bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of pH on the structure of lipoplexes
Authors: Caracciolo Giulio , Pozzi Daniela , Caminiti Ruggero , Marchini Cristina , Montani Maura , Amenitsch Heinz
Published: July 2008

Recently, it has been postulated that a primary importance of the pH is for accomplishing efficient lipid-mediated translocation of nucleic acids across the endosomal membrane into the cytosol for transport to the nucleus. With the aim of providing insight into the postulated correlation between transfection efficiency, phase evolution of lipoplexes upon acidification, and DNA release, we investigated the pH dependence of the structure of low efficiency 1,2-dioleoyl-3-trimethylammonium-propane-dioleoylphosphocholine/DNA and high efficiency 3β-[N-(N′,N′-dimethylaminoethane)-carbamoyl]-cholesterol-dioleoylphosphatidylethanolamine/DNA lipoplexes by high-resolution synchrotron small-angle x-ray diffraction, while the extent of DNA release was estimated by means of electrophoresis on agarose gels. Here we show that upon acidification from physiological to acidic values (as those characteristic of endosomes), (i) the lamellar structure of lipoplexes was preserved with a decrease in the one-dimensional DNA packing density, reflecting a pH-induced contraction of interfacial area of lipid head groups and (ii) DNA was not released from lipoplexes. Distinct levels of transfection between lipoplexes were interpreted in terms of the different DNA-binding capacities of cationic liposomes.
Free vibration of nanorings/arches based on nonlocal elasticity
Authors: Wang C. M. , Duan W. H.
Published: July 2008

This paper deals with the free vibration problem of nanorings/arches. The problem is formulated on the basis of Eringen’s nonlocal theory of elasticity in order to allow for the small length scale effect. Exact vibration frequencies are derived for the nanorings/arches and the effects of small length scale, defects, and elastic boundary conditions are investigated. The small length scale effect lowers the vibration frequencies. The defects and the use of elastic boundary conditions (instead of fixed restraints) also significantly reduce the frequencies and alter the vibration mode shapes of circular rings/arches. The results presented should be useful to engineers who are designing nanorings/arches for microelectromechanical and nanoelectromechanical devices.
Structure and morphology of perylene films grown on different substrates
Authors: Beigmohamadi M. , Niyamakom P. , Farahzadi A. , Effertz C. , Kremers S. , Brueggemann D. , Wuttig M.
Published: July 2008

In this study the structural and morphological properties of perylene films deposited on different substrates have been investigated. The film properties have been studied by employing a range of techniques including atomic force microscopy, x-ray diffraction, and drop shape analysis for the determination of the surface free energy. For films deposited on top of Al2O3, the perylene molecules have a fiber texture ordering perpendicular to the substrate, while for films deposited on top of a Au(111) layer, only for thicker films above 400nm were diffraction peaks observed. The different growths of perylene on top of these substrates are attributed to the different strengths of the molecule-substrate interaction.
Calculating vibrational spectra using modified Shepard interpolated potential energy surfaces
Authors: Evenhuis Christian R. , Manthe Uwe
Published: July 2008

A potential energy interpolation approach based on modified Shepard interpolation and specifically designed for calculation of vibrational states is presented. The importance of the choice of coordinates for the rate of convergence is demonstrated. Studying the vibrational states of the water molecule as a test case, a coordinate system comprised of inverse bond distances and trigonometric functions of the bond angle is found to be particularly efficient. Different sampling schemes used to locate the reference points in the modified Shepard interpolation are investigated. A final scheme is recommended, which allows the construction of potential energy surfaces to sub-wave-number accuracy.
Accurate calculation of transport properties for organic molecular semiconductors with spin-component scaled MP2 and modern density functional theory methods
Authors: Sancho-García J. C. , Pérez-Jiménez A. J.
Published: July 2008

At ambient temperatures, intermolecular hopping of charge carriers dominates the field effect mobility and thus the performance of organic molecular semiconductors for organic-based electronic devices. We have used a wide variety of modern and accurate computational methods to calculate the main parameters associated with charge transport, taking oligoacenes, and its derivatives as the exemplary organic materials. We tackle the problem from a combined inter- and intramolecular approach, in which the parameters are calculated for an isolated single molecule concomitantly with the stability of the dimers found in experimentally determined crystalline structures. Considering that most of the future applications within the field would need a full understanding of the transport mechanism, we assess the reliability of the methods to be employed according to the nature of the problem. Finally, we perform a computationally guided molecular engineering of a new set of materials derived from tetracene (rubrene and highly twisted oligoacenes) which allows to robustly anticipate the reasons for their expected performance in organic-based electronic devices.
Mechanical and thermal properties of single crystals of the type-I clathrate compounds Ba8Ga16Ge30 and Sr8Ga16Ge30
Authors: Okamoto Norihiko L. , Nakano Takahiro , Tanaka Katsushi , Inui Haruyuki
Published: July 2008

The mechanical and thermal properties of single crystals of the type-I clathrate compounds Ba8Ga16Ge30 and Sr8Ga16Ge30 have been investigated by measuring the elastic constants, coefficients of thermal expansion (CTE), and plastic deformation behavior in compression. The values of bulk moduli extrapolated to 0 K for Ba8Ga16Ge30 and Sr8Ga16Ge30 are almost identical to each other, whereas the values of Young and shear moduli for Ba8Ga16Ge30 are larger than those for Sr8Ga16Ge30. The values of polycrystalline moduli for both Ba8Ga16Ge30 and Sr8Ga16Ge30 are all considerably smaller than those for pure germanium. The values of CTE for Ba8Ga16Ge30 and Sr8Ga16Ge30 are virtually identical with each other, but they are considerably larger than that of pure germanium. The feasibility of these two clathrate compounds as a thermoelectric material in terms of mechanical stability under possible thermal stresses is evaluated by calculating thermal stresses that are expected to develop within these compounds when used as a thermoelectric material in thermoelectric devises.
Laser irradiation in Nd3+ doped strontium barium niobate glass
Authors: Haro-González P. , Martín I. R. , Arbelo-Jorge E. , González-Pérez S. , Cáceres J. M. , Núñez P.
Published: July 2008

A local nanocrystalline formation in a neodymium doped strontium barium niobate (SBN) glass has been obtained under argon laser irradiation. The intense emission around 880 nm, originated from the F43/2 (F45/2) thermalized level when the glass structure changes to a glass ceramic structure due to the irradiation of the laser beam, has been studied. The intensities and lifetimes change from this level inside and outside the irradiated area made by the laser excitation. They have been analyzed and demonstrated that the desvitrification process has been successfully achieved. These results confirm that nanocrystals of SBN have been created by the laser action confirming that the transition from glass to glass ceramic has been completed. These results are in agreement with the emission properties of nanocrystals of the bulk glass ceramic sample. The present study also suggests that the SBN nanocrystal has a potential application as temperature detector.
Characterization of out-of-band radiation and plasma parameters in laser-produced Sn plasmas for extreme ultraviolet lithography light sources
Authors: Namba S. , Fujioka S. , Sakaguchi H. , Nishimura H. , Yasuda Y. , Nagai K. , Miyanaga N. , Izawa Y. , Mima K. , Sato K. , Takiyama K.
Published: July 2008

Out-of-band (OOB) radiation, in contrast to the in-band radiation at 13.5 nm in a 2% bandwidth, emitted from dense tin plasmas generated by a laser was investigated for application as an extreme ultraviolet lithography light source. It was found that the continuum spectrum, which overwhelms the atomic and ionic line emissions, is responsible for the intense OOB radiation. The spectral distribution of the continuum emission matches that of blackbody radiation with a temperature of about 10 eV. The OOB radiation can be considerably suppressed by employing a minimum-mass target and short-pulse laser irradiation. Spectroscopic observations were made to examine the spatiotemporal behavior of the plasma immediately after laser irradiation. Prominent line broadening due to the Stark effect in the high-density plasma was observed, from which the variation of the electron density was deduced. The electron density and temperature on the target surface were 1017–1018 cm−3 and a few eV’s, respectively, in the first 200 ns after laser illumination. In addition, the expansion velocities of neutral and singly ionized tin atoms were deduced from time-of-flight transients in the spectral emission.
In situ confocal Raman spectroscopy of lubricants in a soft elastohydrodynamic tribological contact
Authors: Bongaerts J. H. H. , Day J. P. R. , Marriott C. , Pudney P. D. A. , Williamson A.-M.
Published: July 2008

This paper describes an experimental setup that combines friction measurements with in situ confocal Raman microscopy on a lubricating fluid film. The setup allows the determination of film thickness and shear-rate profiles, the position-dependent lubricant composition within the rubbing contact, as well as optical imaging of the contact and its surroundings. The tribological contact can be investigated at continuous rubbing conditions over a range of sliding speeds from 1×10−3to2m∕s. The setup’s capabilities are demonstrated for a soft poly(dimethylsiloxane) sphere pressed and rotated against a quartz flat. Friction forces are presented in the boundary, mixed, and elastohydrodynamic lubrication (EHL) regimes for both a hydrophilic and a hydrophobic flat surface. Optical images of the contact show that, for single-phase aqueous Newtonian lubricants in the EHL regime, starvation of the contact occurs above a critical value of the product of the entrainment speed and the lubricant viscosity. Starvation results in a lower film thickness and lower friction values than predicted by numerical calculations that do not take starvation into account. The capability to determine, using Raman spectroscopy, the composition of the lubricant within the contact is demonstrated for oil-in-water emulsions, stabilized by a nonionic surfactant (Tween 60). The oil concentration was found to depend on both the sliding speed and the position within the rubbing contact. For the investigated tribological conditions, the oil content within the contact was lower than that of the bulk emulsion. This is attributed to the fact that the aqueous matrix phase preferentially wets the hydrophilic quartz slide, which prevents oil droplets from entering the contact if the film thickness is of the order of the droplet size. The ability to determine friction forces, lubricant composition and film thickness, and shear-rate profiles will provide invaluable information in the pursuit of a better understanding of lubrication of soft and hard contacts by multiphase complex fluids.
Electronic structure and morphology of organic/metal/organic junctions
Authors: Watkins N. J. , Kushto G. P. , Mäkinen A. J.
Published: July 2008

We have investigated the electronic structures and morphologies of two organic–metal–organic junctions, relevant for organic bistable devices, photovoltaics, and light emitting devices. Combined photoemission spectroscopy and scanning probe measurements found that in a 3,4,9,10 perylenetetracarboxylic dianhydride bis-benzimidazole (PTCBI)/Ag/Cu-phthalocyanine (CuPc) junction, Ag undergoes a chemical reaction with PTCBI leading to a discontinuous film of Ag clusters. In contrast, Au forms a continuous metal film in the PTCBI/Au/CuPc junction with no observable chemistry occurring. Photoemission spectroscopy suggests that both heterojunctions exhibit barrierless charge recombination leading to very similar functional behavior when integrated in device structures despite significant chemical and morphological differences.
Plastic deformation in Al (Cu) interconnects stressed by electromigration and studied by synchrotron polychromatic x-ray microdiffraction
Authors: Chen Kai , Tamura N. , Valek B. C. , Tu K. N.
Published: July 2008

We report here an in-depth synchrotron radiation based white beam x-ray microdiffraction study of plasticity in individual grains of an Al (Cu) interconnect during the early stage of electromigration (EM). The study shows a rearrangement of the geometrically necessary dislocations (GNDs) in bamboo typed grains during that stage. We find that about 90% of the GNDs are oriented so that their line direction is the closest to the current flow direction. In nonbamboo typed grains, the Laue peak positions shift, indicating that the grains rotate. An analysis in terms of force directions has been carried out and is consistent with observed EM induced grain rotation and bending.
Correlation between site preference of ternary Mn addition in LaAg and superconductivity
Authors: Kumar S. , Kaul S. N. , Fernández J. Rodríguez , Barquín L. Fernández , Henry P. F.
Published: July 2008

Investigation of structure, surface morphology, composition, and the superconducting-normal phase diagram of LaAg1−cMnc superconducting alloys with nominal composition c=0.0, 0.025, 0.05, 0.1, 0.2, and 0.3, reveals the following. The alloys with c=0, 0.025, and 0.05 are essentially single phase alloys with the actual Mn concentration x same as the nominal one, i.e., c=x, whereas in the alloys with c=0.1, 0.2, and 0.3, the actual Mn concentration of the majority phase (crystalline grains) is x=0.050(1), 0.080(1), and 0.100(1), respectively. The ternary Mn addition does not alter the CsCl structure of the parent compound LaAg. Neither a structural phase transition nor a long-range antiferromagnetic order is observed at any temperature in the range 1.8 K≤T≤50 K in any of the Mn containing alloys. Mn has exclusive La (Ag) site preference in the alloy (alloys) with x=c=0.025 (x≥0.05 or c≥0.1) whereas in x=c=0.05, Mn has essentially no site preference in that all the Mn atoms either occupy the La sites or the Ag sites. When Mn substitutes for La at the La sublattice sites (as is the case in x=c=0.025), conventional phonon-mediated (BCS) superconductivity prevalent in the parent LaAg compound is destroyed but when Mn replaces Ag at the Ag sublattice sites (as in x≥0.05), non-BCS superconductivity at elevated temperatures is observed.
Criterion of multiswitching stability for magnetic nanoparticles
Authors: Porrati F. , Huth M.
Published: July 2008

We present a procedure to study the switching and the stability of an array of magnetic nanoparticles in the dynamical regime. The procedure leads to the criterion of multiswitching stability to be satisfied in order to have stable switching. The criterion is used to compare various magnetic-field-induced switching schemes, either present in the literature or suggested in the present work. In particular, we perform micromagnetic simulations to study the magnetization trajectories and the stability of the magnetization after switching for nanoparticles of elliptical shape. We evaluate the stability of the switching as a function of the thickness of the particles and the rise and fall times of the magnetic pulses, both at zero and room temperature. Furthermore, we investigate the role of the dipolar interaction and its influence on the various switching schemes. We find that the criterion of multiswitching stability can be satisfied at room temperature and in the presence of dipolar interactions for pulses shaped according to complementary metal oxide semiconductor specifications, for switching rates in the gigahertz regime.
Magnetic resonance of slotted circular cylinder resonators
Authors: Du Junjie , Liu Shiyang , Lin Zhifang , Chui S. T.
Published: July 2008

By a rigorous full-wave approach, a systemic study is made on the magnetic resonance of slotted circular cylinder resonators (SCCRs) made of a perfect conductor for the lossless case. This is a two-dimensional analog of the split-ring resonator and may serve as an alternative type of essential constituent of electromagnetic metamaterials. It is found that the resonance frequency can be modulated by changing the geometrical parameters and the dielectrics filling in the cavity and the slot. An approximate empirical expression is presented for magnetic resonance frequency of SCCRs from the viewpoint of an L-C circuit system. Finally, it is demonstrated that the SCCR structure can be miniaturized to less than 1/150 resonant wavelength in size with the dielectrics available currently.
Size and frequency dependent gas damping of nanomechanical resonators
Authors: Verbridge Scott S. , Ilic Rob , Craighead H. G. , Parpia Jeevak M.
Published: July 2008

We examine size and frequency dependent gas damping of nanobeam resonators. We find an optimal beam width that maximizes the quality factor at atmospheric pressure, balancing the dissipation that scales with surface-to-volume ratio and dominates at small widths, against the interaction with the underlying substrate via the air that dominates the behavior of the wider devices. This latter interaction is found to affect the Knudsen number corresponding to a transition out of the molecular damping regime. We examine higher order modes and tune tension mechanically to vary the frequency of individual resonators, to resolve size and frequency effects.
Magnetic anisotropy and resonance frequency of patterned soft magnetic strips
Authors: Chen X. , Ma Y. G. , Ong C. K.
Published: July 2008

FeTaN thin films of 100nm with in-plane uniaxial anisotropy were fabricated by rf magnetron sputtering and patterned into strips with width varied from 30to500μm along both the easy and hard magnetization axes of the unpatterned films. The values of effective magnetic anisotropy of the patterned films increase as the strip widths decrease. The ferromagnetic resonance frequency showed a corresponding increment from 1.38to2.58GHz as the strip width varied. It indicates that this method is effective to tune the ferromagnetic resonance frequency of magnetic thin films. The resultant permeability spectra were understood according to the well-known Kittel formula where an averaged shape anisotropy arisen from the patterning was taken into account for the strips patterned along the easy axis. For the strips patterned along the hard axis, the increase in resonance frequency in narrower strips is attributed to the strengthened internal domain pinning effect predicted from the domain patterns numerically calculated.
Free-surface optical scattering as an indicator of the shock-induced solid-liquid phase transition in tin
Authors: Stevens G. D. , Lutz S. S. , Marshall B. R. , Turley W. D. , Veeser L. R. , Furlanetto M. R. , Hixson R. S. , Holtkamp D. B. , Jensen B. J. , Rigg P. A. , Wilke M. D.
Published: July 2008

When highly polished metal surfaces melt upon release after shock loading, they exhibit features that suggest that significant surface changes accompany the phase transition. The reflection of light from such surfaces changes from specular (preshock) to diffuse upon melting. A familiar manifestation of this phenomenon is the loss of signal light observed with a velocity interferometer system for any reflector, which occurs at pressures high enough to melt the free surface. Unlike many other potential material phase-sensitive diagnostics (e.g., reflectometry and conductivity) that show relatively small changes, the specularity of reflection provides a more sensitive and definitive indication of the solid-liquid phase transition. Data are presented that support the hypothesis that specularity changes indicate melt in a way that can be measured easily and unambiguously.
Design and performance of a practical variable-temperature scanning tunneling potentiometry system
Authors: Rozler M. , Beasley M. R.
Published: July 2008

We have constructed a scanning tunneling potentiometry system capable of simultaneously mapping the transport-related electrochemical potential of a biased sample along with its surface topography. Combining a novel sample biasing technique with a continuous current-nulling feedback scheme pushes the noise performance of the measurement to its fundamental limit—the Johnson noise of the scanning tunneling microscope (STM) tunnel junction. The resulting 130nV voltage sensitivity allows us to spatially resolve local potentials at scales down to 2nm, while maintaining angstrom scale STM imaging, all at scan sizes of up to 15μm. A millimeter-range two-dimensional coarse positioning stage and the ability to operate from liquid helium to room temperature with a fast turn-around time greatly expand the versatility of the instrument. By performing studies of several model systems, we discuss the implications of various types of surface morphology for potentiometric measurements.
High-mobility organic single crystal transistors with submicrometer channels
Authors: Kawanishi Takafumi , Fujiwara Takaaki , Akai-Kasaya Megumi , Saito Akira , Aono Masakazu , Takeya Junichi , Kuwahara Yuji
Published: July 2008

We demonstrate high-performance electric-field effects in submicrometer-channel organic transistors with rubrene single crystals. Platinum source and drain electrodes are embedded in silicon dioxide gate insulators to reduce thickness of the dielectrics and to minimize the short-channel effect. The miniaturized devices exhibit typical output characteristics with Ohmic linear region, well-defined current saturation, and on-off ratio of 106. Mobility values are in the range of 0.1–0.3cm2∕Vs, which is comparable to those of the best submicrometer organic transistors. Anisotropy in the mobility is detected, indicating that bandlike transport is responsible for the high transistor performance of the short-channel devices.
New Lennard-Jones metastable phase
Authors: Eshet Hagai , Bruneval Fabien , Parrinello Michele
Published: July 2008

Despite the extensive use of the Lennard-Jones potential in atomistic simulations, we present a metastable phase not reported so far for the Lennard-Jones solid. This phase having space group I4¯3d arises from distortion of the body-centered cubic network. In contrast to the body-centered cubic phase, the I4¯3d phase possesses a noticeable range of metastability.
Calculating effective resistances on underlying networks of association schemes
Authors: Jafarizadeh M. A. , Sufiani R. , Jafarizadeh S.
Published: July 2008

Recently, in the work of Jafarizadeh et al. [J. Phys, A: Math. Theor. 40, 4949 (2007); e-print arXiv:0705.2480], calculation of effective resistances on distance-regular networks was investigated, where in the first paper, the calculation was based on stratification and Stieltjes functions associated with the network, whereas in the latter one a recursive formula for effective resistances was given based on the Christoffel–Darboux identity. In this paper, evaluation of effective resistances on more general networks that are underlying networks of association schemes is considered, where by using the algebraic combinatoric structures of association schemes such as stratification and Bose–Mesner algebras, an explicit formula for effective resistances on these networks is given in terms of the parameters of the corresponding association schemes. Moreover, we show that for particular underlying networks of association schemes with diameter d such that the adjacency matrix A possesses d+1 distinct eigenvalues, all of the other adjacency matrices Ai, i≠0, 1 can be written as polynomials of A, i.e., Ai=Pi(A), where Pi is not necessarily of degree i. Then, we use this property for these particular networks and assume that all of the conductances except for one of them, say, c≡c1=1, are zero to give a procedure for evfor a galuating effective resistances on these networks. The preference of this procedure is that one can evaluate effective resistances by using the structure of their Bose–Mesner algebra without any need to know the spectrum of the adjacency matrices.
Horn–Silver distributions in D-dimensional Euclidean spaces
Authors: Kowalski Kenneth L.
Published: July 2008

A geometric interpretation is found for a class of finite, discrete probability distributions that generalizes the distribution found by Horn and Silver to represent an unusual distribution of neutral versus charged pions in some hadronic production models. Connections to de Finetti’s representation theorem are discussed.
Chaotic advection and the emergence of tori in the Küppers–Lortz state
Authors: Mullowney Paul , Julien Keith , Meiss James D.
Published: July 2008

Motivated by the roll-switching behavior observed in rotating Rayleigh–Bénard convection, we define a Küppers–Lortz (K–L) state as a volume-preserving flow with periodic roll switching. For an individual roll state, the Lagrangian particle trajectories are periodic. In a system with roll-switching, the particles can exhibit three-dimensional, chaotic motion. We study a simple phenomenological map that models the Lagrangian dynamics in a K–L state. When the roll axes differ by 120° in the plane of rotation, we show that the phase space is dominated by invariant tori if the ratio of switching time to roll turnover time is small. When this parameter approaches zero these tori limit onto the classical hexagonal convection patterns, and, as it gets large, the dynamics becomes fully chaotic and well mixed. For intermediate values, there are interlinked toroidal and poloidal structures separated by chaotic regions. We also compute the exit time distributions and show that the unbounded chaotic orbits are normally diffusive. Although the map presumes instantaneous switching between roll states, we show that the qualitative features of the flow persist when the model has smooth, overlapping time-dependence for the roll amplitudes (the Busse–Heikes model).
A high-current microwave ion source with permanent magnet and its beam emittance measurement
Authors: Yao Zeen , Tan Xinjian , Du Hongxin , Luo Ben , Liu Zhanwen
Published: July 2008

The progress of a 2.45GHz high-current microwave ion source with permanent magnet for T(d,n)4He reaction neutron generator is reported in this paper. At 600W microwave power and 22kV extraction voltage, 90mA peak hydrogen ion beam is extracted from a single aperture of 6mm diameter. The beam emittance is measured using a simplified pepper-pot method. The (x,x′) emittance and the (y,y′) emittance for 14keV hydrogen ion beam are 55.3π and 58.2πmmmrad, respectively. The normalized emittances are 0.302π and 0.317πmmmrad, respectively.
Strain, luminescence, and electrical properties of Zn1−xMnxS nanocrystalline films prepared on silicon wafers
Authors: Reddy D. Sreekantha , Kang B. , Yu S. C. , Gunasekhar K. R. , Rao K. Narasimha , Reddy P. Sreedhara
Published: July 2008

Nanocrystalline Zn1−xMnxS films (0≤x≤0.25) were deposited on silicon wafers at 473 K using a simple resistive thermal evaporation technique. Morphological and structural measurements revealed that all the films investigated were nanocrystalline with a cubic structure. The lattice parameter increased linearly with Mn concentration. The surface roughness of all the films was shown to be in the range 1.2–3.5 nm. A blueshift in the photoluminescence was observed in the films with increasing Mn concentration along with an intense ultraviolet emission and orange-yellow emission, which are ascribed to the quantum confinement effect. The composition had a significant influence on the orange-yellow emission intensity as well as peak positions. The excitation wavelength of all the samples was 330 nm and emission wavelengths were observed around 410–560 nm. The presence of many recombination sites, surface areas, and defect types leads to broad band photoluminescence emission lines instead of sharp bands. The electrical resistivity as well as activation energy decreased with increasing Mn concentration.
The failed strength of ceramics subjected to high-velocity impact
Authors: Holmquist Timothy J. , Johnson Gordon R.
Published: July 2008

This article addresses the response of failed ceramics. Under high-velocity impact, ceramics transition from a solid intact material to a fragmented and granular material. This process is often referred to as “damage and failure” and is a complex phenomenon. Because ceramics are very strong in compression, it is difficult to perform laboratory experiments that produce conditions similar to those produced during projectile impact, where the ceramic transitions from an intact material to a granular (failed) material. This limitation generally requires the damage and failed strength to be inferred from computed results that provide good agreement with ballistic penetration experiments. Previous work by the authors [J. Appl. Phys. 97, 093502 (2005)] has suggested a relatively low failed strength for silicon carbide (∼200 MPa) that is generally lower than other published data (although the data vary significantly). Work presented here provides additional evidence for a low failed strength for silicon carbide (and also aluminum nitride and boron carbide). Experimental and computed results of high-velocity penetration into thick ceramic targets exhibit large after-flow penetration (the difference between primary penetration and total penetration) that is strongly influenced by the strength of the material directly in front of the penetrator. The large after-flow observed in the experiments and computed results are consistent with a low failed strength. Similar behavior is also observed for aluminum nitride and boron carbide, suggesting that the failed strength of ceramics may be less a function of the specific material and more a characteristic of granular flow under the conditions of high-velocity impact. To provide additional insight into the response of granular material, an analysis of recent ballistic experiments into silicon carbide powder was performed, where the strength of the powder was determined from the computed results. The analysis indicated that the powder has significant strength under high-velocity impact, much higher than the 200 MPa determined for the failed strength. This suggests that there is something significantly different between the response of powder and that of failed material in the Johnson–Holmquist–Beissel (JHB) model. Possible explanations include, inaccuracies in the computational and numerical techniques, inaccuracies with the JHB model, the tested failed material is not the same as that which exists in the ballistic tests, and/or the failed material exhibits phenomena not included in the JHB model.
Development of eddy current microscopy for high resolution electrical conductivity imaging using atomic force microscopy
Authors: Nalladega V. , Sathish S. , Jata K. V. , Blodgett M. P.
Published: July 2008

We present a high resolution electrical conductivity imaging technique based on the principles of eddy current and atomic force microscopy (AFM). An electromagnetic coil is used to generate eddy currents in an electrically conducting material. The eddy currents generated in the conducting sample are detected and measured with a magnetic tip attached to a flexible cantilever of an AFM. The eddy current generation and its interaction with the magnetic tip cantilever are theoretically modeled using monopole approximation. The model is used to estimate the eddy current force between the magnetic tip and the electrically conducting sample. The theoretical model is also used to choose a magnetic tip–cantilever system with appropriate magnetic field and spring constant to facilitate the design of a high resolution electrical conductivity imaging system. The force between the tip and the sample due to eddy currents is measured as a function of the separation distance and compared to the model in a single crystal copper. Images of electrical conductivity variations in a polycrystalline dual phase titanium alloy (Ti-6Al-4V) sample are obtained by scanning the magnetic tip–cantilever held at a standoff distance from the sample surface. The contrast in the image is explained based on the electrical conductivity and eddy current force between the magnetic tip and the sample. The spatial resolution of the eddy current imaging system is determined by imaging carbon nanofibers in a polymer matrix. The advantages, limitations, and applications of the technique are discussed.
Experimental study of nonlinear solitary waves in two-dimensional dusty plasma
Authors: Sheridan T. E. , Nosenko V. , Goree J.
Published: July 2008

The excitation and propagation of solitary waves is studied experimentally in a two-dimensional strongly coupled dusty (complex) plasma. A single layer with ≈5000 microspheres (8μmdiam) was suspended in an argon plasma with a neutral gas pressure of 3.0mTorr. The measured Debye shielding parameter was κ≈1.6, where κ=a∕λ is the ratio of the lattice constant a to the Debye length λ. Nonlinear, planar longitudinal waves were launched by pushing all the particles in a rectangular region at the center of the crystal in the same direction using an 18W green laser. Compressive solitary waves with density perturbations δn∕n0≲0.8 and widths ≲5a were found to propagate in the forward direction at speeds exceeding the dust acoustic speed. For small amplitude solitary waves, the relations between amplitude, width, and velocity are consistent with those predicted for Korteweg–deVries solitons. Rarefactive perturbations were not observed to evolve into solitary waves. However, oscillatory shocks were seen to move in the backward direction after the laser force was removed.
Transparent organic light emitting diodes using a multilayer oxide as a low resistance transparent cathode
Authors: Yook Kyoung Soo , Jeon Soon Ok , Joo Chul Woong , Lee Jun Yeob
Published: July 2008

Transparent organic light emitting diodes were developed by using a thermally evaporable WO3∕Ag∕WO3 (WAW) as a transparent cathode. A thin Ag layer was introduced as an interlayer between the Li doped electron transport layer and the WAW electrode. A high transparency over 80% was obtained and electron injection was greatly improved by using the thin Ag interlayer between the Li doped layer and the WAW electrode. The driving voltage at 1000cd∕m2 was only 4.5V and the sheet resistance of the WAW electrode was as low as 12Ω∕◻.
Matching pursuit for decomposition and approximation of ultrasonic pulse-echo wavelet and its application in ultrasonic nondestructive evaluation
Authors: Liang Wei , Que Pei-wen , Lei Hua-ming , Chen Liang
Published: July 2008

The estimation of the time of arrival (TOA) and/or time of flight of the ultrasonic echoes is essential in ultrasonic nondestructive evaluation. In this paper, matching pursuit method has been used to analyze noisy ultrasonic pulse-echo wavelet and decompose the noisy pulse-echo wavelet into unit-norm vectors. The error ratio of the mean square is used to monitor the acceptability of the unit-norm vector obtained by the matching pursuit performance. A matched filter can be designed by using the time reversal of the approximation pulse-echo wavelet obtained by the reconstruction result with several large coefficient decomposed unit-norm vectors. After performing the matched filter, the part of the signal related to the electrical noise will be removed leaving only the correlated portion of the ultrasonic echoes. Then no initial phase have been contained in the processed ultrasonic echoes. Therefore, the TOA can be estimated directly from the peaks of the representation of the processed signal. Numerical simulations and experimental results make evidence the good performance of the proposed approach which has good effect of noise suppression and results in much improved accuracy in the estimation of echo arrival time.
Anisotropic diffusion of point defects in metals under a biaxial stress field simulation and theory
Authors: Chan Wai-Lun , Averback Robert S. , Ashkenazy Yinon
Published: July 2008

We study the diffusion anisotropy (DA) of point defects in fcc and bcc metals in the presence of an applied biaxial stress. The DA depends sensitively on both the crystalline structure and the crystallographic direction in which the stress is applied. For example, interstitials in fcc metals diffuse faster in the plane of the compressive stress than normal to it when the stress is applied to the (001) plane, but they diffuse slower in-plane than out-of-plane when the stress is applied in the (111) plane. In contrast, an applied biaxial stress in the (001) plane of a bcc metal does not cause DA. These results can be explained by considering the interaction of the defects at their saddle point configurations with the external field, together with the constraints imposed by the crystal structure on the defect jump directions. Our calculations show that the DA can be large and lead to surprising results in a number of practical situations.
Large-scale silica nanowire array grown on liquid tin and its applications as Hg (II) scavenger
Authors: Wang Hui , Zhang Xiao-Hong , Ma Dorothy Duo Duo , Lee Shuit-Tong
Published: July 2008

Silica (SiO2) nanowire array has been prepared in large scale through thermal evaporation of SiO via oxide-assisted growth on Sn substrate. During growth the Sn substrate became a liquid, whose fluid surface induced the growth of SiO2 (from SiO disproportionation) into an aligned nanowire array on Sn surface. The liquid Sn itself further served as the solvent to dissolve elemental Si produced from SiO disproportionation. The high-density nanowire array is a type of microporous silica with a large surface area, which can serve as an efficient absorbent for scavenging heavy metal ions such as Hg (II) from water after surface modification with mercapto groups (–SH).
Growth and characterization of N-polar GaN films on SiC by metal organic chemical vapor deposition
Authors: Brown David F. , Keller Stacia , Wu Feng , Speck James S. , DenBaars Steven P. , Mishra Umesh K.
Published: July 2008

Smooth, high-quality N-polar GaN films were grown on C-face SiC substrates by metal organic chemical vapor deposition (MOCVD). Growth on substrates misoriented at 4° toward the m-plane suppressed the formation of hexagonal hillocks commonly observed for the growth of N-polar GaN by MOCVD. Aside from the misorientation, the growth temperature of the initial GaN was observed to have a strong impact on the structural and morphological properties of films grown on vicinal substrates as characterized by x-ray diffraction, atomic force microscopy, photoluminescence, and transmission electron microscopy. This strong temperature dependence was discovered to be a consequence of the island growth mode of the initial GaN. A two-step process was developed, which resulted in GaN films that exhibited XRD rocking curves with a full width at half maximum of 203 arc sec for a (0002¯) scan and 497 arc sec for a (202¯1¯) scan.
Mechanically flexible low-leakage multilayer gate dielectrics for flexible organic thin film transistors
Authors: Seol Y. G. , Noh H. Y. , Lee S. S. , Ahn J. H. , Lee N.-E.
Published: July 2008

The incorporation of an ultrathin, atomic layer deposited HfO2 layer in between the spin-coated poly-4-vinyl phenol (PVP) organic layers in the laminated multilayer gate dielectric for pentacene organic thin film transistors on a flexible substrate reduced the gate leakage current by three to four orders of magnitude and thereby significantly enhanced the current on/off ratio up to ≅104-fold. Cyclic bending testing indicated that the electrical characteristics of the device with the PVP∕HfO2∕PVP trilayer gate dielectric stack were superior to those of the device with the single PVP gate dielectrics due to the improved mechanical and electrical stabilities of the gate dielectric.
Reactive epitaxy of metallic hafnium silicide nanocrystals
Authors: Fiorentini G. A. , Leite Marina S. , Pimentel V. L. , Montoro L. A. , Ramirez A. J. , Medeiros-Ribeiro G.
Published: July 2008

Hafnium silicide islanding occurs spontaneously when metallic Hf is deposited on a Si(001) surface and subsequently annealed at 750°C. Different coverages were investigated by scanning tunneling microscopy (STM) in order to verify distinct stages of island formation. Small islands occurred for 0.26 ML of deposited Hf, coalescing into flat top islands for longer deposition times. Atomic resolution STM and nanobeam electron diffraction allowed the determination of the mature islands top face structure, identified as the (061) surface of the HfSi2 in the C49 phase. Finally, scanning tunneling spectroscopy was carried out over the smallest islands, demonstrating their metallic behavior.
Ultrasmall square-lattice zero-cell photonic crystal laser
Authors: Ee Ho-Seok , Jeong Kwang-Yong , Seo Min-Kyo , Lee Yong-Hee , Park Hong-Gyu
Published: July 2008

We report optically pumped room-temperature lasing in a square-lattice photonic crystal cavity consisting of two shifted lattice points and heterogeneous lattices with different air hole sizes. Three-dimensional finite-difference time-domain simulation shows that this cavity has an ultrasmall mode volume of 0.017μm3 ∼1.7 (λ∕2nslab)3. Photoluminescence spectroscopy exhibits pulsed lasing action at 1511nm with a low lasing threshold of ∼130μW. The measured Q factor is ∼2400 that is well agreed with the theoretical Q of 4200. In addition, the computational analysis based on the actual fabricated structural parameters unambiguously presents the successful demonstration of this ultrasmall lasing mode.
Electronic and optical properties of Ti1−xCdxO2: A first-principles prediction
Authors: Zhang X. D. , Guo M. L. , Liu C. L. , Li W. X. , Hong X. F.
Published: July 2008

A first-principles study has been carried out to evaluate the electronic and optical properties of rutile Ti1−xCdxO2 as a possible photocatalytic material. It was found that Cd incorporation lead to the enhancement of p states in the top of valence band and the decrease of band gap. The optical transition between Cd p and O p enhances gradually and shifts to high energy range with increasing Cd concentration. Furthermore, optical absorption of Ti1−xCdxO2 increases in the visible range.
Magnetostatic interactions between magnetic nanotubes
Authors: Escrig J. , Allende S. , Altbir D. , Bahiana M.
Published: July 2008

The investigation of interactions between magnetic nanotubes is complex and often involves substantial simplifications. In this letter an analytical expression for the magnetostatic interaction, taking into account the geometry of the tubes, has been obtained. This expression allows for the definition of a critical vertical separation for relative magnetization between nanotubes and can be used for tailoring barcode-type nanostructures with prospective applications such as biological separation and transport.
Constraints on scalar diffusion anomaly in three-dimensional flows having bounded velocity gradients
Authors: Tran Chuong V.
Published: July 2008

This study is concerned with the decay behavior of a passive scalar θ in three-dimensional flows having bounded velocity gradients. Given an initially smooth scalar distribution, the decay rate d⟨θ2⟩∕dt of the scalar variance ⟨θ2⟩ is found to be bounded in terms of controlled physical parameters. Furthermore, in the zero diffusivity limit, κ→0, this rate vanishes as κα0 if there exists an α0∊(0,1] independent of κ such that ⟨∣(−Δ)α∕2θ∣2⟩<∞ for α⩽α0. This condition is satisfied if in the limit κ→0, the variance spectrum Θ(k) remains steeper than k−1 for large wave numbers k. When no such positive α0 exists, the scalar field may be said to become virtually singular. A plausible scenario consistent with Batchelor’s theory is that Θ(k) becomes increasingly shallower for smaller κ, approaching the Batchelor scaling k−1 in the limit κ→0. For this classical case, the decay rate also vanishes, albeit more slowly—like (lnPr)−1, where Pr is the Prandtl or Schmidt number. Hence, diffusion anomaly is ruled out for a broad range of scalar distribution, including power-law spectra no shallower than k−1. The implication is that in order to have a κ-independent and nonvanishing decay rate, the variance at small scales must necessarily be greater than that allowed by the Batchelor spectrum. These results are discussed in the light of existing literature on the asymptotic exponential decay ⟨θ2⟩∼e−γt, where γ>0 is independent of κ.
Ultraviolet and visible electroluminescence from n-ZnO∕SiOx∕(n,p)-Si heterostructured light-emitting diodes
Authors: Tan S. T. , Sun X. W. , Zhao J. L. , Iwan S. , Cen Z. H. , Chen T. P. , Ye J. D. , Lo G. Q. , Kwong D. L. , Teo K. L.
Published: July 2008

n - Zn O ∕ Si O x ∕ n - Si and n-ZnO∕SiOx∕p-Si heterostructured light-emitting diodes have been fabricated using metal-organic chemical-vapor deposition for a comparison study. n-ZnO∕SiOx∕p-Si heterostructures show diodelike rectifying current-voltage characteristic with low breakdown voltage, while n-ZnO∕SiOx∕n-Si heterostructures show symmetric nonlinear current-voltage behavior due to the double Schottky barriers at the interface. Both types of diodes emit light when a positive bias applied at Si side. Ultraviolet emission at ∼390nm with an orange-emission centered at ∼600nm were observed in electroluminescence spectra of n-ZnO∕SiOx∕n-Si diodes, while whitish emission centered at ∼520nm was observed for n-ZnO∕SiOx∕p-Si diodes. The emission mechanisms were discussed.
Evolution mechanisms of the surface morphology of grains in ZnO thin films grown on p-InP substrates due to thermal annealing
Authors: Yuk J. M. , Lee J. Y. , No Y. S. , Kim T. W. , Choi W. K.
Published: July 2008

Transmission electron microscopy (TEM), high-resolution TEM, and atomic force microscopy images showed that the columnar structure and the surface morphology of grains in ZnO thin films grown on p-InP substrates were changed due to thermal treatment. The surface morphology variation of the ZnO thin films was attributed to the curvature modification of the subpopulations consisting of ZnO grains. While the top surface of the ZnO grains became parallel with the {0001} planes due to thermal treatment, the curvature of the subpopulations in the ZnO grains became rough. Evolution mechanisms of the surface morphology of ZnO thin films are described.
A gate dielectric that enables high ambipolar mobilities in polymer light-emitting field-effect transistors
Authors: Naber R. C. G. , Bird M. , Sirringhaus H.
Published: July 2008

Polymer light-emitting field-effect transistors (LEFETs) require high and balanced electron and hole mobilities to achieve high current densities. Here we demonstrate a novel gate dielectric for polymer LEFETs that enables mobilities of 0.01cm2∕Vs for both electrons and holes. The low-k dielectric polycyclohexylethylene is placed in direct contact with the poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) semiconductor. A second dielectric layer comprising the high-k material poly(vinylidene fluoride-trifluoroethylene) is used to apply a high electric field onto the low-k dielectric layer. The attainable electron-hole recombination current in such optimized polymer LEFETs is measured and the implications for achieving electrically pumped lasing in a LEFET are discussed.