bluuebunny/crossref_metadata_2025 huggingface dataset:
Li ion scattering from Au nanoclusters on TiO2(110)
Authors: Liu G. F. , Sroubek Z. , Karmakar P. , Yarmoff J. A.
Published: August 2006

The neutralization of low energy Li+7 scattered from Au nanoclusters deposited on TiO2(110) was measured with time-of-flight spectroscopy as a function of cluster size, emission angle, and ion energy. The neutralization shows maxima for cluster diameters ∼3nm, and again for thick Au films. The data are compared to previous experiments with Na projectiles. Possible explanations of the observed effects are discussed.
Total dissociative electron attachment cross sections for molecular constituents of DNA
Authors: Aflatooni K. , Scheer A. M. , Burrow P. D.
Published: August 2006

Total cross sections for the dissociative electron attachment process are presented for the DNA bases thymine, cytosine, and adenine and for three compounds used as surrogates for the ribose and phosphate groups, tetrahydrofuran, 3-hydroxytetrahydrofuran, and trimethylphosphate, respectively. Cross section magnitudes are obtained by observation of positive ion production and normalization to ionization cross sections calculated elsewhere using the binary-encounter-Bethe method. The average cross section of the three bases is 3–10 times smaller than the effective cross section per nucleotide reported for single strand breaks in surface-bound supercoiled DNA. Consequently, damage to the bases alone does not appear to account for the major portion of the strand breaks. The presence of an OH group on the ribose surrogate considerably enhances its cross section. Model compounds in which protonation or OH groups are used to terminate bonds may therefore display larger cross sections than in DNA itself.
Nonlinear dynamics of electrostatic ion-temperature-gradient modes in a dust-contaminated plasma with variable charge and sheared ion flows
Authors: Azimi Leila , Mirza Arshad M. , Khan M. Yaqub
Published: August 2006

By employing Braginskii transport equations for ions and Boltzmann distribution for electrons in a dust-contaminated plasma with equilibrium density, temperature, and magnetic field gradients, the nonlinear set of equations are derived. New ion-temperature-gradient driven modes are obtained and various limiting cases are discussed. It is shown that the ion-temperature-gradient driven mode of drift-waves are attenuated in the presence of dust-charge fluctuations. It has been found that dust charging is always dissipative and the growth rate of various modes are damped. Furthermore, the possible stationary solution of the nonlinear mode coupling equations can be represented in the form of dipolar and vortex chains type solutions. The results of the present investigation should be helpful in understanding the fluctuations and transport phenomena in magnetically confined dustcontaminated tokamak plasma.
Optical and mechanical anisotropies of oriented poly(ethylene terephthalate) films
Authors: Schmidegg K. , Sun L. D. , Zeppenfeld P.
Published: August 2006

Reflectance difference spectroscopy (RDS) is used to characterize the optical anisotropy of thin, biaxially oriented poly(ethylene terephthalate) films. Molecular orientation in these films results in birefringence and thus gives rise to a RDS signal, which changes characteristically with external strain. A clear link between the optical and mechanical anisotropies can be established by comparing the azimuthal dependence of the elastic modulus with the RDS signal. Online measurements during tensile tests provide information on the evolution of the optical properties as a function of external strain, thus making RDS a highly sensitive probe for strain in oriented polymers.
High-brightness top-emissive polymer light-emitting diodes utilizing organic oxide/Al∕Ag composite cathode
Authors: Guo Tzung-Fang , Yang Fuh-Shun , Tsai Zen-Jay , Feng Guan-Weng , Wen Ten-Chin , Hsieh Sung-Nien , Chung Chia-Tin , Wu Ching-In
Published: August 2006

This work presents the fabrication of high-brightness (over 30000cd∕m2) top-emissive polymer light-emitting diodes (PLEDs) using a hybrid semitransparent cathode capable of efficient injection of electrons. The composite cathode is comprised of the organic oxide/Al complex as the injection buffer layer covered by a thin Ag overlayer. The anode is made of Ag:Ag2O coated on the glass substrate. The electroluminescence (EL) efficiency of 8.9cd∕A for phenyl-substituted poly(para-phenylene vinylene) copolymer based top-emissive PLED markedly exceeds that of 4.3cd∕A for the control device with the bottom-emissive configuration. The high performance is attributed to the balanced injection of charge carriers and the effective extraction of EL emission from the top cathode. The optical microcavity effect significantly promotes the EL emission in the direction along the surface normal.
Comment on “Coarse-grained interaction potentials for polyaromatic hydrocarbons” [J. Chem. Phys. 124, 054307 (2006)]
Authors: Cinacchi Giorgio
Published: August 2006

It is noticed that optimized potential liquid simulation parameters for benzene show a good transferability in that they give values for interaction energies of polyaromatic molecular systems in good agreement with available experiments. Apart from being employed directly, these all-atom empirical potentials can be used to construct simpler, coarse-grained models for use in classical computer simulations. They might also be of help in calibrating dispersion corrections to a given exchange-correlation density functional for use in quantum chemical molecular dynamics techniques.
Small angle x-ray scattering study of local structure and collapse transition of (1,3)-β-D-glucan-chitosan gels
Authors: Sletmoen Marit , Stokke Bjørn T. , Geissler Erik
Published: August 2006

Scleroglucan is a (1,3)-β-D-glucan polysaccharide produced by the fungus Sclerotium. Dissolved in water at room temperature it adopts a linear, rigid, triple helical structure. Gelation of scleroglucan can be obtained by Schiff-base formation between partly periodate oxidized scleroglucan and the primary amine groups of chitosan. The scleraldehyde-chitosan gels exhibit a collapse transition when exposed to volume fractions of isopropanol, Wp, larger than 65%. The aim of the present study is to provide structural information concerning the local polymer distribution and the collapse transition in (1,3)-β-D-glucan-chitosan gels. Small angle x-ray scattering was used to investigate solutions and gels of scleroglucan in water, as well as in an aqueous mixture containing 65% isopropanol. The results reveal that in aqueous solution, the polysaccharide scleroglucan chains have an approximately cylindrical cross section of external diameter close to 17Å. The gels display the same local structure, but form clusters on a longer distance scale. For the collapsed gels in the water-isopropanol mixture, partial phase separation occurs in which ordered domains of approximate size of 110Å develop. This study indicates that local ordering in liquid-crystalline-type domains is a possible molecular mechanism contributing to the collapse of gels composed of semiflexible polymers. The triple helical structure of the molecule appears not to be conserved in the majority phase in this solvent, but it is conserved in the liquid crystalline domains.
Obtaining reaction coordinates by likelihood maximization
Authors: Peters Baron , Trout Bernhardt L.
Published: August 2006

We present a new approach for calculating reaction coordinates in complex systems. The new method is based on transition path sampling and likelihood maximization. It requires fewer trajectories than a single iteration of existing procedures, and it applies to both low and high friction dynamics. The new method screens a set of candidate collective variables for a good reaction coordinate that depends on a few relevant variables. The Bayesian information criterion determines whether additional variables significantly improve the reaction coordinate. Additionally, we present an advantageous transition path sampling algorithm and an algorithm to generate the most likely transition path in the space of collective variables. The method is demonstrated on two systems: a bistable model potential energy surface and nucleation in the Ising model. For the Ising model of nucleation, we quantify for the first time the role of nuclei surface area in the nucleation reaction coordinate. Surprisingly, increased surface area increases the stability of nuclei in two dimensions but decreases nuclei stability in three dimensions.
Combustion-generated nanoparticles produced in a benzene flame: A multiscale approach
Authors: Violi Angela , Venkatnathan Arun
Published: August 2006

This paper details the multiscale methodology developed to analyze the formation of nanoparticles in a manner that makes it possible to follow the evolution of the structures in a chemically specific way. The atomistic model for particle inception code that combines the strengths of kinetic Monte Carlo and molecular dynamics is used to study the chemical and physical properties of nanoparticles generated in a premixed fuel-rich benzene flame, providing atomistic scale structures (bonds, bond angles, dihedral angles) as soot precursors evolve into a three-dimensional structure. Morphology, density, porosity, and other physical properties are computed. Two heights corresponding to two different times in the benzene flame, experimentally studied by Bittner and Howard [Proc. Combust. Inst. 18, 1105 (1981)], were chosen to examine the influence of different environments on structural properties of the particles formed.
Towards accurate solvation dynamics of divalent cations in water using the polarizable amoeba force field: From energetics to structure
Authors: Piquemal Jean-Philip , Perera Lalith , Cisneros G. Andrés , Ren Pengyu , Pedersen Lee G. , Darden Thomas A.
Published: August 2006

Molecular dynamics simulations were performed using a modified amoeba force field to determine hydration and dynamical properties of the divalent cations Ca2+ and Mg2+. The extension of amoeba to divalent cations required the introduction of a cation specific parametrization. To accomplish this, the Tholé polarization damping model parametrization was modified based on the ab initio polarization energy computed by a constrained space orbital variation energy decomposition scheme. Excellent agreement has been found with condensed phase experimental results using parameters derived from gas phase ab initio calculations. Additionally, we have observed that the coordination of the calcium cation is influenced by the size of the periodic water box, a recurrent issue in first principles molecular dynamics studies.
On the analysis of conformational dynamics in polymers with several rotational isomers
Authors: Canales Manel , Sesé Gemma
Published: August 2006

The ability of different correlation functions to shed some light onto the conformational dynamics of an amorphous polymer has been analyzed. The study has been performed on a polyethylene model polymer, which has been simulated at decreasing temperatures towards its glass transition, via the molecular dynamics technique. Three rotational isomers are allowed by the considered torsional potential. The correlation times associated with the evaluated transition rates have shown to be Arrhenius in nature, with activation energies resulting basically from internal rotation barriers. Overall torsional autocorrelation functions have been calculated. We have observed that they are dominated by slow events. Alternatively, a set of torsional autocorrelation functions associated with every isomeric state has been evaluated. Stretched exponential fits lead to correlation times that display Vogel-Fulcher temperature dependence.
Nanostructures and enhanced thermoelectric properties in Ce-filled skutterudite bulk materials
Authors: Zhai P. C. , Zhao W. Y. , Li Y. , Liu L. S. , Tang X. F. , Zhang Q. J. , Niino M.
Published: August 2006

The nanostructures induced by uniaxial compression in Ce0.29Fe1.40Co2.60Sb11.24 bulk thermoelectric material is reported. High-resolution transmission electron microscope images reveal that the nanostructures consist of Ce0.31Fe1.38Co2.62Sb11.56 crystals and Fe0.34Co0.66Sb1.99 minicrystal/noncrystal with grain sizes about 5–20nm. It is shown that the Seebeck coefficient is increased by 21%, the phonon thermal conductivity is reduced by 19%, while the electrical conductivity rises slightly at 300K. This leads to a surprising increase in the power factor by 51%. The significant increase of Seebeck coefficient and the remarkable reduction of phonon thermal conductivity are believed to be due to quantum effect and size effect of the nanostructures, respectively.
Visualization of drop-on-demand inkjet: Drop formation and deposition
Authors: Dong Hongming , Carr Wallace W. , Morris Jeffrey F.
Published: August 2006

An apparatus developed for visualizing drop-on-demand (DOD) drop formation and impaction on substrates is described. Using a pulsed laser, a low-speed charge coupled device camera, and signal generators, an imaging system based on flash photography is shown to be able to obtain sharp images with a temporal resolution of 200ns and a spatial resolution of 0.81μm∕pixel. Several steps are taken to minimize the “first drop problem” so that excellent reproducibility is achieved; drop formation is reproducible with a positional variation of 1μm. The visualization system coupled with a motorized stage allows imaging of the impaction of micron-size drops on surfaces. A wave form generator and an amplifier are used to produce the required wave form for a given printhead and ejected liquid. Demonstration of the system for study of DOD micron-size drop formation and impaction on a substrate is presented. The effects of signal wave form on DOD drop formation are demonstrated using a Trident printhead driven by four different signal wave forms. The effect of surface wettability on micron-size drop impaction is shown using hydrophilic and hydrophobic surfaces.
Isomeric identification of polycyclic aromatic hydrocarbons formed in combustion with tunable vacuum ultraviolet photoionization
Authors: Qi Fei , Yang Rui , Yang Bin , Huang Chaoqun , Wei Lixia , Wang Jing , Sheng Liusi , Zhang Yunwu
Published: August 2006

Polycyclic aromatic hydrocarbons (PAHs) play important roles in the formation of soot from combustion. The PAHs are formed from incomplete combustion, and are thought to pose a particularly great risk to health. Isomeric identification of PAHs is a big challenge. In this article, we describe an apparatus that combines tunable synchrotron vacuum ultraviolet (VUV) photoionization with molecular-beam mass spectrometry for identifying isomers of PAHs formed in combustion. The isomers of PAHs can be distinguished with measurements of photoionization mass spectrometry and photoionization efficiency spectra. With its unique features, the apparatus provides superior mass and energy resolution and is potentially a powerful tool for the study of formation mechanisms of PAHs and soot in combustion.
Orientation effect on microwave dielectric properties of Si-integrated Ba0.6Sr0.4TiO3 thin films for frequency agile devices
Authors: Kim Hyun-Suk , Hyun Tae-Seon , Kim Ho-Gi , Kim Il-Doo , Yun Tae-Soon , Lee Jong-Chul
Published: August 2006

The effect of texture with (100) and (110) preferred orientations on dielectric properties of Ba0.6Sr0.4TiO3 (BST) thin films grown on SrO (9nm) and CeO2 (70nm) buffered Si substrates, respectively, was investigated. The coplanar waveguide (CPW) phase shifter using (100) oriented BST films on SrO buffered Si exhibited a much-enhanced figure of merit of 24.7°∕dB, as compared to that (10.2°∕dB) of a CPW phase shifter using (110) oriented BST films on CeO2 buffered Si at 12GHz. This work demonstrates that the microwave properties of the Si-integrated BST thin films are highly correlated with crystal orientation.
Magnetic anisotropy energy of antiferromagnetic L1-type equiatomic Mn alloys
Authors: Umetsu R. Y. , Sakuma A. , Fukamichi K.
Published: August 2006

The theoretical calculations of magnetic anisotropy energy (MAE) for antiferromagnetic L10-type MnTM [transition metal (TM)=Ni, Pd, Pt, Rh, and Ir] alloys were carried out on the basis of the local spin-density approximation using linear muffin-tin orbital method with atomic sphere approximation including the spin-orbit interaction. It has been demonstrated that the magnitude of the MAE for L10-type MnIr alloy is highest, −20.77×106J∕m3, and the negative sign of the MAE is consistent with the reported spin structure. On the other hand, the sign of the MAE for L10-type MnPt alloy is positive in accordance with the spin structure. The MAE of the MnPt alloy is sensitive to the electron number, which would be correlated with the fact that only the MnPt alloy system exhibits a complicated magnetic phase diagram.
Tunable quantum cascade lasers with phase-matched third harmonic generation
Authors: Banerjee S. , Spencer P. S. , Shore K. A.
Published: August 2006

The design of two tunable quantum cascade laser structures using intracavity phase-matched third harmonic generation is reported. The transverse mode at the lasing wavelength was phase matched with third order transverse mode of the third harmonic signal in one structure and to the fifth order transverse mode in the second structure. The effects of small perturbations in the widths and refractive indices of the waveguide layers are also studied. This analysis yielded optical-waveguide structures capable of generating third harmonic power with a conversion efficiency of about 1mW∕W.
Vibrational analyses for CHFClBr and CDFClBr based on high level ab initio calculations
Authors: Rauhut Guntram , Barone Vincenzo , Schwerdtfeger Peter
Published: August 2006

Anharmonicity corrections to the harmonic vibrational spectra of CHFClBr and its deuterated isotopomer were computed by means of variational and perturbational approaches. A comparison of both methods is provided. Based on CCSD(T)/aug-cc-pVTZ electronic structure calculations excellent agreement with experimental data was obtained. Absolute mean deviations are in the range of about 4cm−1 for the fundamental modes, while slightly larger values of about 7cm−1 were found for the first vibrational overtones. In addition, vibrationally averaged structural parameters are provided for both molecules. The calculations will serve as a future starting point for parity-violation effects in vibrational transitions in these chiral molecules.
Statistical estimates of electron correlations
Authors: Győrffy W. , Henderson Thomas M. , Greer J. C.
Published: August 2006

While arbitrarily accurate solutions to the many-body Schrödinger equation are possible through a brute force expansion of the wave function, the length of the expansions required renders the approach intractable except for few-electron problems. By considering the form of the energy resulting from truncation of the many-particle expansion space, it is shown that accurate determination of electron correlations may be extracted from estimates of average or effective energy contributions while maintaining a reduced dimension for the expansion space. An energy formula expressed as a rational function of the expansion vector length is determined, allowing for estimates of asymptotic limits of many-body correlations.
Resonance Raman study of the A-band short-time photodissociation dynamics of 2-iodothiophene
Authors: Zhu Hai-Lin , Liu Jian , Zheng Xuming , Phillips David Lee
Published: August 2006

Resonance Raman spectra were obtained for 2-iodothiophene in cyclohexane solution with excitation wavelengths in resonance with the A-band absorption spectrum. These resonance Raman spectra indicate that the Franck-Condon region photodissociation dynamics have multidimensional character with motion mainly along the nominal symmetric CC stretch of the thienyl ring and accompanied by a moderate amount of motion along the nominal symmetric CSC stretch, the nominal antisymmetric CSC stretch, and the nominal C–I stretch vibrational modes. A preliminary resonance Raman intensity analysis was done for the A-band resonance Raman spectra of 2-iodothiophene. These results were compared to previous results for related iodobenzene and iodoalkane molecules that also contain a C–I chromophore and the similarities and differences in the short-time photodissociation dynamics were discussed.
Synthesis and characterization of FeMn-pinned spin valve arrays
Authors: Wang Huixin , Wu Yucheng , Li Qingshan , Wang Ming , Li Guanghai , Zhang Lide
Published: August 2006

We have prepared by electrodeposition nanoscale spin valves into the pores of anodic alumina membranes. Aligned nanometric spin valve arrays, well characterized by field emission scanning electron microscopy, are vertical with respect to the plane of the template and exhibit a perfect two-dimensional array with a hexagonal pattern. The largest value of room temperature giant magnetoresistance (GMR) we achieved is 6.8% at 75Oe. The relatively low saturation fields together with relatively large GMR should make such structures attractive for sensor applications.
Hard x-ray contact microscopy with 250nm spatial resolution using a LiF film detector and a tabletop microsource
Authors: Almaviva S. , Bonfigli F. , Franzini I. , Lai A. , Montereali R. M. , Pelliccia D. , Cedola A. , Lagomarsino S.
Published: August 2006

An innovative route for deep-submicrometer spatial resolution hard x-ray microscopy with tabletop x-ray source is proposed. A film of lithium fluoride (LiF) was used as imaging detector in contact mode. We present here the x-ray images recorded on LiF films of a Fresnel zone plate with submicrometer gold structures and of an onion cataphyll. The images were read with an optical confocal microscope in fluorescence mode. The measured spatial resolution was about 250nm, i.e., close to the resolution limit of the confocal microscope. The advantages and drawbacks, and the possible improvements, of this route are discussed.
Direct observation of the strength of plasmon-longitudinal optical phonon interaction in n-type GaAs
Authors: Altan H. , Xin X. , Matten D. , Alfano R. R.
Published: August 2006

The screening of longitudinal optical phonons by plasmons is investigated by time-resolved visible pump-mid-infrared probe transmission measurements in a series of light to highly doped n-type GaAs wafers. The reduced relaxation of photogenerated carriers is strongly correlated to the coupling between longitudinal optical phonons and background plasmons as suggested by the variation of the phonon strength over the doping range. The critical plasmon density at which the optical phonons were effectively screened, thereby reducing the carrier relaxation, was determined to be Nc∼1×1018cm−3.
Surface recombination velocity in Si wafers by photoinduced thermal emission
Authors: Malyutenko V. , Chyrchyk S.
Published: August 2006

A contactless, all-optical, nondestructive approach is presented for the steady-state measuring of the surface recombination velocity in Si wafers at a low injection level and well above room temperature. The fundamentals and technique of this approach are based on the analysis of the below-band-gap (3–5μm) nonequilibrium thermal emission versus the wavelength of the above-band-gap light pump (<1μm) that generates excess free carriers. Experimental results for Si wafers (400K<T<600K) which were submitted to different surface treatments are reported. Limitations to this technique are also discussed.
Direct observation of ballistic and drift carrier transport regimes in InAs nanowires
Authors: Zhou X. , Dayeh S. A. , Aplin D. , Wang D. , Yu E. T.
Published: August 2006

Conductive atomic force microscopy has been used to characterize distance-dependent electron transport behavior in InAs nanowires grown by metal-organic chemical vapor deposition. Using a conducting diamond-coated tip as a local electrical probe in an atomic force microscope, the resistance of the InAs nanowire has been measured as a function of electron transport distance within the nanowire. Two regimes of transport behavior are observed: for distances of ∼200nm or less, resistance independent of electron transport distance, indicative of ballistic electron transport, is observed; for greater distances, the resistance is observed to increase linearly with distance, as expected for conventional drift transport. These observations are in very good qualitative accord with the Landauer formalism for mesoscopic carrier transport, and the resistance values derived from these measurements are in good quantitative agreement with carrier concentrations and mobilities determined in separate experiments. These results provide direct information concerning distances over which ballistic transport occurs in InAs nanowires as well as demonstrating the ability of the scanning probe techniques employed to characterize nanoscale transport characteristics in semiconductor nanowire structures.
Undissociated screw dislocation in Si: Glide or shuffle set?
Authors: Wang Cai-Zhuang , Li Ju , Ho Kai-Ming , Yip Sidney
Published: August 2006

In diamond and zinc blende crystals, the competition between glide and shuffle-set slips has been intensively studied. In particular, the undissociated screw dislocation in Si seen at low temperature about five years ago was generally believed to be shuffle set. In this letter, the authors have performed tight-binding and density functional theory calculations that show that a glide-set C core has lower energy than the shuffle-set A core after period-doubling reconstruction. Since the C core can cross slip between two glide-set planes, it satisfies all the experimental observations to date, and may play important roles in dislocation cross slip and ductile-to-brittle transition in these materials.
Energy band mixing in core-shell-structured Si∕FeSi2 nanocomposites
Authors: Wu X. L. , Zhang Z. Y. , Xue F. S. , Siu G. G. , Chu Paul K.
Published: August 2006

Using the bandmixing model of direct and indirect gaps, we calculate the energy levels of electrons and holes confined in the Si∕FeSi2 nanocomposite consisting of nanocrystalline Si core, interfacial FeSi2 layer, and outer Fe2O3 crust. The obtained results explain very well the intensity enhancement and pinning behavior of the observed photoluminescence peaks after the nanocomposite is stored in air. We also theoretically obtain the photoluminescence excitation characteristics. Good agreement is achieved between the theory and experiments.
Microwave absorption properties of the carbon-coated nickel nanocapsules
Authors: Zhang X. F. , Dong X. L. , Huang H. , Liu Y. Y. , Wang W. N. , Zhu X. G. , Lv B. , Lei J. P. , Lee C. G.
Published: August 2006

The carbon-coated Ni(C) nanocapsules were prepared by a modified arc-discharge method in methane atmosphere. Its electromagnetic parameters were measured at 2–18GHz. It is observed that the natural resonance which appeared at 5.5GHz is dominant among microwave absorption properties of Ni(C) nanocapsules, as the consequence of the increased surface anisotropic energy for nanosized particles. The measured relative complex permittivity indicates that a high resistivity existed in Ni(C) nanocapsules samples. The maximum reflection loss of Ni(C) nanocomposites can reach 32dB at 13GHz with 2mm in thickness. The microwave absorptive mechanisms of Ni(C) nanocapsule absorbent were discussed.
Development of a deformation-tunable quadrupolar microcavity
Authors: Yang Juhee , Moon Songky , Lee Sang-Bum , Lee Jai-Hyung , An Kyungwon , Shim Jeong-Bo , Lee Hai-Woong , Kim Sang-Wook
Published: August 2006

We have developed a technique for realizing a two-dimensional quadrupolar microcavity with its deformation variable from 0% to 20% continuously. We employed a microjet ejected from a noncircular orifice in order to generate a stationary column with modulated quadrupolar deformation in its cross section. Wavelength redshifts of low-order cavity modes due to shape deformation were measured and were found to be in good agreement with the wave calculation for the same deformation, indicating that the observed deformation is quadrupolar in nature.
Zero-strain GaAs quantum dot molecules as investigated by x-ray diffuse scattering
Authors: Hanke M. , Schmidbauer M. , Grigoriev D. , Schäfer P. , Köhler R. , Metzger T. H. , Wang Zh. M. , Mazur Yu. I. , Salamo G. J.
Published: August 2006

The authors report on x-ray diffuse scattering at nominally strain-free GaAs(001) quantum dot molecules (QDMs). Al0.3Ga0.7As deposited by molecular beam epitaxy on GaAs(001) acts as barrier layer between the GaAs(001) substrate and subsequently grown QDMs; the adjusted thickness of 50nm preserves the in-plane lattice parameter. Pairs of lenselike quantum dots are created with preferential orientation along [11¯0] placed on shallow hills. Grazing incidence diffraction along with kinematical scattering simulations indicate completely strain-free QDs which prove a strongly suppressed intermixing between QDMs and the underlying AlGaAs barrier layer.
Nucleation at the phase transition near 40°C in MnAs nanodisks
Authors: Jenichen B. , Takagaki Y. , Ploog K. H. , Darowski N. , Feyerherm R. , Zizak I.
Published: August 2006

The phase transition near 40°C of both as-grown thin epitaxial MnAs films prepared by molecular beam epitaxy on GaAs(001) and nanometer-scale disks fabricated from the same films is studied. The disks are found to exhibit a pronounced hysteresis in the temperature curve of the phase composition. In contrast, supercooling and overheating take place far less in the samples of continuous layers. These phenomena are explained in terms of the necessary formation of nuclei of the other phase in each of the disks independent from each other. The influence of the elastic strains in the disks is reduced considerably.
Recombination processes in midinfrared InGaAsSb diode lasers emitting at 2.37μm
Authors: O’Brien K. , Sweeney S. J. , Adams A. R. , Murdin B. N. , Salhi A. , Rouillard Y. , Joullié A.
Published: August 2006

The temperature dependence of the threshold current of InGaAsSb∕AlGaAsSb compressively strained lasers is investigated by analyzing the spontaneous emission from working laser devices through a window formed in the substrate metallization and by applying high pressures. It is found that nonradiative recombination accounts for 80% of the threshold current at room temperature and is responsible for the high temperature sensitivity. The authors suggest that Auger recombination involving hot holes is suppressed in these devices because the spin-orbit splitting energy is larger than the band gap, but other Auger processes persist and are responsible for the low T0 values.
Isotropic photonic pseudogap in electrodeposited ZnO inverse opal
Authors: Teh L. K. , Yeo K. H. , Wong C. C.
Published: August 2006

The authors achieved an isotropic photonic stop band in the blue region near the L point in the Brillouin zone of ZnO inverse opal. This was fabricated by electrochemical infiltration in a polystyrene opal template self-assembled with the meniscus method. In the ZnO inverse opal, the stop band is “blueshifted,” widened (compared to its parent opal), and remained fixed at its central frequency along different directions. The extensive spectral overlap of the stop bands indicates a smooth angular dispersion approaching omnidirectionality within a specific range of directions. The isotropic pseudogap could interact with the emission of ZnO material, resulting in useful luminescent behavior.
Current-voltage characteristics of n∕n lateral polarity junctions in GaN
Authors: Aleksov Aleksandar , Collazo Ramon , Mita Seiji , Schlesser Raoul , Sitar Zlatko
Published: August 2006

Lateral Si:N-polar/Si:Ga-polar GaN homojunctions were fabricated using metal organic chemical vapor deposition with nitrogen as the carrier and dilution gas. Nominally undoped N-polar areas are n-type conductive, while nominally undoped Ga-polar areas are insulating with carrier densities below 1×1015cm−3. This allows for the fabrication of selectively doped areas within one growth step that can be used to fabricate novel lateral device structures or enhance existing III-N-device structures. In this letter we investigated the electrical properties of the simplest case of these junctions, namely, when both sides are n-type conductive. The results of the IV measurements show a linear characteristic both for the measurement of a N-polar/Si:Ga-polar junction and a N-polar/Si:Ga-polar/N-polar double junction. This result indicates that, as expected, there are no energy barriers between the N-polar and the Ga-polar material and that these structures can be used to achieve laterally selective doped areas in Ga–N for electronic device applications.
Internal structure of mixed phase hydrogenated silicon thin films made at 39°C
Authors: Bronsveld P. C. P. , Rath J. K. , Schropp R. E. I. , Mates T. , Fejfar A. , Rezek B. , Kočka J.
Published: August 2006

A combined cross-sectional transmission electron microscope (XTEM) and atomic force microscope (AFM) study of a hydrogen to silane dilution series of thin silicon films deposited by very high frequency plasma enhanced chemical vapor deposition at a substrate temperature that is low enough (39°C) to neglect the role of the surface diffusion in the growth process is reported. XTEM images of a mixed amorphous/microcrystalline layer reveal a structure of isolated conically shaped crystalline conglomerates (surface diameter ∼570±75nm) embedded in an amorphous phase of columns with diameters of ∼51±3nm. Detailed closeups of these crystallites, combined with AFM images of the hydrogen dilution dependent evolution of the surface, reveal similarities between the nucleation of amorphous and crystalline columnar structures at this low substrate temperature.
Nonlocal neoclassical transport in tokamak and spherical torus experiments
Authors: Wang W. X. , Rewoldt G. , Tang W. M. , Hinton F. L. , Manickam J. , Zakharov L. E. , White R. B. , Kaye S.
Published: August 2006

Large ion orbits can produce nonlocal neoclassical effects on ion heat transport, the ambipolar radial electric field, and the bootstrap current in realistic toroidal plasmas. Using a global δf particle simulation, it is found that the conventional local, linear gradient-flux relation is broken for the ion thermal transport near the magnetic axis. With regard to the transport level, it is found that details of the ion temperature profile determine whether the transport is higher or lower when compared with the predictions of standard neoclassical theory. Particularly, this nonlocal feature is suggested to exist in the National Spherical Torus Experiment (NSTX) [M. Ono, S. M. Kaye, Y.-K. M. Peng et al., Nucl. Fusion 40, 557 (2000)], being consistent with NSTX experimental evidence. It is also shown that a large ion temperature gradient can increase the bootstrap current. When the plasma rotation is taken into account, the toroidal rotation gradient can drive an additional parallel flow for the ions and then additional bootstrap current, either positive or negative, depending on the gradient direction. Compared with the carbon radial force balance estimate for the neoclassical poloidal flow, our nonlocal simulation predicts a significantly deeper radial electric field well at the location of an internal transport barrier of an NSTX discharge.
Magnetic properties of ZnMnO films grown at low temperature by atomic layer deposition
Authors: Wójcik A. , Kopalko K. , Godlewski M. , Guziewicz E. , Jakieła R. , Minikayev R. , Paszkowicz W.
Published: August 2006

The authors demonstrate that by lowering deposition temperature of ZnMnO films (T<500°C) they can avoid Mn clustering and creation of inclusions of Mn oxides, which are frequently formed in ZnMnO layers grown by high temperature methods. Low temperature growth is achieved using atomic layer deposition and organic zinc and manganese precursors.
Atomic ordering at an amorphous/crystal interface
Authors: Shibata Naoya , Painter Gayle S. , Becher Paul F. , Pennycook Stephen J.
Published: August 2006

In this study, the authors report atomic-resolution images that illustrate the transition from a crystalline Si3N4 grain across the interface into an amorphous Lu–Si–Mg–N–O glassy phase. The interface is not atomically abrupt, but is comprised of sub-nanometer-scale ordered regions that resemble a LuN-like structure. These ordered clusters bind to the prismatic surface of the Si3N4 grains at specific low energy positions for Lu adsorption as predicted by first-principles calculations. The ordered regions are filamentary in nature, extending for at least two atomic layers into the amorphous pockets at multigrain junctions before disappearing.
Atomic layer deposition of W on nanoporous carbon aerogels
Authors: Elam J. W. , Libera J. A. , Pellin M. J. , Zinovev A. V. , Greene J. P. , Nolen J. A.
Published: August 2006

In this study, the authors demonstrate the ability to apply precise, conformal W coatings onto all surfaces of nanoporous carbon aerogels using atomic layer deposition (ALD). The resulting material has a filamentous structure in which the W completely encapsulates the carbon aerogel strands. The material mass increases nonlinearly with W coating, achieving a tenfold increase following ten ALD cycles. The aerogel surface area increases by nearly a factor of 2 after ten W ALD cycles. This conformal metal coating of extremely high aspect ratio nanoporous materials by ALD represents a unique route to forming metal functionalized high surface area materials.
Room temperature defect related electroluminescence from ZnO homojunctions grown by ultrasonic spray pyrolysis
Authors: Du G. T. , Liu W. F. , Bian J. M. , Hu L. Z. , Liang H. W. , Wang X. S. , Liu A. M. , Yang T. P.
Published: August 2006

ZnO homojunction light-emitting diode was grown on single-crystal GaAs (100) substrate by ultrasonic spray pyrolysis. This diode was comprised of N–In codoped p-type ZnO and unintentionally doped n-type ZnO film. Ohmic contact on n-type ZnO layer and GaAs substrate was formed by Zn∕Au and Au∕Ge∕Ni alloyed metal electrodes, respectively. An electroluminescence emission associated with defects was observed from the ZnO homojunction under forward current injection at room temperature. The I-V characteristics of the homojunction showed a threshold voltage of ∼4V under forward bias.
Luminescence from two different crystallographic sites in Sr6BP5O20:Eu2+
Authors: Sohn Kee-Sun , Cho Sang Ho , Park Sung Soo , Shin Namsoo
Published: August 2006

The time resolved luminescence and concentration quenching behavior were examined for Sr6BP5O20:Eu2+ phosphors. The emission spectrum of the Sr6BP5O20:Eu2+ phosphor could be deconvoluted into two well defined Gaussian peaks. This implies that the Eu2+ emission emanates from two different crystallographic sites for Sr2+ ions. Each deconvoluted Gaussian peak was assigned to its corresponding Sr2+ crystallographic site with the assistance of density functional theory (DFT) calculations. The accurate assignment achieved by the DFT calculation will provide some basic guidelines for improving the color purity of Sr6BP5O20:Eu2+ phosphors, when they are to be applied to blue components in plasma display panels.
Nanowire lasers with distributed-Bragg-reflector mirrors
Authors: Chen L. , Towe E.
Published: August 2006

A self-consistent, coupled optoelectronic simulation model is used to study microcavity GaN nanowire lasers. This model is applied to show that it is possible to design a nanowire laser with distributed-Bragg-reflector mirrors. Several advantages are expected to be enjoyed from the use of such mirrors. These include reduction of lasing threshold current density (or pumping density rate), dramatic reduction of cavity losses, and the achievement of stable, single-mode operation.
Spin polarization of quaternary Co2Cr1−xFexAl Heusler alloys
Authors: Karthik S. V. , Rajanikanth A. , Takahashi Y. K. , Okhubo T. , Hono K.
Published: August 2006

Point contact Andreev reflection measurements of Co2Cr1−xFexAl Heusler alloys showed that the spin polarization P decreases with x from P=0.62 for x=0.0toP=0.54 for x=0.4 and then increases slightly to P=0.56 for x=1.0. The deviation in P of the samples with x=0.0 and 0.4 from the theoretical predictions (P>0.9) can be accounted to the phase separation, whereas the deviation in P of the sample with x=1.0 from the theoretical predictions (P=0.3) suggests the lack of accuracy in density of state calculations.
Hybrid devices with high-density nanometallic and nanomagnet dots embedded in the semiconductor
Authors: Holland S. , Gui Y. S. , Kruse J. , Heitmann D. , Hu C.-M. , Chen Y. F. , Cui Z. , Pan G.
Published: August 2006

A homogeneous metallic dot array embedded in an InAs two-dimensional electron system has been fabricated with a dot density as high as 108cm−2. This hybrid device consists of three different areas: one is an antidot array, in another permalloy (Py) is embedded in the holes, and in the third area Ti∕Au is filled into the semiconductor. This design enables to distinguish on the same sample, in situ, the effects of metallic and ferromagnetic behaviors of the embedded nanostructures. A hysteretic magnetoresistance effect and a memory effect in the Py embedded area has been observed at 4.2K while the Ti∕Au area does not show these effects. Considering that there are more than 1×106 nanomagnet dots embedded in the device, the magnetoresistance of 0.25% for an in-plane magnetic field demonstrates the large area homogeneity of the embedded nanodots.
Investigation of micromechanical cantilever sensors with microfocus grazing incidence small-angle x-ray scattering
Authors: Wolkenhauer Markus , Bumbu Gina-Gabriela , Cheng Yajun , Roth Stephan V. , Gutmann Jochen S.
Published: August 2006

Micromechanical cantilever arrays coated with polymer brushes are an alternative to traditional chemical sensors. A critical parameter for the sensor sensitivity is the coating quality, and small changes in the coating uniformity generate a different behavior of the sensors. Microfocus grazing incidence small-angle x-ray scattering proved to be a sensitive tool to characterize coating quality, enabling a structural and chemical coating analysis.
Near infrared broadband emission from Bi5+-doped Al2O3–GeO2–X (X=Na2O,BaO,Y2O3) glasses
Authors: Xia Hai-Ping , Wang Xue-Jun
Published: August 2006

The authors report on near infrared broadband emission with a wavelength ranging from O to S telecommunication bands in Bi5+-doped GeO2–Al2O3–Na2O, GeO2–Al2O3–BaO, and GeO2–Al2O3–Y2O3 glasses. The superbroadband character is ascribed to the emission of Bi5+ ions. The differences among the optical characters of the glasses are due to the Bi5+ affected by the introduction of yttrium and barium. As the integration of yttrium and barium into GeO2–Al2O3–Bi2O3 system would offer a preferable environment for the formation of Bi5+, the integration of yttrium and barium leads to the redshift of absorption edges as well as the accentuation of Bi5+ concentration.
Wave-mode dispersions in a relativistic electron beam with ion-channel guiding
Authors: Rouhani M. H. , Maraghechi B.
Published: August 2006

An analysis of the eigenmodes of a cylindrical metallic waveguide partially filled with a relativistic electron beam, and guided with an ion channel, is presented. The analysis is performed in the rest frame of the beam with self-fields taken into account. Dispersion relations for five families of electrostatic and EM modes are derived and solved numerically to study the characteristics of azimuthally symmetric TM and TE waveguide modes, betatron modes, and space-charge modes. A strong dependence of the frequencies of these electromagnetic-electrostatic waves on the ratio of the radius of the waveguide to the beam radius is revealed. The physical insight given to the electrostatic modes, at large and small wavelengths, equally applies to the Trivelpiece-Gould modes in an electron beam with axial magnetic field guiding. Interactions between the dispersion curves of different modes are used to find the infinite-space dispersion relation from the finite-radius dispersion relation and to explain the transition from a weak to strong ion channel.
Evolution of nonlinear ion-acoustic solitary wave propagation in rotating plasma
Authors: Das G. C. , Nag Apratim
Published: August 2006

A simple unmagnetized plasma rotating around an axis at an angle θ with the propagation direction of the acoustic mode has been taken. The nonlinear wave mode has been derived as an equivalent Sagdeev potential equation. A special procedure, known as the tanh method, has been developed to study the nonlinear wave propagation in plasma dynamics. Further, under small amplitude approximation, the nonlinear plasma acoustic mode has been exploited to study the evolution of soliton propagation in the plasma. The main emphasis has been given to the interaction of Coriolis force on the changes of coherent structure of the soliton. The solitary wave solution finds the different nature of solitons called compressive and rarefactive solitons as well as its explosions or collapses along with soliton dynamics and these have been showing exciting observations in exhibiting a narrow wave packet with the generation of high electric pressure and the growth of high energy which, in turn, yields the phenomena of radiating soliton in dynamics.
Core barrier formation near integer q surfaces in DIII-D
Authors: Austin M. E. , Burrell K. H. , Waltz R. E. , Gentle K. W. , Gohil P. , Greenfield C. M. , Groebner R. J. , Heidbrink W. W. , Luo Y. , Kinsey J. E. , Makowski M. A. , McKee G. R. , Nazikian R. , Petty C. C. , Prater R. , Rhodes T. L. , Shafer M. W. , Van Zeeland M. A.
Published: August 2006

Recent DIII-D experiments have significantly improved the understanding of internal transport barriers (ITBs) that are triggered close to the time when an integer value of the minimum in q is crossed. While this phenomenon has been observed on many tokamaks, the extensive transport and fluctuation diagnostics on DIII-D have permitted a detailed study of the generation mechanisms of q-triggered ITBs as pertaining to turbulence suppression dynamics, shear flows, and energetic particle modes. In these discharges, the evolution of the q profile is measured using motional Stark effect polarimetry and the integer qmin crossings are further pinpointed in time by the observation of Alfvén cascades. High time resolution measurements of the ion and electron temperatures and the toroidal rotation show that the start of improved confinement is simultaneous in all three channels, and that this event precedes the traversal of integer qmin by 5–20ms. There is no significant low-frequency magnetohydrodynamic activity prior to or just after the crossing of the integer qmin and hence magnetic reconnection is determined not to be the precipitant of the confinement change. Instead, results from the GYRO code point to the effects of zonal flows near low order rational q values as playing a role in ITB triggering. A reduction in local turbulent fluctuations is observed at the start of the temperature rise and, concurrently, an increase in turbulence poloidal flow velocity and flow shear is measured with the beam emission spectroscopy diagnostic. For the case of a transition to an enduring internal barrier the fluctuation level remains at a reduced amplitude. The timing and nature of the temperature, rotation, and fluctuation changes leading to internal barriers suggests transport improvement due to increased shear flow arising from the zonal flow structures.
Focusing a multimillijoule soft x-ray laser at 21nm
Authors: Mocek T. , Rus B. , Stupka M. , Kozlová M. , Präg A. R. , Polan J. , Bittner M. , Sobierajski R. , Juha L.
Published: August 2006

The authors focused the beam of currently the most powerful soft x-ray laser at 21nm using a spherical Mo:Si multilayer mirror. Focal spots were directly observed by imaging the fluorescence induced by the soft x-ray beam on a Tb doped phosphor screen. The energy density within the 40×60μm2 focal spot was 48J∕cm2, which corresponds to radiation peak intensity of 5×1011W∕cm2. The first observation of material ablation with a laser at 21nm is reported.