bluuebunny/crossref_metadata_2025 huggingface dataset:
Variable rate data transmission by size of constellation and symbol duration adaptation
Authors: Goot R.E. , Dolev E.
Published: January 2014

In variable rate modulation, maximisation of data rate can be achieved either by fixing the symbol rate with an adaptive control of the constellation size, or by fixing the constellation size and changing the symbol duration. It is proved that for joint optimal control of both types, they should be used separately: the constellation size changes under a constant maximal symbol rate, and when these opportunities have been exhausted the symbol rate adaptation should be used.
Cascaded charge‐domain sampling mixer with embedded sinc3 FIR and four‐path filters for software‐defined radio
Authors: Zhou Yushi , Yuan Fei
Published: January 2014

A charge‐domain downconversion sampling mixer with an embedded four‐path bandpass filter for band selection and a sinc3 finite impulse response (FIR) and a first‐order sinc FIR with a decimation of 4 for channel selection is proposed to provide sufficient attenuation of out‐of‐band blockers and aliasing interferences. The sampling mixer designed in an IBM 130 nm 1.2 V CMOS with a 100 MHz input exhibits an 80 dB suppression of aliasing interferences and a 57 dB stopband attenuation while consuming 1.08 mA.
6.8 mW 15 dBm IIP3 CMOS common‐gate LNA employing post‐linearisation technique
Authors: Guo Benqing , Wen Guangjun , An Shiquan
Published: January 2014

A linearised differential common‐gate CMOS low‐noise amplifier (LNA) is proposed. The linearity is improved by a cross‐coupled post‐distortion technique, employing PMOS in a weak inversion region as an auxiliary field effect transistor to cancel the third‐order nonlinear currents of a common‐gate LNA and impair the related second‐order nonlinear currents. Meanwhile, the resulting noise figure is little affected. The LNA implemented in a 0.18 μm CMOS technology demonstrates that IIP3 and gain have about 8.2 and 1.5 dB improvements in the designed frequency band, respectively. A NF of 3.4 dB is obtained with a power dissipation of 6.8 mW under a 1.8 V power supply.
Achievement of beam steering in terahertz band based on frequency‐scanning grating‐reflector antenna
Authors: Li Shichao , Li Chao , Zhang Xiaojuan , Fang Guangyou
Published: January 2014

A frequency‐scanning grating‐reflector antenna operating in the 0.2 THz band is proposed. The antenna system consists of a planar grating reflector, a thin lens and a feed horn placed at the focal point of the lens. The beam‐scanning property based on the frequency sweeping was analysed and optimised based on Floquet's theorem. A bilayer structure is introduced to improve the diffraction efficiency and to suppress the specular beam. The frequency‐controlled beam steering concept was verified by the quasi‐optical measurements in the 0.2 THz band. The measured scanning range is 13.55° over the frequency band 180–220 GHz. The specular beams are remarkably inhibited with the diffractive efficiency as high as over 89%. The presented antenna has the advantage of the convenience to be extended to a higher frequency with a high directivity.
0.9–10.6 GHz UWB mixer using current bleeding for multi‐band application
Authors: Yeh Chin‐I , Feng Wu‐Shiung , Hsu Chen‐Yu
Published: January 2014

A complementary metal oxide semiconductor (CMOS) downconversion mixer for ultra‐wideband (UWB) applications is presented. The mixer circuit is designed using a VIS 0.25 μm radio frequency (RF) CMOS technology, working at the 0.9–10.6 GHz frequency range; it will be used in applications such as IEEE wireless fidelity (Wi‐Fi), global systems for mobile (GSM) and worldwide interoperability for microwave access (WiMAX). The core of the mixer has been designed based on double‐balanced cell architecture, and uses the current‐bleeding method to increase the linearity and improve the conversion gain. A resistor is put on the drain of the MOS between two RF inputs, which will improve the flatness of the conversion gain. The measured conversion gain of the mixer is 8.6 ± 1 dB. The 1 dB compression point of the intermediate frequency (IF) output power is equal to −8 dBm. The RF and the local oscillator (LO) input return loss are well below −9 dB. The noise figure is 13.1 dB, whereas the IF is 100 MHz, and the mixer core dissipates 9.8 mW under a 1.8 V supply. The analytical, simulated and measured results are mutually consistent.
N1s and O1s double ionization of the NO and N2O molecules
Authors: Hedin L. , Tashiro M. , Linusson P. , Eland J. H. D. , Ehara M. , Ueda K. , Zhaunerchyk V. , Karlsson L. , Pernestål K. , Feifel R.
Published: January 2014

Single-site N1s and O1s double core ionisation of the NO and N2O molecules has been studied using a magnetic bottle many-electron coincidence time-of-flight spectrometer at photon energies of 1100 eV and 1300 eV. The double core hole energies obtained for NO are 904.8 eV (N1s−2) and 1179.4 eV (O1s−2). The corresponding energies obtained for N2O are 896.9 eV (terminal N1s−2), 906.5 eV (central N1s−2), and 1174.1 eV (O1s−2). The ratio between the double and single ionisation energies are in all cases close or equal to 2.20. Large chemical shifts are observed in some cases which suggest that reorganisation of the electrons upon the double ionization is significant. Δ-self-consistent field and complete active space self-consistent field (CASSCF) calculations were performed for both molecules and they are in good agreement with these results. Auger spectra of N2O, associated with the decay of the terminal and central N1s−2 as well as with the O1s−2 dicationic states, were extracted showing the two electrons emitted as a result of filling the double core holes. The spectra, which are interpreted using CASSCF and complete active space configuration interaction calculations, show atomic-like character. The cross section ratio between double and single core hole creation was estimated as 1.6 × 10−3 for nitrogen at 1100 eV and as 1.3 × 10−3 for oxygen at 1300 eV.
Validation and evaluation of a novel time-resolved laser-induced fluorescence technique
Authors: Durot C. J. , Gallimore A. D. , Smith T. B.
Published: January 2014

We present a novel technique to measure time-resolved laser-induced fluorescence signals in plasma sources that have a relatively constant Fourier spectrum of oscillations in steady-state operation, but are not periodically pulsed, e.g., Hall thrusters. The technique uses laser modulation of the order of MHz and recovers signal via a combination of band-pass filtering, phase-sensitive detection, and averaging over estimated transfer functions calculated for many different cycles of the oscillation. Periodic discharge current oscillations were imposed on a hollow cathode. Measurements were validated by comparison with independent measurements from a lock-in amplifier and by comparing the results of the transfer function average to an independent analysis technique triggering averaging over many oscillation cycles in the time domain. The performance of the new technique is analyzed and compared to prior techniques, and it is shown that this new technique has a niche in measurements where the analog photomultiplier signal has a nonwhite noise spectral density and cycles of oscillation are not sufficiently repeatable to allow for reliable triggering or a meaningful average waveform in the time domain.
Resonance ionization laser ion sources for on-line isotope separators (invited)
Authors: Marsh B. A.
Published: January 2014

A Resonance Ionization Laser Ion Source (RILIS) is today considered an essential component of the majority of Isotope Separator On Line (ISOL) facilities; there are seven laser ion sources currently operational at ISOL facilities worldwide and several more are under development. The ionization mechanism is a highly element selective multi-step resonance photo-absorption process that requires a specifically tailored laser configuration for each chemical element. For some isotopes, isomer selective ionization may even be achieved by exploiting the differences in hyperfine structures of an atomic transition for different nuclear spin states. For many radioactive ion beam experiments, laser resonance ionization is the only means of achieving an acceptable level of beam purity without compromising isotope yield. Furthermore, by performing element selection at the location of the ion source, the propagation of unwanted radioactivity downstream of the target assembly is reduced. Whilst advances in laser technology have improved the performance and reliability of laser ion sources and broadened the range of suitable commercially available laser systems, many recent developments have focused rather on the laser/atom interaction region in the quest for increased selectivity and/or improved spectral resolution. Much of the progress in this area has been achieved by decoupling the laser ionization from competing ionization processes through the use of a laser/atom interaction region that is physically separated from the target chamber. A new application of gas catcher laser ion source technology promises to expand the capabilities of projectile fragmentation facilities through the conversion of otherwise discarded reaction fragments into high-purity low-energy ion beams. A summary of recent RILIS developments and the current status of laser ion sources worldwide is presented.
Molecular dispersion energy parameters for alkali and halide ions in aqueous solution
Authors: Reiser S. , Deublein S. , Vrabec J. , Hasse H.
Published: January 2014

Thermodynamic properties of aqueous solutions containing alkali and halide ions are determined by molecular simulation. The following ions are studied: Li+, Na+, K+, Rb+, Cs+, F−, Cl−, Br−, and I−. The employed ion force fields consist of one Lennard-Jones (LJ) site and one concentric point charge with a magnitude of ±1 e. The SPC/E model is used for water. The LJ size parameter of the ion models is taken from Deublein et al. [J. Chem. Phys. 136, 084501 (2012)], while the LJ energy parameter is determined in the present study based on experimental self-diffusion coefficient data of the alkali cations and the halide anions in aqueous solutions as well as the position of the first maximum of the radial distribution function of water around the ions. On the basis of these force field parameters, the electric conductivity, the hydration dynamics of water molecules around the ions, and the enthalpy of hydration is predicted. Considering a wide range of salinity, this study is conducted at temperatures of 293.15 and 298.15 K and a pressure of 1 bar.
A new method for investigating oxidation behavior of liquid metals
Authors: Elahi S. Hossein , Abdi Hadi , Shahverdi H. R.
Published: January 2014

A novel method was introduced to investigate oxidation behavior of melts in different conditions such as flux, alloying element, atmosphere, etc. This approach was based on rheology properties of melts, in which by tracing viscosity variations during oxidation, the behavior of melt can be explored. A modified falling ball viscometer was proposed and used for viscometry. Oxidation of aluminum and Al–Mg alloys was studied using this method. According to the results, it was found that this method was not only a proper approach for investigating oxidation behavior but also could be used as a compliment along conventional approaches.
Design and construction of a cryogenic distillation device for removal of krypton for liquid xenon dark matter detectors
Authors: Wang Zhou , Bao Lei , Hao Xihuan , Ju Yonglin
Published: January 2014

Liquid xenon (Xe) is one of the commendable detecting media for the dark matter detections. However, the small content of radioactive krypton-85 (85Kr) always exists in the commercial xenon products. An efficient cryogenic distillation system to remove this krypton (Kr) from commercial xenon products has been specifically designed, developed, and constructed in order to meet the requirements of the dark matter experiments with high- sensitivity and low-background. The content of krypton in regular commercial xenon products can be reduced from 10−9 to 10−12, with 99% xenon collection efficiency at maximum flow rate of 5 kg/h (15SLPM). The purified xenon gases produced by this distillation system can be used as the detecting media in the project of Panda X, which is the first dark matter detector developed in China.
Ultralow sensing limit in optofluidic micro-bottle resonator biosensor by self-referenced differential-mode detection scheme
Authors: Zhang Xingwang , Liu Liying , Xu Lei
Published: January 2014

Biosensors based on optofluidic micro-bottle resonators are demonstrated. A self-referenced sensing scheme, differential mode sensing, is proposed and used to substantially suppress environmental noises and reach an ultralow noise equal detection limit of 10 fg/ml (∼0.15 fM) for bovine serum albumin molecules. This sensing scheme has high compatibility with any other microcavity sensors to get lower biosensing limit.
Hybrid rotary-translational vibration energy harvester using cycloidal motion as a mechanical amplifier
Authors: Moss Scott D. , Hart Genevieve A. , Burke Stephen K. , Carman Gregory P.
Published: January 2014

This paper reports on a hybrid rotary-translational vibration energy harvesting approach that exploits cycloidal motion to achieve a relatively high power density from an oscillatory kinetic energy harvester operating at frequencies below 10 Hz. The approach uses a rolling magnetic sphere. The rolling motion mechanically amplifies the velocity at which the magnetic pole of the sphere passes a nearby coil transducer, inducing a proportionally larger electro-motive force across the coil. A prototype cycloidal energy harvester is shown to produce a peak power of 201 mW from a host vibration of 500 mg rms at 5.4 Hz.
Electronic stopping powers for heavy ions in SiC and SiO2
Authors: Jin K. , Zhang Y. , Zhu Z. , Grove D. A. , Xue H. , Xue J. , Weber W. J.
Published: January 2014

Accurate information on electronic stopping power is fundamental for broad advances in materials science, electronic industry, space exploration, and sustainable energy technologies. In the case of slow heavy ions in light targets, current codes and models provide significantly inconsistent predictions, among which the Stopping and Range of Ions in Matter (SRIM) code is the most commonly used one. Experimental evidence, however, has demonstrated considerable errors in the predicted ion and damage profiles based on SRIM stopping powers. In this work, electronic stopping powers for Cl, Br, I, and Au ions are experimentally determined in two important functional materials, SiC and SiO2, based on a single ion technique, and new electronic stopping power values are derived over the energy regime from 0 to 15 MeV, where large deviations from the SRIM predictions are observed. As an experimental validation, Rutherford backscattering spectrometry (RBS) and secondary ion mass spectrometry (SIMS) are utilized to measure the depth profiles of implanted Au ions in SiC for energies from 700 keV to 15 MeV. The measured ion distributions by both RBS and SIMS are considerably deeper than the SRIM predictions, but agree well with predictions based on our derived stopping powers.
Observation of strontium segregation in LaAlO3/SrTiO3 and NdGaO3/SrTiO3 oxide heterostructures by X-ray photoemission spectroscopy
Authors: Treske Uwe , Heming Nadine , Knupfer Martin , Büchner Bernd , Koitzsch Andreas , Di Gennaro Emiliano , Scotti di Uccio Umberto , Miletto Granozio Fabio , Krause Stefan
Published: January 2014

LaAlO3 and NdGaO3 thin films of different thicknesses have been grown by pulsed laser deposition on TiO2-terminated SrTiO3 single crystals and investigated by soft X-ray photoemission spectroscopy. The surface sensitivity of the measurements has been tuned by varying photon energy hν and emission angle Θ. In contrast to the core levels of the other elements, the Sr 3d line shows an unexpected splitting for higher surface sensitivity, signaling the presence of a second strontium component. From our quantitative analysis we conclude that during the growth process Sr atoms diffuse away from the substrate and segregate at the surface of the heterostructure, possibly forming strontium oxide.
Discovery of multiple, ionization-created CS2 anions and a new mode of operation for drift chambers
Authors: Snowden-Ifft Daniel P.
Published: January 2014

This paper focuses on the surprising discovery of multiple species of ionization-created CS2 anions in gas mixtures containing electronegative CS2 and O2, identified by their slightly different drift velocities. Data are presented to understand the formation mechanism and identity of these new anions. Regardless of the micro-physics, however, this discovery offers a new, trigger-less mode of operation for the drift chambers. A demonstration of trigger-less operation is presented.
Enhanced magnetoelectric effect in longitudinal-longitudinal mode laminate with cofired interdigitated electrodes
Authors: Yan Yongke , Zhou Yuan , Priya Shashank
Published: January 2014

In this letter, we describe a low-temperature cofiring technique to embed the interdigitated electrodes (IDE) inside the piezoelectric ceramic layer to achieve longitudinally (L) poled magnetoelectric (ME) laminates. The ME voltage coefficient of this L-L mode composite was found to be 4.41 V cm−1 Oe−1 at 1 kHz, which is 340% higher than that of L-T (transversal) mode. IDE based longitudinal poling and piezoelectric response was evaluated by impedance changes and piezoresponse force microscopy, illustrating the effect of inactive/ineffective zone in the range beneath and above the fingers with vanishing fields. At an electric field of 50 V/cm, the composite was found to generate magnetic field on the order of 175 μG at 1 kHz.
Chemical bonding, optical constants, and electrical resistivity of sputter-deposited gallium oxide thin films
Authors: Ramana C. V. , Rubio E. J. , Barraza C. D. , Miranda Gallardo A. , McPeak Samantha , Kotru Sushma , Grant J. T.
Published: January 2014

Gallium oxide (Ga2O3) thin films were made by sputter deposition employing a Ga2O3 ceramic target for sputtering. The depositions were made over a wide range of substrate temperatures (Ts), from 25 to 600 °C. The effect of Ts on the chemical bonding, surface morphological characteristics, optical constants, and electrical properties of the grown films was evaluated using X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), spectroscopic ellipsometry (SE), and four-point probe measurements. XPS analyses indicate the binding energies (BE) of the Ga 2p doublet, i.e., the Ga 2p3/2 and Ga 2p1/2 peaks, are located at 1118.0 and 1145.0 eV, respectively, characterizing gallium in its highest chemical oxidation state (Ga3+) in the grown films. The core level XPS spectra of O 1s indicate that the peak is centered at a BE ∼ 531 eV, which is also characteristic of Ga-O bonds in the Ga2O3 phase. The granular morphology of the nanocrystalline Ga2O3 films was evident from AFM measurements, which also indicate that the surface roughness of the films increases from 0.5 nm to 3.0 nm with increasing Ts. The SE analyses indicate that the index of refraction (n) of Ga2O3 films increases with increasing Ts due to improved structural quality and packing density of the films. The n(λ) of all the Ga2O3 films follows the Cauchy's dispersion relation. The room temperature electrical resistivity was high (∼200 Ω-cm) for amorphous Ga2O3 films grown at Ts = RT-300 °C and decreased to ∼1 Ω-cm for nanocrystalline Ga2O3 films grown at Ts ≥ 500–600 °C. A correlation between growth conditions, microstructure, optical constants, and electrical properties of Ga2O3 films is derived.
Comparison of graphite materials for targets of laser ion source
Authors: Fuwa Y. , Ikeda S. , Kumaki M. , Sekine M. , Munemoto N. , Cinquegrani D. , Romanelli M. , Kanesue T. , Okamura M. , Iwashita Y.
Published: January 2014

To investigate efficient graphite material for carbon ion production in laser ion source, the plasma properties produced from these materials are measured. Comparing acquired current profile and charge state distribution, the distributions of ions in laser induced plasma from isotropic graphite and single crystal of graphite are different. The produced quantity of C6+ from isotropic materials is larger than that from single crystal.
Efficient terahertz wave generation from GaP crystals pumped by chirp-controlled pulses from femtosecond photonic crystal fiber amplifier
Authors: Li Jiang , Chai Lu , Shi Junkai , Liu Bowen , Xu Baozhong , Hu Minglie , Li Yanfeng , Xing Qirong , Wang Chingyue , Fedotov Andrey B. , Zheltikov Aleksei M.
Published: January 2014

A chirp-tunable femtosecond 10 W, 42 MHz photonic-crystal-fiber oscillator-amplifier system that is capable of delivering sub-60 fs light pulses at 1040 nm is used to demonstrate high-efficiency terahertz radiation generation via optical rectification in GaP crystals only a few millimeters in length. The optimization of the chirp of the fiber-laser pulses is shown to radically enhance the terahertz output, indicating one possible way to more efficiently use these extended nonlinear crystals in compact fiber-pumped terahertz radiation sources.
A bootstrapped, low-noise, and high-gain photodetector for shot noise measurement
Authors: Zhou Haijun , Yang Wenhai , Li Zhixiu , Li Xuefeng , Zheng Yaohui
Published: January 2014

We presented a low-noise, high-gain photodetector based on the bootstrap structure and the L-C (inductance and capacitance) combination. Electronic characteristics of the photodetector, including electronic noise, gain and frequency response, and dynamic range, were verified through a single-frequency Nd:YVO4 laser at 1064 nm with coherent output. The measured shot noise of 50 μW laser was 13 dB above the electronic noise at the analysis frequency of 2 MHz, and 10 dB at 3 MHz. And a maximum clearance of 28 dB at 2 MHz was achieved when 1.52 mW laser was illuminated. In addition, the photodetector showed excellent linearities for both DC and AC amplifications in the laser power range between 12.5 μW and 1.52 mW.
Magnetic shielding of a laboratory Hall thruster. I. Theory and validation
Authors: Mikellides Ioannis G. , Katz Ira , Hofer Richard R. , Goebel Dan M.
Published: January 2014

We demonstrate a technique by which erosion of the acceleration channel in Hall thrusters can be reduced by at least a few orders of magnitude. The first principles of the technique, now known as “magnetic shielding,” have been derived based on the findings of 2-D numerical simulations. The simulations, in turn, guided the modification of an existing 6-kW laboratory Hall thruster to test the theory and are the main subject of this Part I article. Part II expands on the results of the experiments. Near the walls of the magnetically shielded (MS) thruster theory and experiment agree that (1) the plasma potential has been sustained at values near the discharge voltage, and (2) the electron temperature has been lowered compared to the unshielded thruster. Erosion rates deduced directly from the wall probes show reductions of at least ∼3 orders of magnitude at the MS inner wall when an ion energy threshold of 30.5 V is used in the sputtering yield model of the channel material. At the outer wall the probes reveal that the ion energy was below the assumed threshold. Using a threshold of 25 V, the simulations predict a minimum reduction of ∼600 at the MS inner wall. At the MS outer wall ion energies are found to be below 25 V. When a 50-V threshold is used the computed ion energies are below the threshold at both sides of the channel. Uncertainties, sensitivities, and differences between theory and experiment are also discussed. The elimination of wall erosion in Hall thrusters solves a problem that has remained unsettled for more than five decades.
Interdot Coulomb correlation effects and spin-orbit coupling in two carbon nanotube quantum dots
Authors: Wang Zhen-Hua , Kuang Xiao-Yu , Zhong Ming-Min , Shao Peng , Li Hui
Published: January 2014

Transport properties of the two-level Kondo effect involving spin, orbital, and pseudospin degrees of freedom are examined in a parallel carbon nanotube double quantum dot with a sufficient interdot Coulomb interaction and small interdot tunneling. The interdot Coulomb correlation effects are taken into account, and it plays an important role in forming bonding and antibonding states. Attached to ferromagnetic leads, the Kondo effect is observed at the interdot Coulomb blockade region with degeneracy of spin, orbital, and pseudospin degrees of freedom. A crossover from a two-level Kondo state involving the fivefold degeneracy of the double quantum dots to an SU(4) spin-orbit Kondo state and to an SU(2) spin-Kondo effect is demonstrated. At finite magnetic field, the splitting of the spin, orbital, and pseudospin Kondo resonance can be restored. For finite intradot Coulomb interaction U, there is a competition between the single-dot Kondo effect and the antiferromagnetic exchange coupling JAFM, resulting in the suppression of the Kondo resonance. Moreover, both the JAFM and the Zeeman interactions compete, leading to need a much higher value of the magnetic field to compensate for the Kondo splitting.
Taheri-Saramad x-ray detector (TSXD): A novel high spatial resolution x-ray imager based on ZnO nano scintillator wires in polycarbonate membrane
Authors: Taheri A. , Saramad S. , Ghalenoei S. , Setayeshi S.
Published: January 2014

A novel x-ray imager based on ZnO nanowires is designed and fabricated. The proposed architecture is based on scintillation properties of ZnO nanostructures in a polycarbonate track-etched membrane. Because of higher refractive index of ZnO nanowire compared to the membrane, the nanowire acts as an optical fiber that prevents the generated optical photons to spread inside the detector. This effect improves the spatial resolution of the imager. The detection quantum efficiency and spatial resolution of the fabricated imager are 11% and <6.8 μm, respectively.
Thermal diffusivity measurements using insulating and isothermal boundary conditions
Authors: Shekhani H. N. , Uchino K.
Published: January 2014

Many methods exist to measure thermal diffusivity using either steady state or transient techniques. Steady state methods yield large experimental error and inaccuracies. Transient techniques, namely, the laser flash method, are expensive and require specialized equipment and advanced data analysis. In this paper, a novel experimental setup is devised to evaluate thermal diffusivity. In this experiment hot isothermal and insulating boundary conditions are imposed on a flat disk sample. The transient temperature profile of the insulated side of the sample is analytically similar to a classic time constant formulation. The thermal diffusivity is proportional to the inverse time constant. This method hosts a variety of advantages over other methods such as accuracy comparable to other methods, low cost, integrated modeling of interface effects, and small sample size. Several materials with low to medium thermal diffusivity (0.1 → 3 mm2/s) have been measured. The diameter of the sample is 32 mm and its thickness ranges from 2 to 6.5 mm. The thermal diffusivity measurements in this experiment have an accuracy of 5% or better in comparison to the literature values.
Line-narrowing effects in the near-infrared spectrum of water and precision determination of spectroscopic parameters
Authors: Amodio Pasquale , Moretti Luigi , Castrillo Antonio , Gianfrani Livio
Published: January 2014

A dual laser absorption spectrometer operating at 1.39 μm was employed to acquire high-quality absorption spectra in coincidence with the 44,1 → 44,0 line of the H218O ν1 + ν3 band, at the temperature of the triple point of water. A rather sophisticated global fitting procedure was developed and used to simultaneously fit spectra across the explored pressure range, roughly between 1 and 4 Torr. Adopting the speed-dependent Galatry profile with a hypergeometric dependence of the collisional width and shift on the absorbers' velocity, we could take into account narrowing effects ascribed to collision-induced velocity changes and speed-dependence of relaxation rates. The global approach allowed us to implement and exploit some relevant physical constraints at the aim of reducing significantly statistical correlation issues among free parameters. Consequently, we demonstrated the possibility of determining pressure broadening and shifting parameters with high precision and accuracy, at the level of one part over 103. Furthermore, even being in the Doppler regime, it was possible to determine the effective frequency of velocity-changing collisions for self-colliding H218O molecules.
Thermal boundary layer near roughnesses in turbulent Rayleigh-Bénard convection: Flow structure and multistability
Authors: Salort J. , Liot O. , Rusaouen E. , Seychelles F. , Tisserand J.-C. , Creyssels M. , Castaing B. , Chillà F.
Published: January 2014

We present global heat-transfer and local temperature measurements, in an asymmetric parallelepiped Rayleigh-Bénard cell, in which controlled square-studs roughnesses have been added. A global heat transfer enhancement arises when the thickness of the boundary layer matches the height of the roughnesses. The enhanced regime exhibits an increase of the heat transfer scaling. Local temperature measurements have been carried out in the range of parameters where the enhancement of the global heat transfer is observed. They show that the boundary layer at the top of the square-stub roughness is thinner than the boundary layer of a smooth plate, which accounts for most of the heat-transfer enhancement. We also report multistability at long time scales between two enhanced heat-transfer regimes. The flow structure of both regimes is imaged with background-oriented synthetic Schlieren and reveals intermittent bursts of coherent plumes.
Statistical and hydrodynamic properties of double-ring polymers with a fixed linking number between twin rings
Authors: Uehara Erica , Deguchi Tetsuo
Published: January 2014

For a double-ring polymer in solution we evaluate the mean-square radius of gyration and the diffusion coefficient through simulation of off-lattice self-avoiding double polygons consisting of cylindrical segments with radius rex of unit length. Here, a self-avoiding double polygon consists of twin self-avoiding polygons which are connected by a cylindrical segment. We show numerically that several statistical and dynamical properties of double-ring polymers in solution depend on the linking number of the constituent twin ring polymers. The ratio of the mean-square radius of gyration of self-avoiding double polygons with zero linking number to that of no topological constraint is larger than 1, in particular, when the radius of cylindrical segments rex is small. However, the ratio is almost constant with respect to the number of vertices, N, and does not depend on N. The large-N behavior of topological swelling is thus quite different from the case of knotted random polygons.
Spin-filtering, giant magnetoresistance, rectifying and negative differential resistance effects in planar four-coordinate Fe complex with graphene nanoribbon electrodes
Authors: Zhao P. , Wu Q. H. , Liu D. S. , Chen G.
Published: January 2014

By using the nonequilibrium Green's function formalism combined with the density functional theory, we have investigated the spin-polarized transport properties of a planar four-coordinate Fe complex sandwiched between two zigzag-edge graphene nanoribbon (ZGNR) electrodes, where the ZGNRs are modulated by external magnetic field. The results show that the system can exhibit perfect dual spin-filtering and spin-rectifying effects at a wide bias range, giant magnetoresistance effect with large magnetoresistance ratio at small bias, and obvious negative differential resistance behavior. The mechanisms are proposed for these phenomena.
Effect of thermal motion on catalytic activity of nanoparticles in polar solvent
Authors: Bonnet Nicéphore , Sugino Osamu , Otani Minoru
Published: January 2014

In this study, we propose that electrode potential fluctuations due to the thermal motion of the solvent may serve to enhance the catalytic activity of nanostructures. The proposed model uses a simple, Marcus-type treatment of the statistical behavior of the solvent and the Butler-Volmer law for the instantaneous catalytic rate as a function of the electrode potential. The rapid development of probing techniques with high spatial and temporal resolution will help to further confirm and characterize the dynamical properties of nanostructures.
Vortex-induced vibration of a circular cylinder of finite length
Authors: Zhao Ming , Cheng Liang
Published: January 2014

Vortex-induced vibration (VIV) of a rigid circular cylinder of finite length subject to uniform steady flow is investigated numerically. The study is focused on the effect of the free end on the response of the cylinder. The vibration of the cylinder is confined only in the cross-flow direction. Three-dimensional Navier-Stokes equations are solved by the Petrov-Galerkin finite element method and the equation of the motion is solved for the cylinder displacement. Simulations are conducted for a constant mass ratio of 2, a constant Reynolds number of 300 and cylinder length to diameter ratios of L/D = 1, 2, 5 10, and 20. It is found that the vortex shedding in the wake of a fixed cylinder is suppressed if the cylinder length is less than 2 cylinder diameters. However, if the cylinder is allowed to vibrate, VIV happens at L/D = 1 and 2 and the response amplitudes at these two cylinder lengths are comparable with that of a 2D-cylinder. The vortices that are shed from a short cylinder of L/D = 1 and 2 are found to be generated from the free-end of the cylinder and convected toward the top end of the cylinder by the upwash velocity. They are found to be nearly perpendicular to the cylinder span. The wake flow in a vibrating cylinder with L/D greater than 5 includes the vortex shedding flow at the top part of the cylinder and the end-induced vortex shedding near the free-end of the cylinder. The phase difference between the sectional lift coefficient and the vibration displacement near the free-end of the cylinder changes from 0° to 180° at higher reduced velocity than that near the top end. Strong variation of the flow along the cylinder span occurs at reduced velocities where the lift coefficient near the free-end and that near the top end are in anti-phase with each other.
Linear stability analysis of miscible two-fluid flow in a channel with velocity slip at the walls
Authors: Ghosh Sukhendu , Usha R. , Sahu Kirti Chandra
Published: January 2014

The linear stability characteristics of pressure-driven miscible two-fluid flow with same density and varying viscosities in a channel with velocity slip at the wall are examined. A prominent feature of the instability is that only a band of wave numbers is unstable whatever the Reynolds number is, whereas shorter wavelengths and smaller wave numbers are observed to be stable. The stability characteristics are different from both the limiting cases of interface dominated flows and continuously stratified flows in a channel with velocity slip at the wall. The flow system is destabilizing when a more viscous fluid occupies the region closer to the wall with slip. For this configuration a new mode of instability, namely the overlap mode, appears for high mass diffusivity of the two fluids. This mode arises due to the overlap of critical layer of dominant instability with the mixed layer of varying viscosity. The critical layer contains a location in the flow domain at which the base flow velocity equals the phase speed of the most unstable disturbance. Such a mode also occurs in the corresponding flow in a rigid channel, but absent in either of the above limiting cases of flow in a channel with slip. The flow is unstable at low Reynolds numbers for a wide range of wave numbers for low mass diffusivity, mimicking the interfacial instability of the immiscible flows. A configuration with less viscous fluid adjacent to the wall is more stable at moderate miscibility and this is also in contrast with the result for the limiting case of interface dominated flows in a channel with slip, where the above configuration is more unstable. It is possible to achieve stabilization or destabilization of miscible two-fluid flow in a channel with wall slip by appropriately choosing the viscosity of the fluid layer adjacent to the wall. In addition, the velocity slip at the wall has a dual role in the stability of flow system and the trend is influenced by the location of the mixed layer, the location of more viscous fluid and the mass diffusivity of the two fluids. It is well known that creating a viscosity contrast in a particular way in a rigid channel delays the occurrence of turbulence in a rigid channel. The results of the present study show that the flow system can be either stabilized or destabilized by designing the walls of the channel as hydrophobic surfaces, modeled by velocity slip at the walls. The study provides another effective strategy to control the flow system.
Water confinement in nanoporous silica materials
Authors: Renou Richard , Szymczyk Anthony , Ghoufi Aziz
Published: January 2014

The influence of the surface polarity of cylindrical silica nanopores and the presence of Na+ ions as compensating charges on the structure and dynamics of confined water has been investigated by molecular dynamics simulations. A comparison between three different matrixes has been included: a protonated nanopore (PP, with SiOH groups), a deprotonated material (DP, with negatively charged surface groups), and a compensated-charge framework (CC, with sodium cations compensating the negative surface charge). The structure of water inside the different pores shows significant differences in terms of layer organization and hydrogen bonding network. Inside the CC pore the innermost layer is lost to be replaced by a quasi bulk phase. The electrostatic field generated by the DP pore is felt from the surface to the centre of pore leading to a strong orientation of water molecules even in the central part of the pore. Water dynamics inside both the PP and DP pores shows significant differences with respect to the CC pore in which the sub-diffusive regime of water is lost for a superdiffusive regime.
Versatile attosecond beamline in a two-foci configuration for simultaneous time-resolved measurements
Authors: Locher R. , Lucchini M. , Herrmann J. , Sabbar M. , Weger M. , Ludwig A. , Castiglioni L. , Greif M. , Hengsberger M. , Gallmann L. , Keller U.
Published: January 2014

We present our attoline which is a versatile attosecond beamline at the Ultrafast Laser Physics Group at ETH Zurich for attosecond spectroscopy in a variety of targets. High-harmonic generation (HHG) in noble gases with an infrared (IR) driving field is employed to generate pulses in the extreme ultraviolet (XUV) spectral regime for XUV-IR cross-correlation measurements. The IR pulse driving the HHG and the pulse involved in the measurements are used in a non-collinear set-up that gives independent access to the different beams. Single attosecond pulses are generated with the polarization gating technique and temporally characterized with attosecond streaking. This attoline contains two target chambers that can be operated simultaneously. A toroidal mirror relay-images the focus from the first chamber into the second one. In the first interaction region a dedicated double-target allows for a simple change between photoelectron/photoion measurements with a time-of-flight spectrometer and transient absorption experiments. Any end station can occupy the second interaction chamber. A surface analysis chamber containing a hemispherical electron analyzer was employed to demonstrate successful operation. Simultaneous RABBITT measurements in two argon jets were recorded for this purpose.
Detectivity enhancement in THz electrooptical sampling
Authors: Ahmed Saima , Savolainen Janne , Hamm Peter
Published: January 2014

We demonstrate and discuss a simple scheme that significantly enhances the detectivity of THz electro-optical sampling by introducing a sequence of Brewster windows that increases the ellipticity of the probe beam. By varying the window material or the number of Brewster windows, the enhancement factor can be adjusted; we demonstrate an enhancement factor of ≈20 with four ZnSe Brewster windows. The scheme is particularly useful when very small THz fields are to be measured in connection with low-repetition rate amplified Ti:S laser systems.
Numerical simulations of sink-flow boundary layers over rough surfaces
Authors: Yuan J. , Piomelli U.
Published: January 2014

Turbulent sink flows over smooth or rough walls with sand-grain roughness are studied using large-eddy and direct numerical simulations. Mild and strong levels of acceleration are applied, yielding a wide range of Reynolds number (Reθ = 372 − 2748) and cases close to the reverse-transitional state. Flow acceleration and roughness are shown to exert opposite effects on boundary-layer integral parameters, on the Reynolds stresses, budgets of turbulent kinetic energy, and properties of turbulent structures in the vicinity of the rough surface; statistics exhibit similarity when plotted using inner scaling for cases with the same roughness Reynolds number, k+. Acceleration leads to a decrease of k+, while roughness increases it. For cases with higher k+, the low-speed streaks become destabilized, and turbulent structures near the wall are distributed more uniformly in the wall-parallel plane; they are less extended in the streamwise direction, but more densely packed. Higher k+ also causes decorrelation of the outer-layer hairpin packets with the near-wall structures, probably due to the direct impact of random roughness elements on the hairpin legs. Wall-similarity applies for the fully turbulent cases, in which the outer-layer turbulent statistics are affected by acceleration only. It is shown that being in the hydraulically smooth regime is a necessary condition for reverse-transition, supporting the idea that relaminarization starts from the inner region, where roughness effects dominate.
Dissociation of internal energy-selected methyl bromide ion revealed from threshold photoelectron-photoion coincidence velocity imaging
Authors: Tang Xiaofeng , Zhou Xiaoguo , Sun Zhongfa , Liu Shilin , Liu Fuyi , Sheng Liusi , Yan Bing
Published: January 2014

Dissociative photoionization of methyl bromide (CH3Br) in an excitation energy range of 10.45–16.90 eV has been investigated by using threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging. The coincident time-of-flight mass spectra indicate that the ground state X2E of CH3Br+ is stable, and both A2A1 and B2E ionic excited states are fully dissociative to produce the unique fragment ion of CH3+. From TPEPICO 3D time-sliced velocity images of CH3+ dissociated from specific state-selected CH3Br+ ion, kinetic energy release distribution (KERD) and angular distribution of CH3+ fragment ion are directly obtained. Both spin-orbit states of Br(2P) atom can be clearly observed in fast dissociation of CH3Br+(A2A1) ion along C–Br rupture, while a KERD of Maxwell-Boltzmann profile is obtained in dissociation of CH3Br+(B2E) ion. With the aid of the re-calculated potential energy curves of CH3Br+ including spin-orbit coupling, dissociation mechanisms of CH3Br+ ion in A2A1 and B2E states along C–Br rupture are revealed. For CH3Br+(A2A1) ion, the CH3+ + Br(2P1/2) channel is occurred via an adiabatic dissociation by vibration, while the Br(2P3/2) formation is through vibronic coupling to the high vibrational level of X2E state followed by rapid dissociation. C–Br bond breaking of CH3Br+(B2E) ion can occur via slow internal conversion to the excited vibrational level of the lower electronic states and then dissociation.
Hybrid Monte Carlo with non-uniform step size
Authors: Holzgräfe Christian , Bhattacherjee Arnab , Irbäck Anders
Published: January 2014

The Hybrid Monte Carlo method offers a rigorous and potentially efficient approach to the simulation of dense systems, by combining numerical integration of Newton's equations of motion with a Metropolis accept-or-reject step. The Metropolis step corrects for sampling errors caused by the discretization of the equations of motion. The integration is usually performed using a uniform step size. Here, we present simulations of the Lennard-Jones system showing that the use of smaller time steps in the tails of each integration trajectory can reduce errors in energy. The acceptance rate is 10–15 percentage points higher in these runs, compared to simulations with the same trajectory length and the same number of integration steps but a uniform step size. We observe similar effects for the harmonic oscillator and a coarse-grained peptide model, indicating generality of the approach.
Size-dependent mobility of gold nano-clusters during growth on chemically modified graphene
Authors: Bell Gavin R. , Dawson Peter M. , Pandey Priyanka A. , Wilson Neil R. , Mulheran Paul A.
Published: January 2014

Gold nano-clusters were grown on chemically modified graphene by direct sputter deposition. Transmission electron microscopy of the nano-clusters on these electron-transparent substrates reveals an unusual bimodal island size distribution (ISD). A kinetic Monte Carlo model of growth incorporating a size-dependent cluster mobility rule uniquely reproduces the bimodal ISD, providing strong evidence for the mobility of large clusters during surface growth. The cluster mobility exponent of −5/3 is consistent with cluster motion via one-dimensional diffusion of gold atoms around the edges of the nano-clusters.
High Mg-content wurtzite MgZnO alloys and their application in deep-ultraviolet light-emitters pumped by accelerated electrons
Authors: Ni Pei-Nan , Shan Chong-Xin , Li Bing-Hui , Shen De-Zhen
Published: January 2014

High Mg-content single-phase wurtzite MgZnO alloys with a bandgap of 4.35 eV have been obtained on sapphire substrate by introducing a composition-gradient MgxZn1−xO buffer layer. By employing the accelerated electrons obtained in a solid-state structure as an excitation source, an emission at around 285 nm, which is originated from the near-band-edge emission of the Mg0.51Zn0.49O active layer, has been observed. The results reported in this paper may provide a promising route to high performance deep-ultraviolet light-emitting devices by bypassing the challenging doping issues of wide bandgap semiconductors.
Near-field optical effect of a core-shell nanostructure in proximity to a flat surface
Authors: Cui Wenping , Li Mingda , Dai Zuyang , Meng Qingping , Zhu Yimei
Published: January 2014

We provide an analytical solution for studying the near-field optical effect of a core-shell nanostructure in proximity to a flat surface, within quasi-static approximation. The distribution of electrostatic potential and the field enhancement in this complex geometry are obtained by solving a set of linear equations. This analytical result can be applied to a wide range of systems associated with near-field optics and surface plasmon polaritons. To illustrate the power of this technique, we study the field-attenuation effect of an oxidized shell in a silver tip in a near-field scanning microscope. The thickness of oxidized layer can be monitored by measuring the intensity of light. We also find a linear relation between resonant frequency and temperature in an Ag–Au core-shell structure, which provides insight for local temperature detection with nm scale resolution. Our results also show good agreement with recent finite element method results.