bluuebunny/crossref_metadata_2025 huggingface dataset:
Phase separation in lead zirconate titanate and bismuth titanate during electrical shorting and fatigue
Authors: Lou Xiaojie , Hu Xiaobing , Zhang Ming , Morrison F. D. , Redfern S. A. T. , Scott J. F.
Published: March 2006

Micro-Raman and electron microprobe techniques are used to show that lead zirconate titanate and samarium-doped bismuth titanate undergo local phase transformations and separation during electrical shorting and in the dendritic precursors to microshorts caused by bipolar fatigue. These precursors for shorts, consisting of dark filaments, were studied just before they completely shorted the sample. The aim of the study was to compare electrical breakdown and breakdown precursors in ABO3 perovskite oxides and related Aurivillius phase layer structures with Bi (A site) substitution and with B-site substitution (e.g., Ti for Zr in PZT). The observation of phase separation and decomposition is related to congruent and incongruent meltings in these materials. Dendritelike shorts and short precursors of a few microns in diameter, produced by extreme bipolar voltage cycling fatigue, are mapped spectroscopically in 1μm2 areas and exhibit almost pure regions of α-PbO, β-PbO, and rutile TiO2. The α-βPbO phase boundary runs from 500°C at 1atm to room temperature at 0.4GPa, easily accessible temperatures and pressures in the dynamical process. Similarly, under large dc voltages the Sm-doped bismuth titanate transforms from a layered-perovskite structure to a pyrochlore structure during filamentary electrical breakdown, with the loss of Bi. The interfacial phase separation has been attributed to the combination of defect aggregation and thermal decomposition effects.
Theory of high gain free-electron lasers operating with segmented undulators
Authors: Dattoli G. , Mezi L. , Ottaviani P. L. , Pagnutti S.
Published: March 2006

The dynamical behavior of high gain free-electron lasers, operating with segmented undulators in the self-amplified spontaneous emission regime, is studied using a set of semianalytical formulas, which reproduce the induced energy spread, the bunching coefficients, and the field evolution along the longitudinal coordinate. The proposed technique is applied to the design of segmented free-electron laser devices, employing undulators with helical or linear polarization. The case of biharmonic sections is discussed too, as well as the possibility of extending the analysis to this “exotic” case.
The detachment of a viscous drop in a viscous solution in the presence of a soluble surfactant
Authors: Jin Fang , Gupta Nivedita R. , Stebe Kathleen J.
Published: March 2006

When a buoyant viscous drop is injected into a viscous fluid, it evolves to form a distended shape that detaches via the rapid formation and pinching of a neck. The effects of surfactants in altering this process are studied numerically. In the absence of surfactants, surface contraction is fastest in the vicinity of the neck. Thus, when surfactants are present, they accumulate there and alter the ensuing dynamics by reducing the surface tension that drives the contraction. The surface tension is described by a nonlinear surface equation of state that accounts for the maximum packing of surfactant in a monolayer. When surfactant adsorption-desorption is very slow, interfaces dilute significantly during drop expansion, and drops form necks which are only slightly perturbed in their dynamics from the surfactant-free case. When adsorption-desorption dynamics are comparable to the rate of expansion, drops thin to form a primary neck at low surfactant coverage, to form both primary and secondary necks at moderate coverages, form only a secondary neck at higher coverages, or fail to neck at elevated coverages. When surfactant adsorption-desorption kinetics are rapid, the surface remains in equilibrium with the surrounding solution, and drops behave like surfactant-free drops with a uniform surface tension. These arguments are used to construct a phase diagram of drop neck shapes as a function of surfactant coverage. A map of neck/no-neck thresholds is also constructed as a function of surfactant coverage and sorption dynamics, suggesting that drop detachment can be used as a means of characterizing surfactant dynamics.
Charge transport across pressure-laminated thin films of molecularly doped polymers
Authors: Klenkler Richard A. , Xu Gu , Graham John F. , Popovic Zoran D.
Published: March 2006

Discrete interfaces between successive layers in an organic semiconducting device simplify any examination of interface barriers for charge transport. To form discrete interfaces between organic layers we propose lamination as an alternate approach to physical vapor deposition. Transient photocurrent measurements as a function of pressure, thickness, and electric field were performed on cells of 1,1-bis[(di-4-tolylamino)phenyl]-cyclohexane (TAPC), N,N-bis(3,4-dimethylphenyl)-4-aminobiphenyl (DMPAB), and N,N’-diphenyl-N,N’-bis(3-methylphenyl)-[1,1’-biphenyl]-4,4’-diamine (TPD). It was found that, in the range 0.8–3.0 MPa, a pressure-laminated interface between two identical materials causes no measurable perturbation to charge transport. This justifies the use of pressure lamination to study interfaces between nonidentical layers.
Atomic scale structure and morphology of (In,Ga)As-capped InAs quantum dots
Authors: McGee W. M. , Krzyzewski T. J. , Jones T. S.
Published: March 2006

The atomic scale morphology of InAs∕GaAs quantum dots (QDs) capped with In0.1Ga0.9As layers of different thickness are studied using in situ scanning tunneling microscopy (STM) and compared to the effects of capping with a pure GaAs layer. QDs capped with a 5nm In0.1Ga0.9As layer exhibit a longer photoluminescence emission wavelength than those capped with GaAs. STM studies show that the QDs capped with In0.1Ga0.9As retain their height during the initial stages of capping (up to ∼2nm), whereas the GaAs-capped QDs collapse as material migrates from their tops onto the cap surface. After deposition of a 25nm GaAs cap the surface is still far from flat, whereas the In0.1Ga0.9As capping layer is planar after just 5nm deposition. High-resolution STM images, supported by reflection high-energy electron diffraction measurements, reveal a (4×3)∕c(4×6) reconstruction for the In0.1Ga0.9As cap, whereas the GaAs cap layer rapidly exhibits a c(4×4) reconstruction after the first few monolayers of deposition. The planar morphology is a consequence of enhanced In adatom diffusion on the InGaAs alloy surface.
A surface hopping method for chemical reaction dynamics in solution described by diabatic representation: An analysis of tunneling and thermal activation
Authors: Yamada Atsushi , Okazaki Susumu
Published: March 2006

We present a surface hopping method for chemical reaction in solution based on diabatic representation, where quantum mechanical time evolution of the vibrational state of the reacting nuclei as well as the reaction-related electronic state of the system are traced simultaneously together with the classical motion of the solvent. The method is effective in describing the system where decoherence between reactant and product states is rapid. The diabatic representation can also give a clear picture for the reaction mechanism, e.g., thermal activation mechanism and a tunneling one. An idea of molecular orbital theory has been applied to evaluate the solvent contribution to the electronic coupling which determines the rate of reactive transition between the reactant and product potential surfaces. We applied the method to a model system which can describe complex chemical reaction of the real system. Two numerical examples are presented in order to demonstrate the applicability of the present method, where the first example traces a chemical reaction proceeded by thermal activation mechanism and the second examines tunneling mechanism mimicking a proton transfer reaction.
Octant analysis based structural relations for three-dimensional turbulent boundary layers
Authors: Ölçmen Semih M. , Simpson Roger L. , Newby Jonathan W.
Published: March 2006

A flow structure based triple-product correlation model developed by Nagano and Tagawa [J. Fluid Mech. 215, 639 (1990)] has been expanded to three-dimensional turbulent flows. Three-dimensional turbulent boundary layer data obtained away from the vortex in a wing-body junction flow are analyzed to calculate the contributions from eight velocity octants to the stresses and higher-order products. The analysis showed that the sweep and ejection modes dominate the flow physics of some shear stresses and some triple products, while the interaction modes are negligible away from the wall. These experimental observations are used together with the extended Nagano-Tagawa mathematical model to obtain relations among the triple products in three-dimensional turbulent boundary layers that can simplify the turbulent diffusion modeling used in Reynolds-averaged Navier-Stokes equations. Results show that û3¯, û2v̂¯, and v̂3¯ triple product correlations can be modeled if an appropriate turbulence model is described for the ûv̂2¯ triple product correlation, and that û2ŵ¯ v̂2ŵ¯ triple products correlations can be modeled if an appropriate turbulence model is described for the ûv̂ŵ¯ triple product correlation.
Transverse electric dominant intersubband absorption in Si-doped GaInAsN∕GaAs quantum wells
Authors: Zhang D. H. , Liu W. , Sun L. , Fan W. J. , Yoon S. F. , Wang S. Z. , Liu H. C.
Published: March 2006

We report observation of transverse electric (TE) dominant intersubband absorption in Si-doped GaInAsN∕GaAs multiple-quantum-well structures. The TE dominant absorption is believed to be caused by the incorporation of nitrogen and the associated nitrogen state. When the confinement is strong in narrow quantum wells, the ground state is pushed up, which enhances the interaction with nitrogen state and significantly changes the nature of the state.
Optical study of ΓL high energy photonic pseudogaps in ZnO inverted opals
Authors: García P. D. , López C.
Published: March 2006

We provide experimental evidence of the opening of two new pseudogaps in the high energy region of the ΓL direction in ZnO inverted opals. The appearance of these new pseudogaps is brought about by the increase of refractive index contrast. The calculated band diagram in the ΓL direction, which corresponds to propagation along the (111) crystallographic direction, accounts for the spectral width and position dependence of the pseudogaps on the ZnO fraction present in the opal. To prove this we have performed an optical study by means of reflection and transmission spectroscopy of ZnO inverted opals with different degrees of infiltration. We also study the pseudogap appearing in the highest energy spectral position as a function of sample thickness. In addition, the band engineering developed here allows us to isolate a nondispersive band from the others.
Online monitoring of alloyed bimetallic nanoparticle formation by optical spectroscopy
Authors: Belotelov V. I. , Carotenuto G. , Nicolais L. , Longo A. , Pepe G. P. , Perlo P. , Zvezdin A. K.
Published: March 2006

An extension of the Mie [Ann. Phys. 25, 377 (1908)] theory for the calculation of optical absorption spectra in bimetallic nanoparticles has been developed. The nanoparticle dielectric function is assumed to be a weighted linear combination of dielectric functions for single particles. Accordingly, analytical expressions for the resonance light absorption frequency, the spectrum maximum value, and the full width at half maximum have been derived, taking into account the interband transitions in the dielectric functions. Experiments have been performed on polymer-embedded Ag∕Au nanoparticles prepared by reducing the presence of poly(vinyl pyrrolidone) at room temperature. Experimental absorption spectra have been compared to numerical curves derived by the model in the case of Ag∕Au systems at different relative compositions, and they are in good agreement. The time dependence of both Ag∕Au nanoparticle size and chemical composition during the growth process has also been investigated.
Polarization fatigue of ferroelectric Pb(Zr0.1Ti0.9)O3 thin films: Temperature dependence
Authors: Wang Y. , Wang K. F. , Zhu C. , Liu J.-M.
Published: March 2006

The polarization switching fatigue behaviors of tetragonal Pb(Zr0.1Ti0.9)O3(PZT0.1) thin films deposited on Pt-coated silicon wafers by the sol-gel method are investigated by testing the fatigue endurance at different temperatures and by measuring the small-signal dielectric loss associated with the fatigued samples. It is observed that the fatigue endurance can be significantly improved at low temperature, while the low-temperature fatigue becomes more serious with increasing magnitude of the electrical pulses for fatigue testing. The fatigued thin films exhibit much bigger dielectric loss than the fresh films over the whole temperature range, due to the domain pinning by the aggregated defects (oxygen vacancies). In addition, the temperature dependence of the fatigue resistance performance of the thin films can be well described by the exponential law. The long-range diffusion and aggregation of the defects and the consequent domain pinning as one of main origins for switching fatigue in the PZT0.1 thin films are demonstrated.
Defect centers and optical absorption edge of degenerated semiconductor ZnO thin films grown by a reactive plasma deposition by means of piezoelectric photothermal spectroscopy
Authors: Sakai K. , Kakeno T. , Ikari T. , Shirakata S. , Sakemi T. , Awai K. , Yamamoto T.
Published: March 2006

Undoped and Ga-doped (3 wt %) n-type ZnO thin films were grown by a reactive plasma deposition method on glass substrates at 200 °C under an oxygen flow rate from 0 to 50 SCCM. In this paper, we report on the defect and band edge related signals in the optical absorption spectra for ZnO thin film by using a piezoelectric photothermal (PPT) spectroscopy, which is effective in observing a nonradiative transition process. The PPT peak around 2.5 eV was observed only for the undoped ZnO samples grown under a low oxygen flow rate. This signal is considered to be related to the oxygen vacancies, because it disappears with the increase of the oxygen flow rates. No corresponding peak was found for the Ga-doped samples. This result indicates that Ga doping inhibits the generation of the oxygen vacancies, and it agrees with that from the first-principle electronic band structure calculations. We have also carried out the theoretical calculation for the optical absorption edge of degenerated ZnO as a function of the carrier concentration. Burstein-Moss effect and band-gap-narrowing effect in ZnO should be considered in the case of high carrier concentration. Comparing the experimental results with the theoretical predictions, we found out that the proposed PPT edge energy coincides well with the Fermi level EF.
Exchange coupling in NiO∕CoFe2 and CoFe2O4∕CoFe2 systems grown by pulsed laser deposition
Authors: Fix T. , Colis S. , Sauvet K. , Loison J. L. , Versini G. , Pourroy G. , Dinia A.
Published: March 2006

Ni O ( 30 nm ) ∕ Co Fe 2 ( 5 nm ) bilayers are grown by pulsed laser deposition on Si (001). After annealing under a magnetic field we observe an exchange bias of −86Oe, corresponding to an exchange coupling of 0.06erg∕cm2. While a similar exchange coupling is observed in NiO(30nm)∕CoFe2(5nm)∕SrTiO3(3nm)∕Ni80Fe20(5nm)∕Ta(5nm), the stack SFMO(30nm)∕SrTiO3(3nm)∕CoFe2(5nm)∕NiO(30nm)∕Ta(5nm) does not provide any exchange bias with NiO on the top electrode. Alternatively, we have used a ferrimagnetic oxide (CoFe2O4) as a bias layer of the top electrode. The stack SFMO(30nm)∕STO(3nm)∕CoFe2(5nm)∕CoFe2O4(70nm) provides a shift of −70Oe for the CoFe2(5nm) layer, corresponding to an exchange coupling of 0.05erg∕cm2.
Near edge x-ray absorption fine structure study of aligned π-bonded carbon structures in nitrogenated ta-C films
Authors: Roy S. S. , McCann R. , Papakonstantinou P. , McLaughlin J. A. , Kirkman I. W. , Bhattacharyya S. , Silva S. R. P.
Published: March 2006

Polarization dependent studies of near edge x-ray absorption fine structure (NEXAFS) show evidence for the presence of aligned π-bonded carbon structures and the formation of a nonplanar CN phase in tetrahedral amorphous nitrogenated carbon (ta-CN) films deposited at room temperature. Moreover, the analysis of NEXAFS data as a function of nitrogen concentration and annealing temperature leads to a comprehensive assignment of the local CN bonding configurations in these ta-CN films and suggests an unusual and almost thermally stable nitrogenated carbon structure.
Film/substrate interface stability in thin films
Authors: Krishnamurthy R. , Srolovitz D. J.
Published: March 2006

We examined the morphological stability of an interface between a misfitting thin film and a substrate within the framework of linear stability theory. An interface instability exists regardless of the mismatch between the elastic properties of the film and the substrate and the magnitude of the interface energy. Stiffer substrates, smaller misfits, and larger interface energies all tend to reduce the range of wave numbers (to smaller values) over which the instability exists. We demonstrate that the film itself is unstable in all circumstances, even under conditions where the free surface remains flat. In other words, a misfitting film on a substrate is always unstable. We also demonstrate that the interface instability is enhanced when the interface can slip than when it cannot.
Analytical integrability and physical solutions of d-KdV equation
Authors: Karmakar P. K. , Dwivedi C. B.
Published: March 2006

A new idea of electron inertia-induced ion sound wave excitation for transonic plasma equilibrium has already been reported. In such unstable plasma equilibrium, a linear source driven Korteweg-de Vries (d-KdV) equation describes the nonlinear ion sound wave propagation behavior. By numerical techniques, two distinct classes of solution (soliton and oscillatory shocklike structures) are obtained. Present contribution deals with the systematic methodological efforts to find out its (d-KdV) analytical solutions. As a first step, we apply the Painleve method to test whether the derived d-KdV equation is analytically integrable or not. We find that the derived d-KdV equation is indeed analytically integrable since it satisfies Painleve property. Hirota’s bilinearization method and the modified sine-Gordon method (also termed as sine-cosine method) are used to derive the analytical results. Perturbative technique is also applied to find out quasistationary solutions. A few graphical plots are provided to offer a glimpse of the structural profiles obtained by different methods applied. It is conjectured that these solutions may open a new scope of acoustic spectroscopy in plasma hydrodynamics.
Exact supersymmetry in the relativistic hydrogen atom in general dimensions—supercharge and the generalized Johnson-Lippmann operator
Authors: Katsura Hosho , Aoki Hideo
Published: March 2006

A Dirac particle in general dimensions moving in a 1∕r potential is shown to have an exact supersymmetry, for which the two supercharge operators are obtained in terms of (a D-dimensional generalization of) the Johnson-Lippmann operator, an extension of the Runge-Lenz-Pauli vector that relativistically incorporates spin degrees of freedom. So the extra symmetry (S(2)) in the quantum Kepler problem, which determines the degeneracy of the levels, is so robust as to accommodate the relativistic case in arbitrary dimensions.
Dynamics of GaN band edge photoluminescence at near-room-temperature regime
Authors: Chen Xiang-Bai , Huso Jesse , Morrison John L. , Bergman Leah
Published: March 2006

In this paper we present an approach based on the known radiative recombination rate model to study the dynamics and characteristics of photoluminescence (PL) transitions at room-temperature (RT) regime of GaN thin film. The model states that the dependence of the PL intensity on the laser excitation intensity is IPL∝Ilaserα in which the value of the exponent α reveals whether the PL is due to an exciton or band gap recombination mechanism. We elaborated on the model and studied the temperature behavior of the exponent α in the range of 180–400K in order to explore the recombination type for that range. It was found that at the temperature range just below RT ∼180–270K the exponent is a slowly increasing function of temperature and has an average value of ∼1.2, implying a free-exciton recombination mechanism. At ∼280K the value of the exponent was found to exhibit a step-function-like behavior with a sharp increase from 1.2 to 1.7. At the temperature range just above RT ∼300–400K the exponent was found again to be a slowly increasing function of temperature with an average value of ∼1.7, implying that at that temperature range the PL involves mainly band gap transitions. From the temperature behavior of the exponent, the activation energy of the free exciton was inferred to be ∼24meV.
Electric field effects on water clusters (n=3–5): Systematic ab initio study of structures, energetics, and transition states
Authors: Choi Young Cheol , Pak Chaeho , Kim Kwang S.
Published: March 2006

The structures, energetics, and transition states of water clusters (trimer to pentamer, n=3–5) are investigated as a function of electric field by using ab initio calculations. With an increasing strength of the field, the most stable cyclic structures of trimer, tetramer, and pentamer open up to align their dipole moments along the direction of the field. For the lower strength (below 0.3V∕Å) of the electric field, the dipole moment of each water monomer is along the same direction with the field, while it retains the cyclic structure. For the higher strength of the field, to have a higher dipole moment for the cluster along the field direction, each cyclic structure opens up to form a linear chain or “water wire.” We have investigated the transition state structures between the cyclic and linear forms for the field strengths of 0.3–0.4V∕Å where both cyclic and linear forms are energetically comparable.
Nonlinear dynamics of the interface of dielectric liquids in a strong electric field: Reduced equations of motion
Authors: Zubarev Nikolay M.
Published: March 2006

The evolution of the interface between two ideal dielectric liquids in a strong vertical electric field is studied. If the ratio of the permittivities of the liquids is inversely proportional to their densities, we find that there is a flow regime for which the velocity and electric field potentials are linearly dependent functions. The corresponding reduced equations of motion are derived for the interface. In the limit of small density ratio, these equations coincide with the well-known equations that describe Laplacian growth.
A temporal approximate deconvolution model for large-eddy simulation
Authors: Pruett C. D. , Thomas B. C. , Grosch C. E. , Gatski T. B.
Published: March 2006

A temporal approximate deconvolution model (TADM) is developed for large-eddy simulation and is demonstrated for plane-channel flow at Reτ=590. The TADM combines explicit causal time-domain filtering with linear deconvolution (defiltering) to approximate unfiltered fields and residual stress to arbitrarily high order. The TADM methodology appears to lead to a robust family of residual-stress models that should provide a viable alternative to conventional (spatial) filtering for applications in which spatial filtering is problematic, e.g., for problems requiring unstructured or highly stretched grids.
Impact of band gap shrinkage on simulated bifurcation routes in directly modulated semiconductor lasers
Authors: Lim C. G. , Iezekiel S. , Snowden C. M.
Published: March 2006

A carrier heating model is derived by taking into account the various temperature processes in the active layer of laser diodes. This model is then used to simulate the static and dynamic characteristics of a directly modulated 1.55μm distributed-feedback laser diode. The calculated results are compared with the measured results of this device as obtained in an earlier work [H. F. Liu and W. F. Ngal, IEEE J. Quantum Electron. 29, 1668 (1993)], and this reveals the significant impact of band gap shrinkage on simulated results. This study also shows that the carrier heating model is a self-consistent model that naturally describes the gain suppression phenomena in directly modulated laser diodes.
Modeling of isotropic backward-wave materials composed of resonant spheres
Authors: Jylhä L. , Kolmakov I. , Maslovski S. , Tretyakov S.
Published: March 2006

A possible realization of isotropic artificial backward-wave materials is theoretically analyzed. An improved mixing rule for the effective permittivity of a composite material consisting of two sets of resonant dielectric spheres in a homogeneous background is presented. The equations are validated using the Mie theory and numerical simulations. The effect of a statistical distribution of sphere sizes on the increase of losses in the operating frequency band is discussed and some examples are shown.
Rayleigh’s instability of Lennard-Jones liquid nanothreads simulated by molecular dynamics
Authors: Min Donghong , Wong Harris
Published: March 2006

A liquid thread of radius R will break up into drops if the axial wavelength of the surface perturbation L>2πR. If L<2πR, the thread is stable and will remain intact. This is Rayleigh’s stability criterion that was derived using a continuum model. We use molecular dynamics to simulate the evolution of Lennard-Jones liquid threads with R=S, 2S, and 3S, where S is the equilibrium distance between two atoms. We find that Rayleigh’s stability criterion holds, even at the molecular scale.
Electronic structure of Pb1−xLaxTiO3 ferroelectric materials from Ti 2p and O 1s soft x-ray absorption spectroscopy
Authors: Mastelaro V. R. , Neves P. P. , de Lazaro S. R. , Longo E. , Michalowicz A. , Eiras J. A.
Published: March 2006

The electronic structure of Pb1−xLaxTiO3 (PLT) compounds for x ranging from 0to30at.% of La is investigated by means of soft x-ray absorption near edge structure (XANES) at the Ti L3,2 and O K edges. The greatest modification in the structure of the Ti 2p XANES spectra of the PLT compounds is observed in the region of the high energy peak of the L3 edge (eg states), which exhibits a splitting in the undoped sample. As the amount of lanthanum increases, this splitting becomes less pronounced. This modification is interpreted as a decrease in the degree of disorder of titanium atoms, which is correlated to the substitution of Pb by La atoms. The structural changes observed at the low energy peaks of the O K-edge XANES spectra of the PLT compounds may be interpreted in terms of hybridization between O 2p, Ti 3d, and Pb 6p orbitals. A decrease in the degree of hybridization observed as Pb atoms are replaced by La atoms may be related to the differences in the ferroelectric properties observed between x=0.0 and x=0.30 compounds.
Dopant regions imaging in scanning electron microscopy
Authors: Morandi Vittorio , Merli Pier Giorgio , Ferroni Matteo
Published: March 2006

An approach to the dopant profiling in bulk specimens with the scanning electron microscope is presented. It will be shown that it is possible to use backscattered electrons, or secondary electrons produced by backscattered electrons, to obtain two dimensional information about the dopant spatial distribution in Sb-implanted silicon. The role of sample preparation, boundary condition, beam energy, and detection strategy will be extensively discussed. Experimental observation and numerical simulation highlight the capability to achieve the sensitivity and spatial resolution required to describe the dopant distribution in the high-dose near-surface region of ultrashallow junctions.
ZnSe interlayer effects on properties of (CdS∕ZnSe)∕BeTe superlattices grown by molecular beam epitaxy
Authors: Li B. S. , Akimoto R. , Akita K. , Hasama T.
Published: March 2006

We study the dependence of structural properties on the thickness of the ZnSe interlayer (IL) in (CdS∕ZnSe)∕BeTe superlattices (SLs); this is crucial for improving the growth mode in this heterostructure. The in situ reflection of high-energy electron diffraction oscillation and high-resolution x-ray diffraction spectra show a perfect structure that is obtained by introducing a ZnSe IL between CdS and BeTe. An intersubband transition (ISB-T) down to 1.57μm with a full width at half maximum of 90meV has been observed in (CdS∕ZnSe)∕BeTe SLs. A strong ISB-T is observed when the ZnSe IL ⩾1 monolayer (ML); however, it completely disappears with the introduction of a 0.5 ML ZnSe IL in (CdS∕ZnSe)∕BeTe SLs. High-resolution transmission electron microscopy images reveal that a sharp interface is formed in the barrier and well transition region in the structure with ZnSe IL ⩾1 ML; however, the interfaces become rough and thick in those with a 0.5 ML ZnSe IL. This indicates that the properties of the interface in (CdS∕ZnSe)∕BeTe SLs play an important role in the performance of ISB absorption. A growth model is proposed to explain two-dimensional growth with the insertion of a ZnSe ML into the BeTe∕CdS heterostructure. Based on this growth model, the properties of the interface in this structure are explained.
Electromagnetic topology quasisolutions for aperture interactions using transmission line matrix
Authors: Kirawanich Phumin , Gleason David , Yakura Susumu J. , Islam N. E.
Published: March 2006

A hybrid simulation technique that integrates transmission line matrix method with electromagnetic topology solutions has been employed to link a field scattering problem at an aperture in order to analyze the frequency and temporal characteristics of electromagnetic pulses. Using this same unified multiconductor transmission line network formulation any subsequent coupling with cables at the other side of the aperture can be integrated into the solution. The hybrid circuit can also be integrated to any existing topological simulation circuit for analyzing very large electrical systems. Incorporation of the compaction technique in the topological simulation reduces the number of simulation grids significantly, resulting in efficient computation without sacrificing accuracy. The simulation results for scattering fields at the aperture compare well with the finite-difference time domain method.
Singularities of Møller-Plesset energy functions
Authors: Sergeev Alexey V. , Goodson David Z.
Published: March 2006

The convergence behavior of Møller-Plesset (MP) perturbation series is governed by the singularity structure of the energy, with the energy treated as a function of the perturbation parameter. Singularity locations, determined from quadratic approximant analysis of high-order series, are presented for a variety of atoms and small molecules. These results can be used as benchmarks for understanding the convergence of low-order methods such as MP4 and for developing and testing summation methods that model the singularity structure. The positions and types of singularities confirm previous qualitative predictions based on functional analysis of the Schrödinger equation.
Damping rate of magnetohydrodynamic vortices at low magnetic Reynolds number
Authors: Ueno Kazuyuki , Moreau René
Published: March 2006

The damping rate of vortices in an electrically conducting fluid submitted to a uniform magnetic field is analyzed for a large Hartmann number Ha. The fluid is contained in a layer of constant thickness h, bounded by two insulating walls that are perpendicular to the magnetic field. The damping times and the eigenfunctions along the magnetic field are obtained from a linear eigenvalue problem. According to the damping times and these eigenfunctions, vortices are classified into several classes by the range of combinations of the mode number m in the magnetic field direction and the wave number k2D in the plane perpendicular to the magnetic field. It is found that the damping rate of vortices in the range of k2D∼[(m+12)πHa]1∕2h−1 and m=0,1,2 is of the same order as that of large-scale two-dimensional vortices. This fact suggests that actual quasi-two-dimensional magnetohydrodynamic turbulent flows include not only m=0 but also higher-mode (m⩾1) eigenfunctions of this wave-number range, although the eigenfunction of m=0 has a 30% variation and the higher-mode eigenfunctions change their sign along the magnetic field.
First and second type self-similar solutions of implosions and explosions containing ultrarelativistic shocks
Authors: Sari Re’em
Published: March 2006

We derive self-similar solutions for ultrarelativistic shock waves propagating into cold material of power law density profile in radius ρ∝r−k. We treat both implosions and explosions in three geometries: planar, cylindrical, and spherical. For spherical explosions these are the first type solutions of Blandford and McKee for k<4; they are the second type solutions found by Best and Sari for k>5−3∕4. In addition we find new, hollow (with evacuated interior), first type solutions that may be applicable for 4<k<17∕4. This “sequence” with increasing k of first type solutions, hollow first type solutions, and then second type solutions is reminiscent of the nonrelativistic sequence. However, although in the nonrelativistic case there is a range of k which corresponds to a “gap”—a range in k with neither first nor second type solution which separates the hollow first type solutions and the second type solutions, here there is an “overlap”: a range of k for which current considerations allow for both hollow first and second type solutions. Further understanding is needed to determine which of the two solutions apply in this overlap regime. We provide similar exploration for the other geometries and for imploding configurations. Interestingly, we find a gap for imploding spherical shocks and exploding planar shocks and an overlap for imploding planar solutions. Cylindrical configurations have no hollow solutions and exhibit a direct transition from first type to second type solutions, without a gap or an overlap region.
Unsteady undular bores in fully nonlinear shallow-water theory
Authors: El G. A. , Grimshaw R. H. J. , Smyth N. F.
Published: March 2006

We consider unsteady undular bores for a pair of coupled equations of Boussinesq-type which contain the familiar fully nonlinear dissipationless shallow-water dynamics and the leading-order fully nonlinear dispersive terms. This system contains one horizontal space dimension and time and can be systematically derived from the full Euler equations for irrotational flows with a free surface using a standard long-wave asymptotic expansion. In this context the system was first derived by Su and Gardner. It coincides with the one-dimensional flat-bottom reduction of the Green-Naghdi system and, additionally, has recently found a number of fluid dynamics applications other than the present context of shallow-water gravity waves. We then use the Whitham modulation theory for a one-phase periodic travelling wave to obtain an asymptotic analytical description of an undular bore in the Su-Gardner system for a full range of “depth” ratios across the bore. The positions of the leading and trailing edges of the undular bore and the amplitude of the leading solitary wave of the bore are found as functions of this “depth ratio.” The formation of a partial undular bore with a rapidly varying finite-amplitude trailing wavefront is predicted for “depth ratios” across the bore exceeding 1.43. The analytical results from the modulation theory are shown to be in excellent agreement with full numerical solutions for the development of an undular bore in the Su-Gardner system.
A note on chirality in NMR spectroscopy
Authors: Harris R. A. , Jameson Cynthia J.
Published: March 2006

Using simple symmetry arguments we give proofs of the derivations of the manifestation of chirality in the chemical shift and spin-spin coupling constant in nuclear magnetic resonance and relate our proofs to earlier discussions.
Observation of difference-frequency generation by mixing of terahertz and near-infrared laser beams in a GaSe crystal
Authors: Shi Wei , Ding Yujie J. , Fernelius Nils , Ken Hopkins F.
Published: March 2006

We have observed difference-frequency generation by mixing terahertz and near-infrared laser beams in a GaSe crystal (i.e., terahertz frequency upconversion). This process has a potential for detecting terahertz waves. Using an InGaAs photodiode operating at room temperature the minimum detectable energy per pulse was 9.75nJ. When a fast photomultiplier tube was used instead, the lowest-energy per pulse was measured to be 245pJ. In addition, the temporal profile of the terahertz pulses was measured with a ns resolution.
Effects of conjugation length, electron donor and acceptor strengths on two-photon absorption cross sections of asymmetric zinc-porphyrin derivatives
Authors: Rubio-Pons Òscar , Luo Yi , Ågren Hans
Published: March 2006

Exceptionally large two-photon absorption cross sections at the infrared region have been revealed by time-dependent density functional theory calculations for asymmetric charge-transfer conjugated zinc-porphyrin derivatives. The largest two-photon cross section is found to be more than one order of magnitude larger than for the conventional two-photon active organic molecules. The calculations show that the formation of strong charge-transfer states depends on the length of the conjugation bridge between the zinc-porphyrin core and the electron donor∕acceptor. The two-photon absorption cross section can be greatly enhanced by increasing the strengths of the electron donor∕acceptor.
Low-switching-energy and high-repetition-frequency all-optical flip-flop operations of a polarization bistable vertical-cavity surface-emitting laser
Authors: Mori Takashi , Yamayoshi Yasuhiro , Kawaguchi Hitoshi
Published: March 2006

Subfemtojoule polarization bistable switching in a vertical-cavity surface-emitting laser (VCSEL) is experimentally demonstrated. All-optical flip-flop operation of the VCSEL was performed using two orthogonally polarized injection light pulses. The optimum wavelengths of the two injection pulses for achieving minimum switching power were different and corresponded to the lasing wavelengths of the two polarization states of the laser. The pulse width/switching frequency dependence of the injection pulses showed that a minimum switching energy was obtained at 1ns∕500MHz. A record low switching energy of 0.3fJ has been achieved as well as a record high switching frequency of 10GHz.
In situ monitoring of GaN epitaxial lateral overgrowth by spectroscopic reflectometry
Authors: Liu C. , Stepanov S. , Shields P. A. , Gott A. , Wang W. N. , Steimetz E. , Zettler J.-T.
Published: March 2006

The application of spectroscopic reflectometry to the monitoring of epitaxial lateral overgrowth of GaN in low pressure metalorganic vapor phase epitaxy has been investigated. Real-time vertical and lateral growth rates and hence thickness and wing width of the growing GaN are extracted. A vertical growth enhancement was clearly observed at an early stage, followed by vertical growth suppression until full coalescence was achieved. The lateral to vertical growth ratio was obtained showing clear time dependent characteristics. The observations were explained by considering the mass transport between the growing (0001) facets and the {112¯0} sidewall facets.
Designing Learning Networks in Peripheral Regions: Comparison of Two Models
Authors: Izushi Hiro , Henderson Dylan
Published: March 2006

In spite of a growth in interest in the implications of networks for regional development, those studies which closely examine applications of learning networks to peripheral regions are still few in number. The authors review how peripheral regions are less favoured in terms of technological adaptation and innovation, and they identify key tasks that network programmes need to perform. The authors consider two typical models of networks that target an industry, one programme calling for participation from all relevant sectors in the industry and the other focusing on a particular firm and its suppliers. Two network programmes in Wales are closely examined. The authors suggest that compared with the other model the ‘supply-chain model’ is a more reliable option for peripheral regions in producing tangible benefits to a wider population of its members in the relatively short
Culture, Communication, and the Information Problem in Contingent Valuation Surveys: A Case Study of a Wildlife Enhancement Scheme
Authors: Burgess Jacquelin , Clark Judy , Harrison Carolyn
Published: March 2006

Contingent valuation (CV) is a technique for providing estimates of the monetary value of public goods which have no market. The authors consider whether the information provided for the hypothetical market enables respondents to express their ‘true’ preference for the ‘good’, or whether their willingness to pay is dependent on the quantity and quality of information provided in the survey. They argue that a cultural perspective in which the CV transaction is viewed as a communicative ‘dialogue-at-a-distance’ between researchers and respondents through the medium of the CV text provides more insight into the encoding and decoding of the ‘good’—in this case an agri-environment policy to enhance nature conservation on an internationally significant wetland in South East England. They argue that, within its own scientific parameters, CV surveys are unable to capture fully all the aspects of the ‘good’ to be valued. The problem is more acute when the ‘good’ represents the uncertain outcomes (in terms of landscape and biodiversity) of a policy. Without a complete specification, which may well be an impossibility for environmental ‘goods’, respondents are able to bring their own readings to their interpretation of the scenario. This means that CV researchers cannot know precisely what ‘good’ respondents were attempting to ‘value’. The authors follow the production of the CV scenario for the valuation of the Pevensey Levels Wildlife Enhancement Scheme; conduct a critical discourse analysis to demonstrate how the linguistic and visual representations inevitably fulfil rhetorical functions; and then present the deliberations of respondents to the CV survey who participated in in-depth discussion groups after completion of the survey.
Voluntary Agreements and the Incentives for Innovation
Authors: Sunnevåg Kjell
Published: March 2006

Various parties have expressed serious concerns regarding the effect of environmental taxes on innovation. The argument is that innovation and investment in emission-abatement technology frequently require capital for investment. Pollution taxes siphon working capital from firms, and new investment becomes more difficult and/or more costly if it is necessary to borrow the capital from banks. In recent years, voluntary agreements have gained increased popularity and have become a central instrument in several countries' environmental policies, the Netherlands perhaps being the most notable example. A core rationale behind voluntary agreements is that they permit collective targets to be set for the entire sector, allowing for a different pace of realization of energy-efficiency improvements among the various individual companies within a sector. This provides the flexibility for firms to set their own goals in a cost-effective manner and to use their own resources to innovate and effect changes in the manner which suits them best. It has been claimed that voluntary agreements provide an improved climate for innovation. How voluntary agreements rate with regard to innovation incentives relative to environmental taxes depends on several factors, in particular the specific design of the agreement and whether the agreements are renegotiated after the market penetration of new technology. The author claims that, whatever advantages voluntary agreements have over the traditional regulatory approach, which makes mandatory particular forms of behavior or specific technological approaches, it is difficult to see that agreements would provide better incentives for innovation. Incentives for innovation may be particularly poor if the regulator requests renegotiation of the agreement with the arrival of new technology that substantially changes marginal conditions. It is proposed that if the authorities for various reasons nevertheless choose to use voluntary agreements as instruments, the agreements should be designed to be incentive-based.
Beauty and the Beast? BP's and Exxon's Positions on Global Climate Change
Authors: Rowlands Ian H
Published: March 2006

The author attempts first to determine whether there are significant differences among the major oil companies' positions on global climate change, and second to discover the reasons behind any differences found. The investigation focuses upon Exxon and BP Amoco—two of the world's largest oil companies. The differences between the two, with regard to their attitudes and actions on global climate change, are striking: whereas Exxon is continuing to act as one might initially expect (resisting proactive policies on climate change), BP Amoco appears more willing to contemplate a world that uses less oil. Attempts to explain these differences focus upon the companies' respective interests, their management structures, and their nationalities. It appears that all factors are important, at least to some degree. The paper concludes with a discussion of research limitations and suggestions.
Urban Planning, Hazardous Installations, and Blight: An Evaluation of Responses to Hazard — Development Conflict
Authors: Walker Gordon
Published: March 2006

Urban planning has played an increasing role in response to threats to health and safety, but this can create problematical conflicts with other planning priorities. The author examines how the UK planning system deals in practice with the safety implications of hazardous installations involving the storage and use of toxic, explosive, and flammable substances. The responses that have been made by local planning authorities to hazard – development conflicts in the vicinity of hazardous installations are evaluated. A distinction is made between those responses focused on development restraint through refusing permission for housing, community facilities, and other sensitive land uses, and those focused on the hazard source. It is argued that these last are becoming increasingly significant despite limitations in the statutory powers available. Through focusing on recent developments and drawing on a wide range of experience, the author adds to the existing research literature on planning and hazardous installations in which the evolution of policy and practice in this area has hitherto been rather sporadically examined. The implications of a recent policy focus on brownfield redevelopment, of new European regulations for hazardous sites, and of wider trends in relationships between industry, regulators, and communities at risk are considered.
Public Choice Theory and the Politics of Urban Containment: Voter-Centred versus Special-Interest Explanations
Authors: Pennington Mark
Published: March 2006

The policy of urban containment has lain at the heart of British land-use planning for over fifty years. The author examines the political dynamics underlying the commitment to this policy through the lens of public choice theory. The analysis suggests that macroelectoral shifts in favour of environmental protection have provided a push towards restrictive land-use planning and an emphasis on urban containment in recent years. Evidence of a ‘voluntary’ approach to regulation in other areas of environmental concern, however, suggests that the peculiar focus on containment is attributable to the political power exerted by a coalition of special interests and public sector bureaucrats who benefit most from this core of the British planning system.