bluuebunny/crossref_metadata_2025 huggingface dataset:
Electrical properties of lightly doped p-type silicon–germanium single crystals
Authors: Gaworzewski P. , Tittelbach-Helmrich K. , Penner U. , Abrosimov N. V.
Published: July 2002

Experimental results are presented on the electrical properties of lightly boron doped bulk Si1−xGex as a function of the Ge content x in the range 0<x<0.13. Calculations of the hole mobility in Si and in Si1−xGex and comparison with experimental results allow us to estimate the averaged scattering potential of the randomly distributed Ge atoms to be 0.55 eV. From Hall effect and capacitance–voltage measurements, a Hall factor around 0.8 at T=300 K is derived. Hall effect measurements in the temperature range 20–300 K enable us to determine the boron acceptor activation energy, which decreases from 45 meV at x=0 down to 32 meV for x=0.13, and to estimate the hole effective masses to be meff/m0≈0.45.
Role of interfacial uncompensated antiferromagnetic spins in unidirectional anisotropy in Ni81Fe19/CoO bilayers (invited)
Authors: Takano Kentaro , Kodama R. H. , Berkowitz A. E. , Cao W. , Thomas G.
Published: July 2002

The uncompensated spins on the surfaces of antiferromagnetic CoO films exhibit a thermoremanent magnetization after field cooling from T>TN that has the same temperature dependence as the exchange field of Ni81Fe19/CoO bilayers after field cooling. This suggests that these interfacial uncompensated spins are responsible for unidirectional anisotropy. A model based on a calculation of the density of these interfacial uncompensated spins predicts the correct magnitude of the exchange field, as well as the observed inverse dependence on average interfacial grain size.
Electrochemical deposition of amorphous FeB films with soft magnetic properties
Authors: Fujita Naoyuki , Inoue Mitsuteru , Arai Ken’ichi , Lim Pang Boey , Fujii Toshitaka
Published: July 2002

To obtain FeB amorphous plated films with soft magnetization and high magnetostriction, effects of three kinds of agents [sodium L-ascorbate (C6H7NaO6), thiourea (NH2CSNH2), and ammonium sulfate ((NH4)2SO4)] on magnetic and magnetoelastic properties of films were investigated systematically. The first two agents, C6H7NaO6 and NH2CSNH2 were found to be ineffective for improving the soft magnetic properties of films. To the contrary, however, (NH4)2SO4 is very effective for the magnetic softening: Amorphous FeB plated films with coercivity less than 4 Oe were obtained by employing the bath composition of FeSO4⋅7H2O (71.9 mM), KBH4 (296.6 mM), NaOH (400 mM), KNaC4H4O6⋅4H2O (600 mM) and (NH4)2SO4 (80 mM). These soft films exhibited a favorable large magnetostriction of 26×10−6, being adequate for micromagnetoelastic devices.
Dependence of exchange field and coercivity on cooling field in NiFe/CoO bilayers
Authors: Ambrose T. , Chien C. L.
Published: July 2002

The effects of different cooling fields (HFC) on both the exchange field and coercivity in a NiFe/CoO bilayer have been investigated. The exchange field is strongly effected by HFC at low field values and levels off at high values, whereas the coercivity is only weakly influenced.
Measurements of the ferromagnetic/antiferromagnetic interfacial exchange energy in CO/CoO and Fe/FeF2 layers (invited)
Authors: Dahlberg E. Dan , Miller Brad , Hill Bradford , Jonsson B. J. , Strom Valter , Rao K. V. , Nogues Josep , Schuller Ivan K.
Published: July 2002

Two measurement techniques, both relying on reversible rotations of the magnetization, have been used to determine the magnitude of the interfacial exchange energy (IEE) between ferromagnetic and antiferromagnetic (F/AF) layers. One technique is to use the anisotropic magnetoresistance to determine rotations of the magnetization away from the unidirectional easy axis, where the rotation is accomplished by applying external magnetic fields less than the effective F/AF exchange field. The second technique uses measurements of the ac susceptibility as a function of the angle between the ac field and the unidirectional exchange field. Both of the reversible process techniques result in values of the IEE larger (by as much as a factor of 10 in Co/CoO bilayers) than the traditional irreversible technique of measuring a shift in the hysteresis loop. The ac susceptibility technique was also used to measure one Fe/FeF2 bilayer. For this sample, the IEE values obtained by reversible and irreversible methods are equivalent.
Measurement of resonant frequency and quality factor of microwave resonators: Comparison of methods
Authors: Petersan Paul J. , Anlage Steven M.
Published: July 2002

Precise microwave measurements of sample conductivity, dielectric, and magnetic properties are routinely performed with cavity perturbation measurements. These methods require the accurate determination of quality factor and resonant frequency of microwave resonators. Seven different methods to determine the resonant frequency and quality factor from complex transmission coefficient data are discussed and compared to find which is most accurate and precise when tested using identical data. We find that the nonlinear least-squares fit to the phase versus frequency is the most accurate and precise when the signal-to-noise ratio is greater than 65. For noisier data, the nonlinear least-squares fit to a Lorentzian curve is more accurate and precise. The results are general and can be applied to the analysis of many kinds of resonant phenomena.
Spectroscopic ellipsometry of composite thin films with embedded Bi nanocrystals
Authors: Serna R. , de Sande J. C. G. , Ballesteros J. M. , Afonso C. N.
Published: July 2002

Spectroscopic ellipsometry together with an effective medium model is used to determine simultaneously the effective refractive index, thickness, and metal volume fraction of thin nanocomposite films. The films are formed by Bi nanocrystals embedded in amorphous matrices, either semiconducting (Ge) or dielectric (Al2O3). For the Bi:Ge films (metal in an absorbing host), the values obtained for both the real and imaginary parts of the refractive index vary continuously from that of Ge to that of Bi. The metal contents determined from the ellipsometry analysis are in excellent agreement with those obtained from direct measurements of the composition. For the Bi:Al2O3 films (metal in a nonabsorbing host), the extinction coefficient (k) exhibits a maximum around 360 nm which is related to the metal plasmon resonance frequency of Bi nanocrystals. The metal content determined from the ellipsometry analysis in this case is underestimated, probably due to interaction of the Bi crystals with the Al2O3 host.
Sintering in a dry snow cover
Authors: Colbeck S. C.
Published: July 2002

The basic shape of bonds in snow is dictated by the geometrical requirements of grain-boundary grooves and is not a simple concave neck as has long been assumed. In fact, all of the earlier work on the theory of sintering in snow was based on an incorrect assumption about the geometry. A theory of the growth of bonds in snow is given here based on observations of their actual shape which is dominated by grain-boundary grooves. The theory describes the growth of the bond by the removal of water molecules from the grain boundary by diffusion due to the stress gradient. Three-dimensional grains are described and the dihedral angle is allowed to increase with time.
Determining the specific area of porous acoustic materials from water extraction data
Authors: Leclaire P. , Swift M. J. , Horoshenkov K. V.
Published: July 2002

The internal pore surface area per unit mass of solid (specific area) is an important parameter for the physical study of sound wave propagation in porous media. It is shown that this parameter can be evaluated with a reasonable accuracy from the integration of the water extraction curve (volume of water extracted versus applied pressure). This method is nonacoustic, modelless and well adapted to acoustic materials. It is a complementary method to the standard BET (Brunauer, Emmet, Teller) technique, generally used for powders in chemistry and the pharmaceutical industry. Experiments are made on loose glass beads of a few hundred microns diameter and results are compared to theoretical values. Results on heterogeneous materials used in civil engineering and in noise control are also given. The parameters necessary for the description of modern acoustic materials are estimated from the specific area measured by this method.
Theoretical evidence for the semi-insulating character of AlN
Authors: Fara Antonella , Bernardini Fabio , Fiorentini Vincenzo
Published: July 2002

We present ab initio density-functional calculations for acceptors, donors, and native defects in aluminum nitride, showing that acceptors are deeper (Be∼0.25 eV, Mg∼0.45 eV) and less soluble than in GaN; at further variance with GaN, both the extrinsic donors SiAl and CAl, and the native donor VN (the anion vacancy) are found to be deep (about 1 to 3 eV below the conduction). We thus predict that doped AlN will generally turn out to be semi-insulating in the normally achieved Al-rich conditions, in agreement with the known doping difficulties of high-x AlxGa1−xN alloys.
Cooperative or noncooperative dielectric relaxation in ionic solids: A discriminating experimental approach
Authors: Henn F. , Vanderschueren J. , Giuntini J. C. , Zanchetta J. V.
Published: July 2002

Calculations, confirmed by experimental evidence on a polymer and on two ionically conducting solids, show that the variation of thermally stimulated depolarization currents (TSDC) with the heating rate depends on the polarization decay function that is assumed. This permits discrimination between models usually proposed for describing the non-Debye behavior of low-frequency dielectric relaxation in solids. It is also demonstrated that consideration of the kinetics involved in nonequilibrium TSDC spectroscopy is an essential, and complementary, experimental tool for dielectric measurements carried out under isothermal conditions in the frequency domain in establishing models of relaxation.
CF
x
radical production and loss in a CF4 reactive ion etching plasma: Fluorine rich conditions
Authors: Booth J. P. , Cunge G. , Chabert P. , Sadeghi N.
Published: July 2002

Space and time resolved laser induced fluorescence, combined with absolute calibration techniques, were used to probe the production and loss mechanisms of CF and CF2 radicals in capacitively coupled 13.56 MHz plasmas in pure CF4 at 50 and 200 mTorr. Under these conditions (pure CF4, with no etched substrate) the gas-phase atomic fluorine concentration is high, minimizing polymer formation on the reactor surfaces. Fluorine-poor conditions will be considered in a following paper. Steady state axial concentration profiles show that, under many circumstances, the (aluminum) rf powered electrode is a net source for these radicals, whereas the grounded (aluminum) reactor surfaces are always a net sink. The summed fluxes of CF and CF2 produced at this surface were found to be comparable to the incident ion flux. We propose therefore that CFx radicals are produced by neutralization, dissociation, and reflection of the incident CFx+ ions under these conditions. This mechanism often predominates over the gas-phase production of these species by direct dissociation of CF4, and accounts for the unexpectedly high concentrations observed. The difference in behavior between the powered and grounded electrode surfaces is explained by the difference in the incident ion energy and mass distributions.
Perpendicular magnetic anisotropy in NdFeB thin films
Authors: Piramanayagam S. N. , Matsumoto M. , Morisako A.
Published: July 2002

The perpendicular magnetic anisotropy (PMA) in NdFeB thin films is investigated. PMA was observed when the c axis of Nd2Fe14B phase orients perpendicular to the film plane. The c-axis orientation and hence the PMA was found to be very sensitive to composition and substrate temperature. The change of composition or substrate temperature leads to a deterioration in the c-axis orientation and PMA. However, the other parameters such as sputter power or deposition rate do not play a very important role over a wide range used in this study. Increasing the thickness of films does not cause a great deterioration of PMA up to 1500 nm, although the trend indicates that at higher values of thickness the films become isotropic.
Using neural networks in the identification of Preisach-type magnetostriction and field-temperature models
Authors: Adly A. A. , Abd-El-Hafiz S. K. , Mayergoyz I. D.
Published: July 2002

It is well known that the identification process of Preisach models is, basically, reduced to the determination of weight functions of the operators upon which the model is built. Recently, two-dimensional Preisach models have been proposed as a possible approach for modeling one-dimensional magnetostriction as well as field-temperature effects on magnetized bodies. In this article, the use of neural network machinery is suggested to solve identification problems of such models. The suggested technique has been implemented and experimentally tested. Testing results suggests that this technique can lead to reasonably accurate simulation results.
Magnetoimpedance effect in CoFeNi plated wire with ac field annealing destabilized domain structure
Authors: Kurlyandskaya G. V. , Barandiarán J. M. , Gutiérrez J. , Garcı́a D. , Vázquez M. , Vas’kovskiy V. O.
Published: July 2002

Giant magnetoimpedance (GMI) behavior at 0.25 MHz has been studied for a composite wire consisting of a 1 μm thick Co6Fe20Ni74 magnetic layer electroplated onto a 0.1 mm diam CuBe nonmagnetic wire. This has been done for as-deposited samples as well as after annealing under dc and ac axial magnetic fields. In the optimum conditions, a maximum GMI of 150% is achieved. The observed GMI hysteresis in the as-prepared state decreases for high driving current used to measure the MI itself. Such hysteresis, to be avoided in technological applications, is also very much reduced after ac field annealing. GMI characteristics are analyzed considering the domain-wall stabilization occurring after the annealing treatments.
Theoretical studies of the field-cooled and zero-field cooled magnetization of interacting fine particles
Authors: Chantrell R. W. , Walmsley N. S. , Gore J. , Maylin M.
Published: July 2002

We have applied a Monte–Carlo model of the properties of an interacting fine particle system to calculations of the field-cooled (FC) and zero-field cooled (ZFC) magnetization. The variation of Tg, the temperature peak, in the ZFC is also calculated. For an interacting system the effective energy barrier distribution is dependent on the packing density, and this is reflected in the Tg vs H curve. An important result is the demonstration that magnetostatic interactions between the particles can give rise to a peak in the FC magnetization. This is in agreement with experiments and is a result of the magnetic order introduced by the magnetostatic interactions.
Accurate method for determining adhesion of cantilever beams
Authors: de Boer M. P. , Michalske T. A.
Published: July 2002

Using surface micromachined samples, we demonstrate the accurate measurement of cantilever beam adhesion by using test structures which are adhered over long attachment lengths. We show that this configuration has a deep energy well, such that a fracture equilibrium is easily reached. When compared to the commonly used method of determining the shortest attached beam, the present method is much less sensitive to variations in surface topography or to details of capillary drying.
A crystalline hydrogenated carbon film obtained by plasma enhanced chemical vapor deposition
Authors: Zhang Z. J. , Narumi K. , Naramoto H. , Wu Z. P. , Yamamoto S. , Miyashita A. , Tamada M.
Published: July 2002

A hydrogenated carbon film in a crystalline form has been prepared on Si (001) using a radio frequency plasma enhanced chemical vapor deposition process. The crystalline film is metastable and has polymer-like features with a mass density of about 1.20 g/cm3 and an optical band gap of about 2.75 eV. The stability of the structure and properties of the film was studied by thermal annealing at successively higher temperatures in the air. Upon heating, the film showed a two-step structure change which resulted in a two-step change in the film properties. The relationship between the properties and the bonded hydrogen in the film was also discussed.
Free electron laser small signal dynamics and inclusion of electron-beam energy phase correlation
Authors: Dattoli G. , Giannessi L. , Ottaviani P. L.
Published: July 2002

We analyze the problems associated with the generation of coherent radiation by an e beam, traversing an undulator magnet, with an initial energy-phase correlation. The mechanisms of the process are explained and the role played by the bunching is clarified. The effect of the correlation on the stimulated part of the emission is also discussed.
An estimate of the linear strain rate dependence of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine
Authors: Conley P. A. , Benson D. J.
Published: July 2002

It is the long term goal of our work to elucidate the microscale mechanisms involved in the initiation of porous energetic materials (EMs) through direct numerical simulation of EM microstructures subjected to dynamic loading. Through this effort it is hoped that we may suggest appropriate continuum level constitutive models (reactive and inert) for porous EMs. A major obstacle in this effort is the lack of reliable parameters for the constitutive models in current use in computation. In the current study, we examine the theoretical dependence of shock structure upon the viscous nature of the deformation. In particular, we attempt to demonstrate that a unique value of the viscous parameter exists for an elastoplastic strength model in concert with a simple Newtonian viscous shear rate dependence, which allows us to accurately simulate shock structures observed in experimental studies of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine. In doing this, a reasonable estimate of the viscous parameter is suggested for use in future modeling efforts.
Secondary ion mass spectrometry study of space-charge formation in thermally poled fused silica
Authors: Alley Thomas G. , Brueck S. R. J. , Wiedenbeck Michael
Published: July 2002

Applying a dc electric field across a fused silica sample at elevated temperatures followed by cooling the sample with the field applied (thermal poling) leads to a second-order nonlinearity that has been linked to the formation of a space-charge region in bulk glass. The first microscopic information on the extent of the space-charge region and its behavior with poling time is reported using secondary ion mass spectrometry to monitor the distribution of charged impurities. Lithium and sodium ions are observed to form depletion regions. Potassium and sodium ions as well as a hydrogenated species appear to be injected from the surface. The extent of the space-charge region evolves approximately logarithmically with poling time well after the nonlinearity as measured by second-harmonic generation has been established. The evolution of the space charge region can be qualitatively understood by an ion-exchange model that allows interaction of two ionic carriers with vastly different mobilities.
Characterization of microwave magnetic narrow band filters by ferromagnetic resonance
Authors: Bousbahi Khaled , Marcelli Romolo
Published: July 2002

Ferromagnetic resonance cavity and microstrip excitation experiments have been performed on a straight edge yttrium iron garnet resonator. Both excitation systems have been modeled as band-stop configurations and a mapping from the swept bias magnetic field domain to the frequency domain has been elaborated to compare them in terms of the dispersion and the amplitude of the magnetostatic wave modes.
Interactions of magnetic resonance imaging radio frequency magnetic fields with elongated medical implants
Authors: Smith Chris D. , Kildishev Alexander V. , Nyenhuis John A. , Foster Kirk S. , Bourland Joe D.
Published: July 2002

Magnetic resonance imaging (MRI) is a well established diagnostic technique, one from which all patients should be able to benefit, including those with implanted medical devices. This paper describes an experimental and numerical study of the temperature rise near the ends of wires by the radio frequency (rf) field in MRI. These wires simulate long wires which may be part of a medical implant. Temperature rise as a function of time was measured for wires of different lengths and diameters in a phantom exposed to the 64 MHz rf field inside a MRI body coil. For wires with no or thin insulation, the maximal rise was about seven times background for a wire length of 20 cm; wires with longer and shorter lengths exhibited less temperature rise. The temperature rise was greater for wires with thicker insulation. Computer simulations using a quasistatic model are in reasonable agreement with the measurements on shorter wires. It is concluded that medical implants with long conducting lead wires may result in potentially unsafe temperature rise during MRI imaging.
Thermodynamics of Cu47Ti34Zr11Ni8, Zr52.5Cu17.9Ni14.6Al10Ti5 and Zr57Cu15.4Ni12.6Al10Nb5 bulk metallic glass forming alloys
Authors: Glade S. C. , Busch R. , Lee D. S. , Johnson W. L. , Wunderlich R. K. , Fecht H. J.
Published: July 2002

The differences in the thermodynamic functions between the liquid and the crystalline states of three bulk metallic glass forming alloys, Cu47Ti34Zr11Ni8, Zr52.5Cu17.9Ni14.6Al10Ti5, and Zr57Cu15.4Ni12.6Al10Nb5, were calculated. The heat capacity was measured in the crystalline solid, the amorphous solid, the supercooled liquid, and the equilibrium liquid. Using these heat capacity data and the heats of fusion of the alloys, the differences in the thermodynamic functions between the liquid and the crystalline states were determined. The Gibbs free energy difference between the liquid and the crystalline states gives a qualitative measure of the glass forming ability of these alloys. Using the derived entropy difference, the Kauzmann temperatures for these alloys were determined.
Effects of ion irradiation on silicon oxidation in electron cyclotron resonance argon and oxygen mixed plasma
Authors: Matsuo S. , Yamamoto M. , Sadoh T. , Tsurushima T. , Gao D. W. , Furukawa K. , Nakashima H.
Published: July 2002

Effects of ion irradiation on oxidation of silicon at a temperature as low as 130 °C in an argon and oxygen mixed plasma excited by electron cyclotron resonance interaction have been investigated. The growth rate of the oxide films increases with increasing incident energy and flux of argon ions, and the thickness increases proportionally to the root square of the oxidation time, which suggests that the growth rate is limited by diffusion of oxidants enhanced by irradiation with argon ions. Effects of substrate bias on the oxidation characteristics have been also investigated. The growth rate increases with increasing positive bias, and the growth kinetics deviate from diffusion limited with increasing thickness. The bias dependence of the growth rate is caused by drift of negative oxidants enhanced by the electric field established in the oxide films. Moreover, it is shown that the electrical properties of the oxide films are improved by applying positive substrate bias. The improvement is due to a reduction of irradiation-damage in the initial oxidation stage. On the basis of the experimental results, it is concluded that the reduction of the incident energy and the flux of argon ions in the initial oxidation stage is essential to improve electrical properties of the oxide films.
Wavelength dependence of the preresonance Raman cross sections of CH3CN, SO42−, ClO4−, and NO3−
Authors: Dudik John M. , Johnson Craig R. , Asher Sanford A.
Published: July 2002

The Raman scattering cross sections of vibrations of SO4−2, NO3−, ClO4−, and CH3CN have been determined between 220–640 nm. The cross sections of the symmetric stretching vibrations of SO4−2 and ClO4− as well as the 918 cm−1 C–C stretching vibration of CH3CN display almost a ν4 excitation frequency dependence. The intensity of the 2249 cm−1 C≡N stretching vibration increases somewhat faster than ν4, but significantly slower than that which would be expected if a dipole allowed C≡N π→π* transition at ∼150 nm dominated the Raman intensity. The intensity of the NO3− symmetric stretch increases with a frequency dependence close to that expected from an Albrecht A term contribution from the ∼200 nm π→π* transition. The fact that the 981 cm−1 SO4−2, 932 cm−1 ClO4−, and 918 cm−1 CH3CN vibrations show essentially only ν4 intensity dependencies indicates that no particular dipole allowed transition dominates the preresonance Raman intensities. These results suggest that conventional preresonance Raman expressions do not accurately model the Raman intensities. The small excitation frequency dependencies of SO4−2, ClO4−, and CH3CN Raman intensities show that these species are ideal internal standards for Raman intensity measurements in the UV spectral region.
Monte Carlo simulation of differences in free energies of hydration
Authors: Jorgensen William L. , Ravimohan C.
Published: July 2002

Perturbation theory has been applied to calculate the relative free energies of hydration of methanol and ethane in dilute soluton. It is demonstrated that only two or three Monte Carlo simulations using double-wide sampling are necessary to obtain results with high precision. The small statistical uncertainty in the computed change in free energy of hydration and the good accord with experimental thermodynamic data are most encouraging for application of the procedure to a wide range of problems. Structural effects accompanying the mutation of methanol to ethane in water are also discussed; hydrogen bonding to the solute is essentialy eliminated by only a 25% reduction in the atomic charges of methanol.
Valence transitions in 1,3-cyclopentadiene, 1,3-cyclohexadiene, and 1,3-cycloheptadiene
Authors: McDiarmid R. , Sabljić A. , Doering J. P.
Published: July 2002

The 300–180 nm (4.1–6.9 eV) optical absorption and 4–10 eV (310–124 nm) electron energy loss spectra of 1,3-cyclopentadiene, 1,3-cyclohexadiene, and 1,3-cycloheptadiene were measured. Three valence and several Rydberg transitions were observed in each molecule. The two strong, optically allowed valence transitions are interpreted as the NV1(B2) and NV3(A+1) transitions (states). The locations of the unobserved A−1 states in cyclic dienes is discussed. Correlations are drawn between the three valence excited states observed here in each cis-diene and those previously reported for trans-butadiene.
Isotope shifts and spin–spin coupling constants in the 13C and 17O NMR spectra of carbon monoxide and carbon dioxide
Authors: Wasylishen Roderick E. , Friedrich Jan O. , Mooibroek Sandra , Macdonald J. Bruce
Published: July 2002

Oxygen isotope shifts on the 13C chemical shifts and carbon isotope shifts on the 17O chemical shifts in carbon monoxide and carbon dioxide are reported. Using models developed by Jameson, shielding derivatives with respect to bond lengths can be calculated using the measured isotope shifts. For carbon monoxide, the derivatives were calculated to be [∂σ (13C)/∂r]e =−456±15 ppm/Å and [∂σ (17O)/∂r]e =−1150±130 ppm/Å. Although earlier coupled Hartree–Fock calculations give a much lower value for [∂σ (17O)/∂r]e, recent ab initio calculations for carbon monoxide agree very well with our experimental results. Furthermore, the observed 18O/16O iostope shift is similar to values measured previously for a series of metal carbonyls. For carbon dioxide the iostope shift gives [∂σ (13C)/∂r]e =−214±17 ppm/Å which is in excellent agreement with the value obtained from a recent variable temperature gas phase NMR study. In addition, scalar spin–spin coupling constants, 1J(13C,17O) were measured to be 16.4±0.1 Hz in carbon monoxide and 16.1±0.1 Hz in carbon dioxide. To our knowledge, these are the first directly measured carbon–oxygen coupling constants to be reported in the literature. From general trends in the periodic table, it seems likely that the sign of these coupling constants is positive.
Multidimensional geometric aspects of the solid–liquid transition in simple substances
Authors: LaViolette Randall A. , Stillinger Frank H.
Published: July 2002

Any molecular system explores significantly different regions of the potential-energy hypersurface as the system is found, respectively, in the solid and liquid phases. We study in detail the multidimensional geometry of these different regions with molecular-dynamics calculations for 256 simple atoms in a fixed volume. The atomic interactions are chosen to represent the noble gases. The stable crystal for this model displays a face-centered cubic structure. We evaluate the local gradient and curvatures of the regions of the hypersurface sampled by the system for a wide range of temperatures. We observe that a significant fraction of the curvatures become negative in the region sampled by the system at temperatures even as low as one-fourth the melting temperature. Further, the curvature distribution changes dramatically with respect to temperature at the melting point. We also construct and evaluate a new distribution for the distance between the atoms in their instantaneous dynamical configurations and those in their corresponding ‘‘quenched’’ configuration (i.e., the configuration found at the corresponding potential-energy minimum). With the help of this new distribution, we conclude that the quenched configurations which are encountered during the melting process are structures which contain vacancy-interstitial defect pairs.
Success of quantum mechanical approximations for molecular geometries and electron–nuclear attraction expectation values: Gift of the Coulomb potential?
Authors: Levy Mel , Perdew John P.
Published: July 2002

A perturbation theory is derived for the study of molecular structure. For a large class of approximate formulations (including Hartree–Fock and higher-level schemes and exact exchange-only density-functional theory), it is proved that the change in the error of the energy, upon geometry change, is zero through the second-order perturbation term, thus helping to explain why approximate energy curves often closely parallel exact curves and give accurate geometries. In contrast, for each non-Coulomb potential considered, the error change is not zero through second order, suggesting that accurate geometries are due, in part, to a special quality of the Coulomb potential. Comparable results are obtained for atomic electron–nuclear attraction expectation values (isoelectronic energy changes), which are exact through second order in the Coulomb cases. The conclusions are supported by a nonperturbative argument and by a numerical example.
The electric conductivity of the lamellar smectic, the micellar nematic, and the isotropic micellar solution of ammonium perfluorononanoate in water
Authors: Photinos P. J. , Saupe A.
Published: July 2002

We present electric conductivity measurements on a mixture of 25 wt. % ammonium perfluorononanoate in H2O. The system shows a lamellar smectic phase up to 26.3 °C, followed by a nematic phase consisting of disk-like aggregates, which at 34.2 °C transforms to an isotropic micellar solution. The relative anisotropy of the conductivity increases from ∼0.07 at 34 °C to ∼0.29 at 26.3 °C. There is no discontinuity, but a small change in slope occurs at the nematic–smectic transition. The experimental results, interpreted in terms of structural models for the two anisotropic phases, shows that the aggregates in the nematic phase have diameter to thickness ratios, decreasing from about 8.3 at the lamellar to nematic transition, to about 4.0 at the transition to the isotropic phase. We estimate that about 30% of the total area are disrupted by holes or channel-like defects. The results are in agreement with previous studies on a mixture of decylammonium chloride/NH4Cl/H2O. It is also indicated that increasing the size of the aggregates increases the binding of the counterions to the aggregates.
Chemical shifts and bond modification effects for some small first-row-atom molecules
Authors: Chesnut D. B. , Foley C. K.
Published: July 2002

Chemical shifts and shift changes with bond modification have been calculated for some small first-row-atom molecules employing a (6311/311/1)=[4s, 3p, d] heavy atom, (31)=[2s] hydrogen basis in the GIAO approach. Shifts and shift anisotropies using this intermediate size basis agree as well with experiment as other, more extensive basis sets; heavy atom shifts are determined reasonably well on an absolute basis and hydrogen shifts are acceptable on a relative scale. Nearly all shift first derivatives are negative with respect to bond lengthening as are the second derivative terms. These results suggest that in most cases a negative temperature coefficient for the chemical shifts of the heavy atoms will be observed, an effect which in fact does obtain in most cases known experimentally.
Inner shell excitation of thiophene and thiolane: Gas, solid, and monolayer states
Authors: Hitchcock A. P. , Horsley J. A. , Stöhr J.
Published: July 2002

The electron energy loss spectra of gaseous thiophene and thiolane in the regions of S 2p, S 2s, and C 1s are presented along with the x-ray photoelectron yield (NEXAFS) spectra of both gases in the region of S 1s excitation. The thiophene spectra are compared to the corresponding NEXAFS spectra of solid (multilayer) and monolayer thiophene on Pt (111). MS-Xα calculations of the C 1s, S 2p, and S 1s excitation spectra of free thiophene are also reported. Intercomparison of the gas, surface, and calculated spectra allows a complete interpretation of the spectral features and facilitates determination of the molecular orientation of thiophene with respect to the surface in both the compressed [thiophene on Pt (111) at 150 K] and relaxed [thiophene on Pt (111) at 180 K] monolayer phases.
Amorphous solid carbon dioxide
Authors: Falk Michael
Published: July 2002

Carbon dioxide gas condensed at 10 K was found to form amorphous solid films whose infrared spectra differ substantially from those of crystalline CO2. With the help of a sample of 13C-enriched CO2, the fundamental absorption bands of different isotopic species of CO2 in the spectra of the amorphous solid, crystalline solid, and of CO2 dispersed in solid nitrogen matrix were analyzed. Some conclusions regarding the structure of the amorphous solid are derived from the spectra.
Raman spectra and phase diagram of fluorine at pressures up to 6 GPa and temperatures between 10 and 320 K
Authors: Schiferl D. , Kinkead S. , Hanson R. C. , Pinnick D. A.
Published: July 2002

The Raman spectra and phase diagram of fluorine were studied in a diamond-anvil cell up to 6 GPa over the temperature range 10–300 K. The sample slowly reacted with the diamond anvils to form CF4. The vibron frequencies in α-F2 and β-F2, as well as the lattice modes in α-F2, were determined as a function of pressure. No new phases were discovered. The α–β phase boundary can be fit with an equation of the Simon form: Pαβ (GPa)=−0.385(10) +4.80(9)×10−4 Tαβ1.75 (K). The melting curve was established to lie between limiting curves: Puppermelt (GPa)=−0.107+1.01×10−4T1.75melt (K) and Plowermelt (GPa)=−0.140+1.32×10−4 T1.75melt (K).
The dynamic quadrupole and dipole–quadrupole polarizabilities of helium
Authors: Bishop David M. , Lam Brenda
Published: July 2002

The dynamic quadrupole and dipole−quadrupole polarizabilities of heliu are calculated, using variation−perturbation theory and electron− correlated wavefunctions.(AIP)
Systematic corrections to Flory–Huggins theory: Polymer–solvent–void systems and binary blend–void systems
Authors: Bawendi M. G. , Freed Karl F.
Published: July 2002

A field theory, presented earlier by us, which is formally an exact mathematical solution of the Flory–Huggins lattice model, is used to evaluate corrections to Flory–Huggins mean field theory in a systematic series expansion in the inverse of the lattice coordination number and in the nearest-neighbor interaction energies. We explicitly determine the first few corrections to the free energy of mixing for polymer–solvent–void systems and for systems containing two kinds of polymers and voids (binary blend–void systems). Applications of the theory to the calculation of equations of state and effective Flory χ parameters are discussed. We compare the result of our theory with the lattice Monte Carlo data of Dickman and Hall for the chain insertion probability and for the pressure in a system of athermal chains and voids. Good quantitative agreement is found. We discuss shortcomings of the lattice model in representing real polymers as well as possible extensions of the model to remedy these deficiencies.
A surface enhanced hyper-Raman scattering study of pyridine adsorbed onto silver: Experiment and theory
Authors: Golab J. T. , Sprague J. R. , Carron K. T. , Schatz G. C. , Van Duyne R. P.
Published: July 2002

This paper presents a combined experimental and theoretical study of the hyper-Raman spectrum of pyridine adsorbed onto roughened silver electrodes. The surface enhanced hyper-Raman spectra (SEHRS) were measured using a focused cw mode-locked Nd:YAG laser with a peak power density of approximately 107 W/cm2 . Dominant bands in the pyridine spectra are the same (totally symmetric) bands as have been seen in the corresponding Raman (SERS) spectrum, although the relative intensities are different. To interpret these spectra, we present a semiempirical molecular orbital method for determining excitation energies, polarizability derivatives, and hyperpolarizability derivatives that is based on the π-electron Pariser–Parr–Pople (PPP) method. An empirical molecular force field is used to derive vibrational information, and the accuracy of the spectra is assessed by comparison with normal Raman spectra for liquid pyridine and with SERS spectra. The resulting SEHRS spectra are in good agreement with the measured spectra, particularly with respect to the intensity changes in the dominant lines in going from SERS to SEHRS. In addition, the theoretical/experimental comparisons indicate that SEHRS is more sensitive to adsorbate orientation than is SERS since the nontotally symmetric modes are predicted to be comparable in SEHRS (but not SERS) intensity to the totally symmetric modes for orientations other than perpendicular. Most important, a comparison of theoretical and experimental SEHRS/SERS ratios suggests that the enhancement factor associated with SEHRS is on the order of 1013 which is much larger than the 106 enhancement seen for SERS.
Optical absorption and fluorescence spectroscopy of stable diacetylene oligomer molecules
Authors: Warta R. , Sixl H.
Published: July 2002

The first fluorescence emission spectra of polydiacetylene oligomer molecules are presented. The spectral features of the low temperature absorption and emission spectra of monomer, dimer, trimer, tetramer, and polymer molecules are discussed. The preparation of the stable oligomer molecules via the reactive intermediates of the solid state polymerization is described.
Surface exclusion effects in adsorption processes
Authors: Schaaf P. , Talbot J.
Published: July 2002

Most of the adsorption experiments of proteins on solid surfaces are interpreted with the Langmuir kinetic equation. We show that this equation does not accurately describe the surface exclusion effect. We propose improvements in two limiting cases; (i) the particles, once adsorbed, can diffuse rapidly on the surface; (ii) the particles can neither diffuse on the surface, nor desorb from it; the so called random sequential adsorption (RSA) model. In the last case, we compare our results with computer simulations.
Analysis of CH2 ã 1A1 (1,0,0) and (0,0,1) Coriolis-coupled states, ã 1A1–X̃ 3B1 spin–orbit coupling, and the equilibrium structure of CH2 ã 1A1 state
Authors: Petek Hrvoje , Nesbitt David J. , Darwin David C. , Ogilby Peter R. , Moore C. Bradley , Ramsay D. A.
Published: July 2002

The symmetric and antisymmetric stretch spectra of ã 1A1 CH2 are measured with Doppler-limited resolution by infrared flash-kinetic spectroscopy between 2600 and 3050 cm−1. The spectra are significantly perturbed by spin–orbit interactions between near-resonant levels of X̃ 3B1 in both lower and upper levels of the transitions, and by Coriolis interaction between symmetric and antisymmetric stretch states. The triplet character of the perturbed levels is detected by observing broadening of MJ structure in a magnetic field of about 1 kG. The levels that are likely to be perturbed by triplet methylene are identified using the experimentally observed and theoretically calculated 3B1 energy level structure. The term values of the remaining symmetric and antisymmetric stretch levels are simultaneously fit to Watson’s Hamiltonian including Coriolis coupling. This analysis provides the remaining information needed to determine the equilibrium rotational constants [Ae =19.8054(206), Be=11.2489(45), and Ce =7.2386(36) cm−1 ], and, therefore, the equilibrium structure (re=1.107 Å, and θe =102.4°) of this fundamentally important carbene. The experimental and theoretical structures are in good agreement.
Liquid theory for band structure in a liquid
Authors: Xu Bing-Chang , Stratt Richard M.
Published: July 2002

When electron correlation effects are small, the set of energy levels available to both the localized and the delocalized individual electrons (the band structure) is the starting place for determining the macroscopic electronic properties of a substance. Calculating the band structure in any disordered medium, however, requires facing the problem that there will always be a distribution of geometries in the material—at least the local parts of which must be accounted for in order to get any reasonable results. In a liquid this requirement means that the liquid structure plays an important role. We show in this paper that the band structure in a liquid is completely and rigorously determined by the equilibrium behavior of an ‘‘effective’’ liquid with artificial internal degrees of freedom. This mapping implies that standard liquid theory methods (which automatically build in the correct liquid structure) can be used to find the electronic energy levels. As illustration, we use the mean-spherical approximation (MSA) to derive a simple expression for the density of states that is accurate at all but the lowest densities. We further show that this particular MSA theory is identical to an apparently different theory derived recently by Logan and Winn—which makes both theories identical to the so–called EMA theory of Roth. An even more general correspondence exists between our exact formalism and the exact formalism of Logan and Winn, though any given approximation might be more natural in one approach then in another.
Coupled cluster energy derivatives. Analytic Hessian for the closed-shell coupled cluster singles and doubles wave function: Theory and applications
Authors: Koch Henrik , Jensen Hans Jo/rgen Aa. , Jo/rgensen Poul , Helgaker Trygve , Scuseria Gustavo E. , Schaefer Henry F.
Published: July 2002

Expressions for coupled cluster molecular energy derivatives up to third order are presented and the molecular gradient and Hessian are implemented in a computer code for the CCSD wave function. Sample calculations on water and hydrogen peroxide indicate that significant savings are obtained by calculating the Hessian analytically rather than by finite differences.
A density functional theory for polymers: Application to hard chain–hard sphere mixtures in slitlike pores
Authors: Woodward Clifford E.
Published: July 2002

A density functional theory for polymer fluids is derived. The form of the theory is identical to a continuum self-consistent-field theory but we show that with the correct ‘‘mean field,’’ the theory is exact. We apply the theory to a polymer fluid made up of tangential hard spheres and borrow from a successful density functional theory for simple hard spheres to obtain a nonlocal functional expression for this mean field. The latter depends only upon the monomer density. This approximation is used to treat the problem of a mixture of such polymers and simple hard spheres, contained in slitlike pores. Comparison with simulations show that the theory, though simple in structure, and containing a single adjustable parameter, is surprisingly accurate.
Interdiffusion and self-diffusion in polymer mixtures: A Monte Carlo study
Authors: Deutsch H. P. , Binder K.
Published: July 2002

A lattice model for dense polymer solutions and polymer mixtures in three dimensions is presented, aiming to develop a model suitable for efficient computer simulation on vector processors, with a qualitatively realistic local dynamics. It is shown that the bond fluctuation algorithm for a suitable set of allowed bond vectors has the property that due to the excluded volume constraint no crossing of bonds by local motions can occur, and entanglement restrictions thus are fully taken into account. For athermal binary (AB) symmetrical polymer mixtures, the dependence of both self-diffusion coefficient and interdiffusion coefficient on polymer density is obtained, simulating a thin film geometry where a film of polymer A is coated with a film of polymer B. For one density, the dependence of the interdiffusion coefficient on an attractive energy between unlike monomers is also studied. For weak attraction an enhancement of interdiffusion proportional to this energy occurs. For strong attraction, however, a rather immobile tightly bound AB layer forms in the interface which hampers further unmixing.
Normal mode analysis of the velocity correlation function in supercooled liquids
Authors: Madan B. , Keyes T. , Seeley G.
Published: July 2002

Further results are given on the application of harmonic normal mode analysis to liquid state dynamics. A new configuration space-normal mode theory is presented for the velocity correlation function. Here, the hopping rate, ωv, of the phase point among the local minima of the potential in configuration space plays a key role. A new relation is given connecting the unstable mode lobe, 〈ρ(ω)〉u, of the averaged density of states to the topology of the potential in configuration space. This relation is used to express the hopping rate in terms of 〈ρ(ω)〉u. With the hopping rate in hand, the velocity correlation function is calculated from the theory in supercooled Lennard-Jones argon at unit density and over the temperature range T=150 K down to T=80 K, where the theory predicts a glass transition. Agreement between theory and computer simulation is excellent. The predicted hopping rates are also in good agreement with the simulation results of Stillinger and Weber.
Activity coefficients of model aqueous electrolyte solutions: Sensitivity to the short range part of the interionic potential
Authors: Ursenbach C. P. , Wei Dongqing , Patey G. N.
Published: July 2002

Activity coefficients are calculated for model aqueous alkali halide solutions. The theoretical results are found to be very sensitive to details of the interionic potential employed and good agreement with experiment is obtained by simply superimposing an additional r −n repulsive interaction upon charged hard sphere potentials. The ‘‘best fits’’ to the experimental data involve interesting changes in the solution structure. In particular, our results suggest that the strong deviations from limiting behavior observed for some alkali halide solutions (e.g., NaCl) are closely associated with structures involving solvent separated as opposed to contact ion pairs.