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Journal of Applied Physics

Issue 7 • Date Oct 2012

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Displaying Results 1 - 25 of 172
  • Electrical conduction in chalcogenide glasses of phase change memory

    Page(s): 071101 - 071101-20
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    Amorphous chalcogenides have been extensively studied over the last half century due to their application in rewritable optical data storage and in non-volatile phase change memory devices. Yet, the nature of the observed non-ohmic conduction in these glasses is still under debate. In this review, we consolidate and expand the current state of knowledge related to dc conduction in these materials. An overview of the pertinent experimental data is followed by a review of the physics of localized states that are peculiar to chalcogenide glasses. We then describe and evaluate twelve relevant transport mechanisms with conductivities that depend exponentially on the electric field. The discussed mechanisms include various forms of Poole-Frenkel ionization, Schottky emission, hopping conduction, field-induced delocalization of tail states, space-charge-limited current, field emission, percolation band conduction, and transport through crystalline inclusions. Most of the candidates provide more or less satisfactory fits of the observed non-linear IV data. Our analysis calls upon additional studies that would enable one to discriminate between the various alternative models. View full abstract»

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  • Disorder induced semiconductor to metal transition and modifications of grain boundaries in nanocrystalline zinc oxide thin film

    Page(s): 073101 - 073101-4
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    This paper report on the disorder induced semiconductor to metal transition (SMT) and modifications of grain boundaries in nanocrystalline zinc oxide thin film. Disorder is induced using energetic ion irradiation. It eliminates the possibility of impurities induced transition. However, it is revealed that some critical concentration of defects is needed for inducing such kind of SMT at certain critical temperature. Above room temperature, the current-voltage characteristics in reverse bias attributes some interesting phenomenon, such as electric field induced charge transfer, charge trapping, and diffusion of defects. The transition is explained by the defects induced disorder and strain in ZnO crystallites created by high density of electronic excitations. View full abstract»

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  • Gap independent coupling into parallel plate terahertz waveguides using cylindrical horn antennas

    Page(s): 073102 - 073102-5
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    We demonstrate how replacing the silicon lenses, traditionally used to couple radiation into parallel plate waveguides, with integrated cylinder-based horn couplers not only greatly improves ease of use and fabrication but also features gap independent coupling. The couplers, created from chords of a cylinder, give reflection free transmission through the waveguide that is on the order of the quasi-optical approach. The gap independent coupling is demonstrated through a precise measurement of the metal conductivity of the THz skin depth layer. View full abstract»

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  • Electro-optic chirality control in MgO:PPLN

    Page(s): 073103 - 073103-4
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    The chirality of MgO-doped periodically poled lithium niobate (MgO:PPLN) by electro-optic effect was studied. It shows that optical propagation is reciprocal in MgO:PPLN under a transverse electric field and quasi-phase-matching condition, which bears similarity to natural optically active material like quartz. The specific rotation is shown to be proportional to the transverse electric field, making large polarization rotation in optically active material with small size possible. We also demonstrate that the chirality of MgO:PPLN can be controlled by the external electric field. View full abstract»

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  • Effect of phase transition on the optoelectronic properties of Zn1-xMgxS

    Page(s): 073104 - 073104-6
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    Density functional calculations are performed to investigate the structural, electronic, and optical properties of Zn1-xMgxS (0 ≤ x ≤ 1). In the present DFT calculations, we used modified Becke-Johnson potential in the exchange and correlation energy, which is effective for the treatment of the d-orbitals. A structural phase transition from zinc-blende to rock-salt is observed at 73% magnesium, which is consistent with the experimental results. Furthermore, the alloy has direct band gap nature for the whole range of Mg concentration in the zinc-blende structure, while the band gap nature for the rock-salt phase is indirect. The zinc-blende crystal structure has many established applications in the UV optoelectronic devices, and therefore the maintenance of the compound in zinc-blende crystal structure for the maximum range of Mg-composition is highly desirable which is dependent on the composition rate, external environment, and thickness of the film. Keeping in view the importance of ZnMgS in UV optical devices, its optical properties like dielectric functions, refractive indices, reflectivity, and energy loss function are also investigated. View full abstract»

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  • Optoelectronic and magnetic properties of Mn-doped indium tin oxide: A first-principles study

    Page(s): 073105 - 073105-8
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    The manganese doped indium tin oxide (ITO) has integrated magnetics, electronics, and optical properties for next generation multifunctional devices. Our first-principles density functional theory (DFT) calculations show that the manganese atom replaces b-site indium atom, located at the second coordination shell of the interstitial oxygen in ITO. It is also found that both anti-ferromagnetic and ferromagnetic behaviors are realizable. The calculated magnetic moment of 3.95μB/Mn as well as the high transmittance of ∼80% for a 150 nm thin film of Mn doped ITO is in good agreement with the experimental data. The inclusion of on-site Coulomb repulsion corrections via DFT + U methods turns out to improve the optical behavior of the system. The optical behaviors of this system reveal its suitability for the magneto-opto-electronic applications. View full abstract»

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  • The direct and indirect bandgaps of unstrained SixGe1-x-ySny and their photonic device applications

    Page(s): 073106 - 073106-8
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    Using empirical pseudopotential theory, the direct (Γ) and indirect bandgaps (L and X) of unstrained crystalline SixGe1-x-ySny have been calculated over the entire xy composition range. The results are presented as energy-contour maps on ternary diagrams along with a ternary plot of the predicted lattice parameters. A group of 0.2 to 0.6 eV direct-gap SiGeSn materials is found for a variety of mid-infrared photonic applications. A set of “slightly indirect” SiGeSn alloys having a direct gap at 0.8 eV (but with a smaller L-Γ separation than in Ge) have been identified. These materials will function like Ge in various telecom photonic devices. Hetero-layered SiGeSn structures are described for infrared light emitters, amplifiers, photodetectors, and modulators (free carrier or Franz-Keldysh). We have examined in detail the optimized design space for mid-infrared SiGeSn-based multiple-quantum-well laser diodes, amplifiers, photodetectors, and quantum-confined Stark effect modulators. View full abstract»

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  • Pulsed laser operated high rate charging of Fe-doped LiNbO3 crystal for electron emission

    Page(s): 073107 - 073107-11
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    High rate 600 ppm Fe-doped LiNbO3 crystal charging for electron emission by pulsed laser radiation is studied theoretically for 532 nm ≈10 ns laser pulse fluence 0.01-6 J/cm2. The previous experimental results are used as a reference point for developing analytical and computational estimates which show that high charging rate under pulsed laser radiation is due to roughly equal contributions from: (i) photogalvanic current, (ii) pyroelectric current, and (iii) electron drift by laser induced electric field. The computational photo-refractive model, verified against experimental data, additionally includes recombination rate non-linearity giving very significant feedback effect for charge density, electric field, and electron energy under laser fluence above 0.5 J/cm2. Theoretical study is finalized by the expressions linking the laser fluence with resulting surface charge density and repulsive energy for emitted electrons. The model gives also the estimate of the laser damage threshold fluence at ≈6 J/cm2 above which the laser induced electric field accelerates the electrons to an energy sufficient for electron impact ionization from the valence band. In operation below this threshold single laser pulse is shown to provide surface charge densities above 10-7 C/cm2 with the repulsive energy close to 106 eV. A high frequency pulsed laser with single pulse fluence below the damage threshold is suggested to achieve the repulsive energies above 106 eV. View full abstract»

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  • X-ray induced Sm3+ to Sm2+ conversion in fluorophosphate and fluoroaluminate glasses for the monitoring of high-doses in microbeam radiation therapy

    Page(s): 073108 - 073108-10
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    Fluorophosphate and fluoroaluminate glasses doped with trivalent samarium were evaluated as sensors of x-ray radiation for microbeam radiation therapy at the Canadian Light Source using the conversion of trivalent Sm3+ to the divalent form Sm2+. Both types of glasses show similar conversion rates and may be used as a linear sensor up to ∼150 Gy and as a nonlinear sensor up to ∼2400 Gy, where saturation is reached. Experiments with a multi-slit collimator show high spatial resolution of the conversion pattern; the pattern was acquired by a confocal fluorescence microscopy technique. The effects of previous x-ray exposure may be erased by annealing at temperatures exceeding the glass transition temperature Tg while annealing at TA < Tg enhances the Sm conversion. This enhancement is explained by a thermally stimulated relaxation of host glass ionic matrix surrounding x-ray induced Sm2+ ions. In addition, some of the Sm3+-doped glasses were codoped with Eu2+-ions but the results show that there is no marked improvement in the conversion efficiency by the introduction of Eu2+. View full abstract»

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  • Nanostructures synthesis by femtosecond laser ablation of glasses

    Page(s): 073109 - 073109-7
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    In this article, we investigate the variations in ablation dynamics that result in diverse nanostructures on SiO2 based glass samples. A three-dimensional fibrous nanoparticle agglomerate was observed on sodalime glass when exposed to femtosecond laser irradiation. The fused nanoparticles have diameters ranging from 30 nm to 70 nm. Long continuous nanofibers of extremely high aspect ratio (certain fibers up to 100 000:1) were obtained by exposing silica glass surface to femtosecond laser irradiation at MHz repetition rate in air. A nanostructure assembly comprising of nanofiber and nanoparticle agglomerates was also observed by ablating silica glass. From our experimental analysis, it was determined that variation in bandgap and material composition alters ablation dynamics and dictates the response of glass to femtosecond laser irradiation, ultimately leading to the formation of structures with varying morphology on silica and sodalime glass. The possible underlying mechanisms that produce such nanostructures on glass specimens have also been explored. View full abstract»

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  • Detection of low frequency hurricane emissions using a ring laser interferometer

    Page(s): 073110 - 073110-10
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    Over the last decade, large horizontally mounted ring laser interferometers have demonstrated the capacity to measure numerous geophysical effects. In this paper, responses from large ring laser interferometers to low frequency hurricane emissions are presented. Hurricanes create a broad spectrum of noise that extends into the millihertz range. In addition to microseisms, hurricanes with established eyewalls were found to create distinct frequency peaks close to 7 mHz as they came ashore or moved over shallow water. Selected emissions from Hurricanes Katrina, Wilma, and Dean are presented. The exact coupling mechanism between the ∼7 mHz hurricane emissions and the ring lasers remains under active investigation. View full abstract»

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  • Effects of excess tellurium and growth parameters on the band gap defect levels in CdxZn1-xTe

    Page(s): 073111 - 073111-13
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    This research summarizes an effective way to understand compensation for use of CdZnTe as ambient temperature radiation detector. The indium doped CdZnTe passivates certain detrimental intrinsic defects and defect complexes in the band gap. This was achieved by using a combination of excess tellurium in the starting material (0% to 7.5% by weight) and the process variables during growth, including the imposed temperature gradient, growth rate, and cool-down process. These studies have shown that a combination of slight excess tellurium as well as the cool-down scheme could control certain intrinsic defect levels and defect level complexes in the band gap of CdZnTe by causing favorable carrier compensation. At a macroscopic level, these manipulations help to minimize thermal instabilities during growth and determine the final grain structure, integrity, and yield of the ingot. Also, these manipulations help to control the formation of certain intrinsic defect levels and defect level complexes in the band gap, which have a direct bearing on the ability of the CdZnTe crystals to function as room temperature radiation detectors. The band-gap defects in CdZnTe were studied using the thermally stimulated current (TSC) technique. The thermal ionization energy and capture cross-section for 8 prominent defect levels (current peaks in the TSC spectrum) were calculated using the variable heating rate method. These fitted values were compared to transition energy levels of possible defects in the band gap of CdTe and purity data of CdZnTe samples used in this study. The theoretical values of the transition energy levels of defects in the band gap of CdTe were determined by the first principle band gap structure studies as well as our earlier studies on defects and defect levels in the band gap of CdTe. We have tentatively associated some prominent current peaks to certain defect levels and defect level complexes in Cd1-xZnxTe. The improvement in the detec- or properties was correlated to the reduction of a proposed deep level defect complex (TeCd + VCd) (thermal ionization energy >0.8 eV and capture cross-section of 10-13 to 10-14 cm2), and the reduction of the ionized species corresponding to an acceptor defect level (thermal ionization energy ∼0.2[03] eV), associated with dislocations/dislocation complexes with Te clusters. The best crystals tested had an average μτe (electrons) of 1.8 × 10-3 cm2/V, a peak-to-valley ratio of 2.0 for the 122 keV x-ray peak using a Co-57 source and bulk resistivity the order of 3 × 1010 Ω cm. The best radiation detector crystals corresponded to those grown with an excess tellurium of 0.5% (by weight in the starting CdZnTe charge) grown at a rate of 0.5 to 0.86 mm/h with an imposed temperature gradient of > 50 °C/in. There was a high yield without any post-processing of the ingots. View full abstract»

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  • Electrical and luminescent properties and deep traps spectra in GaN nanopillar layers prepared by dry etching

    Page(s): 073112 - 073112-6
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    Electrical properties, microcathodoluminescence spectra, and spectra of deep traps were studied for nanopillar structures prepared by dry etching of undoped GaN films using natural masks formed by Ni nanoparticles. It is shown that as-prepared nanopillar structures have low bandedge intensity, very high leakage current of Schottky diodes, their electrical properties are determined by 0.2 eV electron traps or, after etching in aqua regia, 0.14 eV electron traps that are commonly associated with radiation defects. Deep levels transient spectroscopy spectra measured after aqua regia etching are dominated by 1 eV electron traps, other common radiation defects. Annealing at 600 °C is instrumental in eliminating the 0.2 eV and 0.14 eV electron traps, but not the 1 eV traps. A higher temperature annealing at 900 °C is required for strongly suppressing the latter and increasing the bandedge luminescence peak magnitude by 2 times compared to control sample. The best results in terms of luminescence efficiency increase are produced by additional etching in aqueous solution of KOH, but subsequent etching in aqua regia is necessary to suppress excessive surface leakage due to surface contamination by the KOH treatment. View full abstract»

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  • Complementary periodic diffracting metallic nanohole and nanodipole arrays in the mid-infrared range

    Page(s): 073113 - 073113-4
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    Metallic nanohole arrays and metallic nanodipole arrays are fabricated and experimentally characterized. A complementary response is observed in both transmission and reflection. For the metallic nanohole arrays, a peak (dip) in transmission (reflection) is observed at resonance whereas the metallic nanodipole arrays display a dip (peak) in transmission (reflection). The resonant frequency of both the metallic nanohole arrays and the nanodipole arrays depends on the dipole arm length, the incident angle, and the period. The resonant position of the nanohole arrays matches that of its complement, which means that Babinet's principle nearly holds for these structures in the mid-infrared region. View full abstract»

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  • A three-dimensional self-supporting low loss microwave lens with a negative refractive index

    Page(s): 073114 - 073114-4
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    Demonstrations of focusing with metamaterial lenses have predominantly featured two dimensional structures or stacks of planar elements, both limited by losses which hinder realized gain near the focal region. In this study, we present a plano-concave lens built from a 3D self-supporting metamaterial structure featuring a negative refractive index between 10 and 12 GHz. Fabricated using macroscopic layered prototyping, the lens curvature, negative index and low loss contribute to a recognizable focus and free space gains above 13 dB. View full abstract»

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  • Terahertz emission from cubic semiconductor induced by a transient anisotropic photocurrent

    Page(s): 073115 - 073115-8
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    Terahertz emission from the surfaces of narrow-gap semiconductors excited by femtosecond laser pulses was described in terms of a transient interband photoconductivity. It has been found that the nonparabolicity of the electron dispersion law as well as the optical alignment of the photoexcited carrier momenta result in anisotropic photocurrent with a component perpendicular to the surface dc electric field even in semiconductors with a cubic symmetry. This lateral transient photocurrent component is the strongest during the first few hundreds of femtoseconds after the photoexcitation and causes the emission of terahertz radiation pulses with an amplitude dependent on the angle between the optical field and the crystallographic axes. In the case of InAs the contribution of this component explains experimental results of both the azimuthal anisotropy of the emitted terahertz pulse amplitude and its dependence on the exciting photon energy. View full abstract»

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  • Hyperbolic transmission-line metamaterials

    Page(s): 073116 - 073116-5
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    We demonstrate how to realize an indefinite media with hyperbolic isofrequency surfaces in wavevector space by employing two-dimensional metamaterial transmission lines. We classify different types of such media, and visualize the peculiar character of wave propagation by study of the cross-like emission pattern of a current source placed in the lattice center. Our results are supported by a solution of the Kirchhoff equations, an analytical theory, and experimental data. View full abstract»

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  • Transient effects of pump-probe optical response in intersubband transitions of semiconductor quantum wells

    Page(s): 073117 - 073117-11
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    We study theoretically the time evolution of nonlinear optical phenomena of a probe electromagnetic field pulse in intersubband transitions of a semiconductor quantum well structure under the influence of a pump electromagnetic field. More specifically, we take into consideration the effects of electron-electron interactions and consider the interaction of a two-subband system with rectangular electromagnetic fields. The spectral form of the linear absorption, dispersion, and nonlinear optical Kerr effect is described, from the time that the external field is applied up until the dynamics of the system has reached a steady state. In order to describe the dynamics of the system, the effective nonlinear Bloch equations are used, in which renormalized terms for the transition energy and the applied field have been added, due to the effects of electron-electron interactions. These equations are properly combined, and the differential equations of the density matrix elements for the several optical phenomena up to third order are derived. These are solved numerically in the whole time range up to the steady state for a GaAs/AlGaAs quantum well structure. We show that the form of the several optical spectra studied here has a significantly altered evolution over time, according to the value of the frequency of the pump field and of electron sheet density. Even in the case in which the spectral form in the steady state case is almost identical for two different sets of parameters, their evolution can be quite different, according to the precise value of these parameters. View full abstract»

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  • Numerical study of an electrostatic plasma sheath containing two species of charged dust particles

    Page(s): 073301 - 073301-10
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    A multi-fluid model is used to study the dynamics of a dusty plasma sheath consists of electrons, ions, and two species of charged dust particles, i.e., nano-size and micron-size particles. It is found that, when the sheath is dominated by the nano-size dust grains, spatially periodic fluctuations are developed in the profiles of the sheath potential, and the number density and velocity of the plasma and dust particles. Due to inertial effects, the fluctuations in the parameters of the micron-size grains are much lower than those of the other parameters. The competition between the electric and ion drag forces plays the primary role in development of the fluctuations. The spatial period of the fluctuations is approximately a few Debye lengths and their amplitude depends on the plasma and dust parameters. The fluctuations are reduced by the increase in the radius, mass density, and Mach number of the nano-size particles, as well as the density and Mach number of the ions. But, they are enhanced by the increase in the plasma number density and the electron temperature. The sheath thickness demonstrates a non-monotonic behavior against variation of the nanoparticle parameters, i.e., it first decreases quickly, shows a minimum, and then increases. However, the sheath width always decreases with the plasma number density and ion Mach number, while grows linearly with the electron temperature. View full abstract»

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  • Control of ion density distribution by magnetic traps for plasma electrons

    Page(s): 073302 - 073302-10
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    The effect of a magnetic field of two magnetic coils on the ion current density distribution in the setup for low-temperature plasma deposition is investigated. The substrate of 400 mm diameter is placed at a distance of 325 mm from the plasma duct exit, with the two magnetic coils mounted symmetrically under the substrate at a distance of 140 mm relative to the substrate centre. A planar probe is used to measure the ion current density distribution along the plasma flux cross-sections at distances of 150, 230, and 325 mm from the plasma duct exit. It is shown that the magnetic field strongly affects the ion current density distribution. Transparent plastic films are used to investigate qualitatively the ion density distribution profiles and the effect of the magnetic field. A theoretical model is developed to describe the interaction of the ion fluxes with the negative space charge regions associated with the magnetic trapping of the plasma electrons. Theoretical results are compared with the experimental measurements, and a reasonable agreement is demonstrated. View full abstract»

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  • Synthesis of diamond fine particles on levitated seed particles in a rf CH4/H2 plasma chamber equipped with a hot filament

    Page(s): 073303 - 073303-4
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    The first successful growth of diamond layers on levitated seed particles in CH4/H2 plasma is presented. The particles were grown in a rf CH4/H2 plasma chamber equipped with a tungsten hot filament. The seed diamond particles injected in a plasma are negatively charged and levitated under the balance of several forces, and diamond chemical vapor deposition takes place on them. The SEM images show that the crystalline structures are formed after the coagulation of islands. The micro-Raman spectroscopy of the particle grown after several hours shows the clear peak assigned to diamond. View full abstract»

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  • The physical phenomena accompanying the sub-nanosecond high-voltage pulsed discharge in nitrogen

    Page(s): 073304 - 073304-4
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    Results of one-dimensional Particle-in-Cell numerical simulations of mechanism of sub-nanosecond high-voltage pulsed discharge in nitrogen are presented. It is shown that the decrease of the cathode-anode gap changes drastically both the discharge dynamics and mechanism of runaway electrons generation responsible for the discharge initiation. It is obtained that the virtual cathode exists only during tens of picoseconds for short gaps. The conditions when the virtual cathode is not formed are found. Also, the comparison between the experimental [Rybka etal, Tech. Phys. Lett. 38, 653 (2012)] and simulation results indicates the dominant role of the virtual cathode in termination of runaway electrons generation and on separate nature of emission sources from the cathode surface. View full abstract»

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  • Minimum energy structures of faceted, incoherent interfaces

    Page(s): 073501 - 073501-10
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    In this article, we describe a method for quantifying the dislocation distribution in incoherent faceted fcc/bcc interfaces, including details such as the facet length and crystallography and the location, Burgers vector, and line orientation of each interface dislocation. The method is applied to a variety of relaxed equilibrium interface structures obtained from atomistic simulations. The results show that minimum energy forms of faceted interfaces are achieved when the serrated interface planes of the natural lattice are optimally matched such that when joined and relaxed, extended facet faces can form with minimum density of interface dislocations. With a proposed dislocation-based model for the formation energy, we demonstrate that optimal matching corresponds to minimal self-energies of the interfacial dislocations and extended facets (terrace planes). Most importantly, the formation energy of faceted interfaces is found to have no correlation with the net Burgers vector of the interface, which further emphasizes the importance of characterizing the interfacial dislocation distribution. View full abstract»

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  • Surface wettability of titania thin films with increasing Nb content

    Page(s): 073502 - 073502-5
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    TiO2 and TiO2/Nb amorphous thin films were grown on glass substrates by a sol-gel technique (spin coating). Films' surface composition, structure, and morphology were derived from x-ray photoelectron spectroscopy, x-ray diffraction, and atomic force microscopy data. The investigated films showed a smooth surface (roughness values below 5 nm). A separate surface wettability investigation showed that by increasing the Nb amount in pristine titania films results in a decrease of contact angle (CA) values from 40° to nearly 0°, thus, indicating a super-hydrophilic conversion under UV illumination. This conversion rate is greatly enhanced by increasing the Nb content, the surface super-hydrophilic behavior occurring after a couple of minutes in the TiO2/Nb samples, but after 4 h in the pristine titania specimen. The current results are discussed in terms of the optical band gap shift towards higher energies, by increasing the Nb content in the films, a process explained based on small polaron hopping model. View full abstract»

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  • Structural, elastic, and polarization parameters and band structures of wurtzite ZnO and MgO

    Page(s): 073503 - 073503-6
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    Ab initio calculations were carried out to predict lattice constants, elastic stiffness constants, spontaneous polarization, piezoelectric constants, and band structure of virtually wurtzite (wz)-MgO. The ground-state properties for both wz-ZnO and wz-MgO were computed using the pseudopotential-planewave method in conjunction with the local density approximation adding the Hubbard parameter to density functional theory. From the results of fitting to reliable in-plane and out-of-plane lattice constants for strain-free and perfectly pseudomorphic wz-MgxZn1-xO alloys, the elastic stiffness constant C33 of the alloy system is revealed to deviate from Vegard's law. The validity of other calculated results for virtually wz-MgO is discussed based on the physical meaning and accuracy, making a comparison with the results shown in previous reports. View full abstract»

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Aims & Scope

Journal of Applied Physics is the American Institute of Physics' (AIP) archival journal for significant new results in applied physics

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Editor
P. James Viccaro
Argonne National Laboratory