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In situ high-pressure X-ray diffraction and Raman spectroscopy were used to determine the influence of composition on the high-pressure behavior of A2B2O7 pyrochlore (A = Eu, Dy; B = Ti, Zr) up to ∼50 GPa. Based on X-ray diffraction results, all compositions transformed to the high-pressure cotunnite structure. The B-site cation species had a larger effect on the transition pressure than the A-site cation species, with the onset of the phase transformation occurring at ∼41 GPa for B = Ti and ∼16 GPa B = Zr. However, the A-site cation affected the kinetics of the phase transformation, with the transformation for compositions with the smaller ionic radii, i.e., A = Dy, proceeding faster than those with a larger ionic radii, i.e., A = Eu. These results were consistent with previous work in which the radius-ratio of the A- and B-site cations determined the energetics of disordering, and compositions with more similarly sized A- and B-site cations had a lower defect formation energy. Raman spectra revealed differences in the degree of short-range order of the different compositions. Due to the large phase fraction of cotunnite at high pressure for B = Zr compositions, Raman modes for cotunnite could be observed, with more modes recorded for A = Eu than A = Dy. These additional modes are attributed to increased short-to-medium range ordering in the initially pyrochlore structured Eu2Zr2O7 as compared with the initially defect-fluorite structured Dy2Zr2O7.

Epitaxially grown superconductor/dielectric/superconductor trilayers have the potential to form high-performance superconducting quantum devices and may even allow scalable superconducting quantum computing with low-surface-area qubits such as the merged-element transmon. In this work, we measure the power-independent loss and two-level-state (TLS) loss of epitaxial, wafer-bonded, and substrate-removed Al/GaAs/Al trilayers by measuring lumped element superconducting microwave resonators at millikelvin temperatures and down to single-photon powers. The power-independent loss of the device is (4.8 ± 0.1) × 10 − 5 , and the resonator-induced intrinsic TLS loss is (6.4 ± 0.2) × 10 − 5 . Dielectric loss extraction is used to determine a lower bound of the intrinsic TLS loss of the trilayer of 7.2 × 10 − 5 . The unusually high power-independent loss is attributed to GaAs's intrinsic piezoelectricity. [ABSTRACT FROM AUTHOR]

In semiconductors under tightly-focused photocarrier excitation, the lateral variation of carrier concentration induces a lateral variation of photovoltage. In chemically-passivated p-type GaAs films at 300 K, we show experimentally and theoretically that the photovoltage lateral dependence is able to block the photoelectron diffusion, thus reducing the effective charge diffusion constant by a factor of ≈ 5 with respect to surface-free conditions and producing a self-trapping of photoelectrons. This effect is not present for surface-free and for oxidized samples, for which the photovoltage magnitude is strongly reduced. The photovoltage lateral variation also induces a coupling between charge and spin diffusion. Because of this coupling, the effective spin diffusion constant is significantly larger than the effective charge one. [ABSTRACT FROM AUTHOR]

We present electrostatic Micro-Electromechanical System actuators equipped with dimples and contact pads. The introduction of dimples and contact pads is shown to prevent stiction between the actuator and bottom electrodes and minimize dielectric charging and to eliminate multi-valuedness in the response. It also increases the stable travel range by eliminating the pull-in instability and introducing new "tapping mode" oscillations, where the dimples come into regular contact with the contact pads. An experimentally validated lumped-mass model was developed and used to generate the bifurcation diagram of the actuator in the vicinity of tapping mode oscillations. The diagram showed the presence of a chaotic attractor bracketed by a type-I intermittency and a cascade of period-doubling bifurcations. However, these chaotic motions were only present for a limited range of the excitation amplitude and frequency. Provided these ranges are excluded, dimples and contact pads can be deployed to obtain efficient and well regulated electrostatic actuators. We found that these results and conclusions are valid for classes of low-frequency as well as high-frequency actuators. [ABSTRACT FROM AUTHOR]

X-ray diffraction data is presented by demonstrating after high field poling, solid solutions of lead zinc niobate with 8% lead titanate (PZN-PT) consisting of mixture of a dominant monoclinic phase and a remnant tetragonal phase. Diffraction shows a slight distorted version of the rhombohedral zero field solution resulting from the domain walls that consist of a tetragonal phase.

Coupling between two microcavity plasmas in a symmetric, microfabricated dielectric barrier structure has been observed by injecting charge from one of the plasmas into an intervening microchannel. Periodic modulation of the electric field strength in the injector (or electron "donor") cavity has the effect of deforming the acceptor microplasma which exhibits two distinct and stable spatiotemporal modes. Throughout the time interval in which the two microplasmas are coupled electrostatically, the acceptor plasma is elongated and displaced by 75-100 μm (~30% of its diameter) in the direction of the microchannel. The depletion of charge in the microchannel results in an immediate transition of the second (acceptor) microplasma to an equilibrium state in which the plasma is azimuthally symmetric and centered within its microcavity. Switching between these two spatial modes requires a shift (in the plasma centroid) of ~80 μm in <50 ns which corresponds to a velocity of 1.6 km/s. Precise control of this plasma phase transition through device fabrication, and modulation of the donor plasma electric field, suggest applications of this plasma coupling and charge transport mechanism to signal processing, photonics, and electromagnetics. [ABSTRACT FROM AUTHOR]

An in situ Raman monitoring method was used to measure Raman spectra during the deposition of C60 on GaAs(100) surface at varying temperatures. The formation of photopolymerized C60 was then examined with the applied laser irradiation. Contrary to expectation, a maximum value corresponding to pristine C60 was obtained at extremely low coverages. The photoinduced polymerization was calculated at 15 nanometers and was independent of growth temperature, thereby suggesting that a suppressive effect on the polymerization process was at work.

Thermal oxidation of polycrystalline Ge(sub x)Si(sub 1-x) and pseudomorphic GeSi at a favorable temperature ranging from 550 to 900 degrees centigrade reveals that backscattering spectrometry of the high Ge content has a uniform Ge(sub x)Si(sub 1-x)O2 layer. The Si(sub 1-x) layer is produced at high temperatures when the Ge content is very low. The formation of Si(sub 1-y)O2 layer piles up the Ge behind the oxide layers.

Multiferroic single crystals R[Mn.sub.2][O.sub.5] (R = Bi, Eu and Dy) are analyzed by using the temperature dependent Raman scattering study. The anomalous phonon behaviors in bending and stretching modes are consistent with the spin-phonon coupling in a scenario of strong magnetic correlations.

Intrinsic defects in Bi2Te3 were analyzed using first-principles density functional theory calculations. The results demonstrated that antisite defects were dominant in both p- and n-type Bi2Te3. To investigate the characteristics of these defects, n-type Bi2(Te,Se)3 compounds were fabricated using powder prepared by high-energy ball milling and their electrical properties were examined. Electrical resistivity and the Seebeck coefficient increased with ball milling time. These changes in both properties were attributed to a reduction in the carrier concentration, which was supported by the Hall coefficient measurement. The antisite defects were found to be responsible for the changes in carrier concentration. Lattice parameter analysis supported the postulate that variations in the concentration of antisite defects were responsible for the changes in carrier concentration. These findings suggest that optimization of carrier concentration should be considered in the high-energy ball milling process, in which reductions in grain size are expected to improve thermoelectric properties.VC [ABSTRACT FROM AUTHOR]

We have fabricated thin films composed of W-encapsulated Si clusters (WSi10) on Si substrates and investigated their electronic properties using scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). An epitaxial layer was observed at the interface with the Si substrate, and an amorphous layer was on top. The bulk plasmon of the WSi10 cluster film was measured and compared with those of crystalline Si (c-Si) and WSi2 films. We found similar plasmon energies in the epitaxial and amorphous layers of the WSi10 cluster film. The plasmon peak of the WSi10 cluster film is shifted to higher energy compared with that of c-Si, which is related to the electron density increase in the valence band. The Si-L23 absorption edge spectra show that the conduction-band density of states in Si was modified by hybridization between Si and W atoms. [ABSTRACT FROM AUTHOR]

The authors report on the growth mechanism of ZnO homoepitaxy at the low-temperature range of 500 °C, which is unavailable to obtain high-quality ZnO films in heteroepitaxy. One typical set of ZnO films were grown on (0001) ZnO substrates by molecular-beam epitaxy: a standard structure without buffer and two buffered structures with high-temperature (HT) homobuffer and low-temperature (LT) homobuffer. As a result, the LT homobuffered structure had the outstanding material properties: the surface roughness is 0.9 nm, the full width at half maximum of x-ray rocking curve is 13 arcsec, and the emission linewidth of donor-bound excitons is 2.4 meV. In terms of the theoretical interpretation of the experimentally obtained electron mobilities, it was found that the LT homobuffered structure suffers less from the dislocation scattering and the ionized-impurity scattering compared to the HT homobuffered structure. It is proposed that, in the ZnO low-temperature homoepitaxy, the LT homobuffer plays a key role in inducing the complete termination of dislocations in the homointerface and suppressing the outdiffusion of contaminants and point defects on the ZnO surface, which results in the formation of smooth wetting layer on the homointerface. [ABSTRACT FROM AUTHOR]

The influence of a novel 1,2,4-linking hyperbranched poly(arylenevinylene) (1,2,4-hb-PAV) material, designed to feature intramolecular energy-funneling, on the transport and emission properties of organic light emitting diodes (OLEDs) has been studied. A comparison to conventional hyperbranched 1,3,5-linking polymers (1,3,5-hb-PAV), which do not exhibit this effect, has been made. For this purpose, single-layer organic light emitting diodes with a glass/indium-tin oxide/poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate)/active layer/Ca/Al structure and different active layer thicknesses have been fabricated and characterized, using either 1,2,4-hb-PAV or 1,3,5-hb-PAV as active layers. The current-voltage response has been interpreted in terms of a numerical model that includes a field-dependent carrier mobility, which allows us to estimate carrier mobility in the diodes. Stable green emission with Commission Internationale de L'Eclairage coordinates at (0.41,0.56) and a high luminous efficiency of 28 Cd/A has been obtained at very low driving currents (10 μA) for 1,2,4-hb-PAV material, versus 2.1 Cd/A for the conventional 1,3,5-linking material, in spite of the high photoluminescence quantum yield in thin film exhibited by both materials. The significant improvements in the performance of OLEDs based on 1,2,4-hb-PAV with respect to other conventional hyperbranched polymers are attributed to the inherent energy gradient from the shorter branches to the longer conjugated stem in this structure, which enables the characteristic funneling effect. [ABSTRACT FROM AUTHOR]

We discuss two important consequences of recent experiments using surface plasmon polariton (SPP) tomography in a quantum eraser arrangement. In these experiments surface-emission images were modified by manipulating the polarization state of the leakage radiation. We show that SPP tomography does have the potential to produce images that mirror with high fidelity the propagation and interference of SPP beams at the metal-air interface of a sample. We reveal the physical mechanism behind this capability of SPP tomography. In addition, we show how SPP tomography can be used to detect photons passing through the dark fringes of an interference pattern and why photons propagate in such a way that looks like a photon can propagate across a region where it is never observed. [ABSTRACT FROM AUTHOR]

In this study, the effect of Ag or Sb addition on the thermoelectric properties of PbTe including the Seebeck coefficient, the electrical resistivity, and the thermal conductivity, was studied in the temperature range from 323 to 723 K. The major carriers in the Ag-doped and the Sb-doped PbTe are holes and electrons, respectively. A degenerate semiconductor behavior in the electrical transport properties was observed in the Ag-doped, whereas the semi-metallic in the Sb-doped. It was suggested from the results of the Hall effect measurement and the Seebeck coefficient that the effective mass was significantly altered by the Sb-doping. The maximum dimensionless figure of merit, ZT, of Pb1-xAgxTe and Pb1-xSbxTe (x=0.1) alloys showed 0.27 and 0.62 at 723 K, respectively. Based on the analysis of the Seebeck coefficient of the Ag or the Sb-doped PbTe, the interaction between Ag and Sb in PbTe, as expected in the AgPbmSbTem+2 (LAST-m), may be supposed. [ABSTRACT FROM AUTHOR]

We report on the crystallinity, N incorporation efficiency, optical properties, and electrical properties of N and Te codoped ZnO films grown by plasma-assisted molecular-beam epitaxy. Te improves the surface morphology and roughness of ZnO films in terms of both streak reflection high energy electron diffraction pattern and atomic force microscopy observations. Also, N and Te codoping is helpful to improve the crystallinity and N incorporation efficiency simultaneously. We found that; (a) narrower x-ray linewidth and higher N concentration were obtained by codoping. (b) Nitrogen related emission lines including donor-acceptor pair and acceptor-bound exciton dominantly emerged in photoluminescence spectra. (c) Codoping enhanced the carrier compensation of native donors in ZnO films and suppressed the dislocation scattering. As a consequence, we concluded that N and Te codoping is very effective for the growth of reliable p-type ZnO films which fulfill the controversial requirements; high N concentration and high crystallinity, simultaneously. [ABSTRACT FROM AUTHOR]

We suggest a method for the a priori determination of the electrical percolation threshold in carbon nanotube (CNT) networks, of relevance in electronic devices, polymer composites, etc. The variability in the CNT lengths, commonly observed in practical processing and dispersion, was also considered and the resulting probability distribution function determined to be of the Weibull type. Subsequently, the predicted percolation threshold volume fractions for single-walled CNTs, ∼0.000 73 and multiwalled CNTs, ∼0.0193, were found to be in close correspondence to the experimentally determined values of 0.0011 and 0.0147, respectively. [ABSTRACT FROM AUTHOR]

In situ laser-based resonant ultrasound spectroscopy is used to characterize the development of a recrystallized microstructure in a high purity copper sample. The modal shapes, used for mode identification, of several resonant modes are determined before and after annealing by raster scanning the laser interferometric probe. This information is used to isolate the motion of individual modes during high temperature annealing. The evolution of a particular mode during annealing is examined in detail. During recrystallization, the center frequency of this mode shifts by approximately 20% of the original value. Using electron backscatter data it is shown that the majority of this shift is due to changes in the polycrystal average elastic stiffness tensor, driven by changes in texture, and that changes in dislocation density and pinning length are secondary influences. [ABSTRACT FROM AUTHOR]

Silicon-rich silicon oxide (SRSO) and Er-doped SRSO (SRSO:Er) thin films were formed by pulsed laser deposition, and characterized by photoluminescence (PL), x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), and UV-visible transmission in order to clarify the 1.54 μm emission mechanism in the SRSO:Er films. The oxygen content of the films was varied by the adjustment of oxygen partial pressure. The behavior of the 1.54 μm PL with oxygen partial pressure combined with XPS and XRD data show that the 1.54 μm emission intensity is related to the amount of SiOx phase. This conclusion is well supported by the measurements of UV-visible transmission. In many previous papers, the 1.54 μm emission is associated with Si nanocrystals, but in our study the correlation between 1.54 μm emission intensity and the amount of SiOx phase is much clearer. [ABSTRACT FROM AUTHOR]

In this study, the thermoelectric properties of AgPbmSbTem+2 (LAST-m, m=12, 16, 18, 20, 22, and 26), including the Seebeck coefficient, the electrical resistivity, and the thermal conductivity, were studied in the temperature range from 323 to 773 K. The Seebeck coefficient and the electrical resistivity exhibited the behavior of a degenerate semiconductor. The measured results of the carrier concentration supported the conclusions of the electrical transport properties. The lowest electrical resistivity and the thermal conductivity were observed, which leads to the best thermoelectric performance in LAST-18 and the dimensionless figure of merit ZT value of 0.8 at 773 K. [ABSTRACT FROM AUTHOR]

The magnetization depth profiles of three FeCo/GaAs samples grown at different temperatures and measured before and after annealing were obtained using polarized neutron reflectometry. Prior to annealing, the sample grown at 95 °C had the thickest magnetically degraded interfacial region between the FeCo film and the GaAs substrate. For the sample grown at -15 °C, the magnetic interface was sharp. For all samples, annealing promoted thicker interfacial regions with suppressed magnetization and distinct boundaries with the adjoining (FeCo or GaAs) material. Thus, the magnetic structure of the FeCo/GaAs interfacial region was very sensitive to the conditions of growth and annealing. [ABSTRACT FROM AUTHOR]

We present systematic studies on the angle dependence of planar Hall resistance and anisotropic magnetoresistance of ferromagnetic semiconductor (Ga,Mn)As and metallic Permalloy (Py) microdevices. (Ga,Mn)As shows two distinct planar Hall resistance switchings when the magnetic field is applied in the plane, indicating the magnetization reversal by 90° due to the cubic magnetocrystalline anisotropy. Similar magnetization reversal is observed in metallic Py microdevices. The broader planar Hall resistance jump observed in the Py device is attributed to the uniaxial shape anisotropy and complex domain rotation at the voltage probe. Nevertheless the observed planar Hall effect of Py is comparable to that of (Ga,Mn)As. [ABSTRACT FROM AUTHOR]

Intervalley splittings of Si quantum wells are studied using a multivalley effective mass theory and a finite element method. It is found that the contributions to the valley splitting mainly come from the quantum well interfaces and the external field. Especially the derivatives of the confinement potential which appears in the valley coupling Hamiltonian cause the periodic oscillation of the splitting with the well width. Theoretical predictions are in reasonably good agreement with the recent experimental observation of valley splitting in a SiO2/Si/SiO2 quantum well, which proves the validity of our approach. [ABSTRACT FROM AUTHOR]

The structural, magnetic, and electric properties of Dy1-xSrxCoO3-δ perovskite have been investigated systematically over the range of doping, 0.65≤x≤0.90. The Rietveld refinements of x-ray powder diffraction patterns at room temperature indicate that the samples with 0.65≤x≤0.75 show a tetragonal structure with I4/mmm group symmetry, while the compounds with 0.80≤x≤0.90 are cubic with pm3m group symmetry. Zero field-cooled magnetization, M(T), of 0.65≤x≤0.85 samples reveals a cusp at around room temperature. For all samples, M(T) increases rapidly below 50 K due to the paramagnetism of Dy sublattice. The inverse magnetic susceptibility, χ-1(T), was described by using Curie–Weiss law. The resistivity (ρ) data can be explained according to a three-dimensional variable range hopping model in a certain temperature range. The density of states at the vicinity of Fermi level is roughly estimated. [ABSTRACT FROM AUTHOR]

Comparative analysis of optical characteristics of In0.08Ga0.92N/In0.03Ga0.97N multiquantum well (MQW) laser diode structures grown on freestanding GaN and on sapphire substrates is reported. Higher quantum efficiency, higher thermal activation energy, smaller Stokes-like shift, and shorter radiative lifetime are observed for InGaN MQWs on GaN substrate than those of the same MQWs on sapphire substrate. From time-resolved optical analysis, we find that not only an increase in nonradiative lifetime due to reduced dislocation density but also a decrease in radiative lifetime caused by suppressed piezoelectric field play an important role in enhancing optical properties of InGaN MQWs on GaN substrates. [ABSTRACT FROM AUTHOR]

Wave propagation characteristics of apertures were analyzed to explain the light transmission of metallic nanoapertures. Based on Maxwell’s equations, the wave dispersion relations of wave propagation modes in nanoapertures were derived. The resonance frequency shift of the aperture and the variation of the spot size are explained with the dispersion relations. The relationship between near-field and far-field light transmission power throughput and spot size is also shown with the wave mode change predicted by the dispersion relations. [ABSTRACT FROM AUTHOR]

An approach named the defect-induced extension of photonic band gaps (PBGs) is proposed, in which a stratified periodic structure is combined with another stratified structure containing defect layers. For comparison, three structures composed of TiO2 and SiO2 alternating layers, perfect, five-defect and combined-photonic crystals (PCs), were fabricated by using an electron-beam evaporation system. The measured PBG width of combined-PCs extended to 370 nm at the normal incidence, nearly three times more than that of perfect ones, 130 nm, but a few ripples are observed within the PBG. In order to elucidate the origin of the ripples, the influence of the refractive-index contrast on the reflectance was analyzed theoretically. When the refractive-index contrast nH/nL≥1.7, the ripples are eliminated. More importantly, an overdoubled omnidirectional PBG is realized at nH/nL=2.56, covering the visible region completely. This approach is considered to be based on the light localization and a heterostructure resulting from introducing the impurity band and compensating for it using PCs with a strictly periodic structure, in which the thickness of every layer remains unchanged. These characteristics distinguish it from other methods proposed before, with the fabrication much easier. [ABSTRACT FROM AUTHOR]

The structural, magnetic, and transport properties of La0.7(Ca1-ySry)0.3MnO3 films, deposited on a LaAlO3 (001) single crystalline substrate by rf-magnetron sputtering using “soft” (or powder) targets, have been investigated. It was found that at 0.3≤y≤0.5 both the rhombohedral (R3c) and the orthorhombic (Pnma) crystal phases in the form of nanoscale clusters are coexistent at room temperature. The observed structural clustering is accompanied by two-stage magnetic and electronic transitions, and governed by a nonuniform distribution of the lattice strain through the film. It was shown that for the films with 0≤y≤0.5 the nonlinear (almost parabolic) MR(H) dependence is typical while, for 0.65≤y≤1.0, the linear MR(H) behavior is observed at room temperature. The magnetotransport properties of films are explained within the framework of field-dependent activation-energy model. The magnetic phase diagram for La0.7(Ca1-ySry)0.3MnO3 thin-film system is also presented. [ABSTRACT FROM AUTHOR]

Gd2O3 films, in which ZrO2 was incorporated, were epitaxially grown on Si(111) using an electron-beam evaporation and effusion method. The crystalline structure and morphological characteristics were investigated by various measurements. A silicide layer was locally formed during the initial growth stage due to interactions between elemental Gd and Si in the Gd2O3 film, resulting in poor interfacial characteristics and extensive destruction of the crystalline structure. However, the incorporation of ZrO2 influenced the unit-cell structure of Gd2O3, which contains oxygen vacancies that is located diagonally, enhancing the structural stability owing to the effective suppression of the interfacial layer. The effect on the initial growth stage as the result of incorporation improves the crystalline quality of the epitaxial Gd2O3 film and structural coherence between the film and substrate. [ABSTRACT FROM AUTHOR]

Rotational Maker fringes, scaled with respect to χ11(2) of crystalline quartz, were used to determine the second order susceptibilities χ31(2) and χ33(2) for samples of thin AlxGa1-xN films, a thicker GaN film, and a free-standing GaN platelets. The pump wavelength was 1064 nm. The AlxGa1-xN samples, ranging in thickness from roughly 0.5 to 4.4 μm, were grown by metalorganic chemical vapor deposition (MOCVD) and hydride vapor-phase epitaxy (HVPE) on (0001) sapphire substrates. The Al mole fractions x were 0, 0.419, 0.507, 0.618, 0.660, and 0.666, for the MOCVD-grown samples, and x=0, 0.279, 0.363, and 0.593 for the HVPE-grown samples. An additional HVPE-grown GaN sample ∼70 μm thick was also examined. The free-standing bulk GaN platelets consisted of an HVPE grown film ∼226 μm thick removed from its growth substrate, and a crystal ∼160 μm thick grown by high-pressure techniques. For the AlxGa1-xN samples, the magnitudes of χ31(2) and χ33(2) decrease roughly linearly with increasing x and extrapolate to ∼0 for x=1. Furthermore, the constraint expected for a perfect wurtzite structure, namely χ33(2)=-2χ31(2), was seldom observed, and the samples with x=0.660 and x=0.666 showed χ31(2) and χ33(2) having the same sign. These results are consistent with the theoretical studies of nonlinear susceptibilities for AlN and GaN performed by Chen et al. [Appl. Phys. Lett. 66, 1129 (1995)]. The thicker bulk GaN samples displayed a complex superposition of high- and low-frequency Maker fringes due to the multiple-pass interference of the pump and second-harmonic generation beams, and the nonlinear coefficients were approximately consistent with those measured for the thin-film GaN sample. [ABSTRACT FROM AUTHOR]

High energy (MeV) C2+, F2+, and Cl2+ ions were irradiated onto π-conjugated polyaniline emeraldine base (PAN-EB) samples. The energy of an ion beam was controlled to a range of 3–4.5 MeV, with the ion dosage varying from 1×1012 to 1×1016 ions/cm2. The highest dc conductivity (σdc) at room temperature was measured to be ∼60 S/cm for 4.5 MeV Cl2+ ion-irradiated PAN-EB samples with a dose of 1×1016 ions/cm2. We observed the transition of high energy ion-irradiated PAN-EB samples from insulating state to conducting state as a function of ion dosage based on σdc and its temperature dependence. The characteristic peaks of the Raman spectrum of the PAN-EB samples were reduced, while the D-peak (disordered peak) and the G peak (graphitic peak) appeared as the ion dose increased. From the analysis of the D and G peaks of the Raman spectra of the systems compared to multiwalled carbon nanotubes, ion-irradiated graphites, and annealed carbon films, the number of the clusters of hexagon rings with conducting sp2-bonded carbons increased with ion dosage. We also observed the increase in the size of the nanocrystalline graphitic domain of the systems with increasing ion dosage. The intensity of normalized electron paramagnelic resonance signal also increased in correlation with ion dose. The results of this study demonstrate that π-conjugated pristine PAN-EB systems changed from insulating state to carbonized conducting state through high energy ion irradiation with high ion dosage. [ABSTRACT FROM AUTHOR]

Spawned by the finding of efficient quantum algorithms, the development of a scalable quantum computer has emerged as a premiere challenge for nanoscience and nanotechnology in the last years. Spins of electrons and nuclei in [sup 31]P atoms embedded in silicon are promising quantum bit (qubit) candidates. In this article we describe single atom doping strategies and the status of our development of single atom qubit arrays integrated with control gates and readout structures in a “top down” approach. We discuss requirements for [sup 31]P qubit array formation by single ion implantation, and integration with semiconductor processing. [ABSTRACT FROM AUTHOR]

We have performed a detailed contactless electroreflectance study of the interband excitonic transitions on both the Ga and N faces of a 200-μm-thick freestanding hydride-vapor-phase-epitaxy grown wurtzite GaN sample with low defect concentration in the temperature range between 20 and 300 K. The transition energies of the A, B, and C excitons and broadening parameters of the A and B excitons have been determined by least-square fits to the first derivative of a Lorentzian line shape. The energy positions and separations of the excitonic transitions in the sample reveal the existence of residual strain. At 20 K the broadening parameter of A exciton deduced for the Ga (5×10[SUP5] dislocation cm[SUP-2] and N (1×10[SUP7] dislocation cm[SUP-2]) faces are 3 and 7 meV, respectively, indicating a lower defect concentration on the former face. The parameters that describe the temperature dependence of the interband transition energies of the A, B, and C excitons as well as the broadening function of the A and B features are evaluated. The results from an analysis of the temperature dependence of the broadening function of excitons A and B indicate that GaN exhibits a very large exciton-phonon coupling. [ABSTRACT FROM AUTHOR]

High-quality epitaxial thin films of Co-doped anatase TiO[SUB2] (Co:TiO2[SUB2) were grown epitaxially on SrTiO[SUB3] (001) substrates by using pulsed laser deposition with in-situ reflection high-energy electron diffraction. The oxygen partial pressure, P[SUBO2], during the growth was systematically varied. As P[SUBO2] decreased, the growth behavior altered from a two-dimensional layer-by-layer-like growth to a three-dimensional island-like pattern. Electrical conductivity and saturation magnetization increased, seemingly consistent with the picture of carrier-induced ferromagnetism. However, we also found that the spatial distribution of Co ions became highly nonuniform and the chemical state of Co ions changed from ionic to metallic. All of these P[SUBO2] dependences, even including the transport and the magnetic properties, can be explained in terms of the formation of cobalt clusters, whose existence was clearly demonstrated by transmission-electron-microscope studies. Our work clearly indicates that the cobalt clustering will result in the room-temperature ferromagnetism observed in our Co:TiO[SUB2].films. To check the possible spin polarization of carriers in Co:TiO[SUB2] films, we also fabricated a heterojunction composed of a ferromagnetic Co:TiO[SUB2], an insulating SrTiO[SUB3], and a ferromagnetic half-metallic (La,Ba)MnO[SUB3] layer. When the magnetic field was varied, we could not observe any changes in its I-V characteristic curves, which suggests that there might be little spin-polarization effect in the anatase Co:TiO[SUB2] layer. [ABSTRACT FROM AUTHOR]

Deep level electron and hole traps were studied by means of deep level transient spectroscopy with electrical and optical injection on a freestanding thick n-GaN sample with low dislocation density. It is shown that at both the upper and the lower surface of the sample there exists a thin, ~0.5 μm layer of damaged material with lowered concentration of electrons and enhanced density of deep centers. Deep in the bulk of the film the densities of the majority of the electron and hole traps are shown to be very low, but measurably higher on the lower face (N face), which was originally closer to the Al[SUB2]O[SUB3] substrate. The two faces are also shown to similarly differ in the density of deep hole traps whose concentration is deduced from low-temperature capacitance-voltage measurements in the dark and after illumination (such traps were previously associated with dislocation states). The concentration of persistent photocapacitance centers is shown to be very low on both sides and considerably lower than previously reported for other n-GaN samples with higher dislocation densities. Electron beam induced current measurements on both sides of the sample reveal the presence of dark spots whose density roughly correlates with the density of dislocations at the upper and lower faces. The reverse current at high voltages is shown to be considerably higher on the N face. The possible relationship of the observed phenomena to dislocations is discussed. [ABSTRACT FROM AUTHOR]

We used spectroscopic ellipsometry to characterize four different microcrystalline silicon (µc-Si) films, which were fabricated by crystallizing a-Si:H films predeposited on glass substrates using solid phase crystallization (SPC), excimer laser annealing (ELA), Ni induced silicide-mediated crystallization (Ni-SMC), and field enhanced silicide-mediated crystallization (FESMC) method, respectively. A linear regression analysis, which took the effective dielectric function of µc- Si layer into account using effective medium approximation, showed that all these films were homogeneous throughout their thickness except the oxide overlayers, and completely crystallized regardless of the crystallization method. In our linear regression analysis, the complex dielectric function of silicon microcrystallites was represented by the Adachi model dielectric function (MDF) IT. Suzuki and S. Adachi, Jpn. J. Appl. Phys., Partl 32, 4900 (1993)], and the broadening parameters of the critical points (CPs) in MDF were allowed to vary. The dielectric function of silicon microcrystallites showed systematic broadening and shrinking of the peak features corresponding to the E[sub 1] and E[sub 2] CPs, from which we concluded that the average microcrystallite size increased in the order of SPC, ELA, Ni-SMC, and FESMC µc-Si. The Raman spectra and the transmission-electron-microscopy images of these films also supported the idea of systematic variation in the microcrystallite size. [ABSTRACT FROM AUTHOR]

We report a method to fabricate carbon nanotubes (CNT)/conducting polymer composite films for the application of CNTs to field emission displays. The composite was prepared by a combination of electrochemical polymerization of pyrrole and electrophoretic deposition of CNTs. We obtained a uniform CNT/conducting polypyrrole polymer composite film. The CNTs were mainly coated on protrusions of the polypyrrole film and emitted electrons without rubbing and stretching. We realized a triode-type field emission array (FEA) using the CNT/polypyrrol composite. This FEA showed that the emission current was modulated by gate voltage of 30 V. The film morphology and emission characteristics of the CNT/conducting polymer composite were studied using optical microscopy, scanning electron microscopy, and an emission test in vacuum. [ABSTRACT FROM AUTHOR]

We use a combination of in situ and postdeposition experimental probes together with ab initio calculations of strain coefficients and formation energies associated with specific C configurations in the Si lattice to determine C incorporation pathways and lattice site distributions in fully coherent Si[sub 1-y]C[sub y] alloy layers grown by molecular-beam epitaxy on Si(001) as a function of deposition temperature T[sub s] (380 °C–680 °C) and C fraction y (0–0.026). Lattice strain and Raman spectroscopy measurements demonstrate that all C, irrespective of y, is incorporated into substitutional lattice sites in Si[sub 1-y]C[sub y](001) layers grown at T[sub s]≤480 °C. Increasing T[sub s]>=580 °C leads to strong C surface segregation, as shown by in situ angle-resolved x-ray photoelectron spectroscopy, yielding additional pathways for C incorporation. Photoluminescence measurements indicate that an increasing fraction of the incorporated C in the higher-temperature layers resides in dicarbon complexes. Reflection high-energy electron diffraction and cross sectional transmission electron microscopy reveal surface roughening at T[sub s]>=580 °C with the formation of bulk planar structures, interconnected by {113} segments, that are periodic along [001] with a periodicity which decreases with increasing T[sub s]. We interpret the planar structures as layers of C-rich Si[sub 1-y]C[sub y] which form in the presence of excess surface C resulting from segregation. Our ab initio density functional calculations show that substitutional C arranged in an ordered Si[sub 4]C phase is 0.34 eV per C atom more stable than isolated substitutional C atoms. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Epitaxial metastable Ge[sub 1-y]C[sub y] alloy layers with y≤0.045 were grown on Ge(001) by solid-source molecular-beam epitaxy (MBE) at temperatures T[sub s]=200–400 °C. Using calculated strain coefficients and measured layer strains obtained from high-resolution reciprocal lattice maps (HR-RLMs), we determine C lattice site distributions as a function of T[sub s] and total C concentration y. HR-RLMs show that all as-deposited alloys are fully coherent with their substrates. Ge[sub 1-y]C[sub y](001) layers grown at T[sub s]≤350 °C are in a state of in-plane tension and contain C in substitutional sites, giving rise to tensile strain, as well as in nanocluster sites which induce negligible lattice strain. T[sub s]=400 °C layers are strain neutral with negligible substitutional C incorporation. Increasing y and/or T[sub s] leads to a decrease in substitutional C concentration, consistent with Raman spectroscopy results, with a corresponding increase in the C fraction incorporated in nanocluster sites. The latter suggests that nanocluster formation is kinetically limited during film deposition by the C–C adatom encounter probability at the growth surface. Overall, the results show that it is not possible by MBE to obtain fully substitutional C incorporation in Ge[sub 1-y]C[sub y](001) alloys, irrespective of y and T[sub s]. This is consistent with ab initio density functional calculations results showing that C incorporation in nanoclusters sites is energetically favored over incorporation in substitutional Ge lattice sites. Annealing the Ge[sub 1-y]C[sub y](001) layers at T[sub a]=550 °C leads to a significant decrease in the substitutional C fraction and, hence, lower tensile strain. Layers annealed at 650 °C are strain free as all substitutional C has migrated to lower-energy nanocluster sites. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We studied the cathodoluminescence spectra of In[sub x]Ga[sub 1-x]N, focusing on the spatial variation of the spectra. Strong inhomogeneity of cathodoluminescence spectra was observed on 22 nm thick In[sub x]Ga[sub 1-x]N layers, where the peak energy varied up to 400 meV. In a double quantum well with a well width of 10 Å, the luminescence peak in a broad area spectrum was at 3.18 eV, but on some areas the peak was at 3.07 eV, the size of the area being about 1 μm. The variation of the cathodoluminescence spectra clearly indicates the presence of in-plane potential fluctuation in some In[sub x]Ga[sub 1-x]N samples, although the cathodoluminescence spectra of most of our quantum wells optimized for devices were found to be uniform at least within the spatial resolution limit. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The failure behavior of mortar under dynamic impact loading is characterized through a series of plate impact experiments. The analysis focuses on the spall strength and the shear stress carrying capacity in different regions of the specimen under normal impact loading. Special attention is paid to the possible existence of a failure wave phenomenon that has been widely recognized as an important failure mechanism for glasses during plate impact. The experiments are designed to allow the strengths of the medium at locations behind and ahead of a possible failure wave front to be analyzed. The diagnostics used include velocity interferometer system for any reflector (VISAR) recording of the free surface velocities of targets and monitoring of the internal stresses via polyvinylidene fluoride (PVDF) gauges embedded in the specimen. Experiments conducted do not provide evidence for the existence of a failure wave phenomenon for mortar under plate normal impact loading. While the study suggests that a threshold impact stress must be exceeded for failure to occur, a clearly defined failure wave that propagates behind the loading wave is not observed. Instead, a gradual failure process that initiates upon the arrival of the loading wave and progresses thereafter is observed. This gradual failure process in mortar is in contrast to the well-defined failure front and complete loss of tensile strength associated with the failure wave phenomenon reported for glasses. The study also indicates that, under impact involving high levels of input stresses, attenuation of the loading wave occurs as it propagates through the failed medium. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Solid-phase crystallization of Si[sub 1-x]Ge[sub x] (x=0-1.0) alloy layers deposited on a Si (100) substrate was investigated by ellipsometric spectroscopy. From a dispersion analysis of dielectric spectra, we deduced a crystallinity corresponding to the degree of average lattice alignment of the composed polycrystalline Si[sub 1-x]Ge[sub x] layers and investigated the dynamical change in crystallinity during crystallization. We found that the crystallinity and crystallization temperature (T[sub C]) rapidly decreased with increasing Ge concentration (x). When x was small (=0-0.3), the highest crystallinity was ∼0.8 of that for single crystals while the lowest one was considerably below 0.6 when x>0.8. Moreover, the crystallinity decreased with increasing temperature above T[sub C]. We investigated the nucleation rate during crystallization and found that the decrease in crystallinity at both large Ge concentration and high temperature can be explained by a trade-off between the nucleation and crystallization rates; nucleation was dominant under these conditions. An overview of the crystallinity of solid-phase crystallized Si[sub 1-x]Ge[sub x] alloy layers is provided. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Studies the dielectric constant and loss tangent of SrTiO[sub 3] thin films under the influence of an applied dc voltage. Use of a gold resonator with a flip-chip capacitor at cryogenic temperatures for the measurement.

Thermal measurements don on single- crystal R[Mn.sub.2][O.sub.5] (R=Tb, Dy, Ho) along the principal crystallographic axes show distinctive anomalies in the linear thermal expansivities at critical temperatures marking the onset of long-range antiferromagnetic order [T.sub.N1], ferroelectricity [T.sub.C1]. The observations reveal that the coupling between the magnetic orders and the dielectric properties is mediated by the lattice distortion.

Small arrays of microcavity plasma devices have a two stage structure comprising multilayer stacks of nanostructure Al2O3 grown on Al foil with cylindrical microcavities produce intense ultraviolet and visible emission when operated in Ar/3% N2 gas mixtures or Ne and excited by sinusoidal voltage wave form. Two or more coaxial cavities of differing diameters provide a convenient means for confining a microplasma to progressively smaller volumes as the power disposition is increased.

Giant magnetoresistance isolator is modeled for effective transmission of high-speed digital data. The output voltage characteristics depending on pulse width signal, inductor coil turn, and existence of external circuit of the planar inductor is investigated in time domain.

The variation of the magnetization vector with depth into DyFe2/YFe2 superlattice, in which the magnetization of the soft YFe2 dominates the magnetic structure, is investigated. The result showed that the magnetization of the YFe2 component is nearly always saturated, flipping directly from positive to negative saturation, while the net magnetization of the hard DyFe2 component significantly varies in high fields and is probably nonuniform.

Presents information on a study which demonstrated the optical measurements of gain, loss and transparency energy in long-wavelength semiconductors based on a hybrid approach using the fundamental relationship between the gain and the spontaneous emission spectrum. Experimental techniques; Comparison of theoretical and experimental results; Conclusions.

Focuses on a study that investigated the electrical and optical properties of amphoteric silicon-doped gallium arsenide diodes prepared by a single-step liquid-phase-epitaxy technique. Methods; Results; Discussion.