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Journal of Applied Physics

Issue 11 • Date Jun 2011

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Displaying Results 1 - 25 of 117
  • Strain and piezoelectric potential effects on optical properties in CdSe/CdS core/shell quantum dots

    Page(s): 113103 - 113103-4
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    Strain and piezoelectric potential effects on optical properties in CdSe/CdS core/shell quantum dots (QDs) were investigated theoretically using an eight-band strain-dependent k·p Hamiltonian. The strain effect on the shift of the subband energies is found to be larger than the piezoelectric field effect. As a result, interband transition energies are blueshifted with the inclusion of strain and piezoelectric field effects. We know that the theoretical interband transition energy shows a reasonable agreement with the experimental result. The absolute value of the hydrostatic strain in the QD increases with decreasing QD size, whereas that in the barrier decreases with decreasing QD size. View full abstract»

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  • Near-ultraviolet absorption and nanosecond-pulse-laser damage in HfO2 monolayers studied by submicrometer-resolution photothermal heterodyne imaging and atomic force microscopy

    Page(s): 113106 - 113106-7
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    Localized absorption in hafnium dioxide used as a high-index component in multilayer coatings for near-ultraviolet, nanosecond-pulse-laser applications is directly linked to laser-induced damage. The nature of the absorbing species and their physical properties remains unknown because of their extremely small sizes. Previous experimental evidence provided by the atomic force microscopy mapping of damage morphology points to a few-nanometer scale of these absorbers. This work demonstrates the submicrometer-resolution mapping of 355-nm absorption in HfO2 monolayers using a recently developed photothermal heterodyne imaging technique. The comparison of absorption maps with the atomic force microscopy investigation of pulsed-laser-induced damage morphology allows one to better estimate the spatial distribution of nanoscale absorbing defects in hafnia thin films. Possible defect-formation mechanisms are discussed. View full abstract»

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  • Er3+ and Si luminescence of atomic layer deposited Er-doped Al2O3 thin films on Si(100)

    Page(s): 113107 - 113107-9
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    Atomic layer deposition was used to deposit amorphous Er-doped Al2O3 films (0.9–6.2 at. % Er) on Si(100). The Er3+ photoluminescence (PL), Er3+ upconversion luminescence, as well as the Si PL and associated surface passivation properties of the films were studied and related to the structural change of the material during annealing. The PL signals from Er3+ and Si were strongly dependent on the annealing temperature (T = 450–1000 °C), but not directly influenced by the transition from an amorphous to a crystalline phase at T > 900 °C. For T > 650 °C, broad Er3+ PL centered at 1.54 μm (4I13/2) with a full width at half maximum of 55 nm was observed under excitation of 532 nm light. The PL signal reached a maximum for Er concentrations in the range of 2–3 at. %. Multiple photon upconversion luminescence was detected at 660 nm (4F9/2), 810 nm (4I9/2), and 980 nm (4I11/2), under excitation of 1480 nm light. The optical activation of Er3+ was related to the removal of quenching impurities, such as OH (3 at. % H present initially) as also indicated by thermal effusion experiments. In contrast to the Er3+ PL signal, the Si luminescence, and consequently the Si surface passivation, decreased for increasing annealing temperatures. This trade-off between surface passivation quality and Er3+ PL can be attributed to an opposite correlation with the decreasing hydrogen content in the films during thermal treatment. View full abstract»

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  • Luminescence properties and optical dephasing in a glass-ceramic containing sodium-niobate nanocrystals

    Page(s): 113108 - 113108-5
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    Photoluminescence (PL) and degenerate four-wave-mixing (DFWM) experiments were performed in a silica–niobic composite containing NaNbO3 nanocrystals. The PL results indicate the presence of in-gap states attributed to excitons in the nanocrystals and defect centers. The luminescence of the samples becomes more intense at low temperatures, indicating that nonradiative relaxations dominate the dynamics of the in-gap states. The DFWM experiments allowed for measurements of the homogeneous relaxation time, (20 ± 3) fs, of the third-order polarization at room temperature. The main contributions to the dynamics of the electronic response are attributed to the trapping of electrons in the in-gap states and to carrier and phonon scattering. View full abstract»

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  • Hydrogenated amorphous silicon oxide containing a microcrystalline silicon phase and usage as an intermediate reflector in thin-film silicon solar cells

    Page(s): 113109 - 113109-10
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    To further improve the stability of amorphous/microcrystalline silicon (a-Si:H/μc-Si:H) tandem solar cells, it is important to reduce the thickness of the a-Si:H top cell. This can be achieved by introduction of an intermediate reflector between the a-Si:H top and the μc-Si:H bottom cell which reflects light back into the a-Si:H cell and thus, increases its photocurrent at possibly reduced thickness. Microcrystalline silicon oxide (μc-SiOx:H) is used for this purpose and the trade-off between the material’s optical, electrical and structural properties is studied in detail. The material is prepared with plasma enhanced chemical vapor deposition from gas mixtures of silane, carbon dioxide and hydrogen. Phosphorus doping is used to make the material highly conductive n-type. Intermediate reflectors with different optical and electrical properties are then built into tandem solar cells as part of the inner n/p-recombination junction. The quantum efficiency and the reflectance of these solar cells are evaluated to find optical gains and losses due to the intermediate reflector. Suitable intermediate reflectors result in a considerable increase in the top cell current density which allows a reduction of the a-Si:H top cell thickness of about 40% for a tandem cell while keeping the current density of the device constant. View full abstract»

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  • Cavity quantum electrodynamics for photon mediated transfer of quantum states

    Page(s): 113110 - 113110-8
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    An enhanced approach for transferring quantum state between quantum nodes is proposed wherein photons serve as the information carrier. Each node consists of a Rubidium (87Rb) atom trapped inside a two-mode optical cavity. The approach is based on cavity quantum electrodynamics (QED) wherein a system of lasers is applied on the atom in order to generate photon through Raman transition. Logic states ‘0’ and ‘1’ are represented by two subspaces of the hyperfine energy levels with magnetic sub-levels of 87Rb atom. A static magnetic field is applied upon the atoms so that the hyperfine states of 87Rb atom are split into the magnetic sub-levels (due to Zeeman effect). Depending on the logic state of the transmit node, a right- or left-circularly polarized photon with designated frequency is produced through a cavity assisted Raman process. When the photon is received at the receive node via an optical fiber, the logic state of the transmit node is restored (through a cavity QED process) into the receive node. A desirable feature of the approach is that, during the transmission of logic state, the transmit node itself should not significantly change its quantum state; this is successfully validated through simulations. View full abstract»

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  • Dark current mechanisms in quantum dot laser structures

    Page(s): 113111 - 113111-5
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    Current-voltage measurements have been performed on InAs/InGaAs/GaAs quantum dot structures with varying growth and design parameters. These measurements show that the forward and reverse bias dark currents decrease with increasing spacer growth temperature, however, they are relatively insensitive to the number of periods of the quantum dot layers. Temperature dependent current-voltage measurements show that the mechanism that governs the reverse bias leakage current is due to generation-recombination via mid-band traps assisted by the Frenkel-Poole emission of carriers from these traps. View full abstract»

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  • Characterization of a low-pressure chlorine plasma column sustained by propagating surface waves using phase-sensitive microwave interferometry and trace-rare-gas optical emission spectroscopy

    Page(s): 113304 - 113304-10
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    Phase-sensitive microwave interferometry and trace-rare-gas optical emission spectroscopy were used to measure the line-integrated electron density, ne, and electron temperature, Te, in a high-density chlorine plasma sustained in a quartz discharge tube (inner diameter = 6 mm) by an electromagnetic surface wave at 2.45 GHz. For pressures in the 0.1–1 Torr range, ne decreased nearly linearly along the tube’s z-axis down to the critical density for surface wave propagation, where the plasma decayed abruptly. At lower pressures (< 50 mTorr), however, the plasma extended well beyond this critical point, after which ne decreased quasiexponentially toward the end of the plasma column. The length of this expansion region increased with decreasing pressure, going from ∼8 cm at 5 mTorr to ∼1 cm at 50 mTorr. Te was nearly independent of the axial position in the main plasma region and strongly decreased in the expansion region at lower pressures. The Cl2 percent dissociation, τD, obtained from the calibrated Cl2 (306 nm)-to-Xe (828 nm) emission ratio, displayed behavior similar to that of ne and Te. For example, at 5 mTorr, τD was close to 100% near the wave launcher and ∼70% at 0.5 cm from the end of the plasma column. View full abstract»

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  • On the numerical simulation of the diffuse arc in a vacuum interrupter

    Page(s): 113306 - 113306-12
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    A 2D two-fluid numerical model is developed for the description of the diffuse arc in a vacuum interrupter under the possible influence of an axial magnetic field (AMF). This model incorporates the energy balance equations for both ions and electrons and takes into account the three components of the self-generated magnetic field. The possibility of both supersonic (at low current density) and subsonic (at high current density) plasma flow regimes is considered. Data from the literature are used to specify the values of the plasma parameters near the cathode, and the boundary conditions on the anode boundary rely on a simplified model of the anode sheath. Simulation results are presented for both ion flow regimes, with special attention to the current and plasma flow features. It is shown that there is always a contraction of the current flow, whereas the dynamics of the ions throughout the interelectrode gap is strongly influenced by their flow regime near the cathode. Finally, the influence of various operating parameters (presence of an AMF, gap length) on the current constriction at the anode is discussed. View full abstract»

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  • Effect of the reference electrode size on the ionization instability in the plasma sheath of a small positively biased electrode

    Page(s): 113307 - 113307-10
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    It is well known that additional ionization in the vicinity of a positively biased electrode immersed into a weakly ionized plasma is responsible for a hysteresis in the electrode current–voltage characteristics and the current self-oscillations rise. Here we show both experimentally and theoretically that under certain conditions these phenomena cannot be correctly interpreted once considered separately from the reference electrode current–voltage characteristics. It is shown that small electrodes can be separated into three groups according to the relation between the electrode and the reference electrode areas. Each group is characterized by its own dependence of the collected current on the bias voltage. View full abstract»

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  • Ionizing shocks in argon. Part I: Collisional-radiative model and steady-state structure

    Page(s): 113308 - 113308-14
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    A detailed collisional-radiative model is developed and coupled with a single-fluid, two-temperature convection model for the transport of shock-heated argon. The model is used in a systematic approach to examine the effects of the collision cross sections on the shock structure, including the relaxation layer and subsequent radiative-cooling regime. We present a comparison with previous experimental results obtained at the University of Toronto’s Institute of Aerospace Studies and the Australian National University, which serve as benchmarks to the model. It is shown here that ionization proceeds via the ladder-climbing mechanism, in which the upper levels play a dominant role as compared to the metastable states. Taking this into account, the present model is able to accurately reproduce the metastable populations in the relaxation zone measured in previous experiments, which is not possible with a two-step model. Our numerical results of the radiative-cooling region are in close agreement with experiments and have been obtained without having to consider radiative transport. In particular, it found that spontaneous emission involving the upper levels together with Bremsstrahlung emission account for nearly all radiative losses; all other significant radiative processes, resulting in transitions into the ground-state, are mostly self-absorbed and have a lesser impact. The effects of electron heat conduction are also considered and shown to have a large impact on the electron-priming region immediately behind the shock front; however, the overall effect on the induction length, i.e., the distance between the shock front and the electron avalanche, is small. View full abstract»

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  • Ionizing shocks in argon. Part II: Transient and multi-dimensional effects

    Page(s): 113309 - 113309-11
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    We extend the computations of ionizing shocks in argon to the unsteady and multi-dimensional, using a collisional-radiative model and a single-fluid, two-temperature formulation of the conservation equations. It is shown that the fluctuations of the shock structure observed in shock-tube experiments can be reproduced by the numerical simulations and explained on the basis of the coupling of the nonlinear kinetics of the collisional-radiative model with wave propagation within the induction zone. The mechanism is analogous to instabilities of detonation waves and also produces a cellular structure commonly observed in gaseous detonations. We suggest that detailed simulations of such unsteady phenomena can yield further information for the validation of nonequilibrium kinetics. View full abstract»

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  • Pressure induced crystallization in amorphous silicon

    Page(s): 113511 - 113511-7
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    We have investigated the high pressure behavior of amorphous silicon (a-Si) using x-ray diffraction and Raman scattering techniques. Our experiments show that a-Si undergoes a polyamorphous transition from the low density amorphous to the high density amorphous phase, followed by pressure induced crystallization to the primitive hexagonal (ph) phase. On the release path, the sequence of observed phase transitions depends on whether the pressure is reduced slowly or rapidly. Using the results of our first principles calculations, pressure induced preferential crystallization to the ph phase is explained in terms of a thermodynamic model based on phenomenological random nucleation and the growth process. View full abstract»

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  • Effect of vacancy on the sliding of an iron grain boundary

    Page(s): 113512 - 113512-5
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    We perform a first-principles calculation to investigate the effect of a vacancy on the sliding of an iron (Fe) Σ5 (310) [001] tilt grain boundary (GB). We calculate vacancy formation energies in order to determine the site preference of the vacancy at the Fe GB. We show that vacancies can easily segregate to the GB region, which is attributed to special atomic configurations of the GB. We demonstrate that the maximal sliding energy barrier of the GB with a monovacancy is 2.78 J/m2, 15% lower than that of the vacancy-free GB, suggesting that the presence of vacancy at the GB increases the GB mobility. View full abstract»

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  • Localization potentials in AlGaN epitaxial films studied by scanning near-field optical spectroscopy

    Page(s): 113516 - 113516-7
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    Scanning near-field photoluminescence spectroscopy has been applied to evaluate bandgap fluctuations in epitaxial AlGaN films with the AlN molar fraction varying from 0.30 to 0.50. A dual localization pattern has been observed. The potential of the small-scale (<100 nm) localization, evaluated from the width of the photoluminescence spectra, is between 0 and 51 meV and increases with increased Al content. These potential variations have been assigned to small-scale compositional fluctuations occurring due to stress variations, dislocations, and formation of Al-rich grains during growth. Larger area potential variations of 25–40 meV, most clearly observed in the lower Al-content samples, have been attributed to Ga-rich regions close to grain boundaries or atomic layer steps. The density, size, and bandgap energy of these domains were found to be composition dependent. The lower bandgap domains were found to be strongly correlated with the regions with efficient nonradiative recombination. View full abstract»

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  • Formation and structural characterization of Ni nanoparticles embedded in SiO2

    Page(s): 113517 - 113517-7
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    Face-centered cubic Ni nanoparticles were formed in SiO2 by ion implantation and thermal annealing. Small-angle x-ray scattering in conjunction with transmission electron microscopy was used to determine the nanoparticle size as a function of annealing temperature, whereas the local atomic structure was measured with x-ray absorption spectroscopy. The influence of finite-size effects on the nanoparticle structural properties was readily apparent and included a decrease in coordination number and bond length and an increase in structural disorder for decreasing nanoparticle size. Such results are consistent with the non-negligible surface-to-volume ratio characteristic of nanoparticles. In addition, temperature-dependent x-ray absorption spectroscopy measurements showed the mean vibrational frequency (as obtained from the Einstein temperature) decreased with decreasing nanoparticle size. This reduction was attributed to the greater influence of the loosely bound, under-coordinated surface atoms prevailing over the effects of capillary pressure, the former enhancing the low frequency modes of the vibrational density of states. View full abstract»

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  • First principles study of native defects in InI

    Page(s): 113518 - 113518-5
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    Heavy-metal halide semiconductors have attracted much interest recently for their potential applications in radiation detection because the large atomic numbers (high Z) of their constituent elements enable efficient radiation absorption and their large band gaps allow room temperature operation. However, defect properties of these halides and their connection to carrier transport are little known. In this paper, we present first-principles calculations on native defects in InI, which is a promising material for applications in room temperature radiation detection. The important findings are: (1) anion and cation vacancies (Schottky defects) form the dominant low-energy defects that can pin the Fermi level close to midgap, leading to high resistivity that is required for a good radiation detector material; (2) the anion vacancy in InI induces a deep electron trap, which should reduce electron mobility-lifetime product in InI; (3) low diffusion barriers of vacancies could be responsible for the observed polarization phenomenon at room temperature. View full abstract»

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  • Atomistic structural description of the Si(001)/a-SiO2 interface: The influence of different Keating-like potential parameters

    Page(s): 113519 - 113519-12
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    We investigate the influence of force field parameterization on the atomic-level description of the interface structure between Si(001) and its amorphous oxide [Si(001)/a-SiO2] with systematic application of continuous random network model-based Metropolis Monte Carlo (CRN-MMC) simulations. Particular emphasis is given to the nature of the potentials in both the crystalline Si and a-SiO2 phases, especially the quantifiable relative rigidity between phases. To assess their reliability, the energetics and mechanical properties of the interface models generated from the CRN-MMC approach with different Keating-like potential parameters were compared with those calculated using density functional theory. We statistically characterized the structural parameters and interface abruptness from various potential models of varying interface O coverage ratio in terms of bond angle, ring size, and suboxide distributions; lateral Si-O-Si bridge bond interface densities; and strain energy profiles along [001]. Comparison of our simulation results and existing experimental observations shows that a sufficiently hard character of the a-SiO2 phase parameterization is essential in generation of atomically accurate depictions of the Si(001)/a-SiO2 interface. View full abstract»

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  • Ta and Au(Pd) alloy metal film transducers for time-domain thermoreflectance at high pressures

    Page(s): 113520 - 113520-4
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    We studied the pressure dependence of the thermoreflectance and piezo-optical coefficients of metal film transducers—Al, Ta, and Au(Pd) alloy (≈5 at. % Pd)—at a laser wavelength, 785 nm, commonly used in time-domain thermoreflectance (TDTR) and picosecond acoustics experiments. Al has exceptionally high thermoreflectance at ambient pressure, dR/dT ≈ 1.3 × 10-4 K-1, but its applicability at high temperatures is limited by the low melting temperature. The thermoreflectance of Al also has an undesirable zero-crossing near 6 GPa. The thermoreflectance values of Ta and Au(Pd) are comparable to that of Al at ambient conditions but independent of pressure in the pressure range 0 < P < 10 GPa. Ta and Au(Pd) thin film transducers also show strong picosecond acoustic echoes at all pressures in this range. We conclude that Ta and Au(Pd) metal film transducers can replace Al in TDTR experiments and thereby facilitate the extension of TDTR methods to high pressures and temperatures. View full abstract»

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  • Temperature dependence of weak localization effects of excitons in ZnCdO/ZnO single quantum well

    Page(s): 113521 - 113521-4
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    We report on the optical properties of high-quanlity ZnCdO/ZnO single quantum well (SQW) grown on c-sapphire substrates by pulsed laser deposition. The temperature dependent photoluminescence (PL) of ZnO/ZnCdO SQWs exhibits an inconspicuous S-shaped property due to the weak carrier localization effect, as a consequence of the slightly inhomogeneous Cd distribution in the well layer as well as the smooth interfaces. The integrated PL intensity of the higher Cd SQW decreases faster than that of the lower sample with increasing temperature, indicating the presence of interface barrier in high Cd content SQWs. View full abstract»

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  • Blast wave and contraction in Au(111) thin film induced by femtosecond laser pulses. A time resolved x-ray diffraction study.

    Page(s): 113522 - 113522-5
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    We utilize 100 fs optical pulses as a means of heating 150 nm thick single Au (111) crystals, below the melting point and monitor the subsequent structural evolution with subpicosecond time resolution using 0.6 ps, 8.04 KeV x-ray pulses. By monitoring the energy and time dependent modulation of the width and shift of the x-ray diffraction rocking curve, we have obtained information on electron-phonon coupling, photon-lattice interaction, and time resolved kinetics of the crystal disorder. The data show that during the first couple picoseconds after optical excitation, the formation of a “blast force” and lattice contraction, followed by a pressure wave formed 10-20 ps later. Both the blast and pressure wave propagate through the crystal with sonic velocities. These time resolved x-ray diffraction data provide a detail description of the processes induced by ultrafast laser pulses impinging on very thin metallic crystals. View full abstract»

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  • Structural, elastic, and thermal properties of Laves phase ZrV2 under pressure

    Page(s): 113523 - 113523-5
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    The structural, electronic, elastic and some thermodynamic properties of the cubic C15 structure ZrV2 compound under pressure are investigated by first-principles calculations. Our results for the equilibrium unit cell volume, bulk modulus and band structure are consistent with the calculated and experimental results. Cubic ZrV2 is mechanically stable according to the elastic stability criteria and shows ductile with the G/B and Cauchy pressure analysis. Moreover the pressure and temperature dependence of the bulk modulus, specific heat, Debye temperature and thermal expansion coefficient are discussed, among them our calculated Debye temperature is in good agreement with experiments. View full abstract»

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  • Mechanical relaxation studies of α and slow β processes in Nd65Fe15Co10Al10 bulk metallic glass

    Page(s): 113524 - 113524-4
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    The relaxation dynamics of the primary α and secondary β processes in Nd65Fe15Co10Al10 bulk metallic glass has been investigated by using low-frequency mechanical spectroscopy, differential scanning calorimetry (DSC) and x-ray diffraction. From the shift of internal friction peaks by the frequency change, the activation energy of β relaxation (Eβ) is found to be about 1.01 eV, and a correlation between Eβ and glass transition temperature (Tg) is found Eβ ≈ 24 RTg, indicating that the β relaxation is intrinsic in metallic glasses. According to the coupling model, the uncoupled activation energy of α relaxation (Eα*) is found to be about 1.38 eV, still larger than the value of Eβ corresponding to a simple Debye relaxation process. The possible mechanism and the connection between α and β relaxations are discussed. View full abstract»

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  • Time-domain analysis and experimental examination of cumulative second-harmonic generation by primary Lamb wave propagation

    Page(s): 113525 - 113525-12
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    Within the second-order perturbation approximation, the physical process of cumulative second-harmonic generation by the primary Lamb wave propagation has been investigated in the time domain. Based on the preconditions that the transfer of energy from the primary Lamb wave to the double frequency Lamb wave is not zero and that the phase velocity matching condition is satisfied, we focus on analyzing the influence of mismatching of the group velocities on the generation of the second harmonic by propagation of a primary Lamb wave tone burst with a finite duration. Our analysis indicates that the time-domain envelope of the second harmonic generated is dependent on the propagation distance when both the duration of the primary Lamb wave tone burst and the group velocity mismatch are given. Furthermore, it can be concluded that the integrated amplitude of the time-domain second harmonic, which quantifies the efficiency of the second-harmonic generation, grows with the propagation distance even when the group velocity matching condition is not satisfied. The experimental examination has been performed, and it verifies our theoretical analysis. View full abstract»

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  • Spectral conversion for solar cell efficiency enhancement using YVO4:Bi3+,Ln3+ (Ln = Dy, Er, Ho, Eu, Sm, and Yb) phosphors

    Page(s): 113526 - 113526-7
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    Bi3+–Ln3+ (Ln = Dy, Er, Ho, Eu, and Sm) co-doped YVO4 phosphors are proposed as UV-absorbing luminescent converter candidate to enhance the power conversion efficiency and photochemical stability of dye-sensitized solar cells (DSSCs). The phosphors can efficiently convert UV photons in a broad range from 250 to 400 nm into visible emissions, which can be absorbed by DSSCs. Efficient broadband down-conversion UV light into near-infrared emission around 1000 nm was achieved in the YVO4:Bi3+,Yb3+ phosphors. The energy transfer from V5+–Bi3+ charge-transfer state to Yb3+ was shown to be a cooperative down-conversion type by the luminescence spectra, energy transfer efficiency, and luminescence decay curves. The YVO4:Bi3+,Yb3+ phosphors are promising for boosting the efficiency of crystalline silicon solar cells by down-converting the UV part of the solar spectrum to near-infrared photons with a twofold increase in the photon number. This research may open up promising new perspectives for designing novel luminescent materials for photovoltaic cells with high efficiency. 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