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At least 613 records · Page 34

Strangeness electroproduction on the nucleon at CLAS

High-precision measurements of strange particle production from both proton and neutron targets are a core part of the physics program with the CLAS spectrometer in Hall B at Jefferson Laboratory. Measurements have been carried out at beam energies up to 6 GeV in experiments with polarized beams and polarized targets. This talk will focus on the electroproduction measurements that have been completed, which include cross sections and hyperon polarization observables for K{sup +}Y (Y = {Lambda}{Sigma}{sup 0}) final states over a broad kinematic range in momentum transfer Q{sup 2} and invariant energy W, while spanning nearly the full kaon center-of-mass angular range. These data in the strangeness sector are necessary to better understand the different production mechanisms for {Lambda} and {Sigma} hyperons and to disentangle the different resonant and non-resonant amplitudes in the intermediate state. The usefulness of the CLAS electroproduction data as part of a coupled-channel model fit will be discussed as well as an outlook of this program for the future.

43 PARTICLE ACCELERATORS↗

Diffusion of co-implanted carbon and boron in silicon and its effect on excess self-interstitials

Diffusion of co-implanted carbon (C) and boron (B) in silicon (Si) and its effect on excess Si self-interstitials (I's) after annealing at 800 and 1000 deg. C were investigated by means of secondary ion mass spectrometry. The experimental results showed that C diffusion was not significant at 800 and 1000 deg. C but later became visible for longer annealing times at 1000 deg. C. B diffusion was reduced by the presence of C when no significant C diffusion was observed, but it was enhanced when C diffusion was observed. These results indicate that all implanted C atoms form immobile CI clusters with excess I in the amount of implanted C and that these CI clusters are stable and trap I to reduce B diffusion. On the contrary, CI clusters are dissolved to emit I for longer annealing times at 1000 deg. C and both B and C diffusion are enhanced. Diffusion simulation based on these models fits the experimental profiles of B and C.

36 MATERIALS SCIENCE↗

Analysis of soft x-ray/VUV transmission characteristics of Si and Al filters

Ultrathin filters of Al (1500A) and Si (1200A) should exhibit more than 65% transmission above their Labsorption edges in the soft x-ray/vacuum ultra violet region(Si L-edge: 124 A and Al L-edge: 170 A). However, the measured transmission characteristics of these filters showed {approx}40% transmission. The transmission measurements of these filters were carried at the reflectivity beamline of Indus-1 synchrotron source out over a large wavelength range of 120-360A. In order to understand the measured transmission performance a detailed model fitting is performed using the Paratt formalism. It is found that the oxidation of the surface region of the filters is responsible for the reduced transmission performance. Effects of higher harmonics of the toroidal grating monochromator are also considered in the data analysis.

36 MATERIALS SCIENCE↗

Investigation of the effect of bilayer membrane structures and fluctuation amplitudes on SANS/SAXS profile for short membrane wavelength

The effect of bilayer membrane structures and fluctuation amplitudes on small angle neutron scattering (SANS) and small angle X-ray scattering (SAXS) profile is investigated based on harmonic motions of the surfactant bilayers with bending as well as thickness fluctuation motions. In this study we consider the case in which the wavelength of the bilayer membrane is shorter than the thickness of the membrane. We find that the thickness of the surfactant bilayer membrane, d{sub m}, affects both q{sub dip} and q{sub peak} of I(q,0) profile, and that the fluctuation amplitude, a, of the membrane changes the peak of I(q,0). A simple formula is derived to estimate the thickness of the bilayer based on the q{sub dip} of the profile obtained from the simulation. The resulting estimates of the thickness of the bilayer with harmonic motion showed accuracy within 1%. Moreover, the bilayer thicknesses estimated from the proposed formula show an excellent agreement with the SANS and SAXS experimental results available in the literatures. We also propose a curve fit model, which describes the relationship between the fluctuation amplitude and the normalized q{sub peak} ratio. The present results show the feasibility of the simple formula to estimate the fluctuation amplitude based on the SANS and SAXS profiles.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Comparative experimental investigation on the actuation mechanisms of ionic polymer–metal composites with different backbones and water contents

Water-based ionic polymer–metal composites (IPMCs) exhibit complex deformation properties, especially when the water content changes. To explore the general actuation mechanisms, both Nafion and Flemion membranes are used as the polymer backbones. IPMC deformation includes three stages: fast anode deformation, relaxation deformation, and slow anode deformation, which is mainly dependent on the water content and the backbone. When the water content decreases from 21 to 14 wt. %, Nafion–IPMC exhibits a large negative relaxation deformation, zero deformation, a positive relaxation deformation, and a positive steady deformation without relaxation in sequence. Despite the slow anode deformation, Flemion–IPMC also shows a slight relaxation deformation, which disappears when the water content is less than 13 wt. %. The different water states are investigated at different water contents using nuclear magnetic resonance spectroscopy. The free water, which decreases rapidly at the beginning through evaporation, is proven to be critical for relaxation deformation. For the backbone, indirect evidence from the steady current response is correlated with the slow anode deformation of Flemion-IPMC. The latter is explained by the secondary dissociation of the weak acid group –COOH. Finally, we thoroughly explain not only the three deformations by swelling but also their evolvement with decreasing water content. A fitting model is also presented based on a multi-diffusion equation to reveal the deformation processes more clearly, the results from which are in good agreement with the experimental results.

36 MATERIALS SCIENCE↗

Cosmic-ray induced gamma-ray emission from the starburst galaxy NGC 253

Cosmic rays in galaxies interact with the interstellar medium and give us a direct view of nuclear and particle interactions in the cosmos. For example, cosmic-ray proton interactions with interstellar hydrogen produce gamma rays via PcrPism→π{sup 0}→γγ. For a 'normal' star-forming galaxy like the Milky Way, most cosmic rays escape the Galaxy before such collisions, but in starburst galaxies with dense gas and huge star formation rate, most cosmic rays do suffer these interactions [1,2]. We construct a 'thick-target' model for starburst galaxies, in which cosmic rays are accelerated by supernovae, and escape is neglected. This model gives an upper limit to the gamma-ray emission. Only two free parameters are involved in the model: cosmic-ray proton acceleration energy rate from supernova and the proton injection spectral index. The pionic gamma-radiation is calculated from 10 MeV to 10 TeV for the starburst galaxy NGC 253, and compared to Fermi and HESS data. Our model fits NGC 253 well, suggesting that cosmic rays in this starburst are in the thick target limit, and that this galaxy is a gamma-ray calorimeter.

79 ASTRONOMY AND ASTROPHYSICS↗

Ultrafast carrier dynamics in polycrystalline bismuth telluride nanofilm

In this study, the dynamics of energy carriers in polycrystalline bismuth telluride nanofilm are investigated by the ultrafast pump-probe method. The energy relaxation processes are quantitatively analyzed by using the numerical fitting models. The extracted hot carrier relaxation times of photon excitation, thermalization, and diffusion are around sub-picosecond. The initial reflectivity recovery is found to be dominantly determined by the carrier diffusion, electron-phonon coupling, and photo-generated carriers trapping processes. High-frequency and low-frequency oscillations are both observed and attributed to coherent optical phonons and coherent acoustic phonons, respectively.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Method for extracting relevant electrical parameters from graphene field-effect transistors using a physical model

Due to its intrinsic high mobility, graphene has proved to be a suitable material for high-speed electronics, where graphene field-effect transistor (GFET) has shown excellent properties. In this work, we present a method for extracting relevant electrical parameters from GFET devices using a simple electrical characterization and a model fitting. With experimental data from the device output characteristics, the method allows to calculate parameters such as the mobility, the contact resistance, and the fixed charge. Differentiated electron and hole mobilities and direct connection with intrinsic material properties are some of the key aspects of this method. Moreover, the method output values can be correlated with several issues during key fabrication steps such as the graphene growth and transfer, the lithographic steps, or the metalization processes, providing a flexible tool for quality control in GFET fabrication, as well as a valuable feedback for improving the material-growth process.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Microstructure evolution of Al-doped zinc oxide and Sn-doped indium oxide deposited by radio-frequency magnetron sputtering: A comparison

The microstructure and morphology evolution of Al-doped zinc oxide (AZO) and Sn-doped indium oxide (ITO) thin films on borosilicate glass substrates deposited by radio-frequency magnetron sputtering at room temperature (RT) and 300 °C were investigated by X-ray diffraction and atomic force microscopy (AFM). One-dimensional power spectral density (1DPSD) functions derived from the AFM profiles, which can be used to distinguish different growth mechanisms, were used to compare the microstructure scaling behavior of the thin films. The rms roughness R{sub q} evolves with film thickness as a power law, R{sub q} ∼ d{sub f}{sup β}, and different growth exponents β were found for AZO and ITO films. For AZO films, β of 1.47 and 0.56 are obtained for RT and 300 °C depositions, respectively, which are caused by the high compressive stress in the film at RT and relaxation of the stress at 300 °C. While for ITO films, β{sub 1} = 0.14 and β{sub 2} = 0.64 for RT, and β{sub 1} = 0.89 and β{sub 2} = 0.3 for 300 °C deposition are obtained, respectively, which is related to the strong competition between the surface diffusion and shadowing effect and/or grain growth. Electrical properties of both materials as a function of film thickness were also compared. By the modified Fuchs-Sondheimer model fitting of the electrical transport in both materials, different nucleation states are pointed out for both types of films.

36 MATERIALS SCIENCE↗

Surface and interface states of Bi{sub 2}Se{sub 3} thin films investigated by optical second-harmonic generation and terahertz emission

We investigate the surface and interface states of Bi{sub 2}Se{sub 3} thin films by using the second-harmonic generation technique. Distinct from the surface of bulk crystals, the film surface and interface show the isotropic azimuth dependence of second-harmonic intensity, which is attributed to the formation of randomly oriented domains on the in-plane. Based on the nonlinear susceptibility deduced from the model fitting, we determine that the surface band bending induced in a space charge region occurs more strongly at the film interface facing the Al{sub 2}O{sub 3} substrate or capping layer compared with the interface facing the air. We demonstrate that distinct behavior of the terahertz electric field emitted from the samples can provide further information about the surface electronic state of Bi{sub 2}Se{sub 3}.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

Monopole antenna in quantitative near-field microwave microscopy of planar structures

We have developed an analytical model of a near-field microwave microscope based on a coaxial resonator with a sharpened tip probe. The probe interacts with a layered sample that features an arbitrary depth distribution of permittivity. The microscopic tip end with the accumulated charge is regarded as a monopole antenna radiating an electric field in near zone. The impedance of such an antenna is determined within a quasi-static approximation. The proposed model is used for calculating the sample-sensitive parameters of the microscope, specifically, resonance frequency f{sub 0} and quality factor Q{sub 0}, as a function of probe-sample distance h. The theory has been verified experimentally in studies of semiconductor structures, both bulk and thin films. For measurements, we built a ∼2.1 GHz microscope with an effective tip radius of about 100 μm. The theoretical and experimental dependences f{sub 0}(h) and Q{sub 0}(h) were found to be in a good agreement. The developed theory underlies the method for determining sheet resistance R{sub sh} of a semiconductor film on a dielectric substrate proposed in this article. Studies were performed on doped n-GaN films on an Al{sub 2}O{sub 3} substrate. The effective radius and height of the probe determined from calibration measurements of etalon samples were used as the model fitting parameters. For etalon samples, we employed homogeneous sapphire and doped silicon plates. We also performed four-probe dc measurements of R{sub sh}. The corresponding values for samples with R{sub sh} > 1 kΩ were found to be 50% to 100% higher than the microwave results, which are attributed to the presence of microdefects in semiconductor films.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

Resonant tunneling diode based on band gap engineered graphene antidot structures

The present work demonstrates the operation and performance of double barrier Graphene Antidot Resonant Tunnel Diode (DBGA-RTD). Non-Equilibrium Green’s Function (NEGF) frame work with tight-binding Hamiltonian and 2-D Poisson equations were solved self-consistently for device study. The interesting feature in this device is that it is an all graphene RTD with band gap engineered graphene antidot tunnel barriers. Another interesting new finding is that it shows negative differential resistance (NDR), which involves the resonant tunneling in the graphene quantum well through both the electron and hole bound states. The Graphene Antidot Lattice (GAL) barriers in this device efficiently improved the Peak to Valley Ratio to approximately 20 even at room temperature. A new fitting model is developed for the number of antidots and their corresponding effective barrier width, which will help in determining effective barrier width of any size of actual antidot geometry.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

Structure, mechanical, and frictional properties of hydrogenated fullerene-like amorphous carbon film prepared by direct current plasma enhanced chemical vapor deposition

In this study, fullerene like carbon (FL-C) is introduced in hydrogenated amorphous carbon (a-C:H) film by employing a direct current plasma enhanced chemical vapor deposition. The film has a low friction and wear, such as 0.011 and 2.3 × 10{sup −9}mm{sup 3}/N m in the N{sub 2}, and 0.014 and 8.4 × 10{sup −8}mm{sup 3}/N m in the humid air, and high hardness and elasticity (25.8 GPa and 83.1%), to make further engineering applications in practice. It has several nanometers ordered domains consisting of less frequently cross-linked graphitic sheet stacks. We provide new evidences for understanding the reported Raman fit model involving four vibrational frequencies from five, six, and seven C-atom rings of FL-C structures, and discuss the structure evolution before or after friction according to the change in the 1200 cm{sup −1} Raman band intensity caused by five- and seven-carbon rings. Friction inevitably facilitates the transformation of carbon into FL-C nanostructures, namely, the ultra low friction comes from both such structures within the carbon film and the sliding induced at friction interface.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

Measurement of deuterium density profiles in the H-mode steep gradient region using charge exchange recombination spectroscopy on DIII-D

Recent completion of a thirty two channel main-ion (deuterium) charge exchange recombination spectroscopy (CER) diagnostic on the DIII-D tokamak enables detailed comparisons between impurity and main-ion temperature, density, and toroidal rotation. In a H-mode DIII-D discharge, these new measurement capabilities are used to provide the deuterium density profile, demonstrate the importance of profile alignment between Thomson scattering and CER diagnostics, and aid in determining the electron temperature at the separatrix. Sixteen sightlines cover the core of the plasma and another sixteen are densely packed towards the plasma edge, providing high resolution measurements across the pedestal and steep gradient region in H-mode plasmas. Extracting useful physical quantities such as deuterium density is challenging due to multiple photoemission processes. Finally, these challenges are overcome using a detailed fitting model and by forward modeling the photoemission using the FIDASIM code, which implements a comprehensive collisional radiative model. Published by AIP Publishing.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

Heating rate dependent ignition of Al/Pt nanolaminates through pulsed laser irradiation

Direct laser irradiation of sputter deposited Al/Pt nanolaminate multilayers results in rapid local heating and exothermic mixing of reactant layers. Milli- and microsecond pulsed laser irradiation under certain test conditions leads to single-point ignition of rapid, self-propagating, formation reactions. Multilayers having bilayer thicknesses of 328 nm, 164 nm, and 65 nm are characterized by their ignition onset times and temperatures. Smaller bilayer thickness multilayers require less laser intensity for ignition compared with larger bilayer designs (when utilizing a particular pulse duration). The relationship between laser intensity and ignition onset time is used to calibrate an activation energy for ignition within a finite element reactive heat transport model. The local heating rate is varied from 10 4 K/s to 10 6 K/s by selecting a laser intensity. Kissinger analysis was performed on the heating rate-dependent ignition temperatures measured with high speed pyrometry to experimentally determine an activation energy in the foils of (6.2 ± 1.6 × 10 4 J/mole atoms). Furthermore, this value is then compared to an activation energy produced from model fits to an ignition onset time of 7.2 × 10 4 J/mole atoms.

36 MATERIALS SCIENCE↗

Linear theory of instabilities generated by an electron beam in a metamaterial-loaded waveguide

We present the linear theory of the starting current of Cherenkov-cyclotron and Cherenkov instabilities generated by an electron beam passing through a metamaterial-loaded waveguide. Effective medium theory is used to represent the metamaterial structure properties. The theory predicts that the instabilities compete with the Cherenkov-cyclotron mode dominating at a lower magnetic field and the Cherenkov instability at a higher magnetic field. The theoretical results are compared to results from recent experiments at MIT using a 490 kV, 84A electron beam in magnetic fields of 300G to 1500 G. For an effective medium model fitted to the MIT experimental parameters, theory predicts that the Cherenkov-cyclotron mode will dominate below 780G and the Cherenkov mode above 780 G, in good agreement with experimental observations of switching between these modes at 750 G. The analytical theory allows a better understanding of the mode competition and the dependence of the instabilities on key parameters such as voltage, current, and magnetic fields.

43 PARTICLE ACCELERATORS↗

Isotope effects on the high pressure viscosity of liquid water measured by differential dynamic microscopy

In this paper, differential dynamic microscopy is performed in diamond anvil cells to measure the viscosity of water along the 24 °C isotherm to high-pressure by determination of the tracer diffusion coefficient of monodisperse silica spheres of known diameter and application of the Stokes-Einstein-Sutherland equation. This technique allows liquid samples to be compressed to greater pressure prior to freezing than with other viscometry methods. The highest-pressure measurement was made at 1.67 GPa, considerably deeper into the supercompressed regime than previously reported. The effect of isotopic composition is investigated with samples of normal water, heavy water, and partially deuterated water. When data below 0.25 GPa are excluded a free volume model fits the observed viscosities well yielding a theoretical glass transition density close to that observed in very-high-density amorphous ice. The improved fit above 0.25 GPa coincides with the loss of other anomalous behaviors in liquid water caused by hydrogen bonding and represents a transition to properties closer to those of a simple liquid.

74 ATOMIC AND MOLECULAR PHYSICS↗

Defect characterization and charge transport measurements in high-resolution Ni/n-4H-SiC Schottky barrier radiation detectors fabricated on 250 μ m epitaxial layers

Advances in the growth processes of 4H-SiC epitaxial layers have led to the continued expansion of epilayer thickness, allowing for the detection of more penetrative radioactive particles. In this work, we report the fabrication and characterization of high-resolution Schottky barrier radiation detectors on 250 μm thick n-type 4H-SiC epitaxial layers, the highest reported thickness to date. Several 8 × 8 mm 2 detectors were fabricated from a diced 100 mm diameter 4H-SiC epitaxial wafer grown on a conductive 4H-SiC substrate with a mean micropipe density of 0.11 cm -2 From the Mott–Schottky plots, the effective doping concentration was found to be in the range (0.95–1.85) × 10 14 cm -3 , implying that full depletion could be achieved at ~5.7 kV (0.5 MV/cm at the interface). The current-voltage characteristics demonstrated consistently low leakage current densities of 1–3 nA/cm 2 at a reverse bias of -800 V. This resulted in the pulse-height spectra generated using a 241Am alpha source (5486 keV) manifesting an energy resolution of less than 0.5% full width at half maximum (FWHM) for all the detectors at -200 V. The charge collection efficiencies (CCEs) were measured to be 98–99% with no discernable correlation to the energy resolution. A drift-diffusion model fit to the variation of CCE as a function of bias voltage, revealed a minority carrier diffusion length of ~10 μm. Deep level transient spectroscopy measurements on the best resolution detector revealed that the excellent performance was the result of having ultralow concentrations of the order of 1011 cm-3 lifetime limiting defects—Z 1/2 and EH 6/7 .

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗