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Journal of Applied Physics

Issue 9 • Date Nov 2005

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Displaying Results 1 - 25 of 113
  • Issue Cover

    Page(s): c1
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  • Issue Table of Contents

    Page(s): toc1
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  • Sensitive dependence of hydrogen Balmer-alpha laser-induced fluorescence signal from hydrogen neutral beam on background magnetic field

    Page(s): 093101 - 093101-9
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    A previously unreported result for the dependence of a laser-induced fluorescence (LIF) signal from the H-alpha (Balmer-alpha) transition in a hydrogen neutral beam passing through a background of neutral hydrogen gas is presented. The LIF signal from a 30 kV beam is found to be enhanced and the fine-structure line amplitudes in the H-alpha spectrum are seen to vary significantly with an applied perpendicular magnetic field over the range of 0–0.01 T. The phenomenon has also been observed and investigated in a background electric field of ∼0–300 V/cm, and in the presence of crossed perpendicular magnetic and electric fields, demonstrating that the magnetic-field effect is due to the motional Stark electric field perceived in the beam reference frame as it passes through the magnetic field. The effect has been studied with variations of background neutral gas pressure, laser power, and polarization direction and at different locations along the neutral beamline. The phenomenon could be exploited as a low-field diagnostic technique in environments that are not appropriate for magnetic probes. View full abstract»

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  • Silicon carbide photoconductive switch for high-power, linear-mode operations through sub-band-gap triggering

    Page(s): 093102 - 093102-6
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    The analysis of a 6H silicon carbide (SiC) photoconductive switch, designed and packaged for high-power, linear-mode operations, is presented. The switch, fabricated from semi-insulating compensated SiC, is triggered by an optical source with photon energy less than the band-gap energy. Simulation models incorporating the effects of vanadium trap and nitrogen dopant in the compensation material show I-V characteristics that agree with measured values. The photoconductive switch has improved rise-time characteristics as compared to a gallium arsenide (GaAs) switch. The analysis also shows that improved performance at high power is possible through passivation using high-permittivity dielectric near the contact-semiconductor interface and by placing a p+ layer next to the cathode. View full abstract»

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  • Below band-gap optical absorption in GaxIn1-xSb alloys

    Page(s): 093103 - 093103-7
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    The below band-gap optical-absorption characteristics of GaxIn1-xSb alloy system have been reported. The different dependencies of the hole and electron absorption mechanisms on wavelength result in significant changes of the absorption characteristics with alloy composition. In the undoped Ga-rich alloy compositions that are p type in nature (due to residual holes resulting from native defects), the inter-valence-band absorption has been found to be the dominant absorption mechanism. With decreasing Ga (increasing indium) mole fraction, the hole to electron ratio decreases for undoped samples. For such samples, absorption due to electrons becomes significant. With n-type extrinsic doping, intervalley transitions are seen for certain Ga-rich compositions, which also alter the absorption characteristics. The dependencies of various absorption mechanisms as a function of wavelength have been analyzed and discussed in this paper. Based on the theoretical analysis presented in this paper, one can calculate the extrinsic doping level necessary for each alloy composition in order to obtain high optical transparency necessary for infrared optical applications. View full abstract»

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  • Density and production of NH and NH2 in an ArNH3 expanding plasma jet

    Page(s): 093301 - 093301-10
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    The densities of NH and NH2 radicals in an ArNH3 plasma jet created by the expanding thermal plasma source were investigated for various source-operating conditions such as plasma current and NH3 flow. The radicals were measured by cavity ringdown absorption spectroscopy using the (0,0) band of the A 3Π←X 3Σ- transition for NH and the (0,9,0)-(0,0,0) band of the à2A1←X~ 2B1 transition for NH2. For NH, a kinetic gas temperature and rotational temperature of 1750±100 and 1920±100 K were found, respectively. The measurements revealed typical densities of 2.5×1012 cm-3 for the NH radical and 3.5×1012 cm-3 for the NH2 radical. From the combination of the data with ion density and NH3 consumption measurements in the plasma as well as from a simple one-dimensional plug down model, the key production reactions for NH and NH2 are discussed. View full abstract»

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  • Detection of deuterium and hydrogen using laser-induced helium gas plasma at atmospheric pressure

    Page(s): 093302 - 093302-3
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    An experimental study on gas analysis by means of laser-induced breakdown spectroscopy was conducted using a Nd-yttrium aluminum garnet laser (1,064 nm, 120 mJ, 8 ns) and helium host gas at atmospheric pressure on a sample of mixed water (H2O) and heavy water (D2O) in vapor form. It was shown that completely resolved hydrogen (Hα) and deuterium (Dα) emission lines that are separated by only 0.179 nm could be obtained at a properly delayed detection time when the charged particles responsible for the strong Stark broadening effect in the plasma have mostly disappeared. It is argued that the helium metastable excited state plays an important role in the hydrogen excitation process. View full abstract»

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  • A study of spot evolution in hot refractory cathodes of high-pressure arcs

    Page(s): 093303 - 093303-13
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    In this paper the behavior of the cathode spot in atmospheric pressure arc is studied from a dynamic point of view and later applied to a specific case of a tungsten refractory cathode. For this, a model of the cathode region was developed. In this model the tungsten atoms proceeding from the cathode evaporation, which were all supposed to be ionized in the presheath, returned to the cathode surface as ions, provoking a heating of this surface. Therefore, the model allowed us to evaluate the importance of those atoms. Furthermore, different mechanisms of the electron emission from the cathode surface were considered as function of the spot temperature and the electric-field strength. This model of the cathode region allowed getting important parameters in the study of the dynamics of the cathode spot such as total current density, which is necessary for the calculation of Joule heating effect, and the total-energy flux density, which was incorporated as a boundary condition for the solution of the heat conduction equation in the cathode. The dependence of these parameters on the temperature of the cathode surface in contact with the plasma introduced nonlinearities in the equations. The model takes into account the different phase changes that take place in the cathode as well as the thermal ablation of the melted cathode and the dependence on the temperature of the physical magnitudes that characterize the cathode material. In this way, it is possible to get the time evolution of the temperature distribution in the cathode and to study the spot dynamics on the cathode surface. Three different values of the initial cathode voltage drop were used, U0=15, 20, and 25 V, which cover a wide range of working conditions. The cathode spot was assumed to be placed on a crater. For U0=15 V, the crater radius increases in time until it reaches a critical- value when the energy balance principle is broken. This leads to spot death and its jump to another position on the cathode surface. Nevertheless for higher-voltage drops, U0=20 and 25 V, the spot reaches a stationary regime before arriving at that critical state and the spot remains fixed. This way, the maximum spot radius and the possibility of the spot moving or not appear naturally during the numerical development of the model. The results of this model show that the contribution of the evaporated and later ionized cathode atoms is not significant in comparison with the electrons and plasmagen ions contributions. In this sense these atoms need not be taken into account in the cathode region models for hot refractory cathodes in atmospheric pressure arcs. View full abstract»

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  • Thermal inequilibrium of atmospheric helium microwave plasma produced by an axial injection torch

    Page(s): 093304 - 093304-10
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    The population density of several excited states has been obtained spectroscopically in a helium plasma sustained by a torch device at atmospheric pressure as a function of the radius in the plasma for different conditions of microwave power and plasma gas flow. The ground-state atom density is determined from the gas temperature, which is deduced from the rotational temperature of the molecular nitrogen ions. The population distribution is fitted to the theoretical results of a collisional-radiative model that includes particle transport. A large deviation of the measured populations is found from the theoretical populations for local thermodynamic equilibrium. The plasma at any radial position is far from local thermodynamic equilibrium; the equilibrium deviation parameter of the ground state is larger than 10 000. The equilibrium deviation parameters of the measured excited-state populations obey the theoretical pk-6 exponential law. View full abstract»

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  • Characteristics of a low-frequency-driven ferroinductor-coupled discharge

    Page(s): 093305 - 093305-5
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    We present characteristics of a low-frequency (LF) inductively coupled discharge where, instead of using an inductorlike rf antenna, we used a ferromagnetic core with a primary winding (“ferroinductor”). A dense (≫1012 cm-3), highly ionized (30%–40%) plasma was obtained in this ferroinductor at gas pressures as low as 10-4 Torr. In a wide range of comparatively low frequencies the core and winding losses were found to be small compared with the LF driving power delivered to the plasma. The driving frequency could be very low compared with typical inductively coupled discharges. The input impedance was found to be almost purely active (cos φ≈0.9), and it was possible to achieve various input resistances (e.g., 50 Ω) in the whole investigated range of frequencies, powers, and pressures, which made unnecessary any matching box between the LF driver and the ferroinductor-coupled plasma device. Such a combination of properties makes this kind of discharge attractive for many applications. View full abstract»

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  • Effect of helium addition on discharge characteristics in a flat fluorescent lamp

    Page(s): 093306 - 093306-5
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    The discharge characteristics of a multielectrode dual coplanar in a mercury-free flat fluorescent lamp were investigated using brightness-efficiency measurement and the infrared (IR) spectrum and intensified charge coupled device (ICCD) characteristics. The level of brightness was above 14 900 cd/m2 under the conditions of neon-–50% xenon–8% He gas composition, 150 Torr pressure, and 20 kHz alternating current pulse. The ICCD results revealed a faster and wider discharge with a Ne–50% Xe–8% He gas composition. The effect of adding helium (He) to Ne–50% Xe revealed a faster peak emission, as confirmed by ICCD images. From the gated IR emission spectrum, the intensity ratio of I823 nm/I828 nm was ∼8% higher with Ne–50% Xe–8% He than with Ne–50% Xe under the same pressure and applied voltage conditions. View full abstract»

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  • Preliminary analysis of C and H in a “Sangiran” fossil using laser-induced plasma at reduced pressure

    Page(s): 093307 - 093307-8
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    A Nd:YAG laser (1064 nm, 120 mJ, and 8 ns) was focused on various types of fossil samples, including fossilized buffalo horns (around 400 000 and 1×106 yr old, respectively) found in Sangiran, Indonesia. Such fossils represent an important starting point for tracing man’s origin and evolution during the Pleistocene era. Carbon emission was found to decrease significantly with the degree of fossilization and no carbon emission was found in a horn fossil dated at 1×106 yr. Some molecular band spectra were also found in all the fossils examined in this study. It was assumed that by combining information on carbon emission, hydrogen emission, and molecular band spectra that the degree of fossilization might be quantitatively calculated. Further results showed that silicon emission is not detected in old fossils, but it is present as a major constituent. This is probably due to the fact that silicon is strongly bound to other elements in old fossils and is ablated in the form of clusters. In order to prove the above hypothesis, a thin film of an old fossil was deposited on a silver plate substrate by means of a laser ablation technique. The resulting film was then irradiated and atomic emission lines of silicon were clearly detected. A comparative study of the low-pressure plasma introduced in this study was conducted using the well-known laser-induced breakdown spectroscopy technique and the results confirmed that operating conditions at atmospheric pressure are unfavorable for a fossil analysis. View full abstract»

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  • Characterization of the plasma on dielectric fiber (velvet) cathodes

    Page(s): 093308 - 093308-12
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    An investigation of the properties of the plasma and the electron beam produced by velvet cathodes in a diode powered by a ∼200 kV, ∼300 ns pulse is presented. Spectroscopic measurements demonstrated that the source of the electrons is surface plasma with electron density and temperature of ∼4×1014 cm-3 and ∼7 eV, respectively, for an electron current density of ∼50 A/cm2. At the beginning of the accelerating pulse, the plasma expands at a velocity of ∼106 cm/s towards the anode for a few millimeters, where its stoppage occurs. It was shown by optical and x-ray diagnostics that in spite of the individual character and nonuniform cross-sectional distribution of the cathode plasma sources, the uniformity of the extracted electron beam is satisfactory. A mechanism controlling the electron current-density cross-sectional uniformity is suggested. This mechanism is based on a fast radial plasma expansion towards the center due to a magnetic-field radial gradient. Finally, it was shown that the interaction of the electron beam with the stainless-steel anode does not lead to the formation of an anode plasma. View full abstract»

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  • Elasticity of planar fiber networks

    Page(s): 093501 - 093501-9
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    A micromechanics model is proposed for the elasticity of planar fiber networks (FNs). The FN is created by random deposition of linearly elastic straight rods within a region. The rods are bonded rigidly at contacts. Under external in-plane loading, the FN deformation consists of fiber bending, elongation, and contraction. An effective constitutive relation for fiber network is developed by averaging the strain energy dissipated by all possible fiber deformations in all directions. Numerical calculations are performed to analyze the effects of fiber aspect ratio and fiber concentration on the effective stiffness of the planar random FN. Finite element analysis (FEA) is performed and compared with the theoretical predictions of the effective FN moduli at several fiber concentrations. FEA results are in good agreement with theoretical predictions. The present model can be used for the prediction of mechanical properties, scaling analysis, and optimization of fiber assemblies. View full abstract»

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  • Classical nucleation theory applied to molecular orientations in vapor-deposited organic thin films

    Page(s): 093502 - 093502-6
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    Molecular orientational mechanism during physical vapor deposition of various linear long-chain compounds has been interpreted on the basis of a classical nucleation theory modified for the anisotropic nucleus formation. In this model, the anisotropy of the surface energy is considered, and initial small nuclei with two fundamental orientations (normal and lateral) are assumed to be formed and dissociated repeatedly by the thermal energy. The critical energy of nucleus formation for each orientation is calculated as a function of the chemical-potential change of the system. The calculated results are similar to those observed for actual orientational phenomena of long-chain molecules at various chemical-potential changes; i.e., a higher substrate temperature and/or a lower deposition rate result/s in a higher degree of normal orientation. View full abstract»

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  • Magnetic patterning of Fe/Cr/Fe(001) trilayers by Ga+ ion irradiation

    Page(s): 093503 - 093503-6
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    Magnetic patterning of antiferromagnetically coupled epitaxial Fe (10 nm)/Cr (0.7 nm)/Fe (10 nm) (001) trilayers by irradiation with 30 keV Ga+ ions was studied by means of atomic force microscopy, magnetic force microscopy, and Kerr magnetometry. It was found that within a fluence range of (1.25-5)×1016 ions/cm2 a complete transition from antiferromagnetic to ferromagnetic coupling between the two Fe layers can be achieved. The magnetization reversal processes of the nonirradiated, antiferromagnetically coupled areas situated close to the irradiated areas were studied with lateral resolution. Evidence for a lateral coupling mechanism between the magnetic moments of the irradiated and nonirradiated areas was found. Special attention was paid to preserve the flatness of the irradiated samples. Depending on the fluence, topographic steps ranging from +1.5 to -2 nm between the nonirradiated and irradiated areas were observed. At lower fluences the irradiation causes an increase of the surface height, while for higher fluences the height decreases. It was found that for the particular fluence of 2.7×1016 ions/cm2 no height difference between the irradiated and nonirradiated areas occurs. The results suggest that the irradiation of Fe/Cr/Fe trilayers with midenergy ions is an innovative method for magnetic patterning, preserving the initial smoothness of the sample. View full abstract»

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  • Enhanced modulation rate in platinum-diffused resonant-cavity light-emitting diodes

    Page(s): 093504 - 093504-5
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    This study is focused on the modulation response of resonant-cavity light-emitting diodes (RCLEDs). Platinum (Pt) atoms are diffused into the 660 nm RCLED epitaxial layers to increase the concentration of recombination centers and to improve the modulation speed. The RCLED has an AlInGaP multi-quantum-well active layer which was embedded into AlGaAs-distributed Bragg reflectors to form a one-wavelength (1-λ) optical resonator. Afterwards, the deep-level Pt impurity was diffused into the RCLED and an improved average rise time, from 18.07 to 12.21 ns, was obtained. The corresponding modulation frequency can be increased from 19.54 to 30.21 MHz. View full abstract»

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  • The compact microcrystalline Si thin film with structure uniformity in the growth direction by hydrogen dilution profile

    Page(s): 093505 - 093505-6
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    The hydrogen dilution profiling (HDP) technique has been developed to improve the quality and the crystalline uniformity in the growth direction of μc-Si:H thin films prepared by hot-wire chemical-vapor deposition. The high H dilution in the initial growth stage reduces the amorphous transition layer from 30–50 to less than 10 nm. The uniformity of crystalline content Xc in the growth direction was much improved by the proper design of hydrogen dilution profiling which effectively controls the nonuniform transition region of Xc from 300 to less than 30 nm. Furthermore, the HDP approach restrains the formation of microvoids in μc-Si:H thin films with a high Xc and enhances the compactness of the film. As a result the stability of μc-Si:H thin films by HDP against the oxygen diffusion, as well as the electrical property, is much improved. View full abstract»

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  • Surface-plasmon resonance with infrared excitation: Studies of phospholipid membrane growth

    Page(s): 093506 - 093506-6
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    We report on a surface-plasmon resonance (SPR) technique based on a Fourier transform infrared spectrometer for biological and surface-sensitive applications. In contrast with conventional surface-plasmon techniques, which operate at a fixed wavelength and a variable angle of incidence, our setup allows independent variation of the wavelength and the angle of incidence. By the proper choice of these parameters, we achieve optimal coupling to the surface plasmon and high sensitivity. Moreover, by using infrared rather than visible light, we achieve an extremely narrow angular-dependent surface-plasmon resonance. This results in a very sensitive SPR technique that can easily sense one molecular layer. We take advantage of the extremely narrow SPR in the infrared range and use it to study the growth dynamics of the phospholipid layer, which is the main constituent of the biological cell membrane. In particular, we distinguish the difference in the growth dynamics of this artificial membrane from a solution under different conditions of liquid flow (continuous flow or injection). View full abstract»

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  • Raman scattering of polar modes of ZnO crystallites

    Page(s): 093507 - 093507-4
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    One of the key issues of phonon dynamics of nano- and micrometer-scale crystals is the identification of the observed Raman modes. Due to the tilted orientation of small crystallites, the usual Raman selection rules pertaining to the symmetry axes no longer hold, and mixed-symmetry modes need to be considered in order to explain the polar phonon properties of the crystallites. The Raman modes of ZnO crystallites of the wurtzite structure were investigated via micro-Raman scattering. The nonpolar E2 mode was the predominant mode in the spectra for out-of-resonant conditions. In resonance the crystallites exhibited a predominant mode at ∼580 cm-1, intermediate to the frequencies of the A1(LO) and the E1(LO) modes of a reference ZnO single crystal at 568 and 586 cm-1, respectively. Our analysis indicates that the observed frequency of the crystallite ensemble can be explained in terms of Loudon’s model of a quasimode behavior that is due to a preferential orientation of a crystallite ensemble. Additionally, model calculation of the quasi-LO frequency of totally random ensemble is presented. View full abstract»

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  • The role of Si as surfactant and donor in molecular-beam epitaxy of AlN

    Page(s): 093508 - 093508-6
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    The growth of Si-doped AlN(0001) thin films on Al2O3(0001) substrates by plasma-induced molecular-beam epitaxy is reported. We have found that Si positively affects the epitaxy being an effective surfactant for AlN growth with a remarkable impact on the crystal quality. It was proven that the characteristic surface reconstruction sequences frequently related to the Al adatoms are obviously Si induced on AlN(0001) surfaces. It was also observed that heavy doping conditions result in volume segregation of Si on the threading dislocation network and in the formation of an amorphous (AlO)(SiO)N cap layer caused by surface oxidation of the accumulated Al and segregated Si. The electron affinity was measured to be smaller than 0.5 eV on the clean AlN surface after removing of the cap layer using Ar+ sputtering. View full abstract»

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  • Stress relaxation in the GaN/AlN multilayers grown on a mesh-patterned Si(111) substrate

    Page(s): 093509 - 093509-5
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    300×300 μm2 crack-free GaN/AlN multilayers of 2 μm in thickness have been successfully grown on the Si(111) substrate patterned with the SixNy mesh by metal-organic chemical-vapor deposition. The in-plane stress exhibits a U-shape distribution across the “window” region, supported by the Raman shift of the GaN E2(TO) mode. This indicates a stress relaxation abruptly occurring near the edge of the window region due to the freestanding surface (1101) or (1122). The in-plane stress is almost relaxed at the corner of the window region due to three freestanding surfaces (1101), (1122), and (1011). The maximum in-plane stress is located near the surface of the multilayers at the center of the window region, supported by the Raman measurements and the failure observations. The role of the SixNy mesh in the stress relaxation is discussed. View full abstract»

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  • Biexciton lasing of submicron-sized ZnO particle in a Fabry-Perot cavity

    Page(s): 093510 - 093510-5
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    We have observed the lasing of submicron-sized ZnO particle in a Fabry-Perot cavity at low temperatures. A thin alkali-halide crystal including a small amount of ZnO particles are sandwiched between two quartz plates arranged as a pair of opposite mirrors, that is, a Fabry-Perot cavity. Under low excitation due to a He–Cd laser, emissions of free and bound excitons and their phonon replicas are recognized markedly, while a biexciton emission band appears under high-dense excitation by a pulsed N2 laser. Biexciton lasing is demonstrated as a narrowed emission by focusing the excitation light beam on an appropriate position being a ZnO particle in the cavity. When a cavity length satisfies the resonant condition, the lasing occurs in the spectral region of the biexciton emission band and with the very small divergence angle. The obtained threshold of excitation-power density is about 0.50 MW/cm2. We discuss lasing in terms of stimulated emission caused by population inversion between biexciton and exciton states. View full abstract»

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  • Determination of elastic constants of a fiber-textured gold film by combining synchrotron x-ray diffraction and in situ tensile testing

    Page(s): 093511 - 093511-9
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    The elastic behavior of gold thin films deposited onto Kapton substrate has been studied using in situ tensile tester in a four-circle goniometer on a synchrotron beam line (LURE facility, France). The mechanical description of the substrate-thin film composite structure has been developed to determine the stress tensor in the film while the strong {111} fiber texture was taken into account using the crystallite group method (CGM). CGM strain analysis allowed us to forecast the nonlinear relationship between strain and sin2 Ψ obtained for the thin films due to the strong anisotropy of gold. A least-square method was used to fit the overall experimental data with good accuracy and allows determining all single-crystal elastic constants. View full abstract»

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  • Thermodynamically complete equations of state for nickel-titanium alloy

    Page(s): 093512 - 093512-14
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    A thermodynamically complete equation of state for the compression and heating of near-equiatomic NiTi alloy in the CsCl (B2) structure was predicted, based on quantum-mechanical calculations of the electron ground states and a Grüneisen lattice-thermal model. The quantum-mechanical calculations used ab initio pseudopotentials and the local-density approximation; the accuracy of the calculations was investigated for elemental Ni and Ti. These calculations demonstrated that simple averaging techniques do not provide an accurate prediction of the properties of metal alloys, and rigorous treatment of the electron wave functions is needed. Predictions were also made of the behavior of NiTi under uniaxial loading. The pressure-density relation obtained from isotropic compression did not match the mean pressure calculated from uniaxial compression, demonstrating that it is not generally accurate to split the stress response of a material into a scalar equation of state and a stress deviator according to the usual prescription. Polycrystalline NiTi samples were prepared with a range of compositions, in the form of disks from 100 to 400 μm thick and 5 mm in diameter. Flyer impact experiments were performed using a long-pulse laser drive at the TRIDENT facility to obtain shock wave data on the response of NiTi to around 15 GPa; the new data were consistent with the published results from gas gun experiments. The theoretical equation of state was consistent with the shock wave data. 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