bluuebunny/crossref_metadata_2025 huggingface dataset:
Methanol-water solutions: A bi-percolating liquid mixture
Authors: Dougan L. , Bates S. P. , Hargreaves R. , Fox J. P. , Crain J. , Finney J. L. , Réat V. , Soper A. K.
Published: September 2004

An extensive series of neutron diffraction experiments and molecular dynamics simulations has shown that mixtures of methanol and water exhibit extended structures in solution despite the components being fully miscible in all proportions. Of particular interest is a concentration region (methanol mole fraction between 0.27 and 0.54) where both methanol and water appear to form separate, percolating networks. This is the concentration range where many transport properties and thermodynamic excess functions reach extremal values. The observed concentration dependence of several of these material properties of the solution may therefore have a structural origin.
Factors influencing liquid breakup in electrohydrodynamic atomization
Authors: Sung Kian , Lee Chang Sik
Published: September 2004

A series of experiments were conducted to produce fine liquid droplets using electrohydrodynamic atomization. The mode of electrohydrodynamic atomization depends on the flow rate of the liquid, the inner diameter of the needle tip, the distance between the needle tip and the ground electrode, the shape of the ground electrode, and the applied ac power. This work was performed to explore the experimental analysis for the flow pattern visualization of droplets, and the relationship between the voltage application and the behavior of liquid atomization. Uniform droplets of different sizes can be obtained at the inception of the spindle mode by changing the flow rate and the electric field. The droplet size also decreased when the flow rate was raised for the spindle mode. The type of breakup mode, either varicose wave or sinuous wave, was observed using image processing system. For flow rates of 2.0ml∕min, 2.7ml∕min, and 3.5ml∕min, the varicose wave occurred from 4kV, and the sinuous wave and twisting wave occurred over 6kV. The dripping phenomena are investigated when the flow rate and the applied voltage are too low.
Heat conduction calorimeter for massively parallel high throughput measurements with picoliter sample volumes
Authors: Chancellor E. B. , Wikswo J. P. , Baudenbacher F. , Radparvar M. , Osterman D.
Published: September 2004

We have developed a bulk micromachined calorimeter with a sensitivity of 1.5nW∕Hz1∕2 and a 1ms time constant using a thin film thermopile as the sensing element. The thermopile consists of seven titanium and bismuth thermocouples with a total Seebeck coefficient of 574μV∕K. The device is capable of measuring enthalpies in chemical or biological reactions in volumes as small as a few picoliters. The device can be fabricated and operated in a massively parallel fashion in combination with ink-jet printing technologies in air and at room temperature, making it ideally suited for biological and biochemical experiments.
A simple theory of the interaction between polymer brushes immersed in a supercritical fluid
Authors: Chervanyov Alexander I. , Egorov S. A.
Published: September 2004

We use a simple two-order parameter model to describe the interaction between the brushes of polymers terminally attached to flat surfaces immersed in a supercritical solvent. Our approach makes it possible to take into account the high compressibility of the supercritical solvent, which proves to give a significant contribution to the disjoining force acting between polymer brushes. Our theory explains why the interaction between brushes can change from repulsive to attractive with decreasing solvent density. This theoretical finding is verified by making a comparison with recent computer simulations. A reasonably good agreement between the results of the present theory and the simulations is found.
Palladium Schottky barrier contacts to hydrothermally grown n-ZnOand shallow electron states
Authors: Grossner Ulrike , Gabrielsen Stig , Børseth Thomas Moe , Grillenberger Joachim , Kuznetsov Andrej Yu. , Svensson Bengt G.
Published: September 2004

Schottky barrier contacts have been formed by electron beam evaporation of Pd on the (0001¯)-face of hydrothermally grown n-type single-crystalline ZnO. The contacts can be operated under reverse bias voltages up to −5V and in the temperature range between 130 and 350K. A barrier height of 0.83eV is deducted at room temperature, which is in reasonable agreement with the value predicted by fundamental theory. The ideality factor for the current-voltage characteristics varies between 1.01 and 1.03 in the voltage range of −5.0to+0.5V, indicating that thermionic emission is a dominant mechanism for charge-carrier transport. Thermal admittance measurements were performed between 15 and 350K, and two prominent levels were resolved at 0.05(1) and 0.33(2)eV below the conduction band edge, respectively. The origin of these levels is not known, but possible candidates are complexes involving hydrogen, Zn interstitials, and vacancy-oxygen centers.
Amorphization and ultrafine-scale recrystallization in shear bands formed in shock-consolidated Pr2Fe14B∕α-Fe nanocomposite magnets
Authors: Li J. , Jin Z. Q. , Liu J. P. , Wang Z. L. , Thadhani N. N.
Published: September 2004

Amorphization and ultrafine-scale recrystallization within shear bands formed in shock-consolidated Pr2Fe14B∕α-Fe nanocomposite magnetic powder compacts have been observed using transmission electron microscopy. The shear bands span through multiple grain lengths and truncate preexisting ∼25nm hard and soft magnetic phase grains, resulting in further grain size refinement. The shear bands contain nanocrystallites (<10nm size) interdispersed in an amorphous matrix, which suggests the occurrence of shock-induced phase transition in localized regions of the shear bands, and provides insight into the process of deformation of nanocrystalline materials under coupled high-strain-rate and high-pressure conditions.
Probing collective dynamics of active particles using modulation force spectroscopy
Authors: Soni G. V. , Ananthakrishna G. , Shivashankar G. V.
Published: September 2004

In this letter, we report a method of measuring the dynamic viscosity of self-propelled active particles using an intensity-modulated optical tweezer. We have used a 6μm trapped polystyrene bead suspended in a bath of motile bacterial cells as a probe. The response function amplitude of the oscillatory bead directly measures the dynamics of the spatiotemporal structure of the motile particles. We find that unlike passive systems, the viscosity is defined by distributions of response function amplitudes that represent the long-range active temporal structures. Appropriate Langevin equations are set up that capture all these essential features.
Lasing in ZnS nanowires grown on anodic aluminum oxide templates
Authors: Ding J. X. , Zapien J. A. , Chen W. W. , Lifshitz Y. , Lee S. T. , Meng X. M.
Published: September 2004

High-density and uniform-sized gold particle arrays have been prepared electrochemically on anodic aluminum oxide (AAO) templates. The gold particles were used as catalysts to synthesize ZnS nanowires. The as-grown nanowires had a wurtzite single-crystal structure and were aligned perpendicularly to the AAO template. Under high-power density optical excitation (266nm), the nanowire array showed an intense, narrow [full width at half maximum (FWHM) of 2.2nm] photoluminescent peak at 338nm composed of a superposition of optical resonant modes (FWHM ∼0.3nm) resulting from the collective emission of a large number of nanowires. These results indicate that the ZnS nanowires act as optical waveguide resonators.
Solute transport during cyclic flow in saturated porous media
Authors: Goldsztein Guillermo H. , Santamarina Juan C.
Published: September 2004

We consider materials with large pores interconnected by thin long channels saturated with an incompressible fluid. In the absence of fluid flow, solute transport in the porous network is diffusion controlled, however, solute transport can be enhanced when the porous network is subjected to a cyclic flow with zero time average velocity. We develop a mathematical model to analyze this physical phenomenon and obtain an effective macroscale diffusion coefficient for solute transport which dependends on cyclic flow conditions and the geometry of the porous network.
Investigation of surface acoustic wave propagation on a sphere using laser ultrasonics
Authors: Clorennec D. , Royer D.
Published: September 2004

Using the noncontact laser ultrasonic technique, we investigate the propagation of surface acoustic waves on a sphere (SSAW). A finite thermoelastic line source gives rise to combined focusing and reversal effects. As for propagation on a cylinder surface, the reversal of the SSAW pulse is explained by the dispersion of the high frequency components of the laser-generated acoustic pulse. The focusing effect, due to the curvature of the surface, depends on the angular aperture of the source. From diffraction theory, optimal conditions for diffraction free propagation are derived, both in harmonic and pulse regimes. On a steel sphere of radius 25mm, Rayleigh waves excited by a 20ns YAG laser pulse focused onto a 3mm line propagate with a nearly constant amplitude.
Comparison of large-scale layer-relaxation simulations with experiment
Authors: Schwarz K. W. , Cai J. , Mooney P. M.
Published: September 2004

Recent experiments on the strain relaxation of SiGe layers are related to basic dislocation theory via numerical calculations based on discrete dislocation dynamics. Excellent quantitative agreement is obtained, demonstrating that such technologically interesting problems can be treated on a fundamental level.
Scanning tunneling microscope study of capped quantum dots
Authors: Song H. Z. , Kawabe M. , Okada Y. , Yoshizaki R. , Usuki T. , Nakata Y. , Ohshima T. , Yokoyama N.
Published: September 2004

On thinly capped InGaAs∕GaAs quantum dots (QDs), a simultaneous study of both the microscopic and electronic structures was carried out using scanning tunneling microscopy (STM). Although the surface is morphologically flat, the STM image of the embedded QDs can be clearly observed at cryogenic temperatures and is distinguishable up to room temperature. Such images are available in a particular bias range, which corresponds to the occurrence of QD-associated current, as demonstrated in scanning tunneling spectroscopy.
Three-dimensional wavelength-scale confinement in quantum dot microcavity light-emitting diodes
Authors: Zinoni C. , Alloing B. , Paranthoën C. , Fiore A.
Published: September 2004

We introduce a microcavity light-emitting diode (LED) structure that uses submicrometer oxide aperture and a quantum dot active region to achieve strong three-dimensional confinement of both the carrier distribution and the optical field. Light–current curves show optical emission for devices as small as 400nm in diameter. Spectroscopy on electrically pumped LEDs, with apertures ranging from 2.5 down to 0.7μm, show several spectral lines corresponding to cavity modes. A strong blueshift of the resonant modes for smaller apertures demonstrates the role of the oxide aperture in confining laterally the optical wave in a volume comparable to (λ∕n)3. Due to the high quality factors and low mode volumes, the devices could be good candidates for the demonstration of the Purcell effect under electrical pumping.
Development of a biosensor based on laser-fabricatedpolymer microcantilevers
Authors: Zhang X. Richard , Xu Xianfan
Published: September 2004

We develop high-sensitivity biosensors based on microcantilevers. The polymer microcantilevers are fabricated by fast and cost-effective laser machining processes. Polymer film is selected because it gives better sensitivity of deflection measurements than silicon due to its lower Young’s modulus and also its cost is much lower. We demonstrate using these polymer microcantilevers for biological molecular analysis in a DNA hybridization experiment. It is shown that our biosensor is capable of detecting 12 base oligonucleotide with concentrations as low as 0.01μM.
Vortex magnetic field sensor based on ring-type magnetoelectric laminate
Authors: Dong Shuxiang , Li Jie-Fang , Viehland D.
Published: September 2004

It has been found that ring-type magnetoelectric laminate composites of circumferentially magnetized magnetostrictive TERFENOL-D and a circumferentially poled piezoelectric Pb(Zr,Ti)O3 have high sensitivity to a vortex magnetic field. At room temperature, an induced output voltage from this ring laminate exhibited a near-linear response to an alternating current (ac) vortex magnetic field Hac over a wide magnetic field range of 10−9<Hac<10−3T at frequencies between sub-Hz and kHz.
Numerical simulation of tetracene light-emitting transistors: A detailed balance of exciton processes
Authors: Verlaak Stijn , Cheyns David , Debucquoy Maarten , Arkhipov Vladimir , Heremans Paul
Published: September 2004

We assess the possibility to use an ambipolar organic light-emitting transistor structure as gain medium for an electrically pumped laser. Singlet and triplet continuity equations are solved together with Poissons and drift-diffusion equations in two dimensions. The solution allows for a detailed balance between the exciton decay, quenching and generation mechanisms. Simulations of a tetracene light-emitting transistor show that triplets are most dominant in quenching singlets. Singlet–triplet quenching can ultimately prevent pure tetracene crystals or films—when provided with a realistic optical feedback structure, to reach the threshold for stimulated emission.
First principles study of transition-metal substitutions in Sm–Co permanent magnets
Authors: Sabirianov R. F. , Kashyap A. , Skomski R. , Jaswal S. S. , Sellmyer D. J.
Published: September 2004

The microchemistry and magnetism of conventional and high-temperature Sm–Co permanent magnets are investigated by first-principles calculations. Particular emphasis is on the site preference for the substitution of Cu, Ti, and Zr in SmCo5 and Sm2Co17 compounds. Cu substitution is more favorable in the 1:5 phase, in agreement with experimental findings. Titanium and zirconium have positive solution energies for both the phases, with Ti(Zr) having slight preference for the 1:5 (2:17) phase. Some Zr may segregate to the phase boundaries because of its large solution energy. For Ti and Zr the dumbbell site of the 2:17 phase is preferred over the other three inequivalent cobalt sites. These results are used to discuss the observed cellular nanostructure of the high-temperature Sm–Co hard magnets with composition close to the 2:17 phase.
Strong luminescence from dislocation-free GaN nanopillars
Authors: Inoue Y. , Hoshino T. , Takeda S. , Ishino K. , Ishida A. , Fujiyasu H. , Kominami H. , Mimura H. , Nakanishi Y. , Sakakibara S.
Published: September 2004

GaN nanostructures were prepared on Si(111) by a hot-wall epitaxy technique employing the modified two-step growth method. Isolated hexagonal pillar-like GaN nanostructures (GaN nanopillars) with the typical diameter, height, and density of 200–300nm, 0.5–1μm, and 3–4×108cm−2, respectively, are self-organized without intentional pre-processing to the Si substrate. The photoluminescence and cathodoluminescence (CL) measurements show the strong near-band-edge emissions without the yellow band at room temperature. Stronger CL is obtained from the GaN nanopillars in comparison to single-crystalline GaN. The obtained strong CL is related to high crystal quality of the dislocation-free GaN nanopillars.
Nanomechanics of carbon nanofibers: Structural and elastic properties
Authors: Wei Chenyu , Srivastava Deepak
Published: September 2004

A general analytic expression for the Young’s modulus of a range of carbon nanofibers (CNFs) with single or multishell nanocone or cone stacked structures has been developed from continuum elastic theory. The Young’s modulus of a single-shell nanocone is found to be cos4θ of that of an equivalent single-wall carbon nanotube (CNT). The CNFs of short lengths and small tilting angles have very large Young’s modulus comparable to that of single or multiwall CNTs, whereas the inverse is true for the CNFs with long lengths and large tilting angles. The dependence of the stiffness of CNFs on various structural parameters has been predicted by the model, validated through full-scale molecular dynamics simulations, and categorized for scanning probe tip or reinforcing fiber type applications.
Investigation of hard magnetic properties of nanocomposite Fe-Pt magnets by micromagnetic simulation
Authors: Rong Chuan-bing , Zhang Hong-wei , Du Xiao-bo , Zhang Jian , Zhang Shao-ying , Shen Bao-gen
Published: September 2004

Micromagnetic finite element method is used to simulate magnetic properties of FePt∕Fe3Pt exchange-coupled nanocomposites. Numerical results show that the maximum energy product (BH)max about 34.6 MGOe can be obtained for the 3-nm-scale isotropic magnets with the volume fraction of soft Fe3Pt phase vs=15%. The appearance of touched soft grains causes the decrease of reduced remanence mr when vs exceeds 20%. Coercivity decreases with the simultaneous decrease of Ds and Dh, where Ds and Dh are the grain sizes of soft and hard phases, respectively. Only under the prerequisite of sufficiently large Dh, the coercivity can be improved remarkably by decreasing Ds below twice the domain wall width of hard phase.
Photoinduced magnetic memory effect in an iron (II) spin-crossover complex
Authors: Liu Hongwu , Fujishima Akira , Sato Osamu
Published: September 2004

We report a bidirectional reversible light-induced excited spin state trapping (LIESST) effect featuring an associated memorey effect in an Fe(II) spin-crossover complex [FeL(CN)2]·H2O (where L is a Schiff-base macrocyclic ligand). Depending on the illumination history, bidirectional LIESST processes have been successfully observed that are induced by irradiation with the same two wavelengths. A large zero-point energy difference, strong overlapping spectra of the high-spin and low-spin bands and image internal pressure could account for this photomagnetic phenomenon.
Electromodulation of photoinduced charge transfer in polyfluorene bilayer devices
Authors: Dhoot A. S. , Hogan J. A. , Morteani A. C. , Greenham N. C.
Published: September 2004

Photoinduced charge transfer at a conjugated polymer donor-acceptor heterojunction controls the ultimate photogeneration efficiency in photovoltaic devices. We have studied this charge transfer by measurements of electric-field-induced photoluminescence (PL) modulation in polyfluorene-based bilayer devices at room and low temperatures. Depending on the field direction across the interface we observe field-induced quenching or enhancement of the PL, which we ascribe to modulation of the exciton dissociation rate through modification of the energy of the charge-transfer state formed after charge separation.
Evolution of thin Cu films on GaAs(110) towards atomically flat epitaxial overlayers
Authors: Müller J. , Wenderoth M. , Quaas N. , Reusch T. C. G. , Ulbrich R. G.
Published: September 2004

The morphology of thin copper films on cleaved GaAs(110) surfaces, deposited at 80K with subsequent annealing, has been investigated at room temperature by means of scanning tunneling microscopy. Constant-current topographies, taken during 24 days, that illustrate different stages of the film evolution towards an atomically flat, epitaxial metal overlayer are presented here. We compare our results with low-temperature grown silver films on GaAs(110) and find similar morphologies as predicted by the model of “electronic growth.” In contrast to the surface diffusion of Cu on Cu(111) single crystals and the kinetics of similarly prepared silver films on GaAs(110), a strongly increased film stability has been observed, which we ascribe to a high tensile strain in the metal overlayer.
Rapid three-dimensional manufacturing of microfluidic structuresusing a scanning laser system
Authors: Li Biao , Yu Hui , Sharon Andre , Zhang Xin
Published: September 2004

This letter introduces a three-dimensional manufacturing approach to the rapid processing of microfluidic structures using a scanning laser system. This technique takes advantage of the nonuniform distribution of laser power along its incident axis. The laser processing perpendicular to the specimen surface is realized by fine-tuning focus levels and laser intensity. A large number of microfluidic components such as cantilevered valves, embedded channels, and other shapes requiring gaps between layers are demonstrated in a single layer. With this process, a class of microstructures with designed-in functionalities can be developed.
Droplet heteroepitaxy of GaN quantum dots by metal-organic chemical vapor deposition
Authors: Gherasimova M. , Cui G. , Jeon S.-R. , Ren Z. , Martos D. , Han J. , He Y. , Nurmikko A. V.
Published: September 2004

Optically active GaN quantum dots on conductive AlGaN templates are synthesized by droplet heteroepitaxy, whereby the Ga droplets are converted to GaN islands in the presence of ammonia at 600°C. We have investigated the evolution of metallic Ga layers on AlGaN, obtaining the optimal surface densities and size distribution of the Ga droplets. The stability of GaN islands is influenced by the surface kinetics and the initial droplet size; the condition of Ga deposition and subsequent nitrogen exposure is identified, which preserves the initial density of the Ga droplets. A nitrogen-rich environment is identified as a necessary condition for maintaining the optimal GaN morphology by suppressing the Ga surface diffusion and preventing two-dimensional layer growth.
Reversible photodegradation of organic light-emitting diodes
Authors: Kobrin P. , Fisher R. , Gurrola A.
Published: September 2004

A phenomenon in which the electroluminescence from an organic light-emitting diode is suppressed by the absorption of visible light is reported. This at-least partially reversible degradation has a recovery time measured in days at a temperature of 20°C. The absorbed light affects both the I–V characteristics of the device and the electroluminescent quantum efficiency. The degradation is first order in exposure intensity and has been observed in red, green, and blue devices with exposure to as little as 1W∕cm2 of green laser light.
Tunnel junctions for ohmic intra-device contacts on GaSb-substrates
Authors: Dier Oliver , Sterkel Martin , Grau Markus , Lin Chun , Lauer Christian , Amann Markus-Christian
Published: September 2004

A tunnel junction for intradevice contacts on GaSb substrates has been realized. By using solid source molecular beam epitaxy, we have fabricated abrupt, heavily doped homo- and heterojunctions of InAs(Sb) and GaSb to form a low resistive ohmic tunnel junction. The resitivity achieved was as low as 2.6×10−5Ωcm2.
Electrical activation phenomena induced by excimer laser annealingin B-implanted silicon
Authors: Fortunato G. , Mariucci L. , La Magna A. , Alippi P. , Italia M. , Privitera V. , Svensson B. , Monakhov E.
Published: September 2004

The activation process induced by excimer laser annealing (ELA) has been investigated in 10keV B-implanted samples. It is found that for energy densities inducing melt depths of the order or larger of the implanted region the junction depth is controlled by the melt depth, with activation approaching 100% and box-shaped carrier density distributions with abrupt junction profile. For energy densities inducing a melting shallower than the implanted region, two different activation mechanisms have been identified: the first occurring in the molten region and leading to complete B activation; the second occurring in the region immediately below the molten zone and leading to thermal activation of B, induced by the heat wave propagating into the Si wafer. This last process is characterized by an activation energy of 5eV and is not accompanied by B diffusion. As a consequence, a deep tail of active B is produced, preventing the possibility to form abrupt and ultrashallow junctions. These results suggest that for the formation of ultrashallow junctions it is essential to combine ELA with ultralow energy ion implantation.
Domain hierarchy in annealed (001)-oriented Pb(Mg1∕3Nb2∕3)O3-x%PbTiO3 single crystals
Authors: Bai Feiming , Li JieFang , Viehland D.
Published: September 2004

The domain structures of annealed (001)-oriented Pb(Mg1∕3Nb2∕3)O3-x%PbTiO3(PMN-x%PT) crystals for x=10, 20, 30, 35, and 40at.% have been investigated by polarized optical microscopy and scanning force microscopy in the piezoresponse mode. The results demonstrate the presence of a domain hierarchy on various length scales ranging from 40nm to 0.1mm, which varies with x.
White electroluminescence from polyfluorene chemically doped with 1,8-napthalimide moieties
Authors: Tu Guoli , Zhou Quanguo , Cheng Yanxiang , Wang Lixiang , Ma Dongge , Jing Xiabin , Wang Fosong
Published: September 2004

An efficient white light-emitting polymer was developed with blue polyfluorene (PFO) chemically doped with orange fluorescent 1, 8-naphthalimide moieties. The emission spectrum can be easily tuned by varying the content of 1, 8-naphthalimide moieties. A white polymeric light-emitting diode (WPLED) with a structure of indium tin oxide (ITO)/the complex of (3,4-ethylenedioxythiophene) and polystyrene sulfonic acid (PEDOT)/polymer∕Ca∕Al showed a current efficiency of 5.3cd∕A and a power efficiency of 2.8Lm∕W at 6V with the Commission Internationale de L'Eclairage (CIE) coordinates at (0.25,0.35). Moreover, the WPLED from the copolymer showed a very stable white light emission at different driving voltage and brightness. The CIE coordinates of the WPLED were (0.25, 0.35), (0.26, O.36), and (0.26, 0.36) under driving voltages of 6, 8, and 10V, corresponding to the brightness of 82, 3555, and 7530cd∕m2, respectively. This approach for realization of white light emission is promising over the polymer blending system in terms of both efficiency and color stability.
Combinatorial studies of (1−x)Na0.5Bi0.5TiO3−xBaTiO3 thin-film chips
Authors: Cheng Hong-Wei , Zhang Xue-Jin , Zhang Shan-Tao , Feng Yan , Chen Yan-Feng , Liu Zhi-Guo , Cheng Guang-Xi
Published: September 2004

Applying a combinatorial methodology, (1−x)Na0.5Bi0.5TiO3−xBaTiO3 (NBT-BT) thin-film chips were fabricated on (001)-LaAlO3 substrates by pulsed laser deposition with a few quaternary masks. A series of NBT-BT library with the composition of BT ranged from 0 to 44% was obtained with uniform composition and well crystallinity. The relation between the concentration of NBT-BT and their structural and dielectric properties were investigated by x-ray diffraction (XRD), evanescent microwave probe, atomic force microscopy, and Raman spectroscopy. An obvious morphotropic phase boundary (MPB) was established to be about 9% BT by XRD, Raman frequency shift, and dielectric anomaly, different from the well-known MPB of the materials. The result shows the high efficiency of combinatorial method in searching new relaxor ferroelectrics.
Vertically aligned carbon nanotube heterojunctions
Authors: Cassell Alan M. , Li Jun , Stevens Ramsey M. D. , Koehne Jessica E. , Delzeit Lance , Ng Hou Tee , Ye Qi , Han Jie , Meyyappan M.
Published: September 2004

The bottom-up fabrication and electrical properties of end-to-end contacted multiwalled carbon nanotube (MWCNT) heterojunctions are reported. The vertically aligned MWCNT heterojunction arrays are formed via successive plasma-enhanced chemical vapor deposition processing to achieve the layered junction architecture. Electron microscopy and current-sensing atomic force microscopy are used to reveal the physical nature of the junctions. Symmetric, nonlinear I–V curves of the as-fabricated junctions indicate that a tunnel barrier is formed between the end-to-end contacted MWCNTs. Repeated high bias I–V scans of many devices connected in parallel fuses the heterojunctions, as manifested by a shift to linear I–V characteristics.
Effect of chemical functionalization on thermal transport of carbon nanotube composites
Authors: Shenogin S. , Bodapati A. , Xue L. , Ozisik R. , Keblinski P.
Published: September 2004

We use molecular dynamics simulations to analyze the role of chemical bonding between the matrix and the fiber on thermal transport in carbon nanotube organic matrix composites. We find that chemical bonding significantly reduces tube-matrix thermal boundary resistance, but at the same time decreases intrinsic tube conductivity. Estimates based on the effective medium theory predict increase, by about a factor of two, of the composite conductivity due to functionalization of single-walled nanotubes with aspect ratios within 100–1000 range. Interestingly, at high degree of chemical functionalization, intrinsic tube conductivity becomes independent of the bond density.
Observation of a metastable phase during solidification of undercooled Nd–Fe–B alloy melts by in situ diffraction experiments using synchrotron radiation
Authors: Volkmann Thomas , Strohmenger Jörn , Gao Jianrong , Herlach Dieter M.
Published: September 2004

The electromagnetic levitation technique was combined with the diagnostic means at the European Synchrotron Radiation Facility in Grenoble to study in situ the phase selection during solidification of undercooled Nd–Fe–B melts. By energy dispersive diffraction experiments with synchrotron radiation on levitated Nd–Fe–B alloys complete diffraction spectra were recorded within a time interval of a few seconds. Owing to the short measuring time the primary crystallization of a metastable phase was observed that initiates the solidification of hard magnetic Nd2Fe14B1 phase. The metastable phase dissolves subsequently and cannot be detected in the as-solidified sample at the ambient temperature. By analyzing the diffraction spectra the metastable phase is identified as a ternary extension of the rhombohedral Nd2Fe17 phase being stable in binary Nd–Fe alloys.
Modulation derived satellite peaks in x-ray reciprocal mapping on bismuth cuprate superconductor film
Authors: Kaneko Satoru , Shimizu Yoshitada , Akiyama Kensuke , Ito Takeshi , Mitsuhashi Masahiko , Ohya Seishiro , Saito Keisuke , Funakubo Hiroshi , Yoshimoto Mamoru
Published: September 2004

X-ray reciprocal space mapping (XRSM) was employed to investigate epitaxial Bi2Sr2Ca1Cu2Ox(Bi-2212) film. Ordinal cross section XRSM (ω-2θ) and plan view XRSM (ω-ψ) clearly indicated asymmetric intensity distribution of four satellite peaks caused by supercell structure of Bi-2212 film. Modulation vector estimated by XRSM was q=0.2b*+0.9c*. The XRSM image simulated by sawtooth wave vector showed good agreement with asymmetric satellite peaks observed on epitaxial film.
Antifungal activity of polymer-based copper nanocomposite coatings
Authors: Cioffi Nicola , Torsi Luisa , Ditaranto Nicoletta , Sabbatini Luigia , Zambonin Pier Giorgio , Tantillo Giuseppina , Ghibelli Lina , D’Alessio Maria , Bleve-Zacheo Teresa , Traversa Enrico
Published: September 2004

Eukaryotes, such as fungi, can be harmful pathogen agents, and the control of their bioactivity is critical as humans are eukaryote organisms, too. Here, copper∕polymer nanocomposites are proposed as antifungal spinnable coatings with controlled copper-releasing properties. The tests of the bioactivity show that fungal growth is inhibited on the nanocomposite-coated plates, and the antifungal activity can be modulated by controlling the Cu nanoparticle loading.
Self-aligned, buried heterostructure AlInGaAs laser diodesby micro-selective-area epitaxy
Authors: Bour D. , Corzine S. , Perez W. , Zhu J. , Tandon A. , Ranganath R. , Lin C. , Twist R. , Martinez L. , Höfler G. , Tan M.
Published: September 2004

We describe the growth and characteristics of AlInGaAs, self-aligned buried heterostructure quantum well lasers deposited by micro-selective-area growth. The lateral waveguide is defined by metalorganic vapor-phase epitaxy in narrow stripe openings; and the natural tendency to form smooth, no-growth {111} sidewall planes produces a low-loss, single-mode waveguide. By proper adjustment of the growth conditions after the waveguide formation, the waveguide may be buried in p-type InP. A buried heterostructure (BH) laser is there formed in a single growth step, eliminating the deterioration associated with air exposure and etch damage, especially for AlInGaAs active regions. The AlInGaAs BH lasers formed in this simple manner exhibit good performance characteristics, with room-temperature threshold current of 4mA.
Magnetic soliton-based logic with fan-out and crossover functions
Authors: Wadhwa Pooja , Jalil M. B. A.
Published: September 2004

A micromagnetic simulation is performed of a device consisting of arrays of square magnetic nanostructures, which is capable not only of basic logic operations but also controlled information transfer in linear, fan-out, and crossover manner by means of magnetic “solitons.” The square elements exhibit shape anisotropy, with easy axes along the diagonals. This enables the solitons to propagate around a bend in an array, or to fan-out into two distinct directions in Y-shaped arrays, in the presence of particular external field sequence and direction. Finally, independent soliton crossover at the intersection of two arrays is also demonstrated, which completes the device’s logical functionality.
An n- to p-type conductivity transition induced by oxygen adsorption on α-Fe2O3
Authors: Gurlo A. , Bârsan N. , Oprea A. , Sahm M. , Sahm T. , Weimar U.
Published: September 2004

The simultaneous measurements of conductance and work function changes induced by gaseous ambient have been performed on α-Fe2O3 thick film polycrystalline samples kept at 280 °C and exposed to different gaseous atmospheres. The switching from n- to p-type conductivity on α-Fe2O3 is shown to have an electronic origin, which is due to the oxygen adsorption and formation of a surface inversion layer and, therefore, to the inversion of the surface conduction type. The modeling of the n–p switching is described in terms of conductance dependence on the band bending induced by gaseous ambient.
Cooperative polymer gate dielectrics in organic thin-film transistors
Authors: Park S. Young , Park Mikyung , Lee Hong H.
Published: September 2004

A concept is introduced in which two polymer dielectrics are made cooperative in acting as the gate insulator for organic thin-film transistors. The bilayer insulator consists of a thin polymer dielectric in contact with the active layer that can induce good electrical properties and a much thicker dielectric with good dielectric properties. These dielectrics are cooperative in combining the best each can offer for a high mobility and a negligible hysteresis.
Raman gain and nonlinear optical absorption measurements in a low-loss silicon waveguide
Authors: Rong Haisheng , Liu Ansheng , Nicolaescu Remus , Paniccia Mario , Cohen Oded , Hak Dani
Published: September 2004

We fabricated a low-loss (∼0.22dB∕cm) rib waveguide (WG) in silicon-on-insulator with a small effective core area of ∼1.57μm2 and measured the stimulated Raman scattering gain in the WG. We obtained 2.3dB Raman gain in a 4.8-cm-long S-shaped WG using a 1455nm pump laser with a cw power of 0.9W measured before the WG. In addition, we observed nonlinear dependence of Raman gain and optical propagation loss as a function of the pump power. Our study shows that this mainly is due to two-photon absorption (TPA) induced free carrier absorption in the silicon WG. We experimentally determined the TPA induced free carrier lifetime of 25ns, which agrees well with our modeling.
Thickness dependence of the ferroelectric PbTiO3 thin films on the dipolar relaxation in the microwave-frequency range
Authors: Kim Yongjo , Lee Doyoung , Park Byungwoo
Published: September 2004

The effects of film thickness on the dipolar relaxation of ferroelectric PbTiO3 thin films were investigated in the microwave-frequency range. The real and imaginary dielectric constants (ε′–iε″) were measured up to 30GHz using interdigital capacitors on high-quality SiO2. As the polycrystalline PbTiO3 film thickness increased from 42 to 407nm, the dipolar-relaxation frequency reduced with increasing grain size. The observed relaxation behavior for ε′–iε″ was explained in terms of the convolution of Debye relaxation. The relaxation frequency in the thin films was higher than the previous values reported in bulk PbTiO3, due to the smaller grain size of the thin films.
Junction–temperature measurement in GaN ultraviolet light-emitting diodes using diode forward voltage method
Authors: Xi Y. , Schubert E. F.
Published: September 2004

A theoretical model for the dependence of the diode forward voltage (Vf) on junction temperature (Tj) is developed. An expression for dVf∕dT is derived that takes into account all relevant contributions to the temperature dependence of the forward voltage including the intrinsic carrier concentration, the band-gap energy, and the effective density of states. Experimental results on the junction temperature of GaN ultraviolet light-emitting diodes are presented. Excellent agreement between the theoretical and experimental temperature coefficient of the forward voltage (dVf∕dT) is found. A linear relation between the junction temperature and the forward voltage is found.
All-epitaxial mode-confined vertical-cavity surface-emitting laser
Authors: Lu D. , Ahn J. , Huang H. , Deppe D. G.
Published: September 2004

A type of AlGaAs∕GaAs-based vertical-cavity surface-emitting laser is demonstrated that creates a strongly-confined optical mode in an all-epitaxial heterostructure. The mode-confining region in this monolithic device is due to a lithographically defined intracavity phase-shifting layer. Analysis shows that the optical loss in the device can be less than for oxide-confinement.
Off-state luminescence in metal-oxide-semiconductor field-effect transistors and its use as on-chip voltage probe
Authors: Polonsky Stas , Weger Alan
Published: September 2004

Luminescence of deep submicron metal-oxide-semiconductor field-effect transistors in the off state has been detected and characterized. It exponentially depends on voltages applied to the device and behaves similarly to its off-state leakage current. Time-resolved measurements of this luminescence allow extraction of on-chip voltages in complementary metal-oxide semiconductor integrated circuits. We propose and demonstrate the application of this technique to various signal integrity issues, such as characterization of crosstalk and power supply noise.
Precise measurement of the fraction of charged dots in self-assembled quantum dot ensembles using ultrafast pump-probe techniques
Authors: Tartakovskii A. I. , Makhonin M. N. , Cahill J. , Whittaker D. M. , Wells J-P. R. , Fox A. M. , Mowbray D. J. , Skolnick M. S. , Steer M. J. , Groom K. M. , Hopkinson M.
Published: September 2004

We demonstrate that polarization-resolved ultrafast pump-probe spectroscopy can be used to measure accurately (<10%) the fraction of charged dots in quantum dot (QD) ensembles. The method is applied to study the dependence of charging levels in thermally annealed InGaAs QDs, grown both as nominally undoped and n-type modulation-doped structures. We also show that the method can be used to study the spectral distribution of charges (or charging profiles) in quantum dot ensembles.
C
60
thin-film transistors with low work-function metal electrodes
Authors: Chikamatsu Masayuki , Nagamatsu Shuichi , Taima Tetsuya , Yoshida Yuji , Sakai Natsuko , Yokokawa Harumi , Saito Kazuhiro , Yase Kiyoshi
Published: September 2004

We report C60 thin-film transistor characteristics of top-contact structure with low work-function source and drain electrodes. The electron mobility of the Mg electrode device is one order of magnitude higher than that of the Ag electrode device. The depth profile obtained by using secondary-ion mass spectroscopy demonstrates that Mg atoms strongly diffuse into C60 film during Mg deposition. These findings indicate that the improved mobility is due to the reduction of the parasitic resistance under source and drain electrodes by the Mg doping effect.
Action of co-dopant in electron-trapping materials: The case of Sm3+ in Mn2+-activated zinc borosilicate glasses
Authors: Li Chengyu , Su Qiang
Published: September 2004

The action of electron-trapping materials (ETMs) consists of three sequential steps, i.e., the storage, the transfer, and the release of energy. It is generally considered that the co-dopant acts as a trapping center to enhance the storage ability of ETMs, which focus on the first step. In this letter, by the abnormal phosphorescence of Sm3+ in zinc borosilicate glasses doped with samarium and manganese (ZSBMS), we suggest that the co-dopant is able to change the energy transfer in the material.
Defects induced in GaN by europium implantation
Authors: Mamor M. , Matias V. , Vantomme A. , Colder A. , Marie P. , Ruterana P.
Published: September 2004

We have investigated structural and electrical properties of defects introduced during room temperature europium implantation into GaN. Two geometries, random and channeled implantation, were used. Rutherford backscattering and channeling analysis reveals that implantation induces a significantly lower concentration of defects, in the case of channeled implantation. These defects generate a perpendicular expansion of the GaN lattice in the implanted region, as evidenced by x-ray diffraction. From deep-level transient spectroscopy, beside intrinsic defects with energy levels below the conduction band, one additional electron trap, labeled Eu2, is observed at an energy (Ec−0.36eV). It is believed that this defect in n-GaN is europium related.
Signal irreproducibility in high-field solution magnetic resonance experiments caused by spin turbulence
Authors: Huang Susie Y. , Walls Jamie D. , Wang You , Warren Warren S. , Lin Yung-Ya
Published: September 2004

Turbulent spin dynamics arising from the joint action of radiation damping and the distant dipolar field are shown to generate irreproducible measurements in popular high-field, gradient-based magnetic resonance (MR) experiments, undermining the prevailing assumption of essentially predictable observables in MR. Sizeable fluctuations in echo amplitudes are reported and numerically simulated for pulsed gradient spin echo and stimulated echo diffusion measurements. The underlying microscopic dynamical instability is characterized by analysis of the finite-time Lyapunov exponents. Perturbations to the modulated magnetization are shown to render magic-angle gradients ineffective in suppressing signal fluctuations. Alternative approaches are suggested for canceling out the feedback interactions leading to spin turbulence.
Jahn–Teller effect in quasi-two-dimensional doped cuprate antiferromagnets and underdoped high-Tc superconductors (Review)
Authors: Sergeeva G. G. , Soroka A. A.
Published: September 2004

The theoretical aspects of research on the role of Jahn–Teller interactions in quasi-two-dimensional cuprate antiferromagnets and high-Tc superconductors (HTSCs) are analyzed. An analysis is made of results that permit one to establish a link between the quasi-two-dimensional character of the properties of underdoped cuprate HTSCs in the normal and superconducting states and the Jahn–Teller (JT) nature of the divalent copper ions. It is shown that the combination of these two features leads to the existence of a pseudogap state. In underdoped cuprate HTSCs with JT lattice distortions the quasi-two-dimensionality gives rise to two-dimensional local and quasilocal states of the charge carriers. This is manifested in substantial temperature dependence of the number of components of the localized and delocalized states of the charge carriers and in repeated dynamical reduction of the dimensionality of underdoped cuprate HTSCs as the temperature is lowered. Such a HTSC, with doping concentrations less than optimal, is found in a quasi-two-dimensional state in the greater part of its phase diagram, both in the normal and superconducting states. This means that the superconducting state of underdoped cuprate HTSCs differs from the BCS state and is closer in its properties to the state of a two-dimensional Berezinskii–Kosterlitz–Thouless (BKT) superconductor without off-diagonal long-range order (ODLRO). It is shown that the difference primarily consists in the mechanism of superconductivity. In spite of the fact that a strong JT electron–phonon interaction in underdoped cuprate HTSCs plays a key role and leads to the formation of two-site JT polarons, the attraction between holes and such polarons and the formation of a superfluid two-site JT polaron with an antiferromagnetic core are due to compensation of the Coulomb repulsion by the polaron energy shift. The hypothesis that the superconducting state in overdoped cuprate HTSCs is a consequence of the establishment of ODLRO in the three-dimensional BCS model with nonconserved total number of charge carriers and nonzero quantum fluctuations of the number of charge carriers is discussed.
Heat capacity of cylindrically confined helium: theoretical predictions versus experimental data
Authors: Chalyy K. A.
Published: September 2004

Different systems which could exhibit size-dependent second-order phase transitions have been studied experimentally during the last quarter century. The validity of the proposed theoretical results is verified by comparing high-resolution experimental data with an analytical evaluation of the heat capacity of confined He4. It is shown that the theoretical approach to the problem of the finite-size effect gives results that reasonably match the experimental data over a wide range of system sizes, from tens of nanometers up to a few micrometers for the cylindrical type of confinement geometry. The dependences of the shift of transition temperature on the cylinder size and boundary conditions are analyzed. The agreement of the results with finite-size scaling theory is confirmed.
Unified equation of state of He3–4He liquid mixtures over the whole concentration range at temperatures of 2.25–4.2 K and pressures up to 10 MPa
Authors: Sibileva R. M. , Karnatsevich L. V. , Khazhmuradov M. A. , Meriuz A. V.
Published: September 2004

A unified empirical equation of state for He3–4He mixtures is found in analytical form on the basis of the existing experimental P–V–T data for pure He3, He4, and their mixtures in the homogeneous liquid phase in the temperature interval 2.25–4.2 K and pressure interval 0–10 MPa. An algorithm for calculating the fitting coefficients of an approximating expression for P(V,T,c) is constructed. The average absolute error for the determination of P is ±0.015 MPa.
Magnetic and transport properties controlled by structural disorder in La0.7Ca0.3MnO3 films
Authors: Prokhorov V. G. , Komashko V. A. , Kaminsky G. G. , Svetchnikov V. L. , Lee Y. P. , Park S. Y.
Published: September 2004

The magnetic properties of an amorphous, a partially disordered, and a lattice-strained crystalline La0.7Ca0.3MnO3 film are investigated. It is shown that the amorphous film exhibits Curie–Weiss-type paramagnetism with the effective magnetic moment of 4.2 μB/Mn ion and a small ferromagnetic contribution governed by the formation of quasi-two-dimensional crystalline interfacial inclusions. The crystalline film with randomly oriented nanocrystalline inclusions demonstrates a superposition of ferromagnetic character (in the crystalline matrix) and superparamagnetic character (in the inclusions). The fitted average size of the superparamagnetic particles in the case of a Langevin function is coincident with that of the nanocrystalline clusters revealed in high-resolution electron-microscopy images. An increase in the applied magnetic field leads to a reduction in the average magnetic moment of the superparamagnetic particles, which is due to an enhancement of the ferromagnetic coupling between the individual randomly oriented crystallites. The completely crystalline film undergoes only a ferromagnetic transition with a saturation magnetization at 5 K of 2.73 μB/Mn ion.