A cold thermionically emitting dusty air plasma formed by radiativeheating of graphite particulates

Formation of an atmospheric pressure dusty air plasma is explored experimentally in this paper. The plasma is created by seeding an air flow with graphite particles and irradiating the particulates with a focused CO₂ laser beam. The graphite particles are, thus, heated to thermionically emitting temperatures, and average particle temperatures and average particle number densities are measured. The presence of charges is inferred both from these measured quantities using a simple theoretical transient model, and experimentally by applying a dc bias across the irradiated region. It is found that an electron density of ~6.7 × 10⁵ cm^-3 (6.7 × 10¹¹ m^-3) can be produced at steady state in the presence of O₂. This value can be increased to 3.6 × 10⁷ cm^-3 (3.6 × 10¹³ m ^-1) in the ideal case where an electron attachment to O₂ is suppressed and where a lower work function particulate is used     

InAs/GaAs submonolayer quantum dot superluminescent diode emitting around 970 nm

According to the InAs/GaAs submonolayer quantum dot active region, we demonstrate a bent-waveguide superluminescent diode emitting at a wavelength of around 970 nm. At a pulsed injection current of 0.5 A, the device exhibits an output power of 24 mW and an emission spectrum centred at 971 nm with a full width at half maximum of 16 nm.     

Attractive short-range interatomic potential in the lattice dynamics of niobium and tantalum

Summary It is shown in the framework of the pseudopotential approach that there is a sizable attractive short-range component of the interatomic potential due to thes-d interaction which has the same functional form in real space as the Born-Mayer repulsion due to the overlap of core electron wave functions centred on neighbouring ions. The magnitude of this attractive component is such as to completely cancel the conventional Born-Mayer repulsion, making the resultant short-range interatomic potential attractive rather than repulsive. Numerical calculations show that the attractive interatomic potential, which represents the local-field correction, leads to a better understanding of the occurrence of the soft modes in the phonon dispersion curves of niobium and tantalum.

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Riassunto Si mostra, nell'ambito dell'approccio allo pseudopotenziale, che vi è una componente di considerevoli dimensioni attrattiva a corto raggio del potenziale interatomico, dovuta alla interaziones-d, che ha la stessa forma funzionale nello spazio reale della repulsione di Born-Mayer dovuta al sovrapporsi delle funzioni d'onda degli elettroni del nucleo centrate su ioni confinanti. L'ampiezza di questa componente attrattiva è tale da cancellare completamente la repulsione convenzionale di Born-Mayer, rendendo attrattivo, piuttosto che repulsivo, il risultante potenziale interatomico a corto raggio. I calcoli numerici mostrano che il potenziale attrattivo interatomico, che rappresenta la correzione del campo locale, porta ad una migliore comprensione della presenza dei modi molli nelle curve di dispersione dei fononi del niobio e del tantalio.

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Резюме В рамках псевдопотенциального подхода показывается, что заметный короткодействующий вклад в межатомный потенциал притяжения, обусловленныйs-d взаимодействием, имеет такую же функциональную форму в реальном пространстве, как отталкивание Борна-Майера, обусловленное перекрытием электронных волновых функций, центрированных на соседних ионах. Величина этой компоненты притяжения полностью компенсирует обычное отталкивание Борна-Майера, что приводит к результирующему короткодействующему межатомному потенциалу притяжения, а не отталкивания. Численные вычисления показывают, что межатомный потенциал притяжения, который представляет поправку локального поля, приводит к лучшему пониманию существования мягких мод в фононных дисперсионных кривых ниобия и тантала.

     

Finite element modeling of a microparticle manipulator

The contactless movement of microparticles and cells to known locations within a fluid volume is of interest in the fields of microtechnology and life sciences. A device which can position such inhomogeneities suspended in a fluid at multiple locations is described and modeled. The device consists of a thin fluid layer contained in a channel etched into a silicon wafer. Waves are excited by a macro-piezoelectric plate with electrodes on the top and bottom surfaces and, as a result, waves propagate into the adjacent fluid. The result is a pressure field throughout the fluidic volume. When an inhomogeneity in a fluid is exposed to an ultrasonic field the acoustic radiation force results; this is found by integrating the pressure over the surface of the particle, retaining second order terms, and taking the time average. Thus, due to the presence of a pressure field in the fluid in which the particles are suspended, a force field is created. The particles are then collected at the locations of the force potential minima. In the device described here, the force field is used to position particles into lines. The locations of the particles are predicted by using a finite element model of the system. The experimental and modeling results, presented here, are in good agreement.     

Attractive Casimir forces in a closed geometry

We study the Casimir force acting on a conducting piston with arbitrary cross section. We find the exact solution for a rectangular cross section and the first three terms in the asymptotic expansion for small height to width ratio when the cross section is arbitrary. Though weakened by the presence of the walls, the Casimir force turns out to be always attractive. Claims of repulsive Casimir forces for related configurations, like the cube, are invalidated by cutoff dependence.     

Anisotropic diffusion of a magnetically torqued ellipsoidal microparticle

We study the dynamics of an overdamped paramagnetic ellipsoidal particle confined above a plane and subjected to an external rotating magnetic field. Without magnetic forcing, the Brownian ellipsoid exhibits a crossover from a short time anisotropic diffusion to a long time isotropic one. Application of an external static or rotating magnetic field enables controlling and varying the crossover time depending on the field frequency and amplitude. We combine analytical results and numerical simulations in order to explore the diffusive properties of the forced ellipsoid.     

Structure of a microparticle crystal in a radio-frequency discharge plasma

By employing the particle-in-cell method we study the distributions of the electric field strength and of the electron and ion concentrations in the microparticle crystal in the electrode sheath in a radio-frequency discharge in helium. The coordinates and charges of the microparticles are found from the balance condition for the forces acting on the particles and the balance of electron and ion fluxes to the particles. With periodic boundary conditions introduced, we investigate the three-dimensional problem for the unit cell of the microparticle crystal. We examine the dependence on gas pressure and discharge voltage of the main crystal parameters: the critical particle separation (at which a phase transition from a monolayer crystal to a double-layer crystal occurs), the particle potentials, and the distances between the layers in the double-layer crystal. We obtain the critical values of the friction coefficient for the particles in the gas, i.e., values below which the crystal becomes unstable against the development of particle oscillations in the transverse direction, and compare the experimental data on crystal structure and stability with the theoretical results. Finally, we set up an approximate model that makes it possible to calculate the main parameters of the microparticle crystal. Zh. éksp. Teor. Fiz. 115, 877–893 (March 1999)     

A potential red-emitting phosphor with high color-purity for near-UV light emitting diodes

New red tungstates phosphors, Na₅La_1?xLn_x(WO₄)₄ (Ln = Eu, Sm) and Na₅Eu_1?xSm_x(WO₄)₄, were prepared by solid-state reaction technique. And their structure and photo-luminescent properties were investigated. The introduction of Sm³⁺ broadened the excitation band around 400 nm of the phosphors, and strengthened the red emission. And the possible energy transfer process from Sm³⁺ to Eu³⁺ is discussed. The single red LED was fabricated by combining InGaN chip with Na₅Eu_0.94Sm_0.06(WO₄)₄ as red phosphor, intense red light can be observed by naked eyes. Then the phosphor Na₅Eu_0.94Sm_0.06(WO₄)₄ may be a good candidate for red component of near-UV InGaN-based W-LEDs, because of efficient red-emitting with broadened absorption around 400 nm and appropriate CIE chromaticity coordinates (x = 0.65, y = 0.34).     

Three Attractive Osculating Walkers and a Polymer Collapse Transition

Consider n interacting lock-step walkers in one dimension which start at the points {0,2,4,...,2(n?1)} and at each tick of a clock move unit distance to the left or right with the constraint that if two walkers land on the same site their next steps must be in the opposite direction so that crossing is avoided. When two walkers visit and then leave the same site an osculation is said to take place. The space-time paths of these walkers may be taken to represent the configurations of n fully directed polymer chains of length t embedded on a directed square lattice. If a weight λ is associated with each of the i osculations the partition function is $Z_t^{(n)} (\lambda ) = \sum\nolimits_{i = 0}^{\left\lfloor {\tfrac{{(n - 1)t}}{2}} \right\rfloor } {z_{t,i}^{(n)} } \lambda ^i $ where z ⁽ⁿ⁾ _t,i is the number of t-step configurations having i osculations. When λ=0 the partition function is asymptotically equal to the number of vicious walker star configurations for which an explicit formula is known. The asymptotics of such configurations was discussed by Fisher in his Boltzmann medal lecture. Also for n=2 the partition function for arbitrary λ is easily obtained by Fisher's necklace method. For n>2 and λ≠0 the only exact result so far is that of Guttmann and Vöge who obtained the generating function $G^{(n)} (\lambda ,u) \equiv \sum\nolimits_{t = 0}^\infty {Z_t^{(n)} (\lambda )u^t } $ for λ=1 and n=3. The main result of this paper is to extend their result to arbitrary λ. By fitting computer generated data it is conjectured that Z ⁽³⁾ _t (λ) satisfies a third order inhomogeneous difference equation with constant coefficients which is used to obtain $$G^{(n)} (\lambda ,u) = \frac{{(\lambda - 3)(\lambda + 2) - \lambda (12 - 5\lambda + \lambda ^2 )u - 2\lambda ^3 u^2 + 2(\lambda - 4)(\lambda ^2 u^2 - 1){\text{ }}c(2u)}}{{(\lambda - 2 - \lambda ^2 u)(\lambda - 1 - 4\lambda u - 4\lambda ^2 u^2 )}}$$ where $c(u) = \tfrac{{1 - \sqrt {1 - 4u} }}{{2u}}$ , the generating function for Catalan numbers. The nature of the collapse transition which occurs at λ=4 is discussed and extensions to higher values of n are considered. It is argued that the position of the collapse transition is independent of n.     

A potential red emitting K4Ca(PO4)2: Eu3+ phosphor for white light emitting diodes

Europium (III) ions doped red phosphors K₄Ca(PO₄)₂ were prepared first time by high temperature solid state reaction method. The prepared phosphors structure was examined by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) analyses. The thermal properties of the synthesized phosphor were investigated by differential scanning calorimetry (DSC) analysis. Photoluminescence (PL) spectra of K₄Ca(PO₄)₂:Eu³⁺ phosphors have shown strong red emission at 618 nm (⁵D₀→⁷F₂) with near UV an excitation wavelength of λ_exc=394 nm (⁷F₀→⁵L₆). In addition, the decay curves and CIE color coordinate measurements are also carried out. Hence, emission and excitation characterization of synthesized phosphors shows that the phosphors may be a promising red component for the application in the white light emitting diodes (WLEDs).     

Direct observation of the potential distribution within organic light emitting diodes under operation

We show the first direct measurement of the potential distribution within organic light emitting diodes (OLEDs) under operation and hereby confirm existing hypotheses about charge transport and accumulation in the layer stack. Using a focused ion beam to mill holes in the diodes we gain access to the cross section of the devices and explore the spatially resolved potential distribution in situ by scanning Kelvin probe microscopy under different bias conditions. In bilayer OLEDs consisting of tris(hydroxyquinolinato) aluminum (Alq₃)/N, N ′‐bis(naphthalene‐1‐yl)‐N,N ′‐bis(phenyl) benzidine (NPB) the potential exclusively drops across the Alq₃ layer for applied bias between onset voltage and a given transition voltage. These findings are consistent with previously performed capacitance–voltage measurements. The behavior can be attributed to charge accumulation at the interface between the different organic materials. Furthermore, we show the potential distribution of devices with different cathode structures and degraded devices to identify the cathode interface as main culprit for decreased performance. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Phase modulation of charge density wave around a short-range potential by the Aharonov-Bohm effect

The analytic result of the Aharonov-Bohm effect for its influence on the oscillation of a two-dimensional charge density around a short range potential is given. Without losing generality, the cases of δ-function and hard disc potentials were examined. Numerical calculations show that the interferences among quantum particles are greatly influenced by the nonlocal effect, which leads to the modulation of phase and amplitude of the oscillation of charge density. Since the presence of a nonlocal influence of the Aharonov-Bohm effect on charged particles is universal, the results in the specific potentials examined are expected to appear also in other general systems which may be beneficial to the study of nanotechnology.     

Space charge around conductors at high potential in magnetoplasma

Summary We discuss the effect of a magnetic field on the structure of the space charge region around very highly charged bodies in the ionosphere. Deductions are obtained for the values of current collected by such bodies for the ionospheric plasma. Post doctoral fellow from International Centre for Theoretical Physics, Trieste, Italy. On leave from Department of Physics Saurashtra University, Rajkot, India, under programme ?Training and Research? in Italian Laboratories.