bluuebunny/crossref_metadata_2025 huggingface dataset:
Diffuse optical tomographic imager using a single light source
Authors: Biswas Samir Kumar , Rajan K , Vasu R. M.
Published: February 2009

Near-infrared diffuse optical tomography (DOT) technique has the capability of providing good quantitative reconstruction of tissue absorption and scattering properties with additional inputs such as input and output modulation depths and correction for the photon leakage. We have calculated the two-dimensional (2D) input modulation depth from three-dimensional (3D) diffusion to model the 2D diffusion of photons. The photon leakage when light traverses from phantom to the fiber tip is estimated using a solid angle model. The experiments are carried for single (5 and 6 mm) as well as multiple inhomogeneities (6 and 8 mm) with higher absorption coefficient in a homogeneous phantom. Diffusion equation for photon transport is solved using finite element method and Jacobian is modeled for reconstructing the optical parameters. We study the development and performance of DOT system using modulated single light source and multiple detectors. The dual source methods are reported to have better reconstruction capabilities to resolve and localize single as well as multiple inhomogeneities because of its superior noise rejection capability. However, an experimental setup with dual sources is much more difficult to implement because of adjustment of two out of phase identical light probes symmetrically on either side of the detector during scanning time. Our work shows that with a relatively simpler system with a single source, the results are better in terms of resolution and localization. The experiments are carried out with 5 and 6 mm inhomogeneities separately and 6 and 8 mm inhomogeneities both together with absorption coefficient almost three times as that of the background. The results show that our experimental single source system with additional inputs such as 2D input/output modulation depth and air fiber interface correction is capable of detecting 5 and 6 mm inhomogeneities separately and can identify the size difference of multiple inhomogeneities such as 6 and 8 mm. The localization error is zero. The recovered absorption coefficient is 93% of inhomogeneity that we have embedded in experimental phantom.
Local explicitly correlated second-order perturbation theory for the accurate treatment of large molecules
Authors: Adler Thomas B. , Werner Hans-Joachim , Manby Frederick R.
Published: February 2009

A local explicitly correlated LMP2-F12 method is described that can be applied to large molecules. The steep scaling of computer time with molecular size is reduced by the use of local approximations, the scaling with respect to the basis set size per atom is improved by density fitting, and the slow convergence of the correlation energy with orbital basis size is much accelerated by the introduction of terms into the wave function that explicitly depend on the interelectronic distance. The local approximations lead to almost linear scaling of the computational effort with molecular size without much affecting the accuracy. At the same time, the domain error of conventional LMP2 is removed in LMP2-F12. LMP2-F12 calculations on molecules of chemical interest involving up to 80 atoms, 200 correlated electrons, and 2600 contracted Gaussian-type orbitals, as well as several reactions of large biochemical molecules are reported.
Simulation and testing of a lateral, microfabricated electron-impact ion source
Authors: Natarajan Srividya , Gilchrist Kristin H. , Piascik Jeffrey R. , Parker Charles B. , Glass Jeffrey T. , Stoner Brian R.
Published: February 2009

Simulation and experimental testing of a miniaturized, lateral, electron-impact ionization source are reported. Modeling and subsequent optimization of the device design led to a tenfold improvement in the performance of the device in comparison with earlier designs. Increased electron current contributing to ionization and increased ion collection efficiency are believed to be the main factors responsible for this improvement. SIMION software was used to model the behavior of the devices and understand the improvement in performance. The ion source can operate in a wide pressure range from 0.1 to 100 mTorr and generate ion currents in excess of 1 μA.
Simplified CCSD(T)-F12 methods: Theory and benchmarks
Authors: Knizia Gerald , Adler Thomas B. , Werner Hans-Joachim
Published: February 2009

The simple and efficient CCSD(T)-F12x approximations (x=a,b) we proposed in a recent communication [T. B. Adler, G. Knizia, and H.-J. Werner, J. Chem. Phys. 127, 221106 (2007)] are explained in more detail and extended to open-shell systems. Extensive benchmark calculations are presented, which demonstrate great improvements in basis set convergence for a wide variety of applications. These include reaction energies of both open- and closed-shell reactions, atomization energies, electron affinities, ionization potentials, equilibrium geometries, and harmonic vibrational frequencies. For all these quantities, results better than the AV5Z quality are obtained already with AVTZ basis sets, and usually AVDZ treatments reach at least the conventional AVQZ quality. For larger molecules, the additional cost for these improvements is only a few percent of the time for a standard CCSD(T) calculation. For the first time ever, total reaction energies with chemical accuracy are obtained using valence-double-ζ basis sets.
Thermodynamic consistency near the liquid-liquid critical point
Authors: Losada-Pérez Patricia , Pérez-Sánchez Germán , Cerdeiriña Claudio A. , Troncoso Jacobo , Romaní Luis
Published: February 2009

The thermodynamic consistency of the isobaric heat capacity per unit volume at constant composition Cp,x and the density ρ near the liquid-liquid critical point is studied in detail. To this end, Cp,x(T), ρ(T), and the slope of the critical line (dT/dp)c for five binary mixtures composed by 1-nitropropane and an alkane were analyzed. Both Cp,x(T) and ρ(T) data were measured along various quasicritical isopleths with a view to evaluate the effect of the uncertainty in the critical composition value on the corresponding critical amplitudes. By adopting the traditionally employed strategies for data treatment, consistency within 0.01 K MPa−1 (or 8%) is attained, thereby largely improving the majority of previous results. From temperature range shrinking fits and fits in which higher-order terms in the theoretical expressions for Cp,x(T) and ρ(T) are included, we conclude that discrepancies come mainly from inherent difficulties in determining the critical anomaly of ρ accurately: specifically, to get full consistency, higher-order terms in ρ(T) are needed; however, the various contributions at play cannot be separated unambiguously. As a consequence, the use of Cp,x(T) and (dT/dp)c for predicting the behavior of ρ(T) at near criticality appears to be the best choice at the actual experimental resolution levels. Furthermore, the reasonably good thermodynamic consistency being encountered confirms that previous arguments appealing to the inadequacy of the theoretical expression relating Cp,x and ρ for describing data in the experimentally accessible region must be fairly rejected.
Magnetic coupled spin-torque devices for nonvolatile logic applications
Authors: Leem Larkhoon , Harris James S.
Published: February 2009

The magnetic coupled spin-torque device (MCSTD) is a new spintronics device architecture that is free from the difficulties in spin transport and spin detection. It uses spin-torque transfer technique and magnetic coupling to modulate its energy barrier. Requirements for logic device: fast switching speed, inherent gain, inversion capability, and spin interconnection techniques of MCSTD are presented. Logic functionalities such as NAND, NOR, and NOT are successfully demonstrated in micromagnetics simulations.
Magnetic properties of PrCr2Si2C single crystal
Authors: Janatova Marcela , Vejpravova Jana Poltierova , Divis Martin
Published: February 2009

A PrCr2Si2C single crystal has been prepared by the Czochralski method and investigated by specific-heat, ac susceptibility, and magnetization measurements in the temperature range of 2–900K and in magnetic fields up to 9T applied parallel to the principal crystallographic axes. The temperature dependencies of the heat capacity and ac susceptibility exhibit a clear anomaly at ∼30K, attributed to a para-to-ferromagnetic phase transition at the Curie temperature TC. Moreover, a strong magnetocrystalline anisotropy is evident from the magnetization measurements. Electronic structure of PrCr2Si2C was studied by the first-principles calculations based on the density functional theory, which predicts a stable Cr magnetic moment. No clear confirmation of Cr magnetism, however, can be found in our experimental data.
Study of electromagnetic wave absorption characteristics and component parameters of laminated-type magnetic wood with stainless steel and ferrite powder for use as building materials
Authors: Oka Hideo , Tanaka Kazunori , Osada Hiroshi , Kubota Kenji , Dawson Francis P.
Published: February 2009

The purpose of this paper is to propose that a laminated-type magnetic wood using ferrite powder and stainless steel powder can serve as a wood building material of suitable quality for an indoor electromagnetic wave absorber. The experimental results for the gigahertz frequency band show that the laminated-type magnetic wood which has volume content ratio of 20vol%, an inside ratio of stainless steel to ferrite powder 2:3, and a thickness of magnetic layer 4.0mm has an electromagnetic wave absorption characteristic of 45.18dB at 2.62GHz. This laminated board can be used to prevent radio interference for indoor wireless LANs, which mainly use the 2.4GHz frequency band.
A first-principles study on the magnetocaloric compound MnFeP2∕3Si1∕3
Authors: Liu X. B. , Altounian Z.
Published: February 2009

The electronic structure and magnetic properties for MnFeP2∕3Si1∕3 with a hexagonal Fe2P-type structure have been studied by a first-principle density functional theory calculation. The calculated magnetic moments for Fe and Mn are 1.35 and 2.89μB, respectively, leading to a total magnetization of 4.15μB per formula unit due to the small negative moments of P and Si atoms. The total energy calculations show that the Si atoms prefer to occupy the 2c site rather than the 1b site and increase the moment of Fe while decreasing the moment of Mn. The nearest Mn–Fe exchange coupling interaction (JMn–Fe=1.33mRy) is much stronger than for Fe–Fe (JFe–Fe=−0.16mRy) and Mn–Mn atomic pair (JFe–Fe=−0.53mRy) interactions. The competed exchange interactions are responsible for the field induced first order magnetic transition and the large magnetocaloric effect.
Experimental study of time-dependent switching current in MgO based magnetic tunnel junction
Authors: Zhu Wenzhong , Wang Xiaobin , Dimitrov Dimitar
Published: February 2009

Spin-transfer induced magnetization switching in MgO based magnetic tunnel junction has been studied at a wide time range. We observed a discrepancy between experimental data and thermal activation model prediction on switching current mean and variation as a function of pulse width. The discrepancy is mainly due to Joule heating and the theoretical limitation of the analytical model. It can be overcome by using the notion of an effective temperature. We also compare the fitting result of the averaged switching current versus pulse width to that of switching current probability at each individual pulse width. Finally, we present the identically switching currents measured by both finite pulse width and current sweep rate schemes.
Magnetic states for V(001) bcc surface: An ab initio study
Authors: Félix-Medina R. E. , Leyva-Lucero M. A. , Meza-Aguilar S.
Published: February 2009

Recent magneto-optical kerr effect and low energy electron diffraction experiments on V(001) surface probed the existence of surface magnetism. Following these experiments the authors performed an ab initio study for the magnetic states of V(001) surface using a plane wave self-consistent field code. The generalized gradient approximation is used to treat the exchange correlation potential. Structural optimization is considered for the systems. The results obtained from the study of V(001) bcc surface show metastable magnetic states with an energy difference, with respect to the ground state, less than 10meV/atom.
The critical role of growth temperature on the structural and electrical properties of AlGaN/GaN high electron mobility transistor heterostructures grown on Si(111)
Authors: Baron N. , Cordier Y. , Chenot S. , Vennéguès P. , Tottereau O. , Leroux M. , Semond F. , Massies J.
Published: February 2009

This work is dedicated to the study of the growth by ammonia source molecular beam epitaxy of AlxGa1−xN/GaN high electron mobility transistors on (111) oriented silicon substrates. The effect of growth conditions on the structural and electrical properties of the heterostructures was investigated. It is shown that even a slight variation in the growth temperature of the thick GaN buffer on AlN/GaN stress mitigating layers has a drastic influence on these properties via a counterintuitive effect on the dislocation density. Both in situ curvature measurements and ex situ transmission electron microscopy and x-ray diffraction experiments indicate that the relaxation rate of the lattice mismatch stress increases with the growth temperature but finally results in a higher dislocations density. Furthermore, a general trend appears between the final wafer curvature at room temperature and the threading dislocation density. Finally, the influence of the dislocation density on the GaN buffer insulating properties and the two-dimensional electron gas transport properties at the AlxGa1−xN/GaN interface is discussed.
Double excitations in finite systems
Authors: Romaniello P. , Sangalli D. , Berger J. A. , Sottile F. , Molinari L. G. , Reining L. , Onida G.
Published: February 2009

Time-dependent density-functional theory (TDDFT) is widely used in the study of linear response properties of finite systems. However, there are difficulties in properly describing excited states, which have double- and higher-excitation characters, which are particularly important in molecules with an open-shell ground state. These states would be described if the exact TDDFT kernel were used; however, within the adiabatic approximation to the exchange-correlation (xc) kernel, the calculated excitation energies have a strict single-excitation character and are fewer than the real ones. A frequency-dependent xc kernel could create extra poles in the response function, which would describe states with a multiple-excitation character. We introduce a frequency-dependent xc kernel, which can reproduce, within TDDFT, double excitations in finite systems. In order to achieve this, we use the Bethe–Salpeter equation with a dynamically screened Coulomb interaction W(ω), which can describe these excitations, and from this we obtain the xc kernel. Using a two-electron model system, we show that the frequency dependence of W does indeed introduce the double excitations that are instead absent in any static approximation of the electron-hole screening.
Methane dissociation on Ni(111) and Pt(111): Energetic and dynamical studies
Authors: Nave Sven , Jackson Bret
Published: February 2009

Electronic structure studies and quantum scattering methods are used to elucidate the differing reactivities of methane on Ni(111) and Pt(111). For both surfaces the lowest energy pathway to dissociation is over the top site, where the static surface barrier to reaction is about 0.14 eV lower on Pt(111) than on Ni(111). If allowed to relax, both surfaces exhibit a puckering of the metal atoms in the vicinity of the adsorbates and at the transition state. Thus, motion of the lattice can change the barrier to reaction. A quantum model for dissociation is employed that includes several molecular coordinates, and allows for coupling to the lattice motion and puckering of the lattice. We find that on Ni(111) the lattice has time to pucker, increasing the reactivity relative to the static surface case. The more massive atoms on the Pt(111) surface do not have time to pucker during the reaction. As both lattices become vibrationally excited the reactivity increases significantly, particularly at low incident energies where tunneling dominates. Our model suggests that tunneling is important for these large barrier systems, particularly at the relatively low incident energies of the experiments. Our work also suggests that at the large nozzle temperatures of the experiments, there are contributions to the total reactivity from vibrationally excited molecules, particularly for Ni(111). Our model is in reasonable agreement with the experimental results for Ni(111), while we significantly underestimate the reactivity on Pt(111) as well as the difference in reactivity between Ni(111) and Pt(111). This may result from errors in our total-energy calculations and/or effects due to motion (tunneling) of the methyl group at the transition state.
Phenomenological theory of the direct isotropic to hexatic-B phase transition
Authors: Das Asok K. , Mukherjee Prabir K.
Published: February 2009

Monte Carlo simulation studies have demonstrated the possibility of the first order isotropic to smectic-B phase transition. We present a mean-field description of the direct isotropic to hexatic-B phase transition. We analyze the question under which conditions a direct isotropic to hexatic-B phase transition is possible when compared to other phase transitions. Within this model the direct isotropic to hexatic-B phase transition is found to be always strongly first order than the isotropic to nematic and isotropic to smectic-A phase transitions. The presented analysis is in good agreement with Monte Carlo simulation studies.
Quantification of L1 phase volume fraction in annealed [Fe/Pt]n multilayer films
Authors: Yao Bo , Coffey Kevin R.
Published: February 2009

The heat treatment of [Fe/Pt]n multilayer films is a promising approach to form the L10 FePt phase at a reduced temperature, which is of interest for applications as high-density magnetic recording media and high-energy permanent magnets. The volume fraction of the L10 FePt phase in the annealed films strongly influences their magnetic properties. This paper introduces a novel method based on hollow cone dark field transmission electron microscopy imaging to quantify the L10 FePt phase volume fraction. This method is used to characterize two sets of [Fe/Pt]n multilayer films with varying deposition temperature and periodicity. It was found that both the deposition temperature and bilayer periodicity are significant to the structure and magnetic properties of annealed films and to the extent to which the L10 phase is formed. A correlation between L10 phase volume fraction, grain size, and magnetic properties was also observed.
Exploring the dynamics of reactions of oxygen atoms in states P3 and D1 with ethene at collision energy 3 kcal mol−1
Authors: Lee Shih-Huang , Chen Wei-Kan , Huang Wen-Jian
Published: February 2009

In a crossed molecular-beam apparatus, we reacted atomic O in states P3 and D1 with ethene (C2H4) at collision energy 3 kcal mol−1. Employing two mixtures, 20% O2+80% He and 3% O2+12.5% Ar+84.5% He, as discharge media allowed us to generate two sources of oxygen atoms that have the same mean velocity but different ratios of D1/P3 populations, 0.0017 and 0.035. We identified six reactions and recorded time-of-flight spectra of products CH2CHO, CH2CO, and CH3 as a function of laboratory angle. Reaction O(P3)+C2H4→CH2CHO+H has a fraction ft=0.43 of energy release in translation, and product CH2CHO has a maximal probability at scattering angle of 140°. For reaction O(D1)+C2H4→CH2CO+2H, ft=0.26, and the angular distribution of product CH2CO shows a backward preference. For reaction O(P3)+C2H4→CH2CO+H2, ft=0.35, and the angular distribution of product CH2CO has a slight preference for a sideways direction. In contrast, reaction O(D1)+C2H4→CH2CO+H2 has ft=0.26 and an angular distribution with forward and backward peaking and symmetry. Reactions O(P3 and D1)+C2H4→CH3+HCO have ft=0.09 and 0.08, respectively, and angular distributions with forward and backward peaking and nearly symmetric. The reactivity of O D1 with ethene is ca. 38 and 90 times that of O P3 for channels to eliminate H2 and CH3, respectively. For reactions of O D1, the branching ratio for elimination of 2H is ca. 3.3 times that for elimination of H2.
Electromagnetic coupling of localized upper hybrid oscillations in a system of density depletions
Authors: Hall J. O. , Istomin Ya. N. , Leyser T. B.
Published: February 2009

One of the most important effects of pumping the ionospheric plasma by a powerful high frequency electromagnetic wave from the ground is the excitation of filamentary density irregularities. These striations are density depletions which contain localized upper hybrid oscillations. The upper hybrid oscillations radiate Z mode waves that escape from the depletions and can therefore scatter on neighboring depletions, which provides a means for electromagnetic interaction between different striations. The physics of a Z mode wave scattering on a single density depletion is studied analytically in one dimension. At resonance the incident Z mode wave is totally reflected from the irregularity. The treatment of many parallel striations interacting through the Z mode radiation shows that different scattering processes are involved to excite the even and odd modes of the localized upper hybrid oscillations.
Photoacoustic tomography imaging system based on digital B-mode ultrasound diagnosis equipment
Authors: Guo Hua , Yang Sihua
Published: February 2009

A photoacoustic tomography imaging system with B-mode digital ultrasound diagnosis equipment was designed, fabricated, and tested for biomedical imaging. The system for biomedical diagnostics was designed with four parts consisting of an irradiating source, digital B-mode ultrasound diagnostic equipment with a liner transducer array, data acquisition and transmission system, and rotation devices for imaging scanning. Multielement phase-controlled focus technique was applied to reduce the data-acquisition time and enhance signal-to-noise ratio. Under the self-designed working sequence, data-acquisition time of an image can be reduced to 12 s. A series of experiments were performed to estimate the imaging quality of the system. The lateral and elevational resolutions of the system were detected to be 0.25 and 1.4 mm, respectively. The results show that the system has the feasibility to be used as a medical imaging method, and it may provide a new rapid and noninvasive imaging modality for clinical applications.
Accurate calculation of vibrational frequencies using explicitly correlated coupled-cluster theory
Authors: Rauhut Guntram , Knizia Gerald , Werner Hans-Joachim
Published: February 2009

The recently proposed explicitly correlated CCSD(T)-F12x (x=a,b) approximations [T. B. Adler, G. Knizia, and H.-J. Werner, J. Chem. Phys. 127, 221106 (2007)] are applied to compute equilibrium structures and harmonic as well as anharmonic vibrational frequencies for H2O, HCN, CO2, CH2O, H2O2, C2H2, CH2NH, C2H2O, and the trans-isomer of 1,2-C2H2F2. Using aug-cc-pVTZ basis sets, the CCSD(T)-F12a equilibrium geometries and harmonic vibrational frequencies are in very close agreement with CCSD(T)/aug-cc-pV5Z values. The anharmonic frequencies are evaluated using vibrational self-consistent field and vibrational configuration interaction methods based on automatically generated potential energy surfaces. The mean absolute deviation of the CCSD(T)-F12a/aug-cc-pVTZ anharmonic frequencies from experimental values amounts to only 4.0 cm−1.
Basal plane dislocation reduction in 4H-SiC epitaxy by growth interruptions
Authors: Stahlbush R. E. , VanMil B. L. , Myers-Ward R. L. , Lew K-K. , Gaskill D. K. , Eddy C. R.
Published: February 2009

The paths of basal plane dislocations (BPDs) through 4H-SiC epitaxial layers grown on wafers with an 8° offcut were tracked using ultraviolet photoluminescence imaging. The reduction of BPDs by conversion to threading edge dislocations was investigated at ex situ and in situ growth interrupts. For ex situ interrupts, BPDs are imaged after each of several growths. The wafer remains in the reactor for in situ interrupts and BPDs are imaged after the growth is finished. For in situ interrupts, a combination of temperature, propane flow, and duration has been determined, which achieve a BPD reduction of 98%.
Extended high-order harmonics from laser-produced Cd and Cr plasmas
Authors: Ganeev R. A. , Suzuki M. , Baba M. , Kuroda H.
Published: February 2009

We observed the extended harmonic spectra generated in the Cd and Cr plasmas. The mechanism of harmonic extension was attributed to the harmonic generation in doubly charged particles and the influence of broadband ionic resonances in the range of 20–25 nm. The extension of harmonic cutoff up to the 73rd (Cd) and 83rd (Cr) orders was achieved.
Surface modification of organic polymer by dual action of extreme ultraviolet/visible-near infrared ultrashort pulses
Authors: Mocek T. , Polan J. , Homer P. , Jakubczak K. , Rus B. , Kim I. J. , Kim C. M. , Lee G. H. , Nam C. H. , Hájková V. , Chalupský J. , Juha L.
Published: February 2009

We present the experimental evidence of structural surface modifications of poly(methyl methacrylate) (PMMA) caused by simultaneous action of extreme ultraviolet (XUV) (∼21 nm) and visible-near infrared (visible-NIR) (820/410 nm) ultrashort pulses. Although the fluence of each individual beam was far below the surface modification threshold, very efficient and specific material expansion was observed after irradiation of PMMA by more than ∼20 shots of mixed XUV/visible-NIR radiation. As the XUV photons generate free charge carriers, absorption of the optical radiation dramatically increases, which heats up the material and further enhances the XUV induced damage to the polymer chain.
Surface photovoltage spectroscopy in a Kelvin probe force microscope under ultrahigh vacuum
Authors: Streicher F. , Sadewasser S. , Lux-Steiner M. Ch.
Published: February 2009

Surface photovoltage (SPV) spectroscopy is a common method for optoelectronic semiconductor characterization. Kelvin probe force microscopy has developed into a widely used tool for nanoscale characterization of semiconductors, metals, and insulators. We present here a setup for the measurement of local SPV spectra in a Kelvin probe force microscope operated under ultrahigh vacuum conditions. The atomic force microscope tip can be placed to any desired position with nanometer precision and the SPV can then be recorded as a function of the wavelength of the illuminating light. We introduce the realization of the setup and present the SPV spectra on two test systems, an epitaxially grown GaAs/CuGaSe2 junction and a Zn-doped CuInS2 polycrystalline thin film.
Electromigration in Sn–Cu intermetallic compounds
Authors: Wei C. C. , Chen C. F. , Liu P. C. , Chen Chih
Published: February 2009

As the shrinking in bump size continues, the effect of intermetallic compounds (IMCs) on electromigration becomes more pronounced. Electromigration in Sn–Cu intermetallic compounds was examined using edge displacement method. It was found that Cu6Sn5 compounds are more susceptible to electromigration than Cu3Sn compounds. The lower solidus temperature and higher resistivity of the Cu6Sn5 IMCs are responsible for its higher electromigration rate. Length-dependent electromigration behavior was found in the stripes of various lengths and the critical length was determined to be between 5 and 10 μm at 225 °C, which corresponded to a critical product between 2.5 and 5 A/cm. Furthermore, the Sn–Cu compounds were proven to have better electromigration resistance than eutectic SnAgCu solder.
Surface-energy-dependent field-effect mobilities up to 1 cm2/V s for polymer thin-film transistor
Authors: Umeda Tokiyoshi , Kumaki Daisuke , Tokito Shizuo
Published: February 2009

The field-effect mobility of a liquid-crystalline semiconducting polymer, poly(2,5-bis(3-hexadecylthiophene-2-yl)thieno[3,2-b]thiophene) (PB16TTT), has depended significantly on the surface energies of self-assembled monolayers (SAMs) formed on insulating layers. Using a SAM with fluoroalkyl groups, with a low surface energy of 13.3 mN/m, the mobility of PB16TTT reached as high as 1 cm2/V s. These results indicate that an edge-on orientation of the polymer chains progresses more favorably on the surfaces with low surface energies via the liquid-crystalline phase.
Elastic fields of quantum dots in semi-infinite matrices: Green’s function analytical analysis
Authors: Melezhik Evgen , Korotchenkov Oleg
Published: February 2009

We present analytical equations based on Green’s function formalism, which are used to calculate elastic fields produced by a quantum dot buried in a semi-infinite matrix with a rigid surface. The dot of an arbitrary shape as well as quantum dot arrays can be very efficiently carried out within the presented technique. These equations can be exploited for matrices with weak electromechanical coupling, making the computations much faster than other approaches based on Green’s functions.
Multiphoton absorption and refraction in Mn2+ doped ZnS quantum dots
Authors: Chattopadhyay M. , Kumbhakar P. , Tiwary C. S. , Sarkar R. , Mitra A. K. , Chatterjee U.
Published: February 2009

Four-photon absorption and self-defocusing nonlinear refraction are reported for the first time in ZnS and Mn2+ doped ZnS quantum dots of sizes 2–3 nm with 1064 nm radiation from a Q-switched neodymium doped yttrium aluminum garnet laser at a peak intensity of 2.5 GW/cm2.
An O18 study of the interaction between carbon monoxide and dry thermal SiO2 at 1100 °C
Authors: Cavellin Catherine Deville , Trimaille Isabelle , Ganem Jean-Jacques , D’Angelo Marie , Vickridge Ian , Pongracz Anita , Battistig Gabor
Published: February 2009

The mechanisms of oxygen exchange between thermal silicon oxide films and carbon monoxide have been studied using O18 as an isotopic tracer. SiO2 layers of natural isotopic composition, obtained by thermal oxidation of silicon, were exposed at 1100 °C to C13O18 gas at pressures ranging from 50 to 350 mbars. O18 concentration depth profiles were determined using the nuclear narrow resonance profiling technique with the narrow resonance near 151 keV in the reaction O18(p,α)N15. The results show that oxygen exchange takes place via two distinct processes and a mechanism for each process is proposed in the present work. The diffusion coefficient of CO molecules in the silica and the oxygen exchange frequency between CO and the silica are also determined.
Simultaneous observation and analysis of sedimentation and floating motions of microspheres investigated by phase mode–dynamic ultrasound scattering
Authors: Nagao Ayumi , Kohyama Mariko , Norisuye Tomohisa , Tran-Cong-Miyata Qui
Published: February 2009

A high frequency dynamic ultrasound scattering technique was developed to evaluate the mean velocity and the velocity fluctuations of micron-sized beads in highly turbid suspensions. In contrast to the previous study, scattering phase was fully utilized to investigate the dynamics of mixtures consisting of settling and floating microspheres. The velocities of the particles moving upward and downward with respect to gravity were simultaneously measured by a single acquisition. Instantaneous velocities determined by the time derivative of the scattering phase were evaluated as functions of the evolution time and position of the scatterer. (1) The velocity analysis in the time domain, (2) the phase analysis without phase unwrapping, and (3) the effects of noise filtering were discussed. The results were compared with those derived from amplitude mode–dynamic ultrasound scattering.
Reflectance and absorbance of all-dielectric metamaterial composites with fractal boundaries: A numerical investigation
Authors: Mejdoubi A. , Brosseau C.
Published: February 2009

Two-dimensional two-phase heterostructures have been the subject of many investigations including computer simulations. An outstanding issue is the shape analysis of the reflectance for dielectric inclusions of arbitrarily complex geometry. We report on finite element numerical calculations to investigate the effects a fractal-shaped object having an intrinsic complex permittivity with a negative real part embedded in a host matrix with real and positive permittivity can have on reflectance and absorbance of a uniform plane wave impinging on it. The simulations were performed at a wavelength much larger than the length scales of the inhomogeneities. The reflectance and absorbance of the structures are shown to be highly sensitive to the details of the fractal metamaterial environment and to its intrinsic loss. We comment on how these results may be used to provide a quantitative framework for the design, selection, and optimization of artificial metamaterial microwave reflectors and other complex systems yet to be explored.
Producing and quantifying enriched para-H2
Authors: Tom Brian A. , Bhasker Siddhartha , Miyamoto Yuki , Momose Takamasa , McCall Benjamin J.
Published: February 2009

The production of enriched para-H2 is useful for many scientific applications, but the technology for producing and measuring para-H2 is not yet widespread. In this note and in the accompanying auxiliary material, we describe the design, construction, and use of a versatile standalone converter that is capable of producing para-H2 enrichments of up to ≥99.99% at continuous flow rates of up to 0.4 SLM. We also discuss para-H2 storage and back conversion rates, and improvements to three techniques (thermal conductance, NMR, and solid hydrogen impurity spectroscopy) used to quantify the para-H2 enrichment.
Mean-field interacting boson random point fields in weak harmonic traps
Authors: Tamura Hiroshi , Zagrebnov Valentin A.
Published: February 2009

A model of the mean-field interacting boson gas trapped by a weak harmonic potential is considered by the boson random point field methods. We prove that in the weak harmonic trap limit there are two phases distinguished by the boson condensation and by a different behavior of the local particle density. For chemical potentials less than a certain critical value, the resulting random point field (RPF) coincides with the usual boson RPF, which corresponds to a noninteracting (ideal) boson gas. For the chemical potentials greater than the critical value, the boson RPF describes a divergent (local) density, which is due to localization of the macroscopic number of condensed particles. Notice that it is this kind of transition that observed in experiments producing the Bose–Einstein condensation in traps.
Kinetic surface roughening for the Mullins–Herring equation
Authors: Darvish Esmat , Masoudi Amir Ali
Published: February 2009

Using the linearity property of the Mullins–Herring equation when the velocity is zero with a Gaussian noise, we obtain an analytic form for the global mean square surface width and height-height correlation function. This can be used to read the critical exponents in any dimension. In particular, for d=1 we show that although the surface is super rough, the system exhibits Family–Vicsek scaling behavior.
Growth stress in tungsten carbide-diamond-like carbon coatings
Authors: Pujada B. R. , Tichelaar F. D. , Arnoldbik W. M. , Sloof W. G. , Janssen G. C. A. M.
Published: February 2009

Growth stress in tungsten carbide-diamond-like carbon coatings, sputter deposited in a reactive argon/acetylene plasma, has been studied as a function of the acetylene partial pressure. Stress and microstructure have been investigated by wafer curvature and transmission electron microscopy (TEM) whereas composition and energy distribution functions of positive ions were obtained by electron probe microanalyzer, elastic recoil detection analysis, and mass-energy analyzer (MEA). It has been observed that the compressive stress decreases with increasing acetylene partial pressure, showing an abrupt change from −5.0 to −1.6 GPa at an acetylene partial pressure of 0.012 Pa. TEM micrographs show that by increasing the acetylene partial pressure in the plasma from 0 to 0.012 Pa, the microstructure of the coating changes from polycrystalline to amorphous. MEA results show that the most probable energy of positive ions bombarding the substrate during deposition in pure argon and argon/acetylene atmosphere is the same. Based on the results, it is concluded that the huge variation in the compressive stress at low acetylene partial pressures is due to a change in the microstructure of the coating from polycrystalline to amorphous and not to the energy of positive ions bombarding the film.
Metal-insulator transition induced by postdeposition annealing in low doped manganite films
Authors: Sirena M. , Haberkorn N. , Granada M. , Steren L. B. , Guimpel J.
Published: February 2009

We studied the transport and magnetic properties of low-doped manganite films after different oxygenation processes. The oxygen content was adjusted by postdeposition annealing at different oxygen pressures and annealing times. For all the samples we observed an increase in the Curie temperature and the remnant magnetization with the oxygen content. In general, for decreasing number of oxygen vacancies, samples under expansive strain become more homogeneous and their electrical resistivity decreases. A metal-insulator transition is induced in highly oxygenated films grown on SrTiO3, probably related to a shift of the mobility edge crossing below the Fermi energy. We found that the oxygenation dynamics depend critically on the strain field induced by the substrates and also on the Sr doping concentration.
Modeling the dynamical response of ferromagnetic shape memory alloy actuators using a dissipative Euler–Lagrange equation
Authors: Weetman Philip , Akhras George
Published: February 2009

A phenomenological dynamical model of ferromagnetic shape memory alloy based actuators is developed. The parameters of effective mass density, viscosity, and elasticity are defined and used in a dissipative Euler–Lagrange equation to determine the martensite variant fraction and strain as a function of time. These three parameters are determined by fitting our simulations to recent experiments on a NiMnGa based actuator. In addition to the simplicity of only three fitting parameters to model martensite variant evolution, the present model is a convenient formulation of the problem because it incorporates self-consistently all stresses and loads in the system.
Heterodyne lock-in thermal coupling measurements in integrated circuits: Applications to test and characterization
Authors: Altet J. , Aldrete-Vidrio E. , Mateo D. , Salhi A. , Grauby S. , Claeys W. , Dilhaire S. , Perpiñà X. , Jordà X.
Published: February 2009

Heterodyne strategies can be used to characterize thermal coupling in integrated circuits when the electrical bandwidth of the dissipating circuit is beyond the bandwidth of the thermal coupling mechanism. From the characterization of the thermal coupling, two possible applications are described: extraction of characteristics of the dissipating circuit (the determination of the center frequency of a low-noise amplifier) and the extraction of the thermal coupling transfer function.
Thermodynamic properties of lithium amide under hydrogen pressure determined by Raman spectroscopy
Authors: Hino Satoshi , Ogita Norio , Udagawa Masayuki , Ichikawa Takayuki , Kojima Yoshitsugu
Published: February 2009

Partial pressures and equilibrium constants for the reaction from lithium amide and hydrogen to lithium hydride and ammonia at different temperatures were evaluated by Raman spectroscopy to determine standard enthalpy ΔHo and entropy ΔSo of the reaction. Raman intensity measurements were used for estimating ammonia partial pressures in gaseous products after heat treatment of lithium amide under hydrogen atmosphere. The van’t Hoff plot of fractions of the partial pressures of ammonia and hydrogen indicated ΔHo=49.9±9.1 kJ mol−1 and ΔSo=59±16 J mol−1 K−1.
Critical field enhancement in hybrid superconductor/ferromagnet mesoscopic disks
Authors: Schildermans N. , Silhanek A. V. , Sautner J. , Metlushko V. , Vavassori P. , Moshchalkov V. V.
Published: February 2009

We investigated experimentally the nucleation of superconductivity in a mesoscopic hybrid structure, consisting of a thin superconducting disk covered with a ferromagnetic layer with an in-plane magnetic moment. By applying a magnetic field in the plane of the structure, the remanent magnetic state of the ferromagnet can be switched from a flux-closure state where field lines are confined inside the ferromagnet to a polarized state with nonzero stray fields at the edges. This change in the magnetic state causes a drastic modification on the superconductor/normal-state phase boundary of the hybrid sample. In the polarized state a re-entrant transition line and a strong broadening of the phase boundary are observed.
Modification of molecular transitions in fullerene films under ion impacts
Authors: Kumar Amit , Podhorodecki A. , Misiewicz J. , Avasthi D. K. , Pivin J. C.
Published: February 2009

The photoluminescence properties of as-deposited and 90 MeV Si ion irradiated fullerene films are reported. The irradiation of fullerene films leads to some enhancement or quenching in molecular transition intensities (photoluminescence intensities) depending on the ion fluence. These effects are explained in terms of lowering in the energy level symmetry resulting from induced chemical disordering. The structural modifications occurring upon irradiation were investigated using Fourier transform infrared and UV-vis spectroscopies.
Planar hybrid superlattices by compression of rolled-up nanomembranes
Authors: Zander T. , Deneke Ch. , Malachias A. , Mickel Ch. , Metzger T. H. , Schmidt O. G.
Published: February 2009

Hybrid metal/semiconductor superlattices are obtained by the controlled compression of rolled-up metal/semiconductor nanomembranes. The planar superlattice maintains the crystalline quality of the semiconductor and the polycrystalline texture of the metal, allowing the integration of these materials into a multilayer system that cannot be produced by any direct deposition procedure. The superlattice consists of two symmetrically inverted multilayer stacks splitted by a metallic mirror plane. Additional intensity peaks in x-ray reflectivity confirm the periodic structure created by the roll up and compression of nanomembranes.
Experimental observation of whispering gallery modes in sector disk lasers
Authors: Monakhov A. M. , Sherstnev V. V. , Astakhova A. P. , Yakovlev Yu. P. , Boissier G. , Teissier R. , Baranov A. N.
Published: February 2009

The existence of the whispering gallery modes in a sector disk resonator has been demonstrated theoretically and experimentally observed in a disk, half-disk, and quarter-disk semiconductor lasers. The half-disk and quarter-disk lasers are fabricated from 285μm diameter disks emitted near 2.13μm at room temperature with continuous wave threshold currents of 55–60mA. Mode spacing in emission spectra of the half-disk lasers was the same as for the whole disk and doubled for the quarter disk as expected from the theoretical analysis. Half-disk lasers emit two parallel beams from the cleaved facet, while the two output beams are perpendicular in quarter-disk devices.
Photoresponse in large area multiwalled carbon nanotube/polymer nanocomposite films
Authors: Stokes Paul , Liu Liwei , Zou Jianhua , Zhai Lei , Huo Qun , Khondaker Saiful I.
Published: February 2009

We present a near IR photoresponse study of large area multiwalled carbon nanotube/poly(3-hexylthiophene)-block-polystyrene polymer (MWNT/P3HT-b-PS) nanocomposite films for different loading ratio of MWNT into the polymer matrix. We show that the photocurrent strongly depends on the position of the laser spot with maximum photocurrent occurring at the metal-film interface. In addition, compared to the pure MWNT film, the photoresponse is much larger in the MWNT/polymer composite films. The time constant for the photoresponse is slow and varies between 0.6 and 1.2 s. We explain the photoresponse by Schottky barrier modulation at the metal-film interface.
Ultrafast optofluidic gain switch based on conjugated polymer in femtosecond laser fabricated microchannels
Authors: Vishnubhatla Krishna Chaitanya , Clark Jenny , Lanzani Guglielmo , Ramponi Roberta , Osellame Roberto , Virgili Tersilla
Published: February 2009

We report on an optofluidic gain switch based on polydioctylfluorene solution in a femtosecond laser fabricated microchannel. The solution provides a stimulated emission cross section of the order of 10−16 cm2. Depending on the good isolation of polymeric chains we obtain ultrafast gain switching with time constants below 150 fs and high on/off ratios (up to 100%) in a broad spectral region (450–500 nm). This enables potential modulation rates well in the terahertz range. The enhanced optofluidic functionalities combined with the flexibility of femtosecond laser micromachining pave the way to an innovative class of optical devices easily integratable in complex systems.
Formation and manipulation of two-dimensional arrays of micron-scale particles in microfluidic systems by surface acoustic waves
Authors: Wood C. D. , Cunningham J. E. , O’Rorke R. , Wälti C. , Linfield E. H. , Davies A. G. , Evans S. D.
Published: February 2009

The two-dimensional concentration and manipulation of micron-scale particles by orthogonal, surface acoustic, standing waves is demonstrated. The particles are organized by liquid pressure waves in a microfluidic system over a piezoelectric substrate and form a uniform two-dimensional array with a spacing governed by the mechanical nodes of the two orthogonal, surface acoustic, standing waves. The nodal spacing can be controlled in each orthogonal direction independently by adjustment of the radio frequency applied to the separate acoustic wave transducers. This technique could be used to enhance the particle concentrations at sensing locations in DNA or protein array detectors.
Single electron transfer in thermally annealed nanoparticle dropcast thick films
Authors: Pradhan Sulolit , Kang Xiongwu , Mendoza Ernesto , Chen Shaowei
Published: February 2009

A very simple and effective procedure based on thermal annealing was reported in inducing discrete charge transfer in nanoparticle solid films. The particle ensembles were prepared by dropcasting a particle solution onto an interdigitated array electrode. The as-prepared particle films exhibited only linear featureless current-potential profiles in conductivity measurements, whereas after thermal annealing, well-defined staircase features of single electron transfer started to emerge at temperatures higher than 300 K. This was accounted for by the combined consequence of structural rearrangements of nanoparticle cores within the organic protecting matrix and thermal activation of interparticle charge transport.
Impact of near-surface defects and morphology on ZnO luminescence
Authors: Doutt D. , Mosbacker H. L. , Cantwell G. , Zhang J. , Song J. J. , Brillson L. J.
Published: February 2009

We have used depth-resolved cathodoluminescence spectroscopy (DRCLS) to measure the distribution of deep level defects at and below the surface of ZnO crystals grown by vapor phase transport, hydrothermal, and melt-growth methods. DRCLS reveals large variations in defect distributions with depth on a nanometer scale that correlate with maps of potential and surface morphology measured by Kelvin probe force and atomic force (AFM) microscopies, respectively. A strong correlation between the optical emission efficiency of the nanoscale subsurface region and the AFM surface roughness reveals a figure of merit for substrate polishing and etching.