bluuebunny/crossref_metadata_2025 huggingface dataset:
Equivalence classes of perturbations in cosmologies of Bianchi types I and V: Propagation and constraint equations
Authors: Banach Zbigniew , Piekarski Slawomir
Published: July 2002

This is the third in a series of papers [J. Math. Phys. 40, 3978 (1999); 40, 3995 (1999)], the overall objective of which is the demonstration that a set of 26 gauge-invariant variables, denoted collectively by D and referred to as the complete set of basic variables, can be used to describe the equivalence classes of perturbations in a Bianchi type I or type V universe filled with a nonbarotropic perfect fluid. The object here is the derivation of a full system of propagation and constraint equations for these basic variables. We show that the constraint equations, which involve only the spatial derivatives of D, are preserved in time along the unperturbed fluid flow lines, i.e., that the time derivative of each constraint equation is identically satisfied as a consequence of the other equations that hold. Let us put things another way. What we prove is the statement that if the constraints in our system are satisfied at one time and the evolution equations are satisfied at all times, then the constraints are satisfied at all times. A further important point is simply this. When the linearized field equations of Einstein’s gravity theory are re-expressed in a manifestly gauge-invariant form, an open set of equations is obtained for D since there are too many unknowns. Thus this set must be suitably closed by means of accurate “closure” relations. In order to find them, we observe that the definition of basic gauge-invariant variables gives rise to additional geometrical identities from which an exact method of closure can be determined. Our formalism turns out to be especially appropriate for handling the linearized perturbations in a Bianchi type V universe model where the standard approaches conceptually break down.
Forces between like-charged walls in an electrolyte solution: A comparison of McMillan–Mayer results for several models
Authors: Otto Frank , Patey G. N.
Published: July 2002

The interaction of like-charged walls immersed in aqueous solutions with monovalent counterions is investigated at the McMillan–Mayer (MM) level of description. The net pressure acting between the walls is obtained by applying the anisotropic hypernetted-chain theory. The MM approach requires solvent-averaged ion–ion potentials of mean force as input. Results based on “realistic” models for Na+ and Cl− in water are available in the literature and these are used in the present calculations. The wall–wall interactions obtained can differ dramatically from the primitive model (dielectric continuum solvent) case. For some models attractive wall–wall forces are observed at small separations. The MM theory is found to be rather sensitive to details of the counterion–counterion potential of mean force, and different models for the same counterion can give qualitatively different results. At present it is difficult to evaluate the relative accuracy of the different models that have been proposed. However, the results presented here give at least an idea of the interesting possibilities that lie in the physically realistic range.
Reaction cross section for incorporation of ND3 into NH4+(NH3)n−1 (n=3–9) at very low energy collision
Authors: Orii Takaaki , Okada Yoshiki , Takeuchi Kazuo , Ichihashi Masahiko , Kondow Tamotsu
Published: July 2002

Cross sections for the collision-induced reactions between protonated ammonia cluster ions, NH4+(NH3)n−1 (n=3–9), with ND3 were measured at a collision energy ranging from 0.02 to 1.4 eV in the center-of-mass frame with an extremely narrow energy distribution of 0.02 eV. Two types of reaction, incorporation and dissociation, were observed at the same collision energy. The incorporation cross section increased drastically with a decrease in the collision energy, especially in the collision energy region below about 0.2 eV. The incorporation cross section at a collision energy of about 0.02 eV was larger than twice the geometrical reaction cross section. It is suggested that the increase of the incorporation cross section corresponds to the increase of the collision cross section between the cluster ion and the neutral molecule at the low collision energies used in this study because of the presence of the electrostatic attractive force. The reaction probability for incorporation also increased with a decrease of the collision energy. The collision energy dependence of the probability was studied in more detail for NH4+(NH3)4 of which the incorporation cross section can be precisely evaluated, due to its simple spherical structure, by trajectory simulations. This increase was interpreted to be due to the recoil energy of the molecule, which was dependent upon the collision energy and the internal energy of the cluster ion. The size dependence of the incorporation cross section indicated that the incorporation cross section of the pentamer ion NH4+(NH3)4 was at a minimum in the cluster size region (n=3–9) at very low collision energies. The size dependence of the incorporation cross section is discussed in relationship to the structure of the cluster ions.
Binary nucleation of n-octane and i-octane
Authors: Doster G. Jay , Schmitt John L. , Bertrand Gary L.
Published: July 2002

The authors used a Wilson expansion cloud chamber to measure binary homogeneous nucleation rates for pure n-octane, pure i-octane, and 3:1, 1:1, 1:3 (mole ratio) mixtures in a temperature range from 215 to 260 K. The nucleation rates range from approximately 100 to 50 000 drops/cm3 s. Current binary nucleation theory is unable to predict the data for this nearly ideal system.
Triplet states in a series of Pt-containing ethynylenes
Authors: Wilson J. S. , Köhler A. , Friend R. H. , Al-Suti M. K. , Al-Mandhary M. R. A. , Khan M. S. , Raithby P. R.
Published: July 2002

By use of optical steady state and time resolved spectroscopy, we studied the evolution of the triplet excited state in a series of six ethynylenic polymers of the structure [-Pt(PBu3n)2-C≡C-R-C≡C-]n where the spacer unit R is systematically varied to give optical gaps from 1.7–3.0 eV. The inclusion of platinum in the polymer backbone induces a strong spin-orbit coupling such that triplet state emission (phosphorescence) associated with the conjugated system can be detected. Throughout the series we find the S1-T1 (singlet-triplet) energy splitting to be independent of the spacer R, such that the T1 state is always 0.7±0.1 eV below the S1 state. With decreasing optical gap, the intensity and lifetime of the triplet state emission were seen to reduce in accordance with the energy gap law.
Acoustic saturation in bubbly cavitating flow adjacent to an oscillating wall
Authors: Colonius T. , d’Auria F. , Brennen C. E.
Published: July 2002

Bubbly cavitating flow generated by the normal oscillation of a wall bounding a semi-infinite domain of fluid is computed using a continuum two-phase flow model. Bubble dynamics are computed, on the microscale, using the Rayleigh–Plesset equation. A Lagrangian finite volume scheme and implicit adaptive time marching are employed to accurately resolve bubbly shock waves and other steep gradients in the flow. The one-dimensional, unsteady computations show that when the wall oscillation frequency is much smaller than the bubble natural frequency, the power radiated away from the wall is limited by an acoustic saturation effect (the radiated power becomes independent of the amplitude of vibration), which is similar to that found in a pure gas. That is, for large enough vibration amplitude, nonlinear steepening of the generated waves leads to shocking of the wave train, and the dissipation associated with the jump conditions across each shock limits the radiated power. In the model, damping of the bubble volume oscillations is restricted to a simple “effective” viscosity. For wall oscillation frequency less than the bubble natural frequency, the saturation amplitude of the radiated field is nearly independent of any specific damping mechanism. Finally, implications for noise radiation from cavitating flows are discussed.
Calculation of the absolute hydration enthalpy and free energy of H+ and OH−
Authors: Mejı́as J. A. , Lago S.
Published: July 2002

The hydration enthalpy and Gibbs free energy of proton and hydroxide are calculated by means of a combination of ab initio density functional theory and a polarizable continuum model within the self-consistent reaction field method. The ion–water cluster models here used include up to 13 water molecules solvating the ions. This allows the first and second solvation shells to be described explicitly from first principles. Vibrational contributions to the enthalpy and entropy have been taken into account. Our best model of the hydrated proton includes three molecules in the first hydration shell and nine molecules in the second shell. The calculated proton hydration enthalpy is ≈−1150 kJ/mol, which is in rather good agreement with the most recent results from cluster–ion solvation data. The hydration free energy of the proton has a larger error of ≈50–80 kJ/mol as compared to recently reported values. The calculated hydroxide hydration enthalpy, ≈−520 kJ/mol, and hydration free energy, ≈−400 kJ/mol, are consistent with well-established values taken from experiment. Two different sources of error in our calculations, namely, the nature of the hydrated complex and the outlying charge correction, are discussed. Moreover, we compare the results from three slightly different methods for the calculation of hydration energies.
Reynolds number dependence of second-order velocity structure functions
Authors: Antonia R. A. , Pearson B. R. , Zhou T.
Published: July 2002

The Reynolds number dependence of measured (and corrected) second-order longitudinal and transverse velocity structure functions is examined by fitting an expression that extends from the smallest dissipative scales to inertial range scales. Results obtained from fitting to data for decaying grid turbulence and on the centerline of turbulent jets indicate an increase, with respect to the Reynolds number, of both longitudinal and transverse scaling exponents. The results should provide a relatively reliable description for the Reynolds number evolution of Kolmogorov-normalized second-order structure functions from viscous to inertial range scales.
Identification of a Pb-related Ti3+ center in flux-grown KTiOPO4
Authors: Stevens K. T. , Halliburton L. E. , Roth M. , Angert N. , Tseitlin M.
Published: July 2002

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) have been used to identify a new Ti3+ center in KTiOPO4 crystals containing lead impurities. Many of the K+ vacancies in this set of KTP crystals are compensated nonlocally by Pb2+ ions substituting for K+ ions. During exposure to ionizing radiation (either 60 kV x rays or 355 nm photons from a tripled Nd:YAG laser), “free” electrons are trapped on Ti4+ ions near isolated Pb2+ ions, thus forming the perturbed Ti3+ ions observed with EPR and ENDOR. Four distinct Pb-related Ti3+ centers are formed by a 77 K irradiation, but only one remains after a 5 min anneal at 180 K. This latter defect, labeled the [Ti3+–Pb2+]A center, is thermally unstable above 250 K. Angular dependence data were used to determine the g matrix, one Pb207 hyperfine matrix, and two P31 hyperfine matrices for the [Ti3+–Pb2+]A center. More generally, we note that oxygen-vacancy-associated Ti3+ centers could not be formed in these Pb-containing KTP crystals. Replacing oxygen-vacancy-associated Ti3+ centers with the less stable [Ti3+–Pb2+] centers is expected to increase the gray-track resistance of KTP crystals used to generate the second harmonic of high power, near-infrared lasers.
Tunnel switch diode based on AlSb/GaSb heterojunctions
Authors: Cheng X.-C. , Cartoixà X. , Barton M. A. , Hill C. J. , McGill T. C.
Published: July 2002

We report on tunnel switch diodes based on AlSb barriers and GaSb p–n junctions grown by molecular beam epitaxy. These were the devices with thyristor like switching in the GaSb/AlSb system. The characteristic “S” shaped current–voltage curve was found to occur for structures with AlSb barriers less than 300 Å thick. The switching voltage and current density exhibited less sensitivity to barrier and epilayer thickness than was predicted by the punch-through model. The results were correlated with drift diffusion simulations which have been modified to account for the presence of a tunneling contact.
Analysis and application to superconducting quantum interference devices of double barrier superconducting junctions
Authors: Nakayama A. , Furukawa T. , Okabe Y.
Published: July 2002

We have calculated the dc Josephson current through double barrier superconducting junctions. Using a formula derived by Furusaki and Tsukada [Solid State Commun. 78, 299 (1991)], we have numerically estimated the current–phase relationship from the probability amplitude of the Andreev reflections in the junctions, and also discussed the resonant phenomena of quasiparticles. Moreover, using the obtained current–phase relationship, we have analyzed the characteristics of superconducting quantum interference devices containing double barrier superconducting junctions.
Bragg and Laue x-ray diffraction study of dislocations in thick hydride vapor phase epitaxy GaN films
Authors: Ratnikov V. , Kyutt R. , Shubina T. , Paskova T. , Valcheva E. , Monemar B.
Published: July 2002

The dislocation structure of hydride vapor phase epitaxial thick GaN layers grown on sapphire is studied by analysis of the microdistortion tensor components. Symmetrical reflections (including reflections from planes forming a large angle with the basal plane) with two modes of scanning (θ and θ–2θ) in two geometries (Bragg and Laue) are used to obtain the tensor components. The instant connections between the tensor components and major dislocation types are specified. Different types of dislocation distributions have been identified in the thick GaN films grown on sapphire with and without undoped and Si-doped metalorganic chemical vapor deposited templates. Transmission electron microscopy was used to confirm the x-ray results by direct visualization of defect rearrangements.
Bimodal spatial distribution of pores in anodically oxidized aluminum thin films
Authors: Behnke J. F. , Sands T.
Published: July 2002

Though porous anodic aluminum oxide has been the subject of considerable research since the 1950s, little attention has been devoted to the characterization of the self-organization of the pore structures, and fewer of these studies have focused on anodization of thin films. The degree to which these structures self-organize, however, could play a vital role in future applications of porous anodic aluminum oxide. In this study a model is developed to describe pore ordering in thin anodized aluminum films. The model is based on a radial distribution function approach to describe the interpore spacings. Idealized one-dimensional and two-dimensional (2D) radial distribution functions are combined by linear superposition to approximate experimental radial distribution functions. Using these radial distribution functions, an order parameter is developed and an improved definition of pore spacing is constructed. This method confirms that the oxide initially forms with a highly frustrated porous structure and reorganizes toward greater 2D order as the oxide grows into the film.
Optimized exchange biasing by controlled in situ oxidation
Authors: Gruyters M. , Riegel D.
Published: July 2002

A new, simple method is presented for the preparation of thin Co/CoO bilayers exhibiting considerably improved characteristics of the magnetic behavior and large exchange bias effects. The ultrathin antiferromagnetic layer is formed at room temperature by controlled in situ exposure of a clean metal layer to pure oxygen gas. The exposure amounts only 100–10 000 L at an oxygen pressure of 10−5 mbar. High purity gases can be used. This method allows to maintain optimum ultrahigh vacuum conditions in the preparation chamber which is an indispensable condition for the preparation of complex layer systems.
Quantitative two-dimensional carrier profiling of a 400 nm complementary metal–oxide–semiconductor device by Schottky scanning capacitance microscopy
Authors: Tran T. , Nxumalo J. N. , Li Y. , Thomson D. J. , Bridges G. E. , Oliver D. R.
Published: July 2002

Carrier profiling of a 400 nm complementary metal–oxide–semiconductor device has been accomplished by combining metal–semiconductor capacitance–voltage profiling techniques with two-dimensional scanning probe microscopy. When a metal probe is brought into contact with a semiconductor, a space-charged depletion region and therefore a capacitor is formed at the junction. By applying a small ac voltage, the voltage derivative of the contact capacitance can be measured with a lock-in amplifier. The amplitude of the derivative signal is a function of the carrier concentration, and the sign gives the type of carrier. The present work concentrates on the two dimensional (2D) carrier profiling of a 400 nm metal–oxide–semiconductor field effect transistor. The results demonstrate that this technique is capable of quantitative 2D characterization of semiconductor devices.
InAs quantum wires in InP-based microdisks: Mode identification and continuous wave room temperature laser operation
Authors: Seassal C. , Letartre X. , Brault J. , Gendry M. , Pottier P. , Viktorovitch P. , Piquet O. , Blondy P. , Cros D. , Marty O.
Published: July 2002

We present the design, fabrication, and characterization of an optical microsource, which comprises InAs/InP quantum wires embedded in a suspended microdisk. Comparison between photoluminescence measurements and theoretical analysis allows for a clear identification of the whispering gallery modes. cw room temperature lasing operation is demonstrated.
Temperature dependent electrical resistivity in nanocrystalline gold films made by advanced gas deposition
Authors: Ederth J. , Kish L. B. , Olsson E. , Granqvist C. G.
Published: July 2002

Nanocrystalline gold films were made by advanced gas deposition. The mean crystallite diameter lay between ∼10 and ∼80 nm depending on substrate temperature during deposition and annealing post-treatment, as found by x-ray diffractometry and transmission electron microscopy. Temperature dependent resistivity ρ was measured in the 4<T<300 K range. The log ρ versus log T relationship displayed a discontinuity at a temperature Tcr lying between 10 and 7.2 K depending on crystallite size. This can be understood as a result of ρ being dominated by electron scattering against phonons and grain boundaries above and below Tcr, respectively.
High-amplitude high-frequency oscillations of temperature, electron–hole pair concentration, and current in silicon-on-insulator structures
Authors: Dobrovolsky Valentin N. , Ishchuk Larisa V. , Ninidze Georgii K. , Balucani Marco , Ferrari Aldo , Lamedica Giulio
Published: July 2002

Interacting oscillations of the current, lattice temperature, and concentration of thermally generated electron–hole pairs were discovered in silicon films of silicon-on-insulator structures upon their heating with extremely high current. The nature of the oscillations discovered is fundamentally different from what has yet been known. They occur owing to two competing processes: the thermal generation of electron–hole pairs, and the pair concentration reduction caused by the current flowing through the silicon film with nonuniform temperature field. In our experiments the current density reached 1.5×105 A/cm2 and the specific power dissipated in the silicon film exceeded 3.6 GW/cm3. We observed the oscillation frequencies up to 3 MHz and variations of the current and pair concentration were more than tenfold, while the temperature varied from 700–740 to 950–1300 K.
Stress induced magnetic anisotropy and coercivity in Fe73.5Cu1Ta3Si13.5B9 amorphous alloy
Authors: Miguel C. , Murillo N. , González J.
Published: July 2002

The magnetic anisotropy induced by stress annealing (500 MPa) in Fe73.5Cu1Ta3Si13.5B9 amorphous alloy has been investigated. Such thermal treatment was carried out by means of the Joule heating technique (current density ranging from 20 to 40 A/mm2). As a result, a uniaxial in-plane magnetic anisotropy was induced in the samples. The dependence of this induced anisotropy on the current density shows a maximum (1000 J/m3) around 34 A/mm2, which can be connected to the first stages of the crystallization process, followed by a minimum at 37 A/mm2. The coercive field increases with this kind of thermal treatment, and is around 10–15 A/m for the optimum nanocrystalline state, which is one order of magnitude larger than that reported for this nanocrystalline material treated in conventional furnace. The variations of the coercive field after stress annealing seem to be correlated to the induced magnetic anisotropy behavior. The minimum anisotropy occurs for the optimum nanocrystallization process. The induced anisotropy should be mainly understood as of magneto-elastic character arising in the FeSi grains due to internal stresses.
An improved ab initio relativistic zeroth-order regular approximation correct to order 1/c2
Authors: Klopper Wim , van Lenthe Joop H. , Hennum Alf C.
Published: July 2002

The equations of the original ab initio scalar-relativistic zeroth-order regular approximation (ZORA) and the infinite-order regular approximation (IORA) are expanded in orders of 1/c2. It is shown that previous ZORA/IORA implementations in ab initio quantum chemistry programs were not correct to order 1/c2, but contained imperfections leading to fictitious self-interactions. These errors can be avoided by adding exchange-type terms (coupling the large and small components) to the relativistic ZORA correction to the Hamiltonian, yielding improved ab initio relativistic zeroth- and infinite-order regular approximations that are correct to order 1/c2. The new methods have been tested numerically by computing the total energies, orbital energies, and static electric dipole polarizabilities of the rare gas atoms He through Xe.
Structural analysis of interfacial layers in Ti/Ta/Al ohmic contacts to n-AlGaN
Authors: Lim S.-H. , Swider W. , Washburn J. , Liliental-Weber Z.
Published: July 2002

Detailed structure of the interfacial layers of Ti/Ta/Al ohmic contacts to n-type AlGaN/GaN/sapphire are investigated by means of transmission electron microscopy. High-resolution electron microscopy (HREM), optical diffractograms, and computer simulations confirmed that TiN (∼10.0 nm) and Ti3AlN (∼1.4 nm) interfacial layers form at the interface between the Ti layer and the Al0.35Ga0.65N substrate by a solid state reaction during annealing for 3 min in N2 at 950 °C. The orientation relationship between Ti3AlN and Al0.35Ga0.65N was found to be: [011]Ti3AlN∥[21̄1̄0]Al0.35Ga0.65N and (11̄1)Ti3AlN∥(0001)Al0.35Ga0.65N. The cubic Ti3AlN interfacial layer has a lattice parameter of 0.411±0.003 nm with the space group Pm3m matching that of Al0.35Ga0.65N. A model of the atomic configurations of the Ti3AlN/Al0.35Ga0.65N interface is proposed. This model is supported by a good match between the simulated and the experimental HREM image of the Ti3AlN/Al0.35Ga0.65N interface. The formation of TiN and Ti3AlN interfacial layers appears to be responsible for the onset of the ohmic contact behavior in Ti/Ta/Al contacts.
Magnetotransport properties of bismuth films on p-GaAs
Authors: Vereecken P. M. , Sun L. , Searson P. C. , Tanase M. , Reich D. H. , Chien C. L.
Published: July 2002

Polycrystalline Bi films were deposited onto p-GaAs(100) by electrochemical deposition. Annealing resulted in the formation of large grains with a preferred [012] orientation. The p-GaAs/Bi junctions were rectifying and the barrier height and ideality factor decreased with increasing film thickness. For films greater than 0.5 μm in thickness, the barrier height was about 0.56 eV and the ideality factor was between 1.1–1.2 for both as-deposited and annealed films. The resistance of the as-deposited films exhibited a negative temperature coefficient whereas the annealed films exhibited a positive temperature coefficient due to the limiting carrier mean free path. The magnetoresistance (MR) exhibited a quasilinear field dependence with an MR effect as large as 5600 (560 000%) at 5 K and 2.2 (220%) at room temperature.
Electron field emission from Ti-containing tetrahedral amorphous carbon films deposited by filtered cathodic vacuum arc
Authors: Ding Xing-zhao , Li Y. J. , Sun Z. , Tay B. K. , Lau S. P. , Chen G. Y. , Cheung W. Y. , Wong S. P.
Published: July 2002

Titanium-containing tetrahedral amorphous carbon (ta-C:Ti) films with different titanium content were deposited by a filtered cathodic vacuum arc technique. The microstructure of these films was confirmed to be of ta-C+TiCx(x<1) nanocomposite by Rutherford backscattering spectroscopy, x-ray diffraction, and micro-Raman spectroscopy experiments. With the increase of titanium content, the titanium carbide content increased and the sp3 fraction in the residual ta-C phase decreased gradually. In the electron field emission tests, it was found that proper conditioning processes are necessary for all these films in order to get a steady reproducible emission behavior. After conditioning, the emission threshold field of the films is about the same value, around 10 V/μm, except for the film with the lowest titanium content (∼1.2 at%) of which the threshold field is much higher, around 17–18 V/μm. The optimum titanium concentration in the film for field emission, showing the highest emission current and emission site density, is about 12 at%. After field emission testing, graphitization was involved and the titanium carbide phase, at least some of the sub-stoichiometric TiCx phase, in the ta-C:Ti films decomposed.
Relativistic laser guiding in an azimuthal magnetic field in a plasma
Authors: Liu C. S. , Tripathi V. K.
Published: July 2002

Strong azimuthal magnetic field, Bs θ̂, generated in an intense laser plasma interaction, has a focusing effect on the laser. The rays having E⊥Bs propagate as an X mode with a refractive index maximum on the axis and are focused. However, rays with E∥Bs propagate unaffected by the magnetic field. The relativistic mass nonlinearity reinforces additional focusing on the laser. Under the combined effect of both focusing mechanisms a linearly polarized laser emerges with a reduced spot size and elliptical cross section. This process may provide a diagnostic means for measuring the azimuthal magnetic field.
Low-lying excited states of HO2–HONO, HO2–HONO2, and HO2–HO2NO2 complexes
Authors: Li Yumin , Francisco Joseph S.
Published: July 2002

The vertical excitation energies are calculated for the three lowest excited states of HO2 and the HO2–H2O, HO2–HONO, HO2–HONO2, and HO2–HO2NO2 complexes. The first excited state of each complex is found to be similar to HO2, but generally shifted from that of the HO2 monomer. There is also a dependence of the shift on the binding energy of the complex.
Spectroscopic signatures of bond-breaking internal rotation. I. Saddle point induced polyad breakdown
Authors: Jacobson Matthew P. , Child Mark S.
Published: July 2002

With the input of order 10–20 000 cm−1 of vibrational energy, the hydrogen atoms in small gas-phase molecules such as HCN, HCP, and acetylene can undergo internal rotation about the heavy-atom core (CN–CP–CC), breaking and reforming covalent bonds in the process. This article investigates the quantum and classical dynamics of covalent bond-breaking internal rotation, particularly the vibrational energy flow between the hindered internal rotor mode and a stretch mode. The aim is to relate polyad effective Hamiltonian techniques, which have been highly successful in the analysis of high overtone spectra, to the theory of isomerization rates. That is, as approximate constants of motion, polyad numbers constrain vibrational energy flow, and we investigate the extent and mechanism of their breakdown due to nascent bond-breaking internal rotation. Our simple model consists of a spherical pendulum coupled to a harmonic oscillator, which admits a number of analytical results. The central conclusion is that polyad breakdown is a generic consequence of higher order resonances induced by a saddle point but is far from complete, in the sense that the majority of states with energies close to the saddle point can continue to be labeled with polyad numbers; only those with substantial probability density close to the saddle point itself no longer belong to moderately well defined polyads. Our model is particularly relevant to the vibrational structure of HCP, the polyad structure of which has been well studied up to ∼19 000 cm−1; our model predicts systematic polyad breaking at higher energies.
Role of transverse velocity-shear on collisional drift-wave instability in magnetized plasmas
Authors: Chakrabarti Nikhil
Published: July 2002

The effect of sheared poloidal flow on collisional drift-wave instability of magnetized plasma is studied. It is shown that transverse flow shear considerably reduced the growth of the instability. A finite but small amount of viscosity completely stabilized the mode.
Fast disruptions by ballooning mode ridges and fingers in high temperature, low resistivity toroidal plasmas
Authors: Kleva Robert G. , Guzdar Parvez N.
Published: July 2002

The nonlinear evolution of ballooning modes in high temperature, low resistivity tokamaks is investigated. Convection cells driven by unstable ballooning modes rapidly convect the hot central plasma to the wall in ridges whose two-dimensional poloidal projection resembles fingers. As the resistivity η is reduced in magnitude, the number of fingers increases, with the width of each individual finger becoming more narrow. Importantly, the rate at which energy is transported to the wall is unchanged as η is reduced. Because of the increasingly fine scale fingers generated as η is decreased, the time scale for diffusion across the fingers is independent of the magnitude of the resistivity. As a consequence, the magnetic field lines are decoupled from the plasma (not “frozen-in”) even as η approaches zero. The limit of resistive magnetohydrodynamics (MHD) as η→0 is not the same as ideal MHD with η≡0.
Polarization fatigue in Pb(Zn1/3Nb2/3)O3–PbTiO3 ferroelectric single crystals
Authors: Ozgul Metin , Takemura Koichi , Trolier-McKinstry Susan , Randall Clive A.
Published: July 2002

Pb(Zn 1/3 Nb 2/3 ) O 3 –PbTiO 3 (PZN–PT) single crystal ferroelectrics have been studied to determine the relative rates of polarization fatigue. It was recently found that ferroelectrics with the rhombohedral phase in the PZN–PT solid solution have essentially no polarization fatigue in the [001]C directions (all of the orientations in this article will be given in terms of the prototype cubic (m3m) axes, denoted by the subscript C). In this study, we expand upon this observation to correlate fatigue rates more completely with respect to composition, orientation, temperature, and electric field strength. It is inferred that an engineered domain state in relaxor based ferroelectric crystals with the spontaneous polarization inclined to the normal of the electrode is associated with negligible or no fatigue at room temperature. However, if thermal history, temperature, or field strength induces a phase transition that produces a polarization parallel to the normal of electrode, these orientations fatigue. The relative fatigue rates are also studied as a function of temperature. In directions, such as [111]C in the ferroelectric rhombohedral phase, the polarization fatigues at room temperature, but as temperature is increased the fatigue rate systematically decreases. This is explained in terms of a thermally activated process that limits the net fatigue rate of ferroelectrics. In summary, this article gives information on the polarization states and orientation that control fatigue in ferroelectric crystals with a relaxor end member.
Nonexponential dielectric relaxation dynamics in supercooled liquid and glassy states of isoamyl bromide and 2-methylpentane mixtures
Authors: Kalinovskaya O. E. , Vij J. K.
Published: July 2002

The dielectric spectra of mixtures of the polar solute isoamyl bromide in 2-methylpentane have been investigated in the frequency range 1 mHz to 1 MHz and in the temperature range approaching the glass transition temperature. The results obtained from the spectra are compared with those obtained recently [J. Chem. Phys. 111, 10979 (1999)] on pure isoamyl bromide. It is found that on increasing dilution with the nonpolar solvent, the width of the curves of the dielectric spectra increase significantly, and this is reflected in the increase in the nonexponential nature of the relaxation dynamics. This is found to be a consequence of the decrease in the cooperativity of the relaxation dynamics and or an increase in the heterogeneity of the solution. The data are found to fit the Havriliak–Negami equation extremely well. The data at low and high frequencies also fits the “universal law,” since the latter is a low and high frequencies limiting case of the Havriliak–Negami equation. The scaling parameters of this law are calculated for the 25 mol % solution of isoamyl bromide in 2-methylpentane, and these are shown to experimentally relate to the H–N parameters. The stretched exponential parameter, γ, is estimated as a function of the temperature and is shown to follow the equation γ≈a(T−T0). Vogel–Fulcher–Tammann equation fits the data of the relaxation peak frequency as a function of the inverse of absolute temperature for the various mixtures quite well, this being possibly a consequence of the temperature dependence of the stretched exponential parameter. The predictions from the mode coupling theory and those by Bertrand and Souletie are verified with the exception that the exponent is found to be much greater than predicted by these theories. The relative predominance of the Johari–Goldstein process in isoamyl bromide increases initially with dilution with 2-methylpentane and then disappears as the number density of the independent relaxors increases with further dilution.
Kinetics of the self compression and heating of a dense plasma
Authors: Pozwolski Alexandre E.
Published: July 2002

A dense degenerated oxygen plasma at a temperature lower than the equilibrium temperature is considered. Such plasma will reheat because of contraction; the kinetics of such a process is investigated by integration of the Poincaré–Eddington equation. The temperature may increase by a factor four in less than 4 ns.
Direct-fitting approach to the analysis of high-resolution optical spectra: Monte Carlo and experimental studies of OH A(0)→X(0) spectra
Authors: Tellinghuisen Joel
Published: July 2002

The traditional approach for analyzing high-resolution optical spectra involves locating and measuring line frequencies, then assigning and fitting these to an appropriate model for the molecular energy levels. The alternative approach of fitting the spectrum directly is particularly appealing in the case of congested spectra with many overlapped features. The capabilities and limitations of direct fitting are explored in application to the 0–0 band of the OH A→X transition, as recorded on a charge-coupled-device array using a high-pressure Tesla discharge source. Monte Carlo calculations confirm that for Poisson (counting) data, the parameter errors from the variance–covariance matrix are trustworthy, unless weights are neglected, in which case the error estimates can be wildly optimistic. In the fitting of actual spectra, a major barrier is the derivation of a suitable instrumental line-shape function. In particular, the often adopted triangular slit function is grossly inadequate. A systematic procedure has been devised for obtaining the line shape as a sum of Gaussian and Lorentzian components. This method has facilitated the analysis of experimental spectra spanning the region 3071–3103 Å. A model having 46 adjustable parameters determines the temperature (320 K) of the low-N′ levels of the A state with σ < 1 K and corroborates theoretical intensity branching ratios within 2%. It also determines within 10% the contribution to the emission from OD, which is present in natural abundance (H:D≈7000) in the source.
Quasilinear theory of mode interaction in gyrotrons with azimuthally inhomogeneous electron emission
Authors: Nusinovich G. S. , Botton M.
Published: July 2002

A theory is developed which describes the effect of the azimuthal inhomogeneity of the electron current on the gyrotron operation. The theory is simplified by using a quasilinear approach, which allows one to study analytically the stability of oscillations in free-running and phase-locked gyrotron oscillators with inhomogeneous current emission. Also the numerical analysis of nonstationary processes in a three-mode gyrotron oscillator is carried out. The results show that, at low currents, the inhomogeneity may cause excitation of additional modes at the frequency of the operating one. At higher currents, one can observe the onset of phase-locked triplets with equidistant frequency spectrum. As the inhomogeneity increases, this enhances the beating effects in such triplets, e.g., the mode amplitudes exhibit some oscillations whose period is inversely proportional to their frequency separation and whose modulation increases with the inhomogeneity in the beam current.
Magnetized plasma flow through a small orifice
Authors: Gunn J. P.
Published: July 2002

Deuterium plasma flow through a circular hole in a flat conducting plate is simulated by the two-dimensional object-oriented particle-in-cell code (XOOPIC) [J. P. Verboncoeur, A. B. Langdon, and N. T. Gladd, Comp. Phys. Comm. 87, 199 (1995)]. A constant magnetic field is oriented perpendicular to the plate surface, and parallel to the cylindrical axis of the hole. Charge neutralization on the interior surface of the hole leads to attenuation of the ion current throughput. The attenuation is stronger than would be expected from a finite Larmor radius model, due to acceleration of ions by the self-consistent radial electric field. The current attenuation has been measured by comparing two Langmuir probes that were operated simultaneously under a wide range of plasma conditions in Tore Supra [Equipe Tore Supra, IAEA-CN-64/02-2, International Atomic Energy Agency, Vienna, p. I-41 (1996)]. One probe was exposed directly to the plasma and the other was hidden behind a graphite shield pierced with either 3 or 4 mm diameter holes. Both the ion current attenuation and the floating potential drop are in reasonable agreement with the simulation results.
Anisotropic jump model of the rotational dynamics in glasses
Authors: Alessi L. , Andreozzi L. , Faetti M. , Leporini D.
Published: July 2002

Anisotropic jump reorientation occurs in glasses, polymers, and plastic crystals. A general theoretical frame to describe such processes is presented. It generalizes previous work by Ivanov [Sov. Phys. JETP 18, 1041 (1964)]. A tractable model is given by a simple phenomenological assumption on the jump distribution. Analytical results and fast numerical methods to evaluate the relevant quantities are derived. The model is validated by comparing the predictions with ESR experiments on stiff, cylindrical tracers dissolved in the glassformer o-terphenyl.
Exact solutions to magnetized plasma flow
Authors: Wang Zhehui , Barnes Cris W.
Published: July 2002

Exact analytic solutions for steady-state magnetized plasma flow (MPF) using ideal magnetohydrodynamics formalism are presented. Several cases are considered. When plasma flow is included, a finite plasma pressure gradient ∇p can be maintained in a force-free state J×B=0 by the velocity gradient. Both incompressible and compressible MPF examples are discussed for a Taylor-state spheromak B field. A new magnetized nozzle solution is given for compressible plasma when U∥B. Transition from a magnetized nozzle to a magnetic nozzle is possible when the B field is strong enough. No physical nozzle would be needed in the magnetic nozzle case. Diverging-, drum- and nozzle-shaped MPF solutions when U⊥B are also given. The electric field is needed to balance the U×B term in Ohm’s law. The electric field can be generated in the laboratory with the proposed conducting electrodes. If such electric fields also exist in stars and galaxies, such as through a dynamo process, then these solutions can be candidates to explain single and double jets.
Temporal behavior of the wall voltage in a surface-type alternating current plasma display panel cell using laser induced fluorescence spectroscopy
Authors: Kim Jung Hun , Lee Jun Hak , Whang Ki-Woong , Choi Young Wook
Published: July 2002

Electric fields were measured using laser induced fluorescence spectroscopy and the wall voltage was estimated from the measured electric fields in a surface-type alternating current plasma display panel cell with a helium discharge (100 Torr) driven by square sustaining pulses. The wall voltage showed very complicated, temporally dynamic behavior. The polarity of the wall voltage changed rapidly as soon as the plasma was ignited, and its magnitude continuously increased due to the continuous injection of charged particles onto the dielectric surface from the afterglow plasma during the rest of the pulse-on period. When there was a self-erasing discharge at the instant of the pulse turn-off, the wall voltage dropped sharply by about 110 V and decreased continuously owing to the diffusion-induced charge redistribution or leakage. The decay rate of the wall voltage during the pulse-off period was very dependent on the surface condition of the protecting layer of the dielectric.
Quantum-mechanical effects on low energy electron–ion Coulomb bremsstrahlung in dense high-temperature plasmas
Authors: Jung Young-Dae
Published: July 2002

The low energy electron–ion Coulomb bremsstrahlung process is investigated in dense high-temperature plasmas including quantum-mechanical effects. An effective pseudopotential model taking into account both quantum-mechanical effects and plasma screening effects is applied to describe the electron–ion interaction potential in dense high-temperature plasmas. The straight-line trajectory method is applied to the motion of the projectile electron in order to investigate the variation of the bremsstrahlung radiation cross section as a function of the scaled impact parameter, thermal de Broglie wavelength, Debye length, projectile energy, and photon energy. The result shows that the quantum-mechanical effect reduces the bremsstrahlung radiation cross section in dense high-temperature plasmas. It is found that the quantum-mechanical effect is important for small impact parameters and more significant for the soft photon case.
Structural changes during annealing of GaInAsN
Authors: Kurtz Sarah , Webb J. , Gedvilas L. , Friedman D. , Geisz J. , Olson J. , King R. , Joslin D. , Karam N.
Published: July 2002

The alloy GaInAsN has great potential as a lower-band-gap material lattice matched to GaAs, but there is little understanding of what causes its poor optoelectronic properties and why these improve with annealing. This study provides information about the structural changes that occur when GaInAsN is annealed. The Fourier transform infrared spectra exhibit two primary features: a triplet at ∼470 cm−1 (Ga–N stretch) and two or three bands at ∼3100 cm−1 (N–H stretch). The change in the Ga–N stretch absorption can be explained if the nitrogen environment is converted from NGa4 to NInGa3 after annealing. The N–H stretch is also changed after annealing, implying a second, and unrelated, structural change.
Ab initio calculations of excited states in C4H and implications for ultraviolet photodissociation
Authors: Graf Stephan , Geiss Johannes , Leutwyler Samuel
Published: July 2002

Extensive multiconfiguration ab initio calculations on the ground state and electronic excited states of the C4H radical (butadiynyl) are presented. Using multiconfiguration self-consistent field (MCSCF) structure optimization the estimated complete basis set limit for the excitation energy to the first excited state was calculated to be 0.030 eV employing second-order multireference perturbation theory and 0.035 eV using multireference configuration interaction. States up to 8 eV above the 12Σ+ ground state were investigated. Harmonic vibrational frequencies for the ground state and the four lowest Π2 states are reported at the MCSCF level of theory. The 32Σ+ state is the first state above the C–H bond dissociation threshold at 5.71 eV with a large transition dipole moment, so the 32Σ+←12Σ+ transition could lead to dissociation of the C–H bond. The estimated maximum rate for photodissociation is 1.4–3.1×10−6 s−1 at 1 astronomical unit. Thus C4H has a significantly long lifetime in which to be detectable in large parts of cometary comas.
Excited state potential energy surfaces from the inversion of absorption spectra: Removal of a global singularity
Authors: Zhu Wusheng , Rabitz Herschel
Published: July 2002

This paper develops inversion algorithms to extract an excited electronic state potential energy surface based on absorption spectral data from assumed known lower reference electronic states. A global singularity problem is identified which may also play an important role in other inversion algorithms. The means to remove or circumvent the singularity are discussed. Simulations of the proposed inversion algorithms are tested in a one-dimensional model system.
Excitation of the symmetry forbidden bending mode in molecular photoionization
Authors: Miller J. Scott , Poliakoff E. D. , Miller Thomas F. , Natalense Alexandra P. P. , Lucchese Robert R.
Published: July 2002

We present results on the energy dependence of the vibrational branching ratio for the bending mode in CO2 3σu−1 photoionization. Specifically, we determine the v+=(0,1,0)/v+=(0,0,0) intensity ratio by detecting dispersed fluorescence from the electronically excited photoions. The results exhibit large deviations over a very wide energy range, 18<hνexc<190 eV. Production of the v+=(0,1,0) level of the ion from the v0=(0,0,0) ground state is forbidden by symmetry, and while observations of such features are well established in photoelectron spectroscopy, their appearance is normally ascribed to vibronic coupling in the ionic hole state. In this case, we find that such explanations fail to account for the energy dependence of the branching ratio. These deviations indicate that the continuum photoelectron participates in transferring oscillator strength to the nominally forbidden vibrational transition. A theoretical framework is developed for interpreting the experimental data, and Schwinger variational calculations are performed. These calculations demonstrate that the continuum electron is responsible for the observation of the excited bending mode as well as its energy dependence. This is an intrachannel effect that is best described as photoelectron-induced vibronic symmetry breaking. This appears to be a general phenomenon, and it may be useful in illuminating connections between bond angle and photoionization spectroscopies. The magnitude of these deviations display the utility of vibrationally resolved studies, and the extent over which these changes occur underscores the necessity of broad range studies to elucidate slowly varying characteristics in photoionization continua.
Adsorption–desorption kinetics and chemical potential of adsorbed and gas-phase particles
Authors: Zhdanov V. P.
Published: July 2002

In the literature, one can find two alternative ways of using the chemical potential of adsorbed and gas-phase particles, μa and μg, for describing the adsorption–desorption kinetics. According to the first approach, the desorption rate depends only on μa. The second approach, proposed by Ward et al. in a series of papers published in the Journal of Chemical Physics, predicts that the desorption rate is proportional to exp[(μa−μg)/kBT]. Scrutinizing the formalism used by Ward et al., we show that the latter dependence makes no sense because it contradicts the basic principles of the general theory of activated rate processes.