bluuebunny/crossref_metadata_2025 huggingface dataset:
Killer Proteases and Little Strokes—How the Things that do not Kill You Make You Stronger
Authors: O'Duffy Anne E , Bordelon Yvette M , McLaughlin BethAnn
Published: August 2006

The phenomenon of ischemic preconditioning was initially observed over 20 years ago. The basic tenant is that if stimuli are applied at a subtoxic level, cells upregulate endogenous protective mechanisms to block injury induced by subsequent stress. Since this discovery, many conserved signaling mechanisms that contribute to activation of this potent protective program have been identified in the brain. A clinical correlate of this basic research finding can be found in patients with a history of transient ischemic attack (TIA), who have a decreased morbidity after stroke. In spite of multidisciplinary efforts to design safer, more effective stroke therapies, we have thus far failed to translate our understanding of endogenous protective pathways to treatments for neurodegeneration. This review is designed to provide clinicians and basic scientists with an overview of stress biology after TIA and preconditioning, discuss new therapeutic strategies to target the protein dysfunction that follows ischemic injury, and propose enhanced biochemical profiling to identify individuals at risk of stroke after TIA. We pay particular attention to the unanticipated consequences of overly aggressive intervention after TIA in which we have found that traditional cytotoxic agents such as free radicals and apoptosis associated proteases is essential for neuroprotection and communication in the stressed brain. These data emphasize the importance of understanding the complex interplay between chaperones, apoptotic proteases including caspases, and the proteolytic degradation machinery in adaptation to neurological injury.
Optical and structural microanalysis of GaN grown on SiN submonolayers
Authors: Riemann T. , Hempel T. , Christen J. , Veit P. , Clos R. , Dadgar A. , Krost A. , Haboeck U. , Hoffmann A.
Published: August 2006

Lateral overgrowth techniques have demonstrated their ability to strongly reduce the dislocation density in GaN grown on a variety of foreign substrates. The in situ deposition of SiN during metal-organic chemical-vapor phase epitaxy (MOVPE) leads to the formation of a randomly distributed mask layer and induces lateral overgrowth similar to conventional epitaxial lateral overgrowth of GaN. Specifically for GaN on silicon substrate, the insertion of SiN submonolayers is a promising method to reduce not only the dislocation density but also the tensile stress upon Si doping. Besides the advantage of uncomplicated in situ mask formation, it allows complete coalescence and planarization of the overgrown GaN within a layer thickness of about 500 nm depending on the mask thickness, thus reducing the liability to cracking. However, the insertion of ultrathin SiN interlayers and, for thicker GaN stacks, additional stress-compensating low-temperature AIN (LT-AIN) leads to a complicated interplay of stress and dislocation density. We systematically study the impact of different interlayer designs on the evolution of stress and dislocation density in GaN on Si(111). Systematic series of samples comprising different SiN coverages, consecutively increasing GaN overgrowth times, and additionally different vertical positions of the SiN interlayer with respect to the substrate and LT-AIN were prepared by MOVPE. The resulting evolution of stress and dislocation density is assessed and correlated by spatially resolved cathodoluminescence microscopy, Raman spectroscopy, and transmission electron microscopy.
The dependence of polar cluster characteristics on composition in (1−x)Pb(Mg1∕3Nb2∕3)O3−xLa(Mg2∕3Nb1∕3)O3 relaxor dielectrics
Authors: Vogel Petra , Bell Andrew J.
Published: August 2006

The dielectric properties of (1−x)Pb(Mg1∕3Nb2∕3)O3−xLa(Mg2∕3Nb1∕3)O3 have been studied in the range 0<x<0.3. With increasing x there are marked decreases in the value of the maximum in relative permittivity (εm), the temperature of peak permittivity (Tm), the Curie temperature (TC), and the freezing temperature (Tf). From analysis of polarization-field data for the temperatures greater than Tm, the dependence of the polar cluster concentration n and average dipole moment α on composition and temperature have been determined based on a simple superparaelectric model. For 0<x<0.04, α decreases from 4.5×10−27to1.5×10−27Cm, while n increases linearly from 5.5×1025to13×1025m−3. The results have been used to attempt to differentiate between three idealized models of the mapping of polar regions onto the mesostructure defined by the chemical ordering of the perovskite B sites. The confinement of polar regions to the chemically disordered regions alone is shown to be inconsistent with the dielectric data. It is not possible to differentiate between models in which the polar regions inhabit only the chemically ordered regions, or both the ordered and disordered regions. However, it is proposed that the dominant influence on relaxor characteristics in this system is the disruption to polar order by the “nonpolar” La3+ and Mg2+ ions, rather than the partitioning into chemically ordered and disordered regions.
Effects of ferroelectric polarization and converse piezoelectric effect induced lattice strain on the electrical properties of La0.7Sr0.3MnO3 thin films
Authors: Zheng R. K. , Wang J. , Zhou X. Y. , Wang Y. , Chan H. L. W. , Choy C. L. , Luo H. S.
Published: August 2006

Thin films of La0.7Sr0.3MnO3 (LSMO) have been grown on the ferroelectric 0.68Pb(Mg1∕3Nb2∕3)O3–0.32PbTiO3 (PMN-PT) single-crystal substrates. The ferroelectric polarization of the PMN-PT induces a large decrease in the resistivity and an upward shift in the Curie temperature of the LSMO films, which was interpreted in terms of the ferroelectric polarization induced large reduction of the biaxial tensile strain in the LSMO films. The resistivity of the LSMO films can be dynamically tuned by applying dc electric fields across the polarized PMN-PT. The analyses of the results indicate that the electric field induced lattice strain via converse piezoelectric effect in the PMN-PT controls the strain states and hence the resistivity and Curie temperature of the LSMO films.
Formation of a ZnS∕Zn(S,O) bilayer buffer on CuInS2 thin film solar cell absorbers by chemical bath deposition
Authors: Bär M. , Ennaoui A. , Klaer J. , Kropp T. , Sáez-Araoz R. , Allsop N. , Lauermann I. , Schock H.-W. , Lux-Steiner M. C.
Published: August 2006

The application of Zn compounds as buffer layers was recently extended to wide-gap CuInS2 (CIS) based thin film solar cells. Using an alternative chemical deposition route for the buffer preparation aiming at the deposition of a single-layer, nominal ZnS buffer without the need for any toxic reactants such as hydrazine has helped us to achieve a similar efficiency as respective CdS-buffered reference devices. In order to shed light on the differences of other Zn-compound buffers deposited in conventional chemical baths [chemical bath deposition (CBD)] compared to the buffer layers deposited by this alternative CBD process, the composition of the deposited buffers was investigated by x-ray excited Auger electron and x-ray photoelectron spectroscopy to potentially clarify their superiority in terms of device performance. We have found that in the early stages of this alternative CBD process a thin ZnS layer is formed on the CIS, whereas in the second half of the CBD the growth rate is greatly increased and Zn(S,O) with a ZnS∕(ZnS+ZnO) ratio of ∼80% is deposited. Thus, a ZnS∕Zn(S,O) bilayer buffer is deposited on the CIS thin film solar cell absorbers by the alternative chemical deposition route used in this investigation. No major changes of these findings after a postannealing of the buffer/CIS sample series and recharacterization could be identified.
The effect of two-temperature capping on germanium/silicon quantum dots and analysis of superlattices so composed
Authors: Vandervelde Thomas E. , Sun Kai , Merz James L. , Kubis Alan , Hull Robert , Pernell Timothy L. , Bean John C.
Published: August 2006

It is well documented that quantum dots (QDs), grown and subsequently buried under silicon at temperatures greater than 400°C, flatten into a pancakelike shape. Although QD arrays in Si superlattices have been studied for more than a decade, the process of flattening is not well understood. Here, we examine the process by which flattening occurs, using a two-temperature capping technique. Briefly, a 300°C cold-cap layer is deposited, conformally coating the QD, followed by deposition of hot Si at 750°C. Through this process, full or partial shape retention of the buried QDs can be selectively maintained. Interestingly, we found that QDs grown with this technique do not flatten in the traditional way. In fact, the QD truncates without any associated base spreading. The material from this truncation fills in the valleys, thereby flattening the surface of the sample. We analyzed this truncation by growing a series of samples under conditions of varied cold cap thicknesses and base growth temperatures. In addition, we found annealing at or above 650°C sufficient to induce this truncation, while adatom flux was not required for truncation. We examined the samples using cross-sectional transmission electron microscopy (TEM) to determine the degree of shape retention and location of the truncated material. This technique also provided the optimal growth conditions for two additional studies: photoluminescence (PL) analysis, which is used to determine how shape retention affects recombination efficiency. In sharp contrast to earlier studies, PL data indicated that while shape retention could be maximized by low temperature overgrowth, this was accompanied by an unacceptable loss in luminescent intensity. This decrease suggested quenching by QD surface and/or overlayer traps. To retain both QD shape and PL intensity, a low/high temperature procedure was identified. These samples were then analyzed by TEM for the appearance of dislocations, degree of shape retention, and alignment/self-segregation of upper layers compared to lower layers. The findings from all of the above techniques help to elucidate the properties of QDs in complicated structures and imply possible techniques to refine current technological practices.
c
-axis oriented epitaxial BaTiO3 films on (001) Si
Authors: Vaithyanathan V. , Lettieri J. , Tian W. , Sharan A. , Vasudevarao A. , Li Y. L. , Kochhar A. , Ma H. , Levy J. , Zschack P. , Woicik J. C. , Chen L. Q. , Gopalan V. , Schlom D. G.
Published: August 2006

c -axis oriented epitaxial films of the ferroelectric BaTiO3 have been grown on (001) Si by reactive molecular-beam epitaxy. The orientation relationship between the film and substrate is (001) BaTiO3‖(001) Si and [100] BaTiO3‖[110] Si. The uniqueness of this integration is that the entire epitaxial BaTiO3 film on (001) Si is c-axis oriented, unlike any reported so far in the literature. The thermal expansion incompatibility between BaTiO3 and silicon is overcome by introducing a relaxed buffer layer of BaxSr1−xTiO3 between the BaTiO3 film and silicon substrate. The rocking curve widths of the BaTiO3 films are as narrow as 0.4°. X-ray diffraction and second harmonic generation experiments reveal the out-of-plane c-axis orientation of the epitaxial BaTiO3 film. Piezoresponse atomic force microscopy is used to write ferroelectric domains with a spatial resolution of ∼100nm, corroborating the orientation of the ferroelectric film.
Magnetic relaxation measurement in immunoassay using high-transition-temperature superconducting quantum interference device system
Authors: Yang H. C. , Yang S. Y. , Fang G. L. , Huang W. H. , Liu C. H. , Liao S. H. , Horng H. E. , Hong Chin-Yih
Published: August 2006

Due to their ultrahigh sensitivity to magnetic flux, superconducting quantum interference devices (SQUIDs) are able to detect biomagnetic signals. By labeling biotargets with magnetic nanoparticles, several groups have shown that SQUIDs are promising as quantitative probes of biotargets by measuring their magnetic properties. In this work, we describe the design and construction of a high-transition-temperature radio-frequency SQUID magnetometer system for measuring the magnetic relaxation of labeled avidin. We also describe the synthesis of magnetic nanoparticles coated with biotin for use in labeling the avidin. Furthermore, the specifications of the SQUID-based magnetically labeled immunoassay of avidin are explored.
Mechanical properties of nanocrystalline diamond films
Authors: Shen Z. H. , Hess P. , Huang J. P. , Lin Y. C. , Chen K. H. , Chen L. C. , Lin S. T.
Published: August 2006

Nanocrystalline diamond (NCD) films with thicknesses in the range of 0.12–1.5μm were deposited on silicon substrates in CH4∕H2∕O2 gas mixtures by microwave plasma-enhanced chemical vapor deposition. The morphology and structure of these NCD films were analyzed by field-emission scanning electron microscopy, x-ray diffraction (XRD), and ultraviolet-Raman spectroscopy. The lower limit of the grain size in the NCD films was estimated to be 10nm from the XRD measurements. These grains are embedded in a columnar-type structure. The elastic and mechanical properties of the NCD films were determined by measuring the dispersion of laser-induced surface acoustic waves. The densities were in the range of 3.41±0.11g∕cm3 and Young’s moduli varied between 674±34 and 953±48GPa, depending on the growth time and deposition conditions. It is concluded that oxygen may have a significant positive effect on the elastic properties of NCD films. The growth rate decreases sharply for an oxygen content in the source gas in excess of about 4%.
Surface structure of SiC formed by C60 molecules on a Si(001)-2×1 surface at 800°C
Authors: Cheng C.-P. , Huang J.-W. , Pi T.-W. , Lee H.-H.
Published: August 2006

Formation of silicon carbide upon deposition of C60 onto Si(001) at an elevated temperature of 800°C was studied via synchrotron-radiation photoemission and low electron energy diffraction. The molecules are completely decomposed upon hitting on the hot surface, giving rise to a well-order 2×1 pattern. The C 1s and Si 2p core-level spectra, and valence-band spectra indicate characteristics of a Si-terminated β-SiC(001) 2×1 film. Two surface components corresponding to the terminated and ad-dimer Si atoms are well resolved in the Si 2p cores. By the spectral area ratio of both components, the adlayer atoms cover half of the surface, which agrees with the missing-row structure model [W. Lu et al., Phys. Rev. Lett. 81, 2292 (1998)].
Monodisperse spherical core-shell structured SiO2–CaTiO3:Pr3+ phosphors for field emission displays
Authors: Liu Xiaoming , Jia Peiyun , Lin Jun , Li Guangzhi
Published: August 2006

Nanocrystalline CaTiO3:Pr3+ phosphor layers were coated on nonaggregated, monodisperse, and spherical SiO2 particles by the sol-gel method, resulting in the formation of core-shell structured SiO2–CaTiO3:Pr3+ particles. X-ray diffraction, Fourier transform infrared spectroscopy, field emission scanning electron microscopy, transmission electron microscopy, photoluminescence, cathodoluminescence spectra, as well as lifetimes were utilized to characterize the core-shell structured SiO2–CaTiO3:Pr3+ phosphor particles. The obtained core-shell structured phosphors consist of well dispersed submicron spherical particles with a narrow size distribution. The thickness of the CaTiO3:Pr3+ shell could be easily controlled by changing the number of deposition cycles (about 70nm for four deposition cycles). The core-shell SiO2–CaTiO3:Pr3+ particles show a strong red emission corresponding to D21–H43 (612nm) of Pr3+ under the excitation of ultraviolet (326nm) and low voltage electron beams (1–5kV). These particles may be used in field emission displays.
Anticrossing of plasmon resonances and giant enhancement of interlayer terahertz electric field in an asymmetric bilayer of two-dimensional electron strips
Authors: Popov V. V. , Tsymbalov G. M. , Horing N. J. M.
Published: August 2006

We carry out a theoretical analysis of the terahertz response of an asymmetric bilayer of two-dimensional electron strips, modeling the double-quantum-well electron channel of a grid-gated field-effect transistor in which strong terahertz photoresponse was recently observed. We have shown that mixing between optical and acoustic plasmons hugely increases at the anticrossing of optical-like and acousticlike plasma resonances excited by incoming terahertz radiation in such a bilayer. As a result, the amplitude of the interlayer terahertz electric field also increases dramatically in the anticrossing regime, becoming two and a half orders of magnitude greater than the amplitude of electric field of incoming terahertz wave.
Transient sputtering of an amorphous Si surface under low energy O2+ ion bombardment
Authors: Lee Hyung-Ik , Kang Hee Jae , Moon Dae Won
Published: August 2006

The transient sputtering of an amorphous Si surface with 500eV O2+ ion bombardment at surface normal incidence was studied via the in-depth composition profiles for the incorporated oxygen and the sputtering yield change in Si, which were obtained from in situ medium energy ion scattering spectroscopy. As ion doses are increased, the Si surface is thickly oxidized, and the sputtering yield is rapidly reduced. At the initial stage of the sputtering, owing to an incorporation rate of oxygen higher than the sputtering rate of Si, the surface is gradually swelled by the ion dose of ∼1.5×1016 O2+ions∕cm2. A dynamic Monte Carlo calculation, which takes into account both swelling and diffusion effects, has also been carried out to understand the underlying mechanism of the transient processes. In particular, we calculated the sputtered depth as a function of ion dose, and the secondary ion mass spectroscopy depth profiling showed a transient shifting of 2.7nm to the thicker depth.
Absolute electron-impact ionization cross sections of free radicals from threshold to 30eV: CHx=2,3, Si(CH3)x=1,2,3, and H2Si(CH3) radicals
Authors: Jauberteau J. L. , Jauberteau I. , Aubreton J.
Published: August 2006

We report absolute electron-impact ionization cross sections from threshold to 30eV for the formation of the parent ions from free radicals. A microwave discharge is used as a source to produce a large amount of these free radicals, and measurements are mainly performed by means of mass spectrometry. We detail the method used to measure electron-impact ionization cross sections of the main radicals produced. First, this method is tested on well known radicals such as CH3 and CH2, produced in a discharge sustained in an Ar–CH4 gas mixture; then it is applied to the free radicals Si(CH3)3,2,1 and H2Si(CH3), which are produced in a discharge sustained in Ar–Si(CH3)4. The experimental results are compared with the calculations performed using the binary-encounter-Bethe model and using a simplified form of this theory, the total ionization cross section model. Absolute electron-impact ionization cross sections have been measured at 30eV for Si(CH3) and CH3 by means of the selective energy transfer reaction process between Ar(P23) metastable species and the tetramethylsilane molecule [Si(CH3)4]. The value measured for Si(CH3) is used as the reference to calculate absolute values from relative ionization cross sections measured for the other radicals, Si(CH3)3,2,1 and H2Si(CH3), from threshold to 30eV. For example, we find at 30eV the absolute cross section values, 13.9×10−20±1.8×10−20, 2.0×10−20±0.3×10−20, 1.5×10−20±0.1×10−20, and 12.8×10−20±1.7×10−20m2 for m∕z=73, 58, 43, and 45, respectively.
In situ observation of ordering process in FePt films during annealing in a transmission electron microscope
Authors: Kawai K. , Honda S. , Nawate M. , Komatsu M. , Kawabata K.
Published: August 2006

We have investigated the process of the phase transformation from fcc to fct in FePt thin films by in situ observation during annealing in a transmission electron microscope. The coercivity for the in-plane field increases drastically up to about 6.3kOe by annealing at 300°C in the film prepared at 0.5mTorr by multipolar magnetic plasma confinement sputtering. The as-deposited film has a flat surface and very large grains of 8–16nm, which causes the easy ordering at low temperatures. The ordering progresses accompanied with the crystal growth, which occurs laterally by the coalescence with the disordered grains. On the other hand, in the film prepared at 5mTorr by the conventional sputtering, the surface is rough and grains are smaller than 6nm. This small grain in the as-deposited state suppresses the crystal growth and also the ordering even at higher annealing temperatures.
Properties of (Hg,Re)Ba2CaCu2Oy [100]-tilt grain boundary Josephson junctions
Authors: Ogawa A. , Sugano T. , Wakana H. , Kamitani Ai , Adachi S. , Tarutani Y. , Tanabe K.
Published: August 2006

[100]-tilt grain boundary Josephson junctions with misorientation of the c-axis directions have been fabricated using epitaxial (Hg0.9Re0.1)Ba2CaCu2Oy [(Hg,Re)-1212] thin films grown on SrTiO3 bicrystal substrates. Two types of the [100]-tilt junctions, that is, the mountain-type junction and the valley-type junction, of which a-b planes formed a mountain shape and a valley shape along the grain boundaries, respectively, were examined for various misorientation angles (θ). Both the types of junctions showed resistively shunted junction-type current-voltage (I-V) characteristics with relatively high characteristic voltages in a wide temperature range from 4.2to110K. The valley-type junctions exhibited properties superior to those for the mountain-type junctions, such as very low excess current and much homogeneous current distribution, which is attributed to different film growth mode near the grain boundaries. They also exhibited the products of the critical current (Ic) and the normal-state resistance (Rn) as high as 3.0–4.2, 1.0–1.4, and 0.2–0.4mV at 4.2, 77, and 100K, respectively. The misorientation angle dependences of junction parameters for the [100]-tilt junctions were compared with those for conventional [001]-tilt junctions with misorientation of the a-b plane directions. The Rn and the IcRn products for the [100]-tilt junctions were substantially larger than those for the [001]-tilt junctions. Large hysteresis in the magnetic-field dependence of Ic was observed for the applied field larger than a certain threshold value, suggesting flux penetration between the CuO2 planes.
The effects of large signals on charge collection in radiation detectors: Application to amorphous selenium detectors
Authors: Kabir M. Zahangir , Emelianova E. V. , Arkhipov V. I. , Yunus M. , Kasap S. O. , Adriaenssens G.
Published: August 2006

Analytical and numerical models for studying the effects of large signals on charge collection efficiency in radiation detectors are described by considering bimolecular recombination between drifting charge carriers, carrier trapping, and space charge effects. First, an analytical solution is obtained by assuming that the field remains relatively uniform. Then the continuity equations for both holes and electrons, and Poisson’s equation across the photoconductor for a short pulse irradiation are simultaneously solved by the finite difference method, without any assumptions. There is a very good agreement between the approximate analytical and numerical solutions for the charge collection efficiency. The numerical results are also compared with Monte Carlo simulations of carrier transport. The charge collection efficiency model is applied to amorphous selenium x-ray image detectors. The bimolecular-recombination-limited charge collection efficiency depends on the total photogenerated carrier density rather than on its spatial distribution. It is found that the recombination plays practically no role up to the total instantaneous carrier generation Q0 of 109EHPs∕cm2 at the applied electric field of 10V∕μm. The effect of recombination on charge collection increases with decreasing applied electric field strength. For high carrier generation (e.g., Q0 of 1012EHPs∕cm2 for an applied field of 10V∕μm), the electric field distributions vary widely across the photoconductor thickness during the travel of charge carriers towards the electrodes. However, the effect of bimolecular recombination on charge collection efficiency is almost independent of bias polarity and the field distribution. The numerical results are also compared with recent experimental data available in the literature.
Aluminum nanowire polarizing grids: Fabrication and analysis
Authors: Pelletier Vincent , Asakawa Koji , Wu Mingshaw , Adamson Douglas H. , Register Richard A. , Chaikin Paul M.
Published: August 2006

We have produced metal wire grids with 33nm periodicity, using a thin film of a self-assembling diblock copolymer as a template. These grids, supported on fused quartz wafers, function as transmission polarizers for visible and near-ultraviolet lights. Their polarization efficiency is measured to be near 50% in the visible. Quantitative comparison with a new theoretical analysis of such wire grids indicates that they should perform well into the far UV. This analysis also explains the reversal in polarization direction at shorter wavelengths which we observe in our specimens.
Modeling the magnetic behavior of horse spleen ferritin with a two-phase core structure
Authors: Brem Franziska , Stamm Gabriela , Hirt Ann M.
Published: August 2006

The growth of the nanotechnology industry has led to an increased interest in characterizing magnetic nanoparticles. A natural material with well-defined grain size in the nanoparticle range is commercially available—horse spleen ferritin, an iron storage protein. Modeling of the magnetic properties of commercial horse spleen ferritin is often based on the assumption of a single-phase core of ferrihydrite (5Fe2O3∙9H2O). Low temperature hysteresis measurements indicate, however, that the ferritin cores contain at least two magnetic phases. Initial magnetization curves measured at temperatures between 50 and 300K have been modeled using four methods. A model that used a sum of two Langevin functions fitted the data 70% better on average than a model that used a single Langevin function. It was also superior to both a random mean orientation model and a model that takes account of crystalline anisotropy. The two-phase model consists of a phase with a high coercivity that does not undergo saturation and a second phase with a low coercivity and a saturation field of 300mT. The high-coercivity phase is compatible with antiferromagnetic ferrihydrite, while the low-coercivity phase could be magnetite, maghemite, or a mixture of both. The results from this study are consistent with earlier microscopic studies that characterize horse spleen ferritin as a multiphase system with up to 30% of magnetite-maghemitelike cores.
Separate-confinement-oxidation vertical-cavity surface-emitting laser structure
Authors: Sarzała Robert P. , Nakwaski Włodzimierz
Published: August 2006

In the present paper, a comprehensive self-consistent three-dimensional model is used to analyze physical aspects of the operation of oxide-confined vertical-cavity surface-emitting diode lasers (VCSELs) and to optimize their structures. The impact of the built-in radial confining mechanisms created by oxide apertures, i.e., the influence of their diameters and localizations on radial confinements of both the current injection into VCSEL active regions and electromagnetic fields of successive cavity modes, has been investigated. Basically, there are two extreme cases: the index-guided (IG) VCSELs with the aperture localized at the antinode position of the optical standing wave, characterized by a very low lasing threshold but exhibiting rather poor mode selectivity, and the gain-guided (GG) VCSELs with the aperture shifted to the node position, ensuring usually the single-fundamental-mode operation, but at the expense of much higher lasing threshold. In the present paper, the separate-confinement-oxidation VCSEL structure has been proposed exhibiting simultaneously advantages of both the IG VCSELs (the low lasing threshold) and the GG ones (the single-fundamental-mode operation).
Excavation rate of silicon surface nanoholes
Authors: Ohno Yutaka , Takeda Seiji , Ichihashi Toshinari , Iijima Sumio
Published: August 2006

Silicon surface nanoholes, that are small pits introduced spontaneously on electron exit surfaces of silicon foils under electron irradiation, are excavated at a constant rate with increasing electron dose. On the other hand, surface nanoholes on electron entrance surfaces become shallow under electron irradiation. The mechanism for the excavation and shallowing of surface nanoholes is discussed in terms of the movement of surface vacancies assisted by irradiating electrons.
Study of kink instabilities in 1m long, free-standing plasma channels used for ion beam transport
Authors: Neff S. , Knobloch-Maas R. , Tauschwitz A. , Hoffmann D. H. H. , Yu S. S.
Published: August 2006

Experiments at the Gesellschaft für Schwerionenforschung study the transport of heavy-ion beams in plasma channels in collaboration with the Advanced Reactor Innovative Engineering Study for Inertial Fusion Energy. One important aspect of channel transport is the suppression of instabilities. The susceptibility of discharge channels for magnetohydrodynamic instabilities is studied and their growth rate is measured with a fast framing camera. The images are analyzed with a computer code, determining the growth rates of the instability during later stages of the discharge. Magnetohydrodynamical (MHD) instabilities were observed only for high pressure and even then only at late stages of the discharge. The only observed MHD modes were kink instabilities and our measurements indicate that they pose no problem for transport channels with reactor parameters.
High-frequency magnetic and dielectric properties of a three-phase composite of nickel, Co2Z ferrite, and polymer
Authors: Li Bao-Wen , Shen Yang , Yue Zhen-xing , Nan Ce-Wen
Published: August 2006

A three-phase, double-percolating composite with Co2Z ferrite particles and nickel particles embedded in a poly(vinylidene fluoride) matrix was prepared by using a simple low-temperature hot-pressing technique. The large ferrite particles in the composite not only act as the magnetic phase but also present as a high volume fraction discrete (nonpercolating) phase, confining polymer and metallic particles into a continuous double-percolating structure of low volume fraction. With the addition of a small number of magnetic nickel particles, a large enhancement in both initial permeability and dielectric constant of the three-phase composite was observed. This can be explained by effective medium approach and percolation theory. The three-phase composite showed both frequency-independent inductive and capacitive properties in the high-frequency range up to ∼500MHz. Such a multifunctional magnetic-electric three-phase composite could be used in high-frequency communications and electromagnetic interference filters not only as an inductor but also as a capacitor.
Experimental verification of the key properties of a three-dimensional isotropic transmission-line superlens
Authors: Alitalo Pekka , Maslovski Stanislav , Tretyakov Sergei
Published: August 2006

Design and experimental realization of a three-dimensional superlens based on inductively and capacitively loaded transmission lines are presented. Transmission properties of the designed structure are studied experimentally and the observed lens properties are compared with analytical predictions. Backward-wave propagation and amplification of evanescent waves in the prototype structure are verified both analytically and experimentally. Commercially available components and materials are used in the design.
Quantitative determination of domain wall coupling energetics
Authors: Laufenberg M. , Bedau D. , Ehrke H. , Kläui M. , Rüdiger U. , Backes D. , Heyderman L. J. , Nolting F. , Vaz C. A. F. , Bland J. A. C. , Kasama T. , Dunin-Borkowski R. E. , Cherifi S. , Locatelli A. , Heun S.
Published: August 2006

The magnetic dipolar coupling of head-to-head domain walls is studied in 350nm wide NiFe and Co nanostructures by high resolution magnetic imaging. We map the stray field of a domain wall directly with sub-10-nm resolution using off-axis electron holography and find that the field intensity decreases as 1∕r with distance. By using x-ray magnetic circular dichroism photoemission electron microscopy, we observe that the spin structures of interacting domain walls change from vortex to transverse walls, when the distance between the walls is reduced to below (77±5)nm for 27nm thick NiFe and (224±65)nm for 30nm thick Co elements. Using measured stray field values, the energy barrier height distribution for the nucleation of a vortex core is obtained.
Room-temperature photoluminescence in structurally disordered SrWO4
Authors: Anicete-Santos M. , Picon F. C. , Escote M. T. , Leite E. R. , Pizani P. S. , Varela J. A. , Longo E.
Published: August 2006

Intense and broad visible photoluminescent (PL) band in structurally disordered SrWO4 compounds was observed at room temperature. The polycrystalline scheelite strontium tungstate (SrWO4) samples prepared by the polymeric precursor method at different temperatures of annealing were structurally characterized by x-ray diffraction and Fourier transform Raman spectroscopy measurements. Quantum-mechanical calculations showed that the local disorder in the cluster of the network modifiers Sr has a very important role in the charge transfer. The experimental and theoretical results are in good agreement, indicating that the generation of the intense visible PL band can be related to short-range order-disorder degree in the scheelite structure.
Interfacial phonon scattering in semiconductor nanowires by molecular-dynamics simulation
Authors: Becker Brian , Schelling Patrick K. , Phillpot Simon R.
Published: August 2006

We use molecular-dynamics simulations of vibrational wave packets to study the scattering of longitudinal-acoustic modes from interfaces in semiconductor nanowires of varying diameters. The energy transmission coefficient at the interface is found to depend strongly on both the nanowire diameter and the frequency of the incident wave. By analyzing the scattering events, we determine the selection rules for nanowire scattering that can be understood in terms of the representations of the point-group symmetry of the nanowire. Using such symmetry arguments, we predict that the presence of gaps in the phonon spectrum of thin high-symmetry nanowires will result in a complete reflection of phonons at the interfaces. We discuss the implications of our results for interfacial scattering in real systems, including Si∕Ge superlattice nanowires.
Low-temperature high-pressure preparation of transparent nanocrystalline MgAl2O4 ceramics
Authors: Lu T. C. , Chang X. H. , Qi J. Q. , Luo X. J. , Wei Q. M. , Zhu S. , Sun K. , Lian J. , Wang L. M.
Published: August 2006

Transparent MgAl2O4 spinel nanoceramics have been sintered at relatively low-temperature (500–700°C) under high pressure (2–5GPa) using a hydrostatic press with high-temperature-calcined nanopowders. The morphology, nanostructure, optical property, and density of ceramics were analyzed by scanning electron microscopy, transmission electron microscopy, UV-VIS-IR transmission spectrum, and Archimedes draining method. The average grain size (<100nm for all samples sintered) depends on the sintering pressure and temperature. The nanoceramics are highly transparent even though their relative densities are all less than 99%, due to the low or negligible light scattering from the nanosized grains and pores.
Current conduction and saturation mechanism in AlGaN∕GaN ungated structures
Authors: Kuzmík J. , Bychikhin S. , Pogany D. , Gaquière C. , Morvan E.
Published: August 2006

Current conduction and saturation mechanisms in ungated AlGaN∕GaN∕SiC transfer length method test structures with contact-to-contact distance L varying from 2to32μm are investigated. Current-voltage (I-V) characteristics are measured in the pulsed and in the direct current (dc) regimes. The pulsed characteristics were evaluated for the pulse duration of 50ns, and together with the dc ones show that the current saturation value depends on the contact distance and the current saturation occurs at an electrical field ⪡150kV∕cm. This behavior contradicts the theoretical expectation valid for the ungated structures. An analysis of the I-V characteristics shows that the dc saturation current values are up to 5.5 times lower than those under the pulsed conditions for the same L. If the self-heating effect was supposed to be responsible for this observation, an unrealistic temperature rise would be required. This together with the observed ∼1s long transition time into the steady state, which is several orders of magnitude longer than what one can expect for the thermal transient, excludes the dominant role of the thermal effects in the current saturation mechanism. A model of the current conduction and saturation mechanism is suggested where the charge injection from the contacts and charging of the AlGaN surface are responsible for a AlGaN∕GaN channel depletion. Thus the saturation mechanism is similar to that occurring in gated structures. The model explains well the observed dependencies and is further validated by a transient interferometric mapping method. This thermooptical method operated under the pulsed conditions allows to evaluate the free carrier concentration profiles along the structure channel and to make the comparisons with the concentration below Ohmic contacts. A strong channel depletion effect has been observed for the structure with L=32μm in comparison to the negligible surface depletion effect found in the L=2μm structure. This is in full agreement with the suggested model of the current saturation mechanism and with the measured I-V characteristics.
Photoluminescence properties of CdTe∕CdSe core-shell type-II quantum dots
Authors: Wang C. H. , Chen T. T. , Tan K. W. , Chen Y. F. , Cheng C. T. , Chou P. T.
Published: August 2006

We report investigations on the optical properties of type-II CdTe∕CdSe core-shell quantum dots. By varying the core size, we provide an elegant way to verify that the detected emission signal indeed arises from type-II band alignment. The photoluminescence (PL) peak energy increases with a third root of the excitation power. Both of the PL peak energy and linewidth exhibit unique temperature dependence. All these observations can be rationalized by the band bending effect resulting from the spatially separated photoexcited carriers in a type-II band alignment.
Luminescence enhancement of Pb2+ ions in TeO2–PbO–GeO2 glasses containing silver nanostructures
Authors: de Araújo Cid B. , Kassab Luciana R. P. , Kobayashi Renata A. , Naranjo Luz Patrícia , Santa Cruz Petrus A.
Published: August 2006

Te O 2 – Pb O – Ge O 2 glass samples containing silver nanoparticles (NPs) and complex nanostructures were prepared and characterized. Nucleation of NPs and formation of NP structures were obtained following the procedure of powder fusion and thermal annealing at 350°C. Electron microscopy revealed the diffusion paths of the silver NP during the heat-treatment process. Optical experiments were performed to investigate properties of the composite system. Large enhancement of the luminescence from ≈430to≈750nm, associated to aggregates of Pb2+ ions, was observed due to the presence of the silver NPs and their nanostructures.
Far-field background suppression in tip-modulated apertureless near-field optical microscopy
Authors: Gucciardi Pietro Giuseppe , Bachelier Guillaume , Allegrini Maria
Published: August 2006

In apertureless near-field optical microscopy the vertical dithering of the tip, associated with demodulation at higher harmonics (n>1), allows us to suppress the far-field background, providing artifact free elastic scattering images. This paper analyzes, both theoretically and experimentally, the physical origin of the background signal at the different harmonics and the mechanisms underlying its rejection for the general case of propagative-field illumination. We show that Fourier components of the background must be expected at every harmonic, evidencing why demodulation at higher harmonics is not an inherently background-free technique, and assessing the experimental conditions in which it becomes like that. In particular, we put forward the fundamental roles of both the harmonic order and the tip oscillation amplitude in the background suppression mechanisms. Furthermore, we outline how the lock-in detection of the signals amplitude can enhance the nonlinear dependence of the background on the tip-sample distance. Such effect provides a more subtle source of topography artifacts since the optical maps become qualitatively uncorrelated from the topographic counterpart, requiring an upgrade of the criteria to assess the absence of artifacts from the optical maps.
Elastic wave propagation in locally resonant sonic material: Comparison between local interaction simulation approach and modal analysis
Authors: Hirsekorn Martin , Delsanto Pier Paolo , Leung Alan C. , Matic Peter
Published: August 2006

Locally resonant sonic materials exhibit strong sound attenuation bands in the audible frequency range due to resonance scattering of elastic waves. We compare the results of a finite element modal analysis of a single resonant unit with sound attenuation spectra obtained from wave propagation simulations based on the local interaction simulation approach. The modal analysis yields a complete prediction of all resonance modes including information on node locations, mode degeneration, and modes that do not attenuate sound due to geometrical symmetries. Elliptical instead of circular inclusions break the geometric symmetry of the resonators, splitting the attenuation peak of degenerate modes into separate peaks. A small frequency shift is observed between the resonance frequencies and the frequencies of maximum sound attenuation, due to the asymmetric shape of the attenuation peaks and interference between resonance scattering and free propagating waves.
Magnetoelectric properties of multiferroic composites with pseudo-1-3-type structure
Authors: Shi Zhan , Nan Ce-Wen , Zhang Jie , Ma Jing , Li Jing-Feng
Published: August 2006

A pseudo-1-3-type multiferroic composite consisting of Pb(Zr,Ti)O3 (PZT) rod array (with base) and Terfenol-D/epoxy matrix was prepared by the dice-and-fill technique. Simple series and parallel mixture rules well described the measured dielectric and piezoelectric constants. Large magnetoelectric coefficients were observed in the pseudo-1-3-type composite, e.g., over 300mV∕cmOe below 40kHz and over 4500mV∕cmOe at resonant frequency. The magnetoelectric response strongly depends on the magnetostrictive behavior of the matrix and the volume fraction of PZT rods, which gives us two convenient ways to modify their magnetoelectric response. For this pseudo 1-3-type multiferroic composite, the remarkable magnetoelectric response and well-developed fabrication technique are advantageous for their practical applications in piezoelectric-magnetoelectric multifunctional devices and large bandwidth magnetic sensors.
High-tone bulk acoustic resonators on sapphire, crystal quartz, fused silica, and silicon substrates
Authors: Zhang Hao , Pang Wei , Yu Hongyu , Kim Eun Sok
Published: August 2006

High-tone bulk acoustic resonators (HBARs) have been fabricated on four types of substrates: c-axis sapphire, AT-cut crystal quartz, fused silica, and ⟨100⟩ single crystal silicon. Quality factors of the HBARs on the four substrates have been compared. Temperature effect on the Q and resonant frequency of the four types of HBARs has been investigated. This paper reports that acoustic absorption in fused silica decreases with increased temperature. Acoustic absorptions in the other three substrates increase with increased temperature. Also, temperature coefficients of frequency of HBARs on sapphire, crystal quartz, fused silica, and silicon have been investigated. Finally, acoustic velocities in the four substrates have been extracted by measuring the frequency difference between two successive resonances of the HBAR on the corresponding substrate, and their dependences on temperature have been investigated.
Determination of the deposited energy in a silicon volume by n-Si nuclear interaction
Authors: Chabane H. , Vaillé J. R. , Mérelle T. , Saigné F. , Dusseau L. , Dumas M. , Palau J. M. , Barelaud B. , Decossas J. L. , Wrobel F. , Buard N. , Palau M. C.
Published: August 2006

The aim of this work is to validate the results of the Monte Carlo recoil energy determination nuclear physics code used to determine the deposited energy in a silicon volume taking into account the probabilistic approach of the physical phenomenon. A silicon sensor has been used to measure the deposited energy spectrum after an irradiation with a neutron source. The experimental results were then compared with the one obtained by Monte Carlo simulations in the same silicon volume. Experiments and simulations are shown to be in good agreement in the field of interest for soft error rate evaluation that means for deposited energy range leading to memory point upsets in static random access memory.
Information-theoretic analysis of capacitive physical unclonable functions
Authors: Škorić B. , Maubach S. , Kevenaar T. , Tuyls P.
Published: August 2006

Physical unclonable functions (PUFs) can be used as a cost-effective means to store cryptographic key material in an unclonable way. In coating PUFs, keys are generated from capacitance measurements of a coating containing many randomly distributed particles with different dielectric constants. We introduce a physical model of coating PUFs by simplifying the capacitance sensors to a parallel plate geometry. We estimate the amount of information that can be extracted from the coating. We show that the inherent entropy is proportional to n(logn)3∕2, where n is the number of particles that fit between the capacitor plates in a straight line. However, measurement noise may severely reduce the amount of information that can actually be extracted in practice. In the noisy regime the number of extractable bits is, in fact, a decreasing function of n. We derive an optimal value for n as a function of the noise amplitude, the PUF geometry, and the dielectric constants.
Phase transitions and dielectric properties of the Ag1−xBix∕3NbO3 system
Authors: Hu Xing , Valant Matjaz , Suvorov Danilo
Published: August 2006

Ceramics of Ag1−xBix∕3NbO3 (x<0.6) perovskite solid solutions were obtained by solid-state reaction. The tilting systems (a−a−c+ for x=0.1–0.3 and a0a0c+ for x=0.4–0.5) were determined from the splitting of the peaks and the superlattice lines in the x-ray diffraction patterns. The dielectric properties of the Ag1−xBix∕3NbO3 (x=0.1–0.6) ceramics were measured in the range of 100–870K at 1kHz–1MHz. Based on the dielectric measurements, three phase transitions occurred for x=0.1–0.3, and their temperatures decreased with the increasing Bi content. A new diffuse phase transition occurred for x>0.3. A T-x phase diagram was constructed from the dielectric data. Excellent temperature-stable dielectrics were obtained at x=0.25 in the vicinity of the diffuse phase transition.
dc field and temperature dependent acoustic resonances in parallel-plate capacitors based on SrTiO3 and Ba0.25Sr0.75TiO3 films: Experiment and modeling
Authors: Gevorgian S. , Vorobiev A. , Lewin T.
Published: August 2006

Experimentally observed resonant absorption peaks in frequency dependent loss tangent in thin film parallel-plate SrTiO3 and Ba0.25Sr0.75TiO3 capacitors are explained by electrostriction and piezoelectric effects. A simple theory (linear approximation) is proposed to model the electric field and temperature dependences of the acoustic resonant frequencies, and the intensities of the microwave into acoustic transformations. The electric field dependence of the acoustic resonant frequencies may be used to develop tunable thin film bulk acoustic wave resonators (TFBARs) operating at frequencies up to 10GHz and above. The expected tuneability of the TFBARs is more than 5%.