS) simulations are employed to determine the mechanistic details of two competing dynamical pathways to the ground electronic state. In all molecules, these pathways are found to involve as follows: (i) twisting about the central allenic C–C–C axis followed by pyramidalization at one of the terminal carbon atoms and (ii) bending of allene moiety. Importantly, the AIMS trajectory data were used for ab initio simulations of the TRPES, permitting direct comparison with experiment. For each molecule, the decay of the TRPES signal is characterized by short (30 fs, 52 fs, 23 fs) and long (1.8 ps, 3.5 ps, [306 fs, 18 ps]) time constants for 1,2-butadiene, 1,1-dimethylallene, and tetramethylallene, respectively. However, AIMS simulations show that these time constants are only loosely related to the evolution of electronic character and actually more closely correlate to large amplitude motions on the electronic excited state, modulating the instantaneous vertical ionization potentials. Furthermore, the fully substituted tetramethylallene is observed to undergo qualitatively different dynamics, as displacements involving the relatively massive methyl groups impede direct access to the conical intersections which give rise to the ultrafast relaxation dynamics observed in the other species. These results show that the branching between the “twisting” and “bending” pathways can be modified via the selective methylation of the terminal carbon atoms of allene. The interplay between inertial and potential effects is a key to understanding these dynamical branching pathways. The good agreement between the simulated and measured TRPES confers additional confidence to the dynamical picture presented here.
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  • 49
    Publication Date: 2016-01-07
    Description: In order to better understand the behavior and governing characteristics of the wetting transparency phenomenon observed in graphene-coated surfaces, molecular dynamics simulations were coupled with a theoretical model. Graphene-coated silicon was selected for this analysis, due to potential applications of hybrid silicon-graphene materials as detectors in aqueous environments. The results indicate good agreement between the theory and simulations at the macroscopic conditions required to observe wetting transparency. A microscopic analysis was also conducted in order to identify the parameters, such as the interaction potential energy landscape and the interfacial liquid structure that govern the wetting behavior of graphene-coated surfaces. The interfacial liquid structure was found to be different between uncoated Si(100) and the graphene-coated version and very similar between uncoated Si(111) and the graphene-coated version. However, the concentration of liquid particles for both silicon surfaces was found to be very similar under transparent wetting conditions.
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  • 50
    Publication Date: 2016-01-07
    Description: Double-resonance laser spectroscopy via the E F   1 Σ g + , v ′ = 6 , J ′ = 0 –2 state was used to probe the high vibrational levels of the B ′ ′ B ̄   1 Σ u + state of molecular hydrogen. Resonantly enhanced multiphoton ionization spectra were recorded by detecting ion production as a function of energy using a time of flight mass spectrometer. New measurements of energies for the v = 51–66 levels for the B ′ ′ B ̄ state of H 2 are reported, which, taken with previous results, span the v = 46–69 vibrational levels. Results for energy levels are compared to theoretical close-coupled calculations [L. Wolniewicz, T. Orlikowski, and G. Staszewska, J. Mol. Spectrosc. 238 , 118–126 (2006)]. The average difference between the 84 measured energies and calculated energies is −3.8 cm −1 with a standard deviation of 5.3 cm −1 . This level of agreement showcases the success of the theoretical calculations in accounting for the strong rovibronic mixing of the 1 Σ u + and 1 Π u + states. Due to the ion-pair character of the outer well, the observed energies of the vibrational levels below the third dissociation limit smoothly connect with previously observed energies of ion-pair states above this limit. The results provide an opportunity for testing a heavy Rydberg multi-channel quantum defect analysis of the high vibrational states below the third dissociation limit.
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  • 51
    Publication Date: 2016-01-07
    Description: We demonstrate theoretically that laser-induced coherent quantum interference control of asymptotic states of dissociating molecules is possible, starting from a single vibrational eigenstate, after the interaction with two laser pulses—at a fixed time delay—both operating in the weak-field limit. Thus, phase dependence in the interaction with the second fixed-energy phase-modulated pulse persists after the pulse is over. This is illustrated for the nonadiabatic process: I + Br * ←IBr → I + Br, where the relative yield of excited Br * can be changed by pure phase modulation. Furthermore, a strong frequency dependence of the branching ratio is observed and related to the re-crossing dynamics of the avoided crossing in the above-mentioned process.
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  • 52
    Publication Date: 2016-01-08
    Description: The near field meta-molecular interactions in a lattice play an important role in determining the collective behavior of the metamaterials. Here, we exploit the nearest neighbor inter unit cell interactions and the intra near-field coupling in metamolecules to manipulate the co- and the cross-polarized light. We observed large enhancement in the bandwidth and the amplitude of the transmitted light in the strongly coupled meta-molecular lattice. We further show that the proposed metasurface could function as a broadband achromatic quarter-wave plate. The chosen meta-molecular design also enhances the cross-polarized light when integrated with a ground plane to operate in the reflection mode.
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  • 53
    Publication Date: 2016-01-07
    Description: The effect of self focused hollow Gaussian laser beam (HGLB) (carrying null intensity in center) on the excitation of electron plasma wave (EPW) and second harmonic generation (SHG) has been investigated in collisionless plasma, where relativistic-ponderomotive and only relativistic nonlinearities are operative. The relativistic change of electron mass and the modification of the background electron density due to ponderomotive nonlinearity lead to self-focusing of HGLB in plasma. Paraxial ray theory has been used to derive coupled equations for the self focusing of HGLB in plasma, generation of EPW, and second harmonic. These coupled equations are solved analytically and numerically to study the laser intensity in the plasma, electric field associated with the excited EPW, and the power of SHG. Second harmonic emission is generated due to nonlinear coupling between incident HGLB and EPW satisfying the proper phase matching conditions. The results show that the effect of including the ponderomotive nonlinearity is significant on the generation of EPW and second harmonic. The electric field associated with EPW and the power of SHG are found to be highly sensitive to the order of the hollow Gaussian beam.
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  • 54
    Publication Date: 2016-01-07
    Description: Efficient multi-keV x-ray sources can be produced using nanosecond laser pulse-heated middle-Z underdense plasmas generated using gas or foam. Previous experimental results show that an optimal initial target density exists for efficient multi-keV x-ray emission at which the laser ionization wave is supersonic. Here we explore the influence of the laser intensity and the pulse duration on this optimal initial target density via a one-dimensional radiation hydrodynamic simulation. The simulation shows that the optimal initial density is sensitive to both the laser intensity and the pulse duration. However, the speed of the supersonic ionization wave at the end of the laser irradiation is always maintained at 1.5 to 1.7 times that of the ion acoustic wave under the optimal initial density conditions.
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  • 55
    Publication Date: 2016-01-07
    Description: To better understand the two-way coupling between turbulence and chemistry, the changes in turbulence characteristics through a premixed flame are investigated. Specifically, this study focuses on vorticity, ω , which is characteristic of the smallest length and time scales of turbulence, analyzing its behavior within and across high Karlovitz number (Ka) premixed flames. This is accomplished through a series of direct numerical simulations (DNS) of premixed n -heptane/air flames, modeled with a 35-species finite-rate chemical mechanism, whose conditions span a wide range of unburnt Karlovitz numbers and flame density ratios. The behavior of the terms in the enstrophy, ω 2 = ω ⋅ ω , transport equation is analyzed, and a scaling is proposed for each term. The resulting normalized enstrophy transport equation involves only a small set of parameters. Specifically, the theoretical analysis and DNS results support that, at high Karlovitz number, enstrophy transport obtains a balance of the viscous dissipation and production/vortex stretching terms. It is shown that, as a result, vorticity scales in the same manner as in homogeneous, isotropic turbulence within and across the flame, namely, scaling with the inverse of the Kolmogorov time scale, τ η . As τ η is a function only of the viscosity and dissipation rate, this work supports the validity of Kolmogorov’s first similarity hypothesis in premixed turbulent flames for sufficiently high Ka numbers. Results are unaffected by the transport model, chemical model, turbulent Reynolds number, and finally the physical configuration.
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  • 56
    Publication Date: 2016-01-07
    Description: Plasma expansion into a vacuum with an external magnetic field is studied under the ideal magnetohydrodynamic hypothesis. The inclination of the magnetic field with respect to the expansion direction is arbitrary, and both the perpendicular and the oblique cases are separately analyzed. A self-similar solution satisfying the boundary conditions is obtained. The interface with the vacuum is treated as a fluid surface, and jump conditions concerning the momentum conservation are imposed. The effect of the intensity of the magnetic field and its inclination is thoroughly studied, and the consistency of the solution for small and large inclinations is investigated.
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  • 57
    Publication Date: 2016-01-08
    Description: We study transient spatiotemporal structures induced by a weak space-time localized stimulus in an excitable contractile fiber within a two-component globally coupled reaction-diffusion model. The model which we develop allows us to analyze various regimes of excitation spreading and determine origin of the induced structures for various contraction types (defined by the fiber fixation) and global coupling strengths. One of the most notable effects we observed is the after-excitation effect. It leads to emergence of multiple excitation pulses excited by a single external stimulus and can result in long-lasting transient activity and appearance of new oscillatory attractor regimes, including the ones with multiple phase clusters.
    Print ISSN: 1054-1500
    Electronic ISSN: 1089-7682
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  • 58
    Publication Date: 2016-01-07
    Description: In this paper, we present an electronic circuit used to bias a photoconductive antenna that generates terahertz radiation. The working principles and the design process for the device are discussed in detail. The noise and shape of the wave measurements for a built device are considered. Furthermore, their impact on a terahertz pulse and its spectra is also examined. The proposed implementation is simple to build, robust and offers a real improvement over THz instrumentation due to the frequency tuning. Additionally, it provides for galvanic isolation and ESD protection.
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    Topics: Electrical Engineering, Measurement and Control Technology , Physics
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  • 59
    Publication Date: 2016-01-07
    Description: An experimental campaign aiming to investigate electron cyclotron resonance (ECR) plasma X-ray emission has been recently carried out at the ECRISs—Electron Cyclotron Resonance Ion Sources laboratory of Atomki based on a collaboration between the Debrecen and Catania ECR teams. In a first series, the X-ray spectroscopy was performed through silicon drift detectors and high purity germanium detectors, characterizing the volumetric plasma emission. The on-purpose developed collimation system was suitable for direct plasma density evaluation, performed “on-line” during beam extraction and charge state distribution characterization. A campaign for correlating the plasma density and temperature with the output charge states and the beam intensity for different pumping wave frequencies, different magnetic field profiles, and single-gas/gas-mixing configurations was carried out. The results reveal a surprisingly very good agreement between warm-electron density fluctuations, output beam currents, and the calculated electromagnetic modal density of the plasma chamber. A charge-coupled device camera coupled to a small pin-hole allowing X-ray imaging was installed and numerous X-ray photos were taken in order to study the peculiarities of the ECRIS plasma structure.
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  • 60
    Publication Date: 2016-01-08
    Description: We describe results of measurements of the orientational motion of glass microrods in a microchannel flow, following the orientational motion of particles with different shapes. We determine how the orientational dynamics depends on the shape of the particle and on its initial orientation. We find that the dynamics depends so sensitively on the degree to which particle axisymmetry is broken that it is difficult to find particles that are sufficiently axisymmetric so that they exhibit periodic tumbling (“Jeffery orbits”). The results of our measurements confirm earlier theoretical analysis predicting sensitive dependence on particle shape and its initial orientation. Our results illustrate the different types of orientational dynamics for asymmetric particles predicted by theory.
    Print ISSN: 1070-6631
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  • 61
    Publication Date: 2016-01-08
    Description: Separate spin evolution quantum hydrodynamics is generalized to include the Coulomb exchange interaction, which is considered as interaction between the spin-down electrons being in quantum states occupied by one electron. The generalized model is applied to study the non-linear spin-electron acoustic waves. Existence of the spin-electron acoustic soliton is demonstrated. Contributions of concentration, spin polarization, and exchange interaction to the properties of the spin electron acoustic soliton are studied.
    Print ISSN: 1070-664X
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  • 62
    Publication Date: 2016-01-08
    Description: An external meniscus on a narrow blade with a slit-like cross section is studied using the hodograph formulation of the Laplace nonlinear equation of capillarity. On narrow blades, the menisci are mostly shaped by the wetting and capillary forces; gravity plays a secondary role. To describe a meniscus in this asymptotic case, the model of Alimov and Kornev [“Meniscus on a shaped fibre: Singularities and hodograph formulation,” Proc. R. Soc. A 470 , 20140113 (2014)] has been employed. It is shown that at the sharp edges of the blade, the contact line makes a jump. In the wetting case, the contact line sitting at each side of the blade is lifted above the points where the meniscus first meets the blade edges. In the non-wetting case, the contact line is lowered below these points. The contours of the constant height emanating from the blade edges generate unusual singularities with infinite curvatures at some points at the blade edges. The meniscus forms a unique surface made of two mirror-symmetric sheets fused together. Each sheet is supported by the contact line sitting at each side of the blade.
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  • 63
    Publication Date: 2016-01-08
    Description: We have studied the generation of terahertz (THz) radiation via the interaction of intense femtosecond laser pulses with solid targets at a small incidence angle. It is found that preplasma with a moderate density gradient can enhance the emission. We also observe saturation of the THz output with the driving laser energy. We find that THz emission is closely related to the 3/2 harmonics of the driving laser. Particle-in-cell simulations indicate that under the present experimental conditions, the THz emission could be attributed to the transient currents at the plasma-vacuum interface, mainly formed by the two-plasmon-decay instability.
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  • 64
    facet.materialart.
    Unknown
    American Institute of Physics (AIP)
    Publication Date: 2016-01-08
    Description: Various electrostatic twisted modes are re-investigated with finite orbital angular momentum in an unmagnetized collisionless multi-component dusty plasma, consisting of positive/negative charged dust particles, ions, and electrons. For this purpose, hydrodynamical equations are employed to obtain paraxial equations in terms of density perturbations, while assuming the Gaussian and Laguerre-Gaussian (LG) beam solutions. Specifically, approximated solutions for potential problem are studied by using the paraxial approximation and expressed the electric field components in terms of LG functions. The energy fluxes associated with these modes are computed and corresponding expressions for orbital angular momenta are derived. Numerical analyses reveal that radial/angular mode numbers as well as dust number density and dust charging states strongly modify the LG potential profiles attributed to different electrostatic modes. Our results are important for understanding particle transport and energy transfer due to wave excitations in multi-component dusty plasmas.
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  • 65
    Publication Date: 2016-01-08
    Description: In this paper, based on piezoforce measurements, we show the presence of opposite polarization at grains and grain boundaries of Al-doped ZnO (AZO). The polarization can be flipped by 180° in phase by switching the polarity of the applied electric field, revealing the existence of nanoscale pseudoferroelectricity in AZO grown on Pt/TiO 2 /SiO 2 /Si substrate. We also demonstrate an experimental evidence on local band bending at grain boundaries of AZO films using conductive atomic force microscopy and Kelvin probe force microscopy. The presence of an opposite polarization at grains and grain boundaries gives rise to a polarization-driven barrier formation at grain boundaries. With the help of conductive atomic force microscopy, we show that the polarization-driven barrier along with the defect-induced electrostatic potential barrier account for the measured local band bending at grain boundaries. The present study opens a new avenue to understand the charge transport in light of both polarization and electrostatic effects.
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  • 66
    Publication Date: 2016-01-08
    Description: A simple thermodynamic theory is proposed for the quantitative description of giant magnetocaloric effect observed in metamagnetic shape memory alloys. Both the conventional magnetocaloric effect at the Curie temperature and the inverse magnetocaloric effect at the transition from the ferromagnetic austenite to a weakly magnetic martensite are considered. These effects are evaluated from the Landau-type free energy expression involving exchange interactions in a system of a two magnetic sublattices. The findings of the thermodynamic theory agree with first-principles calculations and experimental results from Ni-Mn-In-Co and Ni-Mn-Sn alloys, respectively.
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  • 67
    Publication Date: 2016-01-08
    Description: Neutron diffraction measurements of the Cr intercalated niobium diselenide Cr 1∕3 NbSe 2 together with magnetization measurements have revealed that this compound exhibits ferromagnetic ordering below T C  = 96 K unlike a chiral helimagnetic order observed in the sulfide compound Cr 1∕3 NbS 2 . As derived from neutron diffraction data, the Cr magnetic moments μ Cr  = 2.83 ± 0.03  μ B in Cr 1∕3 NbSe 2 are aligned within basal plane. The discrepancy in the magnetic states of Cr 1∕3 NbS 2 and Cr 1∕3 NbSe 2 is ascribed to the difference in the preferential site occupation of Cr ions in crystal lattices. In Cr 1∕3 NbSe 2 , the Cr ions are predominantly distributed over 2 b Wyckoff site, which determines a centrosymmetric character of the crystal structure unlike Cr 1∕3 NbS 2 , where the Cr ions are mainly located in 2 c position and the crystal structure is non-centrosymmetric.
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  • 68
    Publication Date: 2016-01-08
    Description: In the present paper, investigations of CdSe loaded poly(diallyl dimethyl ammonium chloride) (PDADMAC) nanocomposites and pure PDADMAC synthesized by wet chemical technique have been carried out. Fourier transform infrared and X-ray diffraction analysis have been performed to reveal the structural details of pure polymer and polymer nanocomposite (PNC). The dielectric behavior of pure polymer and PNC has been recorded, which results in higher value of the real and imaginary part of dielectric constant for PNC, as compared with pure PDADMAC. The increase is attributed to the addition of CdSe quantum dots to the pure polymer. The contribution of ionic and electronic polarization has been observed at higher frequency. The theoretical fitting of Cole-Cole function to the experimental data of dielectric constant of PNC and pure PDADMAC results in the determination of relaxation time and conductivity of space charge carriers. The CdSe loaded polymer nanocomposite has been used as an electrolyte in the battery fabrication with configuration Al/PNC/Ag 2 O. The ac conductivity measurements have been carried out for both samples in a frequency window of 1 kHz–5 MHz and at different temperatures varying from 298 K to 523 K. Activation energy ( E a ) has been determined for pure polymer as well as PNC and is found to be less for PNC, as compared with pure polymer. Further, impedance measurement at different temperatures results in two frequency ranges corresponding to ionic conduction and blocking electrode effect. The value of bulk resistance for pure polymer and PNC has been found to be 3660 Ω and 442 Ω, respectively, at 298 K temperature. Electric modulus has been determined and is observed to support the dielectric constant data; it further reveals the deviation from Debye behavior at a higher frequency.
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  • 69
    Publication Date: 2016-01-08
    Description: Accurate optical methods are required to determine the energy bandgap of amorphous semiconductors and elucidate the role of quantum confinement in nanometer-scale, ultra-thin absorbing layers. Here, we provide a critical comparison between well-established methods that are generally employed to determine the optical bandgap of thin-film amorphous semiconductors, starting from normal-incidence reflectance and transmittance measurements. First, we demonstrate that a more accurate estimate of the optical bandgap can be achieved by using a multiple-reflection interference model. We show that this model generates more reliable results compared to the widely accepted single-pass absorption method. Second, we compare two most representative methods (Tauc and Cody plots) that are extensively used to determine the optical bandgap of thin-film amorphous semiconductors starting from the extracted absorption coefficient. Analysis of the experimental absorption data acquired for ultra-thin amorphous germanium (a-Ge) layers demonstrates that the Cody model is able to provide a less ambiguous energy bandgap value. Finally, we apply our proposed method to experimentally determine the optical bandgap of a-Ge/SiO 2 superlattices with single and multiple a-Ge layers down to 2 nm thickness.
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  • 70
    Publication Date: 2016-01-08
    Description: In this paper, we extend the idea of reducing the electromagnetic interactions between transmitting radiators to the case of widely used planar antennas in printed technology based on the concept of mantle cloaking. Here, we show that how lightweight elliptical metasurface cloaks can be engineered to restore the intrinsic properties of printed antennas with strip inclusions. In order to present the novel approach, we consider two microstrip-fed monopole antennas resonating at slightly different frequencies cloaked by confocal elliptical metasurfaces formed by arrays of sub-wavelength periodic elements, partially embedded in the substrate. The presence of the metasurfaces leads to the drastic suppression of mutual near-field and far-field couplings between the antennas, and thus, their radiation patterns are restored as if they were isolated. Moreover, it is worth noting that this approach is not limited to printed radiators and can be applied to other planar structures as well.
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  • 71
    Publication Date: 2016-01-08
    Description: A bilayer graphene film obtained on copper (Cu) foil is known to have a significant fraction of non-Bernal (AB) stacking and on copper/nickel (Cu/Ni) thin films is known to grow over a large-area with AB stacking. In this study, annealed Cu foils for graphene growth were doped with small concentrations of Ni to obtain dilute Cu(Ni) alloys in which the hydrocarbon decomposition rate of Cu will be enhanced by Ni during synthesis of large-area AB-stacked bilayer graphene using atmospheric pressure chemical vapour deposition. The Ni doped concentration and the Ni homogeneous distribution in Cu foil were confirmed with inductively coupled plasma optical emission spectrometry and proton-induced X-ray emission. An electron backscatter diffraction map showed that Cu foils have a single (001) surface orientation which leads to a uniform growth rate on Cu surface in early stages of graphene growth and also leads to a uniform Ni surface concentration distribution through segregation kinetics. The increase in Ni surface concentration in foils was investigated with time-of-flight secondary ion mass spectrometry. The quality of graphene, the number of graphene layers, and the layers stacking order in synthesized bilayer graphene films were confirmed by Raman and electron diffraction measurements. A four point probe station was used to measure the sheet resistance of graphene films. As compared to Cu foil, the prepared dilute Cu(Ni) alloy demonstrated the good capability of growing large-area AB-stacked bilayer graphene film by increasing Ni content in Cu surface layer.
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  • 72
    Publication Date: 2016-01-08
    Description: Using in situ transmission electron microscopy, we investigated the dynamic reconstruction and evolution of ZnO polar and non-polar surfaces under high-energy electron beam irradiation. Electron beam radiolysis creates oxygen vacancies and a Zn rich (0001) surface. Positive polar charges at the (0001) surface expel loosely bonded Zn ions to diffuse away from the (0001) polar surface. As a result, mass loss was observed around the (0001) surface. Dehydration by the electron beam breaks the charge balance on the ( 000 1 ¯ ) polar surface. The negative charges on the ( 000 1 ¯ ) surface suppress the radiolysis effect and further absorb Zn ions to the surface to neutral the polar charges. The ideal stacking sequences of Zn ions in hexagonal ZnO structure can be considered as ABAB… along its c axis, while the absorbed individual Zn ion on the ( 000 1 ¯ ) surface occupies the C site to form three bonds with surface O ions beneath, instead of one bond in the ideal structure. With more Zn ion absorption and surface oxidization, new nanocrystals grow up from the ( 000 1 ¯ ) polar surface. New nanocrystals nucleated at the ( 01 1 ¯ 0 ) non-polar surface are driven by the electric field of the polar charges as well, for the Zn ions were always observed to absorb on the negatively charged [ 000 1 ¯ ] end of the newly formed ( 01 1 ¯ 0 ) surface layer.
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  • 73
    Publication Date: 2016-01-08
    Description: Superconducting microresonators have been successfully utilized as detection elements for a wide variety of applications. With multiplexing factors exceeding 1000 detectors per transmission line, they are the most scalable low-temperature detector technology demonstrated to date. For high-throughput applications, fewer detectors can be coupled to a single wire but utilize a larger per-detector bandwidth. For all existing designs, fluctuations in fabrication tolerances result in a non-uniform shift in resonance frequency and sensitivity, which ultimately limits the efficiency of bandwidth utilization. Here, we present the design, implementation, and initial characterization of a superconducting microresonator readout integrating two tunable inductances per detector. We demonstrate that these tuning elements provide independent control of both the detector frequency and sensitivity, allowing us to maximize the transmission line bandwidth utilization. Finally, we discuss the integration of these detectors in a multilayer fabrication stack for high-speed readout of the D-Wave quantum processor, highlighting the use of control and routing circuitry composed of single-flux-quantum loops to minimize the number of control wires at the lowest temperature stage.
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  • 74
    Publication Date: 2016-01-08
    Description: A photonic bandgap structure was created on the 100 nm thick GaAs barrier layer with Au nanodisks deposited inside the holes. To mitigate the nonradiative surface recombination of GaAs, the Au nanodisks were formed on top of a 15 nm SiO 2 deposited in the holes. A maximum 7.6-fold increase in photoluminescence intensity was obtained at the etch depth of 80 nm. In this configuration, the Au nanodisk is separated from the quantum well by 20 nm of GaAs and 15 nm of SiO 2 . The experimental result was verified by the simulation based on this structure. There was a good agreement between experiments with simulation results.
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  • 75
    Publication Date: 2016-01-08
    Description: Using the effective-mass approximation and Floquet theory, we theoretically investigate the terahertz photon-assisted transport through a ZnSe / Zn 1− x Mn x Se heterostructure under an external magnetic field, an electric field, and a spatially homogeneous oscillatory field. The results show that both amplitude and frequency of the oscillatory field can accurately manipulate the magnitude of the spin-dependent transmission probability and the positions of the Fano-type resonance due to photon absorption and emission processes. Transmission resonances can be enhanced to optimal resonances or drastically suppressed for spin-down electrons tunneling through the heterostructure and for spin-up ones tunneling through the same structure, resonances can also be enhanced or suppressed, but the intensity is less than the spin-down ones. Furthermore, it is important to note that transmission suppression can be clearly seen from both the spin-down component and the spin-up component of the current density at low magnetic field; at the larger magnetic field, however, the spin-down component is suppressed, and the spin-up component is enhanced. These interesting properties may provide an alternative method to develop multi-parameter modulation electron-polarized devices.
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  • 76
    Publication Date: 2016-01-08
    Description: Electrical and luminescent properties and deep trap spectra of Si doped GaN films grown by maskless epitaxial lateral overgrowth (MELO) are reported. The dislocation density in the wing region of the structure was 10 6  cm −2 , while in the seed region it was 10 8  cm −2 . The major electron traps present had activation energy of 0.56 eV and concentrations in the high 10 15  cm −3 range. A comparison of diffusion length values and 0.56 eV trap concentration in MELO GaN and epitaxial lateral overgrowth (ELOG) GaN showed a good correlation, suggesting these traps could be effective in carrier recombination. The doped MELO films were more uniform in their electrical properties than either ELOG films or undoped MELO films. We also discuss the differences in deep trap spectra and luminescence spectra of low-dislocation-density MELO, ELOG, and bulk n-GaN samples grown by hydride vapor phase epitaxy. It is suggested that the observed differences could be caused by the differences in oxygen and carbon contamination levels.
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  • 77
    Publication Date: 2016-01-08
    Description: The dielectric susceptibility of the lead-free relaxor-like perovskite ferroelectric Na 1/2 Bi 1/2 TiO 3 is shown to exhibit the same complicated spinglass-like aging behavior found in several cubic relaxors deep in the glassy state, in contrast to some uniaxial relaxors. The effects include rejuvenation and memory, including the ability to support multiple memories in parallel. Weak dependence of aging memories on changes in the electric field indicates that the aging is associated with relatively non-polar degrees of freedom, not with ferroelectric domains.
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  • 78
    Publication Date: 2016-01-08
    Description: The results of comprehensive theoretical and experimental study of binary GaN/AlN multi-quantum well (MQW) systems oriented along polar c-direction of their wurtzite structure are presented. A series of structures with quantum wells and barriers of various thicknesses were grown by plasma-assisted molecular-beam epitaxy and characterized by x-ray diffraction and transmission electron microscopy. It was shown that in general the structures of good quality were obtained, with the defect density decreasing with increasing quantum well thickness. The optical transition energies in these structures were investigated comparing experimental measurements with ab initio calculations of the entire GaN/AlN MQW structure depending on the QW widths and strains, allowing for direct determination of the energies of optical transitions and the electric fields in wells/barriers by electric potential double averaging procedure. Photoluminescence (PL) measurements revealed that the emission efficiency as well as the shape of luminescence spectra correlated well with their structural quality. Additionally, due to the Quantum-Confined Stark Effect, the emission energy decreased by over 1 eV for quantum well thicknesses increasing from 1 nm up to 6 nm, and this effect was accompanied by the drastic drop of the PL efficiency. The experimental results are consistent with theoretical models. Comparison of experimental data obtained by a number of different characterization techniques with the density functional theory results received on the same geometry structure allowed to prove directly the theoretical models and to determine the polarization and the oscillator strengths in the AlN/GaN nitride systems for the first time.
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  • 79
    Publication Date: 2016-01-08
    Description: In this work, bulk anisotropic Mn-Al-(C) magnets have been produced by hot-deforming arc-melted alloys. The hot-deformed magnets develop a texture in a plane perpendicular to the deformation direction. Optimum magnetic properties have been obtained in a magnet with nominal composition Mn 53.5 Al 44.5 C 2 with values of remnant magnetization, coercivity, and energy product of 2.2 kOe, 3.3 kG, and 1.8 MGOe, respectively. A maximum coercivity of 2.8 kOe has been obtained in the magnet with composition Mn 54.5 Al 43.5 C 2 . Addition of C is effective in increasing the coercivity of the substituted alloy.
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  • 80
    Publication Date: 2016-01-08
    Description: Mott proposed that impurity bands in corundum-symmetry Ti 2 O 3 at high temperatures caused a collapse in the bandgap. However, the origin of the impurity bands has not yet been clarified. We examine the local structural properties of metal-to-insulator-transition Ti 2 O 3 using in-situ x-ray absorption fine structure (XAFS) measurements at the Ti K edge in the temperature range from 288 to 739 K. The Ti 2 O 3 powder is synthesized by using a chemical reaction method. X-ray diffraction (XRD) measurements from Ti 2 O 3 with a Rietveld refinement demonstrate a single-phased R-3c symmetry without additional distortion. Extended-XAFS combined with XRD reveals a zigzag patterned Ti position and an anomalous structural disorder in Ti-Ti pairs, accompanied by a bond length expansion of the Ti-Ti pairs along the c -axis for T 〉 450 K. The local structural distortion and disorder of the Ti atoms would induce impurity levels in the band gap between the Ti 3 d a 1g and e g π bands, resulting in a collapse of the band gap for T 〉 450 K.
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  • 81
    facet.materialart.