bluuebunny/crossref_metadata_2025 huggingface dataset:
Immune Cell Infiltration in Malignant Middle Cerebral Artery Infarction: Comparison with Transient Cerebral Ischemia
Authors: Chu Hannah X , Kim Hyun Ah , Lee Seyoung , Moore Jeffrey P , Chan Christopher T , Vinh Antony , Gelderblom Mathias , Arumugam Thiruma V , Broughton Brad RS , Drummond Grant R , Sobey Christopher G
Published: December 2013

We tested whether significant leukocyte infiltration occurs in a mouse model of permanent cerebral ischemia. C57BL6/J male mice underwent either permanent (3 or 24 hours) or transient (1 or 2 hours+22- to 23-hour reperfusion) middle cerebral artery occlusion (MCAO). Using flow cytometry, we observed ∼15,000 leukocytes (CD45+high cells) in the ischemic hemisphere as early as 3 hours after permanent MCAO (pMCAO), comprising ∼40% lymphoid cells and ∼60% myeloid cells. Neutrophils were the predominant cell type entering the brain, and were increased to ∼5,000 as early as 3 hours after pMCAO. Several cell types (monocytes, macrophages, B lymphocytes, CD8+ T lymphocytes, and natural killer cells) were also increased at 3 hours to levels sustained for 24 hours, whereas others (CD4+ T cells, natural killer T cells, and dendritic cells) were unchanged at 3 hours, but were increased by 24 hours after pMCAO. Immunohistochemical analysis revealed that leukocytes typically had entered and widely dispersed throughout the parenchyma of the infarct within 3 hours. Moreover, compared with pMCAO, there were ∼50% fewer infiltrating leukocytes at 24 hours after transient MCAO (tMCAO), independent of infarct size. Microglial cell numbers were bilaterally increased in both models. These findings indicate that a profound infiltration of inflammatory cells occurs in the brain early after focal ischemia, especially without reperfusion.
Tautomeric preference in polymorphs and pseudopolymorphs of succinylsulfathiazole: fast evaporation screening and thermal studies
Authors: Bag Partha Pratim , Kothur Raghuram Reddy , Malla Reddy C.
Published: December 2013

We employ a fast evaporation crystallization method to identify two new solvates from acetone and tetrahydrofuran of succinylsulfathiazole (SST), as well as determined single crystal structures for previous unsolved forms. SST adopts exclusively the imidine form in all its crystalline forms.
Fast screening of bacterial suspension culture conditions on chips
Authors: Tang Yunfang , Gan Mingzhe , Xie Yifei , Li Xudong , Chen Liwei
Published: December 2013

We present chips for the screening of bacterial suspension culture conditions including pH, nutrient content and temperature.
Critical anomalies of alkaline fading of phenolphthalein in the critical solution of 2-butoxyethanol + water
Authors: Du Zhongyu , Yin Handi , Hao Zhiguo , Zheng Peizhu , Shen Weiguo
Published: December 2013

We have used three-wavelength UV-spectrophotometry to study the reaction of the alkaline fading of phenolphthalein in the critical solution of 2-butoxyethanol + water. It was found that when the temperature was far away from the critical point, the values of the natural logarithm of the rate constant k and the natural logarithm of the chemical equilibrium K determined in our experiments had good linear relationships with the reciprocal of temperature, which served as the backgrounds and were used for correcting k and K in the critical region. The critical slowing down of the reaction and the critical anomaly of the chemical equilibrium were detected near the critical point. The value of the critical exponent characterizing the slowing down effect of the reaction rate was obtained to be 0.156, which was close to the value 0.11 associated with the heat capacity divergence and agreed with the theoretical prediction. The experimental result also confirmed the theoretical prediction of 0.11 for the critical exponent characterizing the weak divergence of the singularity of the chemical equilibrium.
Improvement of a plasma uniformity of the 2nd ion source of KSTAR neutral beam injector
Authors: Jeong S. H. , Kim T. S. , Lee K. W. , Chang D. H. , In S. R. , Bae Y. S.
Published: December 2013

The 2nd ion source of KSTAR (Korea Superconducting Tokamak Advanced Research) NBI (Neutral Beam Injector) had been developed and operated since last year. A calorimetric analysis revealed that the heat load of the back plate of the ion source is relatively higher than that of the 1st ion source of KSTAR NBI. The spatial plasma uniformity of the ion source is not good. Therefore, we intended to identify factors affecting the uniformity of a plasma density and improve it. We estimated the effects of a direction of filament current and a magnetic field configuration of the plasma generator on the plasma uniformity. We also verified that the operation conditions of an ion source could change a uniformity of the plasma density of an ion source.
An in situ accelerator-based diagnostic for plasma-material interactions science on magnetic fusion devices
Authors: Hartwig Zachary S. , Barnard Harold S. , Lanza Richard C. , Sorbom Brandon N. , Stahle Peter W. , Whyte Dennis G.
Published: December 2013

This paper presents a novel particle accelerator-based diagnostic that nondestructively measures the evolution of material surface compositions inside magnetic fusion devices. The diagnostic's purpose is to contribute to an integrated understanding of plasma-material interactions in magnetic fusion, which is severely hindered by a dearth of in situ material surface diagnosis. The diagnostic aims to remotely generate isotopic concentration maps on a plasma shot-to-shot timescale that cover a large fraction of the plasma-facing surface inside of a magnetic fusion device without the need for vacuum breaks or physical access to the material surfaces. Our instrument uses a compact (∼1 m), high-current (∼1 milliamp) radio-frequency quadrupole accelerator to inject 0.9 MeV deuterons into the Alcator C-Mod tokamak at MIT. We control the tokamak magnetic fields – in between plasma shots – to steer the deuterons to material surfaces where the deuterons cause high-Q nuclear reactions with low-Z isotopes ∼5 μm into the material. The induced neutrons and gamma rays are measured with scintillation detectors; energy spectra analysis provides quantitative reconstruction of surface compositions. An overview of the diagnostic technique, known as accelerator-based in situ materials surveillance (AIMS), and the first AIMS diagnostic on the Alcator C-Mod tokamak is given. Experimental validation is shown to demonstrate that an optimized deuteron beam is injected into the tokamak, that low-Z isotopes such as deuterium and boron can be quantified on the material surfaces, and that magnetic steering provides access to different measurement locations. The first AIMS analysis, which measures the relative change in deuterium at a single surface location at the end of the Alcator C-Mod FY2012 plasma campaign, is also presented.
An ultra-thin diamond membrane as a transmission particle detector and vacuum window for external microbeams
Authors: Grilj V. , Skukan N. , Pomorski M. , Kada W. , Iwamoto N. , Kamiya T. , Ohshima T. , Jakšić M.
Published: December 2013

Several applications of external microbeam techniques demand a very accurate and controlled dose delivery. To satisfy these requirements when post-sample ion detection is not feasible, we constructed a transmission single-ion detector based on an ultra-thin diamond membrane. The negligible intrinsic noise provides an excellent signal-to-noise ratio and enables a hit-detection efficiency of close to 100%, even for energetic protons, while the small thickness of the membrane limits beam spreading. Moreover, because of the superb mechanical stiffness of diamond, this membrane can simultaneously serve as a vacuum window and allow the extraction of an ion microbeam into the atmosphere.
Design and validation of a high-voltage levitation circuit for electrostatic accelerometers
Authors: Li G. , Wu S. C. , Zhou Z. B. , Bai Y. Z. , Hu M. , Luo J.
Published: December 2013

A simple high-voltage circuit with a voltage range of 0 to 900 V and an open-loop bandwidth of 11 kHz is realized by using an operational amplifier and a MOSFET combination. The circuit is used for the levitation of a test mass of 71 g, suspended below the top-electrodes with a gap distance of 57 μm, so that the performance of an electrostatic accelerometer can be tested on the ground. The translation noise of the accelerometer, limited by seismic noise, is about 4 × 10−8 m/s2/Hz1/2 at 0.1 Hz, while the high-voltage coupling noise is one-order of magnitude lower.
Operation status of the electron cyclotron resonance ion source at Gunma University
Authors: Souda H. , Yamada S. , Kanai T. , Takeshita E. , Muramatsu M. , Kitagawa A. , Kanazawa M. , Izumiya H. , Kano Y.
Published: December 2013

An ECR ion source of Gunma University Heavy Ion Medical Center, so-called KeiGM [M. Muramatsu, A. Kitagawa, Y. Sakamoto, S. Sato, Y. Sato, H. Ogawa, S. Yamada, H. Ogawa, Y. Yoshida, and A. G. Drentje, Rev. Sci. Instrum. 76, 113304 (2005)], has been operated for cancer therapy and physical/biological experiment since 2010. KeiGM produces typically 230 μA of 10 keV/u C4+ ions from CH4 gases. The vacuum pressure is kept between 1.2 × 10−4 and 1.7 × 10−4 Pa so as to suppress the pulse-to-pulse current fluctuation within ±10%. The extraction electrode is cleaned every 6–8 months in order to remove deposited carbon, which increases the leak current and discharge. In order to investigate the possibility of long-term operation without such maintenances, oxygen aging for the cleaning of the extraction electrode has been tested in the test bench. The same-designed ion sources at National Institute of Radiological Sciences and SAGA Heavy Ion Medical Accelerator in Tosu (SAGA-HIMAT) are also operated with stable C4+ current, which are suitable for the continuous operation for cancer therapy.
The stability of S-states of unit-charge Coulomb three-body systems: From H− to H2+
Authors: King Andrew W. , Longford Frank , Cox Hazel
Published: December 2013

High accuracy non-relativistic quantum chemical calculations of the ground state energies and wavefunctions of symmetric three-particle Coulomb systems of the form $\{ {m_1^ \pm m_2^ \pm m_3^ \mp }\}${m1±m2±m3∓}, m1 = m2, are calculated using an efficient and effective series solution method in a triple orthogonal Laguerre basis set. These energies are used to determine an accurate lower bound to the stability zone of unit-charge three-particle Coulomb systems using an expression for the width of the stability band in terms of g, the fractional additional binding due to a third particle. The results are presented in the form of a reciprocal mass fraction ternary diagram and the energies used to derive a parameterised function g(a3), where $a_3 = m_3^{ - 1} /(m_1^{ - 1} + m_2^{ - 1} + m_3^{ - 1})$a3=m3−1/(m1−1+m2−1+m3−1) is the reciprocal mass of the uniquely charged particle. It is found that the function is not minimal at a3 = 0 which corresponds to ∞H− nor is it minimal at the positronium negative ion (Ps−) the system with the least absolute energetic gain by association with a third particle; the function g(a3) is minimal at m1/m3 = 0.49, and a possible physical interpretation in terms of the transition from atomic-like to molecular-like is provided.
Ensemble simulations with discrete classical dynamics
Authors: Toxvaerd Søren
Published: December 2013

For discrete classical Molecular Dynamics (MD) obtained by the “Verlet” algorithm (VA) with the time increment h there exists (for sufficiently small h) a shadow Hamiltonian \documentclass[12pt]{minimal}\begin{document}$\tilde{H}$\end{document}H̃ with energy \documentclass[12pt]{minimal}\begin{document}$\tilde{E}(h)$\end{document}Ẽ(h), for which the discrete particle positions lie on the analytic trajectories for \documentclass[12pt]{minimal}\begin{document}$\tilde{H}$\end{document}H̃. The first order estimate of \documentclass[12pt]{minimal}\begin{document}$\tilde{E}(h)$\end{document}Ẽ(h) is employed to determine the relation with the corresponding energy, E, for the analytic dynamics with h = 0 and the zero-order estimate E0(h) of the energy for discrete dynamics, appearing in the literature for MD with VA. We derive a corresponding time reversible VA algorithm for canonical dynamics for the \documentclass[12pt]{minimal}\begin{document}$(NV\tilde{T}(h))$\end{document}(NVT̃(h)) ensemble and determine the relations between the energies and temperatures for the different ensembles, including the (NVE0(h)) and (NVT0(h)) ensembles. The differences in the energies and temperatures are proportional with h2 and they are of the order of a few tenths of a percent for a traditional value of h. The relations between \documentclass[12pt]{minimal}\begin{document}$(NV\tilde{E}(h))$\end{document}(NVẼ(h)) and (NVE), and \documentclass[12pt]{minimal}\begin{document}$(NV\tilde{T}(h))$\end{document}(NVT̃(h)) and (NVT) are easily determined for a given density and temperature, and allow for using larger time increments in MD. The accurate determinations of the energies are used to determine the kinetic degrees of freedom in a system of N particles. It is 3N − 3 for a three dimensional system. The knowledge of the degrees of freedom is necessary when simulating small system, e.g., at nucleation.
Domains of analyticity for response solutions in strongly dissipative forced systems
Authors: Corsi Livia , Feola Roberto , Gentile Guido
Published: December 2013

We study the ordinary differential equation \documentclass[12pt]{minimal}\begin{document}$\varepsilon \ddot{x} + \dot{x} + \varepsilon g(x) = \varepsilon f(\bm{\omega }t)$\end{document}ɛẍ+ẋ+ɛg(x)=ɛf(ωt), where g and f are real-analytic functions, with f quasi-periodic in t with frequency vector ω. If \documentclass[12pt]{minimal}\begin{document}$c_{0} \in \mathds {R}$\end{document}c0∈R is such that g(c0) equals the average of f and g′(c0) ≠ 0, under very mild assumptions on ω there exists a quasi-periodic solution close to c0 with frequency vector ω. We show that such a solution depends analytically on ɛ in a domain of the complex plane tangent more than quadratically to the imaginary axis at the origin.
Dynamic stability of running: The effects of speed and leg amputations on the maximal Lyapunov exponent
Authors: Look Nicole , Arellano Christopher J. , Grabowski Alena M. , McDermott William J. , Kram Rodger , Bradley Elizabeth
Published: December 2013

In this paper, we study dynamic stability during running, focusing on the effects of speed, and the use of a leg prosthesis. We compute and compare the maximal Lyapunov exponents of kinematic time-series data from subjects with and without unilateral transtibial amputations running at a wide range of speeds. We find that the dynamics of the affected leg with the running-specific prosthesis are less stable than the dynamics of the unaffected leg and also less stable than the biological legs of the non-amputee runners. Surprisingly, we find that the center-of-mass dynamics of runners with two intact biological legs are slightly less stable than those of runners with amputations. Our results suggest that while leg asymmetries may be associated with instability, runners may compensate for this effect by increased control of their center-of-mass dynamics.
The weak-correlation limits of few-electron harmonium atoms
Authors: Cioslowski Jerzy
Published: December 2013

The weak-correlation asymptotics of electronic properties of harmonium atoms comprising up to four electrons are investigated. In particular, closed-form expressions are derived for the first- and second-order contributions to the Hartree-Fock and correlation energies of eight electronic states that include three singlets, one doublet, two triplets, one quartet, and one quintet, six of which are singly determinantal and two are multi-determinantal. This diversity of states offers a much richer set of benchmarking tools for calibration of approximate electron-correlation methods than the previously published data. The availability of the computed energy contributions due to individual spinorbitals and their pairs present in the dominant Slater determinants further enhances the utility of these benchmarks.
Regimes of streaming potential in cylindrical nano-pores in presence of finite sized ions and charge induced thickening: An analytical approach
Authors: Bandopadhyay Aditya , Goswami Prakash , Chakraborty Suman
Published: December 2013

We obtain approximate analytical expressions for the streaming potential and the effective viscosity in a pure pressure-driven flow through a cylindrical pore with electrokinetic interactions, duly accounting for the finite size effects of the ionic species (steric effects) and charge-induced thickening. Our analytical results show a remarkable agreement with the numerical solution even for high surface potentials and small channel radii. We demonstrate a consistent increment in the predicted value of the streaming potential and effective viscosity when finite size effects of the ionic species are accounted for. In addition to this, we account for the radial variation of in the viscosity of the fluid due to charge-induced thickening. We show that this so-called viscoelectric effect leads to a decrease in the induced streaming potential especially at high steric factors and high surface potentials. However, the viscoelectric effect, which is prominent at high zeta potential and narrow channels, does not cause significant changes in the electrokinetic conversion efficiency. These results shed light on the interesting confluence of the steric factor, the channel radius, the electrical double layer screening length, and the surface charge density in conjunction with the charge induced thickening, and thus provide ion-size dependent analytical framework for accurate system design and better interpretation of electrokinetic data.
Internal energy relaxation in shock wave structure
Authors: Josyula Eswar , Suchyta Casimir J. , Boyd Iain D. , Vedula Prakash
Published: December 2013

The Wang Chang-Uhlenbeck (WCU) equation is numerically integrated to characterize the internal structure of Mach 3 and Mach 5 shock waves in a gas with excitation in the internal energy states for the treatment of inelastic collisions. Elastic collisions are modeled with the hard sphere collision model and the transition rates for the inelastic collisions modified appropriately using probabilities based on relative velocities of the colliding particles. The collision integral is evaluated by the conservative discrete ordinate method [F. Tcheremissine, “Solution of the Boltzmann kinetic equation for high-speed flows,” Comput. Math. Math. Phys. 46, 315–329 (2006); F. Cheremisin, “Solution of the Wang Chang-Uhlenbeck equation,” Dokl. Phys. 47, 487–490 (2002)] developed for the Boltzmann equation. For the treatment of the diatomic molecules, the internal energy modes in the Boltzmann equation are described quantum mechanically given by the WCU equation. As a first step in the treatment of the inelastic collisions by the WCU equation, a two- and three-quantum system is considered to study the effect of the varying of (1) the inelastic cross section and (2) the energy gap between the quantum energy states. An alternative method, the direct simulation Monte Carlo method, is used for the Mach 3 shock wave to ensure the consistency of implementation in the two methods and there is an excellent agreement between the two methods. The results from the WCU implementation showed consistent trends for the Mach 3 and Mach5 standing shock waves simulations. Inelastic contributions change the downstream equilibrium state and allow the flow to transition to the equilibrium state further upstream.
The convection of large and intermediate scale fluctuations in a turbulent mixing layer
Authors: Buxton O. R. H. , de Kat R. , Ganapathisubramani B.
Published: December 2013

The convection velocity of large and intermediate scale velocity fluctuations in a nominally two-dimensional planar mixing layer, and its dependence upon the length scale, is explored by carrying out particle image velocimetry (PIV) experiments. A “global” convection velocity, containing the convection of all the length scales present in the flow, is produced by examining the autocorrelation functions between velocity fluctuations in successive PIV records across the mixing layer. This “global” convection velocity is found to be similar to the mean flow, although fluctuations on the low speed side of the mixing layer on average convect at speeds greater than the mean and fluctuations on the high speed side of the mixing layer are observed to convect at speeds less than the mean. Scale specific convection velocity profiles are then produced by examining the phase difference between the spectral content specific to one wavenumber in streamwise velocity fluctuation traces in successive PIV records, offset by time τ. Probability density functions (pdfs) are produced of this phase difference, which is subsequently converted into a convection displacement, and these show that the convection of single length scale fluctuations exhibits a significant variance, particularly so for larger scale fluctuations and in the high speed side of the mixing layer. Convection velocity profiles are produced from these pdfs using both the mean convection distance and the modal convection distance. It is observed that the convection velocity is relatively insensitive to the length scale of the fluctuations considered, particularly when the mean convection distance is used. A slight sensitivity to length scale is, however, observed for convection velocities based on the modal convection distance. This dependence is primarily observed in the high speed side of the mixing layer, in which smaller length-scale fluctuations convect more quickly than larger length-scale fluctuations. It is also observed that the magnitude of the fluctuation itself affects the convection velocity with larger magnitude fluctuations convecting less rapidly than lower magnitude ones at the largest length scales investigated with this behaviour being reversed at more intermediate length scales.
Structural flexibility of intrinsically disordered proteins induces stepwise target recognition
Authors: Shirai Nobu C. , Kikuchi Macoto
Published: December 2013

An intrinsically disordered protein (IDP) lacks a stable three-dimensional structure, while it folds into a specific structure when it binds to a target molecule. In some IDP-target complexes, not all target binding surfaces are exposed on the outside, and intermediate states are observed in their binding processes. We consider that stepwise target recognition via intermediate states is a characteristic of IDP binding to targets with “hidden” binding sites. To investigate IDP binding to hidden target binding sites, we constructed an IDP lattice model based on the HP model. In our model, the IDP is modeled as a chain and the target is modeled as a highly coarse-grained object. We introduced motion and internal interactions to the target to hide its binding sites. In the case of unhidden binding sites, a two-state transition between the free states and a bound state is observed, and we consider that this represents coupled folding and binding. Introducing hidden binding sites, we found an intermediate bound state in which the IDP forms various structures to temporarily stabilize the complex. The intermediate state provides a scaffold for the IDP to access the hidden binding site. We call this process multiform binding. We conclude that structural flexibility of IDPs enables them to access hidden binding sites and this is a functional advantage of IDPs.
Design and characterization of a compact nano-positioning system for a portable transmission x-ray microscope
Authors: Hwu En-Te , Nazaretski Evgeny , Chu Yong S. , Chen Huang-Han , Chen Yu-Sheng , Xu Weihe , Hwu Yeukuang
Published: December 2013

We have designed and constructed a compact nano-positioning system for a Portable Transmission X-ray Microscope (PTXM). We introduce a concept of PTXM and adopt modular approach which implements identical nano-motion platforms to perform manipulation of PTXM components. Modular design provides higher stiffness of the system and allows for reduction of relative thermal drifts between individual constituents of the PTXM apparatus, ensuring a high degree of stability for nanoscale x-ray imaging. We have measured relative thermal drifts between two identical modules to be as low as 15 nm/h, sufficient to perform nanoscale imaging by TXM. Spatial resolution achieved by developed linear piezo stages was measured to be 3 nm with repeatability of 20 nm over 1 mm travel range.
Integrated fiber-mirror ion trap for strong ion-cavity coupling
Authors: Brandstätter B. , McClung A. , Schüppert K. , Casabone B. , Friebe K. , Stute A. , Schmidt P. O. , Deutsch C. , Reichel J. , Blatt R. , Northup T. E.
Published: December 2013

We present and characterize fiber mirrors and a miniaturized ion-trap design developed to integrate a fiber-based Fabry-Perot cavity (FFPC) with a linear Paul trap for use in cavity-QED experiments with trapped ions. Our fiber-mirror fabrication process not only enables the construction of FFPCs with small mode volumes, but also allows us to minimize the influence of the dielectric fiber mirrors on the trapped-ion pseudopotential. We discuss the effect of clipping losses for long FFPCs and the effect of angular and lateral displacements on the coupling efficiencies between cavity and fiber. Optical profilometry allows us to determine the radii of curvature and ellipticities of the fiber mirrors. From finesse measurements, we infer a single-atom cooperativity of up to 12 for FFPCs longer than 200 μm in length; comparison to cavities constructed with reference substrate mirrors produced in the same coating run indicates that our FFPCs have similar scattering losses. We characterize the birefringence of our fiber mirrors, finding that careful fiber-mirror selection enables us to construct FFPCs with degenerate polarization modes. As FFPCs are novel devices, we describe procedures developed for handling, aligning, and cleaning them. We discuss experiments to anneal fiber mirrors and explore the influence of the atmosphere under which annealing occurs on coating losses, finding that annealing under vacuum increases the losses for our reference substrate mirrors. X-ray photoelectron spectroscopy measurements indicate that these losses may be attributable to oxygen depletion in the mirror coating. Special design considerations enable us to introduce a FFPC into a trapped ion setup. Our unique linear Paul trap design provides clearance for such a cavity and is miniaturized to shield trapped ions from the dielectric fiber mirrors. We numerically calculate the trap potential in the absence of fibers. In the experiment additional electrodes can be used to compensate distortions of the potential due to the fibers. Home-built fiber feedthroughs connect the FFPC to external optics, and an integrated nanopositioning system affords the possibility of retracting or realigning the cavity without breaking vacuum.
Enhanced photodetection in graphene-integrated photonic crystal cavity
Authors: Shiue Ren-Jye , Gan Xuetao , Gao Yuanda , Li Luozhou , Yao Xinwen , Szep Attila , Walker Dennis , Hone James , Englund Dirk
Published: December 2013

We demonstrate the controlled enhancement of photoresponsivity in a graphene photodetector by coupling to slow light modes in a long photonic crystal linear defect cavity. Near the Brillouin zone (BZ) boundary, spectral coupling of multiple cavity modes results in broad-band photocurrent enhancement from 1530 nm to 1540 nm. Away from the BZ boundary, individual cavity resonances enhance the photocurrent eight-fold in narrow resonant peaks. Optimization of the photocurrent via critical coupling of the incident field with the graphene-cavity system is discussed. The enhanced photocurrent demonstrates the feasibility of a wavelength-scale graphene photodetector for efficient photodetection with high spectral selectivity and broadband response.
Insights on cholinium- and piperazinium-based ionic liquids under external electric fields: A molecular dynamics study
Authors: Aparicio Santiago , Atilhan Mert , Pala Nezih
Published: December 2013

The rotational and translational response of cholinium benzoate, cholinium salicylate, piperazinium benzoate, and piperazinium salicylate to static and dynamic external electric fields was studied using non-equilibrium molecular dynamics simulations. The existence of strong intrinsic electric fields in the studied fluids requires intensities larger than 0.25 V Å−1 to obtain remarkable changes in the fluids' properties, such as rotational motion, dipolar alignment, or ion diffusivities. Very effective dipolar alignment with the applied fields is obtained showing rotational motions in the direction of the applied field, increasing with field intensity and decreasing with field frequency. Translational movement is clearly improved by the applied fields specially for strong fields and low frequencies, which lead to ionic diffusivities increasing up to two orders of magnitude for the stronger fields in comparison with zero field situations, and thus, increasing remarkably fluids' electrical conductivity. The effect of external electric fields on the studied ionic liquids is weaker than in common imidazolium-based ionic liquids.
Size and shape effects in the Raman scattering by single GaN nanowires
Authors: Wang J. , Demangeot F. , Péchou R. , Bayon C. , Mlayah A. , Daudin B.
Published: December 2013

The work is focused on the dependence of the Raman scattering properties of single GaN nanowires on their size and aspect ratio. We found that, the Raman scattering by optical phonons of hexagonal GaN nanowires evolves with decreasing nanowire diameter. Below 100 nm diameter, strongly polarized Raman scattering is detected when in-coming and out-going light polarizations are parallel to the nanowire axis, which is interpreted in terms of optical antenna effects. Moreover, we observe additional vibrational modes between the longitudinal and transverse phonon frequencies of GaN which are not present in bulk GaN. Based on a numerical dielectric confinement model, these modes are tentatively assigned to confined and surface phonons. The results presented in this work contribute to the understanding of the optical and vibrational properties of semiconductor nanowires.
Phosphor-free nanopyramid white light-emitting diodes grown on {101¯1} planes using nanospherical-lens photolithography
Authors: Wu Kui , Wei Tongbo , Lan Ding , Wei Xuecheng , Zheng Haiyang , Chen Yu , Lu Hongxi , Huang Kai , Wang Junxi , Luo Yi , Li Jinmin
Published: December 2013

We reported a high-efficiency and low-cost nano-pattern method, the nanospherical-lens photolithography technique, to fabricate a SiO2 mask for selective area growth. By controlling the selective growth, we got a highly ordered hexagonal nanopyramid light emitting diodes with InGaN/GaN quantum wells grown on nanofacets, demonstrating an electrically driven phosphor-free white light emission. We found that both the quantum well width and indium incorporation increased linearly along the {101¯1} planes towards the substrate and the perpendicular direction to the {101¯1} planes as well. Such spatial distribution was responsible for the broadband emission. Moreover, using cathodoluminescence techniques, it was found that the blue emission originated from nanopyramid top, resembling the quantum dots, green emission from the InGaN quantum wells layer at the middle of sidewalls, and yellow emission mainly from the bottom of nanopyramid ridges, similar to the quantum wires.
Experimental study of delay-time statistics of superradiant pulses generated from semiconductor structures
Authors: Vasil'ev Peter P. , Olle Vojtech , Penty Richard V. , White Ian H.
Published: December 2013

We study the delay-time statistics of femtosecond pulses generated from GaAs/AlGaAs multiple contact heterostructures under the superradiant (SR) emission generation. The turn-on time of superradiant pulses was measured by a single-shot streak camera with a few picoseconds temporal resolution using low-jitter reference pulses. The fluctuations of the delay times of SR pulses were found to be by more than one order of magnitude larger as compared to standard lasing and a few times larger than for SR emission from other types of media.
Thermoelectric properties of chalcopyrite type CuGaTe2 and chalcostibite CuSbS2
Authors: Kumar Gudelli Vijay , Kanchana V. , Vaitheeswaran G. , Svane A. , Christensen N. E.
Published: December 2013

Electronic and transport properties of CuGaTe2, a hole-doped ternary copper based chalcopyrite type semiconductor, are studied using calculations within the Density Functional Theory and solving the Boltzmann transport equation within the constant relaxation time approximation. The electronic band structures are calculated by means of the full-potential linear augmented plane wave method, using the Tran-Blaha modified Becke-Johnson potential. The calculated band gap of 1.23 eV is in agreement with the experimental value of 1.2 eV. The carrier concentration- and temperature dependent thermoelectric properties of CuGaTe2 are derived, and a figure of merit of zT = 1.69 is obtained at 950 K for a hole concentration of 3.7·1019 cm−3, in agreement with a recent experimental finding of zT = 1.4, confirming that CuGaTe2 is a promising material for high temperature thermoelectric applications. The good thermoelectric performance of p-type CuGaTe2 is associated with anisotropic transport from a combination of heavy and light bands. Also for CuSbS2 (chalcostibite), a better performance is obtained for p-type than for n-type doping. The variation of the thermopower as a function of temperature and concentration suggests that CuSbS2 will be a good thermoelectric material at low temperatures, similarly to the isostructural CuBiS2 compound.
K and Mn co-doped BaCd2As2: A hexagonal structured bulk diluted magnetic semiconductor with large magnetoresistance
Authors: Yang Xiaojun , Li Yuke , Zhang Pan , Jiang Hao , Luo Yongkang , Chen Qian , Feng Chunmu , Cao Chao , Dai Jianhui , Tao Qian , Cao Guanghan , Xu Zhu-An
Published: December 2013

A bulk diluted magnetic semiconductor was found in the K and Mn co-doped BaCd2As2 system. Different from recently reported tetragonal ThCr2Si2-structured II-II-V based (Ba,K)(Zn,Mn)2As2, the Ba1−yKyCd2−xMnxAs2 system has a hexagonal CaAl2Si2-type structure with the Cd2As2 layer forming a honeycomb-like network. The Mn concentration reaches up to x ∼ 0.4. Magnetization measurements show that the samples undergo ferromagnetic transitions with Curie temperature up to 16 K. With low coercive field of less than 10 Oe and large magnetoresistance of about −70%, the hexagonal structured Ba1−yKyCd2−xMnxAs2 can be served as a promising candidate for spin manipulations.
Effect of packing material on methane activation in a dielectric barrier discharge reactor
Authors: Jo Sungkwon , Hoon Lee Dae , Seok Kang Woo , Song Young-Hoon
Published: December 2013

The conversion of methane is measured in a planar-type dielectric barrier discharge reactor using γ-Al2O3 (sphere), α-Al2O3 (sphere), and γ-Al2O3 (16–20 mesh). Investigations on the surface properties and shape of the three packing materials clearly indicate that methane activation is considerably affected by the material used. Capacitances inside the discharge gap are estimated from charge–voltage plots, and a comparison of the generated and transferred charges for different packing conditions show that the difference in surface properties between γ- and α-phase Al2O3 affects the discharge characteristics. Moreover, all packing conditions show different charge characteristics that are related to the electron density. Finally, the packing material's shape affects the local electron temperature, which is strongly related to methane conversion. The combined results indicate that both microscale and macroscale variations in a packing material affect the discharge characteristics, and a packing material should be considered carefully for effective methane activation.
Investigation of radiation flux in certain band via the preheat of aluminum sample
Authors: Zhang Chen , Wang Zhebin , Zhao Bin , Hu Guangyue , Wang Feng , Peng Xiaoshi , Jiang Shaoen , Ding Yongkun , Zheng Jian
Published: December 2013

Quantitative evaluation of the fractions of high energy x-rays in a hohlraum is crucial to the indirect driven-drive scheme of inertial confinement fusion and many other applications in high energy density physics. Preheat of a sample due to x-rays sensitively depends on optical thin photons. Analyzing the motion of a sample due to preheat can thus provide valuable information of those x-rays. In this article, we propose a method to infer the temporal evolution of the x-ray fluxes in the bands of our interest. By matching the simulation results to the motions of an aluminum sample, we can infer the time-resolved x-ray fluxes around the aluminum K-edge and the gold M-band inside the hohlraum.
Compositional dependence of the magnetic properties of epitaxial FeV/MgO thin films
Authors: Devolder T. , Tahmasebi T. , Eimer S. , Hauet T. , Andrieu S.
Published: December 2013

Owing to their bcc structures and the low lattice misfit with MgO, FeV alloys are interesting for MgO-based magnetic tunnel junctions. We use vector network analyzer ferromagnetic resonance to measure the magnetization, anisotropy, exchange stiffness, and damping of epitaxial FeV/MgO thin alloys of various V contents. The low magnetization, very high exchange stiffness (23 pJ/m) and very low effective damping (<0.0026) of the alloy with 20% V content makes it an interesting candidate for spin torque applications. The ultralow damping is consistent with a spin-orbit origin, which sheds light on the possible strategies to reduce the damping in alloys.
Phonon-limited low-field mobility in silicon: Quantum transport vs. linearized Boltzmann Transport Equation
Authors: Rhyner Reto , Luisier Mathieu
Published: December 2013

We propose to check and validate the approximations made in dissipative quantum transport (QT) simulations solved in the Non-equilibrium Green's Function formalism by comparing them with the exact solution of the linearized Boltzmann Transport Equation (LB) in the stationary regime. For that purpose, we calculate the phonon-limited electron and hole mobility in bulk Si and ultra-scaled Si nanowires for different crystal orientations ⟨100⟩, ⟨110⟩, and ⟨111⟩. In both QT and LB simulations, we use the same sp3d5s* tight-binding model to describe the electron/hole properties and the same valence-force-field approach to account for the phonon properties. It is found that the QT simplifications work well for electrons, but are less accurate for holes, where a renormalization of the phonon scattering strength is proved useful to improve the results.
Unusual dielectric response in cobalt doped reduced graphene oxide
Authors: Akhtar Abu Jahid , Gupta Abhisek , Kumar Shaw Bikash , Saha Shyamal K.
Published: December 2013

Intensive research on cobalt doped reduced graphene oxide (Co-RGO) to investigate the modification in graphene magnetism and spin polarization due to presence of transition metal atom has been carried out, however, its dielectric spectroscopy, particularly, how capacitance changes with impurity levels in graphene is relatively unexplored. In the present work, dielectric spectroscopy along with magneto-dielectric effect are investigated in Co-RGO. Contrary to other materials, here permittivity increases abruptly with frequency in the low frequency region and continues to increase till 107 Hz. This unusual behavior is explained on the basis of trap induced capacitance created due to impurity levels.
Influence of growth stress on the surface morphology of N-polar GaN films grown on vicinal C-face SiC substrates
Authors: Won Dongjin , Weng Xiaojun , Al Balushi Zakaria Y. , Redwing Joan M.
Published: December 2013

In-situ stress measurements were used to monitor the growth of N-polar GaN films on vicinal C-face SiC substrates using a two-step temperature process. A reduction in compressive stress in the N-polar GaN and a corresponding decrease in surface roughness were observed as the initial growth temperature was reduced from 1000 °C to 900 °C. The results suggest that compressive stress in N-polar GaN promotes step bunching and macroscale roughness in films grown on vicinal substrates. The reduction in compressive stress is proposed to originate from tensile thermal stress induced by the temperature change in the two-step process.
Electrode effect on high-detectivity ultraviolet photodetectors based on perovskite oxides
Authors: Zhou Wen-jia , Jin Kui-juan , Guo Hai-zhong , Ge Chen , He Meng , Lu Hui-bin
Published: December 2013

High-detectivity metal-semiconductor-metal ultraviolet photodetectors have been fabricated based on SrTiO3 with different types of metal electrodes (Ag, Ni, and Pt), and the effects of the different types of the electrodes on the photoelectric properties were investigated. Comparing with three types of the metal electrodes, the largest responsivity of 0.517 A/W and the smallest dark current of 2.215 × 10−11 A were obtained by using Ag electrode due to the largest Schottky barrier between Ag electrode and SrTiO3. The detectivities of all the photodetectors are achieved about 1012 cm·HZ1/2/W, which can be comparable to Si-based photodetectors. Furthermore, a transient photovoltaic signal with a rise time of ∼360 ps and a full width at half-maximum of ∼576 ps are obtained in the Ag/SrTiO3/Ag photodetectors under the illumination of a 355 nm laser with 15 ps duration. These results provide a useful guide for designing high-performance photodetectors based on perovskite oxides and the appropriate metal electrode selected.
Improved interfacial and electrical properties of vanadyl-phthalocyanine metal-insulator-semiconductor devices with silicon nitride as gate insulator
Authors: Wang Lijuan , Li Yiping , Song Xiaofeng , Liu Xin , Zhang Long , Yan Donghang
Published: December 2013

We have investigated the interfacial and electrical properties of vanadyl-phthalocyanine (VOPc) metal-insulator-semiconductor devices by the measurement of capacitance and conductance. The devices have been fabricated on ordered para-sexiphenyl (p-6P) layer with silicon nitride (SiNx) as gate insulator. The VOPc/p-6P/SiNx devices have shown a negligible hysteresis, low series resistance, and high operated frequency. Bulk traps have been distinguished from interface traps by two loss peaks in conductance measurement. Trap densities and distribution of trap energy level have been obtained. The improved properties indicate that VOPc/ p-6P devices with SiNx insulator hold a great promise of application in flexible displays.
Intrinsic subpicosecond magnetization reversal driven by femtosecond laser pulses in GdFeCo amorphous films
Authors: Li Shufa , Gao Ruixin , Cheng Chuyuan , Yan Yong , Lai Tianshu
Published: December 2013

Ultrafast magnetization dynamics in GdFeCo films triggered by femtosecond laser pulses with and without an external field applied is studied experimentally for different excitation fluence. It is found that subpicosecond magnetization reversal occurs simultaneously in the ultrafast dynamics of both saturation and remnant magnetization states and almost identical within 13 ps, whereas relatively slow magnetization reversal across compensation point appears only in the dynamics of saturation magnetization state. It shows the subpicosecond magnetization reversal is external field independent, and originates from intrinsic magnetic evolution in ferrimagnetic system. The intrinsic subpicosecond reversal is qualitatively explained by linear reversal.
Selective area growth and characterization of GaN nanocolumns, with and without an InGaN insertion, on semi-polar (11–22) GaN templates
Authors: Bengoechea-Encabo A. , Albert S. , Zuñiga-Perez J. , de Mierry P. , Trampert A. , Barbagini F. , Sanchez-Garcia M. A. , Calleja E.
Published: December 2013

The aim of this work is the selective area growth (SAG) of GaN nanocolumns, with and without an InGaN insertion, by molecular beam epitaxyon semi-polar (11–22) GaN templates. The high density of stacking faults present in the template is strongly reduced after SAG. A dominant sharp photoluminescence emission at 3.473 eV points to high quality strain-free material. When embedding an InGaN insertion into the ordered GaN nanostructures, very homogeneous optical properties are observed, with two emissions originating from different regions of each nanostructure, most likely related to different In contents on different crystallographic planes.
Texture evolution during nitinol martensite detwinning and phase transformation
Authors: Cai S. , Schaffer J. E. , Ren Y. , Yu C.
Published: December 2013

Nitinol has been widely used to make medical devices for years due to its unique shape memory and superelastic properties. However, the texture of the nitinol wires has been largely ignored due to inherent complexity. In this study, in situ synchrotron X-ray diffraction has been carried out during uniaxial tensile testing to investigate the texture evolution of the nitinol wires during martensite detwinning, variant reorientation, and phase transformation. It was found that the thermal martensitic nitinol wire comprised primarily an axial (1¯20), (120), and (102)-fiber texture. Detwinning initially converted the (120) and (102) fibers to the (1¯20) fiber and progressed to a (1¯30)-fiber texture by rigid body rotation. At strains above 10%, the (1¯30)-fiber was shifted to the (110) fiber by (21¯0) deformation twinning. The austenitic wire exhibited an axial (334)-fiber, which transformed to the near-(1¯30) martensite texture after the stress-induced phase transformation.
X-ray driven channeling acceleration in crystals and carbon nanotubes
Authors: Shin Young-Min , Still Dean A. , Shiltsev Vladimir
Published: December 2013

Acceleration of particles channeling in a crystal by means of diffracted x-rays via Bormann anomalous transmission was conceived for heavy ions and muons by Tajima and Cavenago [Phys. Rev. Lett. 59, 1440 (1987)], which potentially offers an appreciably high field gradient on the order of GV/cm. The theoretical model of the high gradient acceleration has been studied in two kinds of atomic structure, crystals and carbon nanotubes (CNTs), with analytic calculations and electromagnetic eigenmode simulations. A range of acceleration gradients and cutoffs of the x-ray power (the lowest power limit to overcome the Bremsstrahlung radiation losses) are characterized in terms of the lattice constants, unit cell sizes, and photon energies. The parametric analysis indicates that the required x-ray power can be reduced to an order of megawatt by replacing crystals with CNTs. Eventually, the equivalent dielectric approximation of a multi-wall nanotube shows that 250–810 MeV muons can be synchronously coupled with x-rays of 0.65–1.32 keV in the accelerating structure.
On the propagation of vorticity in multi-species plasmas
Authors: Núñez Manuel
Published: December 2013

The evolution of plasmas formed by several species is governed by one fluid equation for each species, all of them linked by an electromagnetic forcing and collisional terms, and the Maxwell equations. It is found that in the collisionless case, the field lines of a combination of fluid vorticity and magnetic field are transported by the flow as material points. In consequence, the vorticity propagates at the same velocity as the magnetic field. This is studied in depth for a number of simple configurations, showing that the vorticity travels at a certain fraction of the speed of light, depending on the size of the spatial mode.