bluuebunny/crossref_metadata_2025 huggingface dataset:
Studies of coupled metallic magnetic thin-film trilayers
Authors: Rezende S. M. , Chesman C. , Lucena M. A. , Azevedo A. , de Aguiar F. M. , Parkin S. S. P.
Published: July 2002

Results are reported of a detailed study of static and dynamic responses in symmetric systems consisting of two ferromagnetic films separated by a nonferromagnetic spacer layer. A comparison is made with experimental results for two systems grown by sputter deposition in an UHV chamber, namely, NiFe/Cu/NiFe and Fe/Cr/Fe. First, we present model calculations where the coupling between the magnetic films through magnetic dipolar, bilinear, and biquadratic exchange interactions are fully taken into account, together with surface, in-plane uniaxial, and cubic anisotropies. An analytical expression is given that can readily be used to consistently interpret magnetoresistance, magneto-optical Kerr effect, ferromagnetic resonance, and Brillouin light scattering (BLS) data in such trilayers. Application of the results to BLS data in Ni81Fe19(d)/Cu(25 Å)Ni81Fe19(d), with d=200 and 300 Å, shows that it is essential to treat the dipolar interaction adequately in moderately thick systems. The results are also applied to interpret very interesting data in Fe(40 Å)/Cr(s)/Fe(40 Å), with 5 Å<s<35 Å, investigated by the four techniques mentioned above, at room temperature. It is shown that consistent values for all magnetic parameters can be extracted from the data with a theory that treats both static and dynamic responses on equal footing.
Optical properties of ZnAl2Se4, ZnAl2Se4:Co2+, and ZnAl2Se4:Er3+ single crystals
Authors: Park Tae-Young , Lim Ji-Young , Oh Seok-Kyun , Park Kwang-Ho , Song Ho-Jun , Kim Wha-Tek , Choe Sung-Hyu , Kim Chang-Dae , Yoon Chang-Sun
Published: July 2002

Single crystals of ZnAl2Se4, ZnAl2Se4:Co2+, and ZnAl2Se4:Er3+ were grown by the chemical transport reaction method using iodine as a transporting material. It has been shown that these single crystals have a defect chalcopyrite structure and a direct band gap. The direct band gap at 13 K has been found to be 3.525 eV for ZnAl2Se4, 2.952 eV for ZnAl2Se4:Co2+, and 3.283 eV for ZnAl2Se4:Er3+, respectively. Impurity optical absorption spectra of ZnAl2Se4:Co2+ showed absorption characteristics of Co2+ ions due to electron transitions between their split energy levels under a Td symmetry crystal field. Photoluminescence spectra at 13 K of ZnAl2Se4 showed two emission bands centered at 470 and 799 nm. For ZnAl2Se4:Er3+, we observed sharp photoluminescence peaks due to emission transitions between the energy levels of Er3+ ions with S4 symmetry sites.
Band lineup of layered semiconductor heterointerfaces prepared by van der Waals epitaxy: Charge transfer correction term for the electron affinity rule
Authors: Schlaf R. , Lang O. , Pettenkofer C. , Jaegermann W.
Published: July 2002

The occurrence of quantum dipoles at layered materials semiconductor heterointerfaces was investigated by photoemission spectroscopy (PES). Due to the unique properties of layered compounds the prepared interfaces are essentially free of the structural problems known from the usually investigated heterosystems composed of III–V, IV or II–VI materials allowing the detailed investigation of electronic phenomena at the interfaces. We investigated heterostructures composed of epitaxial layers of SnS2 and SnSe2 on different single crystalline layered chalcogenide substrates (WSe2, MoS2, MoTe2, and GaSe). The epilayers were grown by van der Waals epitaxy (vdWe) on the (0001) plane of the substrate crystals. For every system the valence band offset was determined by careful evaluation of the PES data as a function of the film thickness. Using published values for the band gaps and the experimentally determined work functions and surface potentials the band lineup for each system was determined. The band offsets of all systems were found to differ from the prediction of the electron affinity rule (EAR) by a small systematic deviation which was related to the occurrence of localized quantum dipoles at the interface. This deviation can be expressed as a linear charge transfer correction term added to the original EAR. This corrected EAR is still a linear rule allowing the assignment of “characteristic energies” to each material for the calculation of the band offset. We could demonstrate that the error margin of the corrected EAR lies well within the experimental error of PES experiments, thus proving the general applicability of linear laws for the determination of the band offset in absence of structural dipoles.
Stopping power measurements of 0.5–10.5 MeV 7Li ions in polyimide, vyns, formvar, and polysulfone
Authors: Munnik F. , Väkeväinen K. , Räisänen J. , Wätjen U.
Published: July 2002

Accurate stopping powers for 7Li ions in four (co)polymers were measured using the transmission technique in the energy range of 0.5–10.5 MeV. The estimated uncertainty in the stopping power values is ∼2.5%. A comparison of the experimental data is made with various parametrizations of the stopping power found in the literature. Deviations of the parametrizations from our experimental data range from 5 to 20%.
Mössbauer studies of BaFe11.9Mn0.1O19 by a sol–gel method
Authors: Kim Chul Sung , Lee Seung Wha , An Sung Yong
Published: July 2002

BaFe 11.9 Mn 0.1 O 19 powders were prepared by a sol–gel method. Magnetic and structural properties of the powders were characterized by Mössbauer spectroscopy, x-ray diffractometry, and vibrating sample magnetometry. X-ray diffraction and Mössbauer measurements showed that the BaFe11.9Mn0.1O19 had an M-type hexagonal structure with a0=5.900 Å and c0=23.219 Å. Mössbauer spectroscopy was performed at various temperatures ranging from 13 to 800 K, and each spectrum for a temperature below the Curie temperature (TC=775±5 K) was fitted with five subspectra of Fe sites in the structure (4fVI, 2a, 4fIV, 12k, and 2b). The area fractions of the subspectra at 13 K were 18.0%, 10.2%, 17.5%, 46.1%, and 8.2%, respectively. The 2b site had a very large quadrupole splitting. The isomer shifts indicated that the valence state of the Fe ions was ferric (Fe3+). The saturation magnetization Ms was 58 emu/g, and coercivity Hc was 5141 Oe at room temperature under an applied field of 15 kOe.
The optimization of optical gain in the intersubband quantum well laser
Authors: Tomić Stanko , Tadić Milan , Milanović Vitomir , Ikonić Zoran
Published: July 2002

A systematic procedure is described for the maximization of gain in optically pumped intersubband lasers, via optimal tailoring of the quantum well profile. The procedure relies on using the inverse spectral theory, allowing one to shift the bound states as desired, and additionally to make the isospectral reshaping of the well, with the eventual aim of finding the best combination of those optical dipole and electron–phonon scattering matrix elements which are relevant for the laser gain. Example design is presented for a laser based on the AlxGa1−xAs system, and the band nonparabolicity is accounted for in the final design.
High pressure transformations of NaOH
Authors: Beck Horst P. , Lederer Gunda
Published: July 2002

The structures of two high pressure modifications of NaOH have been determined by in situ x-ray measurements with a diamond anvil cell. Thermodynamic data for the transformation are derived from the P,T diagram. The structures are discussed and especially the influence of the H atoms on characteristic deformations. The compressibility of α-NaOH shows an anomaly, which can be explained by a continuous transformation to yet another new modification, which is isotypic with the low-temperature form of NaOD.
Kinetics of the initial stage of isothermal gas phase formation
Authors: Kashchiev D. , Firoozabadi A.
Published: July 2002

A theoretical description is proposed of the kinetics of the initial stage of gas phase formation in supersaturated liquids at constant temperature. General expressions are given for the time dependence of the number N of bubbles nucleated, of the probability P of appearance of at least one nucleus bubble and of the total volume Vg of the newly formed gas phase in the case of progressive or instantaneous bubble nucleation. Different definitions of the critical supersaturation for gas phase formation are considered and their experimental applicability is discussed. The general results are applied to the particular cases of gas phase formation at constant and steadily declining pressure and the corresponding time dependence of N, P, and Vg is obtained. Expressions for the critical supersaturation are also derived and it is found that this quantity can be rather sensitive to the rate of pressure decline when gas phase formation occurs via instantaneous bubble nucleation.
First-layer formation in ballistic deposition of spherical particles: Kinetics and structure
Authors: Choi H. S. , Talbot J. , Tarjus G. , Viot P.
Published: July 2002

We present a computer simulation and theoretical study of a ballistic deposition process in which spheres are incident on a planar surface. Each incoming sphere follows a path of steepest descent which may involve rolling over the surface of preadsorbed spheres. All particles reaching a stable, elevated position are removed. The frequency of the various rolling mechanisms are evaluated as a function of coverage. The addition mechanism generates clusters of connected spheres by accretion and coalescence. We evaluate the dependence of the cluster size distribution and coalescence probability on coverage. Various peaks in the radial distribution function of the deposited layer provide a signature for the deposition mechanism. The asymptotic approach to saturation is shown to be of the form θ∞−θ(t) ∝exp[−(4/π)Smt]/t2, where Sm=√3/2 is the smallest possible target area. The expression is shown to be consistent with the simulation results. Interpolants, which accurately describe the time-dependent coverage over the entire coverage range, are developed based on the exact expressions for the asymptotic and low coverage kinetics.
Charge reversal seen in electrical double layer interaction of surfaces immersed in 2:1 calcium electrolyte
Authors: Kékicheff P. , Marc̆elja S. , Senden T. J. , Shubin V. E.
Published: July 2002

Interactions between charged surfaces immersed in aqueous calcium solutions were measured using the surface force apparatus and the atomic force microscope. With the surface force apparatus, good agreement with previously reported measurements was found for mica surfaces in dilute solutions up to 0.1 M. However, at higher concentrations large discrepancies were observed. Compared to the earlier work, the strength of the force was lower by two or three orders of magnitude and the range was diminished. Experiments using the atomic force microscope indicated similar force-distance profiles for the interaction between silicon nitride and mica. With this technique concentrations as high as 5 M can be investigated, and owing to the small radius of curvature much higher pressures can be recorded. Results obtained by both methods confirm that the force is strongly attractive at very small surface separations, in agreement with the theoretical predictions based on calculations of ion correlations. Just outside of that interval the interaction is repulsive, and it can be quantitatively explained by taking into account the adsorption of hydrated ions onto the surface (sign reversal of the effective surface charge) and the layering of co- and counterions. At larger surface separations, the behavior indicates a balance between the double layer repulsion and the van der Waals attraction (the presence of a secondary minimum).
Quantum and classical relaxation rates from classical simulations
Authors: Bader Joel S. , Berne B. J.
Published: July 2002

The time correlation function for a harmonic quantum mechanical system can be related to the time correlation function for a corresponding classical system. Although straightforward to derive and well known in other contexts, this relationship has been unappreciated in the context of vibrational relaxation, where time correlation functions obtained from classical molecular dynamics have been used to predict relaxation rates for a quantum solute in a classical solvent. This inconsistent treatment—quantum solute, classical solvent—predicts a relaxation rate which is slower than if the entire system, both solute and solvent, were treated classically. We demonstrate that if the classical time correlation functions are rescaled to account for the ratio of quantum to classical fluctuations, providing a quantum mechanical treatment for the solute and the solvent, the relaxation rates and the entire absorption spectrum are the same as for a purely classical treatment. Our conclusions are valid when the solute and solvent can be described by a set of effective harmonic normal modes, and can also be valid when anharmonicities are present.
Full-dimensional time-dependent treatment for diatom–diatom reactions: The H2+OH reaction
Authors: Zhang Dong H. , Zhang John Z. H.
Published: July 2002

Extending our previous studies for the H2+OH reaction in five mathematical dimensions (5D) [J. Chem. Phys. 99, 5615 (1993); 100, 2697 (1994)], we present in this paper a full-dimensional (6D) dynamics study for the title reaction. The 6D treatment uses the time-dependent wave-packet approach and employs discrete variable representations for three radial coordinates and coupled angular momentum basis functions for three angular coordinates. The present 6D study employs an energy projection method to extract reaction probabilities for a whole range of energies from a single wave-packet propagation, while previous studies produced only energy-averaged reaction probability from a single wave-packet propagation. The application of the energy-projection method allows us to efficiently map out the energy dependence of the reaction probability on a fine grid which revealed surprisingly sharp resonancelike features at low collision energies on the Schatz–Elgersma potential surface. Our calculation shows that the potential-averaged 5D treatment can produce reaction probabilities essentially indistinguishable from the full-dimensional result. We also report initial state-selected reaction cross sections and rate constants which are in good agreement with our previous calculations. The effect of OH vibration on H2+OH reaction is examined in the present study and our calculation shows that the OH vibration can enhance the rate constant by about a factor of 1.7 in good agreement with the experimental estimate of about 1.5.
A structural comparison of random sequential adsorption and equilibrium configurations of spherocylinders
Authors: Ricci S. M. , Talbot J. , Tarjus G. , Viot P.
Published: July 2002

The structure of two-dimensional configurations of spherocylinders (discorectangles) generated by random sequential adsorption (RSA) is analyzed via the circular harmonic expansion of the pair distribution function and compared to that of equilibrium fluids at the same density. The structural differences are minimal for short particles but become more pronounced as the aspect ratio of the particles increases. An analysis of the correlations between particles which adsorb at high coverage with their nearest neighbors in saturated RSA configurations, reveals that the most probable relative orientation for particles with aspect ratio α≊2 is perpendicular. This observation helps to explain the maximum in saturation coverage as a function of particle elongation near the aspect ratio α=2.
Dressed-ion theory for electrolyte solutions: A Debye–Hückel-like reformulation of the exact theory for the primitive model
Authors: Kjellander Roland , Mitchell D. John
Published: July 2002

A detailed derivation of the dressed-ion theory—a formally exact theory for primitive model Coulomb fluids—is presented for the case of bulk electrolyte solutions. It is shown that the exact average electrostatic potential, ψ av(r), in the ion atmosphere around each ion satisfies a linear Poisson–Boltzmann (PB) equation for ‘‘dressed ions,’’ each of which consists of a central ion together with a specific part of the surrounding ion cloud. The dressed-ion charge distribution—a renormalized charge for each ion—takes the role that the bare ionic charge has in the usual PB equation. Apart from this, virtually the only difference between the exact dressed-ion and the approximate Debye–Hückel (DH) theories for the pair distribution function is that the former theory is nonlocal; the spread-out nature of the dressed-ion charge distribution gives rise to a nonlocal polarization response to the average potential. The linear response function relating the polarization and the average potential is investigated in the general case and is found to be intimately related to the dressed-ion charge distributions. A close relationship is also demonstrated between these charge distributions and the electrostatic susceptibility of the electrolyte solution. The theory gives a rigorous definition of the concept of effective point charges for ions. Except at high coupling, the long-range asymptotic behavior of the pair distribution functions is of the Debye–Hückel form, but with effective values of the ionic charges (qi*) and a decay length (κ−1) different from the Debye length (κ−1D). A simple formula relating qi* and κ is derived. Explicit formulae for qi* and κ in terms of κD are given in the limit of low electrolyte concentrations. The long range asymptotic behaviors of the bridge function and the short range parts of the direct and total correlation functions are analyzed in some detail.
Dynamical fluctuating charge force fields: Application to liquid water
Authors: Rick Steven W. , Stuart Steven J. , Berne B. J.
Published: July 2002

A new molecular dynamics model in which the point charges on atomic sites are allowed to fluctuate in response to the environment is developed and applied to water. The idea for treating charges as variables is based on the concept of electronegativity equalization according to which: (a) the electronegativity of an atomic site is dependent on the atom’s type and charge and is perturbed by the electrostatic potential it experiences from its neighbors and (b) charge is transferred between atomic sites in such a way that electronegativities are equalized. The charges are treated as dynamical variables using an extended Lagrangian method in which the charges are given a fictitious mass, velocities, and kinetic energy and then propagated according to Newtonian mechanics along with the atomic degrees of freedom. Models for water with fluctuating charges are developed using the geometries of two common fixed-charge water potentials: the simple point charge (SPC) and the four-point transferable intermolecular potential (TIP4P). Both fluctuating charge models give accurate predictions for gas-phase and liquid state properties, including radial distribution functions, the dielectric constant, and the diffusion constant. The method does not introduce any new intermolecular interactions beyond those already present in the fixed charge models and increases the computer time by only a factor of 1.1, making this method tractable for large systems.
Nucleation in molecular and dipolar fluids: Interaction site model
Authors: Talanquer V. , Oxtoby David W.
Published: July 2002

We consider the effect of molecular anisotropy and dipole moment on the rate of homogeneous and heterogeneous nucleation of molecular and dipolar fluids. Density functional theory is applied to an interaction site model of molecules composed of two tangent or fused hard spheres interacting through Lennard-Jones and coulombic potentials. Interfacial properties are conditioned by entropy vs energy competition. For nucleation on a charged hard sphere, the calculated rates depend strongly on the size and charge of the central seed. Preferred surface orientations of the dipolar molecules are responsible for nucleation rates differing by several orders of magnitude for charges of different sign.
A simple expression for radial distribution functions of pure fluids and mixtures
Authors: Matteoli Enrico , Mansoori G. Ali
Published: July 2002

A simple expression for the radial distribution function (RDF) of pure fluids and mixtures is presented. It satisfies the limiting conditions of zero density and infinite distance imposed by statistical thermodynamics. The equation contains seven adjustable parameters; they have been fitted to extensive literature data of RDF’s for a Lennard-Jones fluid at different values of temperature and density. These in turn have been expressed as functions of reduced temperature and density, thus allowing a complete parametrization with respect to these variables using 21 parameters altogether with fairly good accuracy. The values of the reduced pressure and internal energy calculated by numerical integration of the completely parametrized equation compare fairly with literature molecular dynamics simulation results. The capability of the expression to fit to RDF’s of mixtures has been checked against some of the extensive RDF simulation data of binary mixtures of Lennard-Jones fluids with different diameters available in the literature. Data pertaining to different molar fractions as well as to different εAA/εBB ratios have been considered, and the agreement between calculated and simulation curves has resulted satisfactory. The proposed expression can be used to calculate by integration related quantities such as compressibility, internal energy, pressure and, using the Kirkwood–Buff theory, the chemical potentials and partial molar volumes of the components of mixtures for which RDF data are available.
Laser-induced fluorescence spectroscopy of jet cooled p-aminophenol
Authors: Wategaonkar S. , Doraiswamy S.
Published: July 2002

The laser-induced fluorescence spectra of p-aminophenol both in excitation and emission have been studied in a supersonic jet apparatus. The characterization of the observed spectra was done by comparison with other related substituted anilines and the IR data available in the literature. The excitation spectrum resembles that of aniline with optical activity mainly confined to 6a, 1, and the NH2 inversion mode. In addition, the C–X in-plane bending mode 9b was also found to be optically active. The 6a mode dominates in most of the dispersed fluorescence spectra and shows a strong Franck–Condon activity. Unlike other similar molecules, the Δv=0 transitions were weak in the single vibronic level fluorescence spectra of 6a1 and 11, which has been qualitatively explained in terms of Franck–Condon analysis. The onset of intramolecular vibrational redistribution occurs at 1135 cm−1, which is much higher than many substituted anilines. The van der Waals complexes viz. p-aminophenol–Ar1 and p-aminophenol–Ar2 were observed. A symmetric (1|1) complex for the p-aminophenol–Ar2 is proposed based on the redshift additivity.
Proton spin-lattice relaxation of water molecules in ferrous–ferric/agarose gel system
Authors: Tokuhiro Tadashi , Appleby A. , Leghrouz A. , Metcalf Rodney , Tokarz Robert
Published: July 2002

Proton spin-lattice relaxation time (T1) of water in aqueous solutions of ferrous and ferric ions and in the corresponding agarose gel systems have been studied in the light of NMR relaxation theory. The theoretical analysis of 1/T1’s has revealed that, at the microscopic level, changes in the solvation states of paramagnetic ions in aqueous or gel environment are greater than difference in the paramagnetism between ferric and ferrous ions. The former change is the primary factor for the exhibition of radiation effect. At the phenomenological level, we have confirmed and demonstrated that: (1) Radiation effect is almost exclusively exhibited through changes in 1/T1 caused by the interactions between water proton and ferrous or ferric ions; and (2) fraction of conversion of ferrous to ferric ions induced by radiation is the ‘‘true’’ representation of the spatial distribution of radiation dose.
Elastic modulus and equilibrium swelling of networks crosslinked by end-linking oligodimethylsiloxane at solution state
Authors: Urayama Kenji , Kawamura Takanobu , Kohjiya Shinzo
Published: July 2002

We have investigated the degree of equilibrium swelling and the elastic modulus of networks prepared by end-linking oligo(dimethylsiloxane)s (ODMS) in solution as a function of polymer concentration at crosslinking. The molecular weight of ODMS is so low that entanglement couplings are not formed in uncrosslinked state. It has been found from the preparation concentration dependence of elastic modulus in preparation state that trapped entanglements are formed by the introduction of crosslinks, and those considerably contribute to elastic modulus, even if the prepolymers are not entangled in uncrosslinked state. The experimental results for preparation concentration dependence of the degree of equilibrium swelling and the elastic modulus of equilibrium swollen networks are compared with the theoretical predictions by the two theories, i.e., the affine model and the c* theorem. It has been clearly shown that the affine model describes well the experimental results, while the discrepancies between the experimental results and the predictions by the c* theorem are considerable. These results strongly suggest that preparation concentration should be regarded as a reference state, and the displacement of crosslinks moves affinely on swelling, while the complete disinterpenetration of network chains in equilibrium swollen state, which is a postulate of the c* theorem, does not occur.
Potential models for simulations of the solvated proton in water
Authors: Ojamäe Lars , Shavitt Isaiah , Singer Sherwin J.
Published: July 2002

Analytical potential models are designed for simulations of water with excess protons. The potentials describe both intramolecular and intermolecular interactions, and allow dissociation and formation of the species (H2O)nH+. The potentials are parametrized in the form of interactions between H+ and O2− ions, with additional three-body (H–O–H) interaction terms and self-consistent treatment of the polarizability of the oxygen ions. The screening of electrostatic interactions caused by the overlap of the electron clouds in the real molecules is modeled by functions modifying the electric field at short distances. The model was derived by fitting to the potential surface of the H5O2+ ion and other species, as obtained from ab initio MP2 calculations employing an extensive basis set. Emphasis was put on modeling the potential-energy surface for the proton-transfer reaction. Potential-surface profiles, geometry-optimized structures and formation energies of H5O2+, protonated water clusters [H+(H2O)n, n=2–4] and water clusters [(H2O)n, n=1–6] using these potentials are presented and compared to results using quantum-chemical calculations. The potential models can well reproduce ab initio results for the H5O2+ ion, and can provide formation energies and structures of both protonated-water and water-only clusters that agree favorably with ab initio MP2 calculations.
Pairwise contact potentials are unsuitable for protein folding
Authors: Vendruscolo Michele , Domany Eytan
Published: July 2002

We demonstrate that pairwise contact potentials alone cannot be used to predict the native fold of a protein. Ideally, one would hope that a universal energy function exists, for which the native folds of all proteins are the respective ground states. Here we pose a much more restricted question: Is it possible to find a set of contact parameters for which the energy of the native contact map of a single protein (crambin) is lower than that of all possible physically realizable decoy maps? The set of maps we used was derived by energy minimization (not by threading). We seek such a set of parameters by perceptron learning, a procedure which is guaranteed to find such a set if it exists. We found that it is impossible to fine-tune contact parameters that will assign all alternative conformations higher energy than that of the native map. This finding proves that there is no pairwise contact potential that can be used to fold any given protein. Inclusion of additional energy terms, such as hydrophobic (solvation), hydrogen bond, or multibody interactions may help to attain foldability within specific structural families.
The Re6Se8Cl64− and Re6Se8I64− cluster ions: Another example of luminescent clusters?
Authors: Arratia-Pérez Ramiro , Hernández-Acevedo Lucı́a
Published: July 2002

Dirac molecular orbital calculations on the octahedral Re6Se8Cl64− and Re6Se8I64− cluster ions are reported. These cluster ions have singlet ground state. The estimated clusters electronegativity, χ, and their HOMO-LUMO energy gap suggest that Re6Se8I64− is softer and more reactive than Re6Se8Cl64−. The calculated relativistic molecular orbitals indicate that the manifold of closely spaced unoccupied energy levels is mainly localized on the octahedral rhenium core with some contributions from the μ3-Se ligands, while all the cluster highest occupied molecular orbitals are largely centered on the apical halide ligands. The electronic features of their ground and excited states are similar to the intensely luminescent hexanuclear tungsten halide W6X8X62− cluster ions. Our calculations suggest that the Re6Se8Cl64− and Re6Se8I64− cluster ions should be diamagnetic and luminescent.
Intermolecular energy transfer in liquid water and its contribution to heat conduction: A molecular dynamics study
Authors: Ohara Taku
Published: July 2002

Intermolecular energy transfer (IET) is a dominant factor in heat conduction in liquid. The IET in liquid water and its contribution to macroscopic heat conduction under a temperature gradient were analyzed by a molecular dynamics simulation utilizing the extended simple point charge (SPC/E) potential model. Intermolecular energy exchange rates (IEERs) for both the translational and rotational motion of molecules were defined and their characteristics examined. The IEER of hydrogen-bonded molecules and nonbonded molecules have different characteristics. The IEER oscillates with a high amplitude and its time average, which is much smaller than the magnitude of the IEER, gives the effective rate of the IET that contributes to macroscopic heat conduction. In the present study, the effective rate of the IET was assumed to be proportional to the magnitude of the IEER. Based on the characteristics of the IEER and the above supposition, contributions of the translational and rotational IET between hydrogen-bonded molecules and nonbonded molecules to macroscopic heat conduction were evaluated. The evaluated results were compared with the results of a molecular dynamics (MD) simulation of heat conduction under a constant temperature gradient, and good qualitative agreement between the predicted value and the simulated result was found. The rotational IET was found to be dominant as compared with the translational IET, and the contribution of hydrogen-bonded molecules to heat conduction was found to be relatively small. The possibility of a mechanism that cancels the IET between distant molecules and the development of a precise model for this mechanism were also discussed.
Structural information on the S and S1 state of o-fluorophenol by hole burning and high resolution ultraviolet spectroscopy
Authors: Remmers Karen , Meerts W. Leo , Zehnacker-Rentien Anne , Le Barbu K. , Lahmani F.
Published: July 2002

The electronic transitions of o-fluorophenol situated at 36 799.382 cm−1 and 36 906.710 cm−1, denoted the A and B bands, respectively, have been investigated by high resolution fluorescence excitation spectroscopy. Hole burning studies together with the high resolution spectroscopy results show that both bands originate in the same ground state and can be fitted to the rotational constants of the cis isomer. The rotational constants for the excited states are found to be A′=3231.795 MHz, B′=2207.92 MHz and C′=1313.97 MHz for the A band and A′=3226.945 MHz, B′=2211.24 MHz and C′=1321.03 MHz for the B band. The planarity of the ground state is lost upon electronic excitation, which enhances the activity of an out-of-plane vibration. The A and B band transitions arise from excitations to respectively the zero and first overtone levels in the double-minimum potential of this out-of-plane vibration, which shows similarities to the so-called butterfly mode observed in other benzene derivatives.
Abundant symmetries for the 1+1 dimensional classical Liouville field theory
Authors: Lou Sen-yue
Published: July 2002

Explicit calculation shows us that there exists a rich associated symmetry structure and abundant strong symmetry configuration for the 1+1 dimensional classical Liouville field theory. Starting from these symmetries and strong symmetries, various integrable hierarchies can be obtained. The Liouville I–III hierarchies, the modified Korteweg–de Vries (KdV) hierarchy, the Fordy–Gibbons I, and Fordy–Gibbons II hierarchies are just special cases while the KdV hierarchy, Caudry–Dodd–Gibbon–Sawada–Kotera hierarchy, and Kaup–Kupershmidt hierarchy can be obtained by using a same Miura transformation from the modified KdV, the Fordy–Gibbons I and Fordy–Gibbons II hierarchies, respectively. Two types of the generalized Riccati hierarchies are also presented.
SU(2) coherent-state path integral
Authors: Kochetov E. A.
Published: July 2002

The SU(2) coherent-state path integral is used to represent the matrix element of a propagator in the SU(2) coherent-state basis. It is argued that the continuum representation of this integral is correct provided the necessary boundary term is taken into account. In the case of the SU(2) dynamical symmetry the path integral is explicitly computed by means of a change of variables, the SU(2) motion of the underlying phase space. The correct stationary-phase expansion for the propagator in terms of the total action including boundary term and classical trajectories is obtained.
Note on massive spin 2 in curved space
Authors: Bengtsson Ingemar
Published: July 2002

We give a Hamiltonian formulation of massive spin 2 in arbitrary Einstein space–times. We pay particular attention to Higuchi’s forbidden mass range in deSitter space.
Algebra of pseudodifferential operators and symmetries of equations in the Kadomtsev–Petviashvili hierarchy
Authors: Orlov A. Yu. , Winternitz P.
Published: July 2002

Point symmetries are obtained for all equations in the KP hierarchy. The Lie algebra for each equation is infinite dimensional and involves several arbitrary functions of the corresponding time tN. The symmetry algebra is a semidirect sum of a Virasoro algebra and a Kac–Moody one. The “positive” part of this algebra is embedded into the known W∞ algebra of KP symmetries and into the free fermion algebra ĝl(∞). The corresponding action on the tau-function is presented. The negative part of the point symmetries does not fit into the free fermion algebra, but is embedded into a P∞ algebra, based on the algebra of pseudodifferential operators.
Ocean stirring and chaotic low-order dynamics
Authors: Brown Michael G. , Smith Kevin B.
Published: July 2002

The extent to which results from the study of chaotic low-order dynamical systems might give insight into the process by which passive tracers are laterally mixed in midocean regions is investigated. Three questions relating to this issue are addressed. (1) Are transient passive tracer distributions in the ocean consistent with the efficient stirring associated with chaotic particle trajectories? (2) Do oceanic particle trajectories exhibit extreme sensitivity to their initial conditions? (3) To what extent can oceanic particle trajectories be thought of as being associated with a low-order dynamical system? The answers to these questions suggest, with some uncertainty, that a picture of ocean stirring being associated with a chaotic low-order dynamical system is a useful one.
Effects of ion drift on small-amplitude ion-acoustic solitons
Authors: Kuehl H. H. , Zhang C. Y.
Published: July 2002

Some properties of small-amplitude ion-acoustic solitons in a plasma consisting of nondrifting electrons and drifting ions are investigated. Electron inertia effects are shown to be considerably more important than relativistic effects. It is also shown that ion-acoustic soliton solutions exist only if the ion drift velocity is less than the electron thermal velocity.
Critical error fields for locked mode instability in tokamaks
Authors: La Haye R. J. , Fitzpatrick R. , Hender T. C. , Morris A. W. , Scoville J. T. , Todd T. N.
Published: July 2002

Otherwise stable discharges can become nonlinearly unstable to disruptive locked modes when subjected to a resonant m=2, n=1 error field from irregular poloidal field coils, as in DIII-D [Nucl. Fusion 31, 875 (1991)], or from resonant magnetic perturbation coils as in COMPASS-C [Proceedings of the 18th European Conference on Controlled Fusion and Plasma Physics, Berlin (EPS, Petit-Lancy, Switzerland, 1991), Vol. 15C, Part II, p. 61]. Experiments in Ohmically heated deuterium discharges with q≊3.5, n̄ ≊ 2 × 1019 m−3 and BT ≊ 1.2 T show that a much larger relative error field (Br21/BT ≊ 1 × 10−3) is required to produce a locked mode in the small, rapidly rotating plasma of COMPASS-C (R0 = 0.56 m, f≊13 kHz) than in the medium-sized plasmas of DIII-D (R0 = 1.67 m, f≊1.6 kHz), where the critical relative error field is Br21/BT ≊ 2 × 10−4. This dependence of the threshold for instability is explained by a nonlinear tearing theory of the interaction of resonant magnetic perturbations with rotating plasmas that predicts the critical error field scales as (fR0/BT)4/3n̄2/3. Extrapolating from existing devices, the predicted critical field for locked modes in Ohmic discharges on the International Thermonuclear Experimental Reactor (ITER) [Nucl. Fusion 30, 1183 (1990)] (f=0.17 kHz, R0 = 6.0 m, BT = 4.9 T, n̄ = 2 × 1019 m−3) is Br21/BT ≊ 2 × 10−5. Such error fields could be produced by shifts and/or tilts of only one of the larger poloidal field coils of as little as 0.6 cm with respect to the toroidal field. A means to increase the rotation frequency would obviate the sensitivity to error fields and increase allowable tolerances on coil construction.
Three-dimensional fluid simulations of the nonlinear drift-resistive ballooning modes in tokamak edge plasmas
Authors: Guzdar P. N. , Drake J. F. , McCarthy D. , Hassam A. B. , Liu C. S.
Published: July 2002

A three-dimensional study of the turbulence and sheared flow generated by the drift-resistive ballooning modes in tokamak edge plasmas has been completed. The fluid simulations show that 10%–15% percent density fluctuations can develop in the nonlinear state when the self-consistently generated shear flow is suppressed. These modes are also found to give rise to poloidally asymmetric particle transport. Characteristic scale lengths of these fluctuations are isotropic in the plane transverse to B and smaller than the connection length along the field line. Sheared poloidal flow is self-consistently driven by both the Reynolds stress and the Stringer mechanisms. In the presence of self-consistent shear flow, the transverse spectrum is no longer isotropic transverse to B. The vortices become elongated in the poloidal direction. Also, there is a substantial reduction in both the level of fluctuations of the density and potential and the associated particle transport. These features are in qualitative agreement with L–H transitions observed in tokamaks.
Alignment of vorticity and scalar gradient with strain rate in simulated Navier–Stokes turbulence
Authors: Ashurst Wm. T. , Kerstein A. R. , Kerr R. M. , Gibson C. H.
Published: July 2002

The alignment between vorticity and eigenvectors of the strain-rate tensor in numerical solutions of Navier–Stokes turbulence is studied. Solutions for isotropic flow and homogeneous shear flow from pseudospectral calculations using 1283 grid points have been examined. The Taylor Reynolds number is 83 or greater. In both flows there is an increased probability for the vorticity to point in the intermediate strain direction and at three-fourths of the sample points this strain is positive (extensive). This propensity for vorticity alignment with a positive intermediate strain is a consequence of angular momentum conservation, as shown by a restricted Euler model of the coupling between strain and vorticity. Probability distributions for intermediate strain, conditioned on total strain, change from a symmetric triangular form at small strain to an asymmetric one for large strain. The most probable value of the asymmetric distribution gives strains in the ratios of 3:1: −4. The evolution of the distribution from a symmetric to an asymmetric form as the strain magnitude increases is essentially the same in both flows, indicating a generic structure of intense turbulence. The alignment between the gradient of a passive scalar and eigenvectors of the strain-rate tensor for Prandtl numbers of 0.1, 0.2, 0.5, and 1.0 has also been studied. There is an increased probability for the scalar gradient to align in the most compressive strain direction, and the average gradient is larger when it is pointing in that direction. Estimates for the scalar dissipation from the turbulent kinetic energy, its dissipation, and the root-mean-square scalar value are in reasonable agreement with calculated scalar dissipation if no explicit Prandtl number dependence is used in the estimate. Statistical analysis of scalar dissipation conditioned on energy dissipation yields a power-law relation between conditioned mean values. Both simulated flows are found to obey the qualitative predictions of the Gurvich–Yaglom (lognormal) intermittency model. Energy and scalar intermittency exponents are estimated and compared to measured values.
Tearing modes in toroidal geometry
Authors: Connor J. W. , Cowley S. C. , Hastie R. J. , Hender T. C. , Hood A. , Martin T. J.
Published: July 2002

The separation of the cylindrical tearing mode stability problem into a resistive resonant layer calculation and an external marginal ideal magnetohydrodynamic (MHD) calculation (Δ′ calculation) is generalized to axisymmetric toroidal geometry. The general structure of this separation is analyzed and the marginal ideal MHD information (the toroidal generalization of Δ′) required to discuss stability is isolated. This can then, in principle, be combined with relevant resonant layer calculations to determine tearing mode growth rates in realistic situations. Two examples are given: the first is an analytic treatment of toroidally coupled (m=1, n=1) and (m=2, n=1) tearing modes in a large aspect ratio torus; the second, a numerical treatment of the toroidal coupling of three tearing modes through finite pressure effects in a large aspect ratio torus. In addition, the use of a coupling integral approach for determining the stability of coupled tearing modes is discussed. Finally, the possibility of using initial value resistive MHD codes in realistic toroidal geometry to determine the necessary information from the ideal MHD marginal solution is discussed.
Numerical simulation of a viscous vortex ring interaction with a density interface
Authors: Marcus Daniel L. , Bell John B.
Published: July 2002

The incompressible, variable-density Navier–Stokes equations in axisymmetric coordinates are solved for the interaction of a vortex ring with a density interface. The effect of progressively weakening stratification and variation in Reynolds number are examined. Secondary and tertiary ring formation, vortex rebound, and backflow jets are some of the features observed in the interactions.
Gas flows caused by evaporation and condensation on two parallel condensed phases and the negative temperature gradient: Numerical analysis by using a nonlinear kinetic equation
Authors: Aoki Kazuo , Masukawa Noboru
Published: July 2002

A rarefied gas between its two parallel plane condensed phases with different temperatures is considered, and the steady gas flows caused by evaporation and condensation on the condensed phases are investigated numerically by using the (nonlinear) Boltzmann–Krook–Welander equation. The flow properties are clarified for a wide range of the parameters (the Knudsen number, the ratio of the temperatures of the condensed phases, and the ratio of the saturation gas pressures at these temperatures). In particular, the phenomenon of the negative temperature gradient, which has extensively been studied for weak evaporation and condensation, is confirmed to occur also for nonweak evaporation and condensation. In the course of the numerical analysis, it is demonstrated that the linearized Boltzmann equation does not give the correct description of the problem for very small Knudsen numbers (i.e., Knudsen numbers comparable to or smaller than the flow Mach number), however weak the evaporation and condensation may be. In addition, a high-speed evaporating flow toward a perfectly absorbing wall is investigated as an extreme case of strong evaporation and condensation.
Instability modes excited by natural screech tones in a supersonic rectangular jet
Authors: Raman Ganesh , Rice Edward J.
Published: July 2002

The evolution of hydrodynamic instability modes self-excited by harmonically related natural screech tones was experimentally investigated. A convergent rectangular nozzle with an aspect ratio of 9.63 was used to produce a supersonic shock-containing jet. Measurements in the flow field were made using standard hot-film probes positioned only in the subsonic (outer) portions of the flow. The hydrodynamic instability mode observed in the shear layer at the screech frequency was observed to be antisymmetric (sinuous) about the smaller dimension of the jet, whereas its harmonic was observed to be symmetric (varicose). In addition, the nearfield noise measurements indicated that the radiated screech tone noise was out of phase on either side of the small jet dimension, whereas its harmonic was in phase over the same region. To the authors’ knowledge, such an observation on the nature of the harmonic has thus far gone unreported and therefore is the focus of the present work. The hydrodynamic instability modes occurring at the screech frequency and its harmonic satisfied the conditions for resonance. Detailed measurements of the coherent wave evolution in the streamwise and spanwise directions indicated that strong spanwise variations were present downstream.
Vorticity flux control for a turbulent channel flow
Authors: Koumoutsakos Petros
Published: July 2002

A feedback control algorithm using wall only information has been applied in simulations of low Reynolds number (Reτ=180) turbulent channel flow. The present control scheme is based on the manipulation of the vorticity flux components, which can be obtained as a function of time by measuring the instantaneous pressure at the wall and calculating its gradient. The strength of the unsteady mass transpiration actuators can be derived explicitly by inverting a system of equations whose terms depend on the relative locations of the sensors and actuators. The results of the simulations indicate a large (up to 40%) drag reduction. Moreover it appears that using the present methodology open-loop control laws can be devised.
Stability analysis of perturbed plane Couette flow
Authors: Barkley Dwight , Tuckerman Laurette S.
Published: July 2002

Plane Couette flow perturbed by a spanwise oriented ribbon, similar to a configuration investigated experimentally at the Centre d’Etudes de Saclay, is investigated numerically using a spectral-element code. Two-dimensional (2-D) steady states are computed for the perturbed configuration; these differ from the unperturbed flows mainly by a region of counter-circulation surrounding the ribbon. The 2-D steady flow loses stability to three-dimensional (3-D) eigenmodes at Rec=230, βc=1.3 for ρ=0.086 and Rec≈550, βc≈1.5 for ρ=0.043, where β is the spanwise wave number and 2ρ is the height of the ribbon. For ρ=0.086, the bifurcation is determined to be subcritical by calculating the cubic term in the normal form equation from the time series of a single nonlinear simulation; steady 3-D flows are found for Re as low as 200. The critical eigenmode and nonlinear 3-D states contain streamwise vortices localized near the ribbon, whose streamwise extent increases with Re. All of these results agree well with experimental observations.