bluuebunny/crossref_metadata_2025 huggingface dataset:
Ion irradiation-induced amorphization in the Al2O3–SiO2 system: A comparison with glass formation
Authors: Wang S. X. , Wang L. M. , Ewing R. C. , Doremus R. H.
Published: July 2002

The ion beam-induced crystalline-to-amorphous transition has been investigated for crystalline phases in the Al2O3–SiO2 system: Al2O3, SiO2 (quartz), Al2SiO5 (kyanite, andalusite, sillimanite), and 3Al2O3⋅2SiO2 (mullite). Xe+ 1.5 MeV was used to irradiate samples at temperatures from 15 to 1023 K in situ in a transmission electron microscope to determine the critical amorphization doses. The susceptibility to amorphization is (highest to lowest): quartz, sillimanite, kyanite, andalusite, mullite, and alumina. These data are compared to viscosities and activation energies for viscous flow of melts in this system. The doses required for amorphization by ion irradiation are related to the viscosities of the melts. The activation energies for irradiation-enhanced annealing are qualitatively correlated with the activation energies of viscous flow. These results suggest a parallel between ion beam irradiation-induced amorphization and glass formation. Glass-forming “ability’’ correlates with susceptibility to radiation-induced amorphization.
In-plane spin reorientation transition in ultrathin epitaxial Fe(001) films
Authors: Brockmann M. , Miethaner S. , Onderka R. , Köhler M. , Himmelhuber F. , Regensburger H. , Bensch F. , Schweinböck T. , Bayreuther G.
Published: July 2002

We studied the growth and magnetic properties of ultrathin Fe(001) films on Au(001) buffer layers grown by molecular-beam epitaxy on MgO(001) single crystals. Epitaxial growth could be achieved by use of a Cr seed layer. Epitaxial quality and surface structure were verified in situ by low-energy and reflection high-energy electron diffraction and scanning tunneling microscopy (STM). Magnetic anisotropy of Fe films in the range of 3–155 monolayers (ML) was determined by alternating gradient magnetometry, superconducting quantum interference device, and magneto-optic Kerr effect. The spontaneous magnetization was always in the film plane. Below a critical thickness of 7.3±0.7 ML we observe a rotation of the cubic easy axes by 45° from the 〈100〉 to the 〈110〉 directions within the film plane. This spin reorientation transition is equivalent to a sign reversal of an effective fourth order anisotropy constant K1eff. In addition, a uniaxial in-plane anisotropy is observed, which may be attributed to the step structure of the samples obtained from STM images.
Layer structure analysis of Er δ-doped InP by x-ray crystal truncation rod scattering
Authors: Takeda Yoshikazu , Fujita Keiji , Matsubara Naoteru , Yamada Naoki , Ichiki Satofumi , Tabuchi Masao , Fujiwara Yasufumi
Published: July 2002

The layer structure and crystal structure of an Er δ-doped layer in InP are analyzed in a one monolayer (ML) level by an x-ray crystal truncation rod (CTR) scattering measurement using synchrotron radiation. The Er δ-doped InP sample is prepared by organometallic vapor phase epitaxy using trimethylindium, tertiarybutylphosphine, and tris(methyl-cyclopentadienyl)erbium as source materials. The analysis is made by comparing the measured CTR spectra and theoretically generated ones assuming four possible crystal structures for the Er δ-doped layers. We reveal that the Er atoms in InP form the rocksalt structure ErP. In the analysis of the crystal structure the x-ray interference plays a vital role. Er distribution in the δ-doped layer is clearly resolved in 1 ML level. It is shown that the Er atoms are well confined in 5 ML(15 Å) thickness. The total amount of Er atoms is 0.171 ML, which is quite close to 0.2 ML obtained by Rutherford backscattering measurement on the same sample.
Energy loss spectroscopy of RuO2 thin films
Authors: Mondio G. , Neri F. , Allegrini M. , Iembo A. , Fuso F.
Published: July 2002

The dielectric constant and the reflectivity spectrum of polycrystalline RuO2 films, grown by pulsed laser deposition, are presented as deduced by optical reflection and electron energy-loss spectroscopy. The similarities of these spectra with those obtained on single crystals, suggest that the production of RuO2 by laser ablation is a very good tool in obtaining films with electronic and structural characteristics equivalent to those of the bulk material.
Raman scattering study of surface barriers in GaAs passivated in alcoholic sulfide solutions
Authors: Bessolov Vasily N. , Lebedev Mikhail V. , Zahn Dietrich R. T.
Published: July 2002

Raman scattering has been used to study the variation of surface barriers in GaAs due to sulfur passivation in solutions of ammonium sulfide [(NH4)2S] in different alcohols (ethanol, isopropanol, and tert-butanol). It has been found that the surface barrier height and the depletion layer width decrease considerably with the decrease of the dielectric constant of the passivating solution.
Magneto-optical spectra of long range chemically ordered FePd(001) alloy films
Authors: Armelles G. , Weller D. , Rellinghaus B. , Caro P. , Cebollada A. , Briones F.
Published: July 2002

Magneto-optical and magnetic properties of sputtered epitaxial FePd (001) alloy films have been studied as a function of the film deposition temperature. Characteristic changes in the magnetic anisotropies and Kerr effect spectra are observed and correlated with the degree of long range chemical ordering.
Flux motion during pulsed field magnetization in Y–Ba–Cu–O superconducting bulk magnet
Authors: Itoh Y. , Yanagi Y. , Mizutani U.
Published: July 2002

We have studied the dynamical motion of magnetic-flux lines penetrating into a melt-processed Y–Ba–Cu–O bulk superconductor during pulsed field magnetization. Subsequent flux creep was investigated by measuring signals from three Hall sensors mounted at the center, periphery, and the midpoint on the top surface of the sample at 77 K. The penetration of the magnetic flux into the sample is quite sluggish as long as the applied field is low. However, the magnetic flux penetrates suddenly and moves rapidly in response to the changing external field during the ascending stage of the pulsed field, when the intensity exceeds 1.5–2 T. Furthermore, when the rise time of the pulsed field is very short, the local magnetization measured at the midpoint is found to oscillate with time, as soon as the sudden penetration of the magnetic flux takes place. The fast motion of the magnetic flux generates heat and increases the temperature of the sample. This effect appears as a reduction of the trapped flux, already in the descending stage of the pulsed field and it contributes to the suppression of the flux creep after the pulse. The creep is suppressed more when the applied field is large, and it diminishes most in the center of the sample.
Oxidation of silicon–germanium alloys. II. A mathematical model
Authors: Hellberg P.-E. , Zhang S.-L. , d’Heurle F. M. , Petersson C. S.
Published: July 2002

A mathematical model of oxidation of SixGe1−x alloys is presented. The growth of SiO2 is simulated in conjunction with the determination of silicon distribution in SixGe1−x using numerical methods. The main feature of the model is the assumption of simultaneous oxidation of germanium and silicon when exposing the SixGe1−x to an oxidizing atmosphere. In accordance with thermodynamics, the GeO2 formed is subsequently reduced by the (free) silicon available at the interface between the growing SiO2 and the remaining SixGe1−x through a reduction reaction. Thus, the enhanced oxidation of silicon in the presence of germanium is modeled as a result of the rapid oxidation of germanium followed by the quick reduction of GeO2 by silicon. The growth of a mixed oxide in the form of either (Si,Ge)O2 or SiO2–GeO2 only occurs when the supply of silicon to the SiO2/SixGe1−x interface is insufficient. A comparison is made between simulation and experiment for wet oxidation (in pyrogenic steam) of polycrystalline SixGe1−x films. It is found that the model gives a good account of the oxidation process. Kinetic parameters, i.e., interfacial reaction rate constant for oxidation of germanium and diffusion coefficient of silicon (germanium) in SixGe1−x, are extracted by fitting the simulation to the experiment.
Diamond nucleation under bias conditions
Authors: Stöckel R. , Stammler M. , Janischowsky K. , Ley L. , Albrecht M. , Strunk H. P.
Published: July 2002

The so-called bias pretreatment allows the growth of heteroepitaxial diamond films by plasma chemical vapor deposition on silicon (100) surfaces. We present plan-view and cross-sectional transmission electron micrographs of the substrate surface at different phases of the bias pretreatment. These observations are augmented by measurements of the etch rates of Si, SiC, and different carbon modifications under plasma conditions and the size distribution of oriented diamond crystals grown after bias pretreatment. Based on these results a new model for diamond nucleation under bias conditions is proposed. First, a closed layer of nearly epitaxially oriented cubic SiC with a thickness of about 10 nm is formed. Subplantation of carbon into this SiC layer causes a supersaturation with carbon and results in the subcutaneous formation of epitaxially oriented nucleation centers in the SiC layer. Etching of the SiC during the bias pretreatment as well as during diamond growth brings these nucleation centers to the sample surface and causes the growth of diamonds epitaxially oriented on the Si/SiC substrate.
Thermal fluctuations of the magnetic field over a thin conducting plate
Authors: Roth Bradley J.
Published: July 2002

The thermal fluctuations of the magnetic field over a thin conducting plate are calculated. The plate is represented as a superposition of circuits, where each circuit consists of an array of current loops characterized by a particular spatial frequency. The thermally induced current in each loop is determined from the magnetic energy associated with the circuit and the equipartition theorem. This method gives results that are nearly identical to a previously published analysis of this problem by Varpula and Poutanen [J. Appl. Phys. 55, 4015 (1984)], in the limit when the plate is thin.
Ab initio molecular dynamics simulation of laser melting of graphite
Authors: Silvestrelli Pier Luigi , Parrinello Michele
Published: July 2002

The method of ab initio molecular dynamics, based on finite temperature density functional theory, is used to simulate laser heating of graphite. We find that a sufficiently high concentration of excited electrons dramatically weakens the covalent bond. As a result the system undergoes an ultrafast melting transition to a metallic state. This process appears to be similar to, although considerably faster than, laser melting of silicon. The properties of the laser-induced liquid phase of carbon are found to depend crucially on the level of electronic excitation. All these features are in qualitative agreement with the experimental behavior.
Internal interfaces and intrinsic stress in thin amorphous Cu-Ti and Co-Tb films
Authors: Geyer U. , von Hülsen U. , Kopf H.
Published: July 2002

A combined investigation of intrinsic stress formation by in situ substrate curvature measurements and of surface morphology evolution by scanning tunneling microscopy during the growth of amorphous Cu-Ti and Co-Tb films is reported. Intrinsic tensile stress and surface morphology are clearly correlated: all films that show intrinsic tensile stress formation in the late growth stages exhibit a cluster-like surface morphology and vice versa. Both the magnitude of the intrinsic tensile stress and the cluster size at the surface depend systematically on the reduced substrate temperature during film preparation. This dependence fits Hoffman’s model for tensile stress formation in thin films. Thus, the observed surface clusters are probably the top domes of growth columns, and the atomic mismatch at the column boundaries gives rise to tensile stress formation.
Far-infrared dielectric constant of porous silicon layers measured by terahertz time-domain spectroscopy
Authors: Labbé-Lavigne S. , Barret S. , Garet F. , Duvillaret L. , Coutaz J.-L.
Published: July 2002

We measure the refractive index and the absorption of porous silicon layers in the millimetric and submillimetric wavelength range using the terahertz time-domain spectroscopy technique. For the studied range of porosity (55%–76%), the refractive index of porous silicon is rather well described by mixture theories, in which the refractive index of bulk silicon enters as a main parameter.
Kinetics of phase transformations in real finite systems: Application to switching in ferroelectrics
Authors: Shur Vladimir , Rumyantsev Evgenii , Makarov Sergei
Published: July 2002

The modification of Kolmogorov–Avrami theory for real objects accounting for the finiteness of the transformed object/media is proposed. It takes into account the changing of the domain growth dimensionality (“geometrical catastrophe”) during the switching process. The validity of the proposed approach has been confirmed by model experiments and computer simulation. By this approach we have obtained the essential information about phase kinetics concealed in integrated experimental data. The method has been successfully used for the description of domain kinetics during the fast switching in ferroelectric single crystals and thin films. The scenario of domain evolution and voltage dependence of the main kinetic parameters have been discussed.
Dynamic behavior of carbon ultrathin film formation
Authors: Durand H.-A. , Sekine K. , Etoh K. , Ito K. , Kataoka I.
Published: July 2002

We present here investigations of the initial steps in the formation of ultrathin carbon films on a carbon highly oriented pyrolytic graphite substrate by the deposition of low energy carbon ions at temperatures from ambient to 300 °C. We used an ultrahigh vacuum scanning tunneling microscopy to observe the growth of these films. At a nanometer scale, we revealed a roughening transition of the island’s growth, from lateral to tridimensional growth. Also, here we introduce the scaling analysis of roughness to describe the evolution of the morphology of the interface during growth. The ultrathin film formation mechanisms are studied as a function of ion density and annealing.
Local probe techniques for luminescence studies of low-dimensional semiconductor structures
Authors: Gustafsson Anders , Pistol Mats-Erik , Montelius Lars , Samuelson Lars
Published: July 2002

With the rapid development of technologies for the fabrication of, as well as applications of low-dimensional structures, the demands on characterization techniques increase. Spatial resolution is especially crucial, where techniques for probing the properties of very small volumes, in the extreme case quantum structures, are essential. In this article we review the state-of-the-art in local probe techniques for studying the properties of nanostructures, concentrating on methods involving monitoring the properties related to photon emission. These techniques are sensitive enough to reveal the electronic structure of low-dimensional semiconductor structures and are, therefore, able to give detailed information about the geometrical structure, including fabrication-related inhomogeneities within an ensemble of structures. The local luminescence probe techniques discussed in this review article can be divided into four categories according to the excitation source: (i) spatially localized microphotoluminescence spectroscopy using either strong focusing or masking; (ii) near-field optical microscopy to reach below the diffraction limitation of far-field optics, by either exciting, detecting, or both exciting and detecting in the near field; (iii) cathodoluminescence using focused energetic electrons in an electron microscope; and (iv) scanning tunneling luminescence, using low-energy electrons injected or extracted from the tip of a scanning tunneling microscope.
Photoluminescence spectroscopy of Mg-doped GaN
Authors: Sheu J. K. , Su Y. K. , Chi G. C. , Pong B. J. , Chen C. Y. , Huang C. N. , Chen W. C.
Published: July 2002

We have grown Mg-doped GaN films by metalorganic chemical vapor deposition with various CP2Mg flow rates. After 750 °C postgrowth annealing, p-type GaN films with carrier concentrations and mobilities about 2×1017/cm3 and 10 cm2/V s, respectively, have been achieved. A dominant photoluminescence (PL) line around 2.9 eV was observed at room temperature. By studying the dependence of PL on excitation density at 20 K, the emission line around 2.95 eV can be attributed to a donor-to-acceptor pair transition rather than to a conduction band-to-impurity transition involving the Mg-related deep level. We suggest that the DAP transition line is caused by a Mg related deep level at about 510 meV above the valence band. It is much deeper than the acceptor level at 250 meV commonly produced by the Mg dopants.
Effective thermal conductivity of porous strontium oxide and strontium carbonate samples
Authors: Szelagowski H. , Arvanitidis I. , Seetharaman S.
Published: July 2002

The thermal diffusivity of some pressed powder strontium oxide (SrO) and strontium carbonate (SrCO3) samples has been measured from room temperature to 1400 °C in vacuum and in an argon atmosphere using the laser flash technique. The structures of the samples have been analyzed using a scanning electron microscope and a theoretical model has been successfully used to compare experimental data with model calculations.
Simulation of the particle misalignment of hard magnets after compaction
Authors: Zwick H. , Schrefl T. , Fidler J.
Published: July 2002

The simulation of powder compaction for Nd–Fe–B, Sm–Co, and barium ferrite shows the influence of different static field strengths and different mold filling on particle misalignment. The final magnetization differs up to 20%. The processing determines the pressure activated processes in which the particle overcomes a volume barrier, characterized by elastic energy fluctuations. Isostatic compaction shows less pressure activated processes as compared to uniaxial mold pressing. However, an improved magnetization of about 10% owing to isostatic pressing was observed only for spherical particles. The pressure activated processes determine particle density defects such as cracks and pores, misalignment of c axes at the punch side, at corners and at particle density defects, and internal stress areas with cold plastic flow.
Determination of the magnetic damping constant in NiFe films
Authors: Sandler G. M. , Bertram H. N. , Silva T. J. , Crawford T. M.
Published: July 2002

A two-dimensional, dynamic, micromagnetic model of a thin strip of permalloy is described and used to model the magnetization dynamics of the strip as it is subjected to a transverse step field. The numerical results are compared to experimental data. The experimental precession frequency is matched by varying Hk, the longitudinal, uniaxial, magnetocrystalline anisotropy field. The damping is matched by varying α, the phenomenological damping parameter in the LLG equation. A good fit to the data was obtained with a single damping constant of α=0.013.
Room-temperature optical transitions in Mg-doped cubic GaN/GaAs(100) grown by metalorganic chemical vapor deposition
Authors: Xu Dapeng , Yang Hui , Zhao D. G. , Li S. F. , Wu R. H.
Published: July 2002

The mechanism of room-temperature optical transitions in a Mg-doped cubic GaN epilayer grown on GaAs(100) by metalorganic chemical vapor deposition has been investigated. By examining the dependence of photoluminescence on the excitation intensity (which varied over four orders) at room temperature, four different emissions with different origins were identified. A blue emission at ∼3.037 eV was associated with a shallow Mg acceptor, while three different lower-energy emissions at ∼2.895, ∼2.716, and ∼2.639 eV were associated with a deep Mg complex. In addition to a shallow acceptor at E≅0.213 eV, three Mg-related deep defect levels were also found at around 215, 374, and 570 meV (from the conduction band).
Abnormal temperature dependence of intrinsic coercivity in sintered Sm–Co-based permanent magnets
Authors: Liu Sam , Yang Jie , Doyle George , Potts Gregory , Kuhl G. Edward
Published: July 2002

In search of high-temperature permanent magnet materials, sintered Sm–Co-based magnets with abnormal temperature dependence of intrinsic coercivity have been obtained. These magnets have small negative, or near-zero, or large positive temperature coefficients of intrinsic coercivity. A model of coercivity mechanism has been proposed to explain this abnormal temperature dependence. According to this model, the coercivity in 2:17-type permanent magnets is controlled by two conflicting mechanisms. The first mechanism is thermal activation, which leads to decreased coercivity with increasing temperature. The second mechanism can be explained by a pinning process in which increasing temperature may result in an increased difference of crystalline anisotropy between the 2:17 cell phase and 1:5 cell boundary phase and, in turn, a higher coercivity at high temperature.
Calculation of finite temperature magnetic properties of clusters
Authors: López-Urı́as F. , Pastor G. M. , Bennemann K. H.
Published: July 2002

The finite temperature magnetic properties of small clusters having N⩽6 atoms are determined in the framework of the single-band Hubbard model by using exact diagonalization methods and by taking into account both electronic and structural excitations. Results are given for the average magnetic moment 〈S〉 and for the spin correlation function 〈Si⋅Sj〉. The temperature dependence in weakly, intermediate, and strongly interacting regimes is analyzed and the role of temperature-induced structural changes is discussed.
A study of the extreme ultraviolet spectrum of O2 by electron impact
Authors: Ajello J. M. , Franklin B.
Published: July 2002

We have measured in the laboratory the electron impact emission cross sections for O2 at 200 eV. Included in the study are all emission features in the extreme ultraviolet from 40 to 131 nm at a resolution of 0.5 nm. The features are entirely from the dissociation products (OI, OII, OIII). Additionally we have measured the excitation functions from 0 to 400 eV for characteristic OI multiplets at 98.9 and 102.6 nm and for OII multiplets at 53.9 and 83.3 nm. We find the OI multiplets are formed near the dissociation limit whereas the OII multiplets have a threshold about 10 eV above the dissociation limit. We also determine the total VUV emission cross section of O2 from 40 to 200 nm and indicate the effects of autoionization to the measured emission spectrum.
Time-differential perturbed angular correlations investigation of the (NH4)2ZrF6 thermal decomposition
Authors: Rodríguez A. M. , Martínez J. A. , Caracoche M. C. , Rivas P. C. , López García A. R. , Spinelli S.
Published: July 2002

Via the TDPAC technique and complementary x-ray diffraction analysis, the thermal decomposition of (NH4)2ZrF6 has been observed to occur in the range 400–800 K according to the thermolysis scheme (NH4)2ZrF6→485 KNH4ZrF5→560 KNH4Zr2F9 →620 KZrF4→760 KZrO2. Assuming that the first transformation takes place through the simple chemical reaction (NH4)2ZrF6→NH4ZrF5+NH4F, the corresponding activation energy has been determined to be Ea=(82±6) kJ/mol. The hyperfine parameters of the NH4Zr2F9 decomposition product have been determined between 14 and 560 K. A complete analogy with the isomorphous compound NH4Hf2F9 was observed.
On the application of forced Rayleigh light scattering to mass diffusion measurements
Authors: Urbach W. , Hervet H. , Rondelez F.
Published: July 2002

The newly developed forced Rayleigh light scattering (FRS) method has been applied to the measurement of anisotropic mass diffusion in well-aligned nematic samples of  p-azoxy anisol (PAA) and  p-methoxy benzilidene-p-n-butyl aniline (MBBA). Since both materials are photochromic and can thus be photochemically labeled, self-diffusion can be studied as well as impurity diffusion. In the latter case, the diffusing species was chosen to be methyl red (MR), which is photochromic but had also been used as a regular dye in optical tracer diffusion experiments. The FRS data for self-diffusion are shown to agree well with this earlier determination; a satisfactory result if one considers that MR and both PAA and MBBA have very similar molecular structures. They also agree to within 10% with the Franklin theory of self-diffusion in liquid crystals. On the other hand, the FRS measurements of impurity diffusion with photochemically labeled MR give diffusion coefficients that are considerably smaller. This discrepancy can, in principle, be explained through nonequilibrium thermodynamic considerations which show that the diffusion coefficient actually measured by FRS is different in the two cases of self- and impurity diffusion. However, the observed variation seems anomalously large. We tentatively suggest that photoactivated MR molecules tend to form aggregates, a problem apparently not encountered with photoexcited MBBA or PAA molecules. This demonstrates that, although FRS is undoubtedly a powerful technique to measure mass transport, the possibility of photoinduced spurious intermolecular interactions between the diffusing species must always be considered with care.
Scaling in three-dimensional linear and ring polymers
Authors: Bishop Marvin , Michels J. P. J.
Published: July 2002

Bead size effects on the excluded volume of linear and ring polymers are investigated by Brownian dynamics. It is found that the mean dimensions of the chains follow a scaling relation with scaling variable X=N (σ/l)d/φ, where N is the number of units on the chain, σ is the size of the unit, l is the link length, d is the dimension, and φ is the crossover exponent. The scaling law is 〈R2〉/〈R2〉0 or 〈S2〉/〈S2〉0∼X2ν−1 for X→∞ where ν is the critical exponent for the mean dimensions of an isolated polymer chain.
A theory of percolation in liquids
Authors: DeSimone Tony , Demoulini Sophia , Stratt Richard M.
Published: July 2002

Problems involving percolation in liquids (i.e., involving connectivity of some sort) range from the metal–insulator transition in liquid metals to the properties of supercooled water. A common theme, however, is that connectivity can be distinguished from interaction and that one should not be slighted in order to describe the other. In this paper we suggest a model for percolation in liquids—the model of extended spheres—which permits connectivity to be studied in the context of, but independently from, liquid structure. This model is solved exactly in the Percus–Yevick approximation, revealing the existence of an optimum liquid structure for percolation. We analyze this behavior by first deriving an explicit diagrammatic representation of the Percus–Yevick theory for connectivity and then studying how the various diagrams contribute. The predictions are in excellent qualitative agreement with recent Monte Carlo calculations.
A FT–IRAS study of the vibrational properties of CO adsorbed on Cu/Ru(001). I. The structural and electronic properties of Cu
Authors: Hoffmann F. M. , Paul J.
Published: July 2002

We have used high resolution vibrational spectroscopy (FTIR) and carbon monoxide as a molecular probe to investigate the structural and electronic properties of well annealed copper layers adsorbed on a Ru(001) substrate. Adsorption of CO on annealed Cu–Ru (001) surfaces (θCu<1) at 85 K results in occupation of both Ru and Cu sites which exhibit characteristic C–O stretching frequencies as a function of CO coverage. The latter also indicates formation of Cu islands for 0.25<θCu<1 and chemical modification of the copper film by the underlying Ru, which leads to increased Cu–CO backdonation. From IR line shapes of the C–O stretch from CO-√3×√3-R30° on Ru sites, domain sizes are estimated for the bare Ru patches. Weak chemical modification is still observed for a 3 ML thick Cu film, whereas a 8 ML thick film exhibits all properties of bulk Cu(111), i.e., a coverage dependent red shift (2077–2075 cm−1), a (1×1) LEED pattern and characteristic CO-TDMS features. In contrast to the modification of Cu by Ru, there is little indication of a copper-induced change in the Ru–CO backdonation. The observed decrease in desorption temperature of CO–Ru TDMS features rather is related to a suppression of formation of an ordered √3-CO structure. The mobility of CO from Cu to Ru sites is found to be high and diffusion of CO occurs between 120 and 175 K. We finally observe vibrational coupling between neighboring CO–Cu and CO–Ru which leads to significant intensity transfer and frequency shifts for high θCu, i.e., small size of open Ru patches.
The rheological properties of liquids composed of flexible chain molecules: A molecular dynamics computer simulation study
Authors: Clarke J. H. R. , Brown D.
Published: July 2002

Nonequilibrium molecular dynamics simulations of planar Couette flow have been performed for dense liquids composed of model linear chain molecules each containing six interaction centers. For one liquid the potential parameters were chosen so that it resembles n-hexane. For the other liquid, referred to as flexane, the energy barriers directly opposing internal rotations are set to zero. The rheological properties of these liquids have been examined at 200 and 300 K at constant density. Torsional flexibility gives rise to a dynamic coupling which is associated with a significant reduction (by a factor of 2) in the equilibrium viscosity. A coupling mechanism is proposed based on large angle fluctuations without necessarily involving conformational transitions. This coupling disappears at high shear rates when there are significant structural changes in the liquid associated with shear thinning. Flexibility also causes changes in the shear rate dependent steady-state distribution of conformers and at low temperatures this results in reduced shear dilatancy.
Ferroelectric structure and related properties of Pb5W3O9F1
Authors: Abrahams S. C. , Marsh P. , Ravez J.
Published: July 2002

Lead tungsten oxyfluoride, Pb5W3O9F10, is a new ferroelectric material that crystallizes in the tetragonal system with space group I4 and four formulas in the unit cell. The lattice constants at 295 K are a=14.583(3) and c=7.365(1) Å for λ(MoKα1,2)=0.709 30, 0.713 59 Å. The integrated intensities of all 10 757 reflections within a reciprocal hemisphere of radius (sin θ)/λ ≤0.91 Å−1 were measured on a CAD-4 diffractometer with MoKα radiation and resulted in 7080 F2m ≥2σF2m. Correction for linear anisotropic radiation damage, followed by averaging the symmetry equivalent reflections, gave 2933 independent 〈 F2m〉≥2σ〈 F2m〉 with internal agreement factor Rint〈Fm〉 =0.0289.

The crystal structure was solved from the Patterson function and difference Fourier series and refined on w〈 F2m〉 by the method of least squares. The final agreement factors between measurement and model were R=0.0382 and wR=0.0329 on 〈 Fm〉. The as-grown crystal contained equal volumes of ±Z domains. The three independent tungsten atoms form WO6, WO3F3, and WO2F4 octahedra, and the three independent lead atoms form two different PbO4F5 and a PbO4F4 polyhedron. The major local dipole contribution to the spontaneous polarization is associated with a tungsten atom that is displaced 0.172(43) Å from the center of its octahedron. The corresponding predicted Curie temperature is 590(260) K, in agreement with the experimental value of 785(15) K. All six metal atoms undergo anharmonic thermal vibrations, which are fitted either to fifth or sixth order in a Gram–Charlier expansion of the probability density function. The average interatomic distances are W–F=1.798(8) Å, W–O=1.944(18) Å, Pb–F=2.68(19) Å, and Pb–O=2.72(21) Å: standard deviations are from Bessel’s relationship. The atomic rearrangement required in forming Pb5W3O9F10 from PbF2 and WO3 is discussed, and similarities and contrasts between the related families for which Pb5W3O9F10, SrAlF5, and Sr3(FeF6)2 provide the archetype structures are considered. All three families exhibit ferroelectric behavior.
Shape resonances in the photoionization of N2O
Authors: Braunstein M. , McKoy V.
Published: July 2002

We report the results of studies of the cross sections and photoelectron asymmetry parameters for photoionization of the 7σ level of N2O using Hartree–Fock photoelectron continuum orbitals. These studies were motivated by recent measurements which showed significant non-Franck–Condon vibrational distributions at low photoelectron energies where previously only autoionizing resonances, but no shape resonance, had been identified. Our results establish that there are two σ shape resonances in the 7σ ionization continuum, a pronounced resonance at low photoelectron energies, and another at higher energy which is essentially obscured in the vibrationally unresolved cross sections. The shape resonant structure that emerges from these studies differs significantly from the predictions of previous model studies. Studies in progress reveal a rich and unusual dependence of these resonances on changes in internuclear distances.
Liquid–liquid critical phenomena. The influence of pressure on the coexistence curve of the methanol + cyclohexane system
Authors: Aizpiri Arturo G. , Rubio Ramón G. , Peña Mateo Díaz
Published: July 2002

The liquid–liquid coexistence curve of methanol + cyclohexane has been measured in the range 0.1≤P/MPa≤13.0. The difference in mole fraction between both branches of the coexistence curve has been found to be a more convenient order parameter than the volume fraction difference. The diameter of the coexistence curve has been found to show no anomaly within the experimental uncertainty. The pressure dependence of the correction to simple scaling terms are mostly masked by their estimated uncertainties. Reliable values for xc and Tc have been determined, being xc invariant with pressure within the experimental uncertainty. The data have been found to be compatible with the two-scale-factor universality hypothesis.
The band structure of small iron clusters from Fe1 to Fe6
Authors: Tatewaki Hiroshi , Tomonari Mutsumi , Nakamura Takashi
Published: July 2002

The band structures of Fe clusters Fe1–Fe6 were investigated by symmetry-adapted and symmetry-broken SCF calculations. For Fe2, configuration interaction (CI) calculations were performed at the equilibrium internuclear distance (Re) of 2.02 Å and at another long distance of 2.60 Å. The ground state of Fe2 at Re was hardly described by a single configuration, which was also the case for the ionized states. The Fermi level (the first ionized state) of Fe2 at Re was composed of a 3d electron ionized state (3d−1), and it lay 4.8 eV below the vacuum level by the CI calculations. The result was 1 eV small compared with the experimental value given by Rohlfing and co-workers. On the other hand, the single configuration approximation (SCF) worked for the larger bond distance (2.60 Å). The Fermi level again consisted of the 3d−1 state, but the d hole was localized at one of the constituent atoms. This was also observed in larger clusters. In the SCF approximation, the atomic 3d ionization potential was considerably smaller (2.4 eV) than the experimental value. Therefore, atomic correlation correction was necessary for Fe2–Fe6, and the calculated Fermi levels modified with the atomic correlation correction were 5.3, 5.9, 5.9, 6.4, and 5.9 eV for Fe2, Fe3, Fe4, Fe5, and Fe6, respectively, which were reasonably close to the experimental values of 6.3, 6.4–6.5, 6.3–6.5, 5.9–6.0, and 5.8–6.0 eV. The ionized states with two d holes were found near the Fermi level for larger clusters.
Translational and rotational dynamics of simple dense fluids
Authors: Evans Glenn T.
Published: July 2002

The Mori method and the pseudo-Liouville formalism have been combined in an analysis of the translational and rotational diffusion coefficients of a dense fluid of hard bodies. Correlated recollisions, due to caging, appear naturally in this theory and their effect is to increase the translational friction on a tagged spherical molecule in a hard sphere bath and to increase the rotational friction on a hard nonspherical convex body in a hard sphere fluid. The calculated increase in friction from liquid state correlated caging corrections is in rough accord with the molecular dynamics calculation of Alder et al. on spherical systems and brings kinetic theory predictions for rotational friction coefficients in closer accord with experiment.
Molecular dynamics simulation of polymer liquid and glass. II. Short range order and orientation correlation
Authors: Rigby David , Roe Ryong-Joon
Published: July 2002

Molecular dynamics simulation has been performed with an assembly of n-alkane-like chains subject to potentials restricting bond lengths, bond angles, and trans–gauche dihedral angles and interacting with neighboring chains according to a truncated Lennard-Jones potential. The system, on stepwise cooling under constant pressure, undergoes a transition exhibiting characteristics of glass transition observable with laboratory polymers. The short range order in the system was monitored by evaluating the radial distribution function gk(r), where r is the distance between two short subchains, each comprising k consecutive CH2 units. The orientation correlations between neighboring subchains were monitored by evaluating the function Sk (r), giving the average cosine square of the angle between the end-to-end vectors of two neighboring subchains of k CH2 units separated by a scalar distance r. Integration of Sk(r), weighted by gk(r), gives the correlation volume. Both the short range order and the orientation correlation are seen to improve steadily as the temperature is lowered. This increased order is brought about as a result of increased density, reduced thermal motion, and more extended chain conformation. Extrapolation of the orientation correlation against temperature predicts an isotropic–nematic transition temperature Tc, as suggested by the Landau–deGennes theory. Isothermal annealing just below the predicted Tc indeed produces a highly ordered structure.
Self-consistent theory of polydisperse entangled polymers: Linear viscoelasticity of binary blends
Authors: Rubinstein Michael , Colby Ralph H.
Published: July 2002

The effects of polydispersity on the linear viscoelastic properties of concentrated polymer solutions and melts are analyzed. Existing theories for the dynamics of linear polymers, based on the idea of each polymer confined in a fixed tube, are shown to be incapable of describing observed rheological response of polydisperse polymers. A model is proposed which, in a self-consistent manner, solves the many chain problem given the solution to the single chain problem. Two types of polymer relaxation are incorporated in the model. The first type is escape of a polymer from its tube by motion of the polymer itself. It includes all dynamic modes available to the single chain in a tube—those due to its reptation—as well as other modes, such as fluctuations in tube length. The second type is relaxation of a polymer chain by the motions of the surrounding polymers forming its tube (constraint release). The relaxation modulus is then the product of two functions μ(t) and R(t). μ(t) is the fraction of tube occupied at time t=0 that has not been evacuated at time t, thereby representing escape of the polymer from its tube (solution to the single chain problem). R(t) represents relaxation by the constraint release process, which is modeled by a Rouse chain with random bead mobilities. The probability distribution of these mobilities is determined, in a self-consistent way, from the disentanglement rates due to the tube evacuation processes of the surrounding chains. Thus R(t) is calculated from the spectrum of relaxation rates of the μ(t) processes for the surrounding chains. The predictions of the model with some single chain solutions μ(t) from the literature are compared with oscillatory shear data for binary blends of nearly monodisperse polybutadiene, in which both components are well entangled.
Photofragment angular distributions for HF dimer: Scalar J–J correlations in state-to-state photodissociation
Authors: Dayton D. C. , Jucks K. W. , Miller R. E.
Published: July 2002

Photofragment angular distributions have been measured for HF dimer which show resolved structure that can be assigned to individual fragment rotational channels. This data is used to establish intermolecular scalar correlations between the rotational states of the two HF fragments. The observed angular distributions are strongly dependent upon whether the ‘‘free’’ or ‘‘hydrogen bonded’’ HF stretch is initially excited. Since the infrared spectrum of the parent molecule is highly resolved, these results can be used to determine the relative state-to-state photodissociation cross sections. In addition, the zero point dissociation energy (D0 ) of the HF dimer is accurately determined.
A static ensemble approximation for stochastically modulated quantum systems
Authors: Bittl R. , Schulten K.
Published: July 2002

A short-time approximation for the evolution of quantum systems governed by a Hamiltonian with stochastic time dependence is derived. The evolution operator of the system is replaced by an ensemble of evolution operators with time-independent Hamiltonians, weighted by a distribution function related to the ‘‘line shape function’’ of a randomly modulated harmonic oscillator. The approximation conserves the trace of the density matrix and converges to the exact solution in the case of very slow and in the case of very rapid stochastic modulation. The approximation is applied to hyperfine-induced singlet–triplet transitions of a biradical-like system with fluctuating exchange interaction.
Microscopic model for microemulsions. I. Ground state properties
Authors: Ciach A. , Ho/ye J. S. , Stell G.
Published: July 2002

The ground state of several variants of a new microemulsion model is examined in detail. We find oil–microemulsion–water and oil–surfactant–water equilibria, and coexistence between microemulsions of different geometrical structures (lamellar, tubular, and ‘‘brick’’). We find vanishing or low surface tension between bulk oil and water at the three-phase coexistence in different versions of the model. Our results suggest that the model is capable of reproducing many of the striking features of real microemulsions and deserves a detailed study at nonzero temperatures.
Flexible polymer chain dynamics and rheological properties in steady flows
Authors: Liu Tony W.
Published: July 2002

A stochastic approach equivalent to the kinetic theory of a dilute solution of polymer molecules idealized as Kramers freely jointed bead–rod chains is presented. This stochastic approach forms the basis for a Brownian dynamics simulation algorithm that generates sample trajectories of Kramers chains from which steady-state rheological properties of the model polymer solution can be extracted. In steady shear flow, the solution exhibits both shear-thinning viscosity and first normal stress coefficient. In steady elongational flow, the elongational viscosity of the solution increases sharply with increasing elongation rate and approaches an asymptotic value many times its low elongation rate value. Computer animations of sample trajectories of Kramers chains generated by the simulations also reveal a wealth of information about the rotation, stretching, and alignment of polymer molecules in shear and elongational flows.
Shear-induced homogenization of semidilute solution of polymer mixture and unmixing after cessation of the shear
Authors: Takebe Tomoaki , Sawaoka Ryuji , Hashimoto Takeji
Published: July 2002

Shear-induced homogenization of semidilute solutions of polybutadiene and polystyrene mixtures with nearly critical composition was studied by the flow small-angle light scattering method. The dramatic drop of the homogenization

temperature

ΔTc(S)=Tc(0)−Tc(S) was observed, and studied as a function of shear rate S, where Tc(S) is the homogenization temperature at a given S and Tc(0) is the temperature under the quiescent state. The experimental results showed that ΔTc(S)/Tc(0)=KS0.50±0.02 where the prefactor K was found to be (2.6±0.6)×10−3 for the solutions containing 3.0 wt. % of total polymer weights. This prefactor is much larger than that reported for a case of simple-liquid mixture by 3 or 4 orders of magnitude. The large prefactor is proposed to be due to a long lifetime of the concentration fluctuations in the single-phase state and slow growth rate of the fluctuations in the two-phase state, compared with the corresponding quantities for the simple-liquid mixture. In the shear-induced homogenized state, the light scattered intensity I(qy) perpendicular to the flow direction was found to obey the Ornstein–Zernike equation. The correlation length ξ and the intensity at qy=0, I(0), were found to depend on S, i.e., ξ−2∼S1/2−S1/2c and I(0)−1∼S1/2−S1/2c, and diverge at the critical shear rate Sc. These experimental results on Tc(S), ξ−2 and I(0)−1, suggest the mean-field behavior of the solution of the polymer mixture under shear. The early-stage unmixing behavior after the cessation of the steady-state shear at S>Sc was also examined, the results of which indicate that the hydrodynamic interaction is suppressed, giving rise to the small mode–mode coupling contribution Dhydro/Dapp=0.2 to 0.45, but is not yet completely screened out. Hence the hydrodynamic interaction cannot be still ignored for this system.
Molecular dynamics calculation of the equation of state of liquid propane
Authors: Toxvaerd So/ren
Published: July 2002

Molecular dynamics calculations of the pressure in liquid propane show quantitative agreement with experimentally obtained pressure in the density and temperature intervals for which there exist experimental data. The pressure is obtained for a site–site potential with bond lengths and angles determined spectroscopically and with the intermolecular potential constructed to give the heat of vaporization. The orientational order is found to be of short range even for pressure up to 10 kbar. The self-diffusion constants agree well with otherwise obtained values and demonstrate that the intramolecular vibrations have no effect on the molecular diffusion.
Comparison between the density effects on the levels of the Raman spectra of the Fermi resonance doublet of the 12C16O2 and 13C16O2 molecules
Authors: Garrabos Y. , Echargui M. A. , Marsault-Herail F.
Published: July 2002

Isotropic Raman studies of density effects on the lower Fermi doublet ν1, 2ν2 of the 12C16O2 and 13C16O2 compressed molecules have been made in the gaseous and liquid phases along the saturation line up to the triple point temperature. After a brief bibliography review of the unperturbed frequency ordering for the 12C16O2 case, the problem of the determination of unperturbed levels from our experimental data is treated for both isotopic species. Comparison between 12C and the 13C frequency shifts due to the density effect leads to the conclusion that the order of the ν1, 2ν2 levels is reversed in the liquid and gaseous phase of 12C16O2 with an exact Fermi resonance ν1=2ν2 in the high-density fluid phase. For the first time, bandwidths of the two components of the Fermi dyad are measured in the liquid.