bluuebunny/crossref_metadata_2025 huggingface dataset:
Self-assembled InAs quantum dots formed by molecular beam epitaxy at low temperature and postgrowth annealing
Authors: Zhan H. H. , Nötzel R. , Hamhuis G. J. , Eijkemans T. J. , Wolter J. H.
Published: May 2003

Self-assembled InAs quantum dots are grown at low temperature (LT) by molecular beam epitaxy (MBE) on GaAs substrates. The growth is in situ monitored by reflection high-energy electron diffraction, and ex situ evaluated by atomic force microscopy for the morphological properties, and by high-resolution x-ray diffraction for the structural properties. While two monolayers as-grown LT (250 °C) InAs layers exhibit shallow mounds due to the low adatom migration length at low temperature, well-developed InAs dots are formed after postgrowth annealing above 450 °C. The structural quality of the LT GaAs matrix grown on top and of the embedded InAs dot layer is improved when a 3 nm GaAs interlayer is deposited (at 480 °C) on the InAs dots and subsequently annealed at 580 °C before LT GaAs overgrowth. These high structural quality LT-grown InAs dots are considered for applications in high-speed optical modulators and switches operating at low power by combining the high optical nonlinearity of quantum dots with the ultrafast optical response provided by LT growth in MBE.
Enhanced spontaneous emission from InAs/GaAs self-organized quantum dots in a GaAs photonic-crystal-based microcavity
Authors: Yu Peichen , Bhattacharya Pallab , Cheng Jui-Ching
Published: May 2003

Spontaneous emission characteristics of self-organized InAs/GaAs quantum dots embedded in a photonic-crystal-based microcavity are theoretically investigated. Results are obtained from a three-dimensional quasi-mode analysis of the photonic-crystal microcavity and through calculations of energy levels and eigenstates for electrons and holes in self-organized quantum dots. The spontaneous emission rate from quantum dots is significantly enhanced by up to a factor of 14, compared to that in free space. In addition, the maximum coupling efficiency to the fundamental mode is close to unity.
Simple technique to determine the drain temperature in GaAs metal semiconductor field effect transistor
Authors: Kameche Mohamed , Feham Mohamed
Published: May 2003

The effects of the operating temperature T and the drain voltage on the drain temperature in GaAs metal semiconductor field effect transistor (MESFET) are a serious problem, which do not receive enough attention in the scientific literature in the last years. This article presents a simple technique to determine the drain temperature by setting the equality between the field-dependent diffusivity-to-mobility ratio (nonlinear relation) and the constant ratio linear modeling. So, we investigate the impact of the high-field electron diffusivity model in GaAs on the characteristics of MESFETs. Two dimensional numerical simulation is used to describe significant physics in the characteristics for 0.5 μm MESFET GaAs.
Asymptotic behavior of the exchange-correlation potentials from the linear-response Sham–Schlüter equation
Authors: Niquet Y. M. , Fuchs M. , Gonze X.
Published: May 2003

The linear-response Sham–Schlüter equation can be used to calculate an exchange-correlation potential starting from a given approximation for the self-energy. The asymptotic behavior of these potentials is, however, much debated, a recent work suggesting that they could blow up in finite systems. Here we investigate the asymptotic behavior of the linear-response Sham–Schlüter potentials in the GW and second-order approximations for the self-energy. We show that these potentials do not diverge, and that the correlation potential itself has a −α/(2r4) tail (under appropriate conditions), where α depends on the self-energy. We also provide further justification for the quasiparticle approximation to the linear-response Sham–Schlüter equation, that is much simpler to solve while likely being of comparable accuracy. Calculations for real molecules or solids using this approximation should be within the reach of present computers.
Wave dynamic processes induced by a supersonic projectile discharging from a shock tube
Authors: Jiang Zonglin
Published: May 2003

A numerical study on wave dynamic processes occurring in muzzle blast flows, which are created by a supersonic projectile released from the open-end of a shock tube into ambient air, is described in this paper. The Euler equations, assuming axisymmetric flows, are solved by using a dispersion-controlled scheme implemented with moving boundary conditions. Three test cases are simulated for examining friction effects on the muzzle flow. From numerical simulations, the wave dynamic processes, including two blast waves, two jet flows, the bow shock wave and their interactions in the muzzle blasts, are demonstrated and discussed in detail. The study shows that the major wave dynamic processes developing in the muzzle flow remain similar when the friction varies, but some wave processes, such as shock–shock interactions, shock–jet interactions and the contact surface instability, get more intensive, which result in more complex muzzle blast flows.
Two-way coupling simulations of instabilities in a plane bubble plume
Authors: Caballina Ophélie , Climent Eric , Dušek Jan
Published: May 2003

In the present study we aim at investigating the instabilities in a plane bubble plume by means of two-way coupling simulations. The continuous phase motion is obtained by direct numerical solution of the Navier–Stokes equations forced by the presence of the bubbles. The collective effects induced by the presence of the bubbles are modeled by a spatiotemporal distribution of momentum. Time evolution of the dispersed phase is solved by Lagrangian tracking of all the bubbles. In the present study, the motion of the carrying fluid is initiated and driven by the induced buoyancy of bubbles released from a source located in an initially quiescent fluid layer. A quantitative analysis of the flow transition is thus investigated for several plume widths and for various fluid viscosities over a range of Grashof numbers based on the injection conditions. An analogy is drawn with buoyant single-phase flows for the steady laminar region. Following the similarity formulation of Fujii [Int. J. Heat Mass Transfer 6, 597 (1963)] under boundary layer approximations for free thermal plumes, the velocity profiles can be collapsed to a single self-similar plot. Nevertheless, this analogy with single-phase flow shows some discrepancies in the description of the transition. Numerical data emphasize that the key parameter controlling the height of transition is the Grashof number, which is based on injection conditions of the dispersed phase. Our results concur with the recent experiments of Alam and Arakeri [J. Fluid Mech. 254, 363 (1993)]. Although the Grashof number also determines the transition in thermal plumes [Wakitani and Yosinobu, Fluid Dyn. Res. 2, 363 (1988)], the two-phase configuration is more unstable. These new results underline the important role played by the slip velocity of the bubbles in plume stability. Indeed, it tends to delay the plume transition when the slip velocity increases and approaches the buoyancy-induced velocity. This feature should also be related to the lack of diffusion by the bubble cloud in the Lagrangian transport of the density gradient.
Shape-preserving solutions for quantum vortex motion under localized induction approximation
Authors: Lipniacki Tomasz
Published: May 2003

The motion of a quantum vortex in superfluid helium is considered in the localized induction approximation. In this approximation the instantaneous velocity of quantum vortex is proportional to the local curvature and is parallel to the vector, which is a linear combination of the local binormal and the principal normal to the vortex line. The motion in the direction of the principal normal is specific for a quantum vortex and implies that the vortex shrinks, in contrast to the classical vortex in an ideal fluid. In the present work we deal with two four-parameter classes of shape-preserving solutions (one with increasing and one with decreasing spatial scale) resulting from equations governing the curvature and the torsion. The solutions describe vortex lines whose motion is equivalent to a transformation being a superposition of a homothety and a rotation. In a particular case when the transformation is a pure homothety, we find analytic solutions for the curvature and the torsion. In the general case, when the transformation is a superposition of a nontrivial rotation and a homothety, the asymptotics of the solutions of the first class are given explicitly and are related to the parameters characterizing the transformation. It is found that the solutions of the second class (with decreasing scale) either have asymptotes or are periodic (when the transformation is a pure homothety) or else exhibit chaotic behavior.
Contact line instability and pattern selection in thermally driven liquid films
Authors: Grigoriev Roman O.
Published: May 2003

Liquids spreading over a solid substrate under the action of various forces are known to exhibit a long wavelength contact line instability. We use an example of thermally driven spreading on a horizontal surface to study how the stability of the flow can be altered, or patterns selected, using feedback control. We show that thermal perturbations of certain spatial structure imposed behind the contact line and proportional to the deviation of the contact line from its mean position can completely suppress the instability. Due to the presence of mean flow and a spatially nonuniform nature of spreading liquid films the dynamics of disturbances is governed by a non-normal evolution operator, opening up a possibility of transient amplification and nonlinear instabilities. We show that in the case of thermal driving the non-normality can be significant, especially for small wavenumber disturbances, and trace the origin of transient amplification to a close alignment of a large group of eigenfunctions of the evolution operator. However, for values of noise likely to occur in experiments we find that the transient amplification is not sufficiently strong to either change the predictions of the linear stability analysis or invalidate the proposed control approach.
Perpendicular magnetic anisotropy of Co–TiN composite film with nano-fiber structure
Authors: Chen C. C. , Hashimoto M. , Shi J. , Nakamura Y. , Nittono O. , Barna P. B.
Published: May 2003

Co–Ti–N films have been prepared by sputter deposition of Co and Ti in Ar+N2 atmosphere. Upon thermal anneal at elevated temperatures, Co (face-centered cubic) and TiN were formed in the film and separated from each other. Fiber-like microstructure developed with Co nano-fibers vertical to the substrate surface, and with their lateral size being less than 10 nm. The magnetic anisotropy of such films depends strongly on the film thickness. The Co–TiN films with their thickness above 100 nm show perpendicular magnetic anisotropy, which is explained in terms of shape anisotropy. Considering their microstructure, it is concluded that the diameter to length ratio of Co nano-fibers is an important factor controlling the magnetic anisotropy. For the Co–TiN film to show perpendicular magnetic anisotropy, the diameter to length ratio has to be smaller than 0.07 according to the experimental results. TiN in the films plays an important role in separating Co nano-fibers and thus to reduce the lateral magnetic interaction among them. The nano-scale nature and perpendicular magnetic anisotropy of the Co–TiN nanocomposite film make it a very promising candidate for future ultrahigh magnetic recording media.
Generation of ferroelectric domains in atomic force microscope
Authors: Molotskii M.
Published: May 2003

A theory of an equilibrium shape of domains formed in an electric field of atomic force microscope (AFM) is proposed. The domain shape depends on parameters of the ferroelectric and on the applied voltage. Under low voltages the length and the diameter of the domain are of the same order of magnitude. With voltage increase the ratio between the length and the diameter increases. A correlation between the lateral sizes and the spontaneous polarization value is considered. It is shown that under the same voltage the thinnest domains are formed in ferroelectrics with high spontaneous polarization. The concept of the domain shape invariant as a combination of the domain length and lateral size, which is constant when changing the AFM parameters, is introduced. Results of the calculation of the invariant value in barium titanate as well as of the domain dimensions and the shape in GASH are in good agreement with the experiment.
Microstructure of annealed magnetic tunnel junction by electron microscopy
Authors: Xu Q. Y. , Wang Y. G. , Zhang Z. , You B. , Du J. , Hu A.
Published: May 2003

Transmission electron microscopy, high-resolution electron microscopy, and electron holography were used to study the microstructure of CoFe/AlOx/Co magnetic tunnel junctions (MTJs) isochronally annealed up to 400 °C. A potential barrier across the metal/oxide interfaces was observed for the as-deposited MTJ sample, and was changed into a well for the MTJ samples annealed at 200 and 400 °C, respectively. A shallow potential well was found when the MTJ was annealed at 200 °C and the well became deeper as the annealing temperature increased to 400 °C. The potential change may attribute to the formation of nonmagnetic metallic Al atoms or clusters when the MTJ sample was annealed at 200 °C and the rest content of the barrier layer was more close to Al2O3, which results in the enhancement of tunneling magnetoresistance (TMR). When the MTJ sample was annealed at 400 °C, more Co and Fe atoms or clusters might diffuse from the ferromagnetic layers into the barrier layer, resulting in the deeper well, and thus significantly decrease the TMR value due to the severe spin-flip scattering.
Magnetic properties and the tunneling magnetoresistance effect in Co−MgF2 granular films
Authors: Chi J. H. , Ge S. H. , Liu C. M. , Kunkel H. P. , Zhou X. Z. , Williams G.
Published: May 2003

Co − MgF 2 granular films were deposited on glass substrates by rf co-sputtering at room temperature (RT). The influence of the Co volume fraction, fv, of these granular films on the tunneling magnetoresistance (TMR) and magnetic properties was studied systematically. In a magnetic field of 1.2 T, the TMR value at RT initially increases gradually with decreasing fv, reaches its maximum value of −8% for fv=0.38, and then decreases. The corresponding magnetization curves indicate a change from ferromagnetism to superparamagnetism. A minimum in the coercivity, Hc, (11 Oe) is obtained in the Co50(MgF2)50 granular film which also has a large zero field resistivity. This magnetically soft granular film may consequently be a good candidate for high frequency applications. These variations of the TMR and magnetic properties can be ascribed to gradual changes in the film microstructure with decreasing fv, from interconnected metallic Co grains to nano-scaled Co particles dispersed in a crystallized insulating MgF2 matrix. The zero field cooled (ZFC) and field cooled (FC) curves for the samples were obtained in the temperature range 5–300 K in various magnetic fields. The peak in the ZFC curve shifts gradually towards lower temperature with increasing applied magnetic field and, with increasing fv, the peak temperature decreases more quickly with increasing field. The latter indicates that the magnetic interactions between grains become stronger, consistent with model predictions.
Resonant ionization of laser desorbed silicon
Authors: Gibert T. , Gonthiez T.
Published: May 2003

Soft ultraviolet laser desorption of neutral and ionized Si atoms was investigated at 355 nm for fluences ranging from the desorption threshold (85 mJ/cm2) up to 165 mJ/cm2. The sensitivity of resonance ionization mass spectrometry enabled the number of sputtered particles to be studied at a very low emission level corresponding to only several 100 atoms. For such a low emission yield, the ejected atoms keeps the memory of the laser–surface interaction mechanism during their flight in the ultrahigh vacuum condition. The velocity distribution of neutrals was measured for different fluences and were well fitted by a set of 2 Maxwellian functions at each fluence. From these fits it appears that the primary mechanisms involved in laser desorption are both a thermal process and an electronic one.
Single-chip device for tunneling time measurements
Authors: Dragoman Daniela , Dragoman Mircea
Published: May 2003

A single-chip mesoscopic device is proposed for the measurement of the tunneling time of electrons or holes through various mesoscopic structures. The method is based on the possibility of generating spatially separated electrons and holes that are transported with a controlled velocity and separation over a quantum wire. The electrons and holes, subsequently separated in distinct waveguides, are allowed to recombine after propagating through carefully controlled distances. The electron and hole coincidence requirement is satisfied by compensating the tunneling time of one type of carrier through an engineered mesoscopic structure with the magnetic field induced delay of the other carrier type.
Analysis of the kinetics for interface state generation following hole injection
Authors: Zhang J. F. , Zhao C. Z. , Groeseneken G. , Degraeve R.
Published: May 2003

Interface state generation is a major reliability issue for metal–oxide–semiconductor based devices. The generation can take place not only during stresses, but also after terminating the stress. Our attention is focused on analyzing the dynamic behavior of the generation after substrate hot hole injection. Despite previous efforts in this area, the generation kinetics is not fully understood, and there is insufficient information on the process limiting the generation rate. We start by showing that the normalized generation kinetics is insensitive to either the defect density or the processing condition. We then investigate the effect of various stressing parameters on the kinetics, including the oxide field strength during and posthole injection, the stress time, the energy and current of hot holes. This is followed by examining why the available models are inapplicable in our cases, including hydrogen transportation, trapped hole conversion, and coupling models. Finally, we propose both hydrogen emission and detrapping of a certain type of hole traps as the rate limiting processes, and the relative importance of these two is addressed.
Theoretical calculations of effective exchange integrals by spin projected and unprojected broken-symmetry methods. I. Cluster models of K2NiF4-type solids
Authors: Onishi Taku , Yamaki Daisuke , Yamaguchi Kizashi , Takano Yu
Published: May 2003

Previously, various symmetry-adapted (SA) and broken-symmetry (BS) computations have been performed for strongly correlated transition metal species so as to examine magnetic properties in simple cluster models such as binuclear transition metal oxides. Though SA computations such as the complete active space configuration interaction and CASSCF are desirable for estimating physical constants, these computations are heavy for larger cluster models with strongly correlated electrons. K2NiF4 is known to be the two-dimensional perovskite-type antiferromagnet and to have the electronic configuration similar to that of La2CuO4. Here, we have examined the utility and applicability of the BS spin-polarized hybrid-density functional theory (HUDFT) for cluster models of K2NiF4. As the result, HUDFT calculation such as UB2LYP has provided the reasonable effective exchange integral (Jab) followed by our approximately spin projected scheme, in comparison to the experimental one. It was also found that the square planar tetranuclear model has provided the most reasonable Jab value by HUDFT. In addition, external effects such as putting point charges around cluster models and changing distances between nickel and fluorine have been also examined. The natural orbital analysis by HUDFT has been carried out to obtain natural orbitals and their occupation numbers. Charge density, spin density, and chemical indices expressed by the occupation numbers have been also obtained to elucidate the nature of the chemical bonds in the K2NiF4-type solids.
Fast evaluation of the Coulomb potential for electron densities using multipole accelerated resolution of identity approximation
Authors: Sierka Marek , Hogekamp Annika , Ahlrichs Reinhart
Published: May 2003

A new computational approach is presented that allows for an accurate and efficient treatment of the electronic Coulomb term in density functional methods. This multipole accelerated resolution of identity for J (MARI-J) method partitions the Coulomb interactions into the near- and far-field parts. The calculation of the far-field part is performed by a straightforward application of the multipole expansions and the near-field part is evaluated employing expansion of molecular electron densities in atom-centered auxiliary basis sets (RI-J approximation). Compared to full RI-J calculations, up to 6.5-fold CPU time savings are reported for systems with about 1000 atoms without any significant loss of accuracy. Other multipole-based methods are compared with regard to reduction of the CPU times versus the conventional treatment of the Coulomb term. The MARI-J approach compares favorably and offers speedups approaching two orders of magnitude for molecules with about 400 atoms and more than 5000 basis functions. Our new method shows scalings as favorable as N1.5, where N is the number of basis functions, for a variety of systems including dense three-dimensional molecules. Calculations on molecules with up to 1000 atoms and 7000 to 14 000 basis functions, depending on symmetry, can now be easily performed on single processor work stations. Details of the method implementation in the quantum chemical program TURBOMOLE are discussed.
Viscous-slip, thermal-slip, and temperature-jump coefficients as defined by the linearized Boltzmann equation and the Cercignani–Lampis boundary condition
Authors: Siewert C. E.
Published: May 2003

A polynomial expansion procedure and an analytical discrete-ordinates method are used to evaluate the viscous-slip coefficient, the thermal-slip coefficient, and the temperature-jump coefficient as defined by a rigorous version of the linearized Boltzmann equation for rigid-sphere interactions and the Cercignani–Lampis boundary condition.
Ionic conductivity, glass transition, and local free volume in poly(ethylene oxide) electrolytes: Single and mixed ion conductors
Authors: Bamford D. , Reiche A. , Dlubek G. , Alloin F. , Sanchez J.-Y. , Alam M. A.
Published: May 2003

Ethylene oxide based polymer electrolytes which exhibit single-ion (cation Li+ or anion ClO4−) and mixed ion (from the dissociation of LiClO4 salt) conduction were studied by employing positron lifetime annihilation spectroscopy (PALS) and conductivity (σ) measurements in the temperature range between 170 and 370 K. We present experimental evidence for the validity of (i) the linear expansion of local free volume from PALS, (ii) the Vogel–Tammann–Fulcher (VTF) law for σ, and (iii) the Cohen–Turnbull equation that relates σ to the local free volume. These were found to be valid in the temperature range above the end (or freezing) temperature of the glass transition, Tge≈1.06TgPALS≈1.18TgDSC [TgPALS and TgDSC are the Tg’s from PALS and differential scanning calorimetry (DSC), respectively]. From VTF fits to σ we obtained a Vogel temperature of T0≈TgDSC and pseudoactivations energies of B=3.7–5.7 kJ/mol. These parameters disagree with many data published in the literature but are in perfect agreement with the free volume experiments. Moreover, we found T0=TgPALS−(20–28) K and TgDSC=TgPALS−(25–27) K. Indications for the existence of two relaxation processes near Tg were observed in the free volume expansion curves, which were attributed to the motion of free polymer segments and those interacting with ions. The discrepancy between TgDSC and TgPALS can be attributed to the two-phase microseparation of the polymer electrolytes; DSC responds mainly to the polymer segments in the ion-depleted regions while PALS responds to the polymer segments in the ion-rich regions. From the Cohen–Turnbull plots the critical hole volume required for an elementary jump of an ion was estimated to be γν*≈1 nm3 and was found to be independent of the type of ion. This shows that each type of ionic conductivity is associated with the same segmental mobility. Below TgePALS the conductivity is larger than expected from the (extrapolated) VTF law, but smaller than displayed in the frozen-in free volume.
Model of the plane laminar PbTiO3 thin film domain structure on MgO substrate
Authors: Lahoche L. , Lorman V. , Rochal S. B. , Roelandt J. M.
Published: May 2003

Superstructure formation is examined as a strain relaxation mechanism for polydomain structure in an “utlrathin” film on a cubic substrate. A phenomenological model of the dielectric and mechanical behavior of the ferroelectric PbTiO3 film on an MgO oxide substrate is proposed. The case of a laminar 90° domain structure with the walls tilted to 45° with respect to the film/substrate interface is considered. Taking account an inhomogeneous film–substrate coupling related to the superstructure formation and a dislocation propagation mechanism, we investigate strain relaxation and its effect on the domain structure, electrical and mechanical properties as a function of the film thickness, and temperature. It is shown that evolution of the aa-domain abundance in utlrathin film can be related to the film–substrate coupling mechanism expressed in terms of an external to the film inhomogeneous field. Its variation with the film thickness implies the existence of two distinct growth modes influencing the layer texture. For films with a thickness greater than 250 nm, aa-domains abundance is maximum and equal to 18%. For thinner films, c-domains become more stable and their proportion increases up to ∼100% with the appearance of a single domain structure. Numerical modeling of electrical polarization, total strain in both domains, and mean stress in the film is performed for different thicknesses.
Switching speed limitations in perpendicular magnetic recording media
Authors: Lyberatos A.
Published: May 2003

A method to study switching speed limitations in high-frequency magnetic recording is presented. The characteristic parameter is the minimum irreversible switching time that is evaluated by numerical simulation of the writing of a dibit of two transitions using Landau–Lifshitz dynamics. The dibit is written on a perpendicular polycrystalline thin film medium using a single-pole head with a soft underlayer. We study the physical factors that determine the ultimate data rate and consider the problem of optimization of switching speed by changing the relative head-medium velocity, the magnetocrystalline anisotropy, the rise time of the head field, the head-medium separation, the exchange intergranular coupling, and the dispersion in the direction and magnitude of the local anisotropy.
X-ray absorption and diffraction studies of thin polymer/FePt nanoparticle assemblies
Authors: Anders S. , Toney M. F. , Thomson T. , Farrow R. F. C. , Thiele J.-U. , Terris B. D. , Sun Shouheng , Murray C. B.
Published: May 2003

We have produced assemblies of monodisperse 4 nm FePt nanoparticles using polymer-mediated layer–by–layer deposition at room temperature. The process leads to good control of particle assembly thickness and offers great potential for future fabrication of ultra-high density magnetic storage media. Vibrating sample magnetometry with fields up to 9 T was applied to study the magnetic properties of the particle assemblies as a function of annealing condition while near edge x-ray absorption fine structure (NEXAFS) spectroscopy and x-ray diffraction (XRD) were used to investigate the chemical nature and structural properties within particles. It was found that the coercivity can be as high as 22.7 kOe for samples annealed at 800 °C, the moment density (normalized to the particle volume) for the sample annealed at 650 °C is estimated close to the value for bulk FePt, at 1140 emu/cm3. NEXAFS spectroscopy shows that the Fe in the as-deposited assemblies is partly oxidized, and the oxidation is greatly reduced by annealing. XRD studies on the assemblies annealed at high temperature revealed the increased atomic order and the formation of the high-anisotropy L10 phase within the particles. However, the high-temperature annealing also resulted in nanoparticle agglomeration.
First-principles calculations and experimental study for effect of different inorganic components on solid acid-base properties of inorganic/organic hybrids
Authors: Katayama Shingo , Iwata Keiji , Kubo Yuji , Yamada Noriko , Nonaka Yasushi
Published: May 2003

Solid acidity was found to be generated in methylsiloxane-based inorganic/organic hybrids by incorporating inorganic components other than Si, such as Al and Nb, into methylsiloxane networks by combining theoretical first-principles calculations based on the density functional theory (DFT) with experimental synthesis and characterization. First-principles calculations based on the DFT indicated that the dissociation energy of protons at the surface followed the order Si<Al<Nb. The calculation led to the conclusion that methylsiloxane-based inorganic/organic hybrids were synthesized from metal alkoxides. The solid acidity followed the same order: Si<Al<Nb. The Fourier-transform infrared measurements of pyridine adsorbed on the methylsiloxane-based inorganic/organic hybrids showed that the solid acidity can be attributed to Brønsted acid sites.
On the electronic structure of CmFn (n=1–4) by all-electron Dirac–Hartree–Fock calculations
Authors: Mochizuki Yuji , Tatewaki Hiroshi
Published: May 2003

All-electron Dirac–Hartree–Fock calculations were performed to investigate the electronic structure of curium fluoride molecules (CmFn, n=1–4). The curium–fluorine distances, atomization energies, and Mulliken populations were all evaluated. The bonding was found to be of ionic type with the electron delocalization primarily from F 2p to Cm 6d, where the formal configurations of Cm were 7s25f7, 5f77s1, 5f7, 5f6 for n=1, 2, 3, 4, respectively. The 2p–6d delocalization is significant in the tetrafluoride. For comparison, the isovalent gadolinium fluorides were also studied. CmF4 exists but GdF4 does not, reflecting the fact that 5f7 in CmF3 is less stable than 4f7 in GdF3.
Rydberg–valence interactions in CH2Cl→CH2+Cl photodissociation: Dependence of absorption probability on ground state vibrational excitation
Authors: Levchenko Sergey V. , Demyanenko Andrei V. , Dribinski Vladimir L. , Potter Aaron B. , Reisler Hanna , Krylov Anna I.
Published: May 2003

A strong enhancement of absorption to the lowest A12 state is observed for vibrationally excited chloromethyl radicals. It is demonstrated that this enhancement is due to a significant increase in both electronic and vibrational Franck–Condon factors. Electronic structure calculations of potential energy surfaces (PESs) and transition dipole moments for the ground and the two lowest excited states of A1 symmetry, the 1 2A1 valence and 22A1 Rydberg states, reveal the origin of this effect. The shelflike shape of the 1 2A1 PES in the Franck–Condon region and the strong dependence of the electronic transition dipole moment on C–Cl distance are responsible for the enhancement. Analysis of the shape of the electron density distribution demonstrates that Rydberg–valence interaction in the two lowest excited states causes the changes in the shape of PESs and transition dipoles with C–Cl distance.
Theoretical studies of the first-row transition metal phosphides
Authors: Tong Glenna S. M. , Jeung G. H. , Cheung A. S-C.
Published: May 2003

The electronic structures and spectroscopic parameters of the ground and some low-lying excited states of the first-row transition metal phosphides have been calculated with the density functional theory using the Becke three-parameter hybrid exchange functional with the Lee–Yang–Parr correlation functional (B3LYP). The ground states of the transition metal phosphides are found to be Σ+1 (ScP), Δ2 (TiP), Δ3 (VP), Σ−4 (CrP), Π2 (MnP), Σ+6 (FeP), Δ5 (CoP), Δ4 (NiP), and Σ−3 (CuP). The B3LYP functional predicts an increase in covalent character in the bonds between the metal and the phosphorus across the transition metal series. The energies of the low-lying excited states relative to the ground state for TiP, FeP, and CoP have been found to be so small that many low-lying states are possible candidates to be the ground state.
Tuning of the excited state properties of phenylenevinylene oligomers: A time-dependent density functional theory study
Authors: Grozema F. C. , Telesca R. , Snijders J. G. , Siebbeles L. D. A.
Published: May 2003

This paper discusses a time-dependent density functional theory study of the effect of molecular structure on the excited state polarizability of conjugated molecules. A short phenylenevinylene oligomer containing three phenyl rings (PV2, distyryl benzene) is taken as a model system. Introduction of methyl substituents is shown to have only a small influence on the increase in polarizability upon excitation (the excess polarizability, Δᾱ). Methoxy groups have a much larger effect but in this case Δᾱ depends strongly on the dihedral angle between the side chain and the backbone of the molecule. If the central phenyl ring of PV2 has a meta-configuration rather than para, both the optical absorption spectrum and the excess polarizability change considerably.
Polarization propagator calculations of the polarizability tensor at imaginary frequencies and long-range interactions for the noble gases and n-alkanes
Authors: Norman Patrick , Jiemchooroj Auayporn , Sernelius Bo E.
Published: May 2003

The linear polarization propagator has been computed at imaginary frequencies for He, Ne, Ar, and Kr as well as for the n-alkanes including heptane and its smaller members. It is shown that an effective and direct evaluation of the polarization propagator using standard electronic structure first principle methods can be achieved on the whole imaginary axis without expanding the polarizability in a series of the Cauchy moments. The linear response equation will be complex in this case, but an effective algorithm can be constructed so that the computational cost parallels that of the real propagator. Calculations of the polarizability tensor are used to determine the Casimir–Polder interaction potentials for the molecules under consideration. Theoretical results for the C6 dispersion coefficient are compared with accurate experimental data, and it is shown that results for the extended n-alkanes obtained with density functional theory and the hybrid B3LYP exchange correlation functional are in excellent agreement with experiment. At the same level of theory, on the other hand, there are significant discrepancies for the noble gas atoms. The electron correlation contribution to C6 is less than 9% for the n-alkanes and decreases with the size of the system.
Surface alloying of metals by nanosecond laser pulses under transparent overlays
Authors: Fominski V. Yu. , Romanov R. I. , Smurov I. , Smirnov A. L.
Published: May 2003

A thorough study was made of the processes determining the formation of metastable surface alloys in a film–substrate system [using Au(110 nm)–Ni and Sn(40 nm)–Cr systems as an example] irradiated by laser pulses of nanosecond duration. The irradiation was performed in conventional conditions (in air) and, following the applied method, through a transparent overlay. Scanning/transmission electron microscopy and Rutherford backscattering spectroscopy investigations of alloys were complemented by the measurements of acoustic signals initiated by laser pulses in the substrate. It has been ascertained that the alloy formation in air is accompanied by two competitive processes: laser sputtering of the film substance and its diffusion into the molten substrate layer. As the irradiation intensity goes up, the hydrodynamical sputtering mechanism is replaced by the atomic vaporization. As a result, sufficiently high vapor pressure (up to 102 bar) is generated above the film surface, the film is pressed to the substrate, and the heat and mass transport through the film–substrate interface is realized. However, no more than 20% of the atoms (of their total amount in the film) penetrate into the substrate, and the depth of alloying is within 100 nm. The experiments on the irradiation of the same systems through transparent overlays (water or “liquid glass”) have demonstrated a marked change in the pattern of processes: alloying atoms efficiently penetrate from the film into the substrate, the depth of alloying increases to 400 nm, and defects are formed in deep layers of the substrate.
Effects of assistant anode on planar inductively coupled magnetized argon plasma in plasma immersion ion implantation
Authors: Tang Deli , Chu Paul K.
Published: May 2003

The enhancement of planar radio frequency (RF) inductively coupled argon plasma is studied in the presence of an assistant anode and an external magnetic field at low pressure. The influence of the assistant anode and magnetic field on the efficiency of RF power absorption and plasma parameters is investigated. An external axial magnetic field is coupled into the plasma discharge region by an external electromagnetic coil outside the discharge chamber and an assistant cylindrical anode is inserted into the discharge chamber to enhance the plasma discharge. The plasma parameters and density profile are measured by an electrostatic Langmuir probe at different magnetic fields and anode voltages. The RF power absorption by the plasma can be effectively enhanced by the external magnetic field compared with the nonmagnetized discharge. The plasma density can be further increased by the application of a voltage to the assistant anode. Owing to the effective power absorption and enhanced plasma discharge by the assistant anode in a longitudinal magnetic field, the plasma density can be enhanced by more than a factor of two. Meanwhile, the nonuniformity of the plasma density is less than 10% and it can be achieved in a process chamber with a diameter of 600 mm.
Simulation of the dc plasma in carbon nanotube growth
Authors: Hash David , Bose Deepak , Govindan T. R. , Meyyappan M.
Published: May 2003

A model for the dc plasma used in carbon nanotube growth is presented, and one-dimensional simulations of an acetylene/ammonia/argon system are performed. The effect of dc bias is illustrated by examining electron temperature, electron and ion densities, and neutral densities. Introducing a tungsten filament in the dc plasma, as in hot filament chemical vapor deposition with plasma assistance, shows negligible influence on the system characteristics.
Contact resistance in organic transistors that use source and drain electrodes formed by soft contact lamination
Authors: Zaumseil Jana , Baldwin Kirk W. , Rogers John A.
Published: May 2003

Soft contact lamination of source/drain electrodes supported by gold-coated high-resolution rubber stamps against organic semiconductor films can yield high-performance organic transistors. This article presents a detailed study of the electrical properties of these devices, with an emphasis on the nature of the laminated contacts with the p- and n-type semiconductors pentacene and copper hexadecafluorophthalocyanine, respectively. The analysis uses models developed for characterizing amorphous silicon transistors. The results demonstrate that the parasitic resistances related to the laminated contacts and their coupling to the transistor channel are considerably lower than those associated with conventional contacts formed by evaporation of gold electrodes directly on top of the organic semiconductors. These and other attractive features of transistors built by soft contact lamination suggest that they may be important for basic and applied studies in plastic electronics and nanoelectronic systems based on unconventional materials.
Time- and space-resolved electric potentials in a parallel-plate radio-frequency plasma
Authors: Teii Kungen , Mizumura Michinobu , Matsumura Shousaku , Teii Shinriki
Published: May 2003

A low-input capacitance emissive probe heated by a CO2 laser has been used to investigate frequency-modulated electric potentials in a radio-frequency (rf) glow discharge plasma. An excellent time-response ability of our probe system to a sharp potential change was confirmed by potential measurements with applying a step function voltage to the plasma. The electric field distribution as the first derivative of the potential distribution obtained by the probe method was compared with that by the laser optogalvanic spectroscopy to ensure the reliability of the measurements in the sheath region. The probe method was applied to directly monitor a temporal variation of axial potential and electric field distributions between two parallel-plate electrodes in a rf argon plasma. The formation of ion sheath regions on both electrode surfaces was confirmed at any phase in a rf cycle. The results were consistent with a one-dimensional model for potential and electric field distributions in the sheath region.
Microstructure of nematic amorphous block copolymers: Dependence on the nematic volume fraction
Authors: Hamm M. , Goldbeck-Wood G. , Zvelindovsky A. V. , Fraaije J. G. E. M.
Published: May 2003

We present a model for the structure formation in nematic amorphous copolymers and simulation results for a two-dimensional (2D) implementation. The model is based on a dynamic mean-field method, which allows one to specify the polymer system on two different levels of detail. On the detailed level the nematic amorphous block copolymer molecules are represented by a wormlike chain, characterized by three profiles defining its architecture. The first profile sets the sequence of different monomer types along the chain. The second distinguishes whether individual segments do or do not contribute to the nematic order. The third profile defines how the stiffness varies along the chain. On the coarsened level the system is described in terms of density distributions representing the different monomer species and an orientation distribution for the local alignment of the nematic segments. The simulations investigate how the volume fraction of the nematic component effects the resulting mesostructure. With increasing volume fraction of the nematic block 2D equivalents of “hockey puck” micelles and smectic-C and smectic-A-like structures are found.
Microscopic derivation of hydrodynamic equations for phase-separating fluid mixtures
Authors: Español Pep , Thieulot Cedric
Published: May 2003

The hydrodynamic equations of a phase-separating fluid mixture are derived from the underlying microscopic dynamics of the system. A projection operator method is used in the GENERIC form [H. C. Öttinger, Phys. Rev. E 57, 1416 (1998)]. In this way, the thermodynamic consistency of the final equations is apparent. The microscopic potential is separated into short- and long-range parts, in the spirit of the original work of van der Waals. Explicit expressions for surface tension terms in the hydrodynamic equations are obtained. These terms describe diffuse interfaces in the system. Miscible–immiscible and gas–liquid phase transitions are possible, nonisothermal situations can be studied, and explicit account of cross effects is taken.
A dynamical definition of quasibound molecular clusters
Authors: Harris Sarah A. , Ford Ian J.
Published: May 2003

Molecular configurations which count as snapshots of a quasibound cluster are identified through a retrospective dynamical definition. The trajectory of a molecular cluster is followed, and a clear evaporation event is considered to have occurred when a molecule moves a very long distance away from the others. The cluster is judged to have broken before this condition is satisfied, however, at the instant that the energy of the departing molecule in the center of mass frame becomes positive. The decay of a cluster is therefore defined dynamically as the production of a molecule with positive energy on a separating trajectory. Not all positive energy molecules created by the system follow such a trajectory, hence the need to examine the subsequent behavior in molecular dynamics. We simulate a sequence of decays by repairing broken clusters as they occur. This approach enables us to calculate mean decay rates of isolated Lennard-Jones clusters in what promises to be a physically realistic fashion.
Dielectric constant and proton order and disorder in ice Ih: Monte Carlo computer simulations
Authors: Rick Steven W. , Haymet A. D. J.
Published: May 2003

The dielectric properties of ice Ih are studied using a Monte Carlo algorithm for sampling over proton configurations. The algorithm makes possible the calculation of the dielectric constant and other properties of the proton-disordered crystal. Results are presented for three classical models of water, two commonly used nonpolarizable models (SPC/E and TIP4P) and a polarizable model (TIP4P-FQ).
Dynamics in supercooled liquids and in the isotropic phase of liquid crystals: A comparison
Authors: Cang Hu , Li Jie , Novikov V. N. , Fayer M. D.
Published: May 2003

A comparison is made of the dynamics observed over wide ranges of time and temperature between five supercooled liquids and four isotropic phase liquid crystals that have been previously studied separately. Optical-heterodyne-detected optical Kerr effect (OHD–OKE) measurements were employed to obtain the orientational relaxation dynamics over time scales from sub-ps to tens of ns. For the supercooled liquids, the temperatures range from above the melting point down to ∼Tc, the mode coupling theory critical temperature. For the liquid crystals, the temperatures range from well above the isotropic-to-nematic phase transition temperature TNI down to ∼TNI. For time scales longer than those dominated by intramolecular vibrational dynamics (≳1 ps), the fundamental details of the dynamics are identical. All nine liquids exhibit decays of the OHD–OKE signal that begin (>1 ps) with a temperature-independent power law t−z, where z is somewhat less than or equal to 1. The power law decay is followed in both the supercooled liquids and liquid crystals by a crossover region, modeled as a second power law. The longest time scale decay for all nine liquids is exponential. In supercooled liquids, the exponential decay is the α relaxation (complete structural relaxation). In liquid crystals, the exponential decay is the Landau–de Gennes decay (relaxation of pseudonematic domains). As Tc (supercooled liquids) and TNI (liquid crystals) are approached from above, the time range over which the “intermediate” power law can be observed increases, until near Tc and TNI, the power law can be observed from >1 ps to many ns. The data for all nine liquids are described accurately by the same functional form and exhibit a scaling relation in common. The nature of the dynamics in the liquid crystals is understood in terms of pseudonematic domains that have a correlation length ξ, which increases as TNI is approached. It is conjectured that the similarities between the liquid crystal data and supercooled liquid data are produced by the same underlying physical features: that is, like liquid crystals, supercooled liquid dynamics is a result of structural domains even at relatively high temperature.