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Journal of Applied Physics

Issue 16 • Date Apr 2013

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Displaying Results 1 - 25 of 75
  • Cracks measurement using fiber-phased array laser ultrasound generation

    Page(s): 163101 - 163101-7
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    A phased array laser ultrasound generation system by using fiber optic delivery and a custom-designed focusing objective lens has been developed for crack inspection. The enhancement of crack tip diffraction by using phased array laser ultrasound is simulated with finite element method and validated by experiment. A non-contact and non-destructive measurement of inner-surface cracks by time-of-flight diffraction method using fiber-phased array laser ultrasound generation and electromagnetic acoustic transducer detection has been studied. View full abstract»

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  • Large-area broad band saturable Bragg reflectors using oxidized AlAs in the circular and inverted mesa geometries

    Page(s): 163102 - 163102-5
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    A semiconductor Saturable Bragg Reflector (SBR) is a mirror structure comprising alternating layers of high and low refractive index materials with an incorporated saturable absorber. SBRs can be used to initiate and sustain ultra-short pulses in various laser systems. In order to form ultra-short pulses, SBRs with high reflectivity over a broad wavelength range are required. Furthermore, large-area SBRs facilitate easy integration in a laser cavity. One of the key elements for the realization of broad band SBRs is the development of the thermal oxidation process that creates buried low-index AlxOy layers over large areas. The design, fabrication, characterization, and implementation of broad band, high index contrast III-V/AlxOy SBRs in the form of circular mesas, as well as inverted mesa structures, is presented. View full abstract»

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  • Designing a square invisibility cloak using metamaterials made of stacked positive-negative index slabs

    Page(s): 163103 - 163103-6
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    We present a design of a square invisibility cloak based on triangular transformations. In triangular transformations, certain areas of the cloak are squeezed to be infinitesimal in the virtual space. This results in cloak materials whose constitutive parameters are singular. We show rigorously that these prescribed singular materials can be emulated by metamaterials made of stacked positive-negative index slabs. We use numerical simulations to demonstrate the conceived square cloak, which in principle can be constructed using only homogenous and non-singular materials. The proposed square invisibility cloak suggests another important application of negative index media. View full abstract»

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  • Spectroscopic ellipsometry and multiphonon Raman spectroscopic study of excitonic effects in ZnO films

    Page(s): 163104 - 163104-6
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    The optical properties and room temperature excitonic features of rf-sputtered ZnO films on Si substrates are studied using spectroscopic ellipsometry and multiphonon Raman scattering. The complex dielectric function ε(E) = ε1(E) + iε2(E) in the spectral region of 1.3 to 5.5 eV is measured using variable-angle spectroscopic ellipsometry and a Tauc-Lorentz multi-oscillator model is employed to fit the lineshape of the dielectric function. The values of the refractive index of the as-deposited and annealed samples are obtained. Excitonic features in the dielectric function are observed after thermal annealing of the films. Multiphonon Raman spectra up to 7 orders of the longitudinal-optic phonon mode are observed. The matching of the outgoing resonance condition with the exciton energy indicates a strong exciton-phonon coupling. The shape of the dielectric function as well as the intensities and widths of the multiphonon lines suggest the role of both discrete and continuum excitons in describing the optical properties. The observation of excitonic features and the values of the refractive index suggest the high optical quality of the films. View full abstract»

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  • Deep-ultraviolet tailored- and low-refractive index antireflection coatings for light-extraction enhancement of light emitting diodes

    Page(s): 163105 - 163105-6
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    An omnidirectional antireflection (AR) coating for a deep-ultraviolet (UV) AR band is designed and fabricated on the sapphire substrate of a deep-UV flip-chip light-emitting diode (LED) device. The two-layer AR coating uses the tailored- and low-refractive index nanoporous alumina fabricated by glancing-angle deposition methodology. The AR coating effectively matches the refractive indices between the air and sapphire substrate. At close-to-normal angles of incidence, this AR coating almost completely eliminates the Fresnel reflection at the sapphire/air interface of the deep-UV LED device. The resulting improvement of the light-extraction efficiency by 8% is in good agreement with the simulation results. For a total thickness of 172 nm for the two-layer AR coating, extinction was negligible (<2%). The results show that nanoporous alumina thin films are excellent tailored- and low-refractive index thin film materials for high-performance deep-UV AR coating applications. View full abstract»

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  • Influence of the absorber layer thickness and rod length on the performance of three-dimensional nanorods thin film hydrogenated amorphous silicon solar cells

    Page(s): 163106 - 163106-4
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    Performance of substrate-configured hydrogenated amorphous silicon solar cells based on ZnO nanorod arrays prepared by hydrothermal method has been investigated. The light harvest ability of three-dimensional nanorods solar cells is a compromise between the absorber layer thickness and the nanorods geometry. By optimizing the intrinsic a-Si:H absorber layer thickness from 75 to 250 nm and varying the length of the nanorods from 600 to 1800 nm, the highest energy conversion efficiency of 6.07% is obtained for the nanorods solar cell having thin absorber layer thickness of 200 nm with the rod length of 600 nm. This represents up to 28% enhanced efficiency compared to the conventional flat reference cell with similar absorber layer thickness. View full abstract»

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  • Type II–type I conversion of GaAs/GaAsSb heterostructure energy spectrum under optical pumping

    Page(s): 163107 - 163107-4
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    We present the experimental results of time-resolved photoluminescence spectroscopy in type II GaAs/GaAs0.64Sb0.36 quantum well heterostructures. At moderate optical excitation densities (below 103 W/cm2), we observe blue shift of the photoluminescence peak with increasing pump power which results from band bending at the type II heterointerface due to photo-excited charge carriers. With further increase in the excitation density, the observed peak undergoes red shift accompanied by significant drop in the luminescence decay time (from 10 ns to 1 ns) which is caused by extreme band bending and increasing contribution of type I radiative transitions to the photoluminescence signal. View full abstract»

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  • Modification of dislocation behavior in GaN overgrown on engineered AlN film-on-bulk Si substrate

    Page(s): 163108 - 163108-5
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    The changes that the AlN buffer and Si substrate undergo at each stage of our substrate engineering process, previously shown to lead to a simultaneous and substantial reduction in film crack density and dislocation density in overgrown GaN, are presented. Evidence of ion-implantation assisted grain reorientation for AlN islands coupled with physical isolation from the bulk Si substrate prove to be the dominating driving forces. This is further emphasized with x-ray diffraction analysis that demonstrates a reduction in the in-plane lattice constant of AlN from 3.148 Å to 3.113 Å and a relative change in rotation of AlN islands by 0.135° with regard to the Si substrate after substrate engineering. Misfit dislocations at the AlN-Si interface and disorder that is normally associated with formation of amorphous SiNx at this interface are considered to be two of the major contributors to dislocation nucleation within overgrown GaN. Following our technique, the disappearance of disorder at the AlN-Si interface is observed. Extensive ellipsometry and transmission electron microscopy suggests that larger AlN islands with a smoother surface morphology could further reduce the dislocation density below that previously reported. A 1.2 μm GaN layer deposited on an AlN buffer with larger islands and smoother morphology exhibits a 14× reduction in surface pit density after undergoing the ion-implantation assisted substrate modification technique. View full abstract»

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  • Krypton charge exchange cross sections for Hall effect thruster models

    Page(s): 163301 - 163301-8
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    Following discharge from a Hall effect thruster, charge exchange occurs between ions and un-ionized propellant atoms. The low-energy cations produced can disturb operation of onboard instrumentation or the thruster itself. Charge-exchange cross sections for both singly and doubly charged propellant atoms are required to model these interactions. While xenon is the most common propellant currently used in Hall effect thrusters, other propellants are being considered, in particular, krypton. We present here guided-ion beam measurements and comparisons to semiclassical calculations for Kr+ + Kr and Kr2+ + Kr cross sections. The measurements of symmetric Kr+ + Kr charge exchange are in good agreement with both the calculations including spin-orbit effects and previous measurements. For the symmetric Kr2+ + Kr reaction, we present cross section measurements for center-of-mass energies between 1 eV and 300 eV, which spans energies not previously examined experimentally. These cross section measurements compare well with a simple one-electron transfer model. Finally, cross sections for the asymmetric Kr2+ + Kr → Kr+ + Kr+ reaction show an onset near 12 eV, reaching cross sections near constant value of 1.6 Å2 with an exception near 70–80 eV. View full abstract»

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  • Phase-resolved optical emission spectroscopy for an electron cyclotron resonance etcher

    Page(s): 163302 - 163302-7
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    Phase-resolved optical emission spectroscopy (PROES) is used for the measurement of plasma products in a typical industrial electron cyclotron resonance (ECR) plasma etcher. In this paper, the PROES of oxygen and argon atoms spectral lines are investigated over a wide range of process parameters. The PROES shows a discrimination between the plasma species from gas phase and those which come from the solid phase due to surface etching. The relationship between the micro-wave and radio-frequency generators for plasma creation in the ECR can be better understood by the use of PROES. View full abstract»

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  • The time evolution of turbulent parameters in reversed-field pinch plasmas

    Page(s): 163303 - 163303-8
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    Turbulence is abundant in fully ionized fusion plasmas, with unique turbulent characteristics in different phases of the discharge. Using Fourier and chaos-based techniques, a set of parameters have been developed to profile the time evolution of turbulence in high temperature, fusion plasmas, specifically in self-organized, reversed-field pinch plasma in the Madison Symmetric Torus. With constant density and plasma current, the turbulence profile is measured during ramp-up, magnetic reconnection, and increased confinement phases. During magnetic reconnection, a scan of plasma current is performed with a constant density. Analysis revealed that the energy associated with turbulence (turbulent energy) is found to increase when changes in magnetic energy occur and is correlated to edge ion temperatures. As the turbulent energy increases with increasing current, the rate at which this energy flow between scales (spectral index) and anti-persistence of the fluctuations increases (Hurst exponent). These turbulent parameters are then compared to the ramp-up phase and increased confinement regime. View full abstract»

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  • Characterization of plasma chemistry and ion energy in cathodic arc plasma from Ti-Si cathodes of different compositions

    Page(s): 163304 - 163304-7
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    Arc plasma from Ti-Si compound cathodes with up to 25 at. % Si was characterized in a DC arc system with respect to chemistry and charge-state-resolved ion energy. The plasma ion composition showed a lower Si content, diverging up to 12 at. % compared to the cathode composition, yet concurrently deposited films were in accordance with the cathode stoichiometry. Significant contribution to film growth from neutrals is inferred besides ions, since the contribution from macroparticles, estimated by scanning electron microscopy, cannot alone account for the compositional difference between cathode, plasma, and film. The average ion charge states for Ti and Si were higher than reference data for elemental cathodes. This result is likely related to TiSix phases of higher cohesive energies in the compound cathodes and higher effective electron temperature in plasma formation. The ion energy distributions extended up to ∼200 and ∼130 eV for Ti and Si, respectively, with corresponding average energies of ∼60 and ∼30 eV. These averages were, however, not dependent on Si content in the cathode, except for 25 at. % Si where the average energies were increased up to 72 eV for Ti and 47 eV for Si. View full abstract»

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  • Emission features of femtosecond laser ablated carbon plasma in ambient helium

    Page(s): 163305 - 163305-9
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    We investigated the optical emission features of plasmas produced by 800 nm, 40 fs ultrafast laser pulses on a carbon target in the presence of ambient helium or nitrogen gases at varied pressures. Fast photography employing intensified charge coupled device, optical emission spectroscopy, and temporally spatially resolved optical time of flight emission spectroscopy were used as diagnostic tools. Spatio-temporal contours of excited neutral, ionic, as well as molecular carbon species in the plume were obtained using time of flight emission spectroscopy. These contours provided detailed account of molecular species evolution and expansion dynamics and indicate that three-body recombination is a major mechanism for carbon dimers generation in ultrafast laser ablation plumes in the presence of ambient gas. A systematic comparison of the emission features from ns and fs laser ablation carbon plumes as well as their expansion in ambient helium is also given. C2 vibrational temperatures were estimated during carbon plasma expansion with lower values in ambient helium compared to nitrogen and showed decreasing values with respect to space and ambient gas pressure. View full abstract»

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  • A transition in the nature of the occupancy of the dislocation lines within n-type wurtzite gallium nitride

    Page(s): 163501 - 163501-5
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    Within the framework of the model of Read [Philos. Mag. 45, 775 (1954)], we examine the occupancy of the dislocation lines within n-type wurtzite gallium nitride. In particular, we examine the transition that occurs as the bulk doping concentration is increased, from the depletion limit to the non-depletion limit. We note that an abrupt transitional bulk doping concentration can be defined. The dependence of this transitional bulk doping concentration on the dislocation line density is then determined. We note that existing theoretical results on the occupation statistics of the threading dislocation lines within wurtzite gallium nitride also exhibit such a transition. Since these theoretical results assume different structures for the core of the threading dislocation lines, we conclude that this transition between the depletion and non-depletion domains should be a universal feature, holding true irrespective of the particular structure being assumed for the core of the threading dislocation lines. Conditions under which this abrupt transition smoothens are also discussed. View full abstract»

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  • Evidence for boron diffusion into sub-stoichiometric MgO (001) barriers in CoFeB/MgO-based magnetic tunnel junctions

    Page(s): 163502 - 163502-6
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    Evidence of boron diffusion into the MgO barrier of a CoFeB/MgO based magnetic tunnel junction has been identified using analytical scanning transmission electron microscopy (STEM) and X-ray photoelectron spectroscopy. Structures were deposited by DC/RF-magnetron sputtering, where defective, sub-stoichiometric MgO barriers degrading device performance have been previously mitigated against by deposition of thin Mg layers prior to MgO deposition. We show that despite the protection offered by the Mg layer, disorder in the MgO barrier is still evident by STEM analysis and is a consequence of the oxidation of the Co40Fe40B20 surface during MgO deposition. Evidence of boron diffusion from CoFeB into the MgO barrier in the as-deposited and annealed structure is also presented, which in the as-deposited case we suggest results from the defective structures at the barrier interfaces. Annealing at 375 °C results in the presence of B in the trigonal coordination of [BO3]3- in the MgO barrier and partial crystallization of the top electrode (we presume there is also some boron diffusion into the Ta capping layer). The bottom electrode, however, fails to crystallize and much of the boron is retained in this thicker electrode. A higher annealing temperature or lower initial boron content is required to crystallize the bottom electrode. View full abstract»

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  • Phase selective growth and characterization of vanadium dioxide films on silicon substrates

    Page(s): 163503 - 163503-6
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    We report on selective growth of VO2 films with M1, M2, and intermediate T phases on silicon (Si) substrates by using inductively coupled plasma (ICP)-assisted sputtering (ICPS) under particular conditions. The film composed of M2 phase was proved to be under strong in-plane compressive stress, which is consistent with stress-induced M2 phase. Crystalline structural phase transition (SPT) properties of these films were demonstrated together with infrared light transmittance as a measure of insulator-metal transition (IMT) against temperature. Characteristic correlations between SPT and IMT for films with M2 and intermediate-T phases were reported. Ultraviolet photoelectron spectroscopy measurements probed an energy gap of the film in the M2 phase at around 0.4 eV from the Fermi level indicating the presence of a Mott gap. View full abstract»

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  • On the growth kinetics of Ni(Pt) silicide thin films

    Page(s): 163504 - 163504-8
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    We report on the effect of Pt on the growth kinetics of δ-Ni2Si and Ni1-xPtxSi thin films formed by solid phase reaction of a Ni(Pt) alloyed thin film on Si(100). The study was performed by real-time Rutherford backscattering spectrometry examining the silicide growth rates for initial Pt concentrations of 0, 1, 3, 7, and 10 at. % relative to the Ni content. Pt was found to exert a drastic effect on the growth kinetics of both phases. δ-Ni2Si growth is slowed down tremendously, which results in the simultaneous growth of this phase with Ni1-xPtxSi. Activation energies extracted for the Ni1-xPtxSi growth process exhibit an increase from Ea = 1.35 ± 0.06 eV for binary NiSi to Ea = 2.7 ± 0.2 eV for Ni1-xPtxSi with an initial Pt concentration of 3 at. %. Further increasing the Pt content to 10 at. % merely increases the activation energy for Ni1-xPtxSi growth to Ea = 3.1 ± 0.5 eV. View full abstract»

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  • Kondo effect and non-Fermi liquid behavior in metallic glasses containing Yb, Ce, and Sm

    Page(s): 163505 - 163505-7
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    The low temperature properties of metallic glasses containing different concentrations of ytterbium, cerium, and samarium are studied. It is found that the Kondo effect caused by exchange interactions between the conduction and 4f electrons and non-Fermi liquid behavior appear in the strongly disordered alloys. We study the origins for these unique features and demonstrate that the found Kondo effect is inherited from the crystalline counterparts. The results might have significance on investigating the strong electron-electron interaction systems with structural disorder and be helpful for designing new metallic glasses with functional properties. View full abstract»

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  • Quantitative analysis of the quantum dot superlattice by high-resolution x-ray diffraction

    Page(s): 163506 - 163506-9
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    A new high-resolution x-ray diffraction approach for quantitative analysis of superlattice structures (SLs) with self-assembled quantum dots (QDs) was developed. For numerical simulations of the 2D angular distribution of diffracted x-ray radiation, both the coherent and diffuse scattering components have been calculated. Direct comparison of simulated patterns and experimental results revealed good agreement of the calculated intensity distribution with experimental reciprocal space maps for the superlattice GaAs(001)-AlGaAs-{InAs QDs-GaAs}SL with 20 periods of quantum dots. The simulation procedure allows one to obtain data about the shape, average size, elastic strains around the QDs, average density of the QDs, the presence of short- or long-range order in the arrangement of QDs in the semiconducting matrix, the vertical and lateral correlation lengths of the ensemble of quantum dots, and the parameters of the intermediate GaAs and AlGaAs layers. View full abstract»

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  • Molecular dynamics study on the failure modes of aluminium under decaying shock loading

    Page(s): 163507 - 163507-6
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    We have investigated the failure modes of single crystal aluminium under decaying shock loading by using molecular dynamics simulations. The microstructure evolution during the failure is presented in terms of the central symmetry parameter, and the corresponding pressure and temperature profiles are calculated and discussed. These results explain the failure morphology and mechanical properties under dynamic tension and especially the difference between solid and melted states. In addition, the fracture strength of aluminium is analyzed from surface velocity within acoustic approximation and virial theorem. View full abstract»

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  • Efficient parametric excitation of silicon-on-insulator microcantilever beams by fringing electrostatic fields

    Page(s): 163508 - 163508-11
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    Large amplitude flexural vibrations have been excited in single layer silicon-on-insulator micromechanical cantilever beams in ambient air environment. Our driving approach relies on a single co-planar electrode located symmetrically around the actuated grounded cantilever. Electrostatic forces are created via tailored asymmetries in the fringing fields of deformed mechanical states during their electric actuation, with strong restoring forces acting in a direction opposite to the deflection. This results in an effective increase in the structure stiffness in its elastic regime. The devices had been fabricated using deep reactive ion etching based process and their responses were characterized in a laser Doppler vibrometer under ambient conditions. Harmonic voltages applied to the electrode result in the periodic modulation of the effective stiffness and lead to strong parametric excitation of the structure. As opposed to close gap actuators, where high-amplitude drives are severely limited by pull-in instabilities, squeezed gas damping, and stiction, our resonators exhibit very large vibration amplitudes (up to 8 in terms of the amplitude to thickness ratio in the strong parametric regime), with no apparent damage, via the application of highly tunable distributed forces. A reduced order model, based on the Galerkin decomposition, captures the main dynamical features of the system, and is consistent with the observed beam characteristics. View full abstract»

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  • Electron beam physical vapor deposition of thin ruby films for remote temperature sensing

    Page(s): 163509 - 163509-6
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    Thermographic phosphors (TGPs) possessing temperature-dependent photoluminescence properties have a wide range of uses in thermometry due to their remote access and large temperature sensitivity range. However, in most cases, phosphors are synthesized in powder form, which prevents their use in high resolution micro and nanoscale thermal microscopy. In the present study, we investigate the use of electron beam physical vapor deposition to fabricate thin films of chromium-doped aluminum oxide (Cr-Al2O3, ruby) thermographic phosphors. Although as-deposited films were amorphous and exhibited weak photoluminescence, the films regained the stoichiometry and α-Al2O3 crystal structure of the combustion synthesized source powder after thermal annealing. As a consequence, the annealed films exhibit both strong photoluminescence and a temperature-dependent lifetime that decreases from 2.9 ms at 298 K to 2.1 ms at 370 K. Ruby films were also deposited on multiple substrates. To ensure a continuous film with smooth surface morphology and strong photoluminescence, we use a sapphire substrate, which is thermal expansion coefficient and lattice matched to the film. These thin ruby films can potentially be used as remote temperature sensors for probing the local temperatures of micro and nanoscale structures. View full abstract»

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  • Impact of rapid thermal processing on oxygen precipitation in heavily arsenic and antimony doped Czochralski silicon

    Page(s): 163510 - 163510-7
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    A comparative investigation is performed on the effects of vacancies induced by rapid thermal processing on oxygen precipitation behavior in heavily arsenic- and antimony-doped Czochralski silicon wafers. It is experimentally found that vacancy-assisted oxide precipitate nucleation occurs at 800, 900, and 1000  °C in the Sb-doped wafers, while it only occurs at 800  °C in the As-doped ones. Density functional theory calculations indicate that it is energetically favorable to form AsVO and SbVO complexes in As- and Sb-doped silicon crystals, respectively. These complexes might act as precursors for oxide precipitate nucleation under appropriate conditions. The difference between the effects of rapid thermal processing -induced vacancies on oxide precipitate nucleation in the heavily As- and Sb-doped Cz silicon crystals is tentatively elucidated based on density functional theory calculations revealing the difference in binding energies of AsVO and SbVO complexes. View full abstract»

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  • Low-frequency locally resonant band-gaps in phononic crystal plates with periodic spiral resonators

    Page(s): 163511 - 163511-8
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    In this paper, low-frequency band-gaps (BGs) in a phononic crystal (PC) thin plate with periodic spiral resonators are investigated numerically and experimentally. The formation mechanisms of the BGs in the proposed structure are explained based on the modal analysis. We find that the interaction between the local resonances and the traveling wave modes in the plate is responsible for the formation of the BG in low-frequency range. This interaction strength greatly affects the bandwidth of the BG, of which the lower edge depends on the corresponding local resonance frequency. It is shown that the out-of-plane BG can be modulated by changing the geometrical parameters. The proposed PC plate is demonstrated to possess a broad out-of-plane BG in low-frequency range from 42 Hz to 150 Hz, by combining the numerical calculations with experimental measurements. The structure design and its results provide an effective way for phononic crystals to obtain broad BGs in low-frequency range, which has potential applications in the low-frequency vibration and noise reduction. View full abstract»

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  • Thermopower and resistivity studies of Nd-Na-Mn-O manganites

    Page(s): 163701 - 163701-8
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    The electrical transport properties of the polycrystalline Nd1-xNaxMnO3 (0.05 ≤ x ≤ 0.25) were investigated. All the samples are found to exhibit insulating behavior down to the lowest measured temperature. However, on application of 4 T magnetic field, metal-insulator transitions along with a low temperature resistivity upturn are observed for the samples with x ≥ 0.15. Thermopower values of samples with x = 0.05, 0.10, and 0.20 are negative in the entire temperature range of investigation, while they changes from negative to positive with decreasing temperature in the case of samples with x = 0.15 and 0.25. The resistivity and thermopower data in the high temperature region are explained within the framework of variable range and small polaron hopping models, while the resistivity minima are explained using the combined effect of electron-electron and electron-phonon scattering is considered. 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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P. James Viccaro
Argonne National Laboratory