bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonequilibrium electron distribution functions and nonlinear thermal transport
Authors: Bochkarev S. G. , Bychenkov V. Yu. , Rozmus W.
Published: July 2004

Quasi-self-similar solutions to the stationary electron kinetic equation in diffusive approximation have been found in an inhomogeneous plasma. Electron density and temperature corresponding to these solutions satisfy a steady state plasma profile that is defined by nTa=const (a>1). The derived electron distribution functions describe particle transport, in particular thermal conduction and ambiporal electric field for the arbitrary amplitude of temperature perturbation in the wide range of particle collisionality. The quasi-self-similar solutions display enhanced algebraic tails in the isotropic part and the reduced number of energetic electrons in the anisotropic part of electron distribution functions. The quasi-self-similar theory of electron kinetics is applied to laser plasma heating and heat transport into the overdense region. Calculations of the linear Landau damping rate, growth rate of the return current instability, and dynamical form factor are presented.
Comparison of molecular dynamics with hybrid continuum–molecular dynamics for a single tethered polymer in a solvent
Authors: Barsky Sandra , Delgado-Buscalioni Rafael , Coveney Peter V.
Published: July 2004

We compare a newly developed hybrid simulation method which combines classical molecular dynamics (MD) and computational fluid dynamics (CFD) to a simulation consisting only of molecular dynamics. The hybrid code is composed of three regions: a classical MD region, a continuum domain where the dynamical equations are solved by standard CFD methods, and an overlap domain where transport information from the other two domains is exchanged. The exchange of information in the overlap region ensures that momentum, energy, and mass are conserved. The validity of the hybrid code is demonstrated by studying a single polymer tethered to a hard wall immersed in explicit solvent and undergoing shear flow. In classical molecular dynamics simulation a great deal of computational time is devoted to simulating solvent molecules, although the solvent itself is of no direct interest. By contrast, the hybrid code simulates the polymer and surrounding solvent explicitly, whereas the solvent farther away from the polymer is modeled using a continuum description. In the hybrid simulations the MD domain is an open system whose number of particles is controlled to filter the perturbative density waves produced by the polymer motion. We compare conformational properties of the polymer in both simulations for various shear rates. In all cases polymer properties compare extremely well between the two simulation scenarios, thereby demonstrating that this hybrid method is a useful way to model a system with polymers and under nonzero flow conditions. There is also good agreement between the MD and hybrid schemes and experimental data on tethered DNA in flow. The computational cost of the hybrid protocol can be reduced to less than 6% of the cost of updating the MD forces, confirming the practical value of the method.
The role of intermolecular polarization for the stability of lithium intercalation compounds of α- and β-perylene
Authors: Friedlein R. , Crispin X. , Suess C. , Pickholz M. , Salaneck W. R.
Published: July 2004

Lithium intercalation compounds of α- and β-perylene are investigated by photoelectron spectroscopy. Spectroscopic data together with a Born-Haber cycle provide information on the formation enthalpy of those materials. This approach allows understanding the amount of charge transferred from the alkali metal atoms to the π system, and illuminates the role of molecular versus solid-state properties in the formation of the intercalation compounds. In the bulk of α-perylene material, molecular dimerization survives upon intercalation which reduces the Madelung energy of the intercalation compound but increases the electron-accepting capability of the organic system and facilitates the ionization of lithium atoms in the molecular solid environment. The lower ionization potential results in a larger charge transfer (about two electrons per molecule) in α-perylene compared to the monomeric system, β-perylene.
Dewetting at soft viscoelastic interfaces
Authors: Carbone G. , Persson B. N. J.
Published: July 2004

The dewetting transition of thin liquid films (∼100 nm) at soft viscoelastic interfaces is analyzed theoretically. It is shown that viscoelastic losses in the soft material can drastically increase the time to complete the dewetting. Thus, the influence of the thinning of the liquid film, due to the hydrodynamic drainage caused by the external applied pressure, has to be considered too. The squeezing pressure coupled with the hydrodynamic drainage may slow down the dewetting to almost zero growth rate of the dry zone; in this case a trapped rim of fluid should be observed.
Excited state electronic structures and dynamics of NOCl: A new potential function set, absorption spectrum, and photodissociation mechanism
Authors: Yamashita Takefumi , Kato Shigeki
Published: July 2004

A set of analytical potential energy surfaces (PESs) for six singlet excited states of NOCl are constructed based on multireference configuration interaction calculations. The total absorption cross section at the energy range of 2–7 eV is calculated by quantum dynamics calculations with the present PESs and transition dipole moments. The calculated absorption spectrum agrees well with the experiment. It is also found that the A band with the absorption maximum at 6.3 eV is attributed to the transition to the 4 1A′ state, though the excitations to the 3 1A′ and 3 1A″ states contribute to the spectrum at the energy range between 4 and 5 eV. The spin-forbidden transitions are concluded to be negligibly weak. The mechanism of photodissociation reaction at the energy region corresponding to the A band is examined. The nonadiabatic transition rates from the 4 1A′ state to lower singlet and triplet states are estimated by Fermi’s golden rule, and the transitions to the 1 1A′ and 3 1A′ states induced by vibronic coupling are found to be the predominant dissociation pathways. The experimentally observed energy dependence of the recoil anisotropy of the fragments is discussed based on the calculated nonadiabatic transition rates.
Exact decoupling of the Dirac Hamiltonian. I. General theory
Authors: Reiher Markus , Wolf Alexander
Published: July 2004

Exact decoupling of positive- and negative-energy states in relativistic quantum chemistry is discussed in the framework of unitary transformation techniques. The obscure situation that each scheme of decoupling transformations relies on different, but very special parametrizations of the employed unitary matrices is critically analyzed. By applying the most general power series ansatz for the parametrization of the unitary matrices it is shown that all transformation protocols for decoupling the Dirac Hamiltonian have necessarily to start with an initial free-particle Foldy–Wouthuysen step. The purely numerical iteration scheme applying X-operator techniques to the Barysz–Sadlej–Snijders (BSS) Hamiltonian is compared to the analytical schemes of the Foldy–Wouthuysen (FW) and Douglas–Kroll–Hess (DKH) approaches. Relying on an illegal 1/c expansion of the Dirac Hamiltonian around the nonrelativistic limit, any higher-order FW transformation is in principle ill defined and doomed to fail, irrespective of the specific features of the external potential. It is shown that the DKH method is the only valid analytic unitary transformation scheme for the Dirac Hamiltonian. Its exact infinite-order version can be realized purely numerically by the BSS scheme, which is only able to yield matrix representations of the decoupled Hamiltonian but no analytic expressions for this operator. It is explained why a straightforward numerical iterative extension of the DKH procedure to arbitrary order employing matrix representations is not feasible within standard one-component electronic structure programs. A more sophisticated ansatz based on a symbolical evaluation of the DKH operators via a suitable parser routine is needed instead and introduced in Part II of this work.
Conjugate-gradient optimization method for orbital-free density functional calculations
Authors: Jiang Hong , Yang Weitao
Published: July 2004

Orbital-free density functional theory as an extension of traditional Thomas-Fermi theory has attracted a lot of interest in the past decade because of developments in both more accurate kinetic energy functionals and highly efficient numerical methodology. In this paper, we developed a conjugate-gradient method for the numerical solution of spin-dependent extended Thomas-Fermi equation by incorporating techniques previously used in Kohn-Sham calculations. The key ingredient of the method is an approximate line-search scheme and a collective treatment of two spin densities in the case of spin-dependent extended Thomas-Fermi problem. Test calculations for a quartic two-dimensional quantum dot system and a three-dimensional sodium cluster Na216 with a local pseudopotential demonstrate that the method is accurate and efficient.
Photodissociation of diiodomethane in acetonitrile solution and fragment recombination into iso-diiodomethane studied with ab initio molecular dynamics simulations
Authors: Odelius Michael , Kadi Malin , Davidsson Jan , Tarnovsky Alexander N.
Published: July 2004

Photodissociation of diiodomethane (CH2I2) in acetonitrile solution has been studied with ab initio molecular dynamics simulations, which show how the iso-diiodomethane photoproduct (CH2I-I) can be formed. The first excited state, described by the “restricted open-shell Kohn-Sham” density functional method, is dissociative and photoexcitation of diiodomethane induces a breaking of one of the C-I bonds. In the simulations, we observe that energy dissipation to the surrounding solvent is essential in the formation of a stable iso-diiodomethane molecule. The caging effect of the solvent results in a recombination of the CH2I and I fragments into iso-diiodomethane on a picosecond time scale. The molecular dynamics simulations enable us to study the cage effect as well as the relaxation of intermediates and the distribution of energy. The CH2I fragment is formed vibrationally excited along the C-I stretching mode. After recombination of the CH2I and I fragments, iso-diiodomethane shows a strong vibration excitation in the CH2 group, which could be used as a fingerprint of the proposed mechanism.
First principles simulation of the UV absorption spectrum of ethylene using the vertical Franck-Condon approach
Authors: Hazra Anirban , Chang Hannah H. , Nooijen Marcel
Published: July 2004

A new method which we refer to as vertical Franck-Condon is proposed to calculate electronic absorption spectra of polyatomic molecules. In accord with the short-time picture of spectroscopy, the excited-state potential energy surface is expanded at the ground-state equilibrium geometry and the focus of the approach is more on the overall shape of the spectrum and the positions of the band maxima, rather than the precise position of the 0-0 lines. The Born-Oppenheimer approximation and the separability of the excited-state potential energy surface along the excited-state normal mode coordinates are assumed. However, the potential surface is not necessarily approximated as harmonic oscillator potentials along the individual normal modes. Instead, depending upon the nature of the potential surface along a particular normal mode, it is treated either in the harmonic approximation or the full one-dimensional potential is considered along this mode. The vertical Franck-Condon approach is applicable therefore even in cases where the excited state potential energy surface is highly anharmonic and the conventional harmonic Franck-Condon approach is inadequate. As an application of the method, the ultraviolet spectrum of ethylene between 6.2 eV (50 000 cm−1) and 8.7 eV (70 000 cm−1) is simulated, using the Similarity Transformed Equation of Motion Coupled-Cluster method to describe the required features of the potential energy surfaces. The spectrum is shown to be a result of sharp doublet structures stemming from the π→3s (Rydberg) state superimposed on top of a broad band resulting from the π→π* (valence) state. For the Rydberg state, the symmetric C=C stretch and the torsion mode contribute to the spectrum, while the broad valence band results from excitation into the C=C stretch, CH2 scissors, and the torsion mode. For both states, the potential along the torsion mode is highly anharmonic and the full treatment of the potential along this mode in the vertical Franck-Condon method is required.
The evolution dynamics of model proteins
Authors: Tiana Guido , Dokholyan Nikolay V. , Broglia Ricardo A. , Shakhnovich Eugene I.
Published: July 2004

Explicit simulations of protein evolution, where protein chains are described at a molecular, although simplified, level provide important information to understand the similarities found to exist between known proteins. The results of such simulations suggest that a number of evolutionary-related quantities, such as the distribution of sequence similarity for structurally similar proteins, are controlled by evolutionary kinetics and do not reflect an equilibrium state. An important result for phylogeny is that a subset of the residues of each protein evolve on a much larger time scale than the other residues.
On the measurement of the Hartmann layer thickness in a high magnetic field
Authors: Messadek Karim , Moreau René
Published: July 2004

External measurements of global electric quantities (current and voltage) have been performed in a high magnetic field magnetohydrodynamic experiment. It is shown that, when the magnetic field intensity B is high enough, the electric resistance of the whole fluid domain varies as 1/δ. The classical scaling law for the Hartmann layer δ=1/Bρν/σ=H/Ha (H stands for a typical length along field lines and Ha=σ/ρν BH for the Hartmann number) is experimentally checked with less than 10% error.
A Kirkwood-Buff derived force field for the simulation of aqueous guanidinium chloride solutions
Authors: Weerasinghe Samantha , Smith Paul E.
Published: July 2004

A force field for the simulation of aqueous guanidinium chloride solutions is presented. The force field was parametrized to reproduce the experimental density and Kirkwood-Buff integrals as a function of composition. Consequently, a reasonable description of the salt activity is obtained. The model also performs well for other properties such as the relative permittivity and enthalpy of mixing.
Optically nonlinear energy transfer in light-harvesting dendrimers
Authors: Andrews David L. , Bradshaw David S.
Published: July 2004

Dendrimeric polymers are the subject of intense research activity geared towards their implementation in nanodevice applications such as energy harvesting systems, organic light-emitting diodes, photosensitizers, low-threshold lasers, and quantum logic elements, etc. A recent development in this area has been the construction of dendrimers specifically designed to exhibit novel forms of optical nonlinearity, exploiting the unique properties of these materials at high levels of photon flux. Starting from a thorough treatment of the underlying theory based on the principles of molecular quantum electrodynamics, it is possible to identify and characterize several optically nonlinear mechanisms for directed energy transfer and energy pooling in multichromophore dendrimers. Such mechanisms fall into two classes: first, those where two-photon absorption by individual donors is followed by transfer of the net energy to an acceptor; second, those where the excitation of two electronically distinct but neighboring donor groups is followed by a collective migration of their energy to a suitable acceptor. Each transfer process is subject to minor dissipative losses. In this paper we describe in detail the balance of factors and the constraints that determines the favored mechanism, which include the excitation statistics, structure of the energy levels, laser coherence factors, chromophore selection rules and architecture, possibilities for the formation of delocalized excitons, spectral overlap, and the overall distribution of donors and acceptors. Furthermore, it transpires that quantum interference between different mechanisms can play an important role. Thus, as the relative importance of each mechanism determines the relevant nanophotonic characteristics, the results reported here afford the means for optimizing highly efficient light-harvesting dendrimer devices.
Air-induced fluorescence bursts from single semiconductor nanocrystals
Authors: Müller J. , Lupton J. M. , Rogach A. L. , Feldmann J. , Talapin D. V. , Weller H.
Published: July 2004

We observe a dramatic enhancement of the fluorescence intensity from single core∕shell CdSe∕ZnS nanocrystals upon sudden exposure to air from an evacuated surrounding. Both the number of particles contributing to emission increases as well as the average emission intensity from a single particle, leading to an overall fluorescence rise by a factor of 60. A common power-law distribution of both on- and off times of single nanocrystals is observed independent of shell thickness and environment. We propose that electron transfer to oxygen, which is facilitated by the presence of water, leads to a neutralization of charged, nonemissive nanocrystals.
Colossal magnetoresistance materials-based junctions with antiferromagnetic insulating barriers
Authors: Alldredge L. M. B. , Suzuki Y.
Published: July 2004

We have fabricated epitaxial La0.7Sr0.3MnO3∕La0.35Ca0.65MnO3∕La0.7Sr0.3MnO3 trilayer magnetic tunnel junctions in order to produce high-quality interfaces through the use of an isostructural barrier layer. The barrier we have chosen (La0.35Ca0.65MnO3) is an antiferromagnetic insulator with a Néel temperature of ∼160K. We observed junction magnetoresistances (JMRs) of up to 4% at 5K. The large energy cost to flip a spin in an antiferromagnet preserves some degree of spin orientation during transport through the junctions. However, magnetic coupling between the electrode and the barrier inevitably suppresses the JMR. Despite the magnetic coupling, an antiferromagnetic barrier does not suppress the JMR completely.
Polarization storage by nonlinear orientational hole burning in azo dye-containing polymer films
Authors: Maeda Makoto , Ishitobi Hidekazu , Sekkat Zouheir , Kawata Satoshi
Published: July 2004

We present experimental evidence of polarization storage by nonlinear orientational hole burning in films of poly(methylmethacrylate) containing disperse red one (DR1). The data are written by polarized nonlinear excitation of DR1 by a tightly focused Ti: Sapphire laser and read by polarized confocal laser scanning microscopy in the reflection mode. The bits that are imaged with a light polarization which is parallel to the excitation laser polarization reflect light ∼2.5 times more efficiently than those which are imaged with a light polarization which is perpendicular to it. This finding is consistent with orientational hole burning of chromophores by two-photon excitation.
Ferroelectric domain reversal in LiNbO3 crystals using high-voltage atomic force microscopy
Authors: Agronin A. , Rosenwaks Y. , Rosenman G.
Published: July 2004

High-voltage atomic force microscopy is used for nanoscale polarization reversal in LiNbO3 single crystals. The tailored domain patterns have been observed using piezoelectric force microscopy and etching techniques. A variety of domain shapes preserving the elementary crystallographic symmetry have been obtained. It has been found that the sidewise domain wall motion under the huge electric field near the apex of atomic force microscope tip occurs in isotropic manner. The dependence of the domain equilibrium size on the applied high voltage is analyzed and discussed.
Effects of applied electric field on orientational photorefraction in porphyrin:Zn-doped nematic liquid crystals
Authors: Kim Ki Hyun , Kim Eun Ju , Lee Sang Jo , Lee Jung Hoon , Kwak Chong Hoon , Kim Jeong Eun
Published: July 2004

We observed an orientational photorefractive effect in porphyrin:Zn-doped nematic liquid crystals by measuring two beam coupling gain and diffraction efficiency under the influence of applied electric field. The gain and diffraction efficiency curves against applied electric field typically reveal resonant type curves, rapidly increasing to its maximum values and then gradually decreasing, but have clearly distinct peak positions. Based on the material equations and the torque balance equation of director axis reorientation of liquid crystals, we theoretically derived the expressions for the orientational photorefractive gain and diffraction efficiency, showing good agreement with the experimental results.
Spin-wave spectra of perpendicularly magnetized circular submicron dot arrays
Authors: Kakazei G. N. , Wigen P. E. , Guslienko K. Yu. , Novosad V. , Slavin A. N. , Golub V. O. , Lesnik N. A. , Otani Y.
Published: July 2004

Dynamic microwave properties of arrays of circular Ni and Ni81Fe19 dots were studied by X-band ferromagnetic resonance (FMR) technique. All of the dots had the same radius 0.5μm, thickness 50–70nm, and were arranged into rectangular or square array with different interdot separations. In the case of perpendicular magnetization multiple (up to 8) sharp resonance peaks were observed below the main FMR peak, and the relative positions of these peaks were independent of the interdot separations. Quantitative description of the observed multiresonance FMR spectra is given using the dipole-exchange spin wave dispersion equation for a perpendicularly magnetized film where in-plane wave vector is quantized due to the finite dot radius, and the inhomogenetiy of the intradot static demagnetization field in the nonellipsoidal dot is taken into account.
Liquid-crystal electric tuning of a photonic crystal laser
Authors: Maune Brett , Lončar Marko , Witzens Jeremy , Hochberg Michael , Baehr-Jones Thomas , Psaltis Demetri , Scherer Axel , Qiu Yueming
Published: July 2004

An electrically tuned nematic liquid-crystal (LC) infiltrated photonic crystal (PC) laser is demonstrated. This PC laser represents an emerging class of nanoscale optical adaptive devices enabled by the convergence of nonlinear optical materials, electronics, and fluidics that promise increased functionality and utility over existing technologies. A LC cell is constructed by encasing the PC laser between two indium tin oxide glass plates, which serve as the modulating electrodes. Applying a voltage across the cell realigns the LC, modifies the laser cavity’s optical path length, and blueshifts the lasing wavelength.
Radiative and Auger recombination in 1.3μm InGaAsP and 1.5μm InGaAs quantum-well lasers measured under high pressure at low and room temperatures
Authors: Jin S. R. , Sweeney S. J. , Ahmad C. N. , Adams A. R. , Murdin B. N.
Published: July 2004

We report on the pressure dependence of the threshold current in 1.3μm InGaAsP and 1.5μm InGaAs quantum-well lasers measured at low temperatures ∼100K. It was found that the threshold current of both devices slowly increases with increasing pressure (i.e., increasing band gap) at ∼100K consistent with the calculated variation of the radiative current. In contrast, at room temperature we observed a reduction of the threshold current with increasing pressure. Our low-temperature, high-pressure data confirm the results of previous atmospheric pressure measurements on the same devices which indicated a transition in the dominant recombination mechanism from radiative to Auger as the device temperature is increased from ∼100 to 300K.
Observation of subsurface monolayer thickness fluctuations in InGaN∕GaN quantum wells by scanning capacitance microscopy and spectroscopy
Authors: Zhou X. , Yu E. T. , Florescu D. , Ramer J. C. , Lee D. S. , Armour E. A.
Published: July 2004

Scanning capacitance microscopy and spectroscopy combined with numerical simulations have been used to image nanoscale electronic structures in In0.30Ga0.70N∕GaN quantum-well heterostructures grown by metalorganic chemical vapor deposition. Macroscopic capacitance–voltage spectroscopy and numerical simulations indicate that, depending on the bias voltage applied, either electron or hole accumulation in the n-type quantum-well region can occur. Scanning capacitance microscope images reveal local variations in electronic properties with structure similar to that of monoatomic steps observable in surface topography. Scanning capacitance spectroscopy combined with numerical simulations indicates that the observed features correspond to variations in carrier concentration arising from monolayer fluctuations in the thickness of the subsurface In0.30Ga0.70N quantum-well layer, with thickness variations occurring over distances of tens of nanometers to a micron or more.
Single crystal SrTiO3 field-effect transistor with an atomically flat amorphous CaHfO3 gate insulator
Authors: Shibuya Keisuke , Ohnishi Tsuyoshi , Lippmaa Mikk , Kawasaki Masashi , Koinuma Hideomi
Published: July 2004

We have observed prominent transistor action in a transition-metal oxide which is not traditionally viewed as a semiconductor. We have fabricated a field-effect transistor composed of SrTiO3(100) single crystal as a channel and an amorphous CaHfO3 layer as a gate insulator. The amorphous CaHfO3 gate insulator layer was atomically flat and had a breakdown field of over 5MV∕cm. All electrode and channel patterning was done with simple contact masks. The whole fabrication process took place at room temperature. The device showed a field-effect mobility of 0.4–0.5cm2∕Vs and an on-to-off channel current ratio of ∼105 at room temperature.
Surface plasmons at single nanoholes in Au films
Authors: Yin L. , Vlasko-Vlasov V. K. , Rydh A. , Pearson J. , Welp U. , Chang S.-H. , Gray S. K. , Schatz G. C. , Brown D. B. , Kimball C. W.
Published: July 2004

The generation of surface plasmon polaritons (SPPs) at isolated nanoholes in 100nm thick Au films is studied using near-field scanning optical microscopy. Finite-difference time-domain calculations, some explicitly including a model of the scanning tip, are used to interpret the results. We find the holes act as point-like sources of SPPs and demonstrate that the interference between the SPP and a directly transmitted wave allows determination of the wavelength, phase, and decay length of the SPP. The near-field intensity patterns can be manipulated by varying the angle and polarization of the incident beam.
Hydrogen diffusion as the rate-limiting mechanism of stress development in dielectric films
Authors: Hughey Michael P. , Cook Robert F.
Published: July 2004

The kinetics of irreversible tensile stress development during annealing of dielectric films fabricated by plasma-enhanced chemical vapor deposition (PECVD) are studied, and the hypothesis of a rate-limiting hydrogen diffusion process is tested. Extra-long anneals (∼36h) with in situ stress measurements have been made: experimental observations on a silicon nitride film do not display the characteristics expected of stress development limited by diffusion. Nor do these data imply a limiting first-order reaction process. Infrared spectroscopy results indicate that the amount of bonded hydrogen decreases in proportion with the stress increase, strongly implying stress development is a result of the reduction of bonded hydrogen alone. These findings demonstrate that diffusion in PECVD films does not limit stress development; instead, it is likely governed by nonstraightforward kinetics of hydrogen bond breaking, which is followed by the rapid diffusion of product molecules.
Studies of InGaN∕GaN multiquantum-well green-light-emitting diodes grown by metalorganic chemical vapor deposition
Authors: Ramaiah K. S. , Su Y. K. , Chang S. J. , Chen C. H. , Juang F. S. , Liu H. P. , Chen I. G.
Published: July 2004

InGaN ( 3 nm ) ∕ GaN ( 5 nm ) three period multiquantum green-light-emitting diodes (LEDs) grown by the metalorganic chemical vapor deposition technique have been studied using high-resolution transmission electron microscopy (HRTEM), double crystal high resolution x-ray diffraction (HRXRD) and low temperature photoluminescence. HRTEM analysis showed that the defect density gradually decreased in the growth direction with increasing thickness. Self-assembled quantum dot-like structures in the wells and black lumps between the well and barrier due to In segregation and strain contrast were observed, respectively. The HRXRD spectrum of the green LED structure was simulated using the kinematical theory method to obtain the composition and thickness of the well and barrier. The quantum-well (QW) green emission peak 2.557eV at 10K showed “S” shaped shift like a red–blue–red shift with variation of the temperature in the photoluminescence spectra due to potential fluctuations caused by inhomogeneous alloy distribution in the wells. The activation energy of 49meV obtained from the QW green emission line indicated deepening of the localization of the carriers.
X-ray absorption study of Ti-activated sodium aluminum hydride
Authors: Graetz J. , Reilly J. J. , Johnson J. , Ignatov A. Yu. , Tyson T. A.
Published: July 2004

Ti K-edge x-ray absorption near-edge spectroscopy was used to explore the Ti valence and coordination in Ti-activated sodium alanate. An empirical relationship was established between the Ti valence and the Ti K-edge onset based on a set of standards. This relationship was used to estimate oxidation states of the titanium catalyst in 2 and 4mol% Ti-doped NaAlH4. The results demonstrate that the formal titanium valence is zero in doped sodium alanate and nearly invariant during hydrogen cycling. A qualitative comparison of the edge fine structure suggests that the Ti is present on the surface in the form of amorphous TiAl3.
Attachment losses of high Q oscillators
Authors: Photiadis Douglas M. , Judge John A.
Published: July 2004

Attachment losses can play a role in limiting the quality factors of micro/nanomechanical oscillators. The existing theoretical results in this regard are applicable to highly idealized scenarios. The theory has been extended in two important directions: the width of the cantilever is considered to be small relative to a wavelength as opposed to large, and the base is allowed to have finite thickness. These extensions result in significant, in many cases order of magnitude, changes in the estimates of attachment loss. Simple formulas for Q−1 covering most of the parameter range are given.
Ionizing radiation effects on floating gates
Authors: Cellere G. , Paccagnella A. , Visconti A. , Bonanomi M.
Published: July 2004

Floating gate (FG) memories, and in particular Flash, are the dominant among modern nonvolatile memory technologies. Their performance under ionizing radiation was traditionally studied for the use in space, but has become of general interest in recent years. We are showing results on the charge loss from programmed FG arrays after 10keV x-rays exposure. Exposure to ionizing radiation results in progressive discharge of the FG. More advanced devices, featuring smaller FG, are less sensitive to ionizing radiation that older ones. The reason is identified in the photoemission of electrons from FG, since at high doses it dominates over charge loss deriving from electron/hole pairs generation in the oxides.
Fabrication of high aspect ratio 100nm metallic stamps for nanoimprint lithography using proton beam writing
Authors: Ansari K. , van Kan J. A. , Bettiol A. A. , Watt F.
Published: July 2004

We report a way of fabricating high-quality void-free high-aspect-ratio metallic stamps of 100nm width and 2μm depth, using the technique of proton beam writing coupled with electroplating using a nickel sulfamate solution. Proton beam writing is a one-step direct-write process with the ability to fabricate nanostructures with high-aspect-ratio vertical walls and smooth sides, and as such has ideal characteristics for three-dimensional (3D) stamp fabrication. Nanoindentation and atomic force microscopy measurements of the nickel surfaces of the fabricated stamp show a hardness and side-wall roughness of 5GPa and 7nm, respectively. The fabricated 100nm 3D stamps have been used to transfer test patterns into poly(methylmethacrylate) films, spin coated onto a silicon substrate. Proton beam writing coupled with electroplating offers a process of high potential for the fabrication of high quality metallic 3D nanostamps.
Nature of the acceptor responsible for p-type conduction in liquid encapsulated Czochralski-grown undoped gallium antimonide
Authors: Ling C. C. , Lui M. K. , Ma S. K. , Chen X. D. , Fung S. , Beling C. D.
Published: July 2004

Acceptors in liquid encapsulated Czochralski-grown undoped gallium antimonide (GaSb) were studied by temperature dependent Hall measurement and positron lifetime spectroscopy (PLS). Because of its high concentration and low ionization energy, a level at EV+34meV is found to be the important acceptor responsible for the p-type conduction of the samples. Two different kinds of VGa-related defects (lifetimes of 280ps and 315ps, respectively) having different microstructures were characterized by PLS. By comparing their annealing behaviors and charge state occupancies, the EV+34meV level could not be related to the two VGa-related defects.
Tunable absorption of Au–Al2O3 nanocermet thin films and its morphology
Authors: Hazra S. , Gibaud A. , Sella C.
Published: July 2004

The morphology of Au–Al2O3 nanocermet thin films, prepared by cosputtering Au and Al2O3 on float glass substrates, was studied using surface sensitive x-ray scattering techniques and the results were correlated with the optical absorption of the films measured using ultraviolet visible spectroscopy. The presence of gold nanoparticles in an alumina matrix is evident from both x-ray scattering and spectroscopic studies. The distribution of nanoparticles is obtained from grazing incidence small angle x-ray scattering, while the electron density profile obtained from the analysis of x-ray reflectivity data gives total film thickness, volume fraction (f) of Au and the special arrangement along the growth direction. Optical properties show a linear dependence of the absorption peak position with f, which is interesting for making nanocomposites of tunable absorption.
Biological imaging with a neutron microscope
Authors: Cremer J. T. , Piestrup M. A. , Gary C. K. , Pantell R. H. , Glinka C. J.
Published: July 2004

A simple microscope employing a compound refractive lens (CRL) composed of 100 biconcave lenses was used to image a biological sample with a 9.4× magnification using 10Å cold neutrons. The microscope’s resolution, 0.5mm, was primarily determined by the neutron detector 5.0mm pixel size. Unlike previous work the CRL’s field of view was large (44mm) and increased as the distance between the exit of neutron-waveguide and the specimen decreased. Short source-to-specimen distances allowed the 1.2-cm-diam CRL to view a biological sample with this field of view.
Second-order nonlinearities in the domain walls of periodically poled KTiOPO4
Authors: Fragemann A. , Pasiskevicius V. , Laurell F.
Published: July 2004

The domain wall regions in periodically poled KTiOPO4 crystals were examined and found to give rise to phasematched second harmonic generation in the Čerenkov directions. This phenomenon is caused by the nonlinear coefficients d11 and d12, which are not present in single domain regions, but are nonzero at and close to domain walls. The appearance of these nonlinearities is attributed to strain, produced by the domain inversion process and results in the creation of a dc piezoelectric field.
Synchrotron-radiation deep level transient spectroscopy for defect characterization of semiconductors
Authors: Fujioka H. , Sekiya T. , Kuzuoka Y. , Oshima M. , Usuda H. , Hirashita N. , Niwa M.
Published: July 2004

To simultaneously obtain information on both the electrical properties and the origin of defects, we have developed synchrotron-radiation deep level transient spectroscopy (SR-DLTS) and applied it to characterization of a metal/insulator Si structures. We have confirmed that SR-DLTS can provide element selective information of defects with by using the x-ray absorption edge for each element. In the Al∕AlN∕Si heterostructure, we detected two hole traps, HT1 and HT2, with activation energies of 0.074 and 0.84eV, respectively. We found that HT1 consists of continuous levels just above the valence band and is related to Si atoms at the AlN∕Si interface, whereas HT2 is a discrete level in the AlN film and related to both N and Al.
Where Alonso Meets Sierpinski: An Urban Economic Model of a Fractal Metropolitan Area
Authors: Cavailhès Jean , Frankhauser Pierre , Peeters Dominique , Thomas Isabelle
Published: July 2004

The coexistence of residential and agricultural activities within ‘periurban belts’ characterises many modern metropolitan areas. Unfortunately, few theoretical works in standard urban economics take this type of mixed space into account. This paper is an attempt to fill this gap: we present a residential location model (standard in urban economics) that is based on a support provided by fractal geography. More precisely, on the one hand, a Sierpinski carpet is used to render the nested hierarchies of the rural and urban sites within a metropolitan area. On the other hand, a household maximises, subject to a budget constraint, a Cobb – Douglas/constant elasticity of substitution (CES) utility function, wherein sub-CES functions portray the consumer's taste for variety in urban and rural amenities according to their hierarchical rank. As the household's optimum depends on accessibility to these various amenities, we propose a coding scheme of the sites on the Sierpinski carpet and a procedure for computing the distances between each site and each amenity. The urban equilibrium solution in the case of an open city is analytically determined. Numerical simulations are performed. They reveal the link between the rent gradient and the accessibility to the rural and urban amenities. In particular, we show that the rent is not always monotonous in distance from the origin (central business district) and that the structure of the fractal city is very different from that of the Thünian city when (1) commuting costs are low, (2) preferences for rural amenities are high, or (3) the substitutability of urban (rural) amenities is low.
Surveillance at the Airport: Surveilling Mobility/Mobilising Surveillance
Authors: Adey Peter
Published: July 2004

In this paper the author is concerned with the relationship between mobility and practices of surveillance, examining their interconnections within the modern airport. Recent deliberations about airports define these spaces as free, empty of power and social relationships—open to mobility. The author questions these assumptions and explores the surveillance practices that work to control and differentiate movement, bodies, and identities within the airport. Four examples are discussed, ranging from techniques that ignore mobile passengers towards those that simulate them. The airport is argued to offer perhaps a blueprint for public space, intensifying the surveillance of movement through mobilised and combined forms of monitoring. The author concludes the paper by reflecting upon the implications for the mobility and identity of the passenger as spaces such as airports become increasingly reflexive.
Rural Definitions and Short-Term Dynamics in Rural Areas of Finland in 1989–97
Authors: Muilu Toivo , Rusanen Jarmo
Published: July 2004

In this paper the authors first examine the concept of the rural area and the related typologies and then analyse changes in the internal structure of rural areas in Finland by using georeferenced data. Rural areas seem to have been defined on the basis of land use and population density for as long as they have been a topic of research, but no unambiguous rural – urban dividing line has ever emerged. Statistical areal typologies based on administrative divisions, as employed by the OECD and Eurostat, together with the national classifications prevailing in various countries, are the most commonly used tools for comparisons of spatial structures, but sociocultural analyses can also be used for defining rural areas. The empirical part of this work demonstrates that georeferenced data can be used for studying matters internal to rural areas without recourse to administrative boundaries. The example presented concerns the serious economic recession of the early 1990s, which is studied using the coordinate-based georeferenced census data collected annually by Statistics Finland from population registers. The results indicate that major changes in demographic and occupation structure and in the formation of incomes took place in the rural areas of Finland over a short space of time (1989–97) and that these affected different areas in different ways. Short-term developments of this kind cannot be observed on the basis of traditional census data produced at ten-year intervals.
A Push – Pull Framework for Modelling the Relocation of Retirees to a Retirement Village: The Australian Experience
Authors: Stimson Robert J , McCrea Rod
Published: July 2004

Although most older people prefer to age in place, nonetheless many do relocate, with a small proportion moving to retirement villages, which provide a purpose designed and built residential and lifestyle environment. Using factor analyses, path analyses, and a push – pull framework, the authors model the decision process of retirees in Australia in order to identify relationships between push – pull factors and predictor variables, using data from a national survey of retirement village residents. The push factors relate to change in lifestyle, home maintenance, social isolation, and health and mobility, whereas the pull factors relate to built environment and affordability, the locational attributes of villages, and the desire to maintain an existing lifestyle. The survey data also identify village attributes considered desirable or undesirable, or important or unimportant. Overall, resident satisfaction with moving is high.
Modelling Transboundary Air Pollution in Southeast Asia: Policy Regime and the Role of Stakeholders
Authors: Siddiqui Asif Iqbal , Quah Euston
Published: July 2004

Transboundary air pollution, popularly known as the ‘haze’, is a recent but significant environmental crisis in Southeast Asia, and is primarily the result of Indonesian forest fires. We identify that the fundamental motivation behind the practice of clearing forestland with fire by the polluters is economic in nature. We then argue that the major victims of the forest fires and haze are likely to play a vital role in the solution of the problem, at least by motivating the polluters, even if they cannot force them not to pollute. Thus, the polluters and victims can be regarded as the ‘stakeholders’ who would take interest and participate actively in the policy regime. A regional model is developed to analyze and specifically to take account of the states responsible for and affected by the haze. The problems of the affected resident population, the forest industries, and the role of the regional governments are modeled. The model also highlights the importance of key variables in determining the severity of the haze, namely the magnitude of the investment in fire fighting, risk mitigation, and scale of fire operations; including forecasting weather conditions. We argue that optimal policy options should simultaneously satisfy the economic needs of the key stakeholders to be effective.
Evolutionary dynamics of tumor suppressor gene inactivation
Authors: Nowak Martin A. , Michor Franziska , Komarova Natalia L. , Iwasa Yoh
Published: July 2004

Tumor suppressor genes (TSGs) are important gatekeepers that protect against somatic evolution of cancer. Losing both alleles of a TSG in a single cell represents a step toward cancer. We study how the kinetics of TSG inactivation depends on the population size of cells and the mutation rates for the first and second hit. We calculate the probability as function of time that at least one cell has been generated with two inactivated alleles of a TSG. We find three different kinetic laws: in small, intermediate, and large populations, it takes, respectively, two, one, and zero rate-limiting steps to inactivate a TSG. We also study the effect of chromosomal and other genetic instabilities. Small lesions without genetic instability can take a very long time to inactivate the next TSG, whereas the same lesions with genetic instability pose a much greater risk for cancer progression.
Human eosinophil chemotaxis and selective
in vivo
recruitment by sphingosine 1-phosphate
Authors: Roviezzo Fiorentina , Del Galdo Francesco , Abbate Gianfranco , Bucci Mariarosaria , D'Agostino Bruno , Antunes Edson , De Dominicis Gianfranco , Parente Luca , Rossi Francesco , Cirino Giuseppe , De Palma Raffaele
Published: July 2004

Sphingosine 1-phosphate (S1P) is a sphingolipid mediator that is involved in diverse biological functions. Local administration of S1P causes inflammation coupled to a large eosinophil (EO) recruitment in the rat-paw tissue. The inflammatory response is accompanied by an increase in S1P receptors, namely S1P
1
, S1P
2
, S1P
3
, and by an enhanced expression of CCR3, which is the main chemokine receptor known to be involved in EO function. Human EOs constitutively express S1P
1
and, at a lower extent, S1P
2
, S1P
3
receptors. S1P
in vitro
causes cultured human EO migration and an increase in S1P receptor mRNA copies and strongly up-regulates CCR3 and RANTES (regulated on activation, normal T cell-expressed and secreted) message levels; in particular CCR3 is up-regulated 18,000-fold by S1P. A blocking anti-CCR3 Ab inhibits S1P-induced chemotaxis, implying that S1P acts as specific recruiting signal for EOs not only through its own receptors but also through CCR3. These results show that S1P is involved in EO chemotaxis and contribute to shed light on the complex mechanisms underlying EO recruitment in several diseases such as asthma and some malignancies.
Host–parasite interactions and the evolution of ploidy
Authors: Nuismer Scott L. , Otto Sarah P.
Published: July 2004

Although the majority of animals and plants, including humans, are dominated by the diploid phase of their life cycle, extensive diversity in ploidy level exists among eukaryotes, with some groups being primarily haploid whereas others alternate between haploid and diploid phases. Previous theory has illuminated conditions that favor the evolution of increased or decreased ploidy but has shed little light on which species should be primarily haploid and which primarily diploid. Here, we report a discovery that emerged from host–parasite models in which ploidy levels were allowed to evolve: selection is more likely to favor diploidy in host species and haploidy in parasite species. Essentially, when parasites must evade a host's immune system or defense response, selection favors parasitic individuals that express a narrow array of antigens and elicitors, thus favoring haploid parasites over diploid parasites. Conversely, when hosts must recognize a parasite before mounting a defensive response, selection favors hosts with a broader arsenal of recognition molecules, thus favoring diploid hosts over haploid hosts. These results are consistent with the predominance of haploidy among parasitic protists.