bluuebunny/crossref_metadata_2025 huggingface dataset:
X-ray reciprocal-space mapping of strain relaxation and tilting in linearly graded InAlAs buffers
Authors: Olsen J. A. , Hu E. L. , Lee S. R. , Fritz I. J. , Howard A. J. , Hammons B. E. , Tsao J. Y.
Published: July 2002

The extent of relaxation and orientation of linearly graded InxAl1-xAs (x=0.05–0.25) buffers grown on GaAs were examined using a novel x-ray diffraction reciprocal-space mapping technique (kmap). Samples were grown at temperatures ranging from 370 to 550 °C. The fractional relaxation of the buffers grown between 470 and 550 °C was essentially identical (77%) and symmetric in orthogonal 〈110〉 directions. These buffers are believed to be in equilibrium indicating that the incomplete relaxation is not a kinetic effect. The extent of relaxation was less than that expected for equilibrium relaxation in the absence of dislocation–dislocation interactions indicating that such interactions must be considered to accurately predict the extent of relaxation. The saturation of the relaxation as a function of temperature indicates that at the grading rate used (8% In/μm or 0.69% strain/μm), we are not working in a growth regime where the relaxation is nucleation limited. In addition, all the buffers are slightly tilted with respect to the GaAs substrate about [11̄0] toward the [110] direction suggesting either a bias in the dislocation types in the boule-grown GaAs, or a bias in the way in which α and β dislocations interact with unintentional substrate miscuts.
Studies of electrically active defects in relaxed GeSi films using a near-field scanning optical microscope
Authors: Hsu J. W. P. , Fitzgerald E. A. , Xie Y. H. , Silverman P. J.
Published: July 2002

We study the electrical activity of threading dislocation defects in relaxed GeSi films with a novel, high-resolution optical technique. A near-field scanning optical microscope is used to measure spatially resolved photoresponse while simultaneously imaging the surface topography. We have convincingly established that shallow topographic depressions in these films are electrically active threading dislocations. The apparent sizes of the dislocations in the photovoltage images are in agreement with estimates based on the junction geometry and the near-field optical excitation spot size. We can clearly observe photoresponse changes at ≤100 nm lateral scale, a tenfold improvement from far-field optical techniques. This higher resolution is due to reduction of the excitation volume and of the carrier lifetime near defects.
Equilibrium fluid configurations in low gravity
Authors: Sasges M. R. , Ward C. A. , Azuma H. , Yoshihara S.
Published: July 2002

An examination of the axisymmetric equilibrium configurations of fluid systems in cylindrical containers in an arbitrary gravity field has been undertaken. The derived theory allows the effects of gravity on an interface shape to be quantified. When these effects may be neglected, the resulting predictions for equilibrium are contrary to those of previous theories. The theoretical approach adopted herein leads to the prediction that the equilibrium configuration is dependent on the contact angle and on the amount of fluid in the container. These predictions have been examined through a series of experiments conducted in a drop shaft, and the results support the new theoretical approach.
Voronoi network model of ZnO varistors with different types of grain boundaries
Authors: Bartkowiak M. , Mahan G. D. , Modine F. A. , Alim M. A. , Lauf R. , McMillan A.
Published: July 2002

Electrical transport in zinc oxide varistors is simulated using two-dimensional Voronoi networks. The networks are assumed to contain randomly distributed grain boundaries of three electrical types: (1) high nonlinearity (i.e., ‘‘good’’) junctions; (2) poor nonlinearity (i.e., ‘‘bad’’) junctions; and (3) linear with low-resistivity (i.e., ohmic) junctions. These type classifications are those found in experimental measurements. By varying the type concentrations, the simulated current density versus electric field (J–E) characteristics can be made to conform to the different experimentally observed characteristics of ZnO varistors. These characteristics include the sharpness of switching at the transition between ohmic and nonlinear J–E response (i.e., knee region), as well as the degree of nonlinearity. It is shown that the reduction of the nonlinearity coefficient of bulk varistors, relative to that of isolated grain boundaries, can be explained only by the presence of ‘‘bad’’ varistor junctions.
Influence of a surface film on the particles on the electrorheological response
Authors: Wu C. W. , Conrad H.
Published: July 2002

A conduction model is developed for the dc electrorheological (ER) response of highly conducting particles (e.g., metal particles) suspended in a weakly conducting oil. The numerical analyses show that a surface film with some conductivity is desired, but not a completely insulating film as previously proposed. Increasing the film conductivity leads to an increase in the ER yield stress. However, too high a conductivity will give an unacceptable level of current density. The film should also have an intermediate thickness. A small thickness increases the possibility of electrical breakdown in the film; too large a thickness decreases the ER effect. Good agreement exists between the yield stress and the current density predicted by our model and those measured.
Refractive index as a function of photon energy for AlGaAs between 1.2 and 1.8 eV
Authors: Kokubo Y. , Ohta I.
Published: July 2002

An expression of refractive indices for AlGaAs is presented which agrees reasonably with the experimental results by H. C. Casey Jr., D. D. Shell, and M. B. Panish [Appl. Phys. Lett. 24, 63 (1974)]. This expression is approximately derived by using the method of least squares and is applicable to a determination of the refractive indices of AlxGa1−xAs with x in the range 0<x<0.38 and for photon energy range between 1.2 and 1.8 eV.
Mn
x
Pt
1−x
:
A new exchange bias material for Permalloy
Authors: Farrow R. F. C. , Marks R. F. , Gider S. , Marley A. C. , Parkin S. S. P. , Mauri D.
Published: July 2002

We report exchange biasing of Permalloy in as-grown MnxPt1−x/Permalloy structures grown by coevaporation in ultra-high vacuum. Polycrystalline and epitaxial structures with x between 0.35 and 0.75 were studied. X-ray diffraction confirms the presence of chemical ordering in the MnxPt1−x films (grown at 200 °C) in as-grown polycrystalline and epitaxial structures. An interfacial coupling energy of 0.032 erg/cm2 is found for structures where the MnxPt1−x (x=0.56) is grown at 200 °C onto epitaxial Permalloy (001). Polycrystalline structures have a smaller (0.027 erg/cm2) interfacial coupling energy. No exchange bias is observed for films with x≲0.4. The blocking temperature, for a sample with x=0.56, is >500 K.
Characterization of ferroelectric lead zirconate titanate films by scanning force microscopy
Authors: Zavala Genaro , Fendler Janos H. , Trolier-McKinstry Susan
Published: July 2002

Scanning force microscopy (SFM) has been used for the determination of friction, phase transformation, piezoelectric behavior (in the contact mode), polarization state, and dielectric constant (in the noncontact mode) of nanometer regions of lead zirconate titanate (PZT) films. The use of the SFM tip in the contact mode, to polarize different nanoregions of the PZT film and to apply an oscillating field thereon, led to effective piezoelectric coefficients and piezoelectric loops. The measured effective piezoelectric coefficient was shown to depend appreciably on both the tip contact force and the quality of the tip-to-film electrical contact. In the noncontact mode, application of an ac signal (with a frequency ω) across the tip—PZT film—electrode system produced an oscillation of the tip at frequencies ω (fundamental or first harmonic) and 2ω (second harmonic). The signals at ω and 2ω were related to the state of polarization and the dielectric constant of the PZT film, respectively. Analysis of the combined contact, noncontact and friction force microscopic data provided insight into the structure and into the dielectric, ferroelectric, and piezoelectric properties of distinct nanoregions of the PZT film.
Optical properties of silicon pigmented alumina films
Authors: Tesfamichael T. , Vargas W. E. , Wäckelgård E. , Niklasson G. A.
Published: July 2002

Plates of Al–Si alloy were anodized in a sulfuric acid solution. This treatment provides a Si–Al2O3 coating growing at a rate of 0.14 μm/min. The Si particles had sizes between 1 and 10 μm, as seen by scanning electron microscopy. Optical measurements showed a continuous decrease of reflectance with increasing film thickness. The reflectance of the Si–Al2O3 coated aluminum could be understood from a four flux radiative transfer theory. In order to explain our measurements, it was found necessary to include a free-carrier term in the dielectric permittivity of Si. The free carriers are probably due to doping with Al. Hence, the relaxation time of the free carriers is determined by scattering from the charged Al impurities.
Mechanisms of writing and decay of holographic gratings in semiconducting CdF2:Ga
Authors: Ryskin A. I. , Shcheulin A. S. , Miloglyadov E. V. , Linke R. A. , Redmond I. , Buchinskaya I. I. , Fedorov P. P. , Sobolev B. P.
Published: July 2002

We consider the mechanisms responsible for the photoinduced change in the optical properties of semiconducting CdF2 crystals with metastable Ga impurities forming DX centers. Unlike the case of compound semiconductors with DX centers (GaAlAs:Si, GaAlAs:Te, CdZnTe:Cl), this change is caused not by free electrons but by a redistribution of electrons between deep and shallow localized states. The resulting modification of the refractive index of the crystal allows writing of persistent holographic gratings at temperatures up to 200 K, high for this class of holographic materials. Holographic characteristics of CdF2:Ga crystals such as refractive index change, sensitivity, and grating decay are described.
Mechanical properties of Ti-containing and W-containing diamond-like carbon coatings
Authors: Meng W. J. , Gillispie B. A.
Published: July 2002

Ti-containing and W-containing diamond-like hydrocarbon coatings have been synthesized by glow discharge reactive magnetron sputter deposition in an Ar/CH4 mixture. it is shown that these metal-containing hydrocarbon coatings consist of nanocrystalline TiC and WC embedded in an amorphous hydrocarbon matrix and are thin-film nanocomposite materials. The elastic modulus and hardness of these nanocomposites exhibit systematic dependence on their composition, while the ratio of hardness to modulus remains approximately a constant. It is also shown that the elastic modulus and hardness of these nanocomposites are within macromechanical bounds for two-phase composite materials.
Small angle electron diffraction and Foucault mode Lorentz microscopy of superconducting vortex lattice
Authors: Yoshida T. , Endo J. , Kasai H. , Harada K. , Osakabe N. , Tonomura T. A. , Pozzi G.
Published: July 2002

Small angle electron diffraction patterns from vortex lattices in Nb thin films were observed using a field emission transmission electron microscope. It was possible to obtain diffraction patterns in magnetic fields of up to 1000 G, from selected areas of (1–10) μm in diameter, and in (1–30) s. The degree of order of the vortex lattice was determined from the fine structure of the diffraction pattern. We also demonstrated Foucault mode Lorentz microscopy by using a diffraction technique. Vortices appeared by diffraction contrast in a focused micrograph, and therefore both vortices and material defects were simultaneously observed.
Separate contributions to the pyroelectricity in poly(vinylidene fluoride) from the amorphous and crystalline phases, as well as from their interface
Authors: Rollik D. , Bauer S. , Gerhard-Multhaupt R.
Published: July 2002

New selective poling schemes for polar semicrystalline polymers are described, and appropriate poling fields and temperatures are specified. In the following investigation of the pyroelectric response and the thermally stimulated depolarization (TSD), poly(vinylidene fluoride) is used as an example for a semicrystalline ferroelectric polymer. It is shown that the amorphous and the crystalline phase, as well as the interface between them contribute to the pyroelectric effect. From dynamic pyroelectrical measurements and TSD experiments, the permanent polarization of the dipoles in the crystalline phase, the frozen-in polarization of the dipoles in the amorphous phase, and the effect of the excess charges of the Maxwell– Wagner interface polarization can be identified. With the proposed selective poling schemes (poling temperatures below and/or above the glass transition temperature, as well as poling fields lower and/or higher than the coercive field), it is possible to clearly separate the respective contributions to the pyroelectric effect.
A new molecular gas lubrication theory suitable for head-disk interface modeling
Authors: Kang Soo-Choon , Crone Robert M. , Jhon Myung S.
Published: July 2002

A rigorous generalized Reynolds equation, which governs gas lubrication dynamics at the head disk interface (HDI), has been derived from the Bhatnagar–Gross–Krook kinetic equation. Rarefaction effects for arbitrary surface accommodation coefficients are incorporated by calculating both the Couette and Poiseuille flow rates. New flow rate databases have been generated, which correct and extend the erroneous existing databases currently used for HDI simulations. Furthermore, a new set of system equations for the pitch and roll angles has been constructed by including shear stress changes due to rarefaction phenomena, which becomes critically important in estimating slider dynamics at ultralow flying heights.
Study of interdiffusion in thin Fe film deposited on Si(111) by x-ray reflectivity and secondary ion mass spectrometry
Authors: Banerjee S. , Raghavan G. , Sanyal M. K.
Published: July 2002

We report on the effect of annealing on a thin Fe film deposited on a Si(111) substrate, using x-ray reflectivity and secondary ion mass spectrometry (SIMS) techniques. Using Fourier transform of the x-ray reflectivity data, we have estimated the layer thickness of the film. From the estimated thickness and critical value of the scattering vector qc obtained from the reflectivity data, an initial guess model of the electron density profile of the film is made. Using an iterative inversion technique, based on the Born approximation, with the obtained initial guess model, we have extracted the actual electron density profile of the film as a function of depth from the specular x-ray reflectivity data. On annealing, we observe interdiffusion of Fe and Si resulting in an increase in the thickness of the film. We have also carried out a SIMS measurement on the annealed sample to support the result of the annealing effect observed from the analysis of x-ray reflectivity data. The SIMS analysis indicates that the top of the film is rich in Si which has diffused from the substrate to the surface of the Fe film on annealing.
“Elastic properties” of magnetic fluids
Authors: Nikolaev V. I. , Shipilin A. M. , Shkolnicov E. N. , Zaharova I. N.
Published: July 2002

The results of Mössbauer investigations of the viscous magnetic liquids at temperatures of 100–300 K are discussed. The investigated ferrofluids were the colloidal dispersions of magnetite particles with average diameters of 7.5 and 10 nm in silicone carrier fluid. Supposing that the intensity of the Mössbauer line is determined by the factor f=f′⋅f″ (f′ is Mössbauer factor for magnetite powder; f″ is a similar factor depending on oscillations of particles in a liquid), we estimated the values of the factor f″(T) at various temperatures. To describe the dependence f′(T), the Debye approximation was used. By means of the data on the dependence f″(T) the estimations of frequencies of particles oscillations Ω and “elasticity modulus” E of the investigated substances were obtained.
Characterization of trapped particles in rf plasmas
Authors: Schabel M. , Peterson T. , Sinclair J. , Lynch D.
Published: July 2002

Charged dust particles electrostatically trapped in rf plasmas may exhibit kinetic energies greatly exceeding that of the surrounding gas. The high particle energies are a result of charged particle interactions with local potential fields perturbed by the presence of neighboring particles. Laser Doppler velocimetry is used to experimentally investigate the two-dimensional dynamics of monodisperse 10.2 μm polystyrene latex spherical particles trapped in an argon plasma. Spatially resolved particle velocities and local particle concentrations are reported for variations in rf power and pressure. The experimental results are compared with laser light scatter, often employed for studying particle dynamics. Various theories are reviewed in an attempt to interpret the experimental results.
Small cantilevers for force spectroscopy of single molecules
Authors: Viani Mario B. , Schäffer Tilman E. , Chand Ami , Rief Matthias , Gaub Hermann E. , Hansma Paul K.
Published: July 2002

We have used a simple process to fabricate small rectangular cantilevers out of silicon nitride. They have lengths of 9–50 μm, widths of 3–5 μm, and thicknesses of 86 and 102 nm. We have added metallic reflector pads to some of the cantilever ends to maximize reflectivity while minimizing sensitivity to temperature changes. We have characterized small cantilevers through their thermal spectra and show that they can measure smaller forces than larger cantilevers with the same spring constant because they have lower coefficients of viscous damping. Finally, we show that small cantilevers can be used for experiments requiring large measurement bandwidths, and have used them to unfold single titin molecules over an order of magnitude faster than previously reported with conventional cantilevers.
Film texture evolution in plasma treated TiN thin films
Authors: Ikeda S. , Palleau J. , Torres J. , Chenevier B. , Bourhila N. , Madar R.
Published: July 2002

In semiconductor technology, TiN thin film elements can be used as diffusion barrier between a metallic layer and a silicon oxide dielectric. Plasma application during the growth of TiN thin films modifies the microstructure of these films and consequently alters their physical properties. But details of the effect of plasma application on the evolution of the film microstructure and correlations between this evolution and the physical properties are still unclear. To clarify the correlations, the microstructure of a series of TiN thin films, deposited using an organometallic chemical vapor deposition technique combined with plasma treatments has been analyzed by transmission electron microscopy (TEM). The films were obtained by repeated fabrication sequences consisting of limited film growth followed by the application of a N2/H2 gaseous plasma with various powers and duration times and are actually stackings of plasma-treated elementary layers. TEM analysis shows that these films are made of nanocrystallites and that whereas crystallites are randomly oriented when no plasma is applied, short-time plasma treatments induce a tendency to 〈200〉 texture and longer treatments progressively rotate the direction of texture to 〈220〉.
The Burstein-Moss effect in Cu2GeSe3:Co2+ single crystals
Authors: Lee Jeoung Ju , Yang Chang Soo , Park Young Sin , Kim Kun Ho , Kim Wha Tek
Published: July 2002

Cu 2 GeSe 3 and Cu2GeSe3:Co2+ single crystals are grown using the modified Bridgman method. The grown single crystals have orthorhombic structures. The energy band gaps for Cu2GeSe3 and Cu2GeSe3:Co2+ single crystals measured at 298 K were about 0.789 and 0.798 eV, respectively, and these gaps are due to direct transitions. The Burstein-Moss effect due to overlapping of the energy level of Co2+(Td) and the conduction band of the Cu2GeSe3:Co2+ is observed.
Photoinduced surface deformations on azobenzene polymer films
Authors: Bian Shaoping , Williams John M. , Kim Dong Yu , Li Lian , Balasubramanian Srinivasan , Kumar Jayant , Tripathy Sukant
Published: July 2002

Laser-induced surface deformation and surface relief grating formation at different intensity levels on azobenzene-functionalized polymer films were investigated. The surface deformation profile induced by a Gaussian laser beam at low intensities reveals strong polarization dependence and establishes that an optical-field component in the direction of the optical-field gradient is essential for this deformation process. A theoretical analysis based on the optical-field gradient force model is presented to account for the deformation behavior at low intensities. In the higher-intensity regime, however, a different mechanism is found to dominate the surface deformation process. Simultaneous formation of two different types of topographic gratings is observed in a two-beam interference experiment. The phase relationship between these surface relief gratings and the interference intensity pattern was deduced based on the results from the single-beam experiments. The erasing behaviors of the two distinct surface relief gratings were experimentally investigated to highlight the two mechanisms involved in these grating recordings.
Analysis of the stretched exponential photoluminescence decay from nanometer-sized silicon crystals in SiO2
Authors: Linnros Jan , Lalic Nenad , Galeckas Augustinas , Grivickas Vytautas
Published: July 2002

Time resolved photoluminescence (PL) decays have been measured for Si nanocrystals embedded in silicon dioxide. The nanocrystals were formed by implanting 40 keV Si ions into a 1000 Å thick film of thermally grown SiO2, followed by thermal annealing at 1000–1200 °C. The observed luminescence, peaking at 700–850 nm, is compared to similar measurements performed on porous Si emitting in the same wavelength range. The results show that the PL from the nanocrystals exhibits a stretched exponential decay with characteristic decay time τ in the range 10–150 μs and dispersion factor β in the range 0.7–0.8. Both parameters are, however, higher for the nanocrystals compared to those of porous Si indicating superior passivation of the nanocrystals in the SiO2 matrix. Evidence is also presented for a single exponential behavior at the decay end suggesting a remaining fraction of excitons in isolated nanocrystals. We attribute the highly nonlinear dose dependence of the PL yield to a nucleation process for the nanocrystals and a more curved decay line shape for higher ion doses to a higher crystal density, promoting excitonic migration to nearby nanocrystals. These observations provide strong evidence that the origin of the stretched exponential line shape of the PL decay results from migration and trapping of excitons in a system of randomly distributed and interconnected nanocrystals with a dispersion in size.
Doping of polyaniline by corona discharge
Authors: Job Aldo E. , Giacometti José A. , Herrmann Paulo S. P. , Mattoso Luiz H. C.
Published: July 2002

In the present article it is shown that a corona discharge can be employed to dope thin films of polyaniline (PANI) coated on poly(ethylene terephthalate) films, allowing the electrical conductivity to be tuned within the range 10−10 to 0.3 S cm−1. A study of the effect of different corona conditions, namely corona treatment for positive and negative polarities, air humidity, treatment time, corona current, and the geometry of the corona triode, on the electrical conductivity of the polyaniline is presented. The results indicate that the corona discharge leads to protonic doping of polyaniline similar to that which occurs in conventional protonic acid solution doping. Atomic force microscopic analysis shows that, as the PANI is exposed to the corona discharge, its globular morphology is disrupted leading to the appearance of droplet-like features and a significant decrease in the average height and surface roughness. Doping by corona discharge presents several advantages over the conventional solution method namely that it is a dry process which does not require use of chemicals reagents, and which is both rapid and avoids dopant migration. The latter can be important for applications of PANI in microelectronic devices.
Vibrational dephasing by the exchange mechanism: Some new results
Authors: Nitzan A. , Persson B. N. J.
Published: July 2002

The exchange model of vibrational phase relaxation is studied. The vibrational line profile is obtained analytically in various limiting cases such as high temperature and large friction. Expressions for the first and second moments of the line profile are also presented.
A comparison of perturbative schemes and integral equation theories with computer simulations for fluids at high pressures
Authors: Talbot J. , Lebowitz J. L. , Waisman E. M. , Levesque D. , Weis J.-J.
Published: July 2002

We test some refined perturbation and integral equations theories for predicting the equilibrium properties of spherical fluids, with nonstandard interactions at high densities and temperatures. The perturbation theories are fast and convenient to use and give good results for the thermodynamic properties, but not for the structure. The integral equations require more computer time, but yield thermodynamics and structure that are in very good agreement with simulations. In fact there appears to be no need for computer simulations of classical systems of particles interacting with spherical potentials in the fluid regime—at least away from transitions.
The closed-shell coupled cluster single and double excitation (CCSD) model for the description of electron correlation. A comparison with configuration interaction (CISD) results
Authors: Scuseria Gustavo E. , Scheiner Andrew C. , Lee Timothy J. , Rice Julia E. , Schaefer Henry F.
Published: July 2002

A single and double excitation coupled cluster (CCSD) method restricted to closed-shell single configuration reference functions is described in explicit detail. Some significant simplifications resulting from the restriction to closed-shell systems are exploited to achieve maximum computational efficiency. Comparisons for energetic results and computational requirements are made with the single and double excitation configuration interaction (CISD) method. The specific molecules considered include N2, H2O, H3O+, H5O+2, HSOH, and s-tetrazine (C2N4H2).
Path integral Monte Carlo study of the hydrated electron
Authors: Wallqvist A. , Thirumalai D. , Berne B. J.
Published: July 2002

The structure of an excess electron in water at room temperature is investigated using the Feynman path integral technique. The interaction potential between the electron and water is modeled by an effective potential, made up of three terms: a static potential, a repulsive potential, and a polarization potential. The polarization part is treated in two different ways: approximated as pairwise additive, and exactly with the many body polarization effects treated self-consistently. It is shown that the excess electron forms a cavity with the radius of the electron being 2.24 Å for pairwise additive polarization and 2.11 Å for the self-consistent treatment of the polarization. There is no sharp geometrical coordination number of water molecules around the electron. The water molecules within a distance of about 3.5 Å from the center of the electronic change distribution point their OH bonds towards the electron, and form only three hydrogen bonds. It is also found that the pair correlation function of the solvent molecules close to the electron are considerably different from the corresponding quantities calculated in the bulk. The electron, therefore, is shown to modify the local density to a large extent. There are important structural differences between the many body polarization model and the two body polarization model. It is concluded that for a quantitative description of the structure of the hydrated electron, the self-consistent treatment of the long range (many body) polarization effects are important.
A multiconfigurational self-consistent reaction-field method
Authors: Mikkelsen Kurt V. , Ågren Hans , Jensen Hans Jo/rgen Aa. , Helgaker Trygve
Published: July 2002

We present theory and implementation for a new approach for studying solvent effects: the multiconfigurational self-consistent reaction-field (MCSCRF) method. The atom, molecule, or supermolecule is assumed to be surrounded by a linear, homogeneous, continuous medium described by its macroscopic dielectric constant. The electronic structure of the compound is described by a multiconfigurational self-consistent field (MCSCF) wave function. The wave function is fully optimized with respect to all variational parameters in the presence of the surrounding polarizable dielectric medium. We develop a second-order convergent optimization procedure for the solvent states. The solvent integrals are evaluated by an efficient and general algorithm. The flexible description of the electronic structure allows us to accurately describe ground, excited, or ionized states of the solute. Deficiencies in the calculation can therefore be assigned to the cavity model rather than the description of the solute.
Concentration fluctuations in polymer blend thermodynamics
Authors: Olvera de la Cruz M. , Edwards S. F. , Sanchez I. C.
Published: July 2002

Monomer concentration fluctuations in high molecular weight polymer blends are investigated. The fluctuations reduce the value of the χ interaction term in the Flory–Huggins free energy by a factor of (1-6/π2) and add a concentration and molecular weight contribution which decreases phase stability for off-critical compositions. Except at the critical point, the spinodal values are smaller than those predicted from a Flory–Huggins-type free energy. For an incompressible blend with components of equal size N, the corrections to the spinodal were found to vary as ‖1−2φ0‖4/3N−1/3, where φ0 is the mean concentration.
A simulation based model of NMR T1 relaxation in lipid bilayer vesicles
Authors: Pastor Richard W. , Venable Richard M. , Karplus Martin , Szabo Attila
Published: July 2002

The results of the Brownian dynamics simulation of a hydrocarbon chain in a membrane bilayer described in the preceding paper are used to analyze the 13C NMR T1 relaxation in lipid bilayer vesicles. The analysis shows that the frequency dependence of the relaxation does not arise from gauche–trans isomerization or from axial rotation of the entire lipid molecule. However, a model in which fast axial rotation (D∥≊2×1010 s−1) and slow noncollective diffusive director fluctuations (D⊥≊1–2×108 s−1) are superimposed on the internal motions quantitatively accounts for both the magnitude and frequency dependence of the T1 data. An effective viscosity for the interior of the bilayer in the range of 1 cp, and a director order parameter of 0.5–0.7 are required to fit the NMR data. Collective effects do not appear necessary for explaining the NMR T1 data in vesicles, although they may be important for multilamellar dispersions.
Direct correlations of the capillary wave model and construction of the free energy density functional for the liquid–vapor interface system
Authors: Percus J. K. , Zhang M. Q.
Published: July 2002

We use the capillary wave model of a liquid–vapor interface as a working example to show how to construct the free energy density functional from studies of the direct correlation functions of a reference system, and vise versa.
Isotopic shift in the melting curve of helium: A path integral Monte Carlo study
Authors: Barrat Jean-Louis , Loubeyre Paul , Klein Michael L.
Published: July 2002

The path integral Monte Carlo method is used to estimate the difference in the melting pressures of 4He and 3He. Thermodynamic and structural properties of both isotopes are also computed using an effective pair potential for the interaction between helium atoms. At low temperature, calculations predict the lighter isotope has the higher melting pressure in almost quantitative agreement with available experimental data. The melting curves of 3He and 4He are found to intersect at T≂100 K.
Vibrational spectroscopy of the hydrated hydronium cluster ions H3O+⋅(H2O)n (n=1, 2, 3)
Authors: Yeh L. I. , Okumura M. , Myers J. D. , Price J. M. , Lee Y. T.
Published: July 2002

The gas phase infrared spectra of the hydrated hydronium cluster ions H3O+⋅(H2O)n(n=1, 2, 3) have been observed from 3550 to 3800 cm−1. The new spectroscopic method developed for this study is a two color laser scheme consisting of a tunable cw infrared laser with 0.5 cm−1 resolution used to excite the O–H stretching vibrations and a cw CO2 laser that dissociates the vibrationally excited cluster ion through a multiphoton process. The apparatus is a tandem mass spectrometer with a radio frequency ion trap that utilizes the following scheme: the cluster ion to be studied is first mass selected; spectroscopic interrogation then occurs in the radio frequency ion trap; finally, a fragment ion is selected and detected using ion counting techniques. The vibrational spectra obtained in this manner are compared with that taken previously using a weakly bound H2 ‘‘messenger.’’ A spectrum of H7 O+3 taken using a neon messenger is also presented. Ab initio structure and frequency predictions by Remington and Schaefer are compared with the experimental results.
Lattice dynamics and vibrational spectra of pristine and doped polypyrrole: Effective conjugation coordinate
Authors: Tian B. , Zerbi G.
Published: July 2002

A detailed vibrational analysis of the infrared and Raman spectra of doped and pristine polypyrrole is presented. The theory of the effective conjugation coordinate is applied and fully justifies the observed spectra. The information on the structure and properties of this material derived from the proposed analysis are presented and discussed.
Couplings between the cooperatively rearranging regions of the Adam–Gibbs theory of relaxations in glass-forming liquids
Authors: Ngai K. L. , Rendell R. W. , Plazek D. J.
Published: July 2002

The Adam–Gibbs theory of relaxation for glass-forming liquids is based on an ensemble of independent and equivalent cooperatively rearranging regions (CRR’s). In this work we extend the Adam–Gibbs theory by incorporating the mutual interactions between the CRR’s. This is done by the method of the coupling scheme in which the transition rate for each independent CRR is slowed down by a time-dependent factor. The new combined theory predicts a stretched exponential relaxation function and modifies the Adam–Gibbs parameters for the temperature dependence of the relaxation time making them functions of the stretched exponent β. The model is applied to analyze specific heat and dielectric data of supercooled liquids. It is able to explain a previously proposed correlation of the increasing fragility of glass forming liquids with the size of 1−β at Tg and the rapidity of the temperature variation of β. The model also explains, in hydrogen bonded liquids, the correlation of the increasing fragility with the size of the basic molecular unit and/or the number of hydroxyl groups found by comparison of dielectric relaxations of glycerol, n-propanol, butanediol-1,3, and sorbitol.
Dynamic simulation of an electrorheological fluid
Authors: Bonnecaze R. T. , Brady J. F.
Published: July 2002

A molecular-dynamics-like method is presented for the simulation of a suspension of dielectric particles in a nonconductive solvent forming an electrorheological fluid. The method accurately accounts for both hydrodynamic and electrostatic interparticle interactions from dilute volume fractions to closest packing for simultaneous shear and electric fields. The hydrodynamic interactions and rheology are determined with the Stokesian dynamics methodology, while the electrostatic interactions, in particular, the conservative electrostatic interparticle forces, are determined from the electrostatic energy of the suspension. The energy of the suspension is computed from the induced particle dipoles by a method previously developed [R. T. Bonnecaze and J. F. Brady, Proc. R. Soc. London, Ser. A 430, 285 (1990)]. Using the simulation, the dynamics can be directly correlated to the observed macroscopic rheology of the suspension for a range of the so-called Mason number, Ma, the ratio of viscous to electrostatic forces. The simulation is specifically applied to a monolayer of spherical particles of areal fraction 0.4 with a particle-to-fluid dielectric constant ratio of 4 for Ma=10−4 to ∞. The effective viscosity of the suspension increases as Ma−1 or with the square of the electric field for small Ma and has a plateau value at large Ma, as is observed experimentally. This rheological behavior can be interpreted as Bingham plastic-like with a dynamic yield stress. The first normal stress difference is negative, and its magnitude increases as Ma−1 at small Ma with a large Ma plateau value of zero. In addition to the time averages of the rheology, the time traces of the viscosities are presented along with selected ‘‘snapshots’’ of the suspension microstructure. In particular, at small Ma, the suspension dynamics exhibit two distinct motions: a slow elastic-body-like deformation where electrostatic energy is stored, followed by a rapid microstructural rearrangement where energy is viscously dissipated. It is suggested that the observed dynamic yield stress is associated with these dynamics.
Molecular dynamics simulation of the transport of small molecules across a polymer membrane
Authors: Sok R. M. , Berendsen H. J. C. , van Gunsteren W. F.
Published: July 2002

The transport of small molecules through a polymer membrane is modeled using the computer simulation technique of molecular dynamics (MD). The transport coefficient is derived from a combination of the excess free energy and the diffusion constant. Both properties are derived from MD simulations, applied to helium and methane in polydimethylsiloxane (PDMS). The diffusional process appears to have the character of a jump diffusion for methane and less so for helium. Jumps are allowed by fluctuations of the size and shape of holes. Experimental diffusion constants are well reproduced. The excess free energies, determined by a particle insertion method, are lower by 5–7 kJ/mol than experimental values. It is shown that, as a result of a higher solubility, methane has a higher permeability constant than helium, despite its lower diffusion constant.
Calculation of the cumulative reaction probability via a discrete variable representation with absorbing boundary conditions
Authors: Seideman Tamar , Miller William H.
Published: July 2002

A new method is suggested for the calculation of the microcanonical cumulative reaction probability via flux autocorrelation relations. The Hamiltonian and the flux operators are computed in a discrete variable representation (DVR) and a well-behaved representation for the Green’s operator, G(E+), is obtained by imposing absorbing boundary conditions (ABC). Applications to a one-dimensional-model problem and to the collinear H+H2 reaction show that the DVR-ABC scheme provides a very efficient method for the direct calculation of the microcanonical probability, circumventing the need to compute the state-to-state dynamics. Our results indicate that the cumulative reaction probability can be calculated to a high accuracy using a rather small number of DVR points, confined to the vicinity of the transition state. Only limited information regarding the potential-energy surface is therefore required, suggesting that this method would be applicable also to higher dimensionality problems, for which the complete potential surface is often unknown.
A study of phenylacetylene and styrene, and their argon complexes PA–Ar and ST–Ar with laser threshold photoelectron spectroscopy
Authors: Dyke J. M. , Ozeki H. , Takahashi M. , Cockett M. C. R. , Kimura K.
Published: July 2002

In this work, the molecules styrene (ST) and phenylacetylene (PA), as well as their argon complexes ST–Ar and PA–Ar, have been investigated with (1+1′) resonance enhanced multiphoton ionization (REMPI) threshold photoelectron spectroscopy (TES). The first adiabatic ionization energies of ST, PA, ST–Ar, and PA–Ar have been measured as 68 267±5, 71 175±5, 68 151±5, and 71 027±5 cm−1, respectively. For both ST–Ar and PA–Ar, the first photoelectron band shows structure in the lowest frequency van der Waals (vdW) bending mode in the ground ionic state, with νvdW being measured as 15 cm−1 in each case. For each molecule excitation to a particular vibrational level of the S1 state followed by ionization, allows structure in that mode to be observed in the threshold photoelectron spectrum. This has been achieved for three modes in both styrene and phenylacetylene. The experimental ionic vibrational frequencies thus obtained, have been compared with those known for the S0 and S1 states.
Electron-impact dissociation of oxygen
Authors: Cosby P. C.
Published: July 2002

The electron-impact dissociation of O2 to form two oxygen atoms is observed in a crossed beam experiment at electron energies between 13.5 and 198.5 eV. Detection of the correlated dissociation fragments with a time and position sensitive detector permits detection of both ground and excited state fragments, but excludes interference from dissociative ionization products. The observed translational energy releases in the O2 dissociation are consistent with production of O(1D)+O(3P) fragments following electron impact excitation to the B 3Σu−, B′ 3Σu−, and 2 3Πu states, and production of O(3P)+O(3P) fragments from excitation to the (unresolved) c 1Σu−, A′ 3Δu, and A 3Σu+ states. Absolute cross sections for the electron impact dissociation of O2 are measured.
Time-resolved photoluminescence from poly[2-methoxy, 5-(2′-ethyl-hexyloxy)-p-phenylene-vinylene]: Solutions, gels, films, and blends
Authors: Smilowitz L. , Hays A. , Heeger A. J. , Wang G. , Bowers J. E.
Published: July 2002

We present the results of time-resolved luminescence studies of poly[2-methoxy, 5-(2′-ethyl-hexyloxy)-p-phenylene-vinylene] (MEH-PPV), as a pure film, in solution, in a gel formed by a network of ultrahigh molecular weight polyethylene (UHMW-PE), and in a blend with UHMW-PE. The luminescence has a characteristic lifetime of 200–300 ps at room temperature, increasing to 500–700 ps when the materials are cooled to 80 K; the decay time is approximately the same for all the physical forms of the material (solution, film, gel, blend). The relatively short lifetimes, compared to intrinsic values calculated from absorption and emission spectra, and the observed temperature dependence indicate that the luminescence decay is quenched by nonradiative processes. The time decay of the photoluminescence deviates from a single exponential for most forms of the MEH-PPV. Best fits to stretched exponential and double exponential expressions are presented. Steady state photoluminescence spectra and integrated luminescence vs pump intensity data are presented in order to establish which of the possible mechanisms are most important.
A variational approach to charged polymer chains: Polymer mediated interactions
Authors: Podgornik R.
Published: July 2002

A variational approach, based on a quadratic trial Hamiltonian is used to determine the configurational as well as the mechanical properties of a system composed of two point charges plus a neutralizing charged polymer, considered as a model of polyelectrolyte–macroion interactions. The quadratic variational ansatz used in the context of charged polymers presents a variant of the Feynman–Kleinert approach properly generalized to include polymer self-interactions. Conditions for chain localization in the field of a point charge are derived and its consequences for polymer-mediated forces between two point charges are analyzed. It is established that in three dimensions the polymer can confer long range attraction to nominally equally charged point ions. At small separations, the polymer mediated force is elastic in origin, stemming from the soft electrostatic anchoring of the chain to both point charges, but with a force constant much stronger than expected from entropic elasticity alone. At large separations, the polymer mediated interactions are of long range and are due to polymer-induced charge reversal on one of the external point charges.
Endohedral and exohedral adsorption in C60: An analytical model
Authors: Breton J. , Gonzalez-Platas J. , Girardet C.
Published: July 2002

A simple description of the interaction potential between a small molecule and a fullerene 60 cage is discussed on the basis of the continuum approximation. General conditions are obtained for the validity of this continuum approximation and for the confinement mechanism of the molecule inside (or outside) the cage. Application to the insertion of alkali metal ions and rare gas atoms shows that these conditions can adequately interpret the equilibrium positions of inside (outside) adducts, the adsorption energy and the collision experiments between the ions and atoms and the C60 cage.
Small-angle neutron scattering by partially deuterated polymers and their blends
Authors: Balsara N. P. , Lohse D. J. , Graessley W. W. , Krishnamoorti R.
Published: July 2002

Partially deuterated polymers, made by saturating the double bonds of polydienes with deuterium, have been found to produce weak but significant coherent patterns in small-angle neutron scattering (SANS) experiments. The results suggest the presence of slight differences in deuteration levels among the chains. The scattering profiles for such materials and for blends containing them were calculated with the multicomponent random phase approximation (RPA). The coherent intensity for the deuterated component alone is predicted to be proportional to the variance of its scattering length distribution and to have a q dependence corresponding to the pure component structure factor. The SANS data for a variety of partially deuterated polyolefins are shown to be consistent with these predictions. The theory also predicts that the scattering from their blends with an hydrogenous component is enhanced by this scattering length inhomogeneity, that the enhancement is additive, and that the conventional two component RPA may be used to analyze the data after this contribution is subtracted. Experimentally, we find that uncorrected scattering from blends rich in such deuterated components lead to anomalous estimates of chain dimensions and thermodynamic interactions. The anomalies disappear when the proposed correction is applied.
Detailed balance method for chemical potential determination in Monte Carlo and molecular dynamics simulations
Authors: Fay Patrick J. , Ray John R. , Wolf Ralph J.
Published: July 2002

We present a new, nondestructive, method for determining chemical potentials in Monte Carlo and molecular dynamics simulations. The method estimates a value for the chemical potential such that one has a balance between fictitious successful creation and destruction trials in which the Monte Carlo method is used to determine success or failure of the creation/destruction attempts; we thus call the method a detailed balance method. The method allows one to obtain estimates of the chemical potential for a given species in any closed ensemble simulation; the closed ensemble is paired with a ‘‘natural’’ open ensemble for the purpose of obtaining creation and destruction probabilities. We present results for the Lennard-Jones system and also for an embedded atom model of liquid palladium, and compare to previous results in the literature for these two systems. We are able to obtain an accurate estimate of the chemical potential for the Lennard-Jones system at higher densities than reported in the literature.