bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic interactions in calcium and sodium ladder vanadates
Authors: de Graaf C. , Hozoi L. , Broer R.
Published: December 2003

Magnetic interactions in ladder vanadates are determined with quantum chemical computational schemes using the embedded cluster model approach to represent the material. The available experimental data for calcium vanadate is accurately reproduced and the nature of the interladder interaction is established to be ferromagnetic. An analysis of the main contributions to the magnetic couplings is presented and the role of the covalently bonded apex oxygen is elucidated. In the sodium vanadate, the ground state configuration of the rungs is V-3d1–O-2p5–V-3d1. We show that with this configuration good intrachain coupling constants are obtained for the high-temperature phase. The interchain coupling in NaV2O5 is predicted to be ≈34 K, ferromagnetic in nature.
Electron attachment to chlorouracil: A comparison between 6-ClU and 5-ClU
Authors: Denifl Stephan , Matejcik Stefan , Ptasinska Sylwia , Gstir Bettina , Probst Michael , Scheier Paul , Illenberger Eugen , Mark Tilmann D.
Published: December 2003

Low energy electron impact to the isomers 6-chlorouracil (6-ClU) and 5-chlorouracil (5-ClU) yields a variety of negative ion fragments with surprisingly high cross sections. These ions are dominantly formed via sharply structured resonance features at energies below the threshold for electronic excitation and result from dissociative electron attachment (DEA). The most dominant DEA channel is formation of (M–HCl)−, i.e., ejection of a neutral HCl molecule with the negative charge remaining on the ring. The reaction cross section is 9×10−18 m2 and 5×10−18 m2 for 6-Cl and 5-ClU, respectively, and thus about two orders of magnitude higher than the geometrical cross section of the molecule. Further reactions also operative via low energy resonances (<2.5 eV) are Cl− abstraction, dehydrogenation [formation of (M–H)−, M=ClU], and DEA processes associated with a ring opening. Most of the ion yield curves exhibit remarkably sharp structures which have not been observed before in DEA to a polyatomic system. Although some possibilities on their origin are discussed, their interpretation remains a challenge for theory and further experiments. While electron attachment to both 6-ClU and 5-ClU generates fragments of the same stoichiometric composition, their ion yields and also their relative intensities show some very pronounced differences which can be explained by the different structure but also the different energetic situation in the two isomers.
Intermolecular potential and second virial coefficient of the water–hydrogen complex
Authors: Hodges Matthew P. , Wheatley Richard J. , Schenter Gregory K. , Harvey Allan H.
Published: December 2003

We construct a rigid-body (five-dimensional) potential-energy surface for the water–hydrogen complex using scaled perturbation theory (SPT). An analytic fit of this surface is obtained, and, using this, two minima are found. The global minimum has C2v symmetry, with the hydrogen molecule acting as a proton donor to the oxygen atom on water. A local minimum with Cs symmetry has the hydrogen molecule acting as a proton acceptor to one of the hydrogen atoms on water, where the OH bond and H2 are in a T-shaped configuration. The SPT global minimum is bound by 1097 μEh (Eh≈4.359 744×10−18 J). Our best estimate of the binding energy, from a complete basis set extrapolation of coupled-cluster calculations, is 1076.1 μEh. The fitted surface is used to calculate the second cross virial coefficient over a wide temperature range (100–3000 K). Three complementary methods are used to quantify quantum statistical mechanical effects that become significant at low temperatures. We compare our results with experimental data, which are available over a smaller temperature range (230–700 K). Generally good agreement is found, but the experimental data are subject to larger uncertainties.
Bond and mode selectivity in the reaction of atomic chlorine with vibrationally excited CH2D2
Authors: Bechtel Hans A. , Kim Zee Hwan , Camden Jon P. , Zare Richard N.
Published: December 2003

The title reaction is investigated by co-expanding a mixture of Cl2 and CH2D2 into a vacuum chamber and initiating the reaction by photolyzing Cl2 with linearly polarized 355 nm light. Excitation of the first C–H overtone of CH2D2 leads to a preference for hydrogen abstraction over deuterium abstraction by at least a factor of 20, whereas excitation of the first C–D overtone of CH2D2 reverses this preference by at least a factor of 10. Reactions with CH2D2 prepared in a local mode containing two quanta in one C–H oscillator |2000〉− or in a local mode containing one quantum each in two C–H oscillators |1100〉 lead to products with significantly different rotational, vibrational, and angular distributions, although the vibrational energy for each mode is nearly identical. The Cl+CH2D2|2000〉− reaction yields methyl radical products primarily in their ground state, whereas the Cl+CH2D2|1100〉 reaction yields methyl radical products that are C–H stretch excited. The HCl(v=1) rotational distribution from the Cl+CH2D2|2000〉− reaction is significantly hotter than the HCl(v=1) rotational distribution from the Cl+CH2D2|1100〉 reaction, and the HCl(v=1) differential cross-section (DCS) of the Cl+CH2D2|2000〉− reaction is more broadly side scattered than the HCl(v=1) DCS of the Cl+CH2D2|1100〉 reaction. The results can be explained by a simple spectator model and by noting that the |2000〉− mode leads to a wider cone of acceptance for the reaction than the |1100〉 mode. These measurements represent the first example of mode selectivity observed in a differential cross section, and they demonstrate that vibrational excitation can be used to direct the reaction pathway of the Cl+CH2D2 reaction.
Characterization of hydrated Na+(phenol) and K+(phenol) complexes using infrared spectroscopy
Authors: Vaden Timothy D. , Lisy James M.
Published: December 2003

Hydrated alkali metal ion–phenol complexes were studied to model these species in aqueous solution for M=Na and K. IR predissociation spectroscopy in the O–H stretch region was used to analyze the structures of M+(Phenol)(H2O)n cluster ions, for n=1–4. The onset of hydrogen bonding was observed to occur at n=4. Ab initio calculations were used to qualitatively explore the types of hydrogen-bonded structures of the M+(Phenol)(H2O)4 isomers. By combining the ab initio calculations and IR spectra, several different structures were identified for each metal ion. In contrast to benzene, detailed in a previous study of Na+(Benzene)n(H2O)m [J. Chem. Phys. 110, 8429 (1999)], phenol is able to bind directly to Na+ even in the presence of four waters. This is likely the result of the σ-type interaction between the phenol oxygen and the ion. With K+, the dominant isomers are those in which the phenol O–H group is involved in a hydrogen bond with the water molecules, while with Na+, the dominant isomers are those in which the phenol O–H group is free and the water molecules are hydrogen-bonded to each other. Spectra and ab initio calculations for the M+(Phenol)Ar cluster ions for M=Na and K are reported to characterize the free phenol O–H stretch in the M+(Phenol) complex. While π-type configurations were observed for binary M+(Phenol) complexes, σ-type configurations appear to dominate the hydrated cluster ions.
Semiclassical representations of electronic structure and dynamics
Authors: Van Voorhis Troy , Reichman David R.
Published: December 2003

We use a new formulation of the semiclassical coherent state propagator to derive and evaluate several different approximate representations of electron dynamics. For each representation we examine: (1) its ability to treat quantum effects and electron correlation, (2) its expected scaling with system size, and (3) the types of systems for which it can be used. We also apply two of the methods to a pair of model problems, namely the minimal basis electron dynamics in H2 and the magnetization dynamics in a cluster model of the Kagomè lattice, in order to verify the feasibility of these approaches for realistic systems. Based on all these criteria, we find that the representation that takes the electron spins as the classical variables is particularly promising for the quantitative and qualitative description of large systems.
Photodissociation dynamics of the Kr–HBr cluster: The effect of the rare gas atom substitution
Authors: López-López S. , Garcı́a-Vela A.
Published: December 2003

The ultraviolet photolysis dynamics of Kr–HBr(v=0) is investigated by means of wave packet calculations, focusing on the fragmentation pathway Kr–HBr+ℏω→H+Kr–Br. Photolysis is simulated by starting from two different cluster initial states, namely the ground van der Waals (vdW) and an excited vdW bending state, associated with the Kr–H–Br and Kr–Br–H isomers, respectively. The results show that, for the two initial states of the cluster, the Kr–Br product yield is lower than that of Ar–Br radicals found in previous studies on Ar–HBr photolysis. Despite this decrease, the Kr–Br yield is found to be still rather high, in particular for the initial excited vdW state of Kr–HBr(v=0). In addition, the Kr–Br product state distributions exhibit a remarkably higher excitation (mainly rotational) than the corresponding Ar–Br distributions. The lower yield and higher excitation of Kr–Br as compared to Ar–Br, are attributed to a larger share of the energy available for the radical going to internal excitation in the case of the Kr–Br product. The different partition of the energy available for Kr–Br also causes significant deviations in the photolysis behavior of Kr–HBr when compared to that of Ar–HBr, in the case of the initial excited vdW state of both clusters. A common feature of the photodissociation of Kr–HBr and Ar–HBr is the manifestation of quantum interference effects in the Kr–Br and Ar–Br rotational state distributions, in the form of pronounced structures of supernumerary rotational rainbows.
Many-body effects in molecular photoionization in intense laser fields; time-dependent Hartree–Fock simulations
Authors: Suzuki Masato , Mukamel Shaul
Published: December 2003

The time evolution of the reduced single electron density matrix for eight electrons in a one-dimensional finite box potential driven by an intense laser field is calculated by numerically integrating the time-dependent Hartree–Fock equations. We study the effects of the Coulomb interaction, field intensity, and frequency on the time profile of the ionization process. Our computed saturation ionization intensity (Isat) is in good agreement with experimental results for decatetraene [Ivanov et al. J. Chem. Phys. 117, 1575 (2002)].
Rotation–libration and rotor–rotor coupling in 4-methylpyridine
Authors: Neumann M. A. , Plazanet M. , Johnson M. R. , Trommsdorff H. P.
Published: December 2003

The low temperature rotational dynamics of methyl groups in 4-methylpyridine is analyzed in terms of a model potential including rotation–libration and rotor–rotor coupling. The parameters of the model potential are adjusted by comparison of calculated with published and newly recorded inelastic neutron scattering spectra. Initial evaluations of the potential parameters of the model are obtained from molecular mechanics calculations. Experimental spectra are calculated from these potentials by numerical solution of Schrödinger’s equation for clusters of coupled rotors embedded in a bigger ensemble of rotors treated in the mean field approximation. Adjustment of the potential parameters leads to excellent agreement with the experimental spectra of protonated 4-methylpyridine, measured at well-defined spin temperatures. At higher levels of deuteration, agreement with experiment is qualitative, only. The observed deviations are attributed to the increasing frustration of the system of coupled methyl groups and mutual localization, effects leading to a phase transition around 5.5 K in isotopic mixtures, as shown in diffraction experiments.
Simulation of the reactive scattering of F+D2 on a model family of potential energy surfaces with various topographies: The correlation approach
Authors: Rusin Lev Yu. , Sevryuk Mikhail B. , Toennies J. Peter
Published: December 2003

The connection between the salient features of the potential energy surface (PES) and the dynamical characteristics of the elementary collision process is studied using a correlation approach based on quasiclassical trajectory simulations. The method is demonstrated for the reaction F+D2→D+DF for which the scattering characteristics were calculated on a model family of PES’s based on a London–Eyring–Polanyi–Sato-type five-parameter equation. The correlations between the reactive cross section and the vibrational and rotational quantum numbers and the scattering angle of the DF product, and the various parameters of the collinear and noncollinear PES’s, such as the location and height of the minimal barrier and the Sato coefficients, are reported. Although usually correlations between two variables suffice, in some cases coefficients of correlation among three variables are required. The role of nonlinear parameter dependencies in computing the correlation coefficients is also considered. The correlation approach makes it possible to examine a large set of potential surfaces without intermediate human control and obtain quantitative information.
Phase behavior of binary mixtures of sterically stabilized colloids with large size asymmetry
Authors: Hennequin Yves , Pollard Michael , van Duijneveldt Jeroen S.
Published: December 2003

Experimental phase diagrams of three types of mixtures of sterically stabilized colloids are presented. The size ratios are kept similar, 0.15⩽ξ⩽0.17, while the thickness and the chemical nature of the steric layers are varied. For all particles their effective volume fractions are calculated from their hydrodynamic radii. When their phase behavior is expressed in this way, the experimental liquidus curves all lie slightly above recent computer simulation predictions for the fluid–solid binodal of additive hard sphere mixtures. No dramatic shift of the experimental liquidus curves due to nonadditive particle interactions is observed. The dense phase is in all cases solid, with crystallites of the large spheres visible in some samples.
Electrokinetic microfluidic phenomena by a lattice Boltzmann model using a modified Poisson–Boltzmann equation with an excluded volume effect
Authors: Li Baoming , Kwok Daniel Y.
Published: December 2003

A lattice Boltzmann model (LBM) for electrokinetic microfluidics recently proposed by us [J. Colloid Interface Sci. 263, 144 (2003); Langmuir 19, 3041 (2003)] is employed by consideration of a modified Poisson–Boltzmann equation including an excluded volume effect. In our study, pressure is considered as the only external force for liquid flow. As commonly used, the Poisson–Boltzmann equation assumes only point charge, the predicted microfluidic phenomena of KCl and LiCl electrolyte solutions are theoretically the same; these phenomena, however, have been found to be experimentally different. Our LBM in conjunction with the modified Poisson–Boltzmann equation are capable of explaining this discrepancy and the results are in good agreement with recent experimental data for KCl and LiCl electrolyte solutions in pressure-driven microchannel flow, suggesting that our proposed LBM can be employed to predict the more complex microfluidic systems that might be problematic using conventional methods and electrokinetic models.
An estimate of random close packing density in monodisperse hard spheres
Authors: Jalali Payman , Li Mo
Published: December 2003

Using a simple model for local packing of hard spheres in random close packing state and statistical argument, we have shown that the corresponding packing density on average is 0.64. We assume that a central sphere forms a local cluster with three other spheres. Two of the neighboring ones are in direct contact with the central sphere that in turn support the third one that may or may not be in contact with the central sphere. Based on this cluster configuration the maximum and minimum limits of solid angles between the central sphere and its neighbors are determined. On the other hand, we use the fact that the average nearest neighbors per sphere in hard sphere glass is 12.5 when the close packed (crystal) assembly with 12 nearest-neighbors collapses. Some of these nearest neighbors are in direct contact with the central sphere, forming “touching” neighbors; and the rest may not be in direct contact with the central one but are in contact with other nearest neighbors, forming the “supported” neighbors. The topological model and the statistical data allow us to derive the random close packing density for the monodisperse sphere packing.
Spectral differences in real-space electronic structure calculations
Authors: Jordan D. K. , Mazziotti D. A.
Published: December 2003

Real-space grids for electronic structure calculations are efficient because the potential is diagonal while the second derivative in the kinetic energy may be sparsely evaluated with finite differences or finite elements. In applications to vibrational problems in chemical physics a family of methods known as spectral differences has improved finite differences by several orders of magnitude. In this paper the use of spectral differences for electronic structure is studied. Spectral differences are implemented in two electronic structure programs PARSEC and HARES which currently employ finite differences. Applications to silicon clusters and lattices indicate that spectral differences achieve the same accuracy as finite differences with less computational work.
Solution stability and variability in a simple model of globular proteins
Authors: Sear Richard P.
Published: December 2003

It is well known among molecular biologists that proteins with a common ancestor and that perform the same function in similar organisms, can have rather different amino-acid sequences. Mutations have altered the amino-acid sequences without affecting the function. A simple model of a protein in which the interactions are encoded by sequences of bits is introduced, and used to study how mutations can change these bits, and hence the interactions, while maintaining the stability of the protein solution. This stability is a simple minimal requirement on our model proteins which mimics part of the requirement on a real protein to be functional. The properties of our model protein, such as its second virial coefficient, are found to vary significantly from one model protein to another. It is suggested that this may also be the case for real proteins in vivo.
Ground state and phase transitions in a system of arg-cysteamines self-assembled on a Au(111) crystal surface
Authors: Sadreev Almas F. , Sukhunin Yurii V. , Petoral Rodrigo M. , Uvdal Kajsa
Published: December 2003

The translational and orientation order of arg-cysteamine molecules chemiabsorbed on the Au(111) crystal surface is considered. Couplings between carbon, nitrogen, and hydrogen atoms of the n-alkanethiols are approximated by the Lennard-Jones potential. Moreover, hydrogen bonds between oxygen and nitrogen and dipole–dipole interactions of the dipole moments of different atomic groups are taken into account. It is found that molecules are arranged in a 2×2 lattice and have the total symmetry C6×Z2. The critical temperature of the phase transition to the tilted state Tc1, which breaks the symmetry C6, is estimated to be extremely high. The spontaneous breakdown of the remaining symmetry Z2 leads to the twisted state of the molecules and has the critical temperature Tc2=340 K.
Adsorption, diffusion, and dissociation of molecular oxygen at defected TiO2(110): A density functional theory study
Authors: Rasmussen M. D. , Molina L. M. , Hammer B.
Published: December 2003

The properties of reduced rutile TiO2(110) surfaces, as well as the adsorption, diffusion, and dissociation of molecular oxygen are investigated by means of density functional theory. The O2 molecule is found to bind strongly to bridging oxygen vacancies, attaining a molecular state with an expanded O–O bond of 1.44 Å. The molecular oxygen also binds (with somewhat shortened bond lengths) to the fivefold coordinated Ti atoms in the troughs between the bridging oxygen rows, but only when vacancies are present somewhere in the surface. In all cases, the magnetic moment of O2 is lost upon adsorption. The expanded bond lengths reveal together with inspection of electron density and electronic density of state plots that charging of the adsorbed molecular oxygen is of key importance in forming the adsorption bond. The processes of O2 diffusion from a vacancy to a trough and O2 dissociation at a vacancy are both hindered by relative large barriers. However, we find that the presence of neighboring vacancies can strongly affect the ability of O2 to dissociate. The implications of this in connection with diffusion of the bridging oxygen vacancies are discussed.
The structure of fluids confined in crystalline slitlike nanoscopic pores: Bilayers
Authors: Sałamacha L. , Patrykiejew A. , Sokołowski S. , Binder K.
Published: December 2003

Grand canonical and canonical ensemble Monte Carlo simulation methods are used to study the structure and phase behavior of Lennard-Jones fluids confined between the parallel (100) planes of the face centered cubic crystal. Ultra thin slit pores of the width allowing for the formation of only two adsorbate layers are considered. It is demonstrated that the structure of adsorbed phases is very sensitive to the wall–wall separation and to the strength of the fluid–wall potential. It is also shown that the structure of low temperature (solid) phases strongly depends on the fluid density. In particular, when the surface field is sufficiently strong, then the high density phases may exhibit a domain wall structure, quite the same as found in monolayer films adsorbed at a single substrate wall. On the other hand, the weakening of the surface potential leads to the regime in which only the hexagonally ordered bilayer structure is stable. The phase diagrams for a series of systems are estimated. It is shown that, depending on the pore width and the temperature, the condensation leads to the formation of the commensurate or incommensurate phases. The incommensurate phases may have the domain–wall or the hexagonal structure depending on the pore width and the strength of the fluid–wall potential.
Molecular electrostatic potentials and electron densities in nitroazacubanes
Authors: Dhumal Nilesh R. , Patil Ujwala N. , Gejji Shridhar P.
Published: December 2003

Successive introduction of nitrogen atoms in the cubyl corners instead of C–NO2 groups of octanitrocubane (CNO2)8, the most powerful explosives known to date, leads to a class of energy-rich compounds known as nitroazacubanes. In present work the ab initio Hartree–Fock and hybrid density functional calculations have been carried out on the possible conformers of (CNO2)8−αNα (with α=0–8), nitroazacubanes. The charge distributions in these systems have been derived using the topography of the molecular electrostatic potential and electron density. Molecular electrostatic potential investigations reveal that of different nitroazacubane conformers, the electron-rich regions around nitro oxygens of the lowest energy conformer having face opposite nitrogen atoms within a cube are more delocalized. These conformers are predicted to have the largest difference of the energies of the highest occupied molecular orbital and lowest unoccupied molecular orbital relative to the other conformers. The dipole moments of nitroazacubanes are dependent on the nitrogen sites within a cube, caused by the resultant of C–N bond moments and nearly insensitive to position of the NO2 groups. The lowest frequency vibration (522 cm−1) suggests octa-azacubane having robust structure in the nitroazacubane series. Substitution of nitrogen atom instead of C–NO2 group leads to increase in electron density at the bond critical point of the X–N (X=C or N) bonds in a cube. The heats of formation of different nitroazacubanes were calculated by using the isodesmic reaction approach. The present calculation has shown that for the di- though hexanitroazacubanes the most destabilized conformer possess largest dipole moment and the heat of formation as well. A linear correlation of the electron density at the bond critical point of X–N bonds and the heat of formation has been obtained.
Role of dissimilar interfaces in thin films of cylinder-forming block copolymers
Authors: Lyakhova K. S. , Sevink G. J. A. , Zvelindovsky A. V. , Horvat A. , Magerle R.
Published: December 2003

We study the effect of dissimilar interfaces on the phase behavior of cylinder forming block copolymers in thin films by means of dynamic density-functional theory. In this article, we show that dissimilarity of the interfaces induces hybrid structures. These structures appear when the surface fields at the two interfaces stabilize different surface structures and/or reconstructions. We propose a general classification of hybrid structures and give an unifying description of phase behavior of cylinder forming block copolymer films. Our results are consistent with experimental observations.
Magnetic-field-assisted assembly of metal/polymer/metal junction sensors
Authors: Zhang Haiqian , Boussaad Salah , Ly Nguyen , Tao Nongjian J.
Published: December 2003

We present a method to assemble Au/polyaniline/Au junctions and demonstrate a chemical sensor application. The building blocks consist of an array of microelectrodes on a silicon chip, microfabricated metallic bars, and a thin polyaniline layer deposited on the microelectrodes or on the bars. The individual bars suspended in solution are placed, with the help of a magnetic field, across the microelectrodes to form the junctions. The polyaniline layer is ∼30 nm thick and modified with glycine-glycine-histidine oligopeptides. Strong binding of Cu2+ to the oligopeptide is converted into a conductance change of the junctions, allowing selective detection of trace amounts of Cu+2 ions.
Relationship between long-range charge-transfer excitation energy error and integer discontinuity in Kohn–Sham theory
Authors: Tozer David J.
Published: December 2003

Charge-transfer (CT) electronic excitation energies are known to be very poorly predicted by time-dependent density functional theory (TDDFT) using local exchange-correlation functionals. Insight into this observation is provided by a simple analysis of intermolecular CT excitations at infinite separation. It is argued that the first TDDFT CT excitation energy approximately underestimates the experimental excitation by the average of the integer discontinuities of the donor and acceptor molecules; errors are of the order of several electron volts.
Formation and photodepletion of cluster ion–messenger atom complexes in a cold ion trap: Infrared spectroscopy of VO+, VO2+, and VO3+
Authors: Brümmer Mathias , Kaposta Cristina , Santambrogio Gabriele , Asmis Knut R.
Published: December 2003

A novel experimental technique is described in which radiation from a free electron laser is used to measure infrared spectra of gas-phase cluster ions via vibrational predissociation of the corresponding ion–messenger atom complexes. The weakly bound complexes are formed in a temperature-controllable, radio frequency ion trap. This technique is applied to the study of the vibrational spectroscopy of the monovanadium oxide cluster cations VO+, VO2+, and VO3+.
Epitaxial growth and properties of metastable BiMnO3 thin films
Authors: Moreira dos Santos António F. , Cheetham Anthony K. , Tian Wei , Pan Xiaoqing , Jia Yunfa , Murphy Nathan J. , Lettieri James , Schlom Darrell G.
Published: December 2003

Epitaxial thin films of BiMnO3 were deposited on single-crystal substrates of (100)-oriented SrTiO3 by pulsed-laser deposition. Structural analysis by x-ray diffraction, electron diffraction, and transmission electron microscopy (TEM) indicated that the films were monoclinic and twinned with two dominant orientation relationships. The first is (111) BiMnO3 ∥ (100) SrTiO3 and ∼[1̄01] BiMnO3 ∥ 〈010〉 SrTiO3; the second is (101̄) BiMnO3 ∥ (100) SrTiO3 and ∼[121] BiMnO3 ∥ 〈010〉 SrTiO3. High-resolution TEM images revealed that there is no reaction or appreciable interdiffusion at the substrate/film interface, despite the high temperature of the substrate during deposition (∼1000 K). Magnetic characterization was carried out (both magnetization versus temperature and hysteresis loops) and the results agree with previous reports of a ferromagnetic transition with TC∼105 K. The actual value of TC in the films is a few degrees lower than the bulk material, the discrepancy being attributed to strain, nonstoichiometry, or size effects.
Water absorption and dielectric changes in crystalline poly(vinylidene fluoride-trifluoroethylene) copolymer films
Authors: Jacobson P. A. , Rosa Luis G. , Othon C. M. , Kraemer Kristin L. , Sorokin A. V. , Ducharme Stephen , Dowben P. A.
Published: December 2003

Crystalline Langmuir–Blodgett copolymer films of vinylidene fluoride with trifluoroethylene (70%:30% and 80%:20%) absorb water. Water absorption is accompanied by film swelling, as indicated by an increase in lattice spacing, sometimes by as much as 5%. This water absorption, between 0 and 40 °C, is a result of intercalation or occupation of interstitial sites between the layers of the film, not just water molecules filling voids and defect sites alone. An increase in the film capacitance is observed, although the polymer chains retain all trans configuration of the ferroelectric phase.
CaAl-based bulk metallic glasses with high thermal stability
Authors: Guo F. Q. , Poon S. J. , Shiflet G. J.
Published: December 2003

We report that binary Ca–Al alloys can be readily cast into amorphous rods of 1 mm in diameter. Upon further alloying to depress the liquidus temperature, the amorphous rod diameter is increased to 3 mm. The high glass transition temperature Tg∼210 °C and crystallization onset temperature Tonset∼240 °C or higher observed are attributed to the covalent bonding trend noted in Ca–Al alloys that exhibit complex network structures. Along with a high microhardness value of 200–235 DPH (diamond pyramid hardness) and low mass densities of 2 gm/cm3, these thermally stable light-metal alloys are recognized as potential structural amorphous metals.
Spontaneous recovery of hydrogen-degraded TiO2 ceramic capacitors
Authors: Chen W. P. , Wang Y. , Dai J. Y. , Lu S. G. , Wang X. X. , Lee P. F. , Chan H. L. W. , Choy C. L.
Published: December 2003

Degradation of TiO2-based ceramic capacitors was observed after hydrogen incorporation on the termination electrodes of the capacitors via electrolysis of water. Fourier-transform infrared (FTIR) absorption spectra analysis of polished rutile single crystals clearly showed that hydrogen was incorporated into the TiO2 lattice through the treatment. Hydrogen reduces Ti4+ to Ti3+ and increases the concentration of charge carriers. The degradation was found to exhibit a strong dependence on time at room temperature. The degraded properties were spontaneously recovered through an aging process, showing a spontaneous recovery unique to TiO2-based ceramic capacitors. It is proposed that hydrogen is metastable in TiO2 and that hydrogen-induced degradation has different stabilities among various oxide-based components and devices.
Positive temperature coefficient of resistivity in paraelectric (Ba,Sr)TiO3 thin films
Authors: Jeong Doo Seok , Ahn Kun Ho , Park Woo Young , Hwang Cheol Seong
Published: December 2003

The previously reported positive temperature coefficient of the resistivity (PTCR) observed in paraelectric BST thin films was again analyzed in terms of the non-Ohmic resistance as a result of an anode interface energy barrier induced by the high local charge density and interface energy states. The decrease in the low frequency dielectric constant, which is the dielectric constant relevant to the PTCR effect, with increasing temperature has a larger effect on the anode energy barrier than the temperature increase itself under a high bias voltage. In this case, the slope in the current-voltage curve at higher temperature decreases, which appears as the PTCR effect when the resistance is measured as a function of temperature under high voltages.
Light-emitting transistor: Light emission from InGaP/GaAs heterojunction bipolar transistors
Authors: Feng M. , Holonyak N. , Hafez W.
Published: December 2003

This letter reports the direct observation of the radiative recombination in the graded base layer of InGaP/GaAs heterojunction bipolar transistors (HBTs). For a 1 μm×16 μm emitter HBT, we demonstrate the change of the spontaneous light emission intensity (ΔIout) as the base current (Δib) of the HBT is varied from 0 to 5 mA, i.e., an HBT operating as a light-emitting transistor. We also demonstrate output light modulation from the base layer at 1 MHz with the base current modulated at 1 MHz in normal transistor mode operation of the HBT.
Magnetic bipolar transistor
Authors: Fabian Jaroslav , Žutić Igor , Das Sarma S.
Published: December 2003

A magnetic bipolar transistor is a bipolar junction transistor with one or more magnetic regions, and/or with an externally injected nonequilibrium (source) spin. It is shown that electrical spin injection through the transistor is possible in the forward active regime. It is predicted that the current amplification of the transistor can be tuned by spin.
Temperature memory effect of a nickel–titanium shape memory alloy
Authors: Zheng Yanjun , Cui Lishan , Schrooten Jan
Published: December 2003

An incomplete transformation cycle induces a kinetic stop in the following complete transformation cycle in shape memory alloys. Therefore, the kinetic stop can be regarded as a memory of the previous arrest temperature. Herein, we show that the temperature memory effect of a nickel–titanium shape memory alloy can be expanded to be operational in a very wide temperature range by prestraining and constraining, which may be exploited for various practical applications.
Carrier dynamics of high-efficiency green light emission in graded-indium-content InGaN/GaN quantum wells: An important role of effective carrier transfer
Authors: Sun Yuanping , Cho Yong-Hoon , Suh E.-K. , Lee H. J. , Choi R. J. , Hahn Y. B.
Published: December 2003

Optical properties and carrier dynamics of high-efficiency green-light-emitting InGaN/GaN multiple quantum wells (MQWs) with graded-In-content were studied by photoluminescence (PL), PL excitation, and time-resolved PL techniques. Two separated InGaN-related peaks were clearly found in PL spectra due to strong phase separation in the well of the graded-In-content InGaN MQWs. The integrated intensity of the main InGaN green emission (∼510 nm) decreased by only about a factor of 7 with increasing temperature from 10 to 300 K, indicating strong carrier localization and high quantum efficiency. Strong carrier transfer from low-In-content region with weak carrier localization to high-In-content part with strong carrier localization was observed by time-resolved PL. Therefore, we conclude that the effective carrier transfer from weakly to strongly localized states plays an important role to enhance brightness and quantum efficiency in the green-light-emitting InGaN MQWs with graded-In content.
Fast electro-optic switching in nematic liquid crystals
Authors: Alexe-Ionescu Anca-Luiza , Ionescu Andrei Th. , Barna Emil S. , Barna Valentin , Scaramuzza Nicola
Published: December 2003

The switching between on and off states of nematic liquid crystal pixels can be controlled with the well-known electro-optic effect. This effect however presents a fast response at switching on the electric field but a slow response at switching it off. Here we show a suitable choice of materials, cell geometries, surface preparations, and time dependence of the applied voltage that leads to a switch off response as fast as that at switch on. This is due to the particular conductivity mechanism in polymers with aromatic rings leading to rectifying properties when deposited on top of indium tin oxide surface. A transient negative charge at polymer–liquid crystal interface favors a faster and stronger planar orientation of the nematic molecules diminishing drastically the switch off time and increasing the contrast ratio. These facts, interesting enough from a fundamental point of view, could also lead to important technological consequences.
Brownian modulated optical nanoprobes
Authors: Behrend C. J. , Anker J. N. , Kopelman R.
Published: December 2003

Brownian modulated optical nanoprobes (Brownian MOONs) are fluorescent micro- and nanoparticles that resemble moons: one hemisphere emits a bright fluorescent signal, while an opaque metal darkens the other hemisphere. Brownian motion causes the particles to tumble and blink erratically as they rotate literally through the phases of the moon. The fluctuating probe signals are separated from optical and electronic backgrounds using principal components analysis or images analysis. Brownian MOONs enable microrheological measurements on size scales and timescales that are difficult to study with other methods. Local chemical concentrations can be measured simultaneously, using spectral characteristics of indicator dyes embedded within the MOONs.
Efficient electrophosphorescent polymer light-emitting devices using a Cs/Al cathode
Authors: Nakamura A. , Tada T. , Mizukami M. , Yagyu S.
Published: December 2003

Efficient electrophosphorescent polymer light-emitting devices have been developed using a Cs/Al cathode. The materials used were a molecularly doped poly(9-vinylcarbazole)-emissive layer with electrophosphorescent complexes: bis[2-(2′-benzothienyl)-pyridinato-N,C3′](acetylacetonate)Ir(III) as a red emitter, fac-tris(2-phenylpyridyl)Ir(III) as a green emitter, and bis[(4,6-difluorophenyl)-pyridinato-N,C2](picolinato)Ir(III) as a blue emitter. The red, green, and blue electrophosphorescent emitting devices exhibited efficient emissions of 4, 31, and 14 cd/A, respectively. An inspection of secondary ion mass spectroscopy (SIMS) profiles confirmed that Cs diffuses into both Al and the emissive layer. Also, a Cs concentration of approximately 1 atomic% was estimated to be present at the interface from x-ray photoelectron spectroscopy (XPS) profiles.
Vibrational behaviors of multiwalled-carbon-nanotube-based nanomechanical resonators
Authors: Li Chunyu , Chou Tsu-Wei
Published: December 2003

This letter studies the promising application of carbon nanotubes as nanoresonators. Both single- and double-walled carbon nanotubes are considered and the significant difference in the vibration behavior between them has been identified. The individual tube wall is treated as frame-like structures and simulated by the molecular-structural-mechanics method. The interlayer van der Waals interactions are represented by Lennard–Jones potential and simulated by a nonlinear truss rod model. The results show that fundamental frequencies of double-walled carbon nanotubes are about 10% lower than those of single-walled carbon nanotubes of the same outer diameter. The noncoaxial vibration of double-walled nanotubes begins at the third resonant frequency and does not significantly diminish the value of double-walled nanotubes as high-frequency nanoresonators.
Linewidth enhancement factor and optical gain in (GaIn)(NAs)/GaAs lasers
Authors: Gerhardt N. C. , Hofmann M. R. , Hader J. , Moloney J. V. , Koch S. W. , Riechert H.
Published: December 2003

Experimental results on the linewidth enhancement factor α of 1.3 μm (GaIn)(NAs) lasers are presented and analyzed on the basis of a comparison with theoretical data obtained from a microscopic model. Our experimental data are obtained from the shift of the Fabry–Perot modes with injection current using an approach to eliminate temperature-dependent artifacts. At the emission wavelength at threshold we find a value of 2.5 for α which clamps for varying injection current.
Organic thin-film transistors in sandwich configuration
Authors: Zhang Jian , Wang Jun , Wang Haibo , Yan Donghang
Published: December 2003

Organic thin-film transistors (OTFTs) having source/drain electrodes sandwiched between copper phthalocyanine (CuPc) and cobalt phthalocyanine (CoPc) layers, CuPc/CoPc SC OTFTs, are investigated. Comparing their properties with that of CuPc-based top-contact OTFT, field-effect mobility increases from 0.04 to 0.11 cm2/Vs, threshold voltage shifts from −13.8 to −8.9 V, and the current on/off ratio maintains at a level of 105. A top-contact OTFT with a layer of CuPc and a layer of CoPc (10%)–CuPc mixture reveals that the combination of CuPc and CoPc enhances charge injection from the source electrode into the active layer and increases the off-state current. The sandwich configuration increases the field-effect mobility, reduce the threshold voltage, and improve the on/off ratio at the same time. Our results indicate that using a double-layer of active organic materials in sandwich configuration is an effective way to improve OTFT performance.
Localization versus field effects in single InGaN quantum wells
Authors: Bell A. , Christen J. , Bertram F. , Ponce F. A. , Marui H. , Tanaka S.
Published: December 2003

The optical properties of InxGa1−xN quantum wells (x=0.13) have been studied by cathodoluminescence (CL) spectroscopy. A blueshift of the quantum well emission is observed with increasing excitation density, which can be explained by considering (a) band filling of in-plane potential fluctuations caused by compositional inhomogeneities, or (b) screening of piezoelectric fields inside the well. We have used time-resolved CL spectroscopy to distinguish between the two effects. The onset and decay of the relaxation and recombination kinetics are measured by using rectangular excitation pulses with ultrafast on and off switching and with pulse lengths sufficiently long to ensure excitation into quasi-steady-state conditions. For well widths of Lz⩽6 nm, a redshift is observed after the electron beam is switched on and a further redshift occurs after the electron beam is switched off. For Lz⩾8 nm, a blueshift is observed after the electron beam is switched on and a redshift is observed after the electron beam is switched off. We attribute the different behaviors to the dominance of localization effects for Lz⩽6 nm and the dominance of field effects for Lz⩾8 nm.
Refractive index perturbations in optical fibers resulting from frozen-in viscoelasticity
Authors: Yablon A. D. , Yan M. F. , Wisk P. , DiMarcello F. V. , Fleming J. W. , Reed W. A. , Monberg E. M. , DiGiovanni D. J. , Jasapara J. , Lines M. E.
Published: December 2003

We show that frozen-in viscoelasticity has a significant impact on the refractive index profile of optical fibers. Viscoelastic strains can be frozen into an optical fiber during draw, or by cooling down a drawn fiber from its fictive temperature while under tension. The resulting refractive index perturbation is concentrated in the highest viscosity region of the fiber, exhibits minimal birefringence, and can significantly alter critical performance characteristics. Our quantitative theory for the refractive index perturbation is in excellent agreement with measured data. Frozen-in viscoelasticity can also be harnessed to serve as the basis for optical fiber devices.
Effect of Zn2+ and Mn2+ introduction on the luminescent properties of colloidal ZnS:Mn2+ nanoparticles
Authors: Chen Li , Zhang Jiahua , Luo Yongshi , Lu Shaozhe , Wang Xiaojun
Published: December 2003

We report the observation of photoluminescence (PL) enhancement and quenching for 580 nm emission following the variation of time decay patterns, induced by Zn2+ and Mn2+ introduction, respectively, in colloidal ZnS:Mn2+ nanoparticles (NPs). The activation and quenching processes are analyzed considering surface nonradiative relaxation based on the Langmuir isotherm and Poisson statistics, respectively. The PL enhancement is most likely related to the elimination of dangling bonds from the lone pairs on surface S2− anions. The results indicate that rich Zn2+ and poor Mn2+ in colloidal ZnS:Mn2+ NPs are the key to obtain highly efficient luminescence.
Bias in the temperature of helium nanodroplets measured by an embedded rotor
Authors: Lehmann Kevin K.
Published: December 2003

The rovibrational spectra of molecules dissolved in liquid He4 nanodroplets display rotational structure. Where resolved, this structure has been used to determine a temperature that has been assumed to equal that of the intrinsic excitations of the helium droplets containing the molecules. Consideration of the density of states as a function of energy and total angular momentum demonstrates that there is a small but significant bias of the rotor populations that make the temperature extracted from a fit to its rotational level populations slightly higher than the temperature of the ripplons of the droplet. This bias grows with both the total angular momentum of the droplet and with the moment of inertia of the solute molecule.
Ion Implantation into Presolar Grains: A Theoretical Model
Authors: Verchovsky A. B. , Wright I. P. , Pillinger C. T.
Published: December 2003

A numerical model for ion implantation into spherical grains in free space has been developed. It can be applied to single grains or collections of grains with known grain-size distributions. Ion-scattering effects were taken into account using results of computer simulations. Possible isotope and element fractionation of the implanted species was investigated using this model. The astrophysical significance of the model lies in the possible identification of energetically different components (such as noble gases) implanted into presolar grains (such as diamond and SiC) and in establishing implantation energies of the components.
Evolution of Rotating AGB Stars and the s-Process Nucleosynthesis
Authors: Siess Lionel , Goriely Stéphane , Langer Norbert
Published: December 2003

We present new results on the evolution and nucleosynthesis in rotating AGB stars. We analyse the role of the gradient of mean molecular weight in the mixing process and show that neglecting this component induces a potentially strong third dredge-up. We also quantify the impact of rotation on the structure and conclude that the effects of rotation (1) mainly concern the inner, fast rotating regions of the stars and (2) are relatively weak as long as rotational mixing does not induce a deep third dredge-up. We also focus our investigations on the s-process nucleosynthesis and show that rotational mixing tends to inhibit the production of s-elements. This results from the contamination of the 13C-rich layers responsible for the neutron production by the poisonous 14N. Our calculations also indicate that the distribution of s-process elements depends sensitively on the magnitude of the diffusion coefficient. These results suggest that rotational mixing is not the main mechanism responsible for the production of s-elements in AGB stars, but that it can influence, and in particular reduce, the final enrichment in s-elements.