bluuebunny/crossref_metadata_2025 huggingface dataset:
Phase behavior of polymer/nanoparticle blends near a substrate
Authors: McGarrity E. S. , Frischknecht A. L. , Mackay M. E.
Published: April 2008

We use the recent fluids density functional theory of Tripathi and Chapman [Phys. Rev. Lett. 94, 087801 (2005); J. Chem. Phys. 122, 094506 (2005)] to investigate the phase behavior of athermal polymer/nanoparticle blends near a substrate. The blends are modeled as a mixture of hard spheres and freely jointed hard chains, near a hard wall. There is a first order phase transition present in these blends in which the nanoparticles expel the polymer from the surface to form a monolayer at a certain nanoparticle concentration. The nanoparticle transition density depends on the length of the polymer, the nanoparticle diameter, and the overall bulk density of the system. The phase transition is due to both packing entropy effects related to size asymmetry between the components and to the polymer configurational entropy, justifying the so-called “entropic push” observed in experiments. In addition, a layered state is found at higher densities which resembles that in colloidal crystals, in which the polymer and nanoparticles form alternating discrete layers. We show that this laminar state has nearly the same free energy as the homogeneously mixed fluid in the bulk and is nucleated by the surface.
On the reality of residual entropies of glasses and disordered crystals
Authors: Goldstein Martin
Published: April 2008

We show that the hypothesis that the configurational entropy of a liquid disappears when it is kinetically frozen into a single glass state implies a spontaneous decrease of entropy. We show further that this is not an innocuous exception to the second law that requires a slight modification of that law, but rather implies directly the possibility of an uncompensated conversion of heat to work. We also note that the number of microstates visited in the course of a measurement does not determine the entropy, but rather that this number is always an inconceivably small fraction of the accessible microstates. Various experimental and computational tests of the possible vanishing of configurational entropy on kinetic arrest (e.g., from studies of glass vapor pressure and solubilities, and the coexistence curve between a disordered crystal and the liquid), with the exception of one questionable case, are consistent with the view that it does not vanish. We then show that this result is actually required by the second law. These considerations apply equally to the residual entropy of disordered crystals.
Spatial coherence resonance on diffusive and small-world networks of Hodgkin–Huxley neurons
Authors: Sun Xiaojuan , Perc Matjaž , Lu Qishao , Kurths Jürgen
Published: April 2008

Spatial coherence resonance in a spatially extended system that is locally modeled by Hodgkin–Huxley (HH) neurons is studied in this paper. We focus on the ability of additive temporally and spatially uncorrelated Gaussian noise to extract a particular spatial frequency of excitatory waves in the medium, whereby examining the impact of diffusive and small-world network topology that determines the interactions amongst coupled HH neurons. We show that there exists an intermediate noise intensity that is able to extract a characteristic spatial frequency of the system in a resonant manner provided the latter is diffusively coupled, thus indicating the existence of spatial coherence resonance. However, as the diffusive topology of the medium is relaxed via the introduction of shortcut links introducing small-world properties amongst coupled HH neurons, the ability of additive Gaussian noise to evoke ordered excitatory waves deteriorates rather spectacularly, leading to the decoherence of the spatial dynamics and with it related absence of spatial coherence resonance. In particular, already a minute fraction of shortcut links suffices to substantially disrupt coherent pattern formation in the examined system.
Effect of the sliding orientation on the tribological properties of polyethylene in molecular dynamics simulations
Authors: Heo Seong Jun , Jang Inkook , Barry Peter R. , Phillpot Simon R. , Perry Scott S. , Sawyer W. Gregory , Sinnott Susan B.
Published: April 2008

The role of sliding orientation on the tribological properties of polyethylene (PE) is investigated by using classical molecular dynamics simulations. Cross-linked PE surfaces slide against one another in two different directions: one that is perpendicular to and one that is parallel to the aligned direction of the polymer chains. The results indicate that sliding in the parallel direction occurs with a lower friction coefficient than sliding in the perpendicular direction. In both cases, gross level stick-slip motion is observed to be associated with the sliding of a restrained, corrugated molecular interface. In addition, the simulations demonstrate the way in which the system stores more shear strain energy during sliding in the perpendicular direction. The tribological behavior of these PE surfaces is compared to the behavior of similarly modeled polytetrafluoroethylene surfaces; the differences and similarities between the two systems are discussed.
Vacuum ellipsometry as a method for probing glass transition in thin polymer films
Authors: Efremov Mikhail Yu. , Soofi Shauheen S. , Kiyanova Anna V. , Munoz Claudio J. , Burgardt Peter , Cerrina Franco , Nealey Paul F.
Published: April 2008

A vacuum ellipsometer has been designed for probing the glass transition in thin supported polymer films. The device is based on the optics of a commercial spectroscopic phase-modulated ellipsometer. A custom-made vacuum chamber evacuated by oil-free pumps, variable temperature optical table, and computer-based data acquisition system was described. The performance of the tool has been demonstrated using 20–200nm thick poly(methyl methacrylate) and polystyrene films coated on silicon substrates at 10−6–10−8torr residual gas pressure. Both polymers show pronounced glass transitions. The difficulties in assigning in the glass transition temperature are discussed with respect to the experimental challenges of the measurements in thin polymer films. It is found that the experimental curves can be significantly affected by a residual gas. This effect manifests itself at lower temperatures as a decreased or even negative apparent thermal coefficient of expansion, and is related to the uptake and desorption of water by the samples during temperature scans. It is also found that an ionization gauge—the standard accessory of any high vacuum system—can cause a number of spurious phenomena including drift in the experimental data, roughening of the polymer surface, and film dewetting.
Mini-conference on angular momentum transport in laboratory and nature
Authors: Ji Hantao , Kronberg Philipp , Prager Stewart C. , Uzdensky Dmitri A. ,   Center for Magnetic Self-organization in Laboratory and Astrophysical Plasmas
Published: April 2008

This paper provides a concise summary of the current status of the research and future perspectives discussed in the Mini-Conference on Angular Momentum Transport in Laboratory and Nature. This mini-conference, sponsored by the Topical Group on Plasma Astrophysics, was held as part of the American Physical Society’s Division of Plasma Physics 2007 Annual Meeting (November 12–16, 2007). This mini-conference covers a wide range of phenomena happening in fluids and plasmas, either in laboratory or in nature. The purpose of this paper is not to review these phenomena comprehensively, but to provide a starting point for interested readers to refer to related research in areas other than their own.
Reducible expansions and related sharp crossovers in Feigenbaum’s renormalization field
Authors: Moon Ki-Jung , Choi Sang Don
Published: April 2008

We discuss reducible aspects of Mao and Hu’s multiple scaling expansion [J. Stat. Phys. 46, 111 (1987); Int. J. Mod. Phys. B 2, 65 (1988)] in the framework of renormalization theory. After establishing a suitable form of reduced expansion, we present numerical evidence showing sharp crossovers from Feigenbaum's constant (δ) to Mao and Hu’s constant (δ′) in the first-order reduced expansion. We find that the crossover is caused by the universal scaling relation existing in constant coefficients of Mao and Hu’s expansion. Special attention is paid to constant coefficients corresponding to scaling terms including δ′. We show numerically that they converge to zero in universal ways with convergence ratios larger than δ. Here, the convergence direction is transversal to the unstable eigendirection of the linearized renormalization operator. From this observation, we propose a concise form of expansion for Feigenbaum’s universal function gr(x).
High pressure–high temperature phase diagram of ammonia
Authors: Ninet S. , Datchi F.
Published: April 2008

The high pressure (P)–high temperature (T) phase diagram of solid ammonia has been investigated using diamond anvil cell and resistive heating techniques. The III-IV transition line has been determined up to 20GPa and 500K both on compression and decompression paths. No discontinuity is observed at the expected location for the III-IV-V triple point. The melting line has been determined by visual observations of the fluid-solid equilibrium up to 9GPa and 900K. The experimental data are well fitted by a Simon–Glatzel equation in the covered P-T range. These transition lines and their extrapolations are compared to the reported ab initio calculations.
Time-dependent Pauli equation in the presence of the Aharonov–Bohm effect
Authors: Bouguerra Y. , Bounames A. , Maamache M. , Saadi Y.
Published: April 2008

We use the Lewis–Riesenfeld theory to determine the exact form of the wavefunctions of a two-dimensional Pauli equation of a charged spin 1∕2 particle with time-dependent mass and frequency in the presence of the Aharonov–Bohm effect and a two-dimensional time-dependent harmonic oscillator. We find that the irregular solution at the origin as well as the regular one contributes to the phase of the wavefunction.
Magnetic nozzle and plasma detachment model for a steady-state flow
Authors: Breizman B. N. , Tushentsov M. R. , Arefiev A. V.
Published: April 2008

Plasma propulsion concepts that employ a guiding magnetic field raise the question of how the magnetically controlled plasma can detach from the spacecraft. This paper presents a detachment scenario relevant to high-power thrusters in which the plasma can stretch the magnetic field lines to infinity, similar to the solar wind. In previous work, the corresponding ideal magnetohydrodynamics equations have been solved analytically for a plasma flow in a slowly diverging nozzle. That solution indicates that efficient detachment is feasible if the nozzle is sufficiently long. In order to extend the previous model beyond the idealizations of analytical theory, a Lagrangian code is developed in this work to simulate steady-state kinetic plasma flows and to evaluate nozzle efficiency. The code is benchmarked against the analytical results and then used to examine situations that are not analytically tractable, including plasma behavior in the recent Detachment Demonstration Experiment at the National Aeronautics and Space Administration.
Solving the electron-nuclear Schrödinger equation of helium atom and its isoelectronic ions with the free iterative-complement-interaction method
Authors: Nakashima Hiroyuki , Nakatsuji Hiroshi
Published: April 2008

Our previous paper [J. Chem. Phys. 127, 224104 (2007)] revealed that the Schrödinger equation in the fixed-nucleus approximation could be very accurately solved for helium atom and its isoelectronic ions (Z=1–10) with the free iterative-complement-interaction (ICI) method combined with the variation principle. In this report, the quantum effect of nuclear motion has further been variationally considered by the free ICI formalism for the Hamiltonian including mass-polarization operator. We obtained −2.903304557729580294733816943892697752659273965a.u. for helium atom, which is over 40 digits in accuracy, similarly to the previous result for the fixed-nucleus level. Similar accuracy was also obtained for the helium isoelectronic ions. The present results may be regarded to be the nonrelativistic limits. We have further analyzed the physics of the free ICI wave function by applying it to an imaginary atom called “eneon,” [e−e10+e−]8+, in which both of the quantum effect of nuclear motion and the three-particle collisions are differently important from the helium and its isoelectronic ions. This revealed the accurate physics automatically generated by the free ICI formalism.
Surface relaxation in water clusters: Evidence from theoretical analysis of the oxygen 1s photoelectron spectrum
Authors: Abu-samha M. , Børve K. J.
Published: April 2008

We present a theoretical interpretation of the oxygen 1s photoelectron spectrum published by Öhrwall et al. [J. Chem. Phys. 123, 054310 (2005)]. A water cluster that contains 200molecules was simulated at 215K using the polarizable AMOEBA force field. The force field predicts longer O⋯O distances at the cluster surface than in the bulk. Comparisons to ab initio molecular dynamics (MD) simulations indicate that the force field underestimates the degree of surface relaxation. By comparing cluster lineshape models, computed from MD simulations, to the experimental spectrum we find further evidence of surface relaxation.
Effect of the evanescent modes on ballistic thermal transport in quantum structures
Authors: Xie Fang , Chen Ke-Qiu , Wang Y. G. , Zhang Yan
Published: April 2008

Using the scattering matrix method, we investigate the effect of the evanescent modes on ballistic transport probability and thermal conductance in T-shaped and concavity-shaped quantum structures at low temperatures. The results show that the evanescent modes play different roles in the transport possibility and the thermal conductance in both T-shaped and concavity-shaped quantum structures. The evanescent modes can enhance thermal conductance in the concavity-shaped structure and suppress thermal conductance in the T-shaped structure. A brief analysis of these results is given.
Thermal conductivity of nanofluids: Effects of graded nanolayers and mutual interaction
Authors: Zhou X. F. , Gao L.
Published: April 2008

It has been demonstrated there exists an interfacial nanolayer at the nanoparticles/fluid interface, and the nanoparticles in the base fluid aggregate easily and form clusters. As a result, both the interfacial nanolayer and the mutual interaction between nanoparticles become important. To account for the interfacial nanolayers, we first generalize differential effective dipole approximation to obtain the equivalent thermal conductivity of the coated nanoparticles with graded nanolayers. Then we employ multiple image method to investigate the effect of mutual interaction between nanoparticles on the thermal conductivity of nanofluids. Analytical formulae allow us to check the effect of nanolayer thickness, particle size, and mutual interaction. Numerical results show that when nanoparticles close up and even get touched, the effective thermal conductivity is slightly enhanced in comparison with that predicted from Maxwell–Garnett theory. The nanolayer properties also play a more important role in the conductivity enhancement. In addition, theoretical results on the effective thermal conductivity of nanofluids are in good agreement with experimental data.
Impact of semiconductor/contact metal thickness ratio on organic thin-film transistor performance
Authors: Gowrisanker S. , Ai Y. , Quevedo-Lopez M. A. , Jia H. , Alshareef H. N. , Vogel E. , Gnade B.
Published: April 2008

Pentacene-based organic thin-film transistors have been fabricated using photolithography and the bottom contact structure using parylene as the gate dielectric. Device performance was optimized by varying the thickness ratio of pentacene to Au contacts. Contact resistance dependence on the pentacene/Au thickness ratio (r=tpen∕tAu) was extracted using the transfer-length method [D. K. Schroder, Semiconductor Material and Device Characterization (Wiley, New York, 2006), pp. 184–199; Necliudov et al., Solid-State Electron. 47, 259 (2003)]. In this paper, we show that the effect of parasitic resistance and pentacene film morphology on device performance can be decoupled. Film morphology and microstructure are reported as a function of pentacene thickness and the correlation with transistor field-effect mobility is discussed.
Magnetic exchange couplings in the single-molecule magnet of Mn12–Ac
Authors: Zhang Junrong , He Lunhua , Cao Huibo , Wang Fangwei , Zhang Panlin
Published: April 2008

Four types of isotropic exchange interactions of Mn12–Ac are obtained by using the classical Monte Carlo simulations. The equilibrium susceptibilities are well reproduced in the temperature range between 10 and 100K. The calculated effective spin at 0.1K coincides with the ground-state spin. Our results show that J1 and J2 are strong antiferromagnetic, but the magnitude of J2 is much smaller than that of J1. Both J3 and J4 favor weakly antiferromagnetic couplings. The effects of the exchange couplings on the magnetic properties and ground-state spin are investigated too. The magnetic susceptibilities below 100K depend more on J2 rather than on the stronger J1. The weak exchange couplings J3 and J4 have significant frustration effects on the ground-state configuration.
A theoretical study of a spin polarized transport and giant magnetoresistance: The effect of the number of layers in a magnetic multilayer
Authors: Noh Eun Sun , Lee Hyuck Mo , Lee Seung-Cheol , Ulloa Sergio E.
Published: April 2008

This study presents a quantum-mechanical free electron model for analyzing a spin polarized transport and current-perpendicular-to-the-plane giant magnetoresistance (CPP-GMR) in a more realistic way. The CPP-GMR is evaluated by using three spin resolved conductance parameters based on the Landauer conductance formula. In a ballistic regime, a transfer-matrix method is used to calculate the spin dependent transmission probability as a function of the transverse mode. A spin dependent conduction band structure is constructed by extracting parameters of the free electron model, such as the atomic magnetic moments and the conduction electron densities, from the spin dependent layer-decomposed density of states of the Cu and Co interfacial layers in a Cu5/Co11 slab; these calculations are derived from the density functional theory. As a result, this study shows that the CPP-GMR in a [Cu(5ML)∕Co(11ML)]n magnetic multilayer (n=2–5) with a 35ML×35ML cross section is in the range of 60%–111%. It is qualitatively comparable to the calculation results of first principles. This study also uses transmission probability to explain the increase of spin dependent scattering and CPP-GMR as a function of the number of layers in the [Cu∕Co]n magnetic multilayer. Moreover, the study confirms that modification of the free electron model by quantum-mechanical methods can be applied to calculations of a spin polarized transport and CPP-GMR in a specific material system.
Radial distribution of the fictive temperature in pure silica optical fibers by micro-Raman spectroscopy
Authors: Martinet C. , Martinez V. , Coussa C. , Champagnon B. , Tomozawa M.
Published: April 2008

Micro-Raman spectra are performed at different radial positions in order to determine the fictive temperature distribution in pure silica optical fibers. As-received and annealed optical fibers are compared and Raman results reveal a large decrease in the fictive temperature after annealing. The fictive temperature gradient between the core and the edge of the optical fiber can be eliminated after annealing and the spectroscopic decoupling between stress and the fictive temperature variation is discussed. The micro-Raman is a good probe in determining the spatial distribution of the fictive temperature in the micrometer length scale.
Observation of the three-dimensional distribution of flux pinning centers in Dy-doped YBa2Cu3O7−x coated conductors
Authors: Herrera M. , Ortalan V. , Morgan D. G. , Browning N. D. , Rupich M. W.
Published: April 2008

The spatial distribution of flux pinning centers (such as defects, secondary phases, etc.) has a critical effect on the superconducting properties of YBa2Cu3O7−x (YBCO) coated conductors. To increase the flux pinning in coated conductors, impurities such as Dy can be intentionally added during processing to induce the formation of distributed insulating nanostructures. In this work, we show that the addition of Dy to a particular YBCO coated conductor gives rise to a high density of secondary nanoparticles of composition (YsDy1−s)2Cu2O5 with s∼0.6. Using high-angle annular dark-field tomographic methods, the three-dimensional distribution of these insulating nanoparticles is determined. The size of the nanoparticles and the uniformity of their distribution are found to depend on their interaction with the grain boundary network within the YBCO layer. A bimodal distribution in the size of the particles is seen, where areas away from grain boundaries have a favorable uniform distribution of small particles (∼25 nm) while unfavorable isolated large particles 100–140 nm are seen to intersect the grain boundaries. A grain boundary diffusion model is proposed to account for the agglomeration of the small nanoparticles to form the large ones to the detriment of flux pinning within these local regions of the superconductor.
Microscopic anisotropy revealed by NMR double pulsed field gradient experiments with arbitrary timing parameters
Authors: Özarslan Evren , Basser Peter J.
Published: April 2008

We consider a general double pulsed field gradient experiment with arbitrary experimental parameters and calculate an exact expression for the NMR signal attenuation from restricted geometries, which is valid at long wavelengths, i.e., when the product of the gyromagnetic ratio of the spins, the pulsed gradients’ duration, and their magnitude is small compared to the reciprocal of the pore size. It is possible to observe microscopic anisotropy within the pore space induced by the boundaries of the pore, which can be used to differentiate restricted from free or multicompartmental diffusion and to estimate a characteristic pore dimension in the former case. Explicit solutions for diffusion taking place between parallel plates as well as in cylindrical and spherical pores are provided. In coherently packed cylindrical pores, it is possible to measure simultaneously the cylinders’ orientation and diameter using small gradient strengths. The presence of orientational heterogeneity of cylinders is addressed, and a scheme for differentiating microscopic from ensemble anisotropy is proposed.
Effective dielectric tensor for electromagnetic wave propagation in random media
Authors: Rechtsman M. C. , Torquato S.
Published: April 2008

We derive exact strong-contrast expansions for the effective dielectric tensor εe of electromagnetic waves propagating in a two-phase composite random medium with isotropic components explicitly in terms of certain integrals over the n-point correlation functions of the medium. Our focus is the long-wavelength regime, i.e., when the wavelength is much larger than the scale of inhomogeneities in the medium. Lower-order truncations of these expansions lead to approximations for the effective dielectric constant that depend upon whether the medium is below or above the percolation threshold. In particular, we apply two- and three-point approximations for εe to a variety of different three-dimensional model microstructures, including dispersions of hard spheres, hard oriented spheroids, and fully penetrable spheres as well as Debye random media, the random checkerboard, and power-law-correlated materials. We demonstrate the importance of employing n-point correlation functions of order higher than two for high dielectric-phase-contrast ratio. We show that disorder in the microstructure results in an imaginary component of the effective dielectric tensor that is directly related to the coarseness of the composite, i.e., local-volume-fraction fluctuations for infinitely large windows. The source of this imaginary component is the attenuation of the coherent homogenized wave due to scattering. We also remark on whether there is such attenuation in the case of a two-phase medium with a quasiperiodic structure.
Annealing effects on electrical properties of MgZnO films grown by pulsed laser deposition
Authors: Polyakov A. Y. , Smirnov N. B. , Govorkov A. V. , Kozhukhova E. A. , Belogorokhov A. I. , Kim H. S. , Norton D. P. , Pearton S. J.
Published: April 2008

Electrical properties and deep trap spectra are reported for MgZnO(P) films grown by pulsed laser deposition on undoped n-ZnO substrates. The as-grown films are n type with a “bulk” donor concentration of ∼1018cm−3 and have a compensated high resistivity layer near the surface. Deep trap spectra in these films are dominated by electron traps with an activation energy of 0.3eV, hole traps with an activation energy of 0.14eV, and some unidentified electron traps with a barrier for capture of electrons. After annealing in oxygen at 800°C the MgZnO(P) becomes p type, with the dominant hole traps having an activation energy of 0.2eV. The space charge region of the formed p-n junction is mainly located in the n-ZnO substrate. The main hole traps in this part of the heterojunction have activation energies of 0.14 and 0.84eV, while the main electron traps have activation energies of 0.15 and 0.3eV.
Magnetic microposts for mechanical stimulation of biological cells: Fabrication, characterization, and analysis
Authors: Sniadecki Nathan J. , Lamb Corinne M. , Liu Yaohua , Chen Christopher S. , Reich Daniel H.
Published: April 2008

Cells use force as a mechanical signal to sense and respond to their microenvironment. Understanding how mechanical forces affect living cells requires the development of tool sets that can apply nanoscale forces and also measure cellular traction forces. However, there has been a lack of techniques that integrate actuation and sensing components to study force as a mechanical signal. Here, we describe a system that uses an array of elastomeric microposts to apply external forces to cells through cobalt nanowires embedded inside the microposts. We first biochemically treat the posts’ surfaces to restrict cell adhesion to the posts’ tips. Then by applying a uniform magnetic field (B<0.3T), we induce magnetic torque on the nanowires that is transmitted to a cell’s adhesion site as an external force. We have achieved external forces of up to 45nN, which is in the upper range of current nanoscale force-probing techniques. Nonmagnetic microposts, similarly prepared but without nanowires, surround the magnetic microposts and are used to measure the traction forces and changes in cell mechanics. We record the magnitude and direction of the external force and the traction forces by optically measuring the deflection of the microposts, which linearly deflect as cantilever springs. With this approach, we can measure traction forces before and after force stimulation in order to monitor cellular response to forces. We present the fabrication methods, magnetic force characterization, and image analysis techniques used to achieve the measurements.
Experimental evidence for a surface distribution of two-level systems in superconducting lithographed microwave resonators
Authors: Gao Jiansong , Daal Miguel , Vayonakis Anastasios , Kumar Shwetank , Zmuidzinas Jonas , Sadoulet Bernard , Mazin Benjamin A. , Day Peter K. , Leduc Henry G.
Published: April 2008

We present measurements of the temperature-dependent frequency shift of five niobium superconducting coplanar waveguide microresonators with center strip widths ranging from 3to50μm, taken at temperatures in the range of 100–800mK, far below the 9.2K transition temperature of niobium. These data agree well with the two-level system (TLS) theory. Fits to this theory provide information on the number of TLSs that interact with each resonator geometry. The geometrical scaling indicates a surface distribution of TLSs and the data are consistent with a TLS surface layer thickness of the order of a few nanometers, as might be expected for a native oxide layer.
Competing sigmatropic shift rearrangements in excited allyl radicals
Authors: Stranges D. , O’Keeffe P. , Scotti G. , Di Santo R. , Houston P. L.
Published: April 2008

The competition between rearrangement of the excited allyl radical via a 1,3 sigmatropic shift versus sequential 1,2 shifts has been observed and characterized using isotopic substitution, laser excitation, and molecular beam techniques. Both rearrangements produce a 1-propenyl radical that subsequently dissociates to methyl plus acetylene. The 1,3 shift and 1,2 shift mechanisms are equally probable for CH2CHCH2, whereas the 1,3 shift is favored by a factor of 1.6 in CH2CDCH2. The translational energy distributions for the methyl and acetylene products of these two mechanisms are substantially different. Both of these allyl dissociation channels are minor pathways compared to hydrogen atom loss.
Interfacial strain effects in epitaxial multiferroic heterostructures of PbZrxTi1−xO3∕La0.7Sr0.3MnO3 grown by pulsed-laser deposition
Authors: Vrejoiu Ionela , Ziese Michael , Setzer Annette , Esquinazi Pablo D. , Birajdar Balaji I. , Lotnyk Andriy , Alexe Marin , Hesse Dietrich
Published: April 2008

Ferroelectric PbZrxTi1−xO3 and ferromagnetic La0.7Sr0.3MnO3 films were grown on SrTiO3(100) substrates in order to fabricate multiferroic epitaxial heterostructures. Multilayers of PbZr0.2Ti0.8O3∕La0.7Sr0.3MnO3 with 5nm thin individual layers preserve good magnetic properties and have a transition temperature of ∼320K. The variation of the magnetic coercive field of thin La0.7Sr0.3MnO3 films, sandwiched between PbZrxTi1−xO3 films of increasing Zr content in the same epitaxial heterostructure, demonstrates the influence of the interfacial biaxial strain.
Degradation of InGaN∕GaN laser diodes analyzed by microphotoluminescence and microelectroluminescence mappings
Authors: Rossetti M. , Smeeton T. M. , Tan W.-S. , Kauer M. , Hooper S. E. , Heffernan J. , Xiu H. , Humphreys C. J.
Published: April 2008

The degradation of InGaN∕GaN laser diodes grown by molecular beam epitaxy is analyzed by using surface mapping of the photoluminescence emission on a micrometric scale, which allows the identification of failure regions. This, combined with the mapping of the electroluminescence, suggests two different mechanisms for laser degradation. Increased nonradiative recombination at specific sites along the crystal directions associated with the presence of dislocations lying in the basal plane near the active region is one of these. We also observe an increase in current injection nonuniformities with increasing aging time.
Mitigation of thermal and fatigue behavior in K0.5Na0.5NbO3-based lead free piezoceramics
Authors: Zhang Shujun , Xia Ru , Hao Hua , Liu Hanxing , Shrout Thomas R.
Published: April 2008

K 0.5 Na 0.5 Nb O 3 - ( KNN ) based lead free materials have been found to exhibit good piezoelectric properties (d33∼250pC∕N) due to the orthorhombic-tetragonal polymorphic phase transition (PPT) temperature compositionally shifted downward to near room temperature. However, associated with the PPT are issues of temperature and domain instability, making them impractical for applications. In this work, CaTiO3 (CT) was used to effectively shift the PPT downward in effort to mitigate these issues. As expected, CT modified KNN based materials exhibited nearly temperature independent properties (−50∼200°C) and fatigue-free behavior, together with its relatively high d33 value of ∼200pC∕N, make the CT modified KNN based materials excellent candidates for lead free actuators and transducers.
A streamer-like atmospheric pressure plasma jet
Authors: Sands Brian L. , Ganguly Biswa N. , Tachibana Kunihide
Published: April 2008

The properties of an atmospheric pressure plasma jet (APPJ) are examined in a single-cell dielectric capillary configuration. In contrast to some other flow-driven APPJs, this stable, cold plasma jet is electrically driven, composed of rapidly propagating ionization fronts with speeds of the order of 107cm∕s. Using spatially and temporally resolved optical diagnostics, it is demonstrated that the plasma jet is initiated independent of the dielectric barrier discharge inside the capillary. It is also shown that the properties and dynamics of this APPJ are directly analogous to those of positive corona streamer discharges.
Biophysics in reverse: Using blood cells to accurately calibrate force-microscopy cantilevers
Authors: Heinrich Volkmar , Ounkomol Chawin
Published: April 2008

We report on the refinement and validation of one of the earliest ideas of “reverse” biophysics: the use of individual red blood cells as reliable, ultrasensitive mechanotransducers. Our analysis is based on the numerical prediction of the force exerted by a micropipette-held red cell as it is pushed against a test object. Examining this red-cell transducer, in conjunction with a custom-built “horizontal” force microscope, we were able to soundly corroborate its utility, while at the same time, accurately calibrating the spring constants of atomic-force microscope cantilevers.
Very low sheet resistance and Shubnikov–de-Haas oscillations in two-dimensional electron gases at ultrathin binary AlN∕GaN heterojunctions
Authors: Cao Yu , Wang Kejia , Orlov Alexei , Xing Huili , Jena Debdeep
Published: April 2008

Very low sheet resistance two-dimensional electron gases have been obtained at ultrathin AlN∕GaN heterojunctions. By investigating the molecular beam epitaxy growth conditions, the interface roughness at the heterojunction has been reduced, leading to high room-temperature electron mobilities (μ∼1400–1600cm2∕Vs) and high electron sheet densities due to spontaneous and piezoelectric polarization (ns∼1–3×1013cm−2) depending on the AlN thickness. This combination led to a large reduction of the room-temperature sheet resistance of 148Ω∕◻, a new record. The improved transport properties have made it possible to observe Shubnikov–de-Haas oscillations of the magnetoresistance of the two-dimensional electron gas at ultrathin binary AlN∕GaN heterojunctions for the first time. The results are indicative of the vast potential of high-quality AlN∕GaN structures for a variety of device applications.
Laser switching and sorting for high speed digital microfluidics
Authors: Robert de Saint Vincent Matthieu , Wunenburger Régis , Delville Jean-Pierre
Published: April 2008

We used thermocapillary stresses locally induced by laser on flowing drops to build high throughput drop switchers and sorters for digital microfluidics. Since the laser is disconnected to the chip, the method does not require dedicated micropatterning. We show switching efficiencies of 100% for drop velocities up to 1.3cm∕s, demonstrate the involved mechanism, and apply laser switching for sorting droplets of different natures for lab-on-a-chip applications.
Inversion mode n-channel GaAs field effect transistor with high-k/metal gate
Authors: de Souza J. P. , Kiewra E. , Sun Y. , Callegari A. , Sadana D. K. , Shahidi G. , Webb D. J. , Fompeyrine J. , Germann R. , Rossel C. , Marchiori C.
Published: April 2008

Highly effective passivation of GaAs surface is achieved by a thin amorphous Si (a-Si) cap, deposited by plasma enhanced chemical vapor deposition method. Capacitance voltage measurements show that carrier accumulation or inversion layer is readily formed in response to an applied electrical field when GaAs is passivated with a-Si. High performance inversion mode n-channel GaAs metal-oxide-semiconductor field-effect transistors (MOSFETs) were fabricated with an a-Si/high-k/metal gate stack. Drain current in saturation region of 220mA∕mm with a mobility of 885cm2∕Vs were obtained at a gate overdrive voltage of 3.25V in MOSFETs with 5μm gate length.
Raman spectroscopy of single nanodiamond: Phonon-confinement effects
Authors: Sun K. W. , Wang J. Y. , Ko T. Y.
Published: April 2008

In this paper, we devise techniques for immobilizing and allocating a single nanodiamond on the electron beam (e-beam) patterned smart substrate. The properly designed coordination markers on the semiconductor substrate and the high throughput of the confocal microscope provide us with a convenient tool to single out a nanodiamond with a size less than 100nm and to study Raman spectroscopy. We observe a redshift in energy and broadening in the linewidth of the sp3 bonding Raman peak when the size of the diamond is decreased from 90to35nm. The observed shifts and linewidth broadening arise from the phonon-confinement effects and are in good agreement with calculations reported by [Ager et al. Phys. Rev. B 43, 6491 (1991)] and [Yoshikawa et al. Appl. Phys. Lett. 62, 3114 (1993)].
Current-induced tunnel magnetoresistance due to spin accumulation in Au nanoparticles
Authors: Mitani S. , Nogi Y. , Wang H. , Yakushiji K. , Ernult F. , Takanashi K.
Published: April 2008

Spin-dependent single electron tunneling was investigated in a magnetic double tunnel junction including Au nanoparticles as a center electrode. Tunnel magnetoresistance (TMR) clearly emerged with increasing spin-polarized current injected into Au nanoparticles and reached a maximum value of about 12% at 4.2K. The observation indicates that spin accumulation occurs in Au nanoparticles and causes TMR. The spin relaxation time in Au nanoparticles, as estimated from the critical current for the appearance of TMR, is of the order of 10ns, which is much longer than that in the bulk state.
Fabrication and characteristics of ultrashort-channel carbon nanotube field-effect transistors
Authors: Chiu Po-Wen , Chen Kuei-Jyun , Lin Chih-Yueh
Published: April 2008

We describe a technique for fabricating ultrashort-channel carbon nanotube field-effect transistors, using individual V2O5 nanowires as evaporation masks to define the conduction channel. The flexible and neat nanowires can be readily oriented to cross over the underlying nanotubes and produce a uniform nanogap of ∼10nm. The transport through such ultrashort nanotube devices shows a significant increase of subthreshold slope and leakage current in the off state, both of which are not observed in long-channel regime. These effects become more pronounced as the channel length is further shrunk and set fundamental constraints in transistor function of the current architecture.
High efficiency β radioisotope energy conversion using reciprocating electromechanical converters with integrated betavoltaics
Authors: Duggirala Rajesh , Li Hui , Lal Amit
Published: April 2008

We demonstrate a 5.1% energy conversion efficiency Ni63 radioisotope power generator by integrating silicon betavoltaic converters with radioisotope actuated reciprocating piezoelectric unimorph cantilever converters. The electromechanical energy converter efficiently utilizes both the kinetic energy and the electrical charge of the 0.94μW β radiation from a 9mCi Ni63 thin film source to generate maximum (1) continuous betavoltaic electrical power output of 22nW and (2) pulsed piezoelectric electrical power output of 750μW at 0.07% duty cycle. The electromechanical converters can be potentially used to realize 100year lifetime power sources for powering periodic sampling remote wireless sensor microsystems.
A single electrode room-temperature plasma jet device for biomedical applications
Authors: Lu XinPei , Jiang ZhongHe , Xiong Qing , Tang ZhiYuan , Pan Yuan
Published: April 2008

A single electrode room-temperature atmospheric pressure plasma plume generated between a high-voltage electrode and the surrounding room air is reported. The plasma plume has a peak current of about 360mA. This is highest current carried by a room-temperature plasma plume ever reported. The rotational and vibrational temperature of the plasma plume is about 300 and 2950K, respectively. Emission spectra show that excited species, such as O, OH, N2+, etc., are present in the plasma plume.
A mass balance inventory of mercury in the Arctic Ocean
Authors: Outridge P. M. , Macdonald R. W. , Wang F. , Stern G. A. , Dastoor A. P.
Published: April 2008

Environmental context. Mercury (Hg) occurs at high concentrations in Arctic marine wildlife, posing a possible health risk to northern peoples who use these animals for food. We find that although the dramatic Hg increases in Arctic Ocean animals since pre-industrial times can be explained by sustained small annual inputs, recent rapid increases probably cannot, because of the existing large oceanic Hg reservoir (the ‘flywheel’ effect). Climate change is a possible alternative force underpinning recent trends. Abstract. The present mercury (Hg) mass balance was developed to gain insights into the sources, sinks and processes regulating biological Hg trends in the Arctic Ocean. Annual total Hg inputs (mainly wet deposition, coastal erosion, seawater import, and ‘excess’ deposition due to atmospheric Hg depletion events) are nearly in balance with outputs (mainly shelf sedimentation and seawater export), with a net 0.3% year–1 increase in total mass. Marine biota represent a small fraction of the ocean’s existing total Hg and methyl-Hg (MeHg) inventories. The inertia associated with these large non-biological reservoirs means that ‘bottom-up’ processes (control of bioavailable Hg concentrations by mass inputs or Hg speciation) are probably incapable of explaining recent biotic Hg trends, contrary to prevailing opinion. Instead, varying rates of bioaccumulation and trophic transfer from the abiotic MeHg reservoir may be key, and are susceptible to ecological, climatic and biogeochemical influences. Deep and sustained cuts to global anthropogenic Hg emissions are required to return biotic Hg levels to their natural state. However, because of mass inertia and the less dominant role of atmospheric inputs, the decline of seawater and biotic Hg concentrations in the Arctic Ocean will be more gradual than the rate of emission reduction and slower than in other oceans and freshwaters. Climate warming has likely already influenced Arctic Hg dynamics, with shrinking sea-ice cover one of the defining variables. Future warming will probably force more Hg out of the ocean’s euphotic zone through greater evasion to air and faster Hg sedimentation driven by higher primary productivity; these losses will be countered by enhanced inputs from coastal erosion and rivers.
Mercury cycling in the Arctic - does enhanced deposition flux mean net-input?
Authors: Ebinghaus Ralf
Published: April 2008

Environmental context. Mercury has unique physico-chemical characteristics that include long-range atmospheric transport, transformation into highly toxic methylmercury species, and the bioaccumulation of these compounds, especially in the marine environment. This has motivated intense international research on mercury as a pollutant of global concern. With respect to Polar regions, scientific interest and research activities were even accelerated after the discovery of the so-called atmospheric mercury depletion events (AMDEs), which are supposed to lead to enhanced mercury deposition flux into these pristine environments in the ecologically very sensitive period in polar spring.
A pragmatic assessment of the usefulness of the MODIS (Terra and Aqua) 1-km active fire (MOD14A2 and MYD14A2) products for mapping fires in the fynbos biome
Authors: de Klerk Helen
Published: April 2008

The moderate resolution imaging spectroradiometer (MODIS) fire-affected area data are not currently available for download by the public; therefore, 1-km pixel MODIS active fire data (MOD14A2 and MYD14A2) were assessed as a potential source of fire history data for the fynbos biome. These data mapped between 60.1 and 65.7% of the area within fire boundaries mapped by reserve managers between July 2002 and April 2005. These results should not be taken as a negative reflection on the MODIS team, as active fires do not aim to map fire-affected area. Rather this exercise has helped identify specific limitations in the fynbos, which will provide insight into the fine-tuning of other MODIS fire products for use in the fynbos. Limitations are likely to be primarily due to sun-glint around coastlines; topographic shadows; highly reflective, light soils exposed after a fire; partial burning of pixels; patchy and irregular-shaped fires; and large pixel size. The MODIS active fire data are not sufficient for the purposes of developing a 4-year fire history of the fynbos. The publication of the 500-m MODIS fire-affected area data is eagerly anticipated as a solution.
Meteorologically influenced wildfire impacts on urban particulate matter and visibility in Tucson, Arizona, USA
Authors: Wise Erika K.
Published: April 2008

Fire managers must consider air-quality impacts when planning prescribed burns or devising wildfire containment strategies. Particulate matter (PM) is the primary pollutant of concern: it is the major component of smoke and has known detrimental influences on human health and the environment. The present study examines wildfire–weather–PM interactions and the resulting impacts on urban air quality and visibility in Tucson, Arizona, USA. Few violations of air-quality standards were recorded during large wildfire events in the study area. When examined at a higher-resolution time scale, the impacts of the fires on urban air quality are apparent. The present study also found that extreme PM values were linked to humid and windy conditions, wildfires appear to have a greater impact on PM10 concentrations than PM2.5 concentrations, and PM10 is more closely tied to visibility degradation during fire events than PM2.5. Comparison of actual PM concentrations to those predicted by a regulatory model indicates that the model overestimates standard exceedances, with resulting implications for prescribed burn planning.
Landscape-scale changes in canopy fuels and potential fire behaviour following ponderosa pine restoration treatments
Authors: Roccaforte John P. , Fulé Peter Z. , Covington W. Wallace
Published: April 2008

We evaluated canopy fuels and potential fire behaviour changes following landscape-scale restoration treatments in a ponderosa pine forest at Mt Trumbull, Arizona, USA. The goal of the project was to restore historical forest structure by thinning and burning, thereby reducing canopy fuels and minimising active crown fire potential. We measured 117 permanent plots before (1996–97) and after (2003) treatments. The plots were evenly distributed across the landscape and represented an area of ~1200 ha, about half of which was an untreated control. We compared canopy fuel estimates using three different methods to assess whether fire behaviour modelling outputs were sensitive to the choice of canopy fuel equation. Treatments decreased canopy fuel load by 43–50% from 0.77–1.83 kg m–2 to 0.44–0.91 kg m–2 (the range of values reflects the different canopy fuel equations) and decreased canopy bulk density by 42–61% from 0.038–0.172 kg m–3 to 0.022–0.067 kg m–3 in the treated area, while slight increases occurred in the control. We applied two fire models to estimate potential fire behaviour: FlamMap and NEXUS. These models differ in several important features but predicted outcomes were consistent: under extreme drought and wind conditions, the proportion of the landscape susceptible to active crown fire decreased in the treated area while little change occurred in the control.