bluuebunny/crossref_metadata_2025 huggingface dataset:
Strong temperature-dependent-viscosity effects on a rivulet draining down a uniformly heated or cooled slowly varying substrate
Authors: Wilson S. K. , Duffy B. R.
Published: March 2003

We use the lubrication approximation to investigate the steady locally unidirectional gravity-driven draining of a thin rivulet of Newtonian fluid with temperature-dependent viscosity down a slowly varying substrate that is either uniformly hotter or uniformly colder than the surrounding atmosphere. We consider the situation in which the Biot number (and hence the variation of temperature across the rivulet) is small, but in which the variation of viscosity with temperature is sufficiently strong that thermoviscosity effects appear at leading order in the limit of small Biot number. Three different models for the dependence of viscosity on temperature (specifically, the linear, exponential and Eyring models) are considered, but our attention is concentrated on the more realistic exponential and Eyring models (which coincide at leading order in the limit of small Biot number). We show that the effect of cooling the atmosphere is always to widen and deepen the rivulet, while the effect of heating the atmosphere is always to narrow and shallow it. We interpret our results as describing a slowly varying rivulet draining in the azimuthal direction from the top to the bottom of a large horizontal circular cylinder, and find that the behavior of the rivulet is rather different on the upper and lower parts of the cylinder (i.e., for sessile and pendent rivulets). Specifically, the effect of strong cooling of the atmosphere is to produce a wide rivulet with finite uniform thickness on the upper part of the cylinder, but a deep rivulet with finite semi-width on the lower part of the cylinder. On the other hand, the effect of strong heating of the atmosphere is to produce a narrow and shallow rivulet everywhere except near the top and the bottom of the cylinder.
Improved Lagrangian mixing models for passive scalars in isotropic turbulence
Authors: Fox R. O. , Yeung P. K.
Published: March 2003

Lagrangian data for velocity, scalars, and energy and scalar dissipation from direct numerical simulations are used to validate Lagrangian mixing models for inert passive scalars in stationary isotropic turbulence. The scalar fluctuations are nearly Gaussian, and, as a result of production by uniform mean gradients, statistically stationary. Comparisons are made for Taylor-scale Reynolds numbers in the range 38 to about 240 and Schmidt numbers in the range 1/8 to 1. Model predictions for one-point, one-time Eulerian statistics (Eulerian correspondence) and one-particle, two-time Lagrangian statistics (Lagrangian correspondence) are examined. Two scalar mixing models, namely the Lagrangian Fokker–Planck model and the Lagrangian colored-noise (LCN) model, are proposed and written in terms of stochastic differential equations (SDE) with specified drift and diffusion terms. Both of these models rely on statistics of the scalar field conditioned upon the energy dissipation, as provided by the Lagrangian spectral relaxation (LSR) model. With the exception of the scalar dissipation, the models are shown to capture the Reynolds and Schmidt-number dependence of the Lagrangian integral time scales. However, the LCN model provides a more realistic description of the Lagrangian scalar fluctuations as differentiable time series having the correct form of the scalar autocorrelation function. Further extensions of the new mixing models to non-Gaussian scalars are conceptually straightforward, but require a closure for the scalar-conditioned scalar dissipation rate matrix. Likewise, accurate prediction of joint statistics for differential diffusion between different scalars with unequal molecular diffusivities will require the formulation of a multiscale SDE similar to the LSR model.
A simple model for the calculation of nonlinear optical response functions and femtosecond time-resolved spectra
Authors: Gelin M. F. , Pisliakov A. V. , Egorova D. , Domcke W.
Published: March 2003

A simple model is developed for the calculation of optical response functions for electronically nonadiabatic systems coupled to a heat bath. A phenomenological dissipation ansatz is suggested which describes the major bath-induced relaxation processes, namely excited-state population decay, optical dephasing, and vibrational relaxation. The model is constructed in such a way that it allows one to express the nonlinear response functions for a dissipative system in terms of those for the corresponding bath-free system. The explicit formulas for the third order response functions are given. Several special cases are considered for which the evaluation of the response functions simplifies considerably. To illustrate the performance and validity of the theory, the response functions have been used for the calculation of time and frequency gated spontaneous emission spectra. The time-dependent spectra derived from the model have been compared with those calculated (i) for the standard damped harmonic oscillator model and (ii) for a model nonadiabatic electron-transfer system with Redfield theory. It is concluded that the present model provides qualitatively correct response functions and, therefore, may be useful for the interpretation of observed time-resolved spectra.
Cell multipole method for molecular simulations in bulk and confined systems
Authors: Zheng Jie , Balasundaram Ramkumar , Gehrke Stevin H. , Heffelfinger Grant S. , Goddard William A. , Jiang Shaoyi
Published: March 2003

One of the bottlenecks in molecular simulations is to treat large systems involving electrostatic interactions. Computational time in conventional molecular simulation methods scales with O(N2), where N is the number of atoms. With the emergence of new simulations methodologies, such as the cell multipole method (CMM), and massively parallel supercomputers, simulations of 10-million atoms or more have been performed. In this work, the optimal hierarchical cell level and the algorithm for Taylor expansion were recommended for fast and efficient molecular dynamics simulations of three-dimensional (3D) systems. CMM was then extended to treat quasi-two-dimensional (2D) systems, which is very important for condensed matter physics problems. In addition, CMM was applied to grand canonical ensemble Monte Carlo simulations for both 3D and 2D systems. Under the optimal conditions, our results show that computational time is approximately linear with N for large systems, average error in total potential energy is about 0.05% for 3D and 0.32% for 2D systems, and the RMS force error is 0.27% for 3D and 0.43% for 2D systems when compared with the Ewald summation.
Preferential formation of neutral C10 upon laser vaporized graphite in He gas as studied by photoionization mass spectroscopy with 10.5 eV photons
Authors: Kato Yoshiyasu , Wakabayashi Tomonari , Momose Takamasa
Published: March 2003

Neutral carbon clusters produced from laser-ablated graphite in a supersonic pulsed-helium expansion source were studied by time-of-flight (TOF) mass analysis using single-photon ionization with 10.5 eV photons. Varying the delay time of an ionization laser pulse relative to a vaporization pulse, we found that a signal of C10, along with a weaker signal of C12, was intensified almost exclusively to the other Cn signals with relatively long delay times of 80–250 μs. We observed two distinctly different TOFs for one and the same size, a short TOF at shorter delay times and a long TOF at longer delay times. We attribute the difference in TOF to the difference in initial velocity of the neutral cluster. We also performed the experiment within a high vacuum to find a similar difference in TOF for clusters of the same mass. The bimodal arrival-time distribution from the source to the ionization region indicates that the bunch of laser-ablated clusters separates into two bunches with different group velocities. We attribute this separation to the formation of a relatively dense layer of clusters. During collisions behind this layer, the relatively stable neutral C10, probably of a monocyclic structure, is formed preferentially. This must be the origin of the selective detection of C10 at the longer delay times. Using He as a buffer gas, the signal of the C10 was found to be of a magnitude two orders more pronounced than within the high vacuum.
Steric effect in the endothermic Penning ionization reaction of tert-butyl bromide with Kr(3P)
Authors: Koizumi K. , Ohoyama H. , Okada S. , Kasai T.
Published: March 2003

The steric opacity function (the dependence of reaction cross section upon mutual molecular orientation) for the endothermic Penning ionization channel of Kr(3P)+(CH3)3CBr reaction was determined by using an oriented tert-butyl bromide molecular beam at 0.1 eV average collision energy. A remarkably large steric effect was observed in contrast with the Ar(3P)+CH3Br reaction. We find that the ionization cross section is maximum at sideways and it becomes “zero” at the tert-butyl end. The Br end is found to be unfavorable as compared with sideways approaches. The stereo selectivity observed in the present work is discussed in terms of the electron exchange mechanism, in which the shielding effect by bulky nonreactive tert-butyl group as well as the smearing effect in impact parameter are taken into account.
Broadband population inversion by a frequency-swept pulse beyond the adiabatic approximation
Authors: Mitschang Lorenz , Rinneberg Herbert
Published: March 2003

Frequency swept pulses are used to invert population over a broad band of transition frequencies without the need for a precise calibration of the pulse amplitude. As long as the adiabatic approximation is valid, population is adiabatically inverted at each individual transition frequency of the swept range. Even though this picture fails to be true when the adiabatic approximation breaks down, population inversion can still be achieved for an appreciable range of transition frequencies and field amplitudes. We discuss population inversion in an ensemble of two-level systems by a frequency-swept pulse, the so-called constant adiabaticity pulse, without invoking the adiabatic approximation. The equations of motion are integrated by a seminumerical method to analyze population inversion in the regime where usually the adiabatic approximation is applied. The effects of resonance offset and variable pulse amplitude are described by an average Hamiltonian expansion to discuss pulse performance beyond the validity of the adiabatic approximation. As a function of the adiabaticity parameter (reciprocal of the pulse area), the inversion bandwidth increases in a stepwise fashion due to the consecutive cancellation of average Hamiltonians. The first inversion over a finite range of transition frequencies and pulse amplitudes is shown to occur for an adiabaticity parameter of 1/15.
Dissociation of multiply ionized carbonyl sulfide due to electron impact
Authors: Wang Pengqian , Vidal C. R.
Published: March 2003

The dissociation of singly to triply ionized carbonyl sulfide has been studied through electron impact ionization at an electron energy of 200 eV. Two- and three-dimensional covariance mapping techniques are used, accompanied by a focusing time-of-flight mass spectrometer. The absolute cross sections for the various dissociation channels of up to triply ionized carbonyl sulfide (OCS) have been obtained. The unstable OCS dications dissociate mostly into ion pairs, while the OCS trications dissociate into ion triple or ion pair with comparable probabilities. The total single ionization cross section agrees with the result of the binary-encounter-Bethe method. A metastable decay trace corresponding to OCS2+→CO++S+ is observed on the covariance map.
Ab initio theoretical studies on U3+ and on the structure and spectroscopy of U3+ substitutional defects in Cs2NaYCl6, 5f 26d1 manifold
Authors: Seijo Luis , Barandiarán Zoila
Published: March 2003

In this paper we present the results of spin–orbit relativistic ab initio model potential embedded cluster calculations of the 5f26d1 excited manifold of (UCl6)3− embedded in a reliable representation of the Cs2NaYCl6 elpasolite host. They are aimed at interpreting the 5f3→5f26d1 absorption bands reported by Karbowiak et al. [J. Chem. Phys. 108, 10181 (1998).] An excellent agreement is found between the calculated energies of the absorption transitions from the ground state 5f3 1 Γ8u(4I9/2) and the experimental data, which supports a detailed interpretation of the electronic nature of the absorption spectrum in the energy region 14 000–23 000 cm−1. In particular, the three unidentified electronic origins that had been experimentally detected are now assigned, and the observed bands are interpreted as having multiple electronic origins. From the structural point of view, the excited states of the 5f26d1 manifold are classified in two sets of main configuration 5f26d(t2g)1 and 5f26d(eg)1 with bond distances Re [5f26d(t2g)1]<Re[5f3]<Re[5f26d(eg)1]. The energies of the 5f26d1 manifold of free U3+ have also been calculated; experimental data on them are not available in the literature to the best of our knowledge. These results contribute to show that wave function based ab initio methods can provide useful structural and spectroscopic information, complementary to the experimental data, in studies on actinide ion impurities doping ionic hosts, where large manifolds of 5dn−16d1 excited states are involved.
Orbitally dependent magnetic coupling between cobalt(II) ions: The problem of the magnetic anisotropy
Authors: Palii Andrei V. , Tsukerblat Boris S. , Coronado Eugenio , Clemente-Juan Juan M. , Borrás-Almenar Juan J.
Published: March 2003

A comprehensive theoretical study of the magnetic exchange between Co2+ ions is reported. Using the microscopic background we deduce the general Hamiltonian for a corner shared bioctahedral system involving kinetic exchange, spin–orbit coupling, and low-symmetry local crystal field. This Hamiltonian acting within the orbitally degenerate ground manifold (4T1g)A⊗(4T1g)B of the cobalt pair is expressed in terms of orbital and spin operators. The treatment of the Hamiltonian is performed with the use of the irreducible tensor operator technique. We elucidate the major electronic factors controlling the magnetic anisotropy in the Co(II) pairs. The degree of the exchange anisotropy is shown to depend on the strength of the cubic crystal field and on the relative efficiency of two kinds of electron transfer pathways (e–e and t2–t2) contributing to the kinetic exchange. An unusual role of spin–orbit interaction is revealed. This interaction tends to reduce the anisotropy caused by the orbitally dependent exchange. Special attention is paid on the topical case when spin–orbit coupling exceeds the exchange interaction. In this case the effective Hamiltonian in its general form is projected onto the subspace of low-lying Kramers doublets and in such a way the pseudo-spin-12 Hamiltonian is derived. Unlike the commonly accepted phenomenological approaches based merely on the symmetry arguments the proposed procedure is grounded on the microscopic consideration and hence allows us to establish the interrelation between the idem parameters of the system and the effective parameters of the pseudo-spin-12 Hamiltonian. Finally, we discuss the applicability of the conventional Lines’ model.
Fe valency-induced effect on the magnetic and electrotransport properties in Nd0.67Sr0.33Mn1−xFexO3 polycrystalline system
Authors: Chang Y. L. , Ong C. K.
Published: March 2003

Systematic study is conducted on the effect of Fe valency-induced magnetic and electrotransport properties in Nd0.67Sr0.33Mn1−xFexO3 polycrystalline system. The system is found to display the properties of Nd0.67Sr0.33MnO3 and Nd0.67Sr0.33FeO3 depending on the Fe concentration. It is found that, while Mn3+ and Mn4+ enhance the double exchange in the system, Fe3+ and Fe4+ enhance the superexchange interaction in the system. Although Fe3+ weakens the ferromagnetism, Fe4+ ions enhance the electrical conductivity in the system. The large variations in resistivity with temperature for some of these compounds has potential application in the development of new magnetic recording medias, optoelectronics devices and bolometric detectors.
Mixed quantum-classical dynamics response function approach to spectroscopy
Authors: Toutounji Mohamad
Published: March 2003

Mixed quantum-classical dynamics formulation of Kapral and co-workers has been successfully employed to systems composed of a quantum subsystem coupled to an environment with classical degrees of freedom to study the dynamics of condensed many-body systems. In this formalism the quantum subsystem and the bath dynamics obey the full quantum mechanics, classical mechanics, respectively, whereas the coupling term dynamics is governed by mixed quantum-classical equations. To this end, the linear response function approach in mixed quantum-classical systems is used to derive the optical linear electronic dipole moment time-correlation function of a two-level system coupled to harmonic vibrations in condensed media. The fact that this is an exactly solvable model using full quantum mechanics allows us to test the applicability of the presented approach. An alternative approach to the aforementioned method is also developed as a second method to further test the applicability of the linear response function approach in mixed quantum-classical systems, and to confirm the correctness of the end result when using mixed quantum-classical dynamics formulation of Kapral and co-workers. Both approaches are found to yield identical results. These results are compared to those of the full quantum results in the high temperature limit. Model application of electronic absorption spectra is presented. Optical nonlinear response functions are also obtained in mixed quantum-classical systems with only linear electron–phonon coupling.
On the scaling behavior of the free energetics of nucleation
Authors: Kusaka Isamu
Published: March 2003

We study free energetics of nucleation of simple fluids using density functional theory to assess the validity of recently proposed scaling relations, which point to the existence of a common scaling function that spans across fluid phase nucleation of various materials at different values of the temperature. While particular functional forms found in the literature are of moderate success in capturing the quantitative behavior of the model systems we studied, i.e., square-well potential and truncated and shifted Lennard-Jones potential, some of the essential ingredients of the scaling propositions are found to hold quite well. For example, the free energetics of vapor phase condensation can be described by a single scaling function depending only weakly on the temperature and the details of the model potentials. The same holds for bubble nucleation.
Drop dynamics on the beads-on-string structure for viscoelastic jets: A numerical study
Authors: Li Jie , Fontelos Marco A.
Published: March 2003

It is well known that a viscoelastic jet breaks up much more slowly than a Newtonian jet. Typically, it evolves into the so-called beads-on-string structure, where large drops are connected by thin threads. The slow breakup process provides the viscoelastic jet sufficient time to exhibit some new phenomena. The aim of this paper is to investigate the drop dynamics of the beads-on-string structure. This includes drop migration, drop oscillation, drop merging and drop draining. We will use a 1D Oldroyd-B model for the viscoelastic jet, and solve this model numerically by an explicit finite difference method. Close to exponential draining of the filament, we found that the variation of the axial elastic force in the filament is roughly four times larger than the variation of the capillary force with opposite sign. This fact implies that the elastic force is responsible for the drop migration and oscillation. Our study of the drop draining process shows that the elastic force also plays an important role here, allowing the liquid to flow from smaller drops into larger drops through the filament.
Effects of initial conditions on the passive and active scalar fluxes in unsteady stably stratified turbulence
Authors: Hanazaki H.
Published: March 2003

Passive and active scalar fluxes in unsteady strongly stratified turbulence are analyzed using the rapid distortion theory (RDT). Analytical solutions of the RDT equations show that the initial conditions, i.e., the initial cross correlation between passive scalar and active scalar (density) and the initial potential energy, make a difference between the passive and active scalar flux, giving the difference between the turbulent diffusion coefficients for passive and active scalars. In other words even the initial zero correlation gives the complete correlation in the long time development, in contrast to the previous discussions in the literature based on the equations for stationary turbulence. The difference appears in the components slowly oscillating at half frequency N (N: Brunt–Väisälä frequency) in the passive scalar flux, while the active scalar flux has only the rapidly oscillating components which oscillate at frequency 2N. It is also found that the correlation coefficient between passive and active scalars is not a good measure of identifying the agreement or disagreement of the turbulent diffusion coefficients for these scalars, since the initially large passive scalar variance can lead to a final small correlation coefficient even when the turbulent diffusion coefficients are equal for the active and passive scalars. The results show the importance of unsteady analysis of the initial value problem in the diffusion problem in stratified turbulence.
The settling velocity of heavy particles in an aqueous near-isotropic turbulence
Authors: Yang T. S. , Shy S. S.
Published: March 2003

The ensemble-average settling velocity, Vs, of heavy tungsten and glass particles with different mean diameters in an aqueous near-isotropic turbulence that was generated by a pair of vertically oscillated grids in a water tank was measured using both particle tracking and particle image velocimetries. Emphasis is placed on the effect of the Stokes number, St, a time ratio of particle response to the Kolmogorov scale of turbulence, to the particle settling rate defined as (Vs−Vt)/Vt where Vt is the particle terminal velocity in still fluid. It is found that even when the particle Reynolds number Rep is as large as 25 at which Vt/vk≈10 where vk is the Kolmogorov velocity scale of turbulence, the mean settling rate is positive and reaches its maximum of about 7% when St is approaching to unity, indicating a good trend of DNS results by Wang and Maxey (1993) and Yang and Lei (1998). This phenomenon becomes more and more pronounced as values of Vt/vk decrease, for which DNS results reveal that the settling rate at Vt/vk=1 and Rep<1 can be as large as 50% when St≈1. However, the present result differs drastically with Monte Carlo simulations for heavy particles subjected to nonlinear drag (Rep>1) in turbulence in which the settling rate was negative and decreases with increasing St. Using the wavelet analysis, the fluid integral time (τI), the Taylor microscale (τλ), and two heavy particles’ characteristic times (τc1,τc2) are identified for the first time. For St<1, τc1<τI and τc2<τλ, whereas τc1≈τI and τc2≈τλ for St≈1. This may explain why the settling rate is a maximum near St≈1, because the particle motion is in phase with the fluid turbulent motion only when St≈1 where the relative slip velocities are smallest. These results may be relevant to sediment grains in rivers and aerosol particles in the atmosphere.
Growth and mutual interference of protein seeds under reduced gravity conditions
Authors: Lappa M.
Published: March 2003

This analysis deals with new models and computational methods as well as with novel results on the relative importance of “controlling forces” in macromolecular crystal growth. The attention is focused in particular on microgravity fluid-dynamic aspects and on the case of the simultaneous growth of different seeds. A “kinetic-coefficient-based” volume of fraction method is specifically and carefully developed according to the complex properties and mechanisms of macromolecular protein crystal growth. It is shown that the size and the shape of the growing crystals play a “critical role” in the relative importance of surface effects and in determining the intensity of convection. Convective effects, in turn, are found to impact growth rates, macroscopic structures of precipitates, particle size and morphology as well as the mechanisms driving growth. The face growth rates in particular depend on the complex multicellular structure of the convective field and on associated “pluming phenomena.” The relative importance of mass transport in liquid phase and surface attachment kinetics is investigated. The simulations show that it does not behave as a “fixed” parameter and that different crystallization conditions may occur in the protein chamber due to mutual interference of the growing seeds, complex convective effects and the “finite size” of the reactor.
Numerical simulations of fingering instabilities in miscible magnetic fluids in a Hele-Shaw cell and the effects of Korteweg stresses
Authors: Chen Ching-Yao
Published: March 2003

The first high-accuracy direct numerical simulations of labyrinthine instabilities of miscible magnetic fluids are presented. Great similarities of fingering patterns in a miscible magnetic droplet with the effects of Korteweg stresses to their immiscible equivalents give strong qualitative validations to the numerical results. For conventional modeling that excludes the effects of Korteweg stresses, more vigorous fingerings are observed at stronger magnetic fields and higher viscosity contrast, which are in line with common expectations.
Effect of the Schmidt number on the diffusion of axisymmetric pancake vortices in a stratified fluid
Authors: Godoy-Diana Ramiro , Chomaz Jean-Marc
Published: March 2003

An asymptotic analysis of the equations for quasi-two-dimensional flow in stratified fluids is conducted, leading to a model for the diffusion of pancake-like vortices in cyclostrophic balance. This analysis permits one to derive formally the model for the diffusion of an axisymmetric monopole proposed by Beckers et al. [J. Fluid Mech. 433, 1 (2001)], and to extend their results. The appropriate parameter for the perturbation analysis is identified as the square of the vertical Froude number Fv=U/LvN, where U is the horizontal velocity scale, N is the Brunt–Väisälä frequency, and Lv the vertical length scale. The physical mechanisms involved in the vortex decay are examined under the light of the asymptotic analysis results. In particular we discuss the effects of the Schmidt number, Sc, which measures the balance between the diffusion of momentum and the diffusion of the stratifying agent. Remarkably, the vertical transport due to the slow cyclostrophic adjustment is shown to slowdown the velocity decay when Sc is larger than unity whereas it accelerates it when Sc is smaller than unity.
Kolmogorov turbulence by matched asymptotic expansions
Authors: Lundgren Thomas S.
Published: March 2003

The Kolmogorov [Dokl. Akad. Nauk. SSSR 30, 299 (1941), hereafter K41] inertial range theory is derived from first principles by analysis of the Navier–Stokes equation using the method of matched asymptotic expansions without assuming isotropy or homogeneity and the Kolmogorov (K62) [J. Fluid Mech. 13, 82 (1962)] refined theory is analyzed. This paper is an extension of Lundgren [Phys. Fluids 14, 638 (2002)], in which the second- and third-order structure functions were determined from the isotropic Karman–Howarth [Proc. R. Soc. London, Ser. A 164, 192 (1938)] equation. The starting point for the present analysis is an equation for the difference in velocity between two points, one of which is a Lagrangian fluid point and the second, slaved to the first by a fixed separation r, is not Lagrangian. The velocity difference, so defined, satisfies the Navier–Stokes equation with spatial variable r. The analysis is carried out in two parts. In the first part the physical hypothesis is made that the mean dissipation is independent of viscosity as viscosity tends to zero, as assumed in K41. This means that the mean dissipation is finite as Reynolds number tends to infinity and leads to the K41 inertial range results. In the second part this dissipation assumption is relaxed in an attempt to duplicate the K62 theory. While the K62 structure is obtained, there are restrictions, resulting from the analysis which shows that there can be no inertial range intermittency as Reynolds number tends to infinity, and therefore the mean dissipation has to be finite as Reynolds number tends to infinity, as assumed in part one. Reynolds number-dependent corrections to the K41 results are obtained in the form of compensating functions of r/λ, which tend to zero slowly like Rλ−2/3 as Rλ→∞.
Polarized incandescent light emission from carbon nanotubes
Authors: Li Peng , Jiang Kaili , Liu Ming , Li Qunqing , Fan Shoushan , Sun Jialin
Published: March 2003

Polarized light emission from multiwalled carbon nanotube (MWNT) bundles due to current heating is observed. The spectra of the emitted light fit well with the blackbody radiation distribution. And the emitted light is partially polarized with a degree of 0.33 along the axis of MWNT bundle, which is qualitatively explained in terms of one-dimensional structure of carbon nanotubes (CNTs). Negative temperature-dependent resistance is also observed, which is different from normal metal filaments. The MWNT bundles are very stable at high temperature in vacuum during light emitting, indicating that CNTs can be a good candidate as polarized incandescent light sources.
Probing a strong hydrogen bond with infrared spectroscopy: Vibrational predissociation of BrHBr−⋅Ar
Authors: Pivonka Nicholas L. , Kaposta Cristina , Brümmer Mathias , von Helden Gert , Meijer Gerard , Wöste Ludger , Neumark Daniel M. , Asmis Knut R.
Published: March 2003

The gas phase vibrational spectroscopy of BrHBr−, a prototypical strongly hydrogen-bonded species, has been studied between 6 and 17 μm (590 and 1670 cm−1) by infrared vibrational predissociation of the BrHBr−⋅Ar ion. Infrared excitation was accomplished using the output of the free electron laser for infrared experiments (FELIX). Predissociation spectra were recorded by monitoring depletion of mass-selected BrHBr−⋅Ar ions as a function of excitation wavelength. Four prominent absorption bands are observed at 733, 890, 1048, and 1201 cm−1. They are assigned to the fundamental of the hydrogenic stretching mode ν3 and a sequence of ν3+nν1 combinations (n=1–3). Additional features to the blue of these bands spaced by ∼21 cm−1 are attributed to combination bands involving motion of the Ar messenger atom. Differences in the relative intensities of the ν3+nν1 combinations bands in comparison to previous matrix experiments are rationalized on the basis of the underlying dissociation dynamics.
Near-field scanning optical microscopy of photonic crystal nanocavities
Authors: Okamoto Koichi , Lončar Marko , Yoshie Tomoyuki , Scherer Axel , Qiu Yueming , Gogna Pawan
Published: March 2003

Near-field scanning optical microscopy was used to observe high-resolution images of confined modes and photonic bands of planar photonic crystal (PPC) nanocavities fabricated in active InGaAsP material. We have observed the smallest optical cavity modes, which are intentionally produced by fractional edge dislocation high-Q cavity designs. The size of the detected mode was roughly four by three lattice spacings. We have also observed extended dielectric-band modes of the bulk PPC surrounding the nanocavity by geometrically altering the bands in emission range and eliminating localized modes out of the emission range.
Deep ultraviolet picosecond time-resolved photoluminescence studies of AlN epilayers
Authors: Nam K. B. , Li J. , Nakarmi M. L. , Lin J. Y. , Jiang H. X.
Published: March 2003

AlN epilayers with high optical qualities have been obtained by metalorganic chemical vapor deposition on sapphire substrates. Deep UV picosecond time-resolved photoluminescence (PL) spectroscopy has been employed to study the optical transitions in AlN epilayers. Two PL emission lines associated with the donor bound exciton (D0X, or I2) and free exciton (FX) transitions have been observed, from which the binding energy of the donor bound excitons in AlN epilayers was determined to be around 16 meV. Time-resolved PL measurements revealed that the recombination lifetimes of the I2 and free exciton transitions in AlN epilayers were around 80 and 50 ps, respectively. The temperature dependencies of the free exciton radiative decay lifetime and emission intensity were investigated, from which a value of about 80 meV for the free exciton binding energy in AlN epilayer was deduced. This value is believed to be the largest free exciton binding energy ever reported in semiconductors, implying excitons in AlN are an extremely robust system that would survive well above room temperature. This together with other well-known physical properties of AlN may considerably expand future prospects for the application of III-nitride materials.
High-temperature resistive surface grating for spectral control of thermal radiation
Authors: Sai Hitoshi , Kanamori Yoshiaki , Yugami Hiroo
Published: March 2003

Spectral emittance and thermal stability of two-dimensional W gratings are investigated to obtain high-temperature resistive selective emitters. Numerical calculations based on rigorous coupled-wave analysis are performed to determine the structural profile of gratings with good spectral selectivity. According to the determined parameters, two-dimensional W gratings composed of rectangular microcavities with the period of 1.0 μm are fabricated on single crystalline and polycrystalline W substrates. The grating shows a strong emission peak which can be explained by the confined modes inside the cavities. The grating with 200 nm wall thickness made from a single crystalline W shows very high thermal stability over 1400 K, while the polycrystalline grating is deformed at a high temperature because of the grain growth.
Formation of InAs self-assembled quantum rings on InP
Authors: Raz T. , Ritter D. , Bahir G.
Published: March 2003

Shape transformations of partially capped self-assembled InAs quantum dots grown on InP are studied. Atomic force microscopy images show large anisotropic redistribution of the island material after coverage by a 1-nm-thick InP layer. The anisotropic material redistribution occurs within a few minutes and leads to a change from lens-like to elongated ring-like islands. The shape transformation is not accompanied by dot material compositional change. The formation of InAs/InP quantum rings disagrees with a previous model of InAs/GaAs ring formation that assumes that the driving force for the dot to ring transformation is the difference in surface diffusion velocity of indium and gallium atoms.
Coherent phonon spectroscopy of GaP Schottky diode
Authors: Chang Y.-M.
Published: March 2003

Coherent longitudinal optical and acoustic phonons in GaP Schottky diode are impulsively generated and detected with time-resolved second-harmonic generation. A coherent longitudinal optical (LO) phonon is launched in the near-surface depletion region of GaP. The dephasing time of this coherent LO phonon is ∼10 ps and mainly due to the anharmonic decay into two zone-edge half-energy longitudinal acoustic (LA) phonons. Simultaneously, a coherent LA wave packet is created in the metal–semiconductor interface via transient thermal absorption in the metal thin layer. This acoustic wave packet propagates into the bulk with the sound velocity ∼5.8×105 cm/s of a GaP LA phonon.
Thermally induced oxide crystallinity and interface destruction in Ga2O3–GaAs structures
Authors: Passlack M. , Abrokwah J. K. , Yu Z. , Droopad R. , Overgaard C. , Kawayoshi H.
Published: March 2003

Amorphous Ga2O3 films are deposited onto the GaAs(001) surface using effusive evaporation from a high-purity polycrystalline Ga2O3 source. The Ga2O3–GaAs structure is characterized by cross-sectional transmission electron microscopy, electron diffraction, and photoluminescence intensity measurements. As-deposited Ga2O3 films are found to be amorphous forming an atomically abrupt interface of low interface state density Dit with GaAs. For oxide films with a thickness tox⩾127 Å, rapid thermal annealing above a critical temperature Tc=720 °C induces Ga2O3 bulk crystallization resulting in structural deformation of the Ga2O3–GaAs interface and complete destruction of its low Dit character. Preliminary data suggest that the critical temperature Tc may increase in the limit of very thin Ga2O3 films.
Stable protocrystalline silicon and unstable microcrystalline silicon at the onset of a microcrystalline regime
Authors: Ahn Jun Yong , Jun Kyung Hoon , Lim Koeng Su , Konagai Makoto
Published: March 2003

We investigated the structural, electrical, and optical properties as well as light-induced degradation characteristics of silicon films prepared by photochemical vapor deposition at various hydrogen dilution ratios. The protocrystalline silicon deposited before the onset of the microcrystalline regime was most stable against light soaking. However, the films deposited at the onset of the microcrystalline regime, known to have the most competent device quality and stability, were observed to be less stable. Such instability at the onset of the microcrystalline regime is correlated with the existence of the clustered phase hydrogen that indicates microvoids in the films.
Highly anisotropic morphologies of GaAs(331) surfaces
Authors: Yazdanpanah V. R. , Wang Z. M. , Salamo G. J.
Published: March 2003

The surface morphology of the GaAs(331) surface was investigated by in situ reflection high-energy electron diffraction and scanning tunneling microscopy. It was found, that GaAs(331) A and B surfaces are both faceted on a nanometer scale, containing (110) and (111) facets which are atomically resolved in real space. The resulting highly anisotropic ridge-like surfaces can prove useful in the fabrication of quantum wire structures.
Microstructure and high critical current of powder-in-tube MgB2
Authors: Serquis A. , Civale L. , Hammon D. L. , Coulter J. Y. , Liao X. Z. , Zhu Y. T. , Peterson D. E. , Mueller F. M.
Published: March 2003

We report dc transport and magnetization measurements of the critical current density, Jc, and microstructural analyses of MgB2 wires fabricated by the powder-in-tube method (PIT) using commercial MgB2 powder with 5 at. % Mg powder added as an additional source of magnesium and stainless steel as a sheath material. We identify a weak-link behavior in the as-drawn wire associated with the presence of microcracks. By appropriate heat treatments, we can increase Jc by more than one order of magnitude due to a recrystallization process promoted by the excess Mg, which results in the elimination of most of our microcracks. In contrast, inappropriate annealing conditions result in a deterioration of connectivity due to an inhomogeneous loss of Mg. Grain size and porosity play only a secondary role in determining Jc in PIT MgB2.
Efficient CsF/Yb/Ag cathodes for organic light-emitting devices
Authors: Chan M. Y. , Lai S. L. , Fung M. K. , Tong S. W. , Lee C. S. , Lee S. T.
Published: March 2003

A high-performance cathode consisting of an ultrathin CsF layer and a rare-earth ytterbium (Yb) metal is reported for application in organic electroluminescent devices. Standard tris-(8-hydroxyquinoline) aluminum/α-napthylphenylbiphenyl diamine devices with this bilayer cathode showed dramatically reduced operating voltage and a low turn-on voltage of 2.42 V as compared to 3.75 and 2.95 V in devices using, respectively, the Mg:Ag and single-layer Yb cathodes. At a current density of 200 mA/cm2, devices with the CsF/Yb cathode exhibited high luminance efficiency of 3.45 cd/A and power efficiency of 1.27 lm/W. Analysis by x-ray photoemission spectroscopy suggested that the performance improvement is related to the substantial reduction of electron injection barrier at the cathode/organic interface. It was found that upon Yb deposition, CsF dissociates to liberate low work function Cs metal atoms resulting in a cathode with a lower electron injection barrier and thus a better balance of carriers in the device. Thermodynamically, this exothermic chemical reaction is expected to take place independent of the type of substrates and organic medium, indicating that this bilayer cathode system may be applicable in a wide range of organic electronic/optoelectronic devices.
Spontaneous growth and luminescence of zinc sulfide nanobelts
Authors: Zhu Ying-Chun , Bando Yoshio , Xue Dong-Feng
Published: March 2003

ZnS nanobelts have been synthesized by a simple thermal evaporation method in a N2 atmosphere containing a small amount of CO and H2 gases. The synthesized ZnS nanobelts have a width in the range of 40 to 120 nm, a thickness of 20 nm, and a length of several micrometers. The nanobelts are single crystals with a hexagonal wurtzite structure growing along the [001] direction. A vapor–solid process is proposed for the formation of such nanobelts. The as-prepared nanobelts shows two emission bands, around 450 and 600 nm.
Surface charge accumulation of InN films grown by molecular-beam epitaxy
Authors: Lu Hai , Schaff William J. , Eastman Lester F. , Stutz C. E.
Published: March 2003

A series of thin InN films down to 10 nm in thickness were prepared by molecular-beam epitaxy on either AlN or GaN buffers under optimized growth conditions. By extrapolating the fitted curve of sheet carrier density versus film thickness to zero film thickness, a strong excess sheet charge was derived, which must come from either the surface or the interface between InN and its buffer layer. Since metal contacts, including Ti, Al, Ni, and a Hg probe, can always form an ohmic contact on InN without any annealing, it is determined that at least part of the excess charge is surface charge, which was also confirmed by capacitance–voltage measurements.
Understanding Alternative Food Networks: Exploring the Role of Short Food Supply Chains in Rural Development
Authors: Renting Henk , Marsden Terry K , Banks Jo
Published: March 2003

In this paper we explore the development and incidence of alternative food networks within a European-wide context. By developing a consistent definition of short food supply chains, we address both the morphology and the dynamics of these, and then examine empirical evidence concerning their incidence and rural development impact across seven EU member states. These developments need to be seen as one significant contribution to the current transitions in rural Europe concerning the crisis of conventional intensive and productivist agriculture and the public consumer pressure for a larger variety of distinctive ‘quality’ food products.
Managing Pests, Consumers, and Commitments: The Case of Apple Growers and Pear Growers in Belgium's Lower Meuse Region
Authors: Collet Eric , Mormont Marc
Published: March 2003

The authors analyse the construction of a seed fruit market. The conventional market had led to the ‘anonymousness’ of the fruit and to disconnected temporalities. The process needs new types of coordination and components within the common information channels and actors' network. It also needs the construction of a new contract between producers and consumers, that the conventional market is unable to allow. Producers are experimenting with direct contact with the final consumer to find ways of allowing it. The fruit is replaced in the subject's experience of the final consumer and in the grower's story. The grower's story deals with managing pests, consumers, and commitments through a harmless action: eating a pear or an apple.
Heterosexism and the Geographies of Everyday Life in Belfast, Northern Ireland
Authors: Kitchin Rob , Lysaght Karen
Published: March 2003

In this paper we seek to extend work on the relationship between sexuality, space, and society by providing a nuanced geographical reading of the sexual production of space in Belfast, Northern Ireland. Utilising queer theory, we draw from interviews with thirty gay, lesbian, bisexual, and transgender individuals to illustrate how the discursive and material practices that shape the regulation, self-regulation, and resistance of heterosexism are spatially, temporally, and contextually uneven and unequal. Focusing on the spatial arenas of home, work, and social space we document how each space is produced and experienced in multiple ways, and managed using a variety of sociospatial strategies.
Utilities, Land-Use Change, and Urban Development: Brownfield Sites as ‘Cold-Spots’ of Infrastructure Networks in Berlin
Authors: Moss Timothy
Published: March 2003

This paper explores the interrelationships between urban land use, resource consumption, and utility service provision with a study of brownfield regeneration—from an infrastructure perspective. Drawing on recent research into the spatial strategies of utility companies, after liberalisation and privatisation, I identify disused industrial sites as ‘cold-spots’ of infrastructure systems where energy and water consumption has recently collapsed. Using a case study of Berlin I analyse first the challenges facing the city's three major utilities as a result of shifting patterns of resource consumption and overcapacity in parts of their networks. In the second part I examine the responses of the three utilities to these challenges in the context of recent institutional changes to infrastructure provision; exploring how the utilities are moving towards greater spatial differentiation in their network management and what interest they have in brownfield regeneration.
Local Labour Markets and Individual Transitions into and out of Poverty: Evidence from the British Household Panel Study Waves 1 to 8
Authors: McCulloch Andrew
Published: March 2003

Using data from the British Household Panel Study 1991 to 1998 and the Annual Employment Survey this paper examines the impact of changes in local-labour market conditions on individual poverty exits and entrances. Transitions out of poverty and transitions into poverty are estimated by using discrete duration models that control for individual and family-level characteristics, duration effects, and local labour-market variables. After controlling for individual and family-level characteristics, probabilities of exiting poverty were found to be higher and probabilities of entering poverty were found to be lower for those individuals living in labour markets which experienced increases in the proportion of the population in employment. The relative contribution of labour market variables is substantially smaller than individual and family characteristics in determining transitions out of and into poverty. Whether changes in employment in the local labour-market were biased towards manufacturing or services did not have a systematic influence on poverty exits and entrances. The link between employment conditions in the local labour market and poverty suggests that the poor did share in the gains from economic growth in the 1990s. This finding gives support to the idea that expansionary demand policies at the local level could help to mitigate the economic position of the economically disadvantaged.