bluuebunny/crossref_metadata_2025 huggingface dataset:
Macroscopic analysis of a turbulent round liquid jet impinging on an air/water interface in a confined medium
Authors: Larocque Jerome , Rivière Nicolas , Vincent Stéphane , Reungoat David , Faure Jean-Pierre , Heliot Jean-Philippe , Caltagirone Jean-Paul , Moreau Mathieu
Published: June 2009

In this paper, a submerged vertical water jet impinging on an air/water free surface is experimentally and numerically investigated. On the one hand, a two dimensions-two components particle image velocimetry technique is carried out to perform velocity measurements from the jet injection to the unsteady free surface. The motion of this interface is recorded by a biaxial shadowgraph imaging system. On the other hand, volume of fluid and large eddy simulation methods are coupled to compute the interaction between the jet dynamics and the interface behavior. Numerical and experimental results are confronted in both the single-phase jet area and the impinging region beneath the free surface. A statistical analysis of the interface motion is led through the maximum elevation point to highlight its organized time and spatial evolution in terms of a characteristic lifetime.
Electrical manipulation of spins in the Rashba two dimensional electron gas systems
Authors: Nitta Junsaku , Bergsten Tobias , Kunihashi Yoji , Kohda Makoto
Published: June 2009

We present our theoretical and experimental studies on manipulation of electron spins based on the Rashba spin-orbit interaction (SOI) in semiconductor heterostructures. Quantum well (QW) thickness dependence of the Rashba SOI strength α is investigated in InP/InGaAs/InAlAs asymmetric QWs by analyzing weak antilocalization. Two different QW thicknesses show inverse Ns dependence of |α| in the same heterostructures. This inverse Ns dependence of |α| is explained by the k⋅p perturbation theory. We confirm that narrow wires are effective to suppress the spin relaxation. Spin interference effects due to spin precession are experimentally studied in small array of mesoscopic InGaAs rings. This is an experimental demonstration of a time reversal Aharonov–Casher effect, which shows that the spin precession angle in an InGaAs channel can be controlled by an electrostatic gate.
Unconventional roles of metal catalysts in chemical-vapor syntheses of single-crystalline nanowires
Authors: Kang Kibum , Kim Cheol-Joo , Jo Moon-Ho
Published: June 2009

In this invited contribution at the 29th International Conference on the Physics of semiconductors (ICPS 2008), we review two examples of solid-catalytic nanowire (NW) growth in parallel comparisons to the NW growth from the eutectic liquid catalyst. First, we demonstrated the Cu-catalyzed Ge NW growth using GeH4 vapor precursor at 200 °C, which is far below the Cu–Ge eutectic temperature of 644 °C, with a relatively uniform diameter distribution directly templated from that of the catalysts. We provide evidence that the formation of solid Cu3Ge catalysts and Ge diffusion across the catalysts are responsible for such low-temperature growth of Ge NWs in a size-deterministic manner. Second, we show the spontaneous silicidation of NiSix NWs on continuous Ni bulks using SiH4 vapor precursor at 400 °C. This growth is particularly marked in that NiSix NWs are formed in a self-organized manner without employing the nanocluster catalysts. We discuss this spontaneous growth of NiSix NWs within the frame of the nucleation kinetics in the low supersaturation limit in analogous with the earlier examples of the vapor-condensation at the low vapor pressures.
The influence of the self-consistent mode structure on the Coriolis pinch effect
Authors: Peeters A. G. , Angioni C. , Camenen Y. , Casson F. J. , Hornsby W. A. , Snodin A. P. , Strintzi D.
Published: June 2009

This paper discusses the effect of the mode structure on the Coriolis pinch effect [A. G. Peeters, C. Angioni, and D. Strintzi, Phys. Rev. Lett. 98, 265003 (2007)]. It is shown that the Coriolis drift effect can be compensated for by a finite parallel wave vector, resulting in a reduced momentum pinch velocity. Gyrokinetic simulations in full toroidal geometry reveal that parallel dynamics effectively removes the Coriolis pinch for the case of adiabatic electrons, while the compensation due to the parallel dynamics is incomplete for the case of kinetic electrons, resulting in a finite pinch velocity. The finite flux in the case of kinetic electrons is interpreted to be related to the electron trapping, which prevents a strong asymmetry in the electrostatic potential with respect to the low field side position. The physics picture developed here leads to the discovery and explanation of two unexpected effects: First the pinch velocity scales with the trapped particle fraction (root of the inverse aspect ratio), and second there is no strong collisionality dependence. The latter is related to the role of the trapped electrons, which retain some symmetry in the eigenmode, but play no role in the perturbed parallel velocity.
Density of interface states, electron traps, and hole traps as a function of the nitrogen density in SiO2 on SiC
Authors: Rozen John , Dhar Sarit , Zvanut M. E. , Williams J. R. , Feldman L. C.
Published: June 2009

Nitridation of the SiO2/SiC interface yields a reduction in interface state density, immunity to electron injection, as well as increased hole trapping. It is shown that the accumulation of nitrogen at the oxide/semiconductor interface is solely responsible for these three effects. The evolution of the density of interface states, electron traps, and hole traps is measured in metal-oxide-semiconductor capacitors as a function of the nitrogen content which is varied by adjusting the gate oxide NO annealing time. A rate equation is derived to model the change in the interface state density, observed at various energy levels, in terms of nitrogen binding cross-sections. While the generation of acceptor interface states upon electron injection is suppressed after minimum N incorporation, the density of oxide hole traps appears to scale linearly with the amount of nitrogen. The origin and the properties of the N-induced hole traps resembles those of the defects responsible for enhanced negative bias temperature instability observed in nitrided silicon devices. It is proposed that the binding of nitrogen is not exclusively driven by the passivation of defects at the semiconductor surface but also results in the formation of a silicon oxynitride layer redefining the interface.
Polydispersity effects in poly(isoprene-b-styrene-b-ethylene oxide) triblock terpolymers
Authors: Meuler Adam J. , Ellison Christopher J. , Qin Jian , Evans Christopher M. , Hillmyer Marc A. , Bates Frank S.
Published: June 2009

Four hydroxyl-terminated poly(isoprene-b-styrene) diblock copolymers with comparable molecular weights and compositions (equivalent volume fractions of polyisoprene and polystyrene) but different polystyrene block polydispersity indices (Mw/Mn=1.06,1.16,1.31,1.44) were synthesized by anionic polymerization using either sec-butyllithium or the functional organolithium 3-triisopropylsilyloxy-1-propyllithium. Poly(ethylene oxide) (PEO) blocks were grown from the end of each of these parent diblocks to yield four series of poly(isoprene-b-styrene-b-ethylene oxide) (ISO) triblock terpolymers that were used to interrogate the effects of varying the polydispersity of the middle bridged polystyrene block. In addition to the neat triblock samples, 13 multicomponent blends were prepared at four different compositions from the ISO materials containing a polystyrene segment with Mw/Mn=1.06; these blends were used to probe the effects of increasing the polydispersity of the terminal PEO block. The melt-phase behavior of all samples was characterized using small-angle X-ray scattering and dynamic mechanical spectroscopy. Numerous polydispersity-driven morphological transitions are reported, including transitions from lamellae to core-shell gyroid, from core-shell gyroid to hexagonally packed cylinders, and from network morphologies [either O70 (the orthorhombic Fddd network) or core-shell gyroid] to lamellae. Domain periodicities and order-disorder transition temperatures also vary with block polydispersities. Self-consistent field theory calculations were performed to supplement the experimental investigations and help elucidate the molecular factors underlying the polydispersity effects. The consequences of varying the polydispersity of the terminal PEO block are comparable to the polydispersity effects previously reported in AB diblock copolymers. Namely, domain periodicities increase with increasing polydispersity and domain interfaces tend to curve toward polydisperse blocks. The changes in phase behavior that are associated with variations in the polydispersity of the middle bridged polystyrene block, however, are not analogous to those reported in AB diblock copolymers, as increases in this middle block polydispersity are not always accompanied by (i) increased domain periodicities and (ii) a tendency for domain interfaces to curve toward the polydisperse domain. These results highlight the utility of polydispersity as a tool to tune the phase behavior of ABC block terpolymers.
Quantum efficiency enhancement in nanocrystals using nonradiative energy transfer with optimized donor-acceptor ratio for hybrid LEDs
Authors: Nizamoglu Sedat , Akin Onur , Demir Hilmi Volkan
Published: June 2009

The quantum efficiency enhancement in nanocrystal solids is critically important for their efficient use as luminophors on color-conversion light emitting diodes (LEDs). For this purpose, we investigate energy gradient mixture of nanocrystal solids for recycling their trapped excitons by varying their donor-acceptor nanocrystal ratios and study the resulting quantum efficiency enhancement as a function of the donor-acceptor ratio in the solid film for hybrid LEDs. We achieve a maximum quantum efficiency enhancement of 17% in these nanocrystal solids when the donor-acceptor ratio is 1:1, demonstrating their highly modified time-resolved photoluminescence decays to reveal the kinetics of strong energy transfer between them.
Efficiency of quasiparticle creation in proximized superconducting photon detectors
Authors: Hijmering R. A. , Verhoeve P. , Martin D. D. E. , Kozorezov A. G. , Wigmore J. K. , Venn R. , Groot P. J. , Jerjen I.
Published: June 2009

In previous work using thin superconducting films as photon detectors it has been assumed implicitly that the quasiparticle yield in proximized superconducting bilayers should be the same as for a pure superconducting layer with the same energy gap. The reasoning is that, following the energy down conversion cascade, the resultant quasiparticles will all finish up at the edge of the density of states, which has the same energy throughout the whole structure regardless or whether it is pure or proximized. In this paper we show that, although the energy gap is the same, the actual density of quasiparticle states may vary considerably across a proximized structure, with a secondary peak at the energy of the higher gap material. Our calculations indicate that this peak can give rise to the generation of excess subgap phonons through which a larger portion of the original photon energy is lost from the quasiparticle system. The associated lower quasiparticle yield effectively reduces the responsivity of the proximized detector and affects the limiting energy resolution. The predictions have been confirmed by experimental results obtained with a distributed read out imaging detector (DROID) in which the response to photons absorbed in a pure Ta layer and in a Ta/Al proximized structure could be compared directly.
Electronic structure and optical gain saturation of InAs1−xNx/GaAs quantum dots
Authors: Chen J. , Fan W. J. , Xu Q. , Zhang X. W. , Li S. S. , Xia J. B.
Published: June 2009

The electronic band structures and optical gains of InAs1−xNx/GaAs pyramid quantum dots (QDs) are calculated using the ten-band k⋅p model and the valence force field method. The optical gains are calculated using the zero-dimensional optical gain formula with taking into consideration of both homogeneous and inhomogeneous broadenings due to the size fluctuation of quantum dots which follows a normal distribution. With the variation of QD sizes and nitrogen composition, it can be shown that the nitrogen composition and the strains can significantly affect the energy levels especially the conduction band which has repulsion interaction with nitrogen resonant state due to the band anticrossing interaction. It facilitates to achieve emission of longer wavelength (1.33 or 1.55 μm) lasers for optical fiber communication system. For QD with higher nitrogen composition, it has longer emission wavelength and less detrimental effect of higher excited state transition, but nitrogen composition can affect the maximum gain depending on the factors of transition matrix element and the Fermi–Dirac distributions for electrons in the conduction bands and holes in the valence bands respectively. For larger QD, its maximum optical gain is greater at lower carrier density, but it is slowly surpassed by smaller QD as carrier concentration increases. Larger QD can reach its saturation gain faster, but this saturation gain is smaller than that of smaller QD. So the trade-off between longer wavelength, maximum optical, saturation gain, and differential gain must be considered to select the appropriate QD size according to the specific application requirement.
Contact stress analysis of lightly compressed thin films: Modeling and experimentation
Authors: Terrell Elon J. , Kabir M. Aslam , Higgs C. Fred
Published: June 2009

Although a number of models have been developed to describe the contact between nominally smooth surfaces, very few of these models have been validated with experiment. Therefore, in this study, an asperity-scale experimental contact measurement was conducted and compared to the predictions of two contact models. The experimental component of this study involved a flat diamond punch tip on a nanoindenter, which was used to compress a thin film that was lithographically patterned into isolated raised squares. This experimental method was developed in order to measure the predominantly elastic load response of the surface asperities on one of the isolated raised topography islands. The experimental measurements were compared to the predicted load responses of an existing analytical contact model as well as a finite element contact model that incorporated the topography of the raised island into its formulation. The predictions of both the models were shown to have reasonable agreement with the experimental data. The modeling results were also used to provide greater depth of insight into the physics of the flat punch compression.
Small angle x-ray scattering study of flame soot nanoparticle aggregation and restructuring
Authors: Mitchell J. B. A. , di Stasio S. , LeGarrec J. L. , Florescu-Mitchell A. I. , Narayanan T. , Sztucki M.
Published: June 2009

Results of an in situ small angle x-ray scattering (SAXS) study of soot particles are presented. Scattering data are analyzed using two Fourier transform inversion methods to determine particle volume distributions and by means of the unified function, which allows different structural levels in the scattering objects to be distinguished. We are able to observe the contemporary existence of smaller subprimary particles and larger aggregated particles that usually are classified as primary particles and constitute the building bricks of large fractal-like aggregates with external dimension of above 100 nm. Comparison is made with our previous SAXS and near edge x-ray absorption fine structure spectroscopy results and with those obtained using laser light scattering and particle sampling-electron microscopic methods.
Brownian molecular rotors: Theoretical design principles and predicted realizations
Authors: Schönborn Jan Boyke , Herges Rainer , Hartke Bernd
Published: June 2009

We propose simple design concepts for molecular rotors driven by Brownian motion and external photochemical switching. Unidirectionality and efficiency of the motion is measured by explicit simulations. Two different molecular scaffolds are shown to yield viable molecular rotors when decorated with suitable substituents.
Ultrastable combined atomic force and total internal fluorescence microscope
Authors: Gumpp H. , Stahl S. W. , Strackharn M. , Puchner E. M. , Gaub H. E.
Published: June 2009

Combining atomic force microscope (AFM) with other microscopy techniques has expanded the range of potential applications for single molecule investigations dramatically. Particularly hybrid instruments with total internal reflection fluorescence (TIRF) excitation have opened new routes in life sciences. Here we present a novel design for such a hybrid microscope, which overcomes the limitations of conventional combinations caused by their limited mechanical stability. A thorough analysis of the noise spectra and a comparison of the different designs and the different operation modes are given. With this instrument we demonstrate single molecule manipulation by AFM and simultaneous TIRF imaging.
Mechanics of noncoplanar mesh design for stretchable electronic circuits
Authors: Song J. , Huang Y. , Xiao J. , Wang S. , Hwang K. C. , Ko H. C. , Kim D.-H. , Stoykovich M. P. , Rogers J. A.
Published: June 2009

A noncoplanar mesh design that enables electronic systems to achieve large, reversible levels stretchability (>100%) is studied theoretically and experimentally. The design uses semiconductor device islands and buckled thin interconnects on elastometric substrates. A mechanics model is established to understand the underlying physics and to guide the design of such systems. The predicted buckle amplitude agrees well with experiments within 5.5% error without any parameter fitting. The results also give the maximum strains in the interconnects and the islands, as well as the overall system stretchability and compressibility.
Basins of attraction of tapping mode atomic force microscopy with capillary force interactions
Authors: Hashemi Nastaran , Montazami Reza
Published: June 2009

We perform a large number of simulations over a wide range of system parameters to approximate the basins of attraction of steady oscillating solutions. We find that the basins of attraction vary as a function of system parameters and initial conditions. For large equilibrium separations, the basin of attraction is dominated by the low-amplitude solution. The location of the fixed point is shifted toward the higher values of instantaneous displacement and velocity for larger equilibrium separations. We show that the basin of attraction in the neighborhood of the fixed point is dominated by low-amplitude solutions as relative humidity is increased.
Detection of alignment changes at the open surface of a confined nematic liquid crystal sensor
Authors: McCamley Maureen K. , Ravnik Miha , Artenstein Andrew W. , Opal Steven M. , Žumer Slobodan , Crawford Gregory P.
Published: June 2009

An open surface optical sensor based on nematic liquid crystal contained in square shaped wells was fabricated. Structures and structural transitions associated with changes in surface anchoring conditions were optically detected using polarizing microscopy. A phenomenological Landau–de Gennes model was applied to model the response of the sensor. Simulated optical micrographs were compared with experiments to understand the nematic profiles and to control and optimize the sensor in a specific regime. Distinct optical patterns were detected when the sensor was exposed to air and to water. A structural transition in the nematic structure was observed, driven by changes in the anchoring strength at the open surface of the sensor. Anchoring strength was varied by adding a homeotropic surfactant (sodium dodecyl sulfate) to strongly planar de-ionized water. The transition can be controlled using sensor parameters such as well depth and aspect ratio.
Surface anisotropy, hysteretic, and magnetic properties of magnetite nanoparticles: A simulation study
Authors: Mazo-Zuluaga J. , Restrepo J. , Muñoz F. , Mejía-López J.
Published: June 2009

In this study we address the role of surface anisotropy on the hysteretic properties of magnetite Fe3O4 nanoparticles and the circumstances yielding both horizontal and vertical shifts in the hysteresis loops. Our analysis involves temperature dependence and particle size effects. Different particle sizes ranging from 2 up to 7 nm were considered. Our theoretical framework is based on a three-dimensional classical Heisenberg model with nearest magnetic neighbor interactions involving tetrahedral (A) and octahedral (B) irons. Cubic magnetocrystalline anisotropy for core spins, single-ion site anisotropy for surface spins, and interaction with a uniform external magnetic field were considered. Our results revealed the onset of low temperature exchange bias field, which can be positive or negative at high enough values of the surface anisotropy constant (KS). Susceptibility data, computed separately for the core and the surface, suggest differences in the hard-soft magnetic character at the core-surface interface. Such differences are KS-driven and depend on the system size. Such a hard-soft interplay, via the surface anisotropy, is the proposed mechanism for explaining the observed exchange bias phenomenology. Our results indicate also that the strongly pinned spins at high enough surface anisotropy values are responsible for both the horizontal and vertical shifts in the hysteresis loops. The dependences of the switching and exchange bias fields with the surface anisotropy and temperature are finally discussed.
Effects of nonuniform exchange and magnetostatic interactions on the determination of intrinsic switching field distributions
Authors: Liu Yang , Hovorka Ondrej , Berger Andreas , Dahmen Karin A.
Published: June 2009

A systematic numerical study is performed on the ΔH(M,ΔM) method and its ability to determine the intrinsic switching field distributions of perpendicular recording media in the presence of locally varying intergranular exchange and magnetostatic interactions. It is found by means of evaluating various reliability measures that the overall robustness of the ΔH(M,ΔM) method is only slightly reduced due to the presence of randomness in both the exchange and magnetostatic interactions, at least within the parameter range that is relevant for typical recording media.
Variable-temperature measurements of the dielectric relaxation in carbon black loaded epoxy composites
Authors: Brosseau C. , Achour M. E.
Published: June 2009

Technologically, an understanding of the temperature influence on the transport properties is essential to the study of many random conductor-insulator composites, while fundamentally it is related to a variety of questions in statistical physics, dielectrics, and materials science, to name a few. Variable-temperature measurements of the frequency dependent complex effective permittivity were performed on amine-cured epoxy resins loaded with carbon black (CB). Two series of prepercolative samples differing from the kind of CB particles (Raven 2000 and Raven 5000) mixed in an amine-cured epoxy matrix (diglycidylic ether of bisphenol F) were studied. In this effort to contribute to our understanding of the role of frequency (100 Hz–15 MHz) and temperature (from ambient temperature up to 90 °C) on the complex effective permittivity which describes the linear response of the system to an electromagnetic wave, we investigate these composites with CB loadings below the percolation threshold. Two features are observed. First, our observations cannot be understood in the typical framework of a simple Debye-like dipolar process. In this analysis, we argue that the appearance of the broad temperature and frequency dependent maximum loss can be understood within the heuristic framework proposed by Jonscher which applies to disordered heterogeneous systems. This theoretical framework is consistent with several aspects of the experiments, notably the power-law decays of the real and imaginary parts of the effective permittivity characterized by two fractional exponents m and n. These exponents are both positive and smaller than unity. We further quantified their different temperature variations: while m is strongly decreasing with increasing temperature, n takes a value close to 1. Second, the observed maximum loss frequency found for each CB volume fraction shifts to higher frequencies with increasing temperature and exhibits a non-Arrhenius temperature dependence well represented by a Vogel–Tammam–Fulcher (VTF) fit. Well below the percolation threshold, the associated activation energy and ordering temperature of the VTF fit are not significantly sensitive upon the CB concentration. Such results are compared to previous related work.
A combined experimental and theoretical study on realizing and using laser controlled torsion of molecules
Authors: Madsen C. B. , Madsen L. B. , Viftrup S. S. , Johansson M. P. , Poulsen T. B. , Holmegaard L. , Kumarappan V. , Jørgensen K. A. , Stapelfeldt H.
Published: June 2009

It is demonstrated that strong laser pulses can introduce torsional motion in the axially chiral molecule 3,5-difluoro-3′,5′-dibromobiphenyl. A nanosecond laser pulse spatially aligns the stereogenic carbon-carbon (C–C) bond axis allowing a perpendicularly polarized, intense femtosecond pulse to initiate torsional motion accompanied by a rotation about the fixed axis. We monitor the induced motion by femtosecond time-resolved Coulomb explosion imaging. Our theoretical analysis corroborates the experimental findings and on the basis of these results we discuss future applications of laser-induced torsion, viz., time-resolved studies of deracemization and laser controlled molecular junctions based on molecules with torsion.
Radiation damage in zircon by high-energy electron beams
Authors: Jiang Nan , Spence John C. H.
Published: June 2009

Radiation damage induced by high-energy (200 keV) electron irradiation in zircon has been studied thoroughly using imaging, diffraction, and electron energy-loss spectroscopy techniques in transmission electron microscopy. Both structural and compositional changes during the damage were measured using the above techniques in real time. It was found that the damage was mainly caused by the preferential sputtering of O. The loss of O occurred initially within small sporadic regions with dimension of several nanometers, resulting in the direct transformation of zircon into ZrxSiy. These isolated patches gradually connect each other and eventually cover the whole area of the electron beam. These differ from the previous observations either in the self-irradiated natural and synthetic zircon or in ion-beam irradiated thin zircon specimen.
Two-dimensional patterning of colloidal crystals by means of lateral autocloning in edge-patterned cells
Authors: Emoto Akira , Kamei Tadayoshi , Shioda Tatsutoshi , Kawatsuki Nobuhiro , Ono Hiroshi
Published: June 2009

We report the experimental results of two-dimensional patterning of colloidal crystals using edge-patterned cells. Solvent evaporation of a colloidal suspension from the edge of the cell induces self-organized crystallization of spherical colloidal particles. From a reservoir of colloidal suspension in the cell, different colloidal suspensions are injected repetitively. An edge-patterned substrate is introduced into the cell as an upper substrate. As a result, different colloidal crystals are alternately stacked in the lateral direction according to the edge pattern. The characteristics of cloning formation are specifically showed including deformations from the original pattern. This two-dimensional patterning of three-dimensional colloidal crystals by means of lateral autocloning is promising for the development of photonic crystal arrays for use in optic and photonic devices.
Structural and optical properties of Mn-doped CdS thin films prepared by ion implantation
Authors: Chandramohan S. , Kanjilal A. , Tripathi J. K. , Sarangi S. N. , Sathyamoorthy R. , Som T.
Published: June 2009

We report on structural and optical properties of Mn-doped CdS thin films prepared by 190 keV Mn-ion implantation at different temperatures. Mn-ion implantation in the fluence range of 1×1013–1×1016 ions cm−2 does not lead to the formation of any secondary phase. However, it induces structural disorder, causing a decrease in the optical band gap. This is addressed on the basis of band tailing due to creation of localized energy states and Urbach energy calculations. Mn-doped samples exhibit a new band in their photoluminescence spectra at 2.22 eV, which originates from the d-d (T41→A61) transition of tetrahedrally coordinated Mn2+ ions.
Rate-dependent phase transformations in nanoindented germanium
Authors: Oliver D. J. , Bradby J. E. , Williams J. S. , Swain M. V. , Munroe P.
Published: June 2009

There is considerable controversy over the deformation behavior of germanium (Ge) under nanoindentation using a sharp diamond tip, with a diverse range of observations that suggest competing mechanisms. Here we show the deformation mechanism of Ge can be controlled by the rate of applied load. Loading rate is varied over three orders of magnitude using depth-sensing nanoindentation. At slow loading rates, shear-induced plasticity is observed. At rapid loading rates (>100 mN s−1), pressure-induced phase transformations are detected by ex situ micro-Raman spectroscopy and transmission electron microscopy. This switch in the deformation mechanism is due to the differing rate sensitivities of the respective deformation modes, shear-induced plasticity or pressure-induced phase transformation.
Stable voltage source for Penning trap experiments
Authors: Pinegar David B. , Blaum Klaus , Biesiadzinski Tomasz P. , Zafonte Steven L. , Van Dyck Robert S.
Published: June 2009

A voltage reference has been developed to bias ring electrodes of two Penning traps between −90 and 0 V. For output voltages near −90 V, the Allan deviation of the system’s voltage instability is less than 1 part in 108 over all time scales shorter than 104 s. For averaging times longer than several seconds, the system’s stability is determined almost completely by the noise, drift, and aging of the zener diodes in the array of voltage reference integrated circuits. For shorter averaging times, active filters built into the new system significantly reduce the intrinsic noise of the zener diodes. The system makes it possible to continuously adjust the ring voltages for frequency locking the axial motion in the two Penning traps. By keeping electrical noise highly correlated between the two traps, measurement uncertainty should be reduced for precision experiments such as Penning trap mass spectrometry.
Three-dimensional Ivanov–Petrova manifolds
Authors: Calvaruso Giovanni
Published: June 2009

We introduce a system of partial differential equations, whose solutions determine some large families of three-dimensional Ivanov–Petrova manifolds, both Riemannian and Lorentzian, in terms of explicit metrics on an open subset of R3.
Electron doping and magnetic moment formation in N- and C-doped MgO
Authors: Droghetti A. , Sanvito S.
Published: June 2009

The formation of the magnetic moment in C- and N-doped MgO is the result of a delicate interplay between Hund’s coupling, hybridization, and Jahn–Teller distortion. The balance depends on a number of environmental variables including electron doping. We investigate such a dependence by self-interaction corrected density functional theory and we find that the moment formation is robust with respect to electron doping. In contrast, the local symmetry around the dopant is more fragile and different geometries can be stabilized. Crucially the magnetic moment is always extremely localized, making any carrier mediated picture of magnetism in d0 magnets unlikely.
Intraphase fluctuations in heterogeneous magnetic materials
Authors: Corcolle Romain , Daniel Laurent , Bouillault Frédéric
Published: June 2009

The main purpose of homogenization is the determination of the effective behavior (or macroscopic behavior) of heterogeneous materials. Mean fields per phase are generally used in homogenization and represent sufficient information in most cases. However, more information about the field distribution can be necessary, particularly in nonlinear cases. Then, intraphase fluctuations have to be determined. This paper presents a method, based on homogenization tools, for the determination of both estimates and bounds for the intraphase fluctuations. The presented applications deal with magnetic materials and the results obtained with homogenization are compared to those obtained using a finite element modeling.
Mesoscopic simulations of the counterion-induced electro-osmotic flow: A comparative study
Authors: Smiatek Jens , Sega Marcello , Holm Christian , Schiller Ulf D. , Schmid Friederike
Published: June 2009

We present mesoscopic simulations of the counterion-induced electro-osmotic flow in different electrostatic coupling regimes. Two simulation methods are compared, dissipative particle dynamics (DPD) and coupled lattice-Boltzmann/molecular dynamics (LB/MD). A general mapping scheme to match DPD to LB/MD is developed. For the weak coupling regime, analytic expressions for the flow profiles in the presence of partial-slip as well as no-slip boundary conditions are derived from the Poisson–Boltzmann and Stokes equations, which are in good agreement with the numerical results. The influence of electrofriction and partial slip on the flow profiles is discussed.
Interfacial water: A first principles molecular dynamics study of a nanoscale water film on salt
Authors: Liu Li-Min , Krack Matthias , Michaelides Angelos
Published: June 2009

Density functional theory (DFT) molecular dynamics simulations of a thin (∼15 Å) water film on NaCl(001) have been performed, with the aim of understanding the structural and dynamic properties of this important interfacial water system. The interaction of the water film with the surface orders the water molecules in the immediate vicinity of the interface. This is reflected by oscillations in the planar-averaged water density distribution along the surface normal that extend to about 8 Å from the surface. The interaction with the substrate leaves many of the water molecules in the immediate vicinity with broken hydrogen bonds and as a consequence considerably reduced dipole moments. Indeed a clear correlation between the number of hydrogen bonds which a water molecule is involved in and its dipole moment for both water on NaCl and bulk water is observed. How the DFT results obtained here compare to those obtained with various empirical potentials is briefly discussed.
Frequency dependence of dielectric anomaly around Néel temperature in bilayer manganite Pr(Sr0.1Ca0.9)2Mn2O7
Authors: Ghosh Barnali , Bhattacharya Dipten , Raychaudhuri A. K. , Arumugam S.
Published: June 2009

The anomaly in dielectric constant versus temperature plot, around the Néel temperature TN (∼150 K), is found to be frequency dependent in a single crystal of bilayer manganite Pr(Sr0.1Ca0.9)2Mn2O7. The anomaly in the permittivity (ε′∥c) occurs at a temperature Tf which moves within a temperature window (ΔTf) of ∼40 K around TN for a frequency range of 50 kHz–5 MHz. The capacitive component Cp of the dielectric response exhibits a clear yet broad feature around TN which establishes the intrinsic capacitive nature of the anomaly.
Determination of polarization field in a semipolar (112¯2) InGa∕GaN single quantum well using Franz–Keldysh oscillations in electroreflectance
Authors: Shen H. , Wraback M. , Zhong H. , Tyagi A. , DenBaars S. P. , Nakamura S. , Speck J. S.
Published: June 2009

We present an electroreflectance study of the polarization field in a semipolar (112¯2) oriented In0.12Ga0.88N quantum well. We demonstrate that the polarization field in a quantum well can be determined by the electric field in the barrier region measured by Franz–Keldysh oscillations. The polarization field is found to be 575±150kV∕cm, in the same direction as the built-in field. The corresponding polarization is 0.006±0.002C∕m2. Our experimental result indicates that in the semipolar InGaN∕GaN QW there is a crossover angle where the polarization field vanishes, and this crossover angle is less than 58°.
Quantum mechanics with the basis set guided by Ehrenfest trajectories: Theory and application to spin-boson model
Authors: Shalashilin Dmitrii V.
Published: June 2009

In this article a method of numerical solution of the Schrödinger equation is proposed. The approach corrects the Ehrenfest approximation by using several trajectories/configurations with their amplitudes coupled within and across configurations, thus making the method formally exact. Accurate results are obtained for the spin-boson model with up to 2000 bath modes treated on fully quantum level without approximations.
Temperature modulation in ferrofluid convection
Authors: Singh Jitender , Bajaj Renu
Published: June 2009

Convective instability is suppressed by sinusoidal variation in temperature of horizontal boundaries of a ferrofluid layer subjected to vertical magnetic field. On increasing the amplitude of modulation, instability may arise in the form of oscillations which may have time period equal to that of the temperature modulation or double of it. The effect of frequency of modulation, applied magnetic field, and Prandtl number on the onset of a periodic flow in the ferrofluid layer has been investigated numerically using the Floquet theory. Some theoretical results have also been obtained to discuss the limiting behavior of the underlying instability with the temperature modulation. Depending upon the parameters, the flow patterns at the onset of instability have been found to consist of time-periodically oscillating vertical magnetoconvective rolls.
Measurements of fast electron scaling generated by petawatt laser systems
Authors: Tanimoto Tsuyoshi , Habara H. , Kodama R. , Nakatsutsumi M. , Tanaka Kazuo A. , Lancaster K. L. , Green J. S. , Scott R. H. H. , Sherlock M. , Norreys Peter A. , Evans R. G. , Haines M. G. , Kar S. , Zepf M. , King J. , Ma T. , Wei M. S. , Yabuuchi T. , Beg F. N. , Key M. H. , Nilson P. , Stephens R. B. , Azechi H. , Nagai K. , Norimatsu T. , Takeda K. , Valente J. , Davies J. R.
Published: June 2009

Fast electron energy spectra have been measured for a range of intensities between 1018 and 1021Wcm−2 and for different target materials using electron spectrometers. Several experimental campaigns were conducted on petawatt laser facilities at the Rutherford Appleton Laboratory and Osaka University, where the pulse duration was varied from 0.5to5ps relevant to upcoming fast ignition integral experiments. The incident angle was also changed from normal incidence to 40° in p-polarized. The results confirm a reduction from the ponderomotive potential energy on fast electrons at the higher intensities under the wide range of different irradiation conditions.
Tip-enhanced single molecule fluorescence near-field microscopy in aqueous environment
Authors: Frey Heinrich G. , Paskarbeit Jan , Anselmetti Dario
Published: June 2009

For nanobiophysical applications, scanning near-field optical microscopy must combine high optical resolution and single fluorescent molecule sensitivity with the ability to operate in aqueous solution. These requirements can be achieved using the electric field enhancement at the tip of illuminated silicon probes for atomic force miscroscopy (AFM), whereby single ATTO-740 dye molecules could be imaged at an optical resolution down to 20 nm under ambient conditions as well as in aqueous solution. Two illumination modes have been tested: (a) bottom illumination in a total internal reflection microscopy setup and (b) direct top illumination, both with dedicated phase-sensitive single photon counting technology in dynamic AFM mode of operation.
Molded plasmonic crystals for detecting and spatially imaging surface bound species by surface-enhanced Raman scattering
Authors: Baca Alfred J. , Truong Tu T. , Cambrea Lee R. , Montgomery Jason M. , Gray Stephen K. , Abdula Daner , Banks Tony R. , Yao Jimin , Nuzzo Ralph G. , Rogers John A.
Published: June 2009

This report introduces a type of plasmonic crystal that consists of metal coated nanostructures of relief molded on a polymer film as a substrate for surface-enhanced Raman scattering (SERS). Such crystals exhibit SERS enhancement factors of ∼105, over large areas and with sufficiently high levels of uniformity for precise two-dimensional Raman mapping of surface bound monolayers. The ease of fabrication together with the high sensitivities and spatial resolution that can be achieved suggests an attractive route to SERS substrates for portable chemical warfare agent detection, environmental monitors, noninvasive imaging of biomolecules, and other applications.
Transient phase change effect during the crystallization process in phase change memory devices
Authors: Yeo E. G. , Zhao R. , Shi L. P. , Lim K. G. , Chong T. C. , Adesida I.
Published: June 2009

The transient current waveform during a crystallization process in a phase change memory device was measured and analyzed. It revealed two important time parameters, which were termed as delay time and current recovery time. A link between this transient phase change effect and its crystallization kinetics was established. The delay time was found to be the minimum pulse duration before an onset of resistance change. The current recovery time was the time the device takes to complete its transition from high resistance to low resistance. Real-time crystallization characterization was applied to demonstrate the differences between nucleation and growth dominated materials used in the devices.
Inversion in the temperature coefficient of the optical path length close to the glass transition temperature in tellurite glasses
Authors: Lima S. M. , Andrade L. H. C. , Falcão E. A. , Steimacher A. , Astrath N. G. C. , Medina A. N. , Baesso M. L. , Oliveira R. C. , Moraes J. C. S. , Yukimitu K. , Araújo E. B.
Published: June 2009

In this study, thermal lens spectrometry was applied to determine the thermo-optical properties of fragile tellurite glasses as a function of temperature, close to the glass transition region. The results showed an inversion from positive to negative values in the temperature coefficient of the optical path length occurring after the glass transition temperature, which is the region where structural changes from the TeO4 trigonal bipyramidal unit to a TeO3 trigonal pyramid containing nonbridging oxygen take place. In addition, the thermal diffusivity values as a function of temperature exhibited behaviors that were related to thermodynamic and kinetic structural changes in the glass.
An investigation on the light-emission mechanism of metal-insulator-semiconductor light-emitting diodes with different SiGe quantum well structures
Authors: Liao Milton M. H.
Published: June 2009

The SiGe quantum well (QW) tunneling diodes with different Si cap thickness are designed in this work to investigate the detailed mechanism of light emission for the metal-insulator-semiconductor (MIS) light-emitting diode (LED). At the accumulation (negative) gate bias, the electrons tunnel from the metal gate to the SiGe QW semiconductor substrate (p-type). Meanwhile, the negative gate bias also attracts holes in the interface of insulator and SiGe QW semiconductor and then the tunneling electrons can recombine with holes to emit the infrared. LEDs with different Si/SiGe/Si structure are used to investigate the light-emission mechanism and prove that the region of infrared emission is mainly from the interface of the insulator and semiconductor, not from the neutral region in the semiconductor substrate. The accurate energy band and carrier concentration in the SiGe QW structure with operated bias are also simulated and compared it with the experimental data. It shows that the concentrations of the tunneling electrons, which will be drifted by the gate bias, plays the main role on the light emission and limits the light-emission region near the interface of insulator and semiconductor. This work helps us understand clearly the light-emission mechanism of the MIS LED, developed continuously by our group in the decade.
Kinetics of proton pumping in cytochrome c oxidase
Authors: Smirnov Anatoly Yu. , Mourokh Lev G. , Nori Franco
Published: June 2009

We propose a simple model of cytochrome c oxidase, including four redox centers and four protonable sites, to study the time evolution of electrostatically coupled electron and proton transfers initiated by the injection of a single electron into the enzyme. We derive a system of master equations for electron and proton state probabilities and show that an efficient pumping of protons across the membrane can be obtained for a reasonable set of parameters. All four experimentally observed kinetic phases appear naturally from our model. We also calculate the dependence of the pumping efficiency on the transmembrane voltage at different temperatures and discuss a possible mechanism of the redox-driven proton translocation.
A one-dimensional basic oscillator model of the vircator
Authors: Biswas Debabrata
Published: June 2009

A one-dimensional model of the virtual cathode oscillator (vircator) is proposed keeping only the essential physical processes. The basic model consists of a radiating charge in an oscillating electric field. Using parameters from (realistic) particle-in-cell simulations such as the charge Q and amplitude E1 of the oscillating electric field, the model correctly predicts the amplitude of virtual cathode oscillation and the power radiated. The basic model is then extended to incorporate beam-cavity interaction and the resonance effect.
Experimental investigation of electric and magnetic responses in composites with dielectric resonator inclusions at microwave frequencies
Authors: Giannakopoulou T. , Niarchos D. , Trapalis C.
Published: June 2009

Composites with ferroelectric cubic inclusions are experimentally investigated with coaxial line technique in the microwave frequency range 1–18 GHz. The composites exhibit dispersive effective magnetic and dielectric properties. Strengthening of magnetic resonances with the increase in the inclusions’ number is observed. Electric resonances are damped out significantly due to material losses. The measurement results agree well with theoretical calculations based on Mie theory and Maxwell–Garnett mixing relation. Even number and symmetric arrangement of the inclusions are needed to receive true measurement image in the whole investigated range. Composites utilizing dielectric resonators are attractive in electromagnetic metamaterials fabrication.