bluuebunny/crossref_metadata_2025 huggingface dataset:
Particulate FePt∕Ag(C) films with strong perpendicular anisotropy
Authors: Wan J. , Huang Y. , Zhang Y. , Bonder M. J. , Hadjipanayis G. C. , Weller D.
Published: May 2005

Textured FePt∕Ag and FePt∕C multilayers have been fabricated by dc magnetron sputtering onto single crystal MgO (100) substrates at elevated temperatures from 280to400°C using FePt, Ag, and C targets. Our studies showed that isolated and ordered L10 FePt nanoparticles with perpendicular magnetic anisotropy can be formed in these multilayers. The (FePt∕Ag) multilayers showed a lower transformation temperature from the disordered fcc to the ordered L10 phase than the FePt∕C multilayers. The lowest temperature to form the L10 phase is found to be 280°C in FePt∕Ag multilayers. The c-axis texture can be improved by increasing the thickness of Ag layers. As the deposition temperature is increased, the degree of atomic ordering approaches that of the fully ordered phase. The magnetic and structural properties are consistent with the microstructure studies observed with high-resolution transmission electron microscopy.
The frequency and damping of ion acoustic waves in collisional and collisionless two-species plasma
Authors: Berger R. L. , Valeo E. J.
Published: May 2005

The dispersion properties of ion acoustic waves are sensitive to the strength of ion-ion collisions in multispecies plasma in which the different species usually have differing charge-to-mass ratios. The modification of the frequency and damping of the fast and slow acoustic modes in a plasma composed of light (low-Z) and heavy (high-Z) ions is considered. In the fluid limit where the light ion scattering mean free path λlh is smaller than the acoustic wavelength, λ=2π∕k, the interspecies friction and heat flow carried by the light ions scattering from the heavy ions causes the damping. In the collisionless limit, kλlh⪢1, Landau damping by the light ions provides the dissipation. In the intermediate regime when kλlh∼1, the damping is at least as large as the sum of the collisional and Landau damping.
On the largest singular values of random matrices with independent Cauchy entries
Authors: Soshnikov Alexander , Fyodorov Yan V.
Published: May 2005

We apply the method of determinants to study the distribution of the largest singular values of large m×n real rectangular random matrices with independent Cauchy entries. We show that for a special one-parametric class of statistics the properties of the largest singular values (rescaled by a factor 1∕m2n2) agree in the limit with the statistical properties of the Poisson random point process with the intensity (1∕π)x−3∕2 and, therefore, are different from the Tracy–Widom law. Among other corollaries of our method we show an interesting connection between the mathematical expectations of the determinants of the complex rectangular m×n standard Wishart ensemble and the real rectangular 2m×2n standard Wishart ensemble.
Ultraviolet-induced birefringence in hydrogen-loaded optical fiber
Authors: Canning J. , Deyerl H. J. , Sørensen H. R. , Kristensen M.
Published: May 2005

A precision phase-shifting approach to fabricate various phase-shifted gratings using different combinations of polarized ultraviolet (UV) light is demonstrated. In doing so, the difference between s- and p-polarized light reported by others is confirmed. However, we reveal added complexity for the role of hydrogen and deuterium in the UV-induced process. Previous arguments for the origins are systematically ruled out by reviewing existing literature. We note that the birefringence is made up of at least two components with different thermal stabilities, one consistent simply with molecular hydrogen being present in the system. Overall the birefringence, by deduction, is associated with anisotropy in hydrogen reactions within the fiber. As a result they lead, through known mechanisms of dilation in glass, to anisotropic stress relaxation that can be annealed out, with or without hydrogen remaining, at low temperatures close to 125 °C.
Growth mechanism of InAs quantum dots on GaAs by metal-organic chemical-vapor deposition
Authors: Chung Theodore , Walter Gabriel , Holonyak Nick
Published: May 2005

The growth parameters affecting the deposition of InAs quantum dots (QDs) by metal-organic chemical-vapor deposition are reported. Experiments with arsine pause, gas switching, and hydrogen shroud flow show that a low V∕III ratio is the key to obtaining three-dimensional InAs island formation with high density and uniformity. Based on atomic force microscopy images of InAs QDs deposited under different growth conditions, a physical model for the epitaxial growth of three-dimensional islands is proposed. In this model, the InAs QD growth is governed by two types of arsenic sources at the growth surface: free arsenic atoms arriving at the boundary layer and dangling arsenic bonds available at the GaAs wafer surface. At high V∕III ratio, free arsenic atoms arriving at the boundary layer are the dominant hydride species and produce a low density of InAs islands with irregular shape and polycrystalline defects. At low V∕III ratio arsenic bonds on the GaAs surface are the main sites for indium atoms to attach to, thus producing high island densities and small coherent island sizes.
Similarities in photoluminescence in hafnia and zirconia induced by ultraviolet photons
Authors: Ito Toshihide , Maeda Motohiro , Nakamura Kazuhiko , Kato Hiromitsu , Ohki Yoshimichi
Published: May 2005

Photoluminescence (PL) spectra induced by ultraviolet photons were measured for amorphous hafnia and zirconia deposited by plasma-enhanced chemical-vapor deposition (PECVD), amorphous hafnia deposited by pulse laser deposition, and crystalline yttria-stabilized zirconia. Two kinds of samples were prepared for both hafnia and zirconia deposited by PECVD using different source alkoxides in different deposition chambers. A PL peak was observed around 2.8eV similarly in all hafnia and zirconia samples, irrespective of the difference in crystallinity, oxygen deficiency, source alkoxide, deposition method, or the substrate material. The decay profile of this PL is also similar in all the samples. These facts clearly show that neither impurities, oxygen vacancy, nor defects at the interface between the sample and the substrate are responsible for the PL. It is a luminescence inherent in hafnia and zirconia and is most likely due to radiative recombination between localized states at the band tails. When the samples were annealed in oxygen, a new PL peak appeared around 4.2eV in all the amorphous samples. Its decay profile is also in common with these samples. Vacuum-ultraviolet absorption measurements and PL excitation measurements indicate that the 4.2-eV PL is excited due to the interband absorption.
Size dependence of switching fields of permalloy rings
Authors: Lai Mei-Feng , Wei Zung-Hang , Wu J. C. , Chang C. C. , Chang Ching-Ray , Lai Jun-Yang
Published: May 2005

The size dependence of switching fields of permalloy rings is studied by micromagnetic simulations and by magnetoresistance measurements. Experimentally, the switching fields can be obtained from the discontinuities of the magnetoresistance curve. From the simulated hysteresis loops and the magnetoresistance curves obtained by experiment it is shown that the switching field from the vortex state to the reverse onion state increases considerably as decreasing of the lateral size or as increasing of the thickness. There is, however, no significant correlation between the ring size and the switching field from the onion state to the vortex state.
Breaking a chaos-noise-based secure communication scheme
Authors: Li Shujun , Álvarez Gonzalo , Chen Guanrong , Mou Xuanqin
Published: May 2005

This paper studies the security of a secure communication scheme based on two discrete-time intermittently chaotic systems synchronized via a common random driving signal. Some security defects of the scheme are revealed: 1) The key space can be remarkably reduced; 2) the decryption is insensitive to the mismatch of the secret key; 3) the key-generation process is insecure against known/chosen-plaintext attacks. The first two defects mean that the scheme is not secure enough against brute-force attacks, and the third one means that an attacker can easily break the cryptosystem by approximately estimating the secret key once he has a chance to access a fragment of the generated keystream. Yet it remains to be clarified if intermittent chaos could be used for designing secure chaotic cryptosystems.
Time growth rate of symmetric TM mode of a rod dielectric Cerenkov plasma maser
Authors: Shokri B. , Jazi B.
Published: May 2005

The dispersion relation of symmetrical TM waves (usual operating mode of a traveling wave tube) propagating in a Cerenkov maser including a thin annular relativistic electron beam (TAREB), a strongly magnetized plasma column, and a dielectric rod is investigated. The effects of relative permittivity constant of dielectric rod, accelerating voltage, radii of the rod, TAREB, and plasma column as well, on the frequency spectra and growth-rate coefficient are presented. The conditions under which the growth rates are maximum are studied as well.
Capacitance–voltage characterization of polymer light-emitting diodes
Authors: Shrotriya Vishal , Yang Yang
Published: May 2005

The capacitance–voltage (C–V) characterization of polymer light-emitting diodes (PLEDs) employing poly[5-(2′-ethylhexyloxy)-2-methoxy-1,4-phenylene vinylene], as the light-emitting layer are reported. Several metals, such as calcium (Ca), aluminum (Al), and gold (Cu) were used as the cathode in order to investigate the influence of the charge injection on the C–V characteristics. Under forward bias, the capacitance increases with majority charge carrier injection into the polymer layer, and, afterwards, decreases upon minority charge carrier injection which results in recombination of electrons and holes in the active polymer layer. The increase in the value of capacitance follows the same dependence as the increase in the value of current density through the device, which suggests that the capacitance depends not only on the amount of charge trapped in the polymer near the interface, but also, and mainly, on the amount of charge injection into the polymer layer from the electrodes. The C–V behavior of PLEDs with added amount of ionic defects was also studied. The capacitance values are higher for devices with higher amount of added impurities in form of ionic electrolytes, and, the increase in the capacitance under forward bias also depends on the ionic defect concentration.
Aging behavior in Ba0.7Sr0.3Ti0.97Zr0.03O ceramics
Authors: Portelles J. , Garcia S. , Martinez E. , Raymond O. , Almodovar N. S. , Heiras J. L. , Siqueiros J. M.
Published: May 2005

The polarization aging behavior of the Ba0.7Sr0.3Ti0.97Zr0.03O3 ceramic that presents diffuse phase transition, was investigated at different temperatures. The aging of this system is explained by assuming the existence of two different relaxation regimes, one associated with rapidly relaxing polarized macroregions (τ1) and the other to long time relaxing polarized microregions (τ2). With these considerations, a modified expression to describe the aging behavior is proposed, leading to a good fit to the experimental measurements of the polarization. Hysteresis loop, permittivity, and dielectric loss measurements were performed on the samples to support the aging study.
Thermoelectric properties of potassium-doped β-BaCu2S2 with natural superlattice structure
Authors: Kurosaki Ken , Uneda Hironori , Muta Hiroaki , Yamanaka Shinsuke
Published: May 2005

We prepared polycrystalline samples of β-KxBa1−xCu2S2 (x=0,0.05,0.1) with a natural superlattice structure and measured the thermoelectric properties from room temperature to about 850K. The Seebeck coefficients are positive in the whole temperature range. The electrical resistivity and Seebeck coefficient decrease with potassium content, which indicates that potassium doping introduces charge carriers. The thermal conductivities of potassium-doped samples are slightly higher than those of undoped samples as a result of electronic contributions. The values of thermal conductivities are very low, 0.97Wm−1K−1 at room temperature for β-K0.1Ba0.9Cu2S2. The dimensionless figure of merit ZT can be improved by potassium doping and reaches 0.28 at 820K for β-K0.1Ba0.9Cu2S2.
Two-dimensional simulations of plastic-shell, direct-drive implosions on OMEGA
Authors: Radha P. B. , Goncharov V. N. , Collins T. J. B. , Delettrez J. A. , Elbaz Y. , Glebov V. Yu. , Keck R. L. , Keller D. E. , Knauer J. P. , Marozas J. A. , Marshall F. J. , McKenty P. W. , Meyerhofer D. D. , Regan S. P. , Sangster T. C. , Shvarts D. , Skupsky S. , Srebro Y. , Town R. P. J. , Stoeckl C.
Published: May 2005

Multidimensional hydrodynamic properties of high-adiabat direct-drive plastic-shell implosions on the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] are investigated using the multidimensional hydrodynamic code, DRACO [D. Keller et al., Bull. Am. Phys. Soc. 44, 37 (1999)]. Multimode simulations including the effects of nonuniform illumination and target roughness indicate that shell stability during the acceleration phase plays a critical role in determining target performance. For thick shells that remain integral during the acceleration phase, target yields are significantly reduced by the combination of the long-wavelength (ℓ<10) modes due to surface roughness and beam imbalance and the intermediate modes (20⩽ℓ⩽50) due to single-beam nonuniformities. The neutron-production rate for these thick shells truncates relative to one-dimensional (1D) predictions. The yield degradation in the thin shells is mainly due to shell breakup at short wavelengths (λ∼Δ, where Δ is the in-flight shell thickness). The neutron-rate curves for the thinner shells have significantly lower amplitudes and a fall-off that is less steep than 1D rates. DRACO simulation results are consistent with experimental observations.
Numerical investigation and modeling of stationary plasma thruster low frequency oscillations
Authors: Chable S. , Rogier F.
Published: May 2005

The origin of the oscillations in the range 1–100kHz, which have been observed during the working of stationary plasma thruster (SPT) is not well understood until now. The purpose of this paper is first to clarify the nature of these oscillations and second to propose an explanation using the concept of current instabilities proposed by Buneman. To reach this objective, a spectral study of the linear instabilities has been performed with a stationary quasineutral hybrid model. These computations show unambiguously a relationship between the nonlinear oscillations that appear in the transient simulation and the growth rate of the linear model. A simplified model is also derived, which highlights the role played by the coupling between the electric field and the ion current leading to Buneman’s instabilities. This study suggests to reduce the amplitude of the oscillations controlling the growth rate of the linear mode by modifying the profile of the magnetic field inside the SPT channel. Numerical simulations illustrate the performances of the improvement.
First-principles investigations of nitrogen-doping effects on defect aggregation processes in Czochralski Si
Authors: Taguchi A. , Kageshima H. , Wada K.
Published: May 2005

We theoretically investigated the nitrogen (N)-doping effect on vacancy aggregation and oxygen precipitation processes in Czochralski Si. By using first-principles calculations, we investigated stable atomic configurations and formation energies of various complexes formed by vacancies (V), oxygen (O), and N. It was found that V and O form stable complexes, but some O capture processes are endothermic, suggesting that the V aggregation preferentially occurs, resulting in void growth. It was also found that the {N2–V2} complex, which is already known as the most stable complex among N and V complexes, effectively captures several O atoms, and then forms stable {N2–V2–On} complexes. The stable {N2–V2–On} complexes may become the heterogeneous nucleation sites of the O precipitates, suggesting that the N doping drives the aggregation of O. This is consistent with the experimentally observed enhancement of O precipitate formation due to N doping. The experimentally observed shape change of the void may be due to anisotropic void growth originating from the N and N–O involved in void formation during the growth.
High-resolution transmission electron microscopy study of the interfaces and stacking defects in superconducting/magnetic perovskite superlattices
Authors: Haberkorn N. , Lovey F. , Condó A. M. , Guimpel J.
Published: May 2005

We studied the microstructure of GdBa2Cu3O7−δ∕La0.6Sr0.4MnO3 superlattices grown on (001)MgO substrates by high-resolution transmission electron microscopy. The superlattices show two different epitaxial growths. In addition different types of interfaces between the layers were found. Moreover, several kinds of stacking faults and domain boundaries were observed in both materials. The crystallography of the interfaces and the defects was analyzed and their role on the physical properties of the superlattices is discussed.
High-mobility InGaAs∕InAlAs pseudomorphic heterostructures on InP (001)
Authors: Wallart X. , Pinsard B. , Mollot F.
Published: May 2005

In this work, we study the growth of strained InxGa1−xAs alloys on InP by gas source molecular-beam epitaxy in the 350–500°C range. At low temperatures, we show that the As-rich (2×3) surface reconstruction promotes three-dimensional growth mode whereas the less As-rich (2×4) and cation-rich ones allow keeping two-dimensional growth. For heterostructures with a lattice-matched InAlAs barrier and a strained In0.75Ga0.25As channel, grown at 500°C, the electron mobility rises to 16 000 and 139000cm2∕Vs at 300 and 77K, respectively, for a 140-Åthick channel layer and a 400-Å-thick spacer layer. Both values are among the best ones ever reported for an InGaAs∕InAlAs heterostructure.
Thin-film waveguiding mode light extraction in organic electroluminescent device using high refractive index substrate
Authors: Nakamura Toshitaka , Tsutsumi Naoto , Juni Noriyuki , Fujii Hironaka
Published: May 2005

The redistribution of thin-film waveguiding emitted light from an organic electroluminescent (EL) layer using high refractive index substrates has been investigated. The internally emitted light can be classified in terms of three modes: external, substrate, and indium-tin-oxide (ITO)∕organic modes. According to classical ray optics theory, the ITO∕organic mode emission can be completely redistributed to the substrate mode when the refractive index of the substrate becomes larger than that of the emitting layer. The redistributed substrate mode emission should be able to be easily extracted by an uneven surface, such as a microlens array. With this understanding, we prepared organic EL devices in which the substrate refractive index was varied over a range. However, redistribution of the ITO∕organic mode emission to the substrate mode was not observed experimentally in our EL devices. In addition, there was no difference in the luminous intensity improvement between glass (n=1.52) substrate and high refractive index (n=1.72) resin substrate using identical microlens array. The improvement in luminous intensity was around a factor of 1.5 in both cases.
Magnetic behavior of iron-filled multiwalled carbon nanotubes
Authors: Karmakar S. , Sharma Surinder M. , Mukadam M. D. , Yusuf S. M. , Sood A. K.
Published: May 2005

Using vibrating-sample magnetometry, magnetic properties of iron-filled multiwalled carbon nanotubes have been investigated. The field dependence of dc magnetization at high magnetic fields suggests that these tubes behave as a one-dimensional exchange-coupled ferromagnetic system. At 5K, the saturation magnetization (MS) of the nanowires is found to be 85emu∕g, which is much less than the expected bulk value ∼210emu∕g. The observed exchange bias, in spite of the small fraction of γ-Fe in our samples, implies that γ-Fe may not be the only antiferromagnetic component responsible for the exchange bias in these Fe-filled carbon nanotubes. Quantitative study on the temperature dependence of saturation magnetization, remanent magnetization and coercivity has been carried out.
Anomalous Hall-effect measurement study on Co∕Pt nanosized dot
Authors: Kikuchi N. , Murillo R. , Lodder J. C.
Published: May 2005

Dot arrays made of polycrystalline Co∕Pt multilayer with an average grain size of 20nm are fabricated by using a laser interference lithography and their magnetic properties are examined by detecting anomalous Hall effect. It is revealed that the ratio of dots which have stable single domain state increases from 35% to 85% while the dot diameter decreased from 200to120nm. The energy barrier height of magnetization reversal is estimated as 4.0×10−12erg from sweep rate dependence of the coercivity. The energy corresponds to the switching volume comparable with the volume of a physical grain in the multilayer film.
Stoichiometry dependence of superconductivity and microstructure in mechanically alloyed MgB2
Authors: Perner O. , Eckert J. , Häßler W. , Fischer C. , Acker J. , Gemming T. , Fuchs G. , Holzapfel B. , Schultz L.
Published: May 2005

The correlations among stoichiometry variation, preparation route, and microstructure as well as superconducting properties were investigated in MgB2 bulk samples. We demonstrate that the different initial weight ratios of Mg and B between Mg0.9B2 and Mg1.1B2 are only slightly shifted towards Mg depletion during the mechanical alloying preparation at room temperature followed by a short annealing step. The samples with Mg surplus show the highest critical temperature Tc∼36K, highest critical current density Jc∼1.5×106A∕cm2 at 7.5 K in self-field, and a homogeneous microstructure. In contrast, the Mg-depleted samples exhibit a rather porous and multiphase microstructure with a broad grain size distribution. A direct correlation of stoichiometry variation and superconducting properties was not found, but rather a strong influence of the Mg content on lattice strain and grain connectivity.
Spin gap in low-dimensional magnets (Review)
Authors: Vasil’ev A. N. , Markina M. M. , Popova E. A.
Published: May 2005

The main mechanisms of formation of a spin gap in low-dimensional metaloxide compounds are reviewed. Among the objects in which a singlet ground state is formed are dimers, alternating chains with half-integer spin, uniform chains with integer spin, spin ladders, and two-dimensional ensembles of exchange-coupled spins. In some of these systems the spin gap in the spectrum of magnetic excitations is present from the start, while in others it is formed as a result of magnetoelastic interaction, charge ordering, or orbital ordering.
Novel laser based on magnetic tunneling
Authors: Kadigrobov A. , Shekhter R. I. , Jonson M.
Published: May 2005

A new principle for a compact spin-based solid-state laser is proposed. It operates in the 1–100 THz regime, which is difficult to reach with small size lasers. Spin-flip processes in ferromagnetic conductors form a basis—the mechanism is due to a coupling of light to the exchange interaction in magnetically ordered conductors via the dependence of the exchange constant on the conduction electron momenta. The interaction strength is proportional to the large exchange energy and exceeds the Zeeman interaction by orders of magnitude. A giant lasing effect is predicted in a system where a population inversion has been created by injection of spin-polarized electrons from one ferromagnetic conductor into another through an intermediate tunnel region or weak link; the magnetizations of the two ferromagnets have different orientations. We show that the laser frequency will be in the range 1–100 THz if the experimental data for ferromagnetic manganese perovskites with nearly 100% spin polarization are used. The optical gain is estimated to be gopt∼107 cm−1. This exceeds the gain of conventional semiconductor lasers by 3 or 4 orders of magnitude. An experimental configuration is proposed in order to solve heating problems at a relatively high threshold current density.
Effect of alkyl chain length and size of the headgroups of the surfactant on solvent and rotational relaxation of Coumarin 480 in micelles and mixed micelles
Authors: Chakrabarty Debdeep , Chakraborty Anjan , Seth Debabrata , Hazra Partha , Sarkar Nilmoni
Published: May 2005

The effect of alkyl chain length and size of the headgroups of the surfactant on the solvation dynamics and rotational relaxation of Coumarin 480 (C-480) has been investigated using dynamic Stokes’ shift of C-480 in different types of alkyltrimethylammonium bromide micelles and mixed micelles. The rotational relaxation time increases with increase in alkyl chain length of the surfactant. The increase in the number of alkyl chains of the surfactant leads to the more close packed micelles, hence the microviscosity of the micelles increases and consequently rotational relaxation time increases. Solvation time also increases due to the increase in number of alkyl chains of the surfactant. The change in solvation and rotational relaxation time is more prominent in micelles compared to mixed micelles. The solvation and rotational relaxation time also increase with the increase in size of the headgroup of the surfactant.
Nonlocal nonlinear electrostatic gyrofluid equations: A four-moment model
Authors: Strintzi D. , Scott B. D. , Brizard A. J.
Published: May 2005

Extending a previous single-temperature model, an electrostatic gyrofluid model that includes anisotropic temperatures (T‖≠T⊥) and can treat general nonlinear situations is constructed. The model is based on a Lagrangian formulation of gyrofluid dynamics, which leads to an exact energy conservation law. Diamagnetic cancellations are inserted manually in such a way that energy conservation is preserved. Comparison with previous models shows a very good agreement for zero-Larmor-radius terms in the gyrofluid equations of motion.
Photorefractive performance of a CdSe∕ZnS core/shell nanoparticle-sensitized polymer
Authors: Aslam Farzana , Binks David J. , Rahn Mark D. , West David P. , O’Brien Paul , Pickett Nigel , Daniels Steve
Published: May 2005

We report the photorefractive performance of a polymer composite sensitized by CdSe∕ZnS core/shell nanoparticles, and also comprising poly(N-vinylcarbazole) and an electro-optic chromophore. The nanoparticles are characterized by absorption and photoluminescence spectroscopy, elemental analysis, transmission electron microscopy, and powder x-ray diffraction. The electro-optic response of the composite is measured independently of the photorefractive effect by transmission ellipsometry. An asymmetric two-beam coupling gain of 30.6±0.4cm−1 is obtained, confirming photorefractivity. Degenerate four-wave mixing is used to assess photorefractive performance and, at a poling field of 70Vμm−1, yields a diffraction efficiency of 4.21%±0.03%, a holographic contrast of 3.05×10−4±1×10−6, a space-charge rise time of 25±2s, and a sensitivity of 4.7×10−5±4×10−6cm3J−1. These results constitute a significant improvement on the performance of previous nanoparticle-sensitized photorefractive polymer composites.
A first-principle study of the adsorption of 1-amino-3-cyclopentene on the (100) silicon surface
Authors: Festa Gaetano , Cossi Maurizio , Barone Vincenzo , Cantele Giovanni , Ninno Domenico , Iadonisi Giuseppe
Published: May 2005

The adsorption of 1-amino-3-cyclopentene on the (100) silicon surface has been studied by methods rooted in the density-functional theory using both delocalized (plane waves, PWs) and localized (Gaussian-type orbitals, GTOs) basis functions. The results obtained by modeling the surface by silicon clusters of different sizes are quite similar, thus confirming that the reaction is quite localized. Furthermore, PW and GTO computations give comparable results, provided that the same density functional and carefully chosen computational parameters (contraction of GTO, pseudopotentials, etc.) are used. Slab computations performed in the PW framework show that the cluster results are retrieved when low-coverage adsorption on the surface is considered. On these grounds, we are quite confident that reaction parameters obtained by the more reliable hybrid density functional (PBE0) are essentially converged, our best estimates of reaction and activation free energies are thus −40 and 6kcal∕mol, respectively.
Path-integral Monte Carlo simulations for electronic dynamics on molecular chains. II. Transport across impurities
Authors: Mühlbacher Lothar , Ankerhold Joachim
Published: May 2005

Electron transfer (ET) across molecular chains including an impurity is studied based on a recently improved real-time path-integral Monte Carlo (PIMC) approach [L. Mühlbacher, J. Ankerhold, and C. Escher, J. Chem. Phys. 121 12696 (2004)]. The reduced electronic dynamics is studied for various bridge lengths and defect site energies. By determining intersite hopping rates from PIMC simulations up to moderate times, the relaxation process in the extreme long-time limit is captured within a sequential transfer model. The total transfer rate is extracted and shown to be enhanced for certain defect site energies. Superexchange turns out to be relevant for extreme gap energies only and then gives rise to different dynamical signatures for high- and low-lying defects. Further, it is revealed that the entire bridge compound approaches a steady state on a much shorter time scale than that related to the total transfer. This allows for a simplified description of ET along donor-bridge-acceptor systems in the long-time range.
Plasmon resonances on metal tips: Understanding tip-enhanced Raman scattering
Authors: Demming A. L. , Festy F. , Richards D.
Published: May 2005

Calculations of the electric-field enhancements in the vicinity of an illuminated silver tip, modeled using a Drude dielectric response, have been performed using the finite difference time domain method. Tip-induced field enhancements, of application in “apertureless” Raman scanning near-field optical microscopy (SNOM), result from the resonant excitation of plasmons on the metal tip. The sharpness of the plasmon resonance spectrum and the highly localized nature of these modes impose conditions to better exploit tip plasmons in tip-enhanced apertureless SNOM. The effect of tip-to-substrate separation and polarization on the resolution and enhancement are analyzed, with emphasis on the different field components parallel and perpendicular to the substrate.
A molecular-dynamics study of a model SN1 dissociation reaction at the water liquid/vapor interface
Authors: Winter Nicole , Benjamin Ilan
Published: May 2005

The thermodynamics and dynamics of a model SN1 reaction: t-BuCl→t-Bu++Cl− is studied at the water liquid/vapor interface using molecular-dynamics computer simulations. The empirical valence bond approach is used to couple two diabatic states, covalent and ionic, in the electronically adiabatic limit. Umbrella sampling calculations are used to calculate the potential of mean force along the reaction coordinate (defined as the t-Bu to Cl distance) in bulk water and in several locations at the interface. We find a significant increase of the dissociation barrier height and of the reaction free energy at the interface relative to the bulk. This is shown to be due to the reduced polarity of the interface. Reactive flux correlation function calculations show significant deviation of the rate constant from the transition-state theory: The transmission coefficients range from 0.49 in the bulk to 0.05 above the Gibbs surface. The low transmission coefficient at the interface despite the lower friction is shown to be due to slow vibrational relaxation.
The study of percolation with the presence of impurities
Authors: Kondrat Grzegorz
Published: May 2005

We consider the process of percolation cluster formation for pointlike conductors subjected to random sequential adsorption onto two-dimensional lattice by computer Monte Carlo simulations. The initial presence of impurities disturbs this phenomenon significantly and we study here how the size and density of impurity particles affect the resulting percolation threshold. Some unexpected features such as the nonmonotonicity of the percolation threshold as a function of impurity concentration are discussed.
Gradient-based direct normal-mode analysis
Authors: Kaledin Alexey L.
Published: May 2005

A formulation of a direct, iterative method for obtaining the lowest eigenvalues and eigenvectors of a Hessian matrix is presented. Similar to the iterative schemes in electronic structure configuration interaction calculations (methods due to Lanczos, Davidson, and others), the mass-weighted Hessian matrix K is not constructed explicitly; instead, its operation on a basis vector (a direction in the 3N Cartesian configuration space of the atoms) is computed based on the principles of dynamical equations of motion. By noting that the time derivative of the gradient vector in the harmonic force field is related to the particles’ momenta via dg∕dt=Kp, a Hessian-vector product can be computed on the fly by finite differentiation of the gradient along the direction specified by the p vector. Thus, only two evaluations of the gradient are required per Davidson-like iteration per root, which leads to a linear scaling behavior of the computational effort with the number of atoms (provided that the evaluation of the gradient scales linearly). Preliminary results are presented for a 27 000-atom He4 nanodroplet.
Ionization potentials and electron affinities in the Perdew–Zunger self-interaction corrected density-functional theory
Authors: Vydrov Oleg A. , Scuseria Gustavo E.
Published: May 2005

Using a self-consistent implementation of the Perdew–Zunger self-interaction corrected (PZ-SIC) density-functional theory, we have calculated ionization potentials (IP) and electron affinities (EA) of first- and second-row atoms and a set of small molecules. Several exchange-correlation functionals were tested. IPs and EAs were obtained by two methods: as the difference in self-consistent field (SCF) energies of neutrals and ions (ΔSCF) and as negatives of highest-occupied orbital energies. We found that, except for local spin-density approximation, PZ-SIC worsens ΔSCF IPs and EAs. On the other hand, PZ-SIC brings orbital eigenvalues into much better agreement with electron removal energies. The Perdew–Zunger SIC seems to overcorrect many-electron systems; for molecules it performs worse than for atoms. We also discuss several common approximations to PZ-SIC such as spherical averaging of orbital densities in atoms.
First principles local pseudopotential for silver: Towards orbital-free density-functional theory for transition metals
Authors: Zhou Baojing , Carter Emily A.
Published: May 2005

Orbital-free density-functional theory (OF-DFT) with modern kinetic-energy density functionals (KEDFs) is a linear scaling technique that accurately describes nearly-free-electron-like (main group) metals. In an attempt towards extending OF-DFT to transition metals, here we consider whether OF-DFT can be used effectively to study Ag, a metal with a localized d shell. OF-DFT has two approximations: use of a KEDF and local pseudopotentials (LPSs). This paper reports construction of a reasonably accurate LPS for Ag by means of inversion of the Kohn–Sham (KS) DFT equations in a bulk crystal environment. The accuracy of this LPS is determined within KS-DFT (where the exact noninteracting kinetic energy is employed) by comparing its predictions of bulk properties to those obtained from a conventional (orbital-based) nonlocal pseudopotential (NLPS). We find that the static bulk properties of fcc and hcp Ag predicted within KS-DFT using this LPS compare fairly well to those predicted by an NLPS. With the transferability of the LPS established, we then use this LPS in OF-DFT, where several approximate KEDFs were tested. We find that a combination of the Thomas–Fermi (TTF) and von Weizsäcker (TvW) functionals (TvW+0.4TTF) produces better densities than those from the linear-response-based Wang–Teter KEDF. However, the equations of state obtained from both KEDFs in OF-DFT contain unacceptably large errors. The lack of accurate KEDFs remains the final barrier to extending OF-DFT to treat transition metals.
Computational strategies for a four-component Dirac–Kohn–Sham program: Implementation and first applications
Authors: Belpassi Leonardo , Tarantelli Francesco , Sgamellotti Antonio , Quiney Harry M.
Published: May 2005

An implementation of the generalized gradient approximation within the four-component formulation of relativistic density-functional theory using G-spinor basis sets is presented. This approach is based on the direct evaluation of the relativistic density and its gradient from the G-spinor amplitudes and gradients without explicit reference to the total density matrix. This proves to be a particularly efficient scheme, with an intrinsic computational cost that scales linearly with the number of G-spinor basis functions. In order to validate this new implementation, incorporated in the parallel version of the program BERTHA, a detailed study of the diatomic system CsAu is also reported. The spectroscopic constants De,re,ωe, and xeωe and the dipole moment μ have been calculated and compared with the best available theoretical and experimental data. The sensitivity of our results to the details of the numerical schemes used to evaluate the matrix elements is analyzed in detail. Also presented is a comparative study of molecular properties in the alkali auride series which have been obtained using several standard nonrelativistic density functionals.
Molecular simulation of the homogeneous crystal nucleation of carbon dioxide
Authors: Leyssale Jean-Marc , Delhommelle Jérôme , Millot Claude
Published: May 2005

We report on a molecular simulation study of the homogeneous nucleation of CO2 in the supercooled liquid at low pressure (P=5MPa) and for degrees of supercooling ranging from 32% to 60%. In all cases, regardless of the degree of supercooling, the structure of the crystal nuclei is that of the Pa3 phase, the thermodynamically stable phase. For the more moderate degree of supercooling of 32%, the nucleation is an activated process and requires a method to sample states of high free energy. In this work, we apply a series of bias potentials, which promote the ordering of the centers of mass of the molecules and allow us to gradually grow crystal nuclei. The reliability of the results so obtained is assessed by studying the evolution of the nuclei in the absence of any bias potential, and by determining their probability of growth. We estimate that the size of the critical nucleus, for which the probability of growth is 0.5, is ≈240 molecules. Throughout the nucleation process, the crystal nuclei clearly exhibit a Pa3 structure, in apparent contradiction with Ostwald’s rule of stages. The other polymorphs have a much larger free energy. This makes their formation highly unlikely and accounts for the fact that the nucleation of CO2 proceeds directly in the stable Pa3 structure.
Ab initio-based exciton model of amide I vibrations in peptides: Definition, conformational dependence, and transferability
Authors: Gorbunov Roman D. , Kosov Daniil S. , Stock Gerhard
Published: May 2005

Various aspects of the ab initio-based parametrization of an exciton model of amide I vibrations in peptides are discussed. Adopting “glycine dipeptide” (Ac-Gly-NHCH3) as a simple building-block model that describes the vibrational interaction between two peptide units, we perform comprehensive quantum-chemical calculations to investigate the effect and importance of the level of theory, the choice of local coordinates, and the localization method. A solvent continuum model description turns out important to obtain planar CONH peptide units when a full geometry optimization (which is necessary to obtain the correct frequencies) is performed. To study the conformational dependence of the amide I vibrations, we calculate (ϕ,ψ) maps of the local-mode frequencies and couplings. Performing conformational averages of the (ϕ,ψ) maps with respect to the most important peptide conformational states in solution (α,β,PII, and C5), we discuss the relation between these measurable quantities and the corresponding conformation of the peptide. Finally, the transferability of these maps to dipeptides with hydrophilic and hydrophobic side chains as well as to tripeptides with charged end groups is investigated.
Highly oriented Ni(Pd)SiGe formation at 400 °C
Authors: Jin L. J. , Pey K. L. , Choi W. K. , Fitzgerald E. A. , Antoniadis D. A. , Pitera A. J. , Lee M. L. , Tung C. H.
Published: May 2005

A germanosilicide technology employing Ni0.95Pd0.05 alloy to improve the germanosilicide film texture strucutre on relaxed Si1−xGex substrate has been developed. Highly oriented (Ni0.95Pd0.05)y(Si1−xGex)1−y films where x=0.2, 0.25, 0.3, and y≈0.5 are obtained at 400 °C annealing with (200) as the preferred orientation, as was revealed by cross-sectional transmission electron microscopy and x-ray diffraction results. The formation of the highly oriented (Ni0.95Pd0.05)y(Si1−xGex)1−y film can be explained by interface and surface energies minimization due to the addition of Pd.
Atomic hydrogen densities in capacitively coupled very high-frequency plasmas in H2: Effect of excitation frequency
Authors: Jolly J. , Booth J.-P.
Published: May 2005

Absolute hydrogen atom densities in pure hydrogen capacitive discharges were measured as a function of excitation frequency (13.56, 27.12, and 40.68 MHz), nominal electrical power, and gas pressure (between 0.1 and 1 Torr). Quantitative measurements were made using two-photon absorption laser-induced fluorescence (TALIF), put on an absolute scale by comparison with the TALIF signal from a known density of krypton gas, as proposed by Niemi, Schultz von Gathen, and Döbele [J. Phys. D 34, 2330 (2001)]. The H atom density increases with gas pressure and electrical power, and at a given power and pressure it increases significantly with excitation frequency. The latter can be attributed in part to increased electron density. However, time-resolved TALIF measurements in the afterglow showed that the H atom surface loss probabilities are not constant, becoming somewhat smaller when the sheath voltage is lowered, as is the case when the excitation frequency is increased, contributing to the increase in H density.
Structural characterization of base/collector interfaces for magnetic tunnel transistors grown on Si(001)
Authors: Wang Hua , McCartney Martha R. , Smith David J. , Jiang Xin , Wang Roger , van Dijken Sebastiaan , Parkin Stuart S. P.
Published: May 2005

The microstructure and composition of magnetic tunnel transistors (MTTs), grown on Si(001) collector substrates, have been investigated using high-resolution and analytical electron microscopy. The effects of different seed layers, including a ferromagnetic metal Fe, and noble metals Pd, Ir, Re, and Ru, on the tunnel junction morphology were studied. The Pd seed layers reacted with the Si substrate, and gave MTTs with a small transfer ratio and relatively large leakage current. Devices based on Ir and Re seed layers invariably shorted out, as did most of those with Ru seed layers. Very thin Fe seed layers gave flat and more abrupt seed-layer/Si substrate interfaces, and among the samples studied, the best transport properties were achieved for a thin Fe seed layer in combination with Co70Fe30 base layer.
Magnetohydrodynamic shock wave formation: Effect of area and density variation
Authors: Sujith R. I.
Published: May 2005

The nonlinear steepening of finite amplitude magnetohydrodynamic (MHD) waves propagating perpendicular to the magnetic field is investigated. The nonlinear evolution of a planar fast magnetosonic wave in a homentropic flow field is understood well through simple waves. However, in situations where the wave is moving through a variable area duct or when the flow field is nonhomentropic, the concept of simple waves cannot be used. In the present paper, the quasi-one-dimensional MHD equations that include the effect of area variation and density gradients are solved using the wave front expansion technique. The analysis is performed for a perfectly conducting fluid and also for a weakly conducting fluid. Closed form solutions are obtained for the nonlinear evolution of the slope of the wave front in the limits of infinitely large and small conductivity. A general criterion for a compression wave to steepen into a shock is obtained. An analytical expression for the location of shock formation is derived. The effect of area variation and density gradient on shock formation is studied and examples highlighting the same are presented.
Three-dimensional lattice Boltzmann model for gaseous flow in rectangular microducts and microscale porous media
Authors: Tang G. H. , Tao W. Q. , He Y. L.
Published: May 2005

Microscale fluid dynamics has received intensive interest due to the rapid advances in microelectromechanical systems. In this work, the lattice Boltzmann method is applied to simulate isothermal gaseous slip flow in three-dimensional (3D) rectangular microducts and microscale porous structures. The flow characteristics in 3D microducts—including velocity profile, nonlinear pressure distribution, friction factor, and mass flow rate—are compared with analytical solutions, and the agreement is good. The flow-rate results show that due to the slip-velocity emergence at the walls, the lateral wall influence on the flow rate in 3D rectangular microducts is decreased. The predicted transport characteristics in 3D microscale porous media show that the rarefaction influence increases the gas permeability. The Klinkenberg effect is confirmed and the predicted gas permeability is qualitatively consistent with the experimental results. Furthermore, the nonlinear behavior of the porous flow at relatively higher Reynolds number is also observed. This study demonstrates that the lattice Boltzmann method can be employed to efficiently predict transport characteristics in microducts and microscale porous media.
Using chaotic forcing to detect damage in a structure
Authors: Moniz Linda , Nichols Jon , Trickey Steven , Seaver Mark , Pecora Daniel , Pecora Louis
Published: May 2005

In this work we develop a numerical test for Holder continuity and apply it and another test for continuity to the difficult problem of detecting damage in structures. We subject a thin metal plate with incremental damage to the plate changes, its filtering properties, and therefore the phase space trajectories of the response chaotic excitation of various bandwidths. Damage to the plate changes its filtering properties and therefore the phase space of the response. Because the data are multivariate (the plate is instrumented with multiple sensors) we use a singular value decomposition of the set of the output time series to reduce the embedding dimension of the response time series. We use two geometric tests to compare an attractor reconstructed from data from an undamaged structure to that reconstructed from data from a damaged structure. These two tests translate to testing for both generalized and differentiable synchronization between responses. We show loss of synchronization of responses with damage to the structure.