bluuebunny/crossref_metadata_2025 huggingface dataset:
Density-induced coupling effects on the dispersivity of a flexible chain particle
Authors: Van Dyke Marc , Haber Shimon
Published: May 2004

A model is introduced to investigate the transport properties of an inhomogeneously dense flexible chain particle. The specific model used is a sedimenting non-neutrally buoyant inhomogenously weighted flexible Brownian dumbbell, and it is shown that density inhomogeneity gives rise to a novel coupling effect between the “shape-fluctuation” and “size-fluctuation” dispersion mechanisms. The previously reported shape-fluctuation dispersion term stems from the dumbbell’s nonspherical shape and the ensuing anisotropic mobility tensor, while the already investigated size fluctuation term is the result of the dependence of the overall dumbbell translational mobility on the separation distance between the constitutive spheres. Because the density of the constitutive spheres is unequal, the external force simultaneously reorients and deforms the flexible dumbbell, and it is this mutual dependence between dumbbell orientation and size that induces the coupling. Numerical results are presented for the case of a tethered dumbbell composed of two spheres, identical in size but differing in density. The “weak-field” limit is addressed, where the externally applied torque and particle deformation forces are dominated by the thermal fluctuations associated with rotational and deformation Brownian motion. This numerical solution, obtained by including a large number of higher order hydrodynamic interactions (120 terms), describes the Brownian particle’s long-time transport without resorting to ad hoc approximations, such as preaveraging the hydrodynamic force or incorporating only first-order hydrodynamic interaction effects (such as employing the Burgers–Oseen tensor). Separate analytical solutions, based on these respective approximations, are also presented and it is concluded that in the limit of “long tethers,” where the ratio of tether length to sphere size is greater than seven, no more than 15% error is introduced by neglecting higher-order hydrodynamic interactions. Similarly, the preaveraging approximation introduces no more than a few percent error in the limit of “almost-rigid” dumbbells, where the ratio of tether length to sphere size is less than three. For tethers of “intermediate” length, the full numerical solution must be employed.
Kapitza conductance and phonon scattering at grain boundaries by simulation
Authors: Schelling P. K. , Phillpot S. R. , Keblinski P.
Published: May 2004

We use a nonequilibrium molecular-dynamics method to compute the Kapitza resistance of three twist grain boundaries in silicon, which we find to increase significantly with increasing grain boundary energy, i.e., with increasing structural disorder at the grain boundary. The origin of this Kapitza resistance is analyzed directly by studying the scattering of packets of lattice vibrations of well-defined polarization and frequency from the grain boundaries. We find that scattering depends strongly on the wavelength of the incident wave packet. In the case of a high-energy grain boundary, the scattering approaches the prediction of the diffuse mismatch theory at high frequencies, i.e., as the wavelength becomes comparable to the lattice parameter of the bulk crystal. We discuss the implications of our results in terms of developing a general model of scattering probabilities that can be applied to mesoscale models of heat transport in polycrystalline systems.
Fractal geometry model for effective thermal conductivity of three-phase porous media
Authors: Ma Yongting , Yu Boming , Zhang Duanming , Zou Mingqing
Published: May 2004

An approximate model, a fractal geometry model, for the effective thermal conductivity of three-phase/unsaturated porous media is proposed based on the thermal-electrical analogy technique and on statistical self-similarity of porous media. The proposed thermal conductivity model is expressed as a function of porosity (related to stage n of Sierpı́nski carpet), ratio of areas, ratio of component thermal conductivities, and saturation. The recursive algorithm for the thermal conductivity by the proposed model is presented and found to be quite simple. The model predictions are compared with the existing measurements. Good agreement is found between the present model predictions and the existing experimental data. This verifies the validity of the proposed model.
Low temperature oxidation of SiC preamorphized by ion implantation
Authors: Poggi Antonella , Nipoti Roberta , Solmi Sandro , Barozzi Mario , Vanzetti Lia
Published: May 2004

In this article, we investigate the thermal growth of SiO2 films on 6H-SiC preamorphized by Ar+ ion implantation. The experimental conditions to grow oxide layers on SiC with high oxidation rate and low temperature have been determined. Rutherford back scattering channeling, secondary ion mass spectrometry, and x-ray photoelectron spectroscopy analyses have been used to investigate the oxide characteristics and composition. During wet oxidation at 800 °C, a linear growth rate of about 0.7 nm/min has been found for layers as thick as 240 nm. The presence of a SiO2Cx transition region between a stoichiometric SiO2 layer and the SiC substrate has been evidenced. The thickness of this transition layer increases with the oxidation time up to reach 118 nm for 390 min.
Preferred orientation of DNA oligonucleotide probes on the (2×4) reconstructed surface of (001) GaAs
Authors: Mohaddes-Ardabili Ladan , Martı́nez-Miranda Luz J. , Salamanca-Riba Lourdes G. , Christou Aristos , Silverman Joseph , Bentley William E. , Al-Sheikhly Mohamad
Published: May 2004

Determination of the orientation of the thiol-derivatized DNA oligonucleotide monolayer on arsenic terminated GaAs(001) has been performed using grazing incidence x-ray scattering. DNA oligonucleotides were observed to orient along the trenches of the 2X direction of the GaAs(001)-(2×4) reconstructed surface, at an angle of 33° with respect to the surface. The addition of mercaptohexanol spacer tended to partially align the DNA oligonucleotides at almost 48° with respect to GaAs (001) surface.
Ferromagnetic tunneling junctions at low voltages: Elastic versus inelastic scattering at T=0 °K
Authors: Dartora C. A. , Cabrera G. G.
Published: May 2004

In this article we analyze different contributions to the magnetoresistance of magnetic tunneling junctions at low voltages. A substantial fraction of the resistance drop with voltage can be ascribed to variations of the density of states and the barrier transmission with the bias. However, we found that the anomaly observed at zero bias and the magnetoresistance behavior at very small voltages, point to the contribution of inelastic magnon-assisted tunneling. The latter is described by a transfer parameter TJ, which is one or two orders of magnitude smaller than Td, the direct transmission for elastic currents. Our theory is in excellent agreement with experimental data, yielding estimated values of TJ which are of the order of Td/TJ∼40.
Domain structure in the monoclinic Pm phase of Pb(Mg1/3Nb2/3)O3–PbTiO3 single crystals
Authors: Bokov A. A. , Ye Z.-G.
Published: May 2004

The domain structure of (1−x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 single crystals with composition x≈0.33 in the range of the morphotropic phase boundary (MPB) was studied. Based on the analysis of spontaneous strain compatibility and charge of domain walls, we have established the permissible domain arrangements for the ferroelectric phases of different symmetries, which are expected to occur in the range of the MPB. Examination of (001)-oriented unpoled and electrically poled (along the [001] direction) crystal plates in a polarizing microscope reveals a monophase state with the domain structure compatible with the structure theoretically predicted for the MC monoclinic phase (space group Pm), which was recently discovered in the compositions close to the MPB by x-ray and neutron diffraction studies. In the unpoled crystal, the 180° walls between the domains whose spontaneous polarization vectors are parallel to the plane of the crystal plate (i.e., a-domains) are observed. The domain structure of the poled crystal is predominantly composed of crystallographically prominent Wf walls parallel to (001) (i.e., the plane of the crystal plate) and inclined S walls parallel to [110] or [11̄0] direction. In poled and unpoled samples, an optical rotatory polarization effect is found, which is related to the inhomogeneity of the sample resulting from the mixture of domains.
Confirmation of the “long-lived” tetra-nitrogen (N4) molecule using neutralization-reionization mass spectrometry and ab initio calculations
Authors: Rennie Emma E. , Mayer Paul M.
Published: May 2004

Tetra-nitrogen (N4), which has been the subject of recent controversy [Cacace, d. Petris, and Troiani, Science 295, 480 (2002); Cacace, Chem. Eur. J. 8, 3839 (2002); Nguyen et al., J. Phys. Chem. A 107, 5452 (2003); Nguyen, Coord. Chem. Rev. 244, 93 (2003)] as well as of great theoretical interest, has been prepared from the N4+⋅ cation and then detected as a reionized gaseous metastable molecule with a lifetime exceeding 0.8 μs in experiments based on neutralization-reionization mass spectrometry. Moreover, we have used the nature of the charge-transfer reaction which occurs between a beam of fast N4+⋅ ions (8 keV translational energy) and various stationary gas targets to identify the vertical neutralization energy of the N4+⋅ ion. The measured value, 10.3±0.5, most closely matches that of the lowest energy azidonitrene N4+⋅4Cs(4A′) ion, resulting in the formation of the neutral bound azidonitrene N43Cs(3A″). Neutralization of the global minimum N4+⋅2D∞h(2Σu+) ion leads to a structure 166 kJ mol−1 above the dissociation products [N2(1Σg+)+N2(1Σg+)]; moreover, it was not possible to find a minimum on the N41 neutral potential energy surface for a covalently bonded structure. Ab initio calculations at the G3, QCISD/6-31G(d), and MP2/AUG-cc-pVTZ levels of theory have been used to determine geometries and both vertical neutralization energies of ions (doublet and quartet) and ionization energies of neutrals (singlet and triplet). In addition, we have also described in detail the EI ion source for the Ottawa VG ZAB mass spectrometer [Holmes and Mayer, J. Phys. Chem. A 99, 1366 (1995)] which was modified for high-pressure use, i.e., for the production of dimer and higher number cluster ions.
Positive and negative polarity current pulse characteristics of a helium glow discharge in a cylinder-plane electrode gap at atmospheric pressure
Authors: Radu I. , Bartnikas R. , Wertheimer M. R.
Published: May 2004

The positive and negative polarity discharge current pulse forms were examined between steel cylinder-Al2O3 plane electrodes in helium glow and pseudoglow discharges under atmospheric pressure at 10 kHz. The theoretically determined current pulses agreed very closely in form and magnitude with those recorded experimentally. The electronic and ionic charge carrier components comprising the negative polarity current pulse were found to be very nearly equal in magnitude, in contradistinction to the positive polarity pulse, where the electronic charge carrier component was dominant. This larger electronic charge carrier component was reflected in the magnitude of the corresponding photocurrent pulse, which substantially exceeded that of the negative polarity counterpart. The glow discharge regime current pulse forms were also compared to the pulse forms characterizing pseudoglow discharges, also obtained at 10 kHz but at a more elevated voltage, as well as with those recorded at a reduced frequency of 4 kHz but at the same value of applied voltage. Good agreement was also found to exist between the experimentally determined ultrahigh-speed image patterns of the propagating positive and negative discharges and the corresponding calculated electron density contours within the gap.
Electron mean free path in fused silica and optical fiber preform
Authors: Chua Kang-Bin , Österberg Ulf
Published: May 2004

Electron scattering mean free paths in fused silica and optical fiber preforms were measured for electric fields less than 5×104 V/cm. This was done by fitting the measured photo-injected current as a function of applied voltage to a current–voltage model which takes the effects of scattering in the image force potential well and barrier lowering into account. The mean free paths measured from positive (negative) bias voltage in fused silica and optical fiber perform cladding and core (20 mol % GeO2) are 29±2 Å (29±2Å), 40±3 Å (41±2Å) and 39±2 Å (53±6Å), respectively.
Modeling crosshatch surface morphology in growing mismatched layers. Part II: Periodic boundary conditions and dislocation groups
Authors: Andrews A. M. , LeSar R. , Kerner M. A. , Speck J. S. , Romanov A. E. , Kolesnikova A. L. , Bobeth M. , Pompe W.
Published: May 2004

We present further developments and understanding of the commonly observed crosshatch surface morphology in strain-relaxed heteroepitaxial films. We have previously proposed that the crosshatch morphology is directly related with strain relaxation via threading dislocation glide which results in both surface step and misfit dislocation (MD) formation [see Andrews et al., J. Appl. Phys. 91, 1933 (2002)—now referred to as Part I]. In this article, we have used solutions for the stress fields and displacement fields for periodic MD arrays which include the effects of the free surface. These solutions avoid truncation errors associated with finite dislocation arrays that were used in Part I. We have calculated the surface height profile for relaxed films where the misfit dislocations were introduced randomly or the misfit dislocations were placed in groups with alternating sign of the normal component of their Burgers vector. We have calculated the surface height profiles where the slip step remains at the surface [“slip step only” (SSO)] and where the slip step is eliminated [“slip step eliminated” (SSE)] due to annihilation of opposite sense steps, such as could happen during growth or lateral mass transport. For relaxed films, we find that the surface height undulations, characteristic of crosshatch, increase with increasing film thickness for the SSO case, whereas the surface becomes flatter for the SSE case. Experiments on relaxed In0.25Ga0.75As films on (001) GaAs show that the surface height undulations in the [110] direction increase with increasing film thickness. Thus, we conclude that with increasing film thickness the crosshatch in the slow diffusion [110] direction is best described by the SSO case.
Ferroelectric domain structures in SrBi2Nb2O9 epitaxial thin films: Electron microscopy and phase-field simulations
Authors: Li Y. L. , Chen L. Q. , Asayama G. , Schlom D. G. , Zurbuchen M. A. , Streiffer S. K.
Published: May 2004

Ferroelectric domain structures of (001)SrBi2Nb2O9 epitaxial films, investigated using both transmission electron microscopy and phase-field simulations, are reported. Experiment and numerical simulation both reveal that the domain structures consist of irregularly shaped domains with curved domain walls. It is shown that the elastic contribution to domain structures can be neglected in SrBi2Nb2O9 due to its small ferroelastic distortion, less than 0.0018%. Two-beam dark-field imaging using reflections unique to domains of each of the two 90° polarization axes reveal the domain structure. Phase-field simulation is based on the elastic and electrostatic solutions obtained for thin films under different mechanical and electric boundary conditions. The effects of ferroelastic distortion and dielectric constant on ferroelectric domains are systematically analyzed. It is demonstrated that electrostatic interactions which favor straight domain walls are not sufficient to overcome the domain wall energy which favors curved domains in SrBi2Nb2O9.
Nuclear waste disposal—pyrochlore (A2B2O7): Nuclear waste form for the immobilization of plutonium and “minor” actinides
Authors: Ewing Rodney C. , Weber William J. , Lian Jie
Published: May 2004

During the past half-century, the nuclear fuel cycle has generated approximately 1400 metric tons of plutonium and substantial quantities of the “minor” actinides, such as Np, Am, and Cm. The successful disposition of these actinides has an important impact on the strategy for developing advanced nuclear fuel cycles, weapons proliferation, and the geologic disposal of high-level radioactive waste. During the last decade, there has been substantial interest in the use of the isometric pyrochlore structure-type, A2B2O7, for the immobilization of actinides. Most of the interest has focused on titanate-pyrochlore because of its chemical durability; however, these compositions experience a radiation-induced transition from the crystalline-to-aperiodic state due to radiation damage from the alpha-decay of actinides. Depending on the actinide concentration, the titanate pyrochlore will become amorphous in less than 1000 years of storage. Recently, systematic ion beam irradiations of a variety of pyrochlore compositions has revealed that many zirconate pyrochlores do not become amorphous, but remain crystalline as a defect fluorite structure-type due to disordering of the A- and B-site cations. The zirconate pyrochlores will remain crystalline even to very high doses, greater than 100 displacements per atom. Systematic experimental studies of actinide-doped and ion beam-irradiated pyrochlore, analyses of natural U- and Th-bearing pyrochlore, and simulations of the energetics of the disordering process now provide a rather detailed understanding of the structural and chemical controls on the response of pyrochlore to radiation. These results provide a solid basis for predicting the behavior and durability of pyrochlore used to immobilize plutonium.
Electrical and structural properties of poly-SiGe film formed by pulsed-laser annealing
Authors: Watakabe H. , Sameshima T. , Kanno H. , Sadoh T. , Miyao M.
Published: May 2004

Electrical and structural properties of polycrystalline silicon germanium (poly-SiGe) films fabricated by pulsed-laser annealing were investigated. Observation of laser-induced melt-regrowth of SiGe films using transient conductance measurement revealed that the melt depth and the crystallization velocity increased as Ge concentration increased. The increase of the crystallization velocity resulted in increase of the average size of crystalline grains from 66 to 120 nm at the laser energy density of 360 mJ/cm2 with increasing Ge concentration from 0 to 60%. The crystalline volume ratio obtained by reflectivity spectra in the ultraviolet region also increased from 0.83 to 1.0. Numerical analysis revealed that the density of electrically active defects decreased from 3.5×1018 to 1.1×1018 cm−3 as Ge concentration increased from 0 to 80%. The density of defect states of Si0.8Ge0.2 films were reduced from 3.5×1018 to 1.9×1018 cm−3 by 13.56-MHz hydrogen plasma treatment at 250 °C, 30 W, and 130 Pa for 30 s. However, the plasma treatment did not reduce the defect density for Si0.4Ge0.6 and Si0.2Ge0.8 films.
Polar isomer of formic acid dimers formed in helium nanodroplets
Authors: Madeja Frank , Havenith Martina , Nauta Klaas , Miller Roger E. , Chocholoušová Jana , Hobza Pavel
Published: May 2004

The infrared spectrum of formic acid dimers in helium nanodroplets has been observed corresponding to excitation of the “free” OH and CH stretches. The experimental results are consistent with a polar acyclic structure for the dimer. The formation of this structure in helium, as opposed to the much more stable cyclic isomer with two O–H⋯O hydrogen bonds, is attributed to the unique growth conditions that exist in helium droplets, at a temperature of 0.37 K. Theoretical calculations are also reported to aid in the interpretation of the experimental results. At long range the intermolecular interaction between the two monomers is dominated by the dipole–dipole interaction, which favors the formation of a polar dimer. By following the minimum-energy path, the calculations predict the formation of an acyclic dimer having one O–H⋯O and one C–H⋯O contact. This structure corresponds to a local minimum on the potential energy surface and differs significantly from the structure observed in the gas phase.
Quantum lens in an external electric field: Anomalous photoluminescence behavior
Authors: Rodrı́guez Arezky H. , Trallero-Giner C.
Published: May 2004

Electric field (F) effects in self-assembled quantum dots with a lens geometry have been studied. The optical selection rules and the origin of the anomalous behavior of the photoluminescence lines with the applied field reported by Raymond et al. [Phys. Rev. B 58, R13415 (1998)] are analyzed in terms of the interband oscillator strength and lens symmetry. Also, an explicit analytical representation in the framework of the simple parabolic model for the electronic states as a function of F and lens parameter are given. The excitonic effect has been considered in the strongly confined regime when the excitonic Bohr radius is smaller than the dot dimensions. The influence of the lens geometry on the quantum Stark effect shows an asymmetric energy shift in the electron-hole transition.
Grain growth theories and the isothermal evolution of the specific surface area of snow
Authors: Legagneux Loïc , Taillandier Anne-Sophie , Domine Florent
Published: May 2004

Quantifying the specific surface area (SSA) of snow and its variation during metamorphism is essential to understand and model the exchange of reactive gases between the snowpack and the atmosphere. Isothermal experiments were conducted in a cold room to measure the decay rate of the SSA of four snow samples kept in closed systems at −15 °C. In all cases, a logarithmic law of the form SSA=B−A ln(t+Δt) fits the SSA decrease very well, where A, B and Δt are adjustable parameters. B is closely related to the initial SSA of the snow and A describes the SSA decay rate. These and previous data suggest the existence of a linear relationship between A and B so that it may be possible to predict the decay rate of snow SSA from its initial value. The possibility that grain coarsening theories could explain these observations was investigated. The logarithmic equation was shown to be an approximation of a more general equation, that describes the time evolution of the mean grain radius R in most grain coarsening theories, such as Ostwald ripening: R̄n−R̄0n=Kt. R0¯ is the initial mean grain radius, R̄ is the mean grain radius, n and K are the growth exponent and the growth rate, respectively. Values of n between 2.8 and 5.0 are found. It is concluded that snow metamorphism and Ostwald ripening processes are governed by similar rules. Ostwald ripening theories predict that a steady-state regime is reached after a transient stage, but our results suggest that the steady-state regime is not reached after a few months of isothermal snow metamorphism. This last feature makes is difficult to predict the rate of decrease of snow SSA using the theory of Ostwald ripening.
Radiation tolerance of Si/Si0.6Ge0.4 resonant interband tunneling diodes
Authors: Weaver B. D. , Thompson P. E. , Jin N. , Chung S.-Y. , Rice A. T. , Berger P. R.
Published: May 2004

The effect of 2 MeV proton irradiation on the current–voltage (IV) characteristics of Si/Si0.6Ge0.4 resonant interband tunneling diodes (RITDs) is reported. A fluence of 5×1014 H+/cm2 causes the peak current to increase by about 4% the valley current to nearly double and the peak-to-valley current ratio to be reduced by about half. At comparable fluences, most minority carrier diodes are inoperational. Radiation-induced changes are compared to changes in the IV curves of irradiated Si- and Ge Esaki diodes, GaSb-based RITDs and InP-based resonant tunneling diodes.
Existence of the CuPt-type ordering due to the surface undulation in CdxZn1−xTe epilayers grown on ZnTe buffer layers
Authors: Lee H. S. , Lee J. Y. , Kim T. W. , Lee I. , Park H. L.
Published: May 2004

The relationship between the CuPt-type ordering and the surface undulation in CdxZn1−xTe epitaxial layers grown on ZnTe buffer layers was investigated. The results of selected area electron diffraction pattern and transmission electron microscopy measurements showed that CuPt-type ordered structures were formed in the CdxZn1−xTe epitaxial layers. The atomic force microscopy image showed that the surface undulations were created from lattice mismatch between the CdxZn1−xTe ZnTe thin films and the GaAs substrate. The surface undulations provided the [110] steps, which enhanced the formation of CuPt-type ordering in highly strained CdxZn1−xTe epilayers. These results provide important information on the relationship between the microstructural and surface properties in lattice mismatched heteroepilayers in the CdxZn1−xTe/ZnTe system.
Energy transfer from a laser pulse to a blast wave in reduced-pressure air atmospheres
Authors: Mori Koichi , Komurasaki Kimiya , Arakawa Yoshihiro
Published: May 2004

Focusing a transversely excited atmospheric CO2 laser beam in air atmospheres induced a blast wave. The kinetic energy of a laser-induced blast wave was determined from shadowgraph images of shock wave expansion. Results showed that the fraction of input laser energy that is converted into the blast wave energy decreased from 0.45 to 0.2 concomitant with the decrease in ambient pressure from 100 to 10 kPa. Also, it was insensitive to input laser energy from 4 to 13 J.
Guided propagation in electric-field-controlled hybrid nematic waveguides
Authors: Marangoni M. , Osellame R. , Ramponi R. , Buscaglia M. , Bellini T.
Published: May 2004

The alignment of nematic liquid crystals in a hybrid cell can be easily reconfigured by applying an external electric field. We exploit this feature to realize a waveguiding structure whose refractive index profile can be dynamically reshaped. The mode structure of the guided light, investigated by m-lines spectroscopy at various electric field strenghts, can be well predicted by a parameter-free model based on the elastic, dielectric, and optical properties of the liquid crystal. According to such a model, the field profiles of the modes supported by hybrid nematic waveguides display very peculiar features.
Thermoelectric properties of β-FeSi2 with electrically insulating SiO2 and conductive TiO dispersion by mechanical alloying
Authors: Ito Mikio , Tanaka Takashi , Hara Shigeta
Published: May 2004

The n-type Co-doped β-FeSi2 (Fe0.98Co0.02Si2) with dispersion of an electrically insulating SiO2 and conductive TiO was synthesized by mechanical alloying and subsequent hot pressing. The effects of these oxide dispersions on the thermoelectric properties were investigated. When the SiO2 powder was added, the thermal conductivity (thermal diffusivity) was significantly reduced and the electrical resistivity slightly increased. On the other hand, in the case of the TiO addition, the electrical resistivity decreased compared to that of the sample without addition. The addition of a small amount of SiO2 or TiO was also found to enhance the Seebeck coefficient in the low temperature range. In order to disperse finer SiO2 particles, the addition of Fe2O3, Si, and Co powders to the Fe–Si powder was tried. The precipitation of fine SiO2 particles through a reaction of these additives was significantly effective for enhancing the phonon scattering.
Energy levels of a parabolically confined quantum dot in the presence of spin-orbit interaction
Authors: Kuan W. H. , Tang C. S. , Xu W.
Published: May 2004

We present a theoretical study of the energy levels in a parabolically confined quantum dot in the presence of the Rashba spin-orbit interaction (SOI). The features of some low-lying states in various strengths of the SOI are examined at zero and nonzero magnetic fields. It is shown that the spin-polarized electronic states can be more easily achieved in a weakly confined dot when SOI is greater than a critical value of the confinement strength. The presence of a magnetic field enhances the possibility of the spin polarization and the SOI leads to different energy dependence on magnetic fields applied. Furthermore, in high magnetic fields, the spectra of low-lying states show basic features of Fock–Darwin levels as well as Landau levels.
Phase transition in confined Se nanoclusters
Authors: Li Irene L. , Tang Z. K.
Published: May 2004

Phase transition of Se species incorporated in the AlPO4-5 nanoporous matrix has been investigated by differential scanning calorimetry and temperature dependent vibrational spectra. The thermal adsorption/desorption process shows that the confined Se species have structure transformations at temperature range 320–350 K. At low temperature (85 K), the vibrational peaks are sharp and well dissolved. Two different transformation processes in Se nanosized species have been observed with increasing temperature: (i) the amorphous like selenium in the mesochannles (structural imperfection) is crystallized, forming more stable chain-based trigonal crystalline like selenium and (ii) isolated Se chains become flexible. At low temperature, weak distortion in the dihedral angle occurs. At temperature above 320 K, Se chains change to another phase with strongly disordered structure (“melting” state).
Variable temperature film and contact resistance measurements on operating n-channel organic thin film transistors
Authors: Chesterfield Reid J. , McKeen John C. , Newman Christopher R. , Frisbie C. Daniel , Ewbank Paul C. , Mann Kent R. , Miller Larry L.
Published: May 2004

We report structural and electrical properties in thin films of an n-channel organic semiconductor, N,N′-dipentyl-3,4,9,10-perylene tetracarboxylic dimide (PTCDI–C5). The structure of polycrystalline thin films of PTCDI–C5 was studied using x-ray diffraction and atomic force microscopy. Films order with single crystal-like packing, and the direction of π-π overlap is in the substrate plane. Organic thin film transistors (OTFTs) based on PTCDI–C5 were fabricated on hydrophobic and hydrophilic substrates. OTFTs showed effective mobility as high as 0.1 cm2/V s. Contact resistance of operating OTFTs was studied using resistance versus length plots and a four-probe method for three different contact metals (Au, Ag, Ca). Typical OTFTs had a specific contact resistance of 8×104 Ω cm at high gate voltage. There was no dependence of contact resistance with contact metal. Variable temperature measurements revealed that film resistance in the OTFT was activated in the temperature range 100–300 K, with typical activation energies of 60–80 meV. Contact resistance showed similar activated behavior, implying that the Schottky barrier at the contact is not the limiting resistance for the contact. Film resistance data showed a Meyer–Neldel relationship with characteristic energy EMN=20–25 meV, for various samples. The common TFT instability of threshold voltage shift (TVS) was observed in PTCDI–C5 OTFTs. A model is proposed to explain positive TVS in gate bias stress and oxygen exposure experiments. The model is based on the formation of a metastable complex between PTCDI–C5 and oxygen, which creates a deep acceptor-like trap state.
Temperature dependence of magnetic properties of zinc and niobium doped strontium hexaferrite nanoparticles
Authors: Fang Qingqing , Bao Hengwei , Fang Dongming , Wang Jinzhi
Published: May 2004

Zinc and niobium doped strontium hexaferrite nanoparticles, Sr(Zn0.7Nb0.3)xFe12−xO19 (x=0–1.0), were fabricated using a sol-gel method for high density magnetic recording. The structure and temperature dependence of magnetic properties are investigated. The experiments show that strontium hexaferrite with small Zn and Nb substitutions still remains a hexagonal magnetoplumbite phase. The coercive force is reduced from 6.7 to about 2.3 kOe, while the values of saturation magnetization increased to 67–74 emu/g in the substitution range of x=0–1.0. This indicates that the saturation magnetization and coercivity of strontium hexaferrite nanoparticles can be held over a very wide range by an appropriate amount of Zn and Nb doping contents. Simultaneously, it is found that the doped strontium hexaferrite nanoparticles show a small positive temperature coefficient of coercivity. The substitution of Zn2+ and Nb4+ ions for Fe3+ ions also monitors the temperature dependence of magnetization and Curie temperature, and enhances thermal stability in the measured temperature range. This nanoparticle system is, thus, suitable for high-density recording.
Isotope-concentration dependence of thermal conductivity of germanium investigated by molecular dynamics
Authors: Ishii Hideo , Murakawa Atsushi , Kakimoto Koichi
Published: May 2004

Thermal conductivity of solid germanium as a function of the mole fraction of isotopes was estimated semi-quantitatively by using equilibrium molecular dynamics. The thermal conductivity of isotope-germanium was calculated by using an empirical potential of Stillinger–Weber potential. We employed the molecular dynamics based on Green–Kubo’s formula in which the autocorrelation function of heat flux was integrated as a function of duration time. The results of calculation showed that thermal conductivity of mixed isotope-germanium with large difference of mass is smaller than that with small mass difference, which is similar to experimental results.
Nitrogen-rich indium nitride
Authors: Butcher K. S. A. , Wintrebert-Fouquet M. , Chen P. P.-T. , Tansley T. L. , Dou H. , Shrestha S. K. , Timmers H. , Kuball M. , Prince K. E. , Bradby J. E.
Published: May 2004

Elastic recoil detection analysis, using an incident beam of 200 MeV Au ions, has been used to measure indium nitride films grown by radio-frequency sputtering. It is shown that the films have nitrogen-rich stoichiometry. Nitrogen vacancies are therefore unlikely to be responsible for the commonly observed high background carrier concentration. Ultraviolet Raman and secondary ion mass spectroscopy measurements are used to probe the state of the excess nitrogen. The nitrogen on indium anti-site defect is implicated, though other possibilities for the site of the excess nitrogen, such as molecular nitrogen, or di-nitrogen interstitials cannot be excluded. It is further shown that a shift in the (0002) x-ray diffraction peak correlates with the excess nitrogen, but not with the oxygen observed in some samples.
A technique for the measurement of surface diffusion coefficient and activation energy of Ge adatom on Si(001)
Authors: Kim H. J. , Zhao Z. M. , Liu J. , Ozolins V. , Chang J. Y. , Xie Y. H.
Published: May 2004

A method for determining the surface diffusion coefficient and the activation energy of Ge adatoms on Si(001) has been developed. Ge self-assembled quantum dots (SAQDs) grown on a relaxed SiGe buffer layer preferentially nucleate over a network of buried 60° dislocations. The surface sites over the buried dislocations act as sinks of Ge adatoms. When the average dislocation spacing becomes larger than the surface diffusion length of Ge adatoms, denuded zones free of Ge SAQDs appear on both sides of buried dislocations and separating the preferentially nucleated SAQDs from the randomly nucleated ones. The denuded zone width and the inter-dot spacing of randomly nucleated SAQDs are completely dependent on the surface diffusion coefficient. By varying the substrate temperature during the growth, the activation energy for the surface diffusion can be determined from the equilibrium inter-dot spacing of the randomly nucleated Ge SAQDs. Moreover, the pre-exponential term in the diffusion constant can be determined by using Fick’s first law and the observation that the total incident flux of Ge adatoms impinging on the denuded zones equals the average rate of volume increase of SAQDs over dislocations. The diffusion coefficient of Ge adatoms on Si (001) is found to be Ds≅2.53×10−7 exp(−0.676±0.03 eV/kT) cm2/s in the temperature range from 650 to 725 °C.
Structures of ultra-thin atomic-layer-deposited TaNx films
Authors: Wu Y. Y. , Kohn A. , Eizenberg M.
Published: May 2004

Atomic layer deposition (ALD) is an attractive technique in fabrication of microelectronics presently and in the future, for its accurate thickness control in atomic scale, excellent conformality, and uniformity over large areas at low temperature. It has been adapted and used in deposition of ultrathin TaNx films as diffusion barriers for Cu metallization. In this study, composition, structure, and stability of ultra-thin (1.5–10 nm) atomic layer deposited films are characterized by a set of complementary analytical techniques. The results indicate that the N to Ta atomic concentration ratio in the ALD TaNx films is approximately 2, independent of the film thickness and annealing up to 750 °C. Hydrogen, oxygen, and carbon are detected as impurities within the as-deposited films. The as-deposited ALD TaNx films have an fcc NaCl-type nanocrystalline structure even when the film thickness is 1.5 nm. Following thermal anneal at 600 °C and higher, the films do not undergo a structural change except for an increase in grain size and a decrease in the lattice constant. X-ray photoelectron spectra results indicate that all the Ta atoms in the films are bonded ionically with the surrounding N atoms. An ex situ thermal treatment at 600 °C for 1 h removes the O, which penetrated the layers, by a reduction reaction with the residual H and results in densification of the ALD films. Our analysis of the experimental results indicates that the excess of N atoms of the ALD TaNx films is mainly due to Ta vacancies in the fcc NaCl-type structure. The structural and compositional characteristics of the films explain why the films serve as good diffusion barriers to Cu metallization.
Correlation of conductivity and magnetization in epitaxial La2/3Ca1/3MnO3 thin films
Authors: Morán O. , Hott R. , Grube K. , Fuchs D. , Schneider R. , Baca E. , Saldarriaga W. , Prieto P.
Published: May 2004

We fabricated ∼200 nm thin (001)-oriented films of the ferromagnetic metallic perovskite La2/3Ca1/3MnO3 on single crystal (001)-SrTiO3 substrates by dc-sputtering at high oxygen pressure. The samples feature a Curie temperature TC∼260 K and a magnetic moment μ(T→0 K)∼3 μB per Mn atom. The magnetization loops are nearly square-shaped with a coercive field Hc(5 K)=0.03 T that decreases linearly in temperature down to TC. At low temperature, the magnetization shows a decrease ΔM∼T2 and the resistivity an increase Δρ∼T2 as is expected for itinerant electron ferromagnets where single particle excitations dominate. As a further indication of the high quality of the samples, the resistivity without external magnetic field peaks right at TC with a maximum value ρ(H=0,T=TC) of only ∼2 mΩ cm. The magnetoresistance ratio Δρ/ρ0=[ρ(H,T)−ρ(H=0,T)]/ρ(H=0,T) also reaches its maximum value at TC, with Δρ/ρ0(H=6 T,T=TC)∼−50%. The complete absence of this colossal magnetoresistance effect in the low-temperature limit is a further indication of the high sample quality which may be due to a high degree of oxygenation as a result of our preparation condictions. The relation ρ(H,T)=ρmexp[−M(H,T)/M0] observed below TC as well as the temperature characteristic of the resistivity above TC can both be explained by a simple magnetic polaron hopping tunneling model where the tunneling barrier height depends in a simple way on the relative orientation of the local magnetization at both ends of the hopping path. However, here it must be considered that the polarons completely change their character at TC from large and delocalized to small and highly localized entities.
Heat diffusion and thermolastic vibration influence on the signal of an open photoacoustic cell for two layer systems
Authors: Pichardo-Molina J. L. , Alvarado-Gil J. J.
Published: May 2004

Thermal diffusion and thermoelastic vibration in two layer systems is studied using photoacoustic spectroscopy. Three different two layer systems, copper/lead–tin (Cu/Pb–Sn), aluminum/paint (Al/paint) and aluminum/polyvinyl-chloride (Al/PVC) are studied. A model is developed considering that both layers present thermoelastic vibration and heat diffusion. It is shown that our equations can be used to obtain the theoretical photoacoustic signal independent of the mechanical properties when the thermal and thermomechanical properties of the composing layers are similar. It is shown that this approximation is much better suited for the study of metal/metal systems where the coupling of the thermoelastic bending is smooth. The restrictions of this approach and the possible extensions to study systems with different thermal expansion and mechanical behavior are discussed.
Excitonic nonlinear absorption in CdS nanocrystals studied using Z-scan technique
Authors: He J. , Ji W. , Ma G. H. , Tang S. H. , Elim H. I. , Sun W. X. , Zhang Z. H. , Chin W. S.
Published: May 2004

Irradiance dependence of excitonic nonlinear absorption in cadmium sulfide (CdS) nanocrystals has been studied by using Z-scan method with nanosecond laser pulses. The wavelength dependence of nonlinear absorption has also been measured near the excitonic transition of 1S(e)–1S3/2(h). We observe the saturable absorption, which can be described by a third-order and a fifth-order nonlinear process for both 3.0-nm-sized and 2.3-nm-sized CdS nanocrystals. The experimental results show that the excitonic nonlinear absorption of CdS nanocrystals is greatly enhanced with decreasing particle size. A two-level model is utilized to explain both irradiance and wavelength dependence of the excitonic nonlinearity.
Asymmetrical current distribution in metallized film capacitors
Authors: Joubert Ch. , Béroual A. , Rojat G.
Published: May 2004

The presence of high frequency currents in the vicinity of metallized capacitors can generate eddy currents in the metallizations, with a possible negative impact on the reliability and lifetime of these components. These phenomena are due to the asymmetrical distribution of the field lines. We present a model derived from the physical relations between the current density, the strength of the magnetic field, the electromotive force in a capacitor winding and its plates, and the conductivity of the different materials. This model also takes into account anisotropy due to the structure of the winding. The results obtained from this model can be compared to the measurements of the electromotive force detected by a loop in the vicinity of a capacitor under operation. Finally, the consequences of nonsymmetrical magnetic field distribution are discussed in the light of the results obtained from the model.
High performance n-type bismuth telluride with highly stable thermoelectric figure of merit
Authors: Yamashita Osamu , Tomiyoshi Shoichi
Published: May 2004

The n-type Bi2(Te0.94Se0.06)3 ingots doped with and without 0.010 wt. % CuBr in addition to the codoping of 0.102 wt. % I and 0.026 wt. % Te were grown by the Bridgman method at a rate of 6 cm/h. The thermoelectric properties of these specimens were measured before and after annealing, where annealing was done in the temperature range from 473 up to 673 K for 2–5 h in a vacuum and a hydrogen stream. When the n-type specimens doped without and with CuBr were annealed for 2 h at 473 K in a hydrogen stream and at 573 K in a vacuum, respectively, the thermoelectric figure of merits ZT at 298 K for CuBr nondoped and doped specimens were enhanced by 37% and 20% and reached great values of 1.04 and 0.91, respectively. The thermal stability of the ZT of the former specimen was improved significantly by optimizing the total additional content of I and Te alone, so that the ZT changed little with temperature, at least in the temperature range from 298 to 433 K. In addition, this specimen also exhibited a remarkably high power factor of 6.57×10−3 W/mK2 at 298 K, which is the greatest among the bismuth tellurides reported previously.
Calculation of the rate constant for state-selected recombination of H+O2(v) as a function of temperature and pressure
Authors: Teitelbaum Heshel , Caridade Pedro J. S. B. , Varandas António J. C.
Published: May 2004

Classical trajectory calculations using the MERCURY/VENUS code have been carried out on the H+O2 reactive system using the DMBE-IV potential energy surface. The vibrational quantum number and the temperature were selected over the ranges v=0 to 15, and T=300 to 10 000 K, respectively. All other variables were averaged. Rate constants were determined for the energy transfer process, H+O2(v)→H+O2(v″), for the bimolecular exchange process, H+O2(v)→OH(v′)+O, and for the dissociative process, H+O2(v)→H+O+O. The dissociative process appears to be a mere extension of the process of transferring large amounts of energy. State-to-state rate constants are given for the exchange reaction, and they are in reasonable agreement with previous results, while the energy transfer and dissociative rate constants have never been reported previously. The lifetime distributions of the HO2 complex, calculated as a function of v and temperature, were used as a basis for determining the relative contributions of various vibrational states of O2 to the thermal rate coefficients for recombination at various pressures. This novel approach, based on the complex’s ability to survive until it collides in a secondary process with an inert gas, is used here for the first time. Complete falloff curves for the recombination of H+O2 are also calculated over a wide range of temperatures and pressures. The combination of the two separate studies results in pressure- and temperature-dependent rate constants for H+O2(v)(+Ar)⇄HO2(+Ar). It is found that, unlike the exchange reaction, vibrational and rotational–translational energy are liabilities in promoting recombination.
Electric dipole polarizabilities of copper clusters
Authors: Knickelbein Mark B.
Published: May 2004

The static electric dipole polarizabilities of Cu9–Cu61 have been measured via a molecular beam deflection method. The clusters display per-atom polarizabilities that decrease monotonically with size, from ∼16 Å3 per atom Cu9–10 to ∼5 Å3 (Cu45–61). Absent are any discernible discontinuities or odd–even alternations due to electronic shell filling or electron pairing effects. For the smallest clusters, the experimental polarizabilities are ∼3 times larger than those predicted classically for conducting ellipsoids, and approach the classical values only for clusters containing more than ∼45 atoms.
Prediction of membrane protein structures by replica-exchange Monte Carlo simulations: Case of two helices
Authors: Kokubo Hironori , Okamoto Yuko
Published: May 2004

We test our prediction method of membrane protein structures with glycophorin A transmembrane dimer and analyze the predicted structures in detail. Our method consists of two parts. In the first part, we obtain the amino-acid sequences of the transmembrane helix regions from one of existing WWW servers and use them as an input for the second part of our method. In the second part, we perform a replica-exchange Monte Carlo simulation of these transmembrane helices with some constraints that indirectly represent surrounding lipid and water effects and identify the predicted structure as the global-minimum-energy state. The structure obtained in the case for the dielectric constant ε=1.0 is very close to that from the nuclear magnetic resonance experiments, while that for ε=4.0 is more packed than the native one. Our results imply that the helix–helix interaction is the main driving force for the native structure formation and that the stability of the native structure is determined by the balance of the electrostatic term, van der Waals term, and torsion term, and the contribution of electrostatic energy is indeed important for correct predictions. The inclusion of atomistic details of side chains is essential for estimating this balance accurately because helices are tightly packed.