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In this paper, we present numerical simulations of shock propagation in air over a one-dimensional transverse array of particles. Simulations are carried out by varying the particle spacing and shock Mach number. We compute the unsteady inviscid drag coefficient as a function of time and make relevant comparisons to that for a single particle. We find that deviations in the drag coefficient in time from that of a single particle can be correlated to the acoustic-particle interaction time. Finally, we investigate and classify the interaction of the bow shocks in front of the transverse array of particles. [ABSTRACT FROM AUTHOR]

The acceleration of electron beams with multiple transverse structures in wakefields driven by Laguerre-Gaussian pulses has been studied through three-dimensional (3D) particle-in-cell simulations. Under different laser-plasma conditions, the wakefield shows different transverse structures. In general cases, the wakefield shows a donut-like structure and it accelerates the ring-shaped hollow electron beam. When a lower plasma density or a smaller laser spot size is used, besides the donut-like wakefield, a central bell-like wakefield can also be excited. The wake sets in the center of the donut-like wake. In this case, both a central on-axis electron beam and a ring-shaped electron beam are simultaneously accelerated. Further, reducing the plasma density or laser spot size leads to an on-axis electron beam acceleration only. The research is beneficial for some potential applications requiring special pulse beam structures, such as positron acceleration and collimation. [ABSTRACT FROM AUTHOR]

Electromagnetic properties of Permendur (Fe50Co50 alloy) granular composite materials containing flaky particle have been studied from the RF to microwave frequency range. Properties of the flaky particle composites were compared with the spherical particle ones. The electrical conductivity of the flaky particle composite was higher than that of the spherical particle composite at the same particle content. An insulator to metal transition was observed at the percolation threshold φc in both composites. The φc of the flaky particle composite was lower than that of the spherical one. The relative complex permittivity indicates that the insulating state has dielectric properties. For the spherical particle composite, the permittivity enhancement caused by particle cluster formation can be described by the effective cluster model (ECM). The enhancement of the dielectric constant in the flaky particle composite is larger than the ECM prediction. A negative permittivity spectrum indicating a low frequency plasmonic state was observed in the metallic 70 vol. % flaky particle composite. The relative complex permeability spectra of the flaky particle composite are different from those of the spherical one. The flaky particle composite shows a larger permeability value and lower permeability dispersion frequency than the spherical particle composite. Negative permeability spectra were observed in the both composite materials. The negative permeability frequency band of the flaky particle composite is lower than that of the spherical particle composite owing to the demagnetizing field effect. [ABSTRACT FROM AUTHOR]

Systems of spherical particles moving in Stokes flow are studied for different particle internal structures and boundaries, including the Navier-slip model. It is shown that their hydrodynamic interactions are well described by treating them as solid spheres of smaller hydrodynamic radii, which can be determined from measured single-particle diffusion or intrinsic viscosity coefficients. Effective dynamics of suspensions made of such particles is quite accurately described by mobility coefficients of the solid particles with the hydrodynamic radii, averaged with the unchanged direct interactions between the particles. [ABSTRACT FROM AUTHOR]

The existing solubility data on noble gases in high-temperature silicate melts have been analyzed in terms of Scaling Particle Theory coupled with an ab initio assessment of the electronic, dispersive, and repulsive energy terms based on the Polarized Continuum Model (PCM). After a preliminary analysis of the role of the contracted Gaussian basis sets and theory level in reproducing appropriate static dipole polarizabilities in a vacuum, we have shown that the procedure returns Henry's law constants consistent with the values experimentally observed in water and benzene at T = 25 ◦C and P = 1 bar for the first four elements of the series. The static dielectric constant (ε) of the investigated silicate melts and its optical counterpart (ε ∞) were then resolved through the application of a modified form of the Clausius-Mossotti relation. Argon has been adopted as a probe to depict its high-T solubility in melts through an appropriate choice of the solvent diameter σs, along the guidelines already used in the past for simple media such as water or benzene. The σs obtained was consistent with a simple functional form based on the molecular volume of the solvent. The solubility calculations were then extended to He, Ne, and Kr, whose dispersive and repulsive coefficients are available from theory and we have shown that their ab initio Henry's constants at high T reproduce the observed increase with the static polarizability of the series element with reasonable accuracy. At room temperature (T = 25 ◦C) the calculated Henry's constants of He, Ne, Ar, and Kr in the various silicate media predict higher solubilities than simple extrapolations (i.e., Arrhenius plots) based on high-T experiments and give rise to smooth trends not appreciably affected by the static polarizabilities of the solutes. The present investigation opens new perspectives on a wider application of PCM theory which can be extended to materials of great industrial interest at the core of metallurgical processes, ceramurgy, and the glass industry. [ABSTRACT FROM AUTHOR]

When considering stochastic oscillations of heterogeneous catalyst systems, most researches have focused on the surface of a metal or its oxide catalysts, but there have been few studies on porous catalysts. In this work, the effects of internal noise on oscillations of N2O decomposition over Cu-ZSM-5 zeolites are investigated, using the chemical Langevin equation and a mesoscopic stochastic model. Considering that Cu-ZSM-5 particles are finely divided particles, the number of Cu ions (Ns) is proportional to the particle size at a certain Cu/Al, and the internal noise is inversely proportional to Ns. Stochastic oscillations can be observed outside the deterministic oscillatory region. Furthermore, the performance of the oscillation characterized by the signal-to-noise ratio has a maximum within the optimal size range of 4-8 nm. This suggests that a nanometer-sized zeolite may be best for oscillations. [ABSTRACT FROM AUTHOR]

We have identified the ground state configurations of soft particles (interacting via inverse power potentials) confined between two hard, impenetrable walls. To this end we have used a highly reliable optimization scheme at vanishing temperature while varying the wall separation over a representative range. Apart from the expected layered triangular and square structures (which are compatible with the three-dimensional bulk fcc lattice), we have identified a cascade of highly complex intermediate structures. Taking benefit of the general scaling properties of inverse power potentials, we could identify – for a given softness value – one single master curve which relates the energy to the wall separation, irrespective of the density of the system. Via extensive Monte Carlo simulations, we have performed closer investigations of these intermediate structures at finite temperature: we could provide evidence to which extent these particle arrangements remain stable over a relatively large temperature range. [ABSTRACT FROM AUTHOR]

We investigate the obstructed motion of tracer (test) particles in crowded environments by carrying simulations of two-dimensional Gaussian random walk in model fibrinogen monolayers of different orientational ordering. The fibrinogen molecules are significantly anisotropic and therefore they can form structures where orientational ordering, similar to the one observed in nematic liquid crystals, appears. The work focuses on the dependence between level of the orientational order (degree of environmental crowding) of fibrinogen molecules inside a layer and non-Fickian character of the diffusion process of spherical tracer particles moving within the domain. It is shown that in general particles motion is subdiffusive and strongly anisotropic, and its characteristic features significantly change with the orientational order parameter, concentration of fibrinogens, and radius of a diffusing probe. [ABSTRACT FROM AUTHOR]

Based on the spin-orbit coupling admixture mechanism, we theoretically investigate the spin-flip relaxation via optical phonon scattering in quantum dots by considering the effect of lattice relaxation due to the electron-acoustic phonon deformation potential coupling. The relaxation rate displays a cusp-like structure (or a spin hot spot) that becomes more clearly with increasing temperature. We also calculate the relaxation rate of the spin-conserving process, which follows a Gaussian form and is several orders of magnitude larger than that of spin-flip process. Moreover, we find that the relaxation rate displays the oscillatory behavior due to the interplay effects between the magnetic and spatial confinement for the spin-flip process not for the spin-conserving process. The trends of increasing and decreasing temperature dependence of the relaxation rates for two relaxation processes are obtained in the present model. [ABSTRACT FROM AUTHOR]

We propose a generalized Stoner-Wohlfarth (SW) type model to describe various experimentally observed angular dependencies of the switching field in non-single-domain magnetic particles. Because the nonuniform magnetic states are generally characterized by complicated spin configurations with no simple analytical description, we maintain the macrospin hypothesis and we phenomenologically include the effects of nonuniformities only in the anisotropy energy, preserving as much as possible the elegance of SW model, the concept of critical curve and its geometric interpretation. We compare the results obtained with our model with full micromagnetic simulations in order to evaluate the performance and limits of our approach. [ABSTRACT FROM AUTHOR]

Pr doped Bi7Ti4NbO21 (BTN) multifunctional ceramics were prepared by a conventional sintering technique and their luminescent, dielectric, and ferroelectric properties were investigated. The X-ray diffraction data showed that the samples were single phase, and the scanning electron microscopy image indicated that the ceramics had flake-like grains with uniform thickness. The red emission was observed under the ultraviolet and blue light excitation at room temperature. The thermal quench of luminescence was measured and the result indicated that Pr doped BTN ceramics might be potentially applied in luminescent probes and temperature sensors. With the increasing Pr3+ content, the intensity of the photoluminescence increased initially and then decreased due to concentration quenching; The Curie temperature Tc was slightly shifted to the lower temperature; the ferroelectric properties was almost unchanged by doping. As a new multifunctional material, the Pr doped BTN ceramics could be used for a wide range of application, such as integrated electro-optical devices. [ABSTRACT FROM AUTHOR]

Cation-ordered (111)-oriented epitaxial PbSc0.5Ta0.5O3 (PST) thin films were deposited by pulsed laser deposition on SrRuO3-electroded SrTiO3 (111) substrates at three different temperatures of 525 °C, 550 °C, and 575 °C. All the films were well crystalline and (111)-oriented at all the three growth temperatures; however, the films deposited at the temperatures other than 550 °C exhibited the presence of a pyrochlore phase. X-ray diffraction analysis and transmission electron microscopy measurements revealed that the films were epitaxial and highly cation-ordered. In comparison to (001)-oriented PST films, (111)-oriented films on SrRuO3/SrTiO3 (111) exhibited enhanced ferroelectric and dielectric properties with a broad size distribution of cation-ordered domains (5-100 nm). At a measurement temperature of 100 K, the remnant polarization of PST (111) films is almost [Square_Root]3 times larger than the remnant polarization observed for (001)-oriented PST films, which is attributed to the (111) orientation of the films, as the spontaneous polarization in PST lies close to the [111] direction. The observed dielectric constant and loss at 1 kHz were around 1145 and 0.11, respectively. The dielectric constant is thus almost three times higher than for previously reported (001)-oriented PST thin films, most probably due to the enhancement in cation-ordering. [ABSTRACT FROM AUTHOR]

Demagnetization curves of sub-micron Nd2Fe14B particles with rectangular prism shapes were simulated by micromagnetic simulations. Coercivity decreases with increasing particle thickness for same particle volumes. This unexpected behavior can be explained by different resulting angles of the total effective field when superimposing the applied field vector with the stray field vector at the point of the first reversal event. The angular dependence of the nucleation field follows a Stoner-Wohlfarth like behavior. [ABSTRACT FROM AUTHOR]

We have studied random telegraph signals (RTSs) in a chemically assembled single-electron transistor (SET) at temperatures as low as 300 mK. The RTSs in the chemically assembled SET were investigated by measuring the source-drain current, using a histogram of the RTS dwell time, and calculating the power spectrum density of the drain current-time characteristics. It was found that the dwell time of the RTS was dependent on the drain voltage of the SET, but was independent of the gate voltage. Considering the spatial structure of the chemically assembled SET, the origin of the RTS is attributed to the trapped charges on an alkanethiol-protected Au nanoparticle positioned near the SET. These results are important as they will help to realize stable chemically assembled SETs in practical applications. [ABSTRACT FROM AUTHOR]

We demonstrate a simple generic principle of constructing periodic nano-mesh structures and nano-dot arrays by liquid-bridging in particle self-assemblies. A self-assembly of nano- to micro-sized spheres of polystyrene, produced in a close-packed monolayer on a substrate by a convective method, is used as a template for liquid-bridging of a functional fluid in the pores among the neighboring particles. After the solidification of the fluid, followed by the detachment of the particles from the substrate, a periodic mesh structure is directly obtained. Moreover, deposition of a desired substance onto such mesh structure made of a sacrificial material, which is subsequently removable through a lift-off process, enables to produce a periodic dot array of the substance. Depending on the size of the particle and the nature of liquid bridging in the particle assembly, a variety of nano-mesh structures of silver-ink and nano-dot arrays of gold are constructed. This simple liquid bridging-based methodology is easily applicable for different classes of functional fluids such as soluble organic semiconductors. [ABSTRACT FROM AUTHOR]

Within the general axially nonadiabatic channel approach described in Paper I of this series [M. Auzinsh, E. I. Dashevskaya, I. Litvin, E. E. Nikitin, and J. Troe, J. Chem. Phys. 139, 084311 (2013)], the present article analyzes the simultaneous manifestation of electrostatic and gyroscopic interactions in the quantum capture of dipolar polarizable symmetric top molecules by ions. As a demonstration, the rate coefficients for capture of CH3D and CD3H by H+, D+, and H3+ are calculated. [ABSTRACT FROM AUTHOR]

Ultraviolet photoluminescence, transmission electron microscopy and KOH etching were used to characterize extended defects in 4H-SiC epilayers grown at high growth rates (18 μm/h). Layers exhibited high densities of in-grown stacking faults and dislocation half-loops. The stacking faults were 8H Shockley-type faults. The Burgers vector of the dislocation half-loops was in the (0001) basal plane. Both defects nucleate within the epilayer at early stages of growth. Defect nucleation is directly correlated with high initial growth rate and is not related to any defects/heterogeneities in the substrate or epilayer. Epilayer growth by nucleation of two-dimensional islands is proposed as a possible mechanism for the formation of both defects, through nucleation of faulted Si-C bilayers. [ABSTRACT FROM AUTHOR]

It is demonstrated numerically that a 1-D plasma crystal made of micron size cylindrical dust particles can, in principle, work as a photonic crystal for terahertz waves. The dust rods are parallel to each other and arranged in a linear string forming a periodic structure of dielectric-plasma regions. The dispersion equation is found by solving the waves equation with the boundary conditions at the dust-plasma interface and taking into account the dielectric permittivity of the dust material and plasma. The wavelength of the electromagnetic waves is in the range of a few hundred microns, close to the interparticle separation distance. The band gaps of the 1-D plasma crystal are numerically found for different types of dust materials, separation distances between the dust rods and rod diameters. The distance between levitated dust rods forming a string in rf plasma is shown experimentally to vary over a relatively wide range, from 650 μm to about 1350 μm, depending on the rf power fed into the discharge. [ABSTRACT FROM AUTHOR]

The spontaneous emission rate into Surface Plasmon Polariton (SPP) mode for the InGaN/GaN quantum well (QW) with SP coupling is presented taking into account the electron and hole band structures, the photon density of states, and evanescent fields of SPP. The optical properties of SP-enhanced InGaN QW structure with different QW layer number are investigated in detail by using the formula. It is observed that the energy of electron-hole pairs in the InGaN QW can be efficiently transferred into the SPP modes which will induce the significantly enhancement of the internal quantum efficiency (IQE) of SP-enhanced light emitting diodes (LEDs), especially for the original IQE in the range of 6%-25%. Furthermore, the distribution of electron and hole densities in each well layer can evidently affect the Purcell enhancement factor due to the distance dependence of the intensity of SP-QW coupling. The numerical results also indicate that the SP-enhanced LED can suppress the efficiency droop effect as long as the extraction efficiency of SPP mode is enough large. [ABSTRACT FROM AUTHOR]

The initial growth stages of GaSb epilayers on Si substrates and the role of the AlSb buffer layer were studied by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Heteroepitaxy of GaSb and AlSb on Si both occur by Volmer-Weber (i.e., island mode) growth. However, the AlSb and GaSb islands have distinctly different characteristics as revealed through an atomic-resolution structural study using Z-contrast of HAADF-STEM imaging. While GaSb islands are sparse and three dimensional, AlSb islands are numerous and flattened. The introduction of 3D island-forming AlSb buffer layer facilitates the nucleation of GaSb islands. The AlSb islands-assisted nucleation of GaSb islands results in the formation of drastically higher quality planar film at a significantly smaller thickness of films. The interface of the AlSb and GaSb epilayers with the Si substrate was further investigated with energy dispersive X-ray spectrometry to elucidate the key role of the AlSb buffer layer in the growth of GaSb epilayers on Si substrates. [ABSTRACT FROM AUTHOR]

Two models of particle depositions with different adhesion dynamics have been developed and studied using Monte Carlo simulations. They have been compared for the complete range of Peclet numbers and they differ in the particle-adhesion dynamics, with an adherence-potential barrier in one of them. In general, the time course of the two models is different in ballistic cases, but similar in diffusive cases. Thus, they complement each other, enabling a description of the morphology for most cases. Depending on their deposits, these systems have been characterized morphologically, as compact, homogeneous, or open. They have also been described by the following parameters, which have been analyzed: from the first to the fourth moments, porosity, and the fractal dimension. Also, the time course of the parameters has been studied and described, especially the Anomalous and Family-Vicsek scaling. The third and fourth moments and the fractal dimension have been characterized, defining different stages in their time course, and studying their behavior for the complete range of Peclet numbers. Moreover, the inclusion of an adherence-potential barrier enables the description of new universality class intervals needed to explain recent experimental systems, e.g., at the edges of evaporated drops of colloidal suspensions, electro-convection on nematic liquid crystals, and polynuclear growth model. Finally, the characterization of the systems required not only a dynamic exponent (β) by universality classes but also a third (skewness) and fourth (kurtosis) moments for a complete description. [ABSTRACT FROM AUTHOR]

Although the laser shockwave cleaning process offers a promising alternative to conventional dry-cleaning processes for nanoscale particle removal, its difficulty in removing organic particles has been an unexplained problem. This work elucidates the physics underlying the ineffectiveness of removing organic particles using laser shock cleaning utilizing polystyrene latex particles on silicon substrates. It is found that the shockwave pressure is high enough to deform the particles, increasing the contact radius and consequently the particle adhesion force. The particle deformation has been verified by high-angle scanning electron microscopy. The Maugis-Pollock theory has been applied to predict the contact radius, showing good agreement with the experiment. [ABSTRACT FROM AUTHOR]

An ultrathin metal film deposited on dielectric plate supports low loss THz surface plasma wave (SPW). The SPW field falls off away from the metal film both inside the dielectric as well as in free space. Two lasers normally incident, from free space on the planar structure, exert a difference frequency ponderomotive force on the free electrons of the film and resonantly excite THz SPW. The ratio of SPW amplitude to lasers is 10-3 at laser intensity of 3 × 1012 W/cm2 at 1 μ m wavelength. The growth rate falls monotonically because at higher frequency the coupling of SPW is weak. [ABSTRACT FROM AUTHOR]

In this paper we present a detailed study of the effects of temperature on the two-dimensional electron mobility distribution, using a fully numerical calculation in unintentionally doped AlxGa1-xN/AlN/GaN heterostructures. The analysis of our results clearly indicates that the effect of partial sub-band occupancy is considerable, especially at higher operating temperatures when more than one sub-band is occupied. By correlating with published experimental data, our results are consistent with the observed broadening of the mobility distribution with increasing temperature, which is shown to be a direct consequence of the temperature dependence of polar optical phonons. Although other scattering mechanisms also affect broadening of the mobility distribution, the contribution from these other individual scattering mechanisms is found to be independent of temperature. In particular, a comparison of our calculated results with published experimental data is shown to be in excellent agreement across the temperature range from 100 K to 300 K. By fitting our model to experimental results at low temperatures (95 K-125 K), we are able to extract detailed information related to surface roughness scattering, which is dominant in this temperature range. In particular, the average height of interface roughness is found to be 18 A. [ABSTRACT FROM AUTHOR]

Microstructural evolution of ultrananocrystalline diamond (UNCD) films as a function of substrate temperature (TS) and/or by introducing H2 in Ar/CH4 plasma is investigated. Variation of the sp2 and sp3 carbon content is analyzed using UV-Raman and near-edge X-ray absorption fine structure spectra. Morphological and microstructural studies confirm that films deposited using Ar/CH4 plasma at low TS consist of a random distribution of spherically shaped ultra-nano diamond grains with distinct sp2-bonded grain boundaries, which are attributed to the adherence of CH radicals to the nano-sized diamond clusters. By increasing TS, adhering efficiency of CH radicals to the diamond lattice drops and trans-polyacetylene (t-PA) encapsulating the nano-sized diamond grains break, whereas the addition of 1.5% H2 in Ar/CH4 plasma at low TS induces atomic hydrogen that preferentially etches out the t-PA attached to ultra-nano diamond grains. Both cases make the sp3-diamond phase less passivated. This leads to C2 radicals attaching to the diamond lattice promoting elongated clustered grains along with a complicated defect structure. Such a grain growth model is highly correlated to explain the technologically important functional property, namely, plasma illumination (PI) of UNCD films. Superior PI properties, viz. low threshold field of 0.21 V/μm with a high PI current density of 4.10 mA/cm2 (at an applied field of 0.25 V/μm) and high γ-coefficient (0.2604) are observed for the UNCD films possessing ultra-nano grains with a large fraction of grain boundary phases. The grain boundary component consists of a large amount of sp2-carbon phases that possibly form interconnected paths for facilitating the transport of electrons and the electron field emission process that markedly enhance PI properties. [ABSTRACT FROM AUTHOR]

We report on experimental realization of p-n heterojunctions based on p-type GaN, and an n-type correlated oxide, VO2. The band offsets are evaluated by current-voltage and capacitance-voltage measurements at various temperatures. A band diagram based on the conventional band bending picture is proposed to explain the evolution of the apparent barrier height from electrical measurements and it suggests that the work function of VO2 decreases by ∼0.2 eV when it goes through the insulator to metal transtion, in qualitative agreement with Kelvin force microscopy measurements reported in literature. The frequency-dependent capacitance measurements allow us to differentiate the miniority carrier effect from the interface states and series resistance contributions, and estimate the minority carrier lifetime in insulating phase of VO2 to be of the order of few microseconds. The nitride-oxide based p-n heterojunctions provide a new dimension to study correlated-electron systems and could be of relevance to emerging electronic devices that exploit collective phenomena. [ABSTRACT FROM AUTHOR]

A work function study based on the onset shift (i.e., following low energy cut-off) of secondary electron spectra has been used for the last four decades to monitor the deposition and adsorption in real time, measure the dipole momentum and polarizability of the surface layer, and determine the lateral distribution of the work function. In this work, we show that the onset shift depends on both the coverage of adsorbed species that change the work function and the size of low work function patches. Additionally, the extraction field, which is always applied in these measurements, may also influence the onset shift. Numerical calculations of the potential distributions above different non-uniform surfaces were performed in order to quantitatively determine each of these influences. Depending on the patch size, we define three measurement regimes in which the onset position is related to either the surface average of the work function (small patches), the minimum local work function (large patches), or a value in-between (intermediate size patches). Experimental data have corroborated these findings and demonstrated that manipulating the extraction field intensity enables transition between the measurement regimes. Typical misinterpretations due to neglecting the patch size contribution and the surface non-uniformity to the onset shift are analyzed. Additionally, possible application of work function study for determination of the growth mode in the case of submonolayer deposition was discussed. [ABSTRACT FROM AUTHOR]

The transmittance and the optical constants of SrF2 thin films, a candidate material for multilayer coatings operating in the extreme ultraviolet and soft x-rays, have been determined in the spectral range of 25-780 eV, in most of which no experimental data were previously available. SrF2 films of various thicknesses were deposited by evaporation onto room-temperature, thin Al support films, and their transmittance was measured with synchrotron radiation. The transmittance as a function of film thickness was used to calculate the extinction coefficient k at each photon energy. A decrease in density with increasing SrF2 film thickness was observed. In the calculation of k, this effect was circumvented by fitting the transmittance versus the product of thickness and density. The real part of the refractive index of SrF2 films was calculated from k with Kramers-Krönig analysis, for which the measured spectral range was extended both to lower and to higher photon energies with data in the literature combined with interpolations and extrapolations. With the application of f- and inertial sum rules, the consistency of the compiled data was found to be excellent. [ABSTRACT FROM AUTHOR]

The aims of this study are (i) to demonstrate the synthesis of Cr3+-activated β-Ga2O3 films by metal-organic deposition and (ii) to report the temperature-dependent photoluminescence (PL) properties of such films from 20 to 300 K. An activation energy of ∼0.9 eV for the Cr3+ ions in β-Ga2O3 is determined from a plot of PL intensity vs calcination temperature. The red-line emission doublet R1 and R2 at ∼1.8 eV and the broad emission band with a peak at ∼1.7 eV are ascribed to the Cr3+ ions in the β-Ga2O3 host. The energies of the excited states, i.e., 2E, 4T2, 2T2, 4T1, and 4T1, in Cr3+ are determined from the experimental PL and PL excitation spectra using a newly developed analysis model. The high-energy luminescence tail of the broad 4T2 → 4A2 emission band can be explained by the hot-carrier effect of the photoexcited electrons in the 4T2 state. The relative intensities of the R-line emission doublet can also be explained very well by the population and depopulation of the electron numbers in the [formula] (R1) and [formula] (R2) states. PL properties, such as the temperature-dependent PL intensity, peak energy, and spectral width, are analyzed in detail. [ABSTRACT FROM AUTHOR]

The main objective of the present work is the development of an analytically tractable model of screened electron-electron and electron-exciton interactions in layered systems composed of two parallel semiconductor quantum wells separated by a dielectric layer. These systems are promising for superconductivity with excitons-polaritons, and spin manipulation. Polarization effects induced by the dielectric mismatch in the nanostructure are taken into account using the image charge method. The obtained analytic expressions are used to calculate screened electron-electron and electron-exciton interactions; these are compared to results computed using other recently published models. [ABSTRACT FROM AUTHOR]

X-ray scatter dark field imaging based on the Talbot-Lau interferometer allows for the measurement of ultra-small angle x-ray scattering. The latter is related to the variations in the electron density in the sample at the sub- and micron-scale. Therefore, information on features of the object below the detector resolution can be revealed. In this article, it is demonstrated that scatter dark field imaging is particularly adapted to the study of a material's porosity. An interferometer, optimized for x-ray energies around 50 keV, enables the investigation of aluminum welding with conventional laboratory x-ray tubes. The results show an unprecedented contrast between the pool and the aluminum workpiece. Our conclusions are confirmed due to micro-tomographic three-dimensional reconstructions of the same object with a microscopic resolution. [ABSTRACT FROM AUTHOR]

The transport properties are calculated within the full-potential linearized augmented plane-wave method and the semi-classical Boltzmann theory on the Heusler-type compound Fe2V1-xNbxAl. The obtained Seebeck coefficients are in reasonable agreement with experimental results. The majority carriers of stoichiometric Fe2VAl are holes. When V is slightly replaced by Nb, the Seebeck coefficient becomes negative rapidly resulting in the main carrier changing from hole to electron. σ/τ of stoichiometric Fe2VAl exhibits semiconductor behavior. However, it transits into semimetal when V is replaced by Nb at low temperature and all Nb-substituted samples exhibit semiconducting behavior above 100 K. The power factor S2σ/τ of stoichiometric Fe2VAl is relative small, and it increases greatly after Nb substitution below 360 K. The low temperature power factor of Fe2V1-xNbxAl (x ≠ 0) increases dramatically due to the enhanced Seebeck coefficient. [ABSTRACT FROM AUTHOR]

We investigated origins of the resistivity change during the forming of NiO based resistive random access memories in a nondestructive way using hard x-ray photoelectron spectroscopy. Energy shifts and bandgap states observed after switching suggest that oxygen vacancies are created in the low resistive state. As a result conduction may occur via defects such as electrons traps and metallic nickel impurities. Migration of oxygen atoms seems to be the driving mechanism. This provides concrete evidence of the major role played by oxygen defects in decreasing resistivity. This is a key point since oxygen vacancies are particularly unstable and thus difficult to identify by physico-chemical analyses. [ABSTRACT FROM AUTHOR]

Resonant tunneling through symmetric and asymmetric double barriers based on monolayer graphene at non-normal incidence is investigated. Due to the evanescent modes inside the barrier, the transmission, as a function of the incident energy, has a gap which can be tuned by the height of the barrier and the incident angle of the electrons. In terms of the coupling between the barriers and the well in the symmetric double barriers, several line-type resonance peaks with a unity value appear in the transmission gap, and the number of the resonance peaks is closely related to the incident angle, the height, and the width of the barrier. The resonant conditions in the transmission gap are derived and discussed. However, the line-type resonance peaks are greatly suppressed through the asymmetric double barriers. The authors demonstrate that line-type peaks possess potential applications in resonant tunneling devices and energy filters. [ABSTRACT FROM AUTHOR]

We analyze the mean time tapp that a randomly moving particle spends in a bounded domain (sphere) before it escapes through a small window in the domain’s boundary. A particle is assumed to diffuse freely in the bulk until it approaches the surface of the domain where it becomes weakly adsorbed, and then wanders diffusively along the boundary for a random time until it desorbs back to the bulk, etc. Using a mean-field approximation, we define tapp analytically as a function of the bulk and surface diffusion coefficients, the mean time it spends in the bulk between two consecutive arrivals to the surface and the mean time it wanders on the surface within a single round of the surface diffusion. [ABSTRACT FROM AUTHOR]

Secondary ion mass spectra have been measured for the first time for a liquid ethanol target bombarded by 2.0 MeV He+ ions. Positive and negative ion spectra exhibit evidently a series of cluster ions of the forms [(EtOH)nH]+ and [(EtOH)n-H]-, respectively, in addition to light fragment ions from intact parent molecules. It was found that these cluster ions are produced only from liquid phase ethanol. Both positive and negative secondary ion spectra show similar cluster size distributions with almost the same decay slope. We also present for the first time the cluster ion distribution emitted from the liquid at different liquid temperatures. [ABSTRACT FROM AUTHOR]

We apply a simple model system of patchy particles to study monodisperse self-assembly using the Platonic solids as target structures. We find marked differences between the assembly behaviors of the different systems. Tetrahedra, octahedral, and icosahedra assemble easily, while cubes are more challenging and dodecahedra do not assemble. We relate these differences to the kinetics and thermodynamics of assembly, with the formation of large disordered aggregates a particular important competitor to correct assembly. In particular, the free energy landscapes of those targets that are easy to assemble are funnel-like, whereas for the dodecahedral system the landscape is relatively flat with little driving force to facilitate escape from disordered aggregates. [ABSTRACT FROM AUTHOR]

A typical nucleation-growth process is considered: a system is quenched into a supersaturated state with a small critical radius r*- and is allowed to nucleate during a finite time interval tn, after which the supersaturation is abruptly reduced to a fixed value with a larger critical radius r*+. The size-distribution of nucleated particles f(r,t) further evolves due to their deterministic growth and decay for r larger or smaller than r*+, respectively. A general analytic expressions for f(r,t) is obtained, and it is shown that after a large growth time t this distribution approaches an asymptotic shape determined by two dimensionless parameters, λ related to tn, and Λ=r*+/r*-. This shape is strongly asymmetric with an exponential and double-exponential cutoffs at small and large sizes, respectively, and with a broad near-flat top in case of a long pulse. Conversely, for a short pulse the distribution acquires a distinct maximum at r=rmax(t) and approaches a universal shape exp[ζ-eζ], with ζ∝r-rmax, independent of the pulse duration. General asymptotic predictions are examined in terms of Zeldovich–Frenkel nucleation model where the entire transient behavior can be described in terms of the Lambert W function. Modifications for the Turnbull–Fisher model are also considered, and analytics is compared with exact numerics. Results are expected to have direct implementations in analysis of two-step annealing crystallization experiments, although other applications might be anticipated due to universality of the nucleation pulse technique. [ABSTRACT FROM AUTHOR]

Using molecular dynamics computer simulations we investigate the dynamics of a gel. We start from a fractal structure generated by the diffusion limited cluster aggregation-deflection algorithm, onto which we then impose an interaction potential consisting of a short-range attraction as well as a long-range repulsion. After relaxing the system at zero temperature, we let it evolve at a fixed finite temperature. Depending on the temperature T we find different scenarios for the dynamics. For T>=0.2 the fractal structure is unstable and breaks up into small clusters which relax to equilibrium. For T≤=0.2 the structure is stable and the dynamics slows down with increasing waiting time. At intermediate and low T the mean squared displacement scales as t2/3 and we discuss several mechanisms for this anomalous time dependence. For intermediate T, the self-intermediate scattering function is given by a compressed exponential at small wave vectors and by a stretched exponential at large wave vectors. In contrast, for low T it is a stretched exponential for all wave vectors. This behavior can be traced back to a subtle interplay between elastic rearrangements, fluctuations of chainlike filaments, and heterogeneity. [ABSTRACT FROM AUTHOR]

The multivariable theory of nucleation suggested by Alekseechkin [J. Chem. Phys. 124, 124512 (2006)] is further developed in the context of Langevin’s approach. The use of this approach essentially enhances the capability of the nucleation theory, because it makes possible to consider the cases of small friction which are not taken into account by the classical Zel’dovich–Frenkel theory and its multivariable extensions. The procedure for the phenomenological determination of the nucleation parameters is described. Using the similarity of the Kramers model with that of nucleation, the lifetime of a Brownian particle in potential wells in various dimensionalities is calculated with the help of the expression for the steady state nucleation rate. [ABSTRACT FROM AUTHOR]

Looking back at his research on tachyons in the 1960s and 1970s, the physicist Gerald Feinberg recalled that he started thinking about particles that go faster than light after reading James Blish's 1954 science fiction story "Beep." While the technical conceits of Blish's tale may have stirred Feinberg's curiosity, its literary implications were yet more significant. As a story about faster-than-light messages that travel backward in time, "Beep" thematizes the capacity of speculative fictions to affect the present and reorient the future. For Feinberg, stories like "Beep" and Arthur C. Clarke's 1953 novel Childhood's End offered conceptual resources as well as models for practice, affirming a science fiction way of doing science. By attending to Feinberg's work on tachyons as well as his ventures in futurology, such as The Prometheus Project , this essay shows how Feinberg's reading of science fiction reinforced a speculative approach to knowledge and innovation, an understanding of theoretical science as intimately aligned with science fiction, and a conviction that science fiction was a vital instrument for science policy and social change. [ABSTRACT FROM AUTHOR]

A recent paper1 examines zero field-cooled/field-cooled (ZFC/FC) susceptibility curves for nanoparticle assemblies with a size distribution. It is explained that the 'volume and number weighted distribution are equally valid for the representation of distribution functions in nanoparticle magnetic systems' and the usual modelling approach (abrupt transition from a blocked to a superparamagnetic regime, at a given temperature) is compared to the more elaborate one (the 'progressive crossover model (PCM)') introduced in our previous articles.2-4 The importance of the f0 value is also stressed. In this article, several statements are made in opposition to some of our previously published results. Because we like to believe that these words were driven by a simple 'misunderstanding' of our models and analysis, we would like to clarify some points in the present comment. [ABSTRACT FROM AUTHOR]