bluuebunny/crossref_metadata_2025 huggingface dataset:
Ab initio study of the alternating current impedance of a molecular junction
Authors: Baer Roi , Seideman Tamar , Ilani Shahal , Neuhauser Daniel
Published: February 2004

The small-bias conductance of the C6 molecule, stretched between two metallic leads, is studied using time-dependent density functional theory within the adiabatic local density approximation. The leads are modeled by jellium slabs, the electronic density and the current density are described on a grid, whereas the core electrons and the highly oscillating valence orbitals are approximated using standard norm-conserving pseudopotentials. The jellium leads are supplemented by a complex absorbing potential that serves to absorb charge reaching the edge of the electrodes and hence mimic irreversible flow into the macroscopic metal. The system is rapidly exposed to a ramp potential directed along the C6 axis, which gives rise to the onset of charge and current oscillations. As time progresses, a fast redistribution of the molecular charge is observed, which translates into a direct current response. Accompanying the dc signal, alternating current fluctuations of charge and currents within the molecule and the metallic leads are observed. These form the complex impedance of the molecule and are especially strong at the plasmon frequency of the leads and the lowest excitation peak of C6. We study the molecular conductance in two limits: the strong coupling limit, where the edge atoms of the chain are submerged in the jellium and the weak coupling case, where the carbon atoms and the leads do not overlap spatially.
Calculation of electrostatic and polarization energies from electron densities
Authors: Ma Yuguang , Politzer Peter
Published: February 2004

We investigate procedures for calculating the electrostatic and polarization energies, Ees and Epol, associated with noncovalent interactions. The starting points are the electron densities of the isolated components and the complex; these could be obtained either computationally or experimentally. A slightly modified version of a scheme proposed by Gavezzotti is used to carry out numerical integrations over these electron densities. Our approach to estimating Epol is based upon partitioning the charge distributions of the components into overlapping and nonoverlapping regions. The effects of varying the integration parameters, computational techniques and basis sets are examined in detail for several noncovalently bound molecular dimers. Our results are in good agreement with the values of Ees and Epol produced by other methods, which require analytical integrations over interaction Hamiltonian matrix elements.
Pressure-induced formation of diblock copolymer “micelles” in supercritical fluids. A combined study by small angle scattering experiments and mean-field theory. I. The critical micellization density concept
Authors: Raudino A. , Lo Celso F. , Triolo A. , Triolo R.
Published: February 2004

We developed a simple mean-field theory to describe polymer and AB diblock copolymer phase separation in supercritical (SC) fluids. The highly compressible SC fluid has been described by using a phenomenological hole theory, properly extended to consider the solvent/polymer/vacancy pseudoternary mixture. The model has been applied to describe the phase behavior of AB-diblock copolymers under the assumption of a strong solvent selectivity for just one copolymer chain. In our model the solvent selectivity is a strong function of the external pressure because in compressible fluids vacancies reduce the number of favorable solvent–polymer contacts. The combined effect of the pressure on the average solvent quality and selectivity for a single polymer chain makes the phase behavior of a diblock copolymer in SC fluids quite complex. Small angle neutron and x-ray scattering (SANS and SAXS) measurements have been performed on SC–CO2 solutions of different AB-diblock copolymers containing a perfluorinated chain. The data obtained over a wide range of pressure and temperature confirm our theoretical predictions.
Correlations in simulated model bilayers
Authors: Stecki J.
Published: February 2004

Molecular dynamics simulations of a model bilayer in a solvent are used to determine the structure factors S(q) and other correlations at vanishing lateral tension of the system and at positive and negative tensions as well. The role played by curvature in the absence of surface tension, metastability, and phase transitions, and the dependence on the specific area, are discussed. Possible height-height correlation functions S(q) are examined and tested.
Monte Carlo simulations of flexible polyanions complexing with whey proteins at their isoelectric point
Authors: de Vries R.
Published: February 2004

Electrostatic complexation of flexible polyanions with the whey proteins α-lactalbumin and β-lactoglobulin is studied using Monte Carlo simulations. The proteins are considered at their respective isoelectric points. Discrete charges on the model polyelectrolytes and proteins interact through Debye–Hückel potentials. Protein excluded volume is taken into account through a coarse-grained model of the protein shape. Consistent with experimental results, it is found that α-lactalbumin complexes much more strongly than β-lactoglobulin. For α-lactalbumin, strong complexation is due to localized binding to a single large positive “charge patch,” whereas for β-lactoglobulin, weak complexation is due to diffuse binding to multiple smaller charge patches.
On the efficient path integral evaluation of thermal rate constants within the quantum instanton approximation
Authors: Yamamoto Takeshi , Miller William H.
Published: February 2004

We present an efficient path integral approach for evaluating thermal rate constants within the quantum instanton (QI) approximation that was recently introduced to overcome the quantitative deficiencies of the earlier semiclassical instanton approach [Miller, Zhao, Ceotto, and Yang, J. Chem. Phys. 119, 1329 (2003)]. Since the QI rate constant is determined solely by properties of the (quantum) Boltzmann operator (specifically, by the zero time properties of the flux–flux and delta–delta correlation functions), it can be evaluated by well-established techniques of imaginary time path integrals even for quite complex chemical reactions. Here we present a series of statistical estimators for relevant quantities which can be evaluated straightforwardly with any nonlinear reaction coordinates and general Hamiltonians in Cartesian space. To facilitate the search for the optimal dividing surfaces required by the QI approximation, we introduce a two-dimensional quantum free energy surface associated with the delta–delta correlation function and describe how an adaptive umbrella sampling can be used effectively to construct such a free energy surface. The overall computational procedure is illustrated by the application to a hydrogen exchange reaction in gas phase, which shows excellent agreement of the QI rates with those obtained from quantum scattering calculations.
Formation of HArF in solid Ar revisited: Are mobile vacancies involved in the matrix-site conversion at 30 K?
Authors: Khriachtchev Leonid , Lignell Antti , Räsänen Markku
Published: February 2004

The HArF molecule can occupy in solid Ar thermally unstable and stable configurations, and their microscopic structure is not understood at the moment. We present additional experimental results on the formation of two HArF configurations and analyze them with emphasis on possible reactions of the unstable configuration with matrix vacancies to form the stable configuration. We conclude that the existing computational scenarios do not describe fully the present experimental data. In order to explain qualitatively the experimental results, two tentative models are discussed. The first model is based on local mobility of matrix vacancies produced during photolysis and the second model considers isomerization of the HArF @ Arn supermolecule. More importantly, the present results constitute the experimental basis for future theoretical studies.
The C3-bending levels of the C3–Ar complex studied by optical spectroscopy and ab initio calculation
Authors: Zhang Guiqiu , Lin B.-G. , Wen S.-M. , Hsu Yen-Chu
Published: February 2004

The Ã−X̃ electronic transition of C3–Ar, near 405 nm, has been studied by both laser-induced fluorescence and wavelength-resolved emission techniques. Emission spectra have been recorded from 14 vibrational levels of the Ã state of C3–Ar; these spectra consist of progressions in the ground state v2 and v4 vibrations (the in- and out-of-plane C3-bending motions, respectively). With increasing bending excitation, these ground state levels shift progressively downwards compared to those of free C3, indicating that the van der Waals complexes are becoming more tightly bound. The level structure of the two vibrations of C3–Ar has been fitted to a perturbed harmonic oscillator model, where the potential function has the form V=V1r cos θ+V2r2 cos 2θ (r is the amplitude of the C3-bending motion and θ gives the orientation of the rare gas atom relative to the plane of the bent C3 molecule). Ab initio calculations have been carried out for C3–Ar at the coupled-cluster singles, doubles (and triples)/correlation consistent polarization valence quadruple-zeta level. They predict that the C3–Ar complex is nearly T shaped at equilibrium, and that as the C3 molecule bends away from the linear configuration, the preferred orientation is “arrow” shaped. From the results of the best fit to the model and the emission spectral intensities, the relative orientation of the out-of-plane π electron of the Ã-state complex and the Ar atom has been estimated. No bands of the Ar complex were found near the C3, Ã−X̃, (0,0) band, consistent with the fact that the Ã 1Πu, υ=0 level of free C3 is strongly perturbed by triplet levels. In the excitation spectra of the Ar complex, the bands with υb′>0 show redshifts of about 16–36 cm−1 compared to those of free C3, indicating that the Ã-state complex in these levels is more tightly bonded than the X̃-state complex.
Auxiliary field Monte Carlo for charged particles
Authors: Maggs A. C.
Published: February 2004

This article describes Monte Carlo algorithms for charged systems using constrained updates for the electric field. The method is generalized to treat inhomogeneous dielectric media, electrolytes via the Poisson–Boltzmann equation and considers the problem of charge and current interpolation for off lattice models. We emphasize the differences between this algorithm and methods based on the electrostatic potential, calculated from the Poisson equation.
Solid–liquid phase coexistence of the Lennard-Jones system through phase-switch Monte Carlo simulation
Authors: Errington Jeffrey R.
Published: February 2004

The phase-switch Monte Carlo method of Wilding and Bruce [Phys. Rev. Lett. 85, 5138 (2000)] is extended to enable calculation of solid–liquid phase coexistence for soft potentials. The method directly accesses coexistence information about a system while avoiding simulation of the interfacial region. Order parameters are introduced that allow one to define a path that connects liquid and crystalline phases. Transition matrix methods are employed to bias the sampling such that both phases are sampled in a rapid and efficient manner. Coexistence properties are determined through an analysis of specific volume probability distributions, which are generated naturally during a biased simulation. The approach is demonstrated with the Lennard-Jones system. Finite-size effects are examined and compared to those for the hard sphere system. In addition, two techniques are considered for accounting for long-range interactions. The methodology presented here is general and therefore provides a basis for its application to other soft systems.
Toward a theory of the magnetoimpurity states of electrons in conductors
Authors: Ermolaev A. M. , Rashba G. I.
Published: February 2004

Conductors containing impurity atoms capable of localizing electrons in a magnetic field are considered with the use of functional methods for investigating electronic magnetoimpurity states. These states are taken into account in the Keldysh formalism. The theory is illustrated for the example of a two-dimensional electron gas characterizing impurity atoms in a quantizing magnetic field. The characteristics of the magnetoimpurity states are calculated in the case of a Gaussian separable impurity potential.
Influence of the magnetic dipole interaction on the properties of magnetic vortices in particles of small size
Authors: Kovalev A. S. , Prilepsky J. E.
Published: February 2004

Two type of plaquette systems are proposed for modeling a vortex in an easy-plane two-dimensional Heisenberg ferromagnet with the dipole–dipole interaction of the magnetic moments of the lattice taken into account. The first plaquette describes a system with the boundary spins fixed in the easy plane perpendicular to the “surface” of the magnet, while in the second plaquette the boundary spins are fixed parallel to the “surface.” The properties of static vortex ordering of the magnetization and the transformation of the spectra of eigenmodes of the plaquettes upon a change in the parameters of the easy-plane exchange anisotropy and dipole–dipole interaction are investigated. It is shown that the region of stability of the various vortex solutions on the plane of the anisotropy and dipole–dipole interaction parameters is substantially different for these two plaquettes. The differences between the results obtained for the dynamics and structure of the vortex solutions with the dipole interaction taken into account and those obtained in a treatment including only the exchange interaction are analyzed.
Strain hardening of metals and alloys in the superconducting state
Authors: Pustovalov V. V. , Kusmenko I. N. , Isaev N. V. , Fomenko V. S. , Shumilin S. É.
Published: February 2004

The strain hardening of single crystals of pure Al and a Pb–In alloy in the normal (N) and superconducting (S) states is investigated. It is found that the strain hardening coefficients satisfy the inequality θS>θN irrespective of whether the SN transition is brought about by an external magnetic field or temperature.
Nonlinear acoustic effects arising from dislocations in niobium single crystals
Authors: Pal-Val P. P. , Natsik V. D. , Pal-Val L. N. , Semerenko Yu. A.
Published: February 2004

The temperature dependence of the logarithmic decrement δ and dynamic Young’s modulus E in niobium single crystals of high purity is investigated in a wide interval of low temperatures 3 K⩽T⩽300 K. Measurements are made by means of a two-component composite vibrator method under excitation of longitudinal vibrations with a frequency of 88 kHz and a relative amplitude of the acoustic strain varying in the range 5×10−9<ε0<7×10−5. In the temperature interval 50–200 K a nonlinear effect is registered: an amplitude-dependent contribution to the decrement and Young’s modulus is observed when the amplitude of the ultrasound reaches a threshold value ε0c≳10−5. The dependence of the threshold value ε0c on temperature and on preliminary plastic deformation of the sample is established. The amplitude dependences δ(ε0) and E(ε0) at ε0>ε0c and their transformation with temperature are investigated in detail. Analysis shows that the nonlinear effects are due to the breakaway of ultrasonically excited dislocations from impurity atoms. The experimental results are in satisfactory agreement with the conclusions of the Granato–Lücke theory of athermal dislocation hysteresis and its generalization to the case of thermally activated hysteresis proposed by Indenbom and Chernov.
On the magnetization of a low-dimensional electron gas at high magnetic fields
Authors: Gokhfeld V. M.
Published: February 2004

The magnetic moment of an electron gas is calculated for two-dimensional and one-dimensional models of the energy spectrum of the particles in the quantum cases, when only the lower magnetic subbands are filled.
Work Function of the (100) Surface of Tungsten-Rhenium Alloys
Authors: Abey A. E.
Published: February 2004

The work functions of the (100) surfaces of tungsten, tungsten with 2% bulk rhenium, and tungsten with 6% bulk rhenium single crystals were measured using the Shelton method. The values of 4.55 V, 4.56 V, and 4.59 V were obtained for the three samples, respectively. The A values for the three samples were 100, 110, 130 A/cm2 °K2, respectively. Electron-beam microprobe results indicated a surface concentration of 0.67% rhenium in the 2% bulk sample and 2% rhenium in the 6% bulk sample. The loss of rhenium from the surface has been attributed to the preferential sublimation during outgassing of the samples. The relative changes in work functions are consistent with the shift in the voltage knees on the ln i vs VR curves. The actual shape of the ln i vs VR curve is discussed.
Linear Unfolding Methods and Optimization for X-Ray and Similar Spectra
Authors: Porteus J. O.
Published: February 2004

The problem of correcting x-ray spectra and similar data for linear smearing or resolution loss, i.e., unfolding, is reexamined with emphasis on accuracy limitations. The advantage of optimum smoothing or filtering in minimizing the effects of experimental uncertainties is demonstrated by comparing three methods when applied to a simple example. The related roles of nonstationarity and the sample span in optimization are considered in detail with reference to such common difficulties as flattening of line peaks, excessive noise in line tails, and spurious oscillations near large changes in spectral intensity. Refinements tending to alleviate these difficulties are discussed, along with ultimate limitations. Also discussed is a means of sampling integrated spectra, which is useful when the unfolding includes a differentiation, as in processing electron energy-loss spectra measured with a retarding field.
Electron-Transport Phenomena in Thermionic Converter Plasmas
Authors: Nighan William L.
Published: February 2004

An analysis of electron transport phenomena is developed in a form consistent with conditions known to exist in thermionic converter plasmas. Particular attention is given to the contribution of electron-ion collisions to the electron momentum transfer collision frequency, and the resultant dependence of the plasma transport coefficients on ion density which itself depends upon interelectrode position. Experimentally determined spatial variations of electron density and electron temperature are analyzed numerically for a variety of converter operating conditions and quantitative estimates are made of: the spatial variation of electric field intensity and electrostatic potential in the plasma, the spatial variation of electron kinetic energy flux, and the relative importance of electron energy gain and loss processes in the interelectrode region. On the basis of these results, an interpretation is made of the role played by the cesium neutralization plasma in determining converter performance.
Electron-Optical Properties of a Quadrupole Quadruplet Projector Lens
Authors: Kawakatsu Hisazo , Vosburgh Kirby G. , Siegel Benjamin M.
Published: February 2004

A magnetic quadrupole quadruplet projector system has been designed, built, and studied, which has excellent characteristics for high-resolution and high-energy electron microscopes. The focal length is continuously variable from 0.7 mm to 3.6 mm by adjusting two current supplies. The third-order distortion aberrations and first-order chromatic aberration were calculated using measured field distributions. These aberrations change very little over the whole range of focal lengths and are always lower than these aberrations in the best set of rotationally symmetric lenses designed to cover this range of focal lengths.
Effect of Neutron Irradiation at 4.5°K on Guinier-Preston Zone Formation in Aluminum-Zinc Alloys
Authors: Horak J. A. , Blewitt T. H. , Fine M. E.
Published: February 2004

The effect of fast neutron irradiation at 4.5°K on Guinier-Preston zone formation in the reverted as well as quenched alloys of Al-1.70 to 5.27 at.% Zn has been studied by means of electrical resistivity measurements. There is no indication of zone formation during irradiation nor during stage I and stage II of the isochronal annealing, but there is a large enhancement in the kinetics of zone formation during stage III annealing. The large increase in resistivity associated with zone formation begins at about 150° with the resistivity maximum occurring at about 260°K. Comparison with stage III recovery in pure aluminum suggests that vacancy migration, which controls Guinier-Preston zone formation in these alloys, results in stage III recovery in pure aluminum. In addition, there is an indication of solute-atom-vacancy binding and/or vacancy trapping by the zones since stage III extends to higher temperatures in the alloys than in pure aluminum.
Magnetic Behavior of Reduced Nickel Oxide Single Crystals
Authors: Srivastava Ramji
Published: February 2004

Magnetic behavior of antiferromagnetic nickel oxide single crystals, both in normal and reduced state, has been studied by magnetic susceptibility, anisotropy in susceptibility, and torque measurements. The observed magnetic behavior is interpreted in terms of T- and S-type domains, which are normally present in a nickel oxide single crystal.
Electrical Properties of Single-Crystal Gallium Phosphide Doped with Zinc
Authors: Cohen Marvin M. , Bedard Fernand D.
Published: February 2004

Hall measurements have been made on epitaxially grown, zinc-doped gallium phosphide, from which the gallium arsenide substrate had been removed. The measurements were made in the temperature range from 300° to 4°K on samples whose room-temperature carrier concentration varied from 7×1015 to 7×1018 cm−3. Using both van der Pauw and conventional Hall techniques on those samples which show a high degree of compensation, the variation of the ionization energy of zinc with concentration was determined. Room-temperature hole mobility varied from 60 to 100 cm2· V−1· sec−1. These observed hole mobilities are in good agreement with the calculated values of mobility due to reduced scattering by acoustical and optical phonons, and are shown to be too small to be accounted for by the mechanism of impurity scattering below 100°K. Resistivity measurements made in the temperature range 77°–4.2°K show that in the impurity band conduction range, gallium phosphide behaves in a manner similar to impure germanium.
Computed Shock Response of Porous Aluminum
Authors: Hofmann Ronald , Andrews Dudley J. , Maxwell D. E.
Published: February 2004

The shock response of powdered aluminum compacted by a driver plate was simulated with a one-dimensional Lagrangian continuum mechanics computer program. The porous aluminum was mocked up by a series of flat plates separated by gaps to obtain an initial density of 1.35 g/cm3 for the ``powder.'' The compaction process was followed in detail for two cases of driver-plate conditions corresponding to two Stanford Research Institute experiments. The calculations showed an approach to equilibrium behind the shock in each of the porous samples. The equilibrium states established were found to be consistent with the Rankine-Hugoniot jump conditions applied to aluminum of density 1.35 g/cm3. These states did not lie on the Hugoniot curve of solid-density aluminum. This was shown to be due to the significant internal-energy dependency in the equation of state. The calculated results were in good agreement with the SRI experiments. Application of this computational model to more complex cases is indicated.
X-Ray Measurements of Faulting in Ag–Pd Alloys
Authors: Rao P. Rama , Rao K. Krishna
Published: February 2004

A detailed study of x-ray line broadening has been made in severely cold-worked filings of pure Pd and five Ag–Pd alloys containing 20, 40, 60, 75, and 80 wt.% Pd. Debye-Scherrer line shapes recorded in a proportional counter diffractometer have been analyzed both by Fourier and integral breadth methods for the evaluation of effective particle sizes (De)hkl and rms strains (〈ε2〉hkl1/2). It has been found that both (De)hkl and 〈ε2〉hkl1/2 are anisotropic and vary little with composition. The importance of faulting in the particle-size broadening could also be clearly observed. The intrinsic fault densities (α) estimated from peak shifts, although small, show a marked variation with solute content. Following Otte's analysis, intrinsic fault energies (γ) have been deduced using measured α, 〈ε2〉hkl1/2, and available shear modulus values. γ values thus arrived at have been compared with those obtained for these alloys by Harris et al. using a texture method. It was concluded that interaction of d electrons has a marked influence in enhancing the fault energy values and consequently on the stability of the fcc structure. The increase in the number of s electrons from 0.6 to 1 per atom in alloys ranging in composition from 40% Pd to 0% Pd does not appear to affect γ value.
Proton Blocking in Cubic Crystals
Authors: Barrett C. S. , Mueller R. M. , White W.
Published: February 2004

Photographic recording of low-energy (∼100 keV) protons backscattered from thick bcc, fcc, and diamond cubic crystals has been used to obtain blocking patterns characteristic of crystal type. Photographs of blocking patterns symmetric about the [001] direction are presented, together with the corresponding standard linear (gnomonic) projections of the directions and planes of the crystals. The projections have been drawn to facilitate assignment of planar and directional indices corresponding to the lines and spots, respectively, of the blocking patterns for cubic crystals. Simple densitometric and visual measurements of the planar intensities and angular widths of the spots, as taken directly from nuclear emulsion film, or photographic enlargements, are shown to correlate with crystal structure.
Birefringence Effects in Unplasticized and Plasticized Polyvinyl Chloride
Authors: Andrews R. D. , Kazama Y.
Published: February 2004

Some basic birefringence effects in a PVC homopolymer have been investigated as a function of temperature, stress or strain level, and plasticizer content. The behavior of the stress-optical coefficient (SOC) has been investigated over a broad temperature range extending down to liquid-nitrogen temperature for both unplasticized polymer and polymer containing weight fractions of 0.1, 0.3, and 0.5 dioctyl phthalate (DOP), 0.3 dioctyl adipate (DOA), and 0.3 tricresyl phosphate (TCP). The SOC of unplasticized PVC is negative below the glass transition and shows a sign change to large positive values above the glass transition. This progressive change from negative to positive values is also observed when stress level is increased in the glassy state below Tg, or when plasticizer content is progressively increased. The SOC goes to large positive values above Tg for all plasticized polymers. A positive SOC can therefore be considered characteristic of rubbery state behavior of PVC, and a positive sign will consequently be characteristic of orientation birefringence as well. Orientation birefringence increases with increasing temperature in all cases; however, its temperature coefficient (fractional change per °C) is strongly dependent on both plasticizer content and plasticizer type. The theoretical significance of these results is discussed.
Peierls Stress for Screw Dislocations in Polyethylene
Authors: Peterson James M.
Published: February 2004

The Peierls stress for the motion of screw dislocations with [001] Burgers vectors in orthorhombic polyethylene has been investigated. Assumptions made are that a polymer chain moves along [001] as a unit and no intramolecular interactions need be considered, that the chains do not twist during dislocation motion, and that material displacements around the dislocation are the same as in continuum anisotropic theory. The force between chains is shown to vary nearly sinusoidally with relative displacement along the chain direction. The shear moduli C44 and C55 are derived. The Peierls stresses are expressed relative to C55. The results show that for reasonable values of C44/C55, the Peierls stress for screw dislocation glide on a (100) slip plane is about 5×10−5 C55, while the Peierls stress for dislocation glide on a (010) slip plane is about 0.1 C55. For screw dislocation glide, it is concluded that dislocation motion on a (100) slip plane is probable, but that dislocation motion on a (010) slip plane is highly unlikely. Any slip plane other than (100) for these screw dislocations would necessitate crossing (100) planes, and Peierls stresses arising from glide on (010) slip planes would then be felt. Hence it is concluded that (100) is the only likely slip plane for the glide of such screw dislocations.
Lattice-Frequency Studies of Crystalline and Fold Structure in Polyethylene
Authors: Bank M. I. , Krimm S.
Published: February 2004

The assignment of the 71 cm−1 band in the infrared spectrum of polyethylene to the B1u translational lattice vibration has been confirmed by dichroism studies on an a-axis oriented sample. This permits confident use of the Tasumi-Krimm calculations for the dependence of this frequency of unit-cell parameters. The results of these calculations have been applied to the analysis of the observed lattice frequency differences between odd and even n-paraffins, and it is shown that the frequency shifts are interpretable in terms of changes in unit cell parameters resulting from different methyl end-group packing. The variation of the lattice frequency with degree of branching observed in a series of low-density polyethylenes can be understood on the basis of incorporation of branches in the lattice. The dependence of the lattice frequency in high-density polyethylene on the physical state of the specimen can be correlated with different constraints imposed by the fold regions on the chain packing. These results indicate that the folds must be considered to be ``tight'' rather than ``loose.''
Profile Measurements of Plasma Columns using Microwave Resonant Cavities
Authors: Vlachos Michael A. W. , Hsuan Hulbert C. S.
Published: February 2004

The density profile of a positive column of a gas discharge is measured by the comparison of two microwave cavity measurements. The two cavity modes used are the TE011 and TM020.
Measurement of Plasma Electron Distributions using Microwave Cavities
Authors: Burkley C. J. , Sexton M. C.
Published: February 2004

A new technique is presented whereby electron radial distributions in the positive column of a mercury plasma were determined using simultaneous measurements from TM020 and TE011 microwave cavities. Experimental results are compared with a comprehensive set of theoretical radial distributions.
Nonmixing Cells due to Crucible Rotation during Czochralski Crystal Growth
Authors: Carruthers J. R. , Nassau K.
Published: February 2004

The presence of a stationary crystal in a rotating liquid gives rise to a stationary liquid cell beneath the crystal as predicted from the Taylor-Proudman theorem. Outside this cell, the liquid rotates as an almost solid body, while inside the cell it is shown that a predictable inner flow must arise. The effects of combined crystal and crucible rotations produce a resultant force field which controls the convection. The liquid flow patterns are discussed in detail. The presence and form of thermal convection in rotating liquids are discussed specifically for typical conditions occurring during Czochralski growth. The observation of such flow behavior must be made carefully, since the relative observer motion effect can obscure the essential flow patterns. Previously observed effects as well as observations and measurements from new simulation experiments are explained by the treatment. Stagnation surfaces which prevent complete bulk liquid mixing in the crucible exist for all cases, except when the crucible rotation rate is zero. When crystal and crucible rotate in the same direction one stagnation surface separates two nonintermixing regions; during counterrotation there can be two stagnation surfaces and three nonintermixing regions. Practically speaking, the stagnation surfaces are absent when the crucible rotation rate is very small compared to the crystal-rotation rate.
Double Schottky-Barrier Capacitance on Trigonal Selenium
Authors: Lakatos A. I. , Roberts G. G.
Published: February 2004

The low-frequency parallel capacitance and resistance of back-to-back metal-semiconductor Schottky barriers with a series bulk semiconductor resistance Rs≠0 between them has been measured on trigonal Se with In electrodes. Differential capacitance vs bias-voltage measurements show a sharp minimum close to V = 0, similar to that observed for Ge–Si (n-n) isotype heterojunctions. [C. van Opdorp and H. K. J. Kanerva, Solid-State Electron. 10, 401 (1967).] This minimum is due to the relative voltage dependence of the two Schottky barriers. Ionized acceptor concentrations of 1.8–7.5×1015 cm−3 and a low-frequency bulk conductivity of trigonal Se of 1.6×10−6 Ω−1 cm−1 are obtained from an equivalent circuit analysis.
Ultrahigh-Frequency Discharge in Atmospheric-Pressure Gas
Authors: Murayama Seiichi
Published: February 2004

A 2469 MHz discharge was generated at an open end of a coaxial waveguide, through which atmospheric-pressure gas flowed. Argon served as a plasma-forming gas, hydrogen and calcium as probe elements for the measurement of electron density and gas temperature, respectively, and sodium as a seed for increasing electron density. The density of excited hydrogen atoms, the excitation temperature, the electron density, the ionization temperature, and the gas temperature were measured at operating powers of 50, 200, and 400 W. The plasma in the close vicinity of the electrode (0 to 0.5 mm from the terminus of the inner conductor) was not in the local thermal equilibrium at any power level investigated, independently of the sodium seeding. At a distance between 1 and 3 mm from the electrode, on the contrary, the nonseeded plasma approached the local thermal equilibrium when the power was increased, while the seeded plasma exhibited a deviation from the local thermal equilibrium. The deviation from the local thermal equilibrium in the nonseeded plasma can be interpreted to be due to inhomogeneity of the plasma. In the outer zone of the seeded plasma, the excitation temperature was equal to the gas temperature but not to the ionization temperature. A separation of plasma components occurred which was depressed by the sodium seeding.
Electron Radiation Temperature Measurements in a CO2 Laser Amplifier
Authors: Noon J. H. , Blaszuk P. R. , Holt E. H.
Published: February 2004

An experiment is described which shows that no significant variation in average electron energy is associated with changes in optical gain in a 10.6 μm laser amplifier containing CO2 mixed with N2 and He. A microwave technique for measuring the average electron energy has been developed, using a 4-mm-bore discharge tube sealed by NaCl windows and enclosed in an X-band waveguide section.
LEED Studies of the (0001) Face of α-Alumina
Authors: Chang Chuan C.
Published: February 2004

The (0001) face of α-Al2O3 has been studied with low-energy electron diffraction. A surface etched with hot phosphoric acid showed the two-dimensional hexagonal 1×1 lattice expected of the bulk (0001) plane. This 1×1 lattice coincides with the lattice of aluminum atoms in this plane. Heating a freshly etched crystal to some temperature below 1400°C produced a surface with any of the following structures: the 1×1, a (3×3) rotated 30°, a (33×33) rotated 30°, or a [(31)1/2×(31)1/2] rotated ±tan−1 (3/11). Because of their rotations, all three superstructures were in registry with the 1×1. The ±tan−1 (3/11), almost ±9°, rotations of the (31)1/2 structure represent two domains rotated in opposite senses; each domain was 10−3 to 10−5 cm across. Above 1400°C, the entire surface was converted to the (31)1/2 structure. Once formed, all the observed structures were stable down to room temperature if the incident electron energy was kept below about 150 eV. Electron bombardment at above 150 eV converted all the superstructures to the 1×1 at crystal temperatures above 700°C. Above 1000°C, the (31)1/2 structure was regenerated fast enough so that the electron beam could not destroy it. The 3 and 33 structures were never observed to return once they were removed. In contrast, the (31)1/2 structure could be converted to the 1×1 either by irradiation with electrons while the crystal was held at 800°C, or by in situ etching at 1000°C with silicon, and regenerated by heating to 1200°C. The (31)1/2 structure is probably characteristic of the uncontaminated (0001) face.
A Peak-Stress Gauge for Determining the Yield of Underground Nuclear Detonations
Authors: Larson D. B. , Sisemore C. J.
Published: February 2004

A spherically shaped, stress-sensitive plastic gauge designed by Power and Lombard in 1966 is described in terms of a phenomenological model. The predictions presented are based upon a semiempirical description given by Allison and Hauver in their work on uniaxially loaded flat Plexiglas disks. The model description is shown to be consistent with both the data of Power and Lombard and data reported in this paper. The anomalous gauge behavior reported by Power and Lombard is attributed to effects of the external circuit. This observation has led to a new circuit design which allows a sensitive measure of stress from 50 to 230 kbar. These results suggest that this gauge would be useful for obtaining hydrodynamic energy yield when used in conjunction with computer-code calculations of shock-wave effects associated with underground nuclear detonations.
The Diffusion of Lead and Bismuth into Single Crystals of Undoped and Bismuth-Doped Lead Sulfide at 700°C
Authors: Zanio K. R. , Wagner J. B.
Published: February 2004

The simultaneous diffusion of 210Pb and 210Bi into undoped single crystals of lead sulfide and single crystals doped with 4×1018 and 4×1019 bismuth atoms/cm3 has been studied as a function of deviations from stoichiometry at 700°C. The diffusion coefficients for lead and for bismuth were identical. The identical values of the lead and bismuth diffusion coefficients suggest that Coulombic attraction between VPb′ and BiPb is negligible. Although the addition of 4×1018 atoms of bismuth to lead sulfide does not change the diffusion coefficients of 210Pb and of 210Bi relative to undoped lead sulfide for −6.5<log sulfur pressure (logps2) <0.0, the addition of 4×1019 atoms of bismuth does. This result and other evidence indicates a higher concentration of native lattice defects than previously considered. For logps2>−3.9, the diffusion coefficients in undoped and lightly doped (4×1018 Bi) material are proportional to ¼ logps2. Because the concentration of singly ionized cation vacancies is also proportional to ¼ logps2, it is concluded that the singly ionized cation vacancy is the predominant defect through which 210Pb and 210Bi diffuse and may be the majority cation defect in lead sulfide at 700°C. For a logps2<−3.9, the diffusion coefficients are independent of composition presumably because the as-grown crystals contain microprecipitates of lead which dissolve extremely slowly under low sulfur pressures. Hence, these crystals contained essentially a constant concentration of defects.
Thickness Measurements of Silicon Dioxide Films Over Small Geometries
Authors: Fried L. J. , Froot H. A.
Published: February 2004

A simple nondestructive interferometric technique to measure to thickness of transparent films on reflecting substrates in the range of 100–20 000 Å is described. Measurements can be made over areas as small as 0.1 mil in diameter. The method utilizes a microspectrophotometer, operating in the visible region of the spectrum, to measure the intensity of reflected light. The film thickness is calculated by one of two techniques depending upon whether the thickness is greater or less than 3000 Å. The error is less than 50 Å.
Theory of the DeBlois Experiment. II. Surface Imperfection in an Infinite Coil
Authors: Aharoni Amikam
Published: February 2004

The demagnetizing field caused by surface roughness is approximated by the field of a straight-cut, cylindrically symmetric, scratch or protrusion, in an infinite circular cylinder. Only the values of this field along the axis of symmetry, r=0, are taken into account, and are assumed to be the same for every value of r. For this simplified model, the nucleation field can be calculated by a one-dimensional integration, which is done numerically. The results are of the same order of magnitude as the experimental values, and the general shape of the theoretical curve is rather similar to the experimental one.
Low-Energy Electron Scattering from Clean and Hydrogen-Covered Nb (110) Surfaces
Authors: Haas T. W.
Published: February 2004

Low-energy electron diffraction (LEED) and inelastic electron scattering (IES) measurements are used to characterize the chemisorption of hydrogen onto the Nb (110) surface. It is found that hydrogen chemisorbs readily at room temperature, causing no rearrangement or faceting of the surface, and with only small changes in the work function. Distinctive changes in the intensity curves indicate that the hydrogen is chemisorbed in a crystalline layer with the same lattice structure as the Nb (110). Removal of the hydrogen is easily accomplished by heating to 400°C, and residual gas measurements show a higher mass 1 peak than usual, indicating that the hydrogen is adsorbed as atoms. Changes in both the LEED intensity curves and in the IES measurements both suggest that the beam penetrates more deeply into the crystal when hydrogen is chemisorbed than for the clean surface.
Effect of Hydrostatic Pressure on the Mn, As, and Sb NMR in Ferromagnetic MnAs and MnSb
Authors: Schirber J. E. , Anderson D. H.
Published: February 2004

The effect of hydrostatic pressure to about 4 kbar on the Mn, As, and Sb zero-field NMR frequencies in the ferromagnetic compounds MnAs and MnSb has been determined in powder samples from 4°K to about room temperature. Low-temperature data are required because these pressure derivatives are quite temperature dependent and decrease in a nonlinear fashion with decreasing temperature. The derivatives tend to constant values for T/TC⪝0.3 of d ln ν55/dP= −5.2(±0.2), d ln ν75/dP= −9.3(±0.3) in MnAs and d ln ν55/dP= −3.5(±0.1), d ln ν128/dP= −10.9(±0.3) in MnSb (all in units of 10−3 kbar−1).
Spin-Density Space Oscillations and Hyperfine Interaction in RbCoF3
Authors: Petrov M. P. , Nedlin G. M.
Published: February 2004

An experimental investigation of NMR on the 19F, 87Rb, 59Co nuclei in polycrystalline RbCoF3 has been performed.

From these data the spin-density distribution was obtained in the discrete points of the crystal and it was found that the spin density oscillates, and a change of sign is observed in the space between the F− and Rb+ ions.

The spin density was examined theoretically on the 19F and 87Rb ions taking into consideration that the ground state of Co2+ in RbCoF3 is the superposition of (egt2g2) and (eg2t2g) states. Most attention was paid to the successive definition of the spin density when it was necessary to account for the spin-orbit coupling in a paramagnetic ion. This question had not been considered earlier.

The negative sign of the spin density on 87Rb implies the predominant role of bonding orbitals in the hyperfine interaction.
Observation of Ferri- and Antiferromagnetic Resonance in Insulating Magnetic Spiral Structures
Authors: Stickler J. J. , Zeiger H. J.
Published: February 2004

Microwave resonance has been observed in MnCr2O4 and CoCr2O4 (ferrimagnetic spirals), ZnCr2Se4 and MnI2 (flat antiferromagnetic spirals), and ZnCr2S4 (a complex spiral). Spectra were observed in powders and some single crystals, as a function of frequency and temperatures down to 4°K or lower. MnCr2O4 shows a uniform-mode ferrimagnetic resonance with g=2. But CoCr2O4, with a small net moment, shows complex spectra in powders and single crystals, which mirror roughly the phase transitions observed in neutron diffraction, and predicted by molecular field theory. Powder spectra of ZnCr2S4 and ZnCr2Se4 yield effective internal fields which vary with temperature qualitatively as the average moment per magnetic ion. Resonance has been observed in single crystals of MnI2, from the transition temperature of ∼3.4°K to ∼1.5°K. Theoretical models of the spectra are discussed.
Antiferromagnetic Resonance and Origin of Canting in Orthorhombic Crystals
Authors: Gurevich A. G. , Golovenchits E. I. , Sanina V. A.
Published: February 2004

AFMR in orthorhombic weak ferromagnet NaNiF3 is studied for magnetic field applied along a and c axes. Measurements were performed by pulse techniques in 1-–10 mm wavelength range at 77°K. Some phenomenological parameters are determined.
Spin-Density-Wave Distributions in Chromium and Chromium Alloys
Authors: Street R. , Munday B. C. , Window B. , Williams I. R.
Published: February 2004

Two aspects of spin-density-wave ordering in chromium and its alloys are considered. First, hyperfine fields of magnitudes up to 100 kG have been observed to act on 119Sn nuclei incorporated as probes in a 0.4% manganese in chromium alloy. Mössbauer spectra obtained for both commensurate and incommensurate spin-density-wave phases are consistent with the assumption that the magnitude of the hyperfine field is directly proportional to the spin-density-wave magnetization at the nucleus. Second, measurements of the temperature dependence of the elastic properties of chromium have been made in fields up to 150 kG. Neutron-diffraction observations on the effect of magnetic fields and stress on the distribution of spin-density-wave domains in chromium provide a satisfactory basis for discussion of elasticity data. It has been shown that a sufficiently high magnetic field applied to the longitudinally polarized spin-density-wave phase may result in a reversible transition to the transversely polarized spin-density-wave phase. It follows from the results that the spin-flip transition of chromium is of the first kind.
Energy Levels, Wavefunctions, and Relaxation Times of Tb3+ in Yttrium Iron Garnet
Authors: Knight T. D. , Huber D. L.
Published: February 2004

The energy levels, wavefunctions, and relaxation times of the Tb3+ ion in yttrium iron garnet are investigated with particular emphasis on the ``near-crossing'' regions. The calculations are based on an extended point-charge model for the crystal field parameters with an exchange integral equal to that of Gd3+. It is shown that the near crossing comes about through a two-step process. First, the tetragonal component projects out a nearly degenerate |±6〉 doublet from the lower six cubic levels. Second, the exchange field splits this doublet and separates off a singlet from the remaining four levels. There is a near-crossing when the singlet intersects the lower doublet level. The wavefunctions in the near-crossing region are utilized in the calculation of the relaxation time. It is found that at low temperatures the spin-magnon process is dominant and τ=10−10 sec (T=0°K) at the point of closest approach. A calculation of the FMR linewidth which is based on the slow relaxation model with parameters inferred from the energy level analysis is outlined and compared with experiment. Quantitative agreement is obtained with respect to the angular dependence, temperature dependence, and over-all magnitude of the low-temperature peaks reported by Solev'ev and Gurevich.
High-Resolution Lorentz Microscopy
Authors: Cohen Mitchell S.
Published: February 2004

High-resolution Lorentz micrographs of magnetic films can be obtained with a commercial electron microscope if it is properly modified. A high-resolution picture of a cross-tie wall is presented as an example of such micrographs.