bluuebunny/crossref_metadata_2025 huggingface dataset:
Compressed exponential relaxation in liquid silicon: Universal feature of the crossover from ballistic to diffusive behavior in single-particle dynamics
Authors: Morishita Tetsuya
Published: July 2012

We report a first-principles molecular-dynamics study of the relaxation dynamics in liquid silicon (l-Si) over a wide temperature range (1000–2200 K). We find that the intermediate scattering function for l-Si exhibits a compressed exponential decay above 1200 K including the supercooled regime, which is in stark contrast to that for normal “dense” liquids which typically show stretched exponential decay in the supercooled regime. The coexistence of particles having ballistic-like motion and those having diffusive-like motion is demonstrated, which accounts for the compressed exponential decay in l-Si. An attempt to elucidate the crossover from the ballistic to the diffusive regime in the “time-dependent” diffusion coefficient is made and the temperature-independent universal feature of the crossover is disclosed.
Electron beam evaporated cadmium sulphide thin films for solar cell applications
Authors: Geng Lei , Xu Ling , Su Weining , Yu Yao , Xiao Jinrong , Tong Liang , Xu Jun , Ma Zhongyuan
Published: July 2012

Cadmium sulphide (CdS) thin films were deposited by electron beam evaporation at different substrate temperatures. The structural characteristics were studied by the x-ray diffraction pattern. The CdS films are polycrystalline, exhibiting a preferred orientation along the (002) plane. Atomic force microscopy reveals that the root mean square roughness of the films increases as the substrate temperature increases. The optical band gap values of CdS films increase slightly with the increasing substrate temperature, in the range of 2.42–2.48 eV. The electrical characteristics of CdS/p-Si heterojunctions were investigated in dark condition and under the illumination intensity of 100 mW/cm2. Excellent rectifying and photovoltaic properties were observed.
Thermally activated polarization dynamics under the effects of lattice mismatch strain and external stress in ferroelectric film
Authors: Zhang Y. , Zhong X. L. , Vopson M. , Wang J. B. , Zhou Y. C.
Published: July 2012

The strain, inevitably existing in ferroelectric films and devices, is an important factor for determining their polarization properties and performances. It requires an understanding of the effects of strain on polarization dynamics of ferroelectric film. Considering the strain effect, an analytic thermally activated electric polarization-switching model was developed for the study of the polarization properties of ferroelectric films. Using this model, we successfully simulated the time dependent polarization reversal, the coercive field, the polarization switching time, and the polarization hysteresis loops as a function of the lattice mismatch, external applied stress, applied electric field, and temperature. Theoretical predications of the model are in good agreement to experimental observations and they reinforce the possibility of using sophisticated thin film fabrication techniques such as lattice mismatch strain to engineer ferroelectric thin films structures and devices with controlled properties. Our study offers clear analytical relations that can be easily used in future studies of ferroelectrics.
Illumination effects on electrical characteristics of GaN/AlGaN/GaN heterostructures and heterostructure field effect transistors and their elimination by proper surface passivation
Authors: Fagerlind M. , Rorsman N.
Published: July 2012

The effect of ambient illumination is investigated for differently processed GaN/AlGaN/GaN heterostructure materials. For samples of the same material with different passivation, the difference in sheet resistance of illuminated and non-illuminated material can be as large as 130% (for annealed heterostructure without passivation) and as small as 3% (for heterostructure passivated with low pressure chemical vapor deposition (LPCVD) silicon nitride). The time constant for the decay of the persistent photoconductance (PPC) is also very different for the differently processed samples. The majority of the effect on the conductance is from photons with energies between 3.1 and 3.7 eV. The investigation indicates that delayed recombination of electrons emitted from surface states and from deep level states in the AlGaN layer dominates the PPC. A theory is formulated by which the difference in illumination sensitivity for the differently passivated materials can be explained by different distributions of electrons between the channel two dimensional electron gas and an accumulation layer formed in the cap layer. For practical heterostructure field effect transistor (HFET) measurements, the illumination sensitivity is generally lower than that of the Hall measurements. Furthermore, HFETs fabricated with the LPCVD silicon nitride passivation are practically illumination invariant.
Oxygen isotope fractionation in the vacuum ultraviolet photodissociation of carbon monoxide: Wavelength, pressure, and temperature dependency
Authors: Chakraborty Subrata , Davis Ryan D. , Ahmed Musahid , Jackson Teresa L. , Thiemens Mark H.
Published: July 2012

Several absorption bands exist in the vacuum ultraviolet (VUV) region of carbon monoxide (CO). Emission spectra indicate that these bands are all predissociative. Experimental results of CO photodissociation by vacuum ultraviolet photons (90 to 108 nm; ∼13 to 11 eV) from the Advanced Light Source synchrotron by measurement of the oxygen isotopic composition of the products are presented here. A large (few hundred per mil) range of oxygen isotopic compositions are observed in the CO photodissociation product and are wavelength dependent. Slope values (δ′17O/δ′18O) ranging from 0.72 to 1.36 were observed in the oxygen three-isotope space (δ′18O vs. δ′17O), which anti-correlated with increasing synchrotron photon energy, and indicates a dependency on the upper electronic state specific dissociation dynamics (e.g., perturbation and coupling associated with a particular state). An unprecedented magnitude in isotope fractionation was observed for photodissociation at 105 and 107 nm and is found to be associated with accidental predissociation of the vibrational states (ν = 0 and 1) for the upper electronic state E1Π. A significant temperature dependency in oxygen isotopic fractionation was observed, indicating a rotational level dependency in the predissociation process.
Joint strength measurements of individual fiber-fiber bonds: An atomic force microscopy based method
Authors: Schmied Franz J. , Teichert Christian , Kappel Lisbeth , Hirn Ulrich , Schennach Robert
Published: July 2012

We are introducing a method to measure tensile strength of individual fiber-fiber bonds within a breaking force range of 0.01 mN–1 mN as well as the energy consumed during breaking. Until now, such a method was not available. Using a conventional atomic force microscope and a specifically designed sample holder, the desired force and the breaking behavior can be analyzed by two different approaches. First, dynamic loading can be applied, where force-versus-distance curves are employed to determine the proportions of elastic energy and energy dissipated in the bond. Second, static loading is utilized to study viscoelastic behavior and calculate viscoelastic energy contributions. To demonstrate the capability of the proposed method, we are presenting results for breaking strength of kraft pulp fiber-fiber bonds in tensile opening mode. The procedure is by no means restricted to cellulose fibers, it has the potential to quantify joint strength of micrometer-sized fibers in general.
Pb nanowire formation on Al/lead zirconate titanate surfaces in high-pressure hydrogen
Authors: Alvine Kyle J. , Shutthanandan Vaithiyalingam , Arey Bruce W. , Wang Chongmin , Bennett Wendy D. , Pitman Stan G.
Published: July 2012

Piezoelectric systems are well known to degrade in hydrogen because of various mechanisms including loss of polarization, Pb migration into the electrode, and surface blistering. Understanding damage mechanisms is crucial for potential high-pressure applications such as injectors for hydrogen-fueled vehicles. In this paper, we report on a previously unreported form of high-pressure hydrogen damage resulting from growth of surface Pb nanowires from an Al electrode on lead zirconate titanate. Wires were observed with roughly 80 nm diameter and with length that varied between 5 and 100 μm. Microscopy to characterize the nanowires and ion-scattering measurements to quantify concurrent Pb surface migration and hydrogen absorption effects are described.
A new Hamiltonian for the topological BF phase with spinor networks
Authors: Bonzom Valentin , Livine Etera R.
Published: July 2012

We describe fundamental equations which define the topological ground states in the lattice realization of the \documentclass[12pt]{minimal}\begin{document}$\operatorname{SU}(2)$\end{document}SU(2) BF phase. We introduce a new scalar Hamiltonian, based on recent works in quantum gravity and topological models, which is different from the plaquette operator. Its gauge-theoretical content at the classical level is formulated in terms of spinors. The quantization is performed with Schwinger's bosonic operators on the links of the lattice. In the spin network basis, the quantum Hamiltonian yields a difference equation based on the spin 1/2. In the simplest case, it is identified as a recursion on Wigner 6j-symbols. We also study it in different coherent states representations, and compare with other equations which capture some aspects of this topological phase.
Dziobek configurations of the restricted (N + 1)-body problem with equal masses
Authors: Tsai Ya-lun
Published: July 2012

In this article, we count the number of (N − 1)-dimensional central configurations of the restricted (N + 1)-body problem with equal masses. For N = 4, 5, there are 25, 56 central configurations. For N = 3 or N ⩾ 6, the number of central configurations is 2N + N − 1.
Deformed noncommutative tori
Authors: Arnlind Joakim , Grosse Harald
Published: July 2012

We recall a construction of non-commutative algebras related to a one-parameter family of (deformed) spheres and tori, and show that in the case of tori, the *-algebras can be completed into C*-algebras isomorphic to the standard non-commutative torus. As the former was constructed in the context of matrix (or fuzzy) geometries, it provides an important link to the framework of non-commutative geometry, and opens up for a concrete way to study deformations of non-commutative tori. Furthermore, we show how the well-known fuzzy sphere and fuzzy torus can be obtained as formal scaling limits of finite-dimensional representations of the deformed algebras, and their projective modules are described together with connections of constant curvature.
Fokker-Planck equations for nonlinear dynamical systems driven by non-Gaussian Lévy processes
Authors: Sun Xu , Duan Jinqiao
Published: July 2012

The Fokker-Planck equations describe time evolution of probability densities of stochastic dynamical systems and are thus widely used to quantify random phenomena such as uncertainty propagation. For dynamical systems driven by non-Gaussian Lévy processes, however, it is difficult to obtain explicit forms of Fokker-Planck equations, because the adjoint operators of the associated infinitesimal generators usually do not have exact formulation. In the present paper, Fokker-Planck equations are derived for nonlinear stochastic differential equations with non-Gaussian Lévy processes. A few examples are presented to illustrate the method.
The Ricci flow of asymptotically hyperbolic mass and applications
Authors: Balehowsky T. , Woolgar E.
Published: July 2012

We consider the evolution of the asymptotically hyperbolic mass under the curvature-normalized Ricci flow of asymptotically hyperbolic, conformally compactifiable manifolds. In contrast to asymptotically flat manifolds, for which Arnowitt-Deser-Misner (ADM) mass is constant during Ricci flow, we show that the mass of an asymptotically hyperbolic manifold of dimension n ⩾ 3 decays smoothly to zero exponentially in the flow time. From this, we obtain a no-breathers theorem and a Ricci flow based, modified proof of the scalar curvature rigidity of zero-mass asymptotically hyperbolic manifolds. We argue that the nonconstant time evolution of the asymptotically hyperbolic mass is natural in light of a conjecture of Horowitz and Myers, and is a test of that conjecture. Finally, we use a simple parabolic scaling argument to produce a heuristic “derivation” of the constancy of ADM mass under asymptotically flat Ricci flow, starting from our decay formula for the asymptotically hyperbolic mass under the curvature-normalized flow.
Influence of impurity propagation and concomitant enhancement of impurity spread on excitation profile of doped quantum dots
Authors: Kr Datta Nirmal , Pal Suvajit , Ghosh Manas
Published: July 2012

We investigate the excitation behavior of a repulsive impurity doped quantum dot under the combined influence of dopant drift and associated time variation in its spatial spread. We have considered Gaussian impurity centers. In order to make the investigation rational, the time-dependence of the spatial spread has been connected with the instantaneous location of the dopant. Looking at the general applicability of the findings, we have considered linear and random propagation of the dopant. For a systematic analysis, we have proceeded in a stepwise manner during the investigation. Thus, at first we have not considered the time-dependence of spatial spread of dopant and concentrated only on the dopant drift. Subsequently, we have introduced time-dependence in the spatial spread and observed the outcome. Although the incorporation of time-dependence in spatial stretch makes the calculation much more tedious and involved, yet this adequately describes the role played by the time-varying impurity domain exclusively in modulating the excitation rate. The varied nature of dopant propagation interplays delicately with the time-dependent modulation of its spatial stretch giving some important insight into the physics underlying the excitation process.
Yet another position-dependent mass quantum model
Authors: Lima Jonas R. F. , Vieira M. , Furtado C. , Moraes F. , Filgueiras Cleverson
Published: July 2012

The quantum dynamics of particles with mass dependent on the position is a problem of interest since the effective-mass approach to charge carriers in conductors and semiconductors began to be used. These problems have been solved using the Hamiltonian \documentclass[12pt]{minimal}\begin{document}$H=\frac{1}{2}m^\alpha (x) p m^\beta (x) p m^\alpha (x)$\end{document}H=12mα(x)pmβ(x)pmα(x), where α and β are real parameters which satisfy the condition 2α + β = −1. It has been verified that the choice α = 0, β = −1 is compatible with Galilean invariance. In this work we propose a new Hamiltonian, \documentclass[12pt]{minimal}\begin{document}$\hat{H}=\frac{1}{6}\left[\hat{m}(\hat{x})^{-1}\hat{p}^2+\hat{p}\hat{m}(\hat{x})^{-1}\hat{p}+p^2\hat{m}(\hat{x})^{-1}\right]$\end{document}Ĥ=16m̂(x̂)−1p̂2+p̂m̂(x̂)−1p̂+p2m̂(x̂)−1, to describe variable mass systems. We considered every permutation among the operators, taking into account that the mass is now an operator. We verified that this Hamiltonian is Hermitian and is compatible with Galilean invariance. For comparison, we used both Hamiltonians to calculate the band structure for a quantum particle with mass varying periodically. Although qualitatively equivalent, the results turn out to produce different numerical values.
Individual degrees of freedom and the solvation properties of water
Authors: Bren Urban , Janežič Dušanka
Published: July 2012

Using molecular dynamics simulations in conjunction with home-developed Split Integration Symplectic Method we effectively decouple individual degrees of freedom of water molecules and connect them to corresponding thermostats. In this way, we facilitate elucidation of structural, dynamical, spectral, and hydration properties of bulk water at any given combination of rotational, translational, and vibrational temperatures. Elevated rotational temperature of the water medium is found to severely hinder hydration of polar molecules, to affect hydration of ionic species in a nonmonotonous way and to somewhat improve hydration of nonpolar species. As proteins consist of charged, polar, and nonpolar amino-acid residues, the developed methodology is also applied to critically evaluate the hypothesis that the overall decrease in protein hydration and the change in the subtle balance between hydration of various types of amino-acid residues provide a plausible physical mechanism through which microwaves enhance aberrant protein folding and aggregation.
Conformational effect on small angle neutron scattering behavior of interacting polyelectrolyte solutions: A perspective of integral equation theory
Authors: Shew Chwen-Yang , Do Changwoo , Hong Kunlun , Liu Yun , Porcar Lionel , Smith Gregory S. , Chen Wei-Ren
Published: July 2012

We present small angle neutron scattering (SANS) measurements of deuterium oxide (D2O) solutions of linear and star sodium poly(styrene sulfonate) (NaPSS) as a function of polyelectrolyte concentration. Emphasis is on understanding the dependence of their SANS coherent scattering cross section I(Q) on the molecular architecture of single polyelectrolyte. The key finding is that for a given concentration, star polyelectrolytes exhibit more pronounced characteristic peaks in I(Q), and the position of the first peak occurs at a smaller Q compared to their linear counterparts. Based on a model of integral equation theory, we first compare the SANS experimental I(Q) of salt-free polyelectrolyte solutions with that predicted theoretically. Having seen their satisfactory qualitative agreement, the dependence of counterion association behavior on polyelectrolyte geometry and concentration is further explored. Our predictions reveal that the ionic environment of polyelectrolyte exhibits a strong dependence on polyelectrolyte geometry at lower polyelectrolyte concentration. However, when both linear and star polyelectrolytes exceed their overlap concentrations, the spatial distribution of counterion is found to be essentially insensitive to polyelectrolyte geometry due to the steric effect.
Quantum Painlevé-Calogero correspondence
Authors: Zabrodin A. , Zotov A.
Published: July 2012

The Painlevé-Calogero correspondence is extended to auxiliary linear problems associated with Painlevé equations. The linear problems are represented in a new form which has a suggestive interpretation as a “quantized” version of the Painlevé-Calogero correspondence. Namely, the linear problem responsible for the time evolution is brought into the form of non-stationary Schrödinger equation in imaginary time, \documentclass[12pt]{minimal}\begin{document}$\partial _t \psi =(\frac{1}{2}\, \partial _x^2 +V(x,t))\psi$\end{document}∂tψ=(12∂x2+V(x,t))ψ, whose Hamiltonian is a natural quantization of the classical Calogero-like Hamiltonian \documentclass[12pt]{minimal}\begin{document}$H=\frac{1}{2}\, p^2 +V(x,t)$\end{document}H=12p2+V(x,t) for the corresponding Painlevé equation. In present paper, we present explicit constructions for the first five equations from the Painlevé list.
Near field enhancement in silver nanoantenna-superlens systems
Authors: Liu Zhengtong , Li Erping , Shalaev Vladimir M. , Kildishev Alexander V.
Published: July 2012

We demonstrate near field enhancement generation in silver nanoantenna-superlens systems via numerical modeling. Using near-field interference and global optimization algorithms, we can design nanoantenna-superlens systems with mismatched permittivities, whose performance can match those with matched permittivities. The systems studied here may find broad applications in the fields of sensing, such as field-enhanced fluorescence and surface-enhanced Raman scattering, and the methodology used here can be applied to the designing and optimization of other devices, such as two-dimensional near field focusing lens.
Pinned, optically aligned diagnostic dock for use on the Z facility
Authors: Gomez M. R. , Rochau G. A. , Bailey J. E. , Dunham G. S. , Kernaghan M. D. , Gard P. , Robertson G. K. , Owen A. C. , Argo J. W. , Nielsen D. S. , Lake P. W.
Published: July 2012

The pinned optically aligned diagnostic dock (PODD) is a multi-configuration diagnostic platform designed to measure x-ray emission on the Z facility. The PODD houses two plasma emission acquisition (PEA) systems, which are aligned with a set of precision machined pins. The PEA systems are modular, allowing a single diagnostic housing to support several different diagnostics. The PEA configurations fielded to date include both time-resolved and time-integrated, 1D spatially resolving, elliptical crystal spectrometers, and time-integrated, 1D spatially resolving, convex crystal spectrometers. Additional proposed configurations include time-resolved, monochromatic mirrored pinhole imagers and arrays of filtered x-ray diodes, diamond photo-conducting diode detectors, and bolometers. The versatility of the PODD system will allow the diagnostic configuration of the Z facility to be changed without significantly adding to the turn-around time of the machine. Additionally, the PODD has been designed to allow instrument setup to be completed entirely off-line, leaving only a refined alignment process to be performed just prior to a shot, which is a significant improvement over the instrument the PODD replaces. Example data collected with the PODD are presented.
Deformation of inherent structures to detect long-range correlations in supercooled liquids
Authors: Mosayebi Majid , Del Gado Emanuela , Ilg Patrick , Öttinger Hans Christian
Published: July 2012

We propose deformations of inherent structures as a suitable tool for detecting structural changes underlying the onset of cooperativity in supercooled liquids. The non-affine displacement (NAD) field resulting from the applied deformation shows characteristic differences between the high temperature liquid and supercooled state, which are typically observed in dynamic quantities. The average magnitude of the NAD is very sensitive to temperature changes in the supercooled regime and is found to be strongly correlated with the inherent structure energy. In addition, the NAD field is characterized by a correlation length that increases upon lowering the temperature towards the supercooled regime.
A photodiode-based neutral particle bolometer for characterizing charge-exchanged fast-ion behavior
Authors: Clary R. , Smirnov A. , Dettrick S. , Knapp K. , Korepanov S. , Ruskov E. , Heidbrink W. W. , Zhu Y.
Published: July 2012

A neutral particle bolometer (NPB) has been designed and implemented on Tri Alpha Energy's C-2 device in order to spatially and temporally resolve the charge-exchange losses of fast-ion populations originating from neutral beam injection into field-reversed configuration plasmas. This instrument employs a silicon photodiode as the detection device with an integrated tungsten filter coating to reduce sensitivity to light radiation. Here we discuss the technical aspects and calibration of the NPB, and report typical NPB measurement results of wall recycling effects on fast-ion losses.
Observation and control of the surface kinetics of InGaN for the elimination of phase separation
Authors: Moseley Michael , Gunning Brendan , Greenlee Jordan , Lowder Jonathan , Namkoong Gon , Alan Doolittle W.
Published: July 2012

The growth of InGaN alloys via Metal-Modulated Epitaxy has been investigated. Transient reflection high-energy electron diffraction intensities for several modulation schemes during the growth of 20% InGaN were analyzed, and signatures associated with the accumulation, consumption, and segregation of excess metal adlayers were identified. A model for shuttered, metal-rich growth of InGaN was then developed, and a mechanism for indium surface segregation was elucidated. It was found that indium surface segregation only occurs after a threshold of excess metal is accumulated, and a method of quantifying this indium surface segregation onset dose is presented. The onset dose of surface segregation was found to be indium-composition dependent and between 1 and 2 monolayers of excess metal. Below this surface threshold off excess metal, metal-rich growth can occur without indium surface segregation. Since at least 2 monolayers of excess metal will accumulate in the case of metal-rich, unshuttered growth of InGaN at the low temperatures required to suppress thermal and spinodal decomposition, this study reveals that some form of modulation must be employed to maintain this adlayer thickness. These theories were applied in the growth of InGaN with varying compositions using Metal-Modulated Epitaxy. Single-phase, high-quality InGaN films with compositions throughout the miscibility gap with root mean square roughnesses less than 0.8 nm were obtained, demonstrating the feasibility of shuttered, metal-rich InGaN growth.
A modified repulsive bridge correction to accurate evaluation of solvation free energy in integral equation theory for molecular liquids
Authors: Kido Kentaro , Yokogawa Daisuke , Sato Hirofumi
Published: July 2012

Integral equation theory for molecular liquids is one of the powerful frameworks to evaluate solvation free energy (SFE). Different from molecular simulation methods, the theory computes SFE in an analytical manner. In particular, the correction method proposed by Kovalenko and Hirata [Chem. Phys. Lett. 290, 237 (1998); Kovalenko and Hirata J. Chem. Phys. 113, 2793 (2000)]10.1063/1.1305885 is quite efficient in the accurate evaluation of SFE. However, the application has been limited to aqueous solution systems. In the present study, an improved method is proposed that is applicable to a wide range of solution systems. The SFE of a variety of solute molecules in chloroform and benzene solvents is evaluated. A key is the adequate treatment of excluded volume in SFE calculation. By utilizing the information of chemical bonds in the solvent molecule, the accurate computation of SFE is achieved.
Turbulent mixing measurements in the Richtmyer-Meshkov instability
Authors: Weber Christopher , Haehn Nicholas , Oakley Jason , Rothamer David , Bonazza Riccardo
Published: July 2012

The Richtmyer-Meshkov instability is experimentally investigated in a vertical shock tube using a new type of broadband initial condition imposed on an interface between a helium-acetone mixture and argon (A = 0.7). The initial condition is created by first setting up a gravitationally stable stagnation plane between the gases and then injecting the same two gases horizontally at the interface to create a shear layer. The perturbations along the shear layer create a statistically repeatable broadband initial condition. The interface is accelerated by a M = 1.6 planar shock wave, and the development of the ensuing turbulent mixing layer is investigated using planar laser induced fluorescence. By the latest experimental time, 2.1 ms after shock acceleration, the layer is shown to be fully turbulent, surpassing both turbulent transition criteria based on the Reynolds number and shear layer scale. Mixing structures are nearly isotropic by the latest time, as seen by the probability density function of gradient angles within the mixing layer. The scalar variance energy spectrum suggests a k−5/3 inertial range by the latest time and an exponential region at higher wavenumbers.
Detection of turbulent thermal diffusion of particles in numerical simulations
Authors: Haugen Nils Erland L. , Kleeorin Nathan , Rogachevskii Igor , Brandenburg Axel
Published: July 2012

The phenomenon of turbulent thermal diffusion in temperature-stratified turbulence causing a non-diffusive turbulent flux (i.e., non-counter-gradient transport) of inertial and non-inertial particles in the direction of the turbulent heat flux is found using direct numerical simulations (DNS). In simulations with and without gravity, this phenomenon is found to cause a peak in the particle number density around the minimum of the mean fluid temperature for Stokes numbers less than 1, where the Stokes number is the ratio of particle Stokes time to turbulent Kolmogorov time at the viscous scale. Turbulent thermal diffusion causes the formation of inhomogeneities in the spatial distribution of inertial particles whose scale is large in comparison with the integral scale of the turbulence. The strength of this effect is maximum for Stokes numbers around unity, and decreases again for larger values. The dynamics of inertial particles is studied using Lagrangian modelling in forced temperature-stratified turbulence, whereas non-inertial particles and the fluid are described using DNS in an Eulerian framework.
Simultaneous imaging/reflectivity measurements to assess diagnostic mirror cleaning
Authors: Skinner C. H. , Gentile C. A. , Doerner R.
Published: July 2012

Practical methods to clean ITER's diagnostic mirrors and restore reflectivity will be critical to ITER's plasma operations. We describe a technique to assess the efficacy of mirror cleaning techniques and detect any damage to the mirror surface. The method combines microscopic imaging and reflectivity measurements in the red, green, and blue spectral regions and at selected wavelengths. The method has been applied to laser cleaning of single crystal molybdenum mirrors coated with either carbon or beryllium films 150–420 nm thick. It is suitable for hazardous materials such as beryllium as the mirrors remain sealed in a vacuum chamber.
Collinearity alignment of probe beams in a laser-based Faraday effect diagnostic
Authors: Lin L. , Ding W. X. , Brower D. L.
Published: July 2012

Two counter-rotating circularly polarized beams are used in a laser-based polarimetry diagnostic providing a phase measurement of the Faraday effect. Collinearity of these beams is a key issue that affects measurement accuracy. Spatial offset from even small misalignment induces systematic error due to density gradient and path length difference. Here, we report an alignment technique using a rotating dielectric wedge, which is capable of reducing spatial offset of two probe beams below 0.1 mm for beams with 40 mm diameter. With optimized alignment, 0.05° Faraday effect fluctuations associated with global tearing modes are resolved with an uncertainty below 0.01°.
Initial measurements of fast ion loss in KSTAR
Authors: Kim Junghee , Kim Jun Young , Yoon S. W. , García-Muñoz M. , Isobe M. , Kim W. C.
Published: July 2012

A fast ion loss detector (FILD) has been installed and tested in Korea Superconducting Tokamak Advanced Research (KSTAR). KSTAR FILD measures the energy and the pitch-angle of the escaping ions with the striking positions on the scintillator plane. Measurements of the fast ion loss have been performed for the neutral beam heated plasmas. Initial experimental results indicate the prompt losses from neutral beam are dominant and the effects of the resonant magnetic perturbation on the fast ion loss are investigated. In addition, further design change of the detector-head in order to avoid excessive heat load and to detect the fusion products or the fast ions having order of MeV of energy is also discussed.
Investigating the performance of an ion luminescence probe as a multichannel fast-ion energy spectrometer using pulse height analysis
Authors: Zurro B. , Baciero A. , Jiménez-Rey D. , Rodríguez-Barquero L. , Crespo M. T.
Published: July 2012

We investigate the capability of a fast-ion luminescent probe to operate as a pulse height ion energy analyzer. An existing high sensitivity system has been reconfigured as a single channel ion detector with an amplifier to give a bandwidth comparable to the phosphor response time. A digital pulse processing method has been developed to determine pulse heights from the detector signal so as to obtain time-resolved information on the ion energy distribution of the plasma ions lost to the wall of the TJ-II stellarator. Finally, the potential of this approach for magnetic confined fusion plasmas is evaluated by studying representative TJ-II discharges.
Gas gun shock experiments with single-pulse x-ray phase contrast imaging and diffraction at the Advanced Photon Source
Authors: Luo S. N. , Jensen B. J. , Hooks D. E. , Fezzaa K. , Ramos K. J. , Yeager J. D. , Kwiatkowski K. , Shimada T.
Published: July 2012

The highly transient nature of shock loading and pronounced microstructure effects on dynamic materials response call for in situ, temporally and spatially resolved, x-ray-based diagnostics. Third-generation synchrotron x-ray sources are advantageous for x-ray phase contrast imaging (PCI) and diffraction under dynamic loading, due to their high photon fluxes, high coherency, and high pulse repetition rates. The feasibility of bulk-scale gas gun shock experiments with dynamic x-ray PCI and diffraction measurements was investigated at the beamline 32ID-B of the Advanced Photon Source. The x-ray beam characteristics, experimental setup, x-ray diagnostics, and static and dynamic test results are described. We demonstrate ultrafast, multiframe, single-pulse PCI measurements with unprecedented temporal (<100 ps) and spatial (∼2 μm) resolutions for bulk-scale shock experiments, as well as single-pulse dynamic Laue diffraction. The results not only substantiate the potential of synchrotron-based experiments for addressing a variety of shock physics problems, but also allow us to identify the technical challenges related to image detection, x-ray source, and dynamic loading.
10 ps resolution, 160 ns full scale range and less than 1.5% differential non-linearity time-to-digital converter module for high performance timing measurements
Authors: Markovic B. , Tamborini D. , Villa F. , Tisa S. , Tosi A. , Zappa F.
Published: July 2012

We present a compact high performance time-to-digital converter (TDC) module that provides 10 ps timing resolution, 160 ns dynamic range and a differential non-linearity better than 1.5% LSBrms. The TDC can be operated either as a general-purpose time-interval measurement device, when receiving external START and STOP pulses, or in photon-timing mode, when employing the on-chip SPAD (single photon avalanche diode) detector for detecting photons and time-tagging them. The instrument precision is 15 psrms (i.e., 36 psFWHM) and in photon timing mode it is still better than 70 psFWHM. The USB link to the remote PC allows the easy setting of measurement parameters, the fast download of acquired data, and their visualization and storing via an user-friendly software interface. The module proves to be the best candidate for a wide variety of applications such as: fluorescence lifetime imaging, time-of-flight ranging measurements, time-resolved positron emission tomography, single-molecule spectroscopy, fluorescence correlation spectroscopy, diffuse optical tomography, optical time-domain reflectometry, quantum optics, etc.
Instantaneous vibrational frequencies of diffusing and desorbing adsorbates: CO/Pt(111)
Authors: Inoue Ken-ichi , Watanabe Kazuya , Matsumoto Yoshiyasu
Published: July 2012

Electronic excitation of metal by intense laser pulses stimulates nuclear motions of adsorbates through nonadiabatic coupling, resulting in diffusion and desorption of adsorbates. These processes take place via precursor states: adsorbates whose vibrational modes with respect to substrate are highly excited. This paper reports the dynamics of precursor states of CO on Pt(111) probed by use of infrared-visible sum frequency generation with phase-sensitive detection, which allows us to obtain the second-order nonlinear susceptibility and thus the vibrational response function. Without pump pulses at 400 nm, the inverse Fourier transformation of the vibrational response function reveals a free induction decay of vibrational polarization of C–O stretching created by a short infrared pulse. The free induction decay is perturbed when an intense 400-nm pump pulse following the infrared pulse is irradiated, because diffusion and desorption of CO are induced by the pump pulse. The time evolution of instantaneous C–O stretching frequency retrieved from the perturbed free induction decay shows a redshift followed by a rapid reverse shift when the fluence of pump pulse is high enough to desorb CO. This indicates that the frustrated modes of CO is first substantially excited and then the parallel momentum of CO is converted to the normal one through mutual collisions, leading to substantial excitation of the external stretching mode of CO.
Design of a millimeter-wave polarimeter for NSTX-Upgrade and initial test on DIII-D
Authors: Zhang J. , Peebles W. A. , Carter T. A. , Crocker N. A. , Doyle E. J. , Kubota S. , Nguyen X. , Rhodes T. L. , Wannberg C. , Zeng L.
Published: July 2012

Polarimetry is a powerful diagnostic technique to probe plasma equilibria and magnetic fluctuations in fusion plasmas. In a high beta plasma such as the National Spherical Torus eXperiment (NSTX), these measurements are important to understand plasma stability and anomalous transport. A 288 GHz polarimeter operating along a major radial chord in retroreflection geometry has been developed and is being tested on the DIII-D tokamak to prepare for future implementation on NSTX-Upgrade. The system launches a rotating linearly polarized beam and detects the phase shift directly related to the polarization change caused by the plasma. To accomplish this, a pair of orthogonal linearly polarized beams with a stable difference frequency is generated using a single sideband modulation technique, then combined and transformed to be counter-rotating circularly polarized. To improve phase resolution, quasi-optical isolation, using Faraday rotators and polarizers, is utilized to eliminate a multi-path feedback effect, which is found to be the primary source of phase error. The bench tests in the laboratory and DIII-D power supply test discharges indicate ≤1° phase resolution.
Optical pyrometer system for collisionless shock experiments in high-power laser-produced plasmas
Authors: Morita T. , Sakawa Y. , Kuramitsu Y. , Dono S. , Ide T. , Shibata S. , Aoki H. , Tanji H. , Sano T. , Shiroshita A. , Waugh J. N. , Gregory C. D. , Woolsey N. C. , Takabe H.
Published: July 2012

A temporally and spatially resolved optical pyrometer system has been fielded on Gekko XII experiments. The system is based on the self-emission measurements with a gated optical imager (GOI) and a streaked optical pyrometer (SOP). Both detectors measure the intensity of the self-emission from laser-produced plasmas at the wavelength of 450 nm with a bandpass filter with a width of ∼10 nm in FWHM. The measurements were calibrated with different methods, and both results agreed with each other within 30% as previously reported [T. Morita et al., Astrophys. Space Sci. 336, 283 (2011)]10.1007/s10509-010-0525-5. As a tool for measuring the properties of low-density plasmas, the system is applicable for the measurements of the electron temperature and density in collisionless shock experiments [Y. Kuramitsu et al., Phys. Rev. Lett. 106, 175002 (2011)]10.1103/PhysRevLett.106.175002.
Formation of stress-controlled, highly textured, α-SiC thin films at 950 °C
Authors: Habermehl S. , Rodriguez M. , Simmons B.
Published: July 2012

We report the formation of α-SiC thin films via low-pressure chemical vapor deposition from mixtures of dichlorosilane and dicholorethylene at 950 °C. Pole figure x-ray diffraction (XRD) analysis indicates that the films are composed primarily of highly c-axis out-of-plane textured α-SiC polytype mixed with a smaller volume of highly textured β-SiC(111). Depending upon temperature and source gas ratio, the residual film stress can be tailored from −350 to 200 MPa. Analysis of the x-ray diffraction data indicates that the intrinsic film stress is correlated to the volume ratio of α-SiC to β-SiC. Stress modulation as a manifestation of carbon vacancy formation and polytype mixing is discussed. Furthermore, a kinetic mechanism for low temperature α-SiC formation is proposed based upon control of C/Si surface saturation.
Experimental observation on the Fermi level shift in polycrystalline Al-doped ZnO films
Authors: Jia Junjun , Takasaki Aiko , Oka Nobuto , Shigesato Yuzo
Published: July 2012

The shift of the Fermi level in polycrystalline aluminum doped zinc oxide (AZO) films was studied by investigating the carrier density dependence of the optical band gap and work function. The optical band gap showed a positive linear relationship with the two-thirds power of carrier density ne2/3. The work function ranged from 4.56 to 4.73 eV and showed a negative linear relationship with ne2/3. These two phenomena are well explained on the basis of Burstein-Moss effect by considering the nonparabolic nature of the conduction band, indicating that the shift of Fermi level exhibits a nonparabolic nature of the conduction band for the polycrystalline AZO film. The variation of work function with the carrier density reveals that the shift of the surface Fermi level can be tailored by the carrier density in the polycrystalline AZO films. The controllability between the work function and the carrier density in polycrystalline AZO films offers great potential advantages in the development of optoelectronic devices.
Anomalous structural phase transition properties in ReSe2 and Au-doped ReSe2
Authors: Kao Yu-Cheng , Huang Tony , Lin Der-Yuh , Huang Ying-Sheng , Tiong Kwong-Kau , Lee Hsin-Yi , Lin Jhih-Min , Sheu Hwo-Shuenn , Lin Chih-Ming
Published: July 2012

The high pressure induced phase transition in rhenium diselenides (ReSe2) and gold-doped rhenium diselenides (Au-ReSe2) at ambient temperature have been investigated using angular-dispersive x-ray diffraction (ADXRD) under high pressure up to around 10.50 and 9.98 GPa, respectively. In situ ADXRD measurements found that the phase transition pressures of ReSe2 and Au-ReSe2 began at 9.98 and 8.52 GPa, respectively. Compressibilities analysis shows the relationship of along c-axis > along a-axis > along b-axis. The linear compressibility of the pressure dependence of α, β, and γ of ReSe2 shows that a phase transition can be related to a counterclockwise rotational trend of the selenium atoms around the chain of Re4 atoms during the decrease of the c-axis distance by a combination of stresses due to the bending effect of α and stretching effect of β. The cause of the reduction of the phase transition pressure of Au-ReSe2 is attributed mainly to a structural distortion as evidenced by the observation of a weak clockwise rotational trend of Se atoms around the chain of Re4 atoms in the pressure range 3.99–4.99 GPa which subsequently reversed to counterclockwise rotation under higher pressure.
Poisoning of magnetism in silicon doped with Re, caused by a charge transfer from interstitials to substitutionals, by means of the self-interaction corrected density-functional approach
Authors: Wierzbowska Małgorzata
Published: July 2012

The self-interaction corrected density-functional calculations are performed for Re impurities and their pairs in silicon. Rhenium ions form in the host crystal not very tight pairs, with impurities separated by one Si atom or by a distance close to two silicon bonds. Comparison of formation energies for various pairs of substitutionals, interstitials, and mixed-site impurities favours the last type. Electron transfer from the interstitial into the substitutional impurity makes the both Re sites nonmagnetic, but the p-type and the n-type co-doping revives magnetism again, the latter more efficiently.
Nonlinear induction detection of electron spin resonance
Authors: Bachar Gil , Suchoi Oren , Shtempluck Oleg , Blank Aharon , Buks Eyal
Published: July 2012

We present an approach to the induction detection of electron spin resonance (ESR) signals exploiting the nonlinear properties of a superconducting resonator. Our experiments employ a yttrium barium copper oxide superconducting stripline microwave (MW) resonator integrated with a microbridge. A strong nonlinear response of the resonator is thermally activated in the microbridge when exceeding a threshold in the injected MW power. The responsivity factor characterizing the ESR-induced change in the system’s output signal is about 100 times larger when operating the resonator near the instability threshold, compared to the value obtained in the linear regime of operation. Preliminary experimental results, together with a theoretical model of this phenomenon are presented. Under appropriate conditions, nonlinear induction detection of ESR can potentially improve upon the current capabilities of conventional linear induction detection ESR.
Strain induced magnetic domain evolution and spin reorientation transition in epitaxial manganite films
Authors: Singh Gyanendra , Rout P. K. , Porwal Rajni , Budhani R. C.
Published: July 2012

The evolution of magnetic domain structure in epitaxial La0.625Ca0.375MnO3 films on (001) NdGaO3 is monitored as a function of temperature and magnetic field using magnetic force microscopy. We see two distinct regions of magnetic orientational order: one in-plane displaying contrast-less image and the other tilted away from the film plane forming a distinct stripe pattern. A strong domain splitting is observed at the boundary of two regions, which is resilient to reorientation with temperature and magnetic field. We propose a model magnetic free energy functional to explain the mechanism of domain splitting seen in manganite films.
Differential scanning calorimeter and infrared imaging for electrocaloric characterization of poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer
Authors: Sebald Gael , Seveyrat Laurence , Capsal Jean-Fabien , Cottinet Pierre-Jean , Guyomar Daniel
Published: July 2012

Electrocaloric properties of poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) [P(VDF-TrFE-CFE)] terpolymer were determined by two methods. A modified differential scanning calorimeter measures the entropy variation when applying an electric field under isothermal conditions. Alternative technique consists of an infrared imaging camera that gives direct information on temperature variation in pseudo-adiabatic condition. Both techniques give similar results with a heat capacity of 1500 J/(kg K). For an electric field of 80 V/μm, entropy variation was measured at 15.1 J/(kg K) and a temperature variation of 2.75 K. High frequency measurement is possible using infrared imaging, and a strong frequency dependence of the electrocaloric effect was observed.
Epitaxial strain-induced changes in the cation distribution and resistivity of Fe-doped CoFe2O4
Authors: Moyer J. A. , Kumah D. P. , Vaz C. A. F. , Arena D. A. , Henrich V. E.
Published: July 2012

The distribution of cations in Fe-doped cobalt ferrite (Co1−xFe2+xO4) is investigated as a function of epitaxial strain through x-ray absorption measurements of samples grown on SrTiO3, MgO, and CoCr2O4-buffered MgAl2O4 (001). In agreement with recent theoretical calculations, compressive (tensile) strain results in the films having a larger (smaller) degree of cation inversion for iron doping levels up to x = 0.62. Measurements of the resistivity further conclude that the degree of cation inversion has a direct effect on the size of the bandgap for stoichiometric CoFe2O4, an effect that is reduced as the iron doping level is increased.
Lipid bilayer array for simultaneous recording of ion channel activities
Authors: Hirano-Iwata Ayumi , Nasu Tomohiro , Oshima Azusa , Kimura Yasuo , Niwano Michio
Published: July 2012

This paper describes an array of stable and reduced-solvent bilayer lipid membranes (BLMs) formed in microfabricated silicon chips. BLMs were first vertically formed simultaneously and then turned 90° in order to realize a horizontal BLM array. Since the present BLMs are mechanically stable and robust, the BLMs survive this relatively tough process. Typically, a ∼60% yield in simultaneous BLM formation over 9 sites was obtained. Parallel recordings of gramicidin channel activities from different BLMs were demonstrated. The present system has great potential as a platform of BLM-based high throughput drug screening for ion channel proteins.